lessonName,TopicID,Text
earth science and its branches,T_0016,"Geology is the study of the solid Earth. Geologists study how rocks and minerals form. The way mountains rise up is part of geology. The way mountains erode away is another part. Geologists also study fossils and Earths history. There are many other branches of geology. There is so much to know about our home planet that most geologists become specialists in one area. For example, a mineralogist studies minerals, as seen in (Figure 1.11). Some volcanologists brave molten lava to study volcanoes. Seismologists monitor earthquakes worldwide to help protect people and property from harm (Figure 1.11). Paleontologists are interested in fossils and how ancient organisms lived. Scientists who compare the geology of other planets to Earth are planetary geologists. Some geologists study the Moon. Others look for petroleum. Still others specialize in studying soil. Some geologists can tell how old rocks are and determine how different rock layers formed. There is probably an expert in almost anything you can think of related to Earth! Geologists might study rivers and lakes, the underground water found between soil and rock particles, or even water that is frozen in glaciers. Earth scientists also need geographers who explore the features of Earths surface and work with cartographers, who make maps. Studying the layers of rock beneath the surface helps us to understand the history of planet Earth (Figure 1.12). "
earth science and its branches,T_0017,"Oceanography is the study of the oceans. The word oceanology might be more accurate, since ology is the study of. Graph is to write and refers to map making. But mapping the oceans is how oceanography started. More than 70% of Earths surface is covered with water. Almost all of that water is in the oceans. Scientists have visited the deepest parts of the ocean in submarines. Remote vehicles go where humans cant. Yet much of the ocean remains unexplored. Some people call the ocean the last frontier. Humans have had a big impact on the oceans. Populations of fish and other marine species have been overfished. Contaminants are polluting the waters. Global warming is melting the thick ice caps and warming the water. Warmer water expands and, along with water from the melting ice caps, causes sea levels to rise. There are many branches of oceanography. Physical oceanography is the study of water movement, like waves and ocean currents (Figure 1.13). Marine geology looks at rocks and structures in the ocean basins. Chemical oceanography studies the natural elements in ocean water. Marine biology looks at marine life. "
earth science and its branches,T_0018,"Meteorologists dont study meteors they study the atmosphere! The word meteor refers to things in the air. Meteorology includes the study of weather patterns, clouds, hurricanes, and tornadoes. Meteorology is very important. Using radars and satellites, meteorologists work to predict, or forecast, the weather (Figure 1.14). The atmosphere is a thin layer of gas that surrounds Earth. Climatologists study the atmosphere. These scientists work to understand the climate as it is now. They also study how climate will change in response to global warming. The atmosphere contains small amounts of carbon dioxide. Climatologists have found that humans are putting a lot of extra carbon dioxide into the atmosphere. This is mostly from burning fossil fuels. The extra carbon dioxide traps heat from the Sun. Trapped heat causes the atmosphere to heat up. We call this global warming (Figure 1.15). "
earth science and its branches,T_0019,Environmental scientists study the ways that humans affect the planet we live on. We hope to find better ways of living that can also help the environment. Ecologists study lifeforms and the environments they live in (Figure 1.16). They try to predict the chain reactions that could occur when one part of the ecosystem is disrupted. 
earth science and its branches,T_0020,"Astronomy and astronomers have shown that the planets in our solar system are not the only planets in the universe. Over 530 planets were known outside our solar system in 2011. And there are billions of other planets! The universe also contains black holes, other galaxies, asteroids, comets, and nebula. As big as Earth seems, the entire universe is vastly more enormous. Earth is just a tiny part of our universe. Astronomers use many tools to study things in space. Earth-orbiting telescopes view stars and galaxies from the darkness of space (Figure 1.17). They may have optical and radio telescopes to see things that the human eye cant see. Spacecraft travel great distances to send back information on faraway places. Astronomers ask a wide variety of questions. How do strong bursts of energy from the Sun, called solar flares, affect communications? How might an impact from an asteroid affect life on Earth? What are the properties of black holes? Astronomers ask bigger questions too. How was the universe created? Is there life on other planets? Are there resources on other planets that people could use? Astronomers use what Earth scientists know to make comparisons with other planets. "
erosion and deposition by flowing water,T_0021,"Flowing water is a very important agent of erosion. Flowing water can erode rocks and soil. Water dissolves minerals from rocks and carries the ions. This process happens really slowly. But over millions of years, flowing water dissolves massive amounts of rock. Moving water also picks up and carries particles of soil and rock. The ability to erode is affected by the velocity, or speed, of the water. The size of the eroded particles depends on the velocity of the water. Eventually, the water deposits the materials. As water slows, larger particles are deposited. As the water slows even more, smaller particles are deposited. The graph in Figure 10.1 shows how water velocity and particle size influence erosion and deposition. "
erosion and deposition by flowing water,T_0022,"Faster-moving water has more energy. Therefore, it can carry larger particles. It can carry more particles. What causes water to move faster? The slope of the land over which the water flows is one factor. The steeper the slope, the faster the water flows. Another factor is the amount of water thats in the stream. Streams with a lot of water flow faster than streams that are nearly dry. "
erosion and deposition by flowing water,T_0023,"The size of particles determines how they are carried by flowing water. This is illustrated in Figure 10.2. Minerals that dissolve in water form salts. The salts are carried in solution. They are mixed thoroughly with the water. Small particles, such as clay and silt, are carried in suspension. They are mixed throughout the water. These particles are not dissolved in the water. Somewhat bigger particles, such as sand, are moved by saltation. The particles move in little jumps near the stream bottom. They are nudged along by water and other particles. The biggest particles, including gravel and pebbles, are moved by traction. In this process, the particles roll or drag along the bottom of the water. "
erosion and deposition by flowing water,T_0024,"Flowing water slows down when it reaches flatter land or flows into a body of still water. What do you think happens then? The water starts dropping the particles it was carrying. As the water slows, it drops the largest particles first. The smallest particles settle out last. "
erosion and deposition by flowing water,T_0025,"Water that flows over Earths surface includes runoff, streams, and rivers. All these types of flowing water can cause erosion and deposition. "
erosion and deposition by flowing water,T_0026,"When a lot of rain falls in a short period of time, much of the water is unable to soak into the ground. Instead, it runs over the land. Gravity causes the water to flow from higher to lower ground. As the runoff flows, it may pick up loose material on the surface, such as bits of soil and sand. Runoff is likely to cause more erosion if the land is bare. Plants help hold the soil in place. The runoff water in Figure 10.3 is brown because it eroded soil from a bare, sloping field. Can you find evidence of erosion by runoff where you live? What should you look for? Much of the material eroded by runoff is carried into bodies of water, such as streams, rivers, ponds, lakes, or oceans. Runoff is an important cause of erosion. Thats because it occurs over so much of Earths surface. "
erosion and deposition by flowing water,T_0027,"Streams often start in mountains, where the land is very steep. You can see an example in Figure 10.4. A mountain stream flows very quickly because of the steep slope. This causes a lot of erosion and very little deposition. The rapidly falling water digs down into the stream bed and makes it deeper. It carves a narrow, V-shaped channel. "
erosion and deposition by flowing water,T_0028,"Mountain streams may erode waterfalls. As shown in Figure 10.5, a waterfall forms where a stream flows from an area of harder to softer rock. The water erodes the softer rock faster than the harder rock. This causes the stream bed to drop down, like a step, creating a waterfall. As erosion continues, the waterfall gradually moves upstream. "
erosion and deposition by flowing water,T_0029,"Rivers flowing over gentle slopes erode the sides of their channels more than the bottom. Large curves, called meanders, form because of erosion and deposition by the moving water. The curves are called meanders because they slowly wander over the land. You can see how this happens in Figure 10.6. As meanders erode from side to side, they create a floodplain. This is a broad, flat area on both sides of a river. Eventually, a meander may become cut off from the rest of the river. This forms an oxbow lake, like the one in Figure 10.6. "
erosion and deposition by flowing water,T_0030,"When a stream or river slows down, it starts dropping its sediments. Larger sediments are dropped in steep areas, but smaller sediments can still be carried. Smaller sediments are dropped as the slope becomes less steep. Alluvial Fans In arid regions, a mountain stream may flow onto flatter land. The stream comes to a stop rapidly. The deposits form an alluvial fan, like the one in Figure 10.7. Deltas Deposition also occurs when a stream or river empties into a large body of still water. In this case, a delta forms. A delta is shaped like a triangle. It spreads out into the body of water. An example is shown in Figure 10.7. "
erosion and deposition by flowing water,T_0031,"A flood occurs when a river overflows it banks. This might happen because of heavy rains. Floodplains As the water spreads out over the land, it slows down and drops its sediment. If a river floods often, the floodplain develops a thick layer of rich soil because of all the deposits. Thats why floodplains are usually good places for growing plants. For example, the Nile River in Egypt provides both water and thick sediments for raising crops in the middle of a sandy desert. Natural Levees A flooding river often forms natural levees along its banks. A levee is a raised strip of sediments deposited close to the waters edge. You can see how levees form in Figure 10.8. Levees occur because floodwaters deposit their biggest sediments first when they overflow the rivers banks. "
erosion and deposition by flowing water,T_0032,"Some water soaks into the ground. It travels down through tiny holes in soil. It seeps through cracks in rock. The water moves slowly, pulled deeper and deeper by gravity. Underground water can also erode and deposit material. "
erosion and deposition by flowing water,T_0033,"As groundwater moves through rock, it dissolves minerals. Some rocks dissolve more easily than others. Over time, the water may dissolve large underground holes, or caves. Groundwater drips from the ceiling to the floor of a cave. This water is rich in dissolved minerals. When the minerals come out of solution, they are deposited. They build up on the ceiling of the cave to create formations called stalactites. A stalactite is a pointed, icicle-like mineral deposit that forms on the ceiling of a cave. They drip to the floor of the cave and harden to form stalagmites. A stalagmite is a more rounded mineral deposit that forms on the floor of a cave (Figure 10.9). Both types of formations grow in size as water keeps dripping and more minerals are deposited. "
erosion and deposition by flowing water,T_0034,"As erosion by groundwater continues, the ceiling of a cave may collapse. The rock and soil above it sink into the ground. This forms a sinkhole on the surface. You can see an example of a sinkhole in Figure 10.10. Some sinkholes are big enough to swallow vehicles and buildings. "
erosion and deposition by waves,T_0035,"All waves are the way energy travels through matter. Ocean waves are energy traveling through water. They form when wind blows over the surface of the ocean. Wind energy is transferred to the sea surface. Then, the energy is carried through the water by the waves. Figure 10.11 shows ocean waves crashing against rocks on a shore. They pound away at the rocks and anything else they strike. Three factors determine the size of ocean waves: 1. The speed of the wind. 2. The length of time the wind blows. 3. The distance the wind blows. The faster, longer, and farther the wind blows, the bigger the waves are. Bigger waves have more energy. "
erosion and deposition by waves,T_0036,"Runoff, streams, and rivers carry sediment to the oceans. The sediment in ocean water acts like sandpaper. Over time, they erode the shore. The bigger the waves are and the more sediment they carry, the more erosion they cause. "
erosion and deposition by waves,T_0037,Erosion by waves can create unique landforms (Figure 10.12). Wave-cut cliffs form when waves erode a rocky shoreline. They create a vertical wall of exposed rock layers. Sea arches form when waves erode both sides of a cliff. They create a hole in the cliff. Sea stacks form when waves erode the top of a sea arch. This leaves behind pillars of rock. 
erosion and deposition by waves,T_0038,"Eventually, the sediment in ocean water is deposited. Deposition occurs where waves and other ocean motions slow. The smallest particles, such as silt and clay, are deposited away from shore. This is where water is calmer. Larger particles are deposited on the beach. This is where waves and other motions are strongest. "
erosion and deposition by waves,T_0039,"In relatively quiet areas along a shore, waves may deposit sand. Sand forms a beach, like the one in Figure 10.13. Many beaches include bits of rock and shell. You can see a close-up photo of beach deposits in Figure 10.14. "
erosion and deposition by waves,T_0040,Most waves strike the shore at an angle. This causes longshore drift. Longshore drift moves sediment along the shore. Sediment is moved up the beach by an incoming wave. The wave approaches at an angle to the shore. Water then moves straight offshore. The sediment moves straight down the beach with it. The sediment is again picked up by a wave that is coming in at an angle. This motion is show in Figure 10.15 and at the link below. 
erosion and deposition by waves,T_0041,Deposits from longshore drift may form a spit. A spit is a ridge of sand that extends away from the shore. The end of the spit may hook around toward the quieter waters close to shore. You can see a spit in Figure 10.16. Waves may also deposit sediments to form sandbars and barrier islands. You can see examples of these landforms in Figure 10.17. 
erosion and deposition by waves,T_0042,"Shores are attractive places to live and vacation. But development at the shore is at risk of damage from waves. Wave erosion threatens many homes and beaches on the ocean. This is especially true during storms, when waves may be much larger than normal. "
erosion and deposition by waves,T_0043,"Barrier islands provide natural protection to shorelines. Storm waves strike the barrier island before they reach the shore. People also build artificial barriers, called breakwaters. Breakwaters also protect the shoreline from incoming waves. You can see an example of a breakwater in Figure 10.18. It runs parallel to the coast like a barrier island. "
erosion and deposition by waves,T_0044,"Longshore drift can erode the sediment from a beach. To keep this from happening, people may build a series of groins. A groin is wall of rocks or concrete that juts out into the ocean perpendicular to the shore. It stops waves from moving right along the beach. This stops the sand on the upcurrent side and reduces beach erosion. You can see how groins work in Figure 10.19. "
erosion and deposition by glaciers,T_0054,"Glaciers form when more snow falls than melts each year. Over many years, layer upon layer of snow compacts and turns to ice. There are two different types of glaciers: continental glaciers and valley glaciers. Each type forms some unique features through erosion and deposition. An example of each type is pictured in Figure 10.27. A continental glacier is spread out over a huge area. It may cover most of a continent. Today, continental glaciers cover most of Greenland and Antarctica. In the past, they were much more extensive. A valley glacier is long and narrow. Valley glaciers form in mountains and flow downhill through mountain river valleys. "
erosion and deposition by glaciers,T_0055,"Like flowing water, flowing ice erodes the land and deposits the material elsewhere. Glaciers cause erosion in two main ways: plucking and abrasion. Plucking is the process by which rocks and other sediments are picked up by a glacier. They freeze to the bottom of the glacier and are carried away by the flowing ice. Abrasion is the process in which a glacier scrapes underlying rock. The sediments and rocks frozen in the ice at the bottom and sides of a glacier act like sandpaper. They wear away rock. They may also leave scratches and grooves that show the direction the glacier moved. "
erosion and deposition by glaciers,T_0056,"Valley glaciers form several unique features through erosion. You can see some of them in Figure 10.28. As a valley glacier flows through a V-shaped river valley, it scrapes away the sides of the valley. It carves a U-shaped valley with nearly vertical walls. A line called the trimline shows the highest level the glacier reached. A cirque is a rounded hollow carved in the side of a mountain by a glacier. The highest cliff of a cirque is called the headwall. An arte is a jagged ridge that remains when cirques form on opposite sides of a mountain. A low spot in an arte is called a col. A horn is a sharp peak that is left behind when glacial cirques are on at least three sides of a mountain. "
erosion and deposition by glaciers,T_0057,"Glaciers deposit their sediment when they melt. They drop and leave behind whatever was once frozen in their ice. Its usually a mixture of particles and rocks of all sizes, called glacial till. Water from the melting ice may form lakes or other water features. Figure 10.29 shows some of the landforms glaciers deposit when they melt. Moraine is sediment deposited by a glacier. A ground moraine is a thick layer of sediments left behind by a retreating glacier. An end moraine is a low ridge of sediments deposited at the end of the glacier. It marks the greatest distance the glacier advanced. A drumlin is a long, low hill of sediments deposited by a glacier. Drumlins often occur in groups called drumlin fields. The narrow end of each drumlin points in the direction the glacier was moving when it dropped the sediments. An esker is a winding ridge of sand deposited by a stream of meltwater. Such streams flow underneath a retreating glacier. A kettle lake occurs where a chunk of ice was left behind in the sediments of a retreating glacier. When the ice melted, it left a depression. The meltwater filled it to form a lake. "
fossils,T_0064,"Fossils are preserved remains or traces of organisms that lived in the past. Most preserved remains are hard parts, such as teeth, bones, or shells. Examples of these kinds of fossils are pictured in Figure 11.1. Preserved traces can include footprints, burrows, or even wastes. Examples of trace fossils are also shown in Figure 11.1. "
fossils,T_0065,The process by which remains or traces of living things become fossils is called fossilization. Most fossils are preserved in sedimentary rocks. 
fossils,T_0066,Most fossils form when a dead organism is buried in sediment. Layers of sediment slowly build up. The sediment is buried and turns into sedimentary rock. The remains inside the rock also turn to rock. The remains are replaced by minerals. The remains literally turn to stone. Fossilization is illustrated in Figure 11.2. 
fossils,T_0067,"Fossils may form in other ways. With complete preservation, the organism doesnt change much. As seen below, tree sap may cover an organism and then turn into amber. The original organism is preserved so that scientists might be able to study its DNA. Organisms can also be completely preserved in tar or ice. Molds and casts are another way organisms can be fossilized. A mold is an imprint of an organism left in rock. The organisms remains break down completely. Rock that fills in the mold resembles the original remains. The fossil that forms in the mold is called a cast. Molds and casts usually form in sedimentary rock. With compression, an organisms remains are put under great pressure inside rock layers. This leaves behind a dark stain in the rock. You can read about them in Figure 11.3. "
fossils,T_0068,"Its very unlikely that any given organism will become a fossil. The remains of many organisms are consumed. Remains also may be broken down by other living things or by the elements. Hard parts, such as bones, are much more likely to become fossils. But even they rarely last long enough to become fossils. Organisms without hard parts are the least likely to be fossilized. Fossils of soft organisms, from bacteria to jellyfish, are very rare. "
fossils,T_0069,"Of all the organisms that ever lived, only a tiny number became fossils. Still, scientists learn a lot from fossils. Fossils are our best clues about the history of life on Earth. "
fossils,T_0070,"Fossils give clues about major geological events. Fossils can also give clues about past climates. Fossils of ocean animals are found at the top of Mt. Everest. Mt. Everest is the highest mountain on Earth. These fossils show that the area was once at the bottom of a sea. The seabed was later uplifted to form the Himalaya mountain range. An example is shown in the Figure 11.4. Fossils of plants are found in Antarctica. Currently, Antarctica is almost completely covered with ice. The fossil plants show that Antarctica once had a much warmer climate. "
fossils,T_0071,"Fossils are used to determine the ages of rock layers. Index fossils are the most useful for this. Index fossils are of organisms that lived over a wide area. They lived for a fairly short period of time. An index fossil allows a scientist to determine the age of the rock it is in. Trilobite fossils, as shown in Figure 11.5, are common index fossils. Trilobites were widespread marine animals. They lived between 500 and 600 million years ago. Rock layers containing trilobite fossils must be that age. Different species of trilobite fossils can be used to narrow the age even more. "
relative ages of rocks,T_0072,The study of rock strata is called stratigraphy. The laws of stratigraphy can help scientists understand Earths past. The laws of stratigraphy are usually credited to a geologist from Denmark named Nicolas Steno. He lived in the 1600s. The laws are illustrated in Figure 11.6. Refer to the figure as you read about the laws below. 
relative ages of rocks,T_0073,"Superposition refers to the position of rock layers and their relative ages. Relative age means age in comparison with other rocks, either younger or older. The relative ages of rocks are important for understanding Earths history. New rock layers are always deposited on top of existing rock layers. Therefore, deeper layers must be older than layers closer to the surface. This is the law of superposition. You can see an example in Figure 11.7. "
relative ages of rocks,T_0074,"Rock layers extend laterally, or out to the sides. They may cover very broad areas, especially if they formed at the bottom of ancient seas. Erosion may have worn away some of the rock, but layers on either side of eroded areas will still match up. Look at the Grand Canyon in Figure 11.8. Its a good example of lateral continuity. You can clearly see the same rock layers on opposite sides of the canyon. The matching rock layers were deposited at the same time, so they are the same age. "
relative ages of rocks,T_0075,"Sediments were deposited in ancient seas in horizontal, or flat, layers. If sedimentary rock layers are tilted, they must have moved after they were deposited. "
relative ages of rocks,T_0076,"Rock layers may have another rock cutting across them, like the igneous rock in Figure 11.9. Which rock is older? To determine this, we use the law of cross-cutting relationships. The cut rock layers are older than the rock that cuts across them. "
relative ages of rocks,T_0077,"Geologists can learn a lot about Earths history by studying sedimentary rock layers. But in some places, theres a gap in time when no rock layers are present. A gap in the sequence of rock layers is called an unconformity. Look at the rock layers in Figure 11.10. They show a feature called Huttons unconformity. The unconformity was discovered by James Hutton in the 1700s. Hutton saw that the lower rock layers are very old. The upper layers are much younger. There are no layers in between the ancient and recent layers. Hutton thought that the intermediate rock layers eroded away before the more recent rock layers were deposited. Huttons discovery was a very important event in geology! Hutton determined that the rocks were deposited over time. Some were eroded away. Hutton knew that deposition and erosion are very slow. He realized that for both to occur would take an extremely long time. This made him realize that Earth must be much older than people thought. This was a really big discovery! It meant there was enough time for life to evolve gradually. "
relative ages of rocks,T_0078,"When rock layers are in the same place, its easy to give them relative ages. But what if rock layers are far apart? What if they are on different continents? What evidence is used to match rock layers in different places? "
relative ages of rocks,T_0079,"Some rock layers extend over a very wide area. They may be found on more than one continent or in more than one country. For example, the famous White Cliffs of Dover are on the coast of southeastern England. These distinctive rocks are matched by similar white cliffs in France, Belgium, Holland, Germany, and Denmark (see Figure 11.11). It is important that this chalk layer goes across the English Channel. The rock is so soft that the Channel Tunnel connecting England and France was carved into it! "
relative ages of rocks,T_0080,"Like index fossils, key beds are used to match rock layers. A key bed is a thin layer of rock. The rock must be unique and widespread. For example, a key bed from around the time that the dinosaurs went extinct is very important. A thin layer of clay was deposited over much of Earths surface. The clay has large amount of the element iridium. Iridium is rare on Earth but common in asteroids. This unusual clay layer has been used to match rock up layers all over the world. It also led to the hypothesis that a giant asteroid struck Earth and caused the dinosaurs to go extinct. "
relative ages of rocks,T_0081,Index fossils are commonly used to match rock layers in different places. You can see how this works in Figure 
relative ages of rocks,T_0082,Earth formed 4.5 billion years ago. Geologists divide this time span into smaller periods. Many of the divisions mark major events in life history. 
relative ages of rocks,T_0083,"Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic Period is named for the Jura Mountains in France and Switzerland, for example. "
relative ages of rocks,T_0084,"To create the geologic time scale, geologists correlated rock layers. Stenos laws were used to determine the relative ages of rocks. Older rocks are at the bottom and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late 1800s changed that. Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years. "
relative ages of rocks,T_0085,"The largest blocks of time on the geologic time scale are called eons. Eons are split into eras. Each era is divided into periods. Periods may be further divided into epochs. Geologists may just use early or late. An example is late Jurassic, or early Cretaceous. Figure 11.13 shows you what the geologic time scale looks like. "
relative ages of rocks,T_0086,The geologic time scale may include illustrations of how life on Earth has changed. Major events on Earth may also be shown. These include the formation of the major mountains or the extinction of the dinosaurs. Figure 11.14 is a different kind of the geologic time scale. It shows how Earths environment and life forms have changed. 
relative ages of rocks,T_0087,"We now live in the Phanerozoic Eon, the Cenozoic Era, the Quaternary Period, and the Holocene Epoch. Phanero- zoic means visible life. During this eon, rocks contain visible fossils. Before the Phanerozoic, life was microscopic. The Cenozoic Era means new life. It encompasses the most recent forms of life on Earth. The Cenozoic is sometimes called the Age of Mammals. Before the Cenozoic came the Mesozoic and Paleozoic. The Mesozoic means middle life. This is the age of reptiles, when dinosaurs ruled the planet. The Paleozoic is old life. Organisms like invertebrates and fish were the most common lifeforms. "
absolute ages of rocks,T_0088,"Radioactive decay is the breakdown of unstable elements into stable elements. To understand this process, recall that the atoms of all elements contain the particles protons, neutrons, and electrons. "
absolute ages of rocks,T_0089,"An element is defined by the number of protons it contains. All atoms of a given element contain the same number of protons. The number of neutrons in an element may vary. Atoms of an element with different numbers of neutrons are called isotopes. Consider carbon as an example. Two isotopes of carbon are shown in Figure 11.15. Compare their protons and neutrons. Both contain 6 protons. But carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Almost all carbon atoms are carbon-12. This is a stable isotope of carbon. Only a tiny percentage of carbon atoms are carbon-14. Carbon-14 is unstable. Figure 11.16 shows carbon dioxide, which forms in the atmosphere from carbon-14 and oxygen. Neutrons in cosmic rays strike nitrogen atoms in the atmosphere. The nitrogen forms carbon- 14. Carbon in the atmosphere combines with oxygen to form carbon dioxide. Plants take in carbon dioxide during photosynthesis. In this way, carbon-14 enters food chains. "
absolute ages of rocks,T_0090,"Like other unstable isotopes, carbon-14 breaks down, or decays. For carbon-14 decay, each carbon-14 atom loses an alpha particle. It changes to a stable atom of nitrogen-14. This is illustrated in Figure 11.17. The decay of an unstable isotope to a stable element occurs at a constant rate. This rate is different for each isotope pair. The decay rate is measured in a unit called the half-life. The half-life is the time it takes for half of a given amount of an isotope to decay. For example, the half-life of carbon-14 is 5730 years. Imagine that you start out with 100 grams of carbon-14. In 5730 years, half of it decays. This leaves 50 grams of carbon-14. Over the next 5730 years, half of the remaining amount will decay. Now there are 25 grams of carbon-14. How many grams will there be in another 5730 years? Figure 11.18 graphs the rate of decay of carbon-14. "
absolute ages of rocks,T_0091,The rate of decay of unstable isotopes can be used to estimate the absolute ages of fossils and rocks. This type of dating is called radiometric dating. 
absolute ages of rocks,T_0092,"The best-known method of radiometric dating is carbon-14 dating. A living thing takes in carbon-14 (along with stable carbon-12). As the carbon-14 decays, it is replaced with more carbon-14. After the organism dies, it stops taking in carbon. That includes carbon-14. The carbon-14 that is in its body continues to decay. So the organism contains less and less carbon-14 as time goes on. We can estimate the amount of carbon-14 that has decayed by measuring the amount of carbon-14 to carbon-12. We know how fast carbon-14 decays. With this information, we can tell how long ago the organism died. Carbon-14 has a relatively short half-life. It decays quickly compared to some other unstable isotopes. So carbon- 14 dating is useful for specimens younger than 50,000 years old. Thats a blink of an eye in geologic time. But radiocarbon dating is very useful for more recent events. One important use of radiocarbon is early human sites. Carbon-14 dating is also limited to the remains of once-living things. To date rocks, scientists use other radioactive isotopes. "
absolute ages of rocks,T_0093,"The isotopes in Table 11.1 are used to date igneous rocks. These isotopes have much longer half-lives than carbon- 14. Because they decay more slowly, they can be used to date much older specimens. Which of these isotopes could be used to date a rock that formed half a million years ago? Unstable Isotope Decays to At a Half-Life of (years) Potassium-40 Uranium-235 Uranium-238 Argon-40 Lead-207 Lead-206 1.3 billion 700 million 4.5 billion Dates Rocks Aged (years old) 100 thousand - 1 billion 1 million - 4.5 billion 1 million - 4.5 billion "
the origin of earth,T_0094,"Our solar system began about 5 billion years ago. The Sun, planets and other solar system objects all formed at about the same time. "
the origin of earth,T_0095,"The Sun and planets formed from a giant cloud of gas and dust. This was the solar nebula. The cloud contracted and began to spin. As it contracted, its temperature and pressure increased. The cloud spun faster, and formed into a disk. Scientists think the solar system at that time looked like these disk-shaped objects in the Orion Nebula (Figure "
the origin of earth,T_0096,"Temperatures and pressures at the center of the cloud were extreme. It was so hot that nuclear fusion reactions began. In these reactions hydrogen fuses to make helium. Extreme amounts of energy are released. Our Sun became a star! Material in the disk surrounding the Sun collided. Small particles collided and became rocks. Rocks collided and became boulders. Eventually planets formed from the material (Figure 12.2). Dwarf plants, comets, and asteroids formed too (Figure 12.3). "
the origin of earth,T_0097,Material at a similar distances from the Sun collided together to form each of the planets. Earth grew from material in its part of space. Moons origin was completely different from Earths. 
the origin of earth,T_0098,"Earth formed like the other planets. Different materials in its region of space collided. Eventually the material made a planet. All of the collisions caused Earth to heat up. Rock and metal melted. The molten material separated into layers. Gravity pulled the denser material into the center. The lighter elements rose to the surface (Figure 12.4). Because the material separated, Earths core is made mostly of iron. Earths crust is made mostly of lighter materials. In between the crust and the core is Earths mantle, made of solid rock. "
the origin of earth,T_0099,"This model for how the Moon formed is the best fit of all of the data scientists have about the Moon. In the early solar system there was a lot of space debris. Asteroids flew around, sometimes striking the planets. An asteroid the size of Mars smashed into Earth. The huge amount of energy from the impact melted most of Earth. The asteroid melted too. Material from both Earth and the asteroid was thrown out into orbit. Over time, this material smashed together to form our Moon. The lunar surface is about 4.5 billion years old. This means that the collision happened about 70 million years after Earth formed. "
the origin of earth,T_0100,An atmosphere is the gases that surround a planet. The early Earth had no atmosphere. Conditions were so hot that gases were not stable. 
the origin of earth,T_0101,"Earths first atmosphere was different from the current one. The gases came from two sources. Volcanoes spewed gases into the air. Comets carried in ices from outer space. These ices warmed and became gases. Nitrogen, carbon dioxide, hydrogen, and water vapor, or water in gas form, were in the first atmosphere (Figure 12.5). Take a look at the list of gases. Whats missing? The early atmosphere had almost no oxygen. "
the origin of earth,T_0102,"Earths atmosphere slowly cooled. Once it was cooler, water vapor could condense. It changed back to its liquid form. Liquid water could fall to Earths surface as rain. Over millions of years water collected to form the oceans. Water began to cycle on Earth as water evaporated from the oceans and returned again as rainfall. "
early earth,T_0103,"The earliest crust was probably basalt. It may have resembled the current seafloor. This crust formed before there were any oceans. More than 4 billion years ago, continental crust appeared. The first continents were very small compared with those today. "
early earth,T_0104,"Continents grow when microcontinents, or small continents, collide with each other or with a larger continent. Oceanic island arcs also collide with continents to make them grow. "
early earth,T_0105,"There are times in Earth history when all of the continents came together to form a supercontinent. Supercontinents come together and then break apart. Pangaea was the last supercontinent on Earth, but it was not the first. The supercontinent before Pangaea is called Rodinia. Rodinia contained about 75% of the continental landmass that is present today. The supercontinent came together about 1.1 billion years ago. Rodinia was not the first supercontinent either. Scientists think that three supercontinents came before Rodina, making five so far in Earth history. "
early earth,T_0106,"Since the early Earth was very hot, mantle convection was very rapid. Plate tectonics likely moved very quickly. The early Earth was a very active place with abundant volcanic eruptions and earthquakes. The remnants of these early rocks are now seen in the ancient cores of the continents. "
early earth,T_0107,For the first 4 billion years of Earth history there is only a little evidence of life. Organisms were tiny and soft and did not fossilize well. But scientists use a variety of ways to figure out what this early life was like. 
early earth,T_0108,"Life probably began in the oceans. No one knows exactly how or when. Life may have originated more than once. If life began before the Moon formed, that impact would have wiped it out and it would have had to originate again. Eventually conditions on Earth became less violent. The planet could support life. The first organisms were made of only one cell (Figure 12.6). The earliest cells were prokaryotes. Prokaryotic cells are surrounded by a cell membrane, but they do not have a nucleus. The cells got their nutrients directly from the water. The cells needed to use these nutrients to live and grow. The cells also needed to be able to make copies of themselves. To do this they stored genetic information in nucleic acids. The two nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Nucleic acids pass "
early earth,T_0109,"Early cells took nutrients from the water. Eventually the nutrients would have become less abundant. Around 3 billion years ago, photosynthesis began. Organisms could make their own food from sunlight and inorganic molecules. From these ingredients they made chemical energy that they used. Oxygen is a waste product of photosynthesis. That first oxygen combined with iron to create iron oxide. Later on, the oxygen entered the atmosphere. Some of the oxygen in the atmosphere became ozone. The ozone layer formed to protect Earth from harmful ultraviolet radiation. This made the environment able to support more complex life forms. "
early earth,T_0110,The first organisms to photosynthesize were cyanobacteria. These organisms may have been around as far back as 3.5 billion years and are still alive today (Figure 12.7). Now they are called blue-green algae. They are common in lakes and seas and account for 20% to 30% of photosynthesis today. 
early earth,T_0111,"Eukaryotes evolved about 2 billion years ago. Unlike prokaryotes, eukaryotes have a cell nucleus. They have more structures and are better organized. Organelles within a eukaryote can perform certain functions. Some supply energy; some break down wastes. Eukaryotes were better able to live and so became the dominant life form. "
early earth,T_0112,"For life to become even more complex, multicellular organisms needed to evolve. Prokaryotes and eukaryotes can be multicellular. Toward the end of the Precambrian, the Ediacara Fauna evolved (Figure 12.8). These are the fossils discovered by Walcott in the introduction to the next section. The Ediacara was extremely diverse. They appeared after Earth defrosted from a worldwide glaciation. The Ediacara fauna seem to have died out. Other multicellular organisms appeared in the Phanerozoic. "
water on earth,T_0131,"Water is a simple chemical compound. Each molecule of water contains two hydrogen atoms (H2 ) and one oxygen atom (O). Thats why the chemical formula for water is H2 O. If water is so simple, why is it special? Water is one of the few substances that exists on Earth in all three states of matter. Water occurs as a gas, a liquid and a solid. You drink liquid water and use it to shower. You breathe gaseous water vapor in the air. You may go ice skating on a pond covered with solid water ice in the winter. "
water on earth,T_0132,"Earth is often called the water planet. Figure 13.1 shows why. If astronauts see Earth from space, this is how it looks. Notice how blue the planet appears. Thats because oceans cover much of Earths surface. Water is also found in the clouds that rise above the planet. Most of Earths water is salt water in the oceans. As Figure 13.2 shows, only 3 percent of Earths water is fresh. Freshwater is water that contains little or no dissolved salt. Most freshwater is frozen in ice caps and glaciers. Glaciers cover the peaks of some tall mountains. For example, the Cascades Mountains in North America and the Alps Mountains in Europe are capped with ice. Ice caps cover vast areas of Antarctica and Greenland. Chunks of ice frequently break off ice caps. They form icebergs that float in the oceans. "
water on earth,T_0133,"Did you ever wonder where the water in your glass came from or where its been? The next time you take a drink of water, think about this. Each water molecule has probably been around for billions of years. Thats because Earths water is constantly recycled. "
water on earth,T_0134,"Water is recycled through the water cycle. The water cycle is the movement of water through the oceans, atmo- sphere, land, and living things. The water cycle is powered by energy from the Sun. Figure 13.3 diagrams the water cycle. "
water on earth,T_0135,"Water keeps changing state as it goes through the water cycle. This means that it can be a solid, liquid, or gas. How does water change state? How does it keep moving through the cycle? As Figure 13.3 shows, several processes are involved. Evaporation changes liquid water to water vapor. Energy from the Sun causes water to evaporate. Most evaporation is from the oceans because they cover so much area. The water vapor rises into the atmosphere. Transpiration is like evaporation because it changes liquid water to water vapor. In transpiration, plants release water vapor through their leaves. This water vapor rises into the atmosphere. Condensation changes water vapor to liquid water. As air rises higher into the atmosphere, it cools. Cool air can hold less water vapor than warm air. So some of the water vapor condenses into water droplets. Water droplets may form clouds. Precipitation is water that falls from clouds to Earths surface. Water droplets in clouds fall to Earth when they become too large to stay aloft. The water falls as rain if the air is warm. If the air is cold, the water may freeze and fall as snow, sleet, or hail. Most precipitation falls into the oceans. Some falls on land. Runoff is precipitation that flows over the surface of the land. This water may travel to a river, lake, or ocean. Runoff may pick up fertilizer and other pollutants and deliver them to the water body where it ends up. In this way, runoff may pollute bodies of water. Infiltration is the process by which water soaks into the ground. Some of the water may seep deep under- ground. Some may stay in the soil, where plants can absorb it with their roots. In all these ways, water keeps cycling. The water cycle repeats over and over again. Who knows? Maybe a water molecule that you drink today once quenched the thirst of a dinosaur. "
surface water,T_0136,Look at the pictures of flowing water in Figure 13.4. A waterfall tumbles down a mountainside. A brook babbles through a forest. A river slowly meanders through a broad valley. What do all these forms of flowing water have in common? They are all streams. 
surface water,T_0137,"A stream is a body of freshwater that flows downhill in a channel. The channel of a stream has a bottom, or bed, and sides called banks. Any size body of flowing water can be called a stream. Usually, though, a large stream is called a river. "
surface water,T_0138,"All streams and rivers have several features in common. These features are shown in (Figure 13.5). The place where a stream or river starts is its source. The source might be a spring, where water flows out of the ground. Or the source might be water from melting snow on a mountain top. A single stream may have multiple sources. A stream or river probably ends when it flows into a body of water, such as a lake or an ocean. A stream ends at its mouth. As the water flows into the body of water, it slows down and drops the sediment it was carrying. The sediment may build up to form a delta. Several other features of streams and rivers are also shown in Figure 13.5. Small streams often flow into bigger streams or rivers. The small streams are called tributaries. A river and all its tributaries make up a river system. At certain times of year, a stream or river may overflow its banks. The area of land that is flooded is called the floodplain. The floodplain may be very wide where the river flows over a nearly flat surface. A river flowing over a floodplain may wear away broad curves. These curves are called meanders. "
surface water,T_0139,"All of the land drained by a river system is called its basin, or watershed. One river systems basin is separated from another river systems basin by a divide. The divide is created by the highest points between the two river basins. Precipitation that falls within a river basin always flows toward that river. Precipitation that falls on the other side of the divide flows toward a different river. Figure 13.6 shows the major river basins in the U.S. You can watch an animation of water flowing through a river basin at this link: http://trashfree.org/btw/graphics/watershed_anim.gif "
surface water,T_0140,"After a heavy rain, you may find puddles of water standing in low spots. The same principle explains why water collects in ponds and lakes. Water travels downhill, so a depression in the ground fills with standing water. A pond is a small body of standing water. A lake is a large body of standing water. Most lakes have freshwater, but a few are salty. The Great Salt Lake in Utah is an example of a saltwater lake. The water in a large lake may be so deep that sunlight cannot penetrate all the way to the bottom. Without sunlight, water plants and algae cannot live on the bottom of the lake. Thats because plants need sunlight for photosynthesis. The largest lakes in the world are the Great Lakes. They lie between the U.S. and Canada, as shown in Figure 13.7. How great are they? They hold 22 percent of all the worlds fresh surface water! "
surface water,T_0141,Ponds and lakes may get their water from several sources. Some falls directly into them as precipitation. Some enters as runoff and some from streams and rivers. Water leaves ponds and lakes through evaporation and also as outflow. 
surface water,T_0142,"The depression that allows water to collect to form a lake may come about in a variety of ways. The Great Lakes, for example, are glacial lakes. A glacial lake forms when a glacier scrapes a large hole in the ground. When the glacier melts, the water fills the hole and forms a lake. Over time, water enters the lake from the sources mentioned above as well. Other lakes are crater lakes or rift lakes, which are pictured in Figure 13.8. Crater lakes form when volcanic eruptions create craters that fill with water. Rift lakes form when movements of tectonic plates create low places that fill with water. "
surface water,T_0143,"Some of Earths freshwater is found in wetlands. A wetland is an area that is covered with water, or at least has very soggy soil, during all or part of the year. Certain species of plants thrive in wetlands, and they are rich ecosystems. Freshwater wetlands are usually found at the edges of steams, rivers, ponds, or lakes. Wetlands can also be found at the edges of seas. "
surface water,T_0144,"Not all wetlands are alike, as you can see from Figure 13.9. Wetlands vary in how wet they are and how much of the year they are soaked. Wetlands also vary in the kinds of plants that live in them. This depends mostly on the climate where the wetland is found. Types of wetlands include marshes, swamps, and bogs. A marsh is a wetland that is usually under water. It has grassy plants, such as cattails. A swamp is a wetland that may or may not be covered with water but is always soggy. It has shrubs or trees. A bog is a wetland that has soggy soil. It is generally covered with mosses. "
surface water,T_0145,"People used to think that wetlands were useless. Many wetlands were filled in with rocks and soil to create lands that were then developed with roads, golf courses, and buildings. Now we know that wetlands are very important. Laws have been passed to help protect them. Why are wetlands so important? Wetlands have great biodiversity. They provide homes or breeding sites to a huge variety of species. Because so much wetland area has been lost, many of these species are endangered. Wetlands purify water. They filter sediments and toxins from runoff before it enters rivers, lakes, and oceans. Wetlands slow rushing water. During hurricanes and other extreme weather, wetlands reduce the risk of floods. Although the rate has slowed, wetlands are still being destroyed today. "
surface water,T_0146,"A flood occurs when so much water enters a stream or river that it overflows its banks. Flood waters from a river are shown in Figure 13.10. Like this flood, many floods are caused by very heavy rains. Floods may also occur when deep snow melts quickly in the spring. Floods are a natural part of the water cycle, but they can cause a lot of damage. Farms and homes may be lost, and people may die. In 1939, millions of people died in a flood in China. Although freshwater is needed to grow crops and just to live, too much freshwater in the same place at once can be deadly. "
groundwater,T_0147,"Freshwater below Earths surface is called groundwater. The water infiltrates, or seeps down into, the ground from the surface. How does this happen? And where does the water go? "
groundwater,T_0148,"Water infiltrates the ground because soil and rock are porous. Between the grains are pores, or tiny holes. Since water can move through this rock it is permeable. Eventually, the water reaches a layer of rock that is not porous and so is impermeable. Water stops moving downward when it reaches this layer of rock. Look at the diagram in Figure 13.11. It shows two layers of porous rock. The top layer is not saturated; it is not full of water. The next layer is saturated. The water in this layer has nowhere else to go. It cannot seep any deeper into the ground because the rock below it is impermeable. "
groundwater,T_0149,"The top of the saturated rock layer in Figure 13.11 is called the water table. The water table isnt like a real table. It doesnt remain firmly in one place. Instead, it rises or falls, depending on how much water seeps down from the surface. The water table is higher when there is a lot of rain and lower when the weather is dry. "
groundwater,T_0150,"An underground layer of rock that is saturated with groundwater is called an aquifer. A diagram of an aquifer is shown in Figure 13.12. Aquifers are generally found in porous rock, such as sandstone. Water infiltrates the aquifer from the surface. The water that enters the aquifer is called recharge. "
groundwater,T_0151,"Most land areas have aquifers beneath them. Many aquifers are used by people for freshwater. The closer to the surface an aquifer is, the easier it is to get the water. However, an aquifer close to the surface is also more likely to become polluted. Pollutants can seep down through porous rock in recharge water. An aquifer that is used by people may not be recharged as quickly as its water is removed. The water table may lower and the aquifer may even run dry. If this happens, the ground above the aquifer may sink. This is likely to damage any homes or other structures built above the aquifer. "
groundwater,T_0152,"One of the biggest aquifers in the world is the Ogallala aquifer. As you can see from Figure 13.13, this aquifer lies beneath parts of eight U.S. states. It covers a total area of 451,000 square kilometers (174,000 square miles). In some places, it is less than a meter deep. In other places, it is hundreds of meters deep. The Ogallala aquifer is an important source of freshwater in the American Midwest. This is a major farming area, and much of the water is used to irrigate crops. The water in this aquifer is being used up ten times faster than it is recharged. If this continues, what might happen to the Ogallala aquifer? "
groundwater,T_0153,"The top of an aquifer may be high enough in some places to meet the surface of the ground. This often happens on a slope. The water flows out of the ground and creates a spring. A spring may be just a tiny trickle, or it may be a big gush of water. One of the largest springs in the world is Big Spring in Missouri, seen in Figure 13.14. Water flowing out of the ground at a spring may flow downhill and enter a stream. Thats what happens to the water that flows out of Big Spring in Missouri. If the water from a spring cant flow downhill, it may spread out to form a pond or lake instead. Lake George in New York State, which is pictured in Figure 13.15, is a spring-fed lake. The lake basin was carved by a glacier. "
groundwater,T_0154,"Some springs have water that contains minerals. Groundwater dissolves minerals out of the rock as it seeps through the pores. The water in some springs is hot because it is heated by hot magma. Many hot springs are also mineral springs. Thats because hot water can dissolve more minerals than cold water. Grand Prismatic Spring, shown in Figure 13.16, is a hot mineral spring. Dissolved minerals give its water a bright blue color. The edge of the spring is covered with thick orange mats of bacteria. The bacteria use the minerals in the hot water to make food. "
groundwater,T_0155,"Heated groundwater may become trapped in spaces within rocks. Pressure builds up as more water seeps into the spaces. When the pressure becomes great enough, the water bursts out of the ground at a crack or weak spot. This is called a geyser. When the water erupts from the ground, the pressure is released. Then more water collects and the pressure builds up again. This leads to another eruption. Old Faithful is the best-known geyser in the world. You can see a picture of it in Figure 13.17. The geyser erupts faithfully every 90 minutes, day after day. During each eruption, it may release as much as 30,000 liters of water! "
groundwater,T_0156,"Most groundwater does not flow out of an aquifer as a spring or geyser. So to use the water thats stored in an aquifer people must go after it. How? They dig a well. A well is a hole that is dug or drilled through the ground down to an aquifer. This is illustrated in Figure 13.18. People have depended on water from wells for thousands of years. To bring water to the surface takes energy because the force of gravity must be overcome. Today, many wells use electricity to pump water to the surface. However, in some places, water is still brought to the surface the old-fashioned way  with human labor. The well pictured in Figure 13.19 is an example of this type of well. A hand-cranked pulley is used to lift the bucket of water to the surface. "
introduction to the oceans,T_0157,"When Earth formed 4.6 billion years ago, it would not have been called the water planet. There were no oceans then. In fact, there was no liquid water at all. Early Earth was too hot for liquid water to exist. Earths early years were spent as molten rock and metal. "
introduction to the oceans,T_0158,"Over time, Earth cooled. The surface hardened to become solid rock. Volcanic eruptions, like the one in Figure 14.1, brought lava and gases to the surface. One of the gases was water vapor. More water vapor came from asteroids and comets that crashed into Earth. As Earth cooled still more, the water vapor condensed to make Earths first liquid water. At last, the oceans could start to form. "
introduction to the oceans,T_0159,"Earths crust consists of many tectonic plates that move over time. Due to plate tectonics, the continents changed their shapes and positions during Earth history. As the continents changed, so did the oceans. About 250 million years ago, there was one huge land mass known as Pangaea. There was also one huge ocean called Panthalassa. You can see it in Figure 14.2. By 180 million years ago, Pangaea began to break up. The continents started to drift apart. They slowly moved to where they are today. The movement of the continents caused Panthalassa to break into smaller oceans. These oceans are now known as the Pacific, Atlantic, Indian, and Arctic Oceans. The waters of all the oceans are connected. "
introduction to the oceans,T_0160,"Oceans cover more than 70 percent of Earths surface and hold 97 percent of its surface water. Its no surprise that the oceans have a big influence on the planet. The oceans affect the atmosphere, climate, and living things. "
introduction to the oceans,T_0161,"Oceans are the major source of water vapor in the atmosphere. Sunlight heats water near the sea surface, as shown in Figure 14.3. As the water warms, some of it evaporates. The water vapor rises into the air, where it may form clouds and precipitation. Precipitation provides the freshwater needed by plants and other living things. Ocean water also absorbs gases from the atmosphere. The most important are oxygen and carbon dioxide. Oxygen is needed by living things in the oceans. Much of the carbon dioxide sinks to the bottom of the seas. Carbon dioxide is a major cause of global warming. By absorbing carbon dioxide, the oceans help control global warming. "
introduction to the oceans,T_0162,"Coastal areas have a milder climate than inland areas. They are warmer in the winter and cooler in the summer. Thats because land near an ocean is influenced by the temperature of the oceans. The temperature of ocean water is moderate and stable. Why? There are two major reasons: 1. Water is much slower to warm up and cool down than land. As a result, oceans never get as hot or as cold as land. 2. Water flows through all the worlds oceans. Warm water from the equator mixes with cold water from the poles. The mixing of warm and cold water makes the water temperature moderate. Even inland temperatures are milder because of oceans. Without oceans, there would be much bigger temperature swings all over Earth. Temperatures might plunge hundreds of degrees below freezing in the winter. In the summer, lakes and seas might boil! Life as we know it could not exist on Earth without the oceans. "
introduction to the oceans,T_0163,"The oceans provide a home to many living things. In fact, a greater number of organisms lives in the oceans than on land. Coral reefs, like the one in Figure 14.4, have more diversity of life forms than almost anywhere else on Earth. "
introduction to the oceans,T_0164,You know that ocean water is salty. But do you know why? How salty is it? 
introduction to the oceans,T_0165,"Ocean water is salty because water dissolves minerals out of rocks. This happens whenever water flows over or through rocks. Much of this water and its minerals flow in rivers that end up in the oceans. Minerals dissolved in water form salts. When the water evaporates, it leaves the salts behind. As a result, ocean water is much saltier than other water on Earth. "
introduction to the oceans,T_0166,"Have you ever gone swimming in the ocean? If you have, then you probably tasted the salts in the water. By mass, salts make up about 3.5 percent of ocean water. Figure 14.5 shows the most common minerals in ocean water. The main components are sodium and chloride. Together they form the salt known as sodium chloride. You may know the compound as table salt or the mineral halite. The amount of salts in ocean water varies from place to place. For example, near the mouth of a river, ocean water may be less salty. Thats because river water contains less salt than ocean water. Where the ocean is warm, the water may be more salty. Can you explain why? (Hint: More water evaporates when the water is warm.) "
introduction to the oceans,T_0167,"In addition to the amount of salts, other conditions in ocean water vary from place to place. One is the amount of nutrients in the water. Another is the amount of sunlight that reaches the water. These conditions depend mainly on two factors: distance from shore and depth of water. Oceans are divided into zones based on these two factors. The ocean floor makes up another zone. Figure 14.6 shows all the ocean zones. "
introduction to the oceans,T_0168,"There are three main ocean zones based on distance from shore. They are the intertidal zone, neritic zone, and oceanic zone. Distance from shore influences how many nutrients are in the water. Why? Most nutrients are washed into ocean water from land. Therefore, water closer to shore tends to have more nutrients. Living things need nutrients. So distance from shore also influences how many organisms live in the water. "
introduction to the oceans,T_0169,"Two main zones based on depth of water are the photic zone and aphotic zone. The photic zone is the top 200 meters of water. The aphotic zone is water deeper than 200 meters. The deeper you go, the darker the water gets. Thats because sunlight cannot penetrate very far under water. Sunlight is needed for photosynthesis. So the depth of water determines whether photosynthesis is possible. There is enough sunlight for photosynthesis only in the photic zone. Water also gets colder as you go deeper. The weight of the water pressing down from above increases as well. At great depths, life becomes very difficult. The pressure is so great that only specially adapted creatures can live there. "
ocean movements,T_0170,"Most ocean waves are caused by winds. A wave is the transfer of energy through matter. A wave that travels across miles of ocean is traveling energy, not water. Ocean waves transfer energy from wind through water. The energy of a wave may travel for thousands of miles. The water itself moves very little. Figure 14.9 shows how water molecules move when a wave goes by. "
ocean movements,T_0171,"Figure 14.9 also shows how the size of waves is measured. The highest point of a wave is the crest. The lowest point is the trough. The vertical distance between a crest and a trough is the height of the wave. Wave height is also called amplitude. The horizontal distance between two crests is the wavelength. Both amplitude and wavelength are measures of wave size. The size of an ocean wave depends on how fast, over how great a distance, and how long the wind blows. The greater each of these factors is, the bigger a wave will be. Some of the biggest waves occur with hurricanes. A hurricane is a storm that forms over the ocean. Its winds may blow more than 150 miles per hour! The winds also travel over long distances and may last for many days. "
ocean movements,T_0172,"Figure 14.10 shows what happens to waves near shore. As waves move into shallow water, they start to touch the bottom. The base of the waves drag and slow. Soon the waves slow down and pile up. They get steeper and unstable as the top moves faster than the base. When they reach the shore, the waves topple over and break. "
ocean movements,T_0173,"Not all waves are caused by winds. A shock to the ocean can also send waves through water. A tsunami is a wave or set of waves that is usually caused by an earthquake. As we have seen in recent years, the waves can be enormous and extremely destructive. Usually tsunami waves travel through the ocean unnoticed. But when they reach the shore they become enormous. Tsunami waves can flood entire regions. They destroy property and cause many deaths. Figure 14.11 shows the damage caused by a tsunami in the Indian Ocean in 2004. "
ocean movements,T_0174,"Tides are daily changes in the level of ocean water. They occur all around the globe. High tides occur when the water reaches its highest level in a day. Low tides occur when the water reaches its lowest level in a day. Tides keep cycling from high to low and back again. In most places the water level rises and falls twice a day. So there are two high tides and two low tides approximately every 24 hours. In Figure 14.12, you can see the difference between high and low tides. This is called the tidal range. "
ocean movements,T_0175,"Figure 14.13 shows why tides occur. The main cause of tides is the pull of the Moons gravity on Earth. The pull is greatest on whatever is closest to the Moon. Although the gravity pulls the land, only the water can move. As a result: Water on the side of Earth facing the Moon is pulled hardest by the Moons gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide. Earth itself is pulled harder by the Moons gravity than is the ocean on the side of Earth opposite the Moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide. With water bulging on two sides of Earth, theres less water left in between. This creates low tides on the other two sides of the planet. "
ocean movements,T_0176,"The Suns gravity also pulls on Earth and its oceans. Even though the Sun is much larger than the Moon, the pull of the Suns gravity is much less because the Sun is much farther away. The Suns gravity strengthens or weakens the Moons influence on tides. Figure 14.14 shows the position of the Moon relative to the Sun at different times during the month. The positions of the Moon and Sun relative to each other determines how the Sun affects tides. This creates spring tides or neap tides. Spring tides occur during the new moon and full moon. The Sun and Moon are in a straight line either on the same side of Earth or on opposite sides. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range. Neap tides occur during the first and third quarters of the Moon. The Moon and Sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range. This animation shows the effect of the Moon and Sun on the tides: "
ocean movements,T_0177,"Another way ocean water moves is in currents. A current is a stream of moving water that flows through the ocean. Surface currents are caused mainly by winds, but not the winds that blow and change each day. Surface currents are caused by the major wind belts that blow in the same direction all the time. The major surface currents are shown in Figure 14.15. They flow in a clockwise direction in the Northern Hemi- sphere. In the Southern Hemisphere, they flow in the opposite direction. "
ocean movements,T_0178,"Winds and surface currents tend to move from the hot equator north or south toward the much cooler poles. Thats because of differences in the temperature of air masses over Earths surface. But Earth is spinning on its axis underneath the wind and water as they move. The Earth rotates from west to east. As a result, winds and currents actually end up moving toward the northeast or southeast. This effect of Earths rotation on the direction of winds and currents is called the Coriolis effect. "
ocean movements,T_0179,"Large ocean currents can have a big impact on the climate of nearby coasts. The Gulf Stream, for example, carries warm water from near the equator up the eastern coast of North America. Look at the map in Figure 14.16. It shows how the Gulf Stream warms both the water and land along the coast. "
ocean movements,T_0180,"Currents also flow deep below the surface of the ocean. Deep currents are caused by differences in density at the top and bottom. Density is defined as the amount of mass per unit of volume. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense? Water becomes more dense when it is colder and when it has more salt. In the North Atlantic Ocean, cold winds chill the water at the surface. Sea ice grows in this cold water, but ice is created from fresh water. The salt is left behind in the seawater. This cold, salty water is very dense, so it sinks to the bottom of the North Atlantic. Downwelling can take place in other places where surface water becomes very dense (see Figure 14.17). When water sinks it pushes deep water along at the bottom of the ocean. This water circulates through all of the ocean basins in deep currents. "
ocean movements,T_0181,"Sometimes deep ocean water rises to the surface. This is called upwelling. Figure 14.18 shows why it happens. Strong winds blow surface water away from shore. This allows deeper water to flow to the surface and take its place. When water comes up from the deep, it brings a lot of nutrients with it. Why is deep water so full of nutrients? Over time, dead organisms and other organic matter settle to the bottom water and collect. The nutrient-rich water that comes to the surface by upwelling supports many living things. "
the ocean floor,T_0182,Scientists study the ocean floor in various ways. Scientists or their devices may actually travel to the ocean floor. Or they may study the ocean floor from the surface. One way is with a tool called sonar. 
the ocean floor,T_0183,"Did you ever shout and hear an echo? If you did, thats because the sound waves bounced off a hard surface and back to you. The same principle explains how sonar works. A ship on the surface sends sound waves down to the ocean floor. The sound waves bounce off the ocean floor and return to the surface, like an echo. Figure 14.19 show how this happens. Sonar can be used to measure how deep the ocean is. A device records the time it takes sound waves to travel from the surface to the ocean floor and back again. Sound waves travel through water at a known speed. Once scientists know the travel time of the wave, they can calculate the distance to the ocean floor. They can then combine all of these distances to make a map of the ocean floor. Figure 14.20 shows an example of this type of map. "
the ocean floor,T_0184,"Only a specially designed vehicle can venture beneath the sea surface. But only very special vehicles can reach the ocean floor. Three are described here and pictured in Figure 14.21: In 1960, scientists used the submersible Trieste to travel into the Mariana Trench. They succeeded, but the trip was very risky. Making humans safe at such depths costs a lot of money. People have not traveled to this depth again. In 2012, the film director, James Cameron, dove to the bottom of the Mariana Trench by himself in a submersible that he had built for the purpose. The vehicle named Alvin was developed soon after Trieste. The submersible has made over 4,000 dives deep into the ocean. People can stay underwater for up to 9 hours. Alvin has been essential for developing a scientific understanding the worlds oceans. Today, remote-control vehicles, called remotely operated vehicles (ROVs) go to the deepest ocean floor. They dont have any people on board. However, they carry devices that record many measurements. They also collect sediments and take photos. "
the ocean floor,T_0185,"Scientists have learned a lot about the ocean floor. For example, they know that Earths tallest mountains and deepest canyons are on the ocean floor. The major features on the ocean floor are described below. They are also shown in Figure 14.22. The continental shelf is the ocean floor nearest the edges of continents. It has a a gentle slope. The water over the continental shelf is shallow. The continental slope lies between the continental shelf and the abyssal plain. It has a steep slope with a sharp drop to the deep ocean floor. The abyssal plain forms much of the floor under the open ocean. It lies from 3 to 6 kilometers (1.9 to 3.7 miles) below the surface. Much of it is flat. An oceanic trench is a deep canyon on the ocean floor. Trenches occur where one tectonic plate subducts under another. The deepest trench is the Mariana Trench in the Pacific Ocean. It plunges more than 11 kilometers (almost 7 miles) below sea level. A seamount is a volcanic mountain on the ocean floor. Seamounts that rise above the water surface are known as islands. There are many seamounts dotting the seafloor. The mid-ocean ridge is a mountain range that runs through all the worlds oceans. It is almost 64,000 kilometers (40,000 miles) long! It forms where tectonic plates pull apart. Magma erupts through the ocean floor to make new seafloor. The magma hardens to create the ridge. "
the ocean floor,T_0186,The ocean floor is rich in resources. The resources include both living and nonliving things. 
the ocean floor,T_0187,"The ocean floor is home to many species of living things. Some from shallow water are used by people for food. Clams and some fish are among the many foods we get from the ocean floor. Some living things on the ocean floor are sources of human medicines. For example, certain bacteria on the ocean floor produce chemicals that fight cancer. "
the ocean floor,T_0188,"Oil and natural gas lie below some regions of the seafloor. Large drills on floating oil rigs must be used to reach them. This is risky for workers on the rigs. It is also risky for the ocean and its living things. An oil rig explosion caused a massive oil leak in the Gulf of Mexico in 2010. Oil poured into the water for several months. The oil caused great harm to habitats and living things, both in the water and on the coast. The oil spill also hurt the economy of Gulf Coast states. The effects of the oil spill are still being tallied. There are many minerals on the ocean floor. Some settle down from the water above. Some are released in hot water through vents, or cracks, in the seafloor. The minerals in hot water settle out and form metallic chimneys, as in Figure 14.23. These metals could be mined, but they are very deep in the sea and very far from land. This means that mining them would be too expensive and not worth the effort. Some types of minerals form balls called nodules. Nodules may be tiny or as big as basketballs. They contain manganese, iron, copper, and other useful minerals. As many as 500 billion tons of nodules lie on the ocean floor! However, mining them would be very costly and could be harmful to the ocean environment. "
ocean life,T_0189,"When you think of life in the ocean, do you think of fish? Actually, fish are not the most common life forms in the ocean. Plankton are the most common. Plankton make up one of three major groups of marine life. The other two groups are nekton and benthos. Figure 14.24 shows the three groups. "
ocean life,T_0190,Plankton are living things that float in the water. Most plankton are too small to see with the unaided eye. Some examples are shown in Figure 14.25. Plankton are unable to move on their own. Ocean motions carry them along. There are two main types of plankton: 1. Phytoplankton are plant-like plankton. They make food by photosynthesis. They live in the photic zone. Most are algae. 2. Zooplankton are animal-like plankton. They feed on phytoplankton. They include tiny animals and fish larvae. 
ocean life,T_0191,"Nekton are living things that swim through the water. They may live at any depth, in the photic or aphotic zone. Most nekton are fish, although some are mammals. Fish have fins and streamlined bodies to help them swim. Fish also have gills to take oxygen from the water. Figure 14.26 shows examples of nekton. "
ocean life,T_0192,Benthos are living things on the ocean floor. Many benthic organisms attach themselves to rocks and stay in one place. This protects them from crashing waves and other water movements. Some benthic organisms burrow into sediments for food or protection. Benthic animals may crawl over the ocean floor. Examples of benthos include clams and worms. Figure 14.27 shows two other examples. Some benthos live near vents on the deep ocean floor. Tubeworms are an example (see Figure 14.28). Scalding hot water pours out of the vents. The hot water contains chemicals that some specialized bacteria can use to make food. Tubeworms let the bacteria live inside them. The bacteria get protection and the tubeworms get some of the food. 
ocean life,T_0193,"Figure 14.29 shows a marine food chain. Phytoplankton form the base of the food chain. Phytoplankton are the most important primary producers in the ocean. They use sunlight and nutrients to make food by photosynthesis. Small zooplankton consume phytoplankton. Larger organisms eat the small zooplankton. Larger predators eat these consumers. In an unusual relationship, some enormous whales depend on plankton for their food. They filter tremendous amounts of these tiny creatures out of the water. The bacteria that make food from chemicals are also primary producers. These organisms do not do photosynthesis since there is no light at the vents. They do something called chemosynthesis. They break down chemicals to make food. When marine organisms die, decomposers break them down. This returns their nutrients to the water. The nutrients can be used again to make food. Decomposers in the oceans include bacteria and worms. Many live on the ocean floor. Do you know why? "
energy in the atmosphere,T_0210,What explains all of these events? The answer can be summed up in one word: energy. Energy is defined as the ability to do work. Doing anything takes energy. A campfire obviously has energy. You can feel its heat and see its light. 
energy in the atmosphere,T_0211,"Heat and light are forms of energy. Other forms are chemical and electrical energy. Energy cant be created or destroyed. It can change form. For example, a piece of wood has chemical energy stored in its molecules. When the wood burns, the chemical energy changes to heat and light energy. "
energy in the atmosphere,T_0212,Energy can move from one place to another. It can travel through space or matter. Thats why you can feel the heat of a campfire and see its light. These forms of energy travel from the campfire to you. 
energy in the atmosphere,T_0213,Almost all energy on Earth comes from the Sun. The Suns energy heats the planet and the air around it. Sunlight also powers photosynthesis and life on Earth. 
energy in the atmosphere,T_0214,The Sun gives off energy in tiny packets called photons. Photons travel in waves. Figure 15.7 models a wave of light. Notice the wavelength in the figure. Waves with shorter wavelengths have more energy. 
energy in the atmosphere,T_0215,"Energy from the Sun has a wide range of wavelengths. The total range of energy is called the electromagnetic spectrum. You can see it in Figure 15.8. Visible light is the only light that humans can see. Different wavelengths of visible light appear as different colors. Radio waves have the longest wavelengths. They also have the least amount of energy. Infrared light has wavelengths too long for humans to see, but we can feel them as heat. The atmosphere absorbs the infrared light. Ultraviolet (UV) light is in wavelengths too short for humans to see. The most energetic UV light is harmful to life. The atmosphere absorbs most of this UV light from the Sun. Gamma rays have the highest energy and they are the most damaging rays. Fortunately, gamma rays dont penetrate Earths atmosphere. "
energy in the atmosphere,T_0216,"Energy travels through space or material. Heat energy is transferred in three ways: radiation, conduction, and convection. "
energy in the atmosphere,T_0217,"Radiation is the transfer of energy by waves. Energy can travel as waves through air or empty space. The Suns energy travels through space by radiation. After sunlight heats the planets surface, some heat radiates back into the atmosphere. "
energy in the atmosphere,T_0218,"In conduction, heat is transferred from molecule to molecule by contact. Warmer molecules vibrate faster than cooler ones. They bump into the cooler molecules. When they do they transfer some of their energy. Conduction happens mainly in the lower atmosphere. Can you explain why? "
energy in the atmosphere,T_0219,"Convection is the transfer of heat by a current. Convection happens in a liquid or a gas. Air near the ground is warmed by heat radiating from Earths surface. The warm air is less dense, so it rises. As it rises, it cools. The cool air is dense, so it sinks to the surface. This creates a convection current, like the one in Figure 15.9. Convection is the most important way that heat travels in the atmosphere. "
energy in the atmosphere,T_0220,"Different parts of Earths surface receive different amounts of sunlight. You can see this in Figure 15.10. The Suns rays strike Earths surface most directly at the equator. This focuses the rays on a small area. Near the poles, the Suns rays strike the surface at a slant. This spreads the rays over a wide area. The more focused the rays are, the more energy an area receives and the warmer it is. "
energy in the atmosphere,T_0221,"When sunlight heats Earths surface, some of the heat radiates back into the atmosphere. Some of this heat is absorbed by gases in the atmosphere. This is the greenhouse effect, and it helps to keep Earth warm. The greenhouse effect allows Earth to have temperatures that can support life. Gases that absorb heat in the atmosphere are called greenhouse gases. They include carbon dioxide and water vapor. Human actions have increased the levels of greenhouse gases in the atmosphere. This is shown in Figure 15.11. The added gases have caused a greater greenhouse effect. How do you think this affects Earths temperature? "
layers of the atmosphere,T_0222,"Air temperature changes as altitude increases. In some layers of the atmosphere, the temperature decreases. In other layers, it increases. You can see this in Figure 15.12. Refer to this figure as you read about the layers below. "
layers of the atmosphere,T_0223,"The troposphere is the lowest layer of the atmosphere. In it, temperature decreases with altitude. The troposphere gets some of its heat directly from the Sun. Most, however, comes from Earths surface. The surface is heated by the Sun and some of that heat radiates back into the air. This makes the temperature higher near the surface than at higher altitudes. "
layers of the atmosphere,T_0224,"Look at the troposphere in Figure 15.12. This is the shortest layer of the atmosphere. It rises to only about 12 kilometers (7 miles) above the surface. Even so, this layer holds 75 percent of all the gas molecules in the atmosphere. Thats because the air is densest in this layer. "
layers of the atmosphere,T_0225,"Air in the troposphere is warmer closer to Earths surface. Warm air is less dense than cool air, so it rises higher in the troposphere. This starts a convection cell. Convection mixes the air in the troposphere. Rising air is also a main cause of weather. All of Earths weather takes place in the troposphere. "
layers of the atmosphere,T_0226,"Sometimes air doesnt mix in the troposphere. This happens when air is cooler close to the ground than it is above. The cool air is dense, so it stays near the ground. This is called a temperature inversion. An inversion can trap air pollution near the surface. Temperature inversions are more common in the winter. Can you explain why? "
layers of the atmosphere,T_0227,At the top of the troposphere is a thin layer of air called the tropopause. You can see it in Figure 15.12. This layer acts as a barrier. It prevents cool air in the troposphere from mixing with warm air in the stratosphere. 
layers of the atmosphere,T_0228,The stratosphere is the layer above the troposphere. The layer rises to about 50 kilometers (31 miles) above the surface. 
layers of the atmosphere,T_0229,"Air temperature in the stratosphere layer increases with altitude. Why? The stratosphere gets most of its heat from the Sun. Therefore, its warmer closer to the Sun. The air at the bottom of the stratosphere is cold. The cold air is dense, so it doesnt rise. As a result, there is little mixing of air in this layer. "
layers of the atmosphere,T_0230,"The stratosphere contains a layer of ozone gas. Ozone consists of three oxygen atoms (O3 ). The ozone layer absorbs high-energy UV radiation. As you can see in Figure 15.14, UV radiation splits the ozone molecule. The split creates an oxygen molecule (O2 ) and an oxygen atom (O). This split releases heat that warms the stratosphere. By absorbing UV radiation, ozone also protects Earths surface. UV radiation would harm living things without the ozone layer. "
layers of the atmosphere,T_0231,At the top of the stratosphere is a thin layer called the stratopause. It acts as a boundary between the stratosphere and the mesosphere. 
layers of the atmosphere,T_0232,The mesosphere is the layer above the stratosphere. It rises to about 85 kilometers (53 miles) above the surface. Temperature decreases with altitude in this layer. 
layers of the atmosphere,T_0233,There are very few gas molecules in the mesosphere. This means that there is little matter to absorb the Suns rays and heat the air. Most of the heat that enters the mesosphere comes from the stratosphere below. Thats why the mesosphere is warmest at the bottom. 
layers of the atmosphere,T_0234,"Did you ever see a meteor shower, like the one in Figure 15.15? Meteors burn as they fall through the mesosphere. The space rocks experience friction with the gas molecules. The friction makes the meteors get very hot. Many meteors burn up completely in the mesosphere. "
layers of the atmosphere,T_0235,At the top of the mesosphere is the mesopause. Temperatures here are colder than anywhere else in the atmosphere. They are as low as -100 C (-212 F)! Nowhere on Earths surface is that cold. 
layers of the atmosphere,T_0236,The thermosphere is the layer above the mesosphere. It rises to 600 kilometers (372 miles) above the surface. The International Space Station orbits Earth in this layer as in Figure 15.16. 
layers of the atmosphere,T_0237,"Temperature increases with altitude in the thermosphere. Surprisingly, it may be higher than 1000 C (1800 F) near the top of this layer! The Suns energy there is very strong. The molecules absorb the Suns energy and are heated up. But there are so very few gas molecules, that the air still feels very cold. Molecules in the thermosphere gain or lose electrons. They then become charged particles called ions. "
layers of the atmosphere,T_0238,"Have you ever seen a brilliant light show in the night sky? Sometimes the ions in the thermosphere glow at night. Storms on the Sun energize the ions and make them light up. In the Northern Hemisphere, the lights are called the northern lights, or aurora borealis. In the Southern Hemisphere, they are called southern lights, or aurora australis. "
layers of the atmosphere,T_0239,"The exosphere is the layer above the thermosphere. This is the top of the atmosphere. The exosphere has no real upper limit; it just gradually merges with outer space. Gas molecules are very far apart in this layer, but they are really hot. Earths gravity is so weak in the exosphere that gas molecules sometimes just float off into space. "
world climates,T_0304,"Major climate types are based on temperature and precipitation. These two factors determine what types of plants can grow in an area. Animals and other living things depend on plants. So each climate is associated with certain types of living things. A major type of climate and its living things make up a biome. As you read about the major climate types below, find them on the map in Figure 17.9. "
world climates,T_0305,"Tropical climates are found around the equator. As youd expect, these climates have warm temperatures year round. Tropical climates may be very wet or wet and dry. Tropical wet climates occur at or very near the equator. They have high rainfall year round. Tropical rainforests grow in this type of climate. Tropical wet and dry climates occur between 5 and 20 latitude and receive less rainfall. Most of the rain falls in a single season. The rest of the year is dry. Few trees can withstand the long dry season, so the main plants are grasses (see Figure 17.10). "
world climates,T_0306,"Dry climates receive very little rainfall. They also have high rates of evaporation. This makes them even drier. The driest climates are deserts. Most occur between about 15 and 30 latitude. This is where dry air sinks to the surface in the global circulation cells. Deserts receive less than 25 centimeters (10 inches) of rain per year. They may be covered with sand dunes or be home to sparse but hardy plants (see Figure 17.11). With few clouds, deserts have hot days and cool nights. Other dry climates get a little more precipitation. They are called steppes. These regions have short grasses and low bushes (see Figure 17.11). Steppes occur at higher latitudes than deserts. They are dry because they are in continental interiors or rain shadows. "
world climates,T_0307,"Temperate climates have moderate temperatures. These climates vary in how much rain they get and when the rain falls. You can see different types of temperate climates in Figure 17.12. Mediterranean climates are found on the western coasts of continents. The latitudes are between 30 and 45. The coast of California has a Mediterranean climate. Temperatures are mild and rainfall is moderate. Most of the rain falls in the winter, and summers are dry. To make it through the dry summers, short woody plants are common. Marine west coast climates are also found on the western coasts of continents. They occur between 45 and 60 latitude. The coast of Washington State has this type of climate. Temperatures are mild and theres plenty of rainfall all year round. Dense fir forests grow in this climate. Humid subtropical climates are found on the eastern sides of continents between about 20 and 40 latitude. The southeastern U.S. has this type of climate. Summers are hot and humid, but winters are chilly. There is moderate rainfall throughout the year. Pine and oak forests grow in this climate. "
world climates,T_0308,"Continental climates are found in inland areas. They are too far from oceans to experience the effects of ocean water. Continental climates are common between 40 and 70 north latitude. There are no continental climates in the Southern Hemisphere. Can you guess why? The southern continents at this latitude are too narrow. All of their inland areas are close enough to a coast to be affected by the ocean! Humid continental climates are found between 40 and 60 north latitude. The northeastern U.S. has this type of climate. Summers are warm to hot, and winters are cold. Precipitation is moderate, and it falls year round. Deciduous trees grow in this climate. They lose their leaves in the fall and grow new ones in the spring. Subarctic climates are found between 60 and 70 north latitude. Much of Canada and Alaska have this type of climate. Summers are cool and short. Winters are very cold and long. Little precipitation falls, and most of it falls during the summer. Conifer forests grow in this climate (see Figure 17.13). "
world climates,T_0309,"Polar climates are found near the North and South Poles. They also occur on high mountains at lower latitudes. The summers are very cool, and the winters are frigid. Precipitation is very low because its so cold. You can see examples of polar climates in Figure 17.14. Polar tundra climates occur near the poles. Tundra climates have permafrost. Permafrost is layer of ground below the surface that is always frozen, even in the summer. Only small plants, such as mosses, can grow in this climate. Alpine tundra climates occur at high altitudes at any latitude. They are also called highland climates. These regions are very cold because they are so far above sea level. The alpine tundra climate is very similar to the polar tundra climate. Ice caps are areas covered with thick ice year round. Ice caps are found only in Greenland and Antarctica. Temperatures and precipitation are both very low. What little snow falls usually stays on the ground. It doesnt melt because its too cold. "
world climates,T_0310,"A place might have a different climate than the major climate type around it. This is called a microclimate. Look at Figure 17.15. The south-facing side of the hill gets more direct sunlight than the north side of a hill. This gives the south side a warmer microclimate. A microclimate can be due to a place being deeper. Since cold air sinks, a depression in the land can be a lot colder than the land around it. "
climate change,T_0311,Earths climate has changed many times through Earths history. Its been both hotter and colder than it is today. 
climate change,T_0312,"Over much of Earths past, the climate was warmer than it is today. Picture in your mind dinosaurs roaming the land. Theyre probably doing it in a pretty warm climate! But ice ages also occurred many times in the past. An ice age is a period when temperatures are cooler than normal. This causes glaciers to spread to lower latitudes. Scientists think that ice ages occurred at least six times over the last billion years alone. How do scientists learn about Earths past climates? "
climate change,T_0313,"The last major ice age took place in the Pleistocene. This epoch lasted from 2 million to 14,000 years ago. Earths temperature was only 5 C (9 F) cooler than it is today. But glaciers covered much of the Northern Hemisphere. In Figure 17.17, you can see how far south they went. Clearly, a small change in temperature can have a big impact on the planet. Humans lived during this ice age. "
climate change,T_0314,"Since the Pleistocene, Earths temperature has risen. Figure 17.18 shows how it changed over just the last 1500 years. There were minor ups and downs. But each time, the anomaly (the difference from average temperature) was less than 1 C (1.8 F). Since the mid 1800s, Earth has warmed up quickly. Look at Figure 17.19. The 14 hottest years on record have all occurred since 1900. Eight of them have occurred since 1998! This is what is usually meant by global warming. "
climate change,T_0315,Natural processes caused earlier climate changes. Human beings are the main cause of recent global warming. 
climate change,T_0316,"Several natural processes may affect Earths temperature. They range from sunspots to Earths wobble. Sunspots are storms on the Sun. When the number of sunspots is high, the Sun gives off more energy than usual. Still, there is little evidence for climate changing along with the sunspot cycle. Plate movements cause continents to drift closer to the poles or the equator. Ocean currents also shift when continents drift. All these changes can affect Earths temperature. Plate movements trigger volcanoes. A huge eruption could spew so much gas and ash into the air that little sunlight would reach the surface for months or years. This could lower Earths temperature. A large asteroid hitting Earth would throw a lot of dust into the air. This could block sunlight and cool the planet. Earth goes through regular changes in its position relative to the Sun. Its orbit changes slightly. Earth also wobbles on its axis of rotation. The planet also changes the tilt on its axis. These changes can affect Earths temperature. "
climate change,T_0317,Recent global warming is due mainly to human actions. Burning fossil fuels adds carbon dioxide to the atmosphere. Carbon dioxide is a greenhouse gas. Its one of several that human activities add to the atmosphere. An increase in greenhouse gases leads to greater greenhouse effect. The result is increased global warming. Figure 17.20 shows the increase in carbon dioxide since 1960. 
climate change,T_0318,"As Earth has gotten warmer, sea ice has melted. This has raised the level of water in the oceans. Figure 17.21 shows how much sea level has risen since 1880. "
climate change,T_0319,"Earths temperature will keep rising unless greenhouse gases are curbed. The temperature in 2100 may be as much as 5 C (9 F) higher than it was in 2000. Since the glacial periods of the Pleistocene, average temperature has risen about 4 C. Thats just 4 C from abundant ice to the moderate climate we have today. How might a 5 C increase in temperature affect Earth in the future? Warming will affect the entire globe by the end of this century. The map in Figure 17.22 shows the average temperature in the 2050s. This is compared with the average temperature in 1971 to 2000. In what place is the temperature increase the greatest? Where in the United States is the temperature increase the highest? As temperature rises, more sea ice will melt. Figure 17.23 shows how much less sea ice there may be in 2050 if temperatures keep going up. This would cause sea level to rise even higher. Some coastal cities could be under water. Millions of people would have to move inland. How might other living things be affected? "
climate change,T_0320,"Youve probably heard of El Nio and La Nia. These terms refer to certain short-term changes in climate. The changes are natural and occur in cycles. To understand the changes, you first need to know what happens in normal years. This is shown in Figure 17.24. "
climate change,T_0321,"During an El Nio, the western Pacific Ocean is warmer than usual. This causes the trade winds to change direction. The winds blow from west to east instead of east to west. This is shown in Figure 17.25. The warm water travels east across the equator, too. Warm water piles up along the western coast of South America. This prevents upwelling. Why do you think this is true? These changes in water temperature, winds, and currents affect climates worldwide. The changes usually last a year or two. Some places get more rain than normal. Other places get less. In many locations, the weather is more severe. "
climate change,T_0322,La Nia generally follows El Nio. It occurs when the Pacific Ocean is cooler than normal. Figure 17.26 shows what happens. The trade winds are like they are in a normal year. They blow from east to west. But in a La Nia the winds are stronger than usual. More cool water builds up in the western Pacific. These changes can also affect climates worldwide. 
climate change,T_0323,Some scientists think that global warming is affecting the cycle of El Nio and La Nia. These short-term changes seem to be cycling faster now than in the past. They are also more extreme. 
cycles of matter,T_0337,"Carbon is an element. By itself, its a black solid. You can see a lump of carbon in Figure 18.10. Carbon is incredibly important because of what it makes when it combines with many other elements. Carbon can form a wide variety of substances. For example, in the air, carbon combines with oxygen to form the gas carbon dioxide. In living things, carbon combines with several other elements. For example, it may combine with nitrogen and "
cycles of matter,T_0338,"In the carbon cycle, carbon moves through living and nonliving things. Carbon actually moves through two cycles that overlap. One cycle is mainly biotic; the other cycle is mainly abiotic. Both cycles are shown in Figure 18.11. "
cycles of matter,T_0339,Producers such as plants or algae use carbon dioxide in the air to make food. The organisms combine carbon dioxide with water to make sugar. They store the sugar as starch. Both sugar and starch are carbohydrates. Consumers get carbon when they eat producers or other consumers. Carbon doesnt stop there. Living things get energy from food in a process called respiration. This releases carbon dioxide back into the atmosphere. The cycle then repeats. 
cycles of matter,T_0340,"Carbon from decaying organisms enters the ground. Some carbon is stored in the soil. Some carbon may be stored underground for millions of years. This will form fossil fuels. When volcanoes erupt, carbon from the mantle is released as carbon dioxide into the air. Producers take in the carbon dioxide to make food. Then the cycle repeats. The oceans also play an important role in the carbon cycle. Ocean water absorbs carbon dioxide from the air. In fact, the oceans contain 50 times more carbon than the atmosphere. Much of the carbon sinks to the bottom of the oceans, where it may stay for hundreds of years. "
cycles of matter,T_0341,"Human actions are influencing the carbon cycle. Burning of fossil fuels releases the carbon dioxide that was stored in ancient plants. Carbon dioxide is a greenhouse gas and is a cause of global warming. Forests are also being destroyed. Trees may be cut down for their wood, or they may be burned to clear the land for farming. Burning wood releases more carbon dioxide into the atmosphere. You can see how a tropical rainforest was cleared for farming in Figure 18.12. With forests shrinking, there are fewer trees to remove carbon dioxide from the air. This makes the greenhouse effect even worse. "
cycles of matter,T_0342,"Living things also need nitrogen. Nitrogen is a key element in proteins. Like carbon, nitrogen cycles through ecosystems. You can see the nitrogen cycle in Figure 18.13. "
cycles of matter,T_0343,"Air is about 78 percent nitrogen. Decomposers release nitrogen into the air from dead organisms and their wastes. However, producers such as plants cant use these forms of nitrogen. Nitrogen must combine with other elements before producers can use it. This is done by certain bacteria in the soil. Its called fixing nitrogen. "
cycles of matter,T_0344,"Nitrogen is one of the most important nutrients needed by plants. Thats why most plant fertilizers contain nitrogen. Adding fertilizer to soil allows more plants to grow. As a result, a given amount of land can produce more food. So far, so good. But what happens next? Rain dissolves fertilizer in the soil. Runoff carries it away. The fertilizer ends up in bodies of water, from ponds to oceans. The nitrogen is a fertilizer in the water bodies. Since there is a lot of nitrogen it causes algae to grow out of control. Figure 18.14 shows a pond covered with algae. Algae may use up so much oxygen in the water that nothing else can grow. Soon, even the algae die out. Decomposers break down the dead tissue and use up all the oxygen in the water. This creates a dead zone. A dead zone is an area in a body of water where nothing grows because there is too little oxygen. There is a large dead zone in the Gulf of Mexico. You can see it Figure 18.14. "
the human population,T_0345,"A population usually grows when it has what it needs. If theres plenty of food and other resources, the population will get bigger. Look at Table 18.1. It shows how a population of bacteria grew. A single bacteria cell was added to a container of nutrients. Conditions were ideal. The bacteria divided every 30 minutes. After just 10 hours, there were more than a million bacteria! Assume the bacteria population keeps growing at this rate. How many bacteria will there be at 10.5 hours? Or at 12 hours? Time (hours) 0 0.5 Number of Bacteria 1 2 Time (hours) 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10 Number of Bacteria 4 8 16 32 64 128 256 512 1,024 2,048 4,096 8,192 16,384 32,768 65,536 131,072 262,144 524,288 1,048,576 "
the human population,T_0346,"The population growth rate is how fast a population is growing. The letter r stands for the growth rate. The growth rate equals the number of new members added to the population in a year for each 100 members already in the population. The growth rate includes new members added to the population and old members removed from the population. Births add new members to the population. Deaths remove members from the population. The formula for population growth rate is: r = b - d, where b = birth rate (number of births in 1 year per 100 population members) d = death rate (number of deaths in 1 year per 100 population members) If the birth rate is greater than the death rate, r is positive. This means that the population is growing bigger. For example, if b = 10 and d = 8, r = 2. This means that the population is growing by 2 individuals per year for every 100 members of the population. This may not sound like much, but its a fairly high rate of growth. A population growing at this rate would double in size in just 35 years! If the birth rate is less than the death rate, r is negative. This means that the population is becoming smaller. What do you think might cause this to happen? "
the human population,T_0347,"A population cant keep growing bigger and bigger forever. Sooner or later, it will run out of things it needs. For a given species, there is a maximum population that can be supported by the environment. This maximum is called the carrying capacity. When a population gets close to the carrying capacity, it usually grows more slowly. You can see this in Figure 18.16. When the population reaches the carrying capacity, it stops growing. "
the human population,T_0348,"Figure 18.17 shows how the human population has grown. It grew very slowly for tens of thousands of years. Then, in the 1800s, something happened to change all that. The human population started to grow much faster. "
the human population,T_0349,"The industrial revolution is what happened. The industrial revolution began in the late 1700s in Europe, North America, and a few other places. In these places, the human population grew faster. While there had always been a lot of births, the population grew because the death rate fell. It fell for several reasons: 1. New farm machines were invented. They increased the amount of food that could be produced. With more food, people were healthier and could live longer. 2. Steam engines and railroads were built. These machines could quickly carry food long distances. This made food shortages less likely. 3. Sanitation was improved. Sewers were dug to carry away human wastes (see Figure 18.18). This helped reduce the spread of disease. With better food and less chance of disease, the death rate fell. More children lived long enough to reach adulthood and have children of their own. As the death rate fell, the birth rate stayed high for a while. This caused rapid population growth. However, the birth rate in these countries has since fallen to a rate close to that of the low death rate. The result was slow population growth once again. These changes are called the demographic transition. "
the human population,T_0350,"More recently, the death rate has fallen because of the availability of more food and medical advances: A green revolution began in the mid 1900s. New methods and products increased how much food could be grown. For example, chemicals were developed that killed weeds without harming crops. Pesticides were developed that killed pests that destroyed crops. Vaccinations were developed that could prevent many diseases (see Figure 18.19). Antibiotics were discov- ered that could cure most infections caused by bacteria. Together, these two advances saved countless lives. Today in many countries, death rates have gone down but birth rates remain high. This means that the population is growing. Figure 18.20 shows the growth rates of human populations all over the world. "
the human population,T_0351,The growth of the human population has started to slow down. You can see this in Figure 18.21. It may stop growing by the mid 2000s. Scientists think that the human population will peak at about 9 billion people. What will need to change for the population to stop growing then? 
the human population,T_0352,"Are 9 billion people the human carrying capacity? It looks that way in Figure 18.21. But some people think there are too many of us already. Thats because we are harming the environment. Supplying all those people with energy creates a lot of pollution. For example, huge oil spills have killed millions of living things. Burning fossil fuels pollutes the air. This also increases causes global warming. Fossil fuels and other resources are being used up. We may run out of oil by the mid 2000s. Many other resources will run out sooner or later. People are killing too many animals for food. For example, some of the best fishing grounds in the oceans have almost no fish left. People have destroyed many habitats. For example, theyve drained millions of acres of wetlands. Wetlands have a great diversity of species. As wetlands shrink, species go extinct. People have allowed alien or invasive species - species originally from a different area - to invade new habitats. Often, the aliens have no natural enemies in their new home. They may drive native species extinct. Figure People themselves are also affected by the large size of the human population. A minority of people use most of the worlds energy and other resources. Many other people lack resources. Many dont have enough to eat or live with "
the human population,T_0353,"Is it possible for all the worlds people to live well and still protect the planet? Thats the aim of sustainable development. Its goals are to: 1. Distribute resources fairly. 2. Conserve resources so they wont run out. 3. Use resources in ways that wont harm ecosystems. A smaller human population may be part of the solution. Better use of resources is another part. For example, when forests are logged, new trees should be planted. Everyone can help in the effort. What will you do? "
pollution of the land,T_0362,Love Canal gained worldwide attention in the late 1970s when the press started covering its story. The story is outlined below and illustrated in Figure 19.9. 
pollution of the land,T_0363,"The Love Canal disaster actually began back in the mid 1900s. The disaster continues even today. Starting in the early 1940s, a big chemical company put thousands of barrels of chemical waste into an old canal. Over the next 10 years, the company dumped almost 22,000 tons of chemicals into the ground! In the early 1950s, the company covered over the barrels in the canal with soil. Then they sold the land to the city for just a dollar. The city needed the land in order to build an elementary school. The company warned the city that toxic waste was buried there. But they thought the waste was safe. The school and hundreds of homes were also built over the old canal. As it turned out, the cheap price was no bargain. Chemicals started leaking from the barrels. Chemicals seeped into basements. Chemicals bubbled up to the surface of the ground. In some places, plants wouldnt even grow on the soil. People noticed bad smells. Many got sick, especially the children. Residents wanted to know if the old chemicals were the cause. But they had a hard time getting officials to listen. So they demonstrated and demanded answers. Finally, the soil was tested and was found to be contaminated with harmful chemicals. For example, it contained a lot of lead and mercury. Both can cause permanent damage to the human nervous system. The school was closed. More than 200 homes were evacuated. Much of the Love Canal neighborhood was bulldozed away. The area had a massive clean-up effort. The cleanup cost millions of dollars. More than three decades later, much of Love Canal is still too contaminated to be safe for people. "
pollution of the land,T_0364,"Love Canal opened peoples eyes to toxic waste burial. They realized there must be other Love Canals all over the country. Thousands of contaminated sites were found. The Superfund Act was passed in 1980. The law required that money be set aside for cleanup of toxic waste sites, like the Elizabeth Copper Mine in Vermont (see the far-right image in Figure 19.9). The law also required safer disposal of hazardous waste in the future. "
pollution of the land,T_0365,"Love Canal highlighted the problem of pollution by hazardous waste. Hazardous waste is any waste that is dangerous to the health of people or the environment. It may be dangerous because it is toxic, corrosive, flammable, or explosive. Toxic waste is poisonous. Toxic waste may cause cancer or birth defects in people. It may also harm other living things. Corrosive waste is highly reactive with other substances. Corrosive waste may cause burns or destroy other materials that it touches. Flammable waste can burn easily. It may also give off harmful fumes when it burns. Explosive waste is likely to explode. The risk of explosion may be greater if the waste is mixed with other substances. Table 19.1 shows some examples of hazardous waste. Look closely. Are any of these examples lurking around your home? Example Description Cars contain toxic fluids such as brake fluid. The fluids may also be corrosive and flammable. This photo shows one way the fluids can end up in the ground. Cars use gas and oil. These materials are toxic and flammable. They pollute the land when they leak or spill. Batteries contain toxic and corrosive materials. People often toss them in the trash, but they should be disposed of properly. Electronics, such as old computers, contain toxic chem- icals. They may be sent to landfills where the toxic materials end up in the ground. Medical waste can contain many hazards: Human body fluids may cause disease; old thermometers may contain toxic mercury; and pharmaceuticals may be toxic to people and other living things. Example Description Paints can be both toxic and flammable. Paints may spill on the ground or be thrown improperly in the trash. Chemicals are applied to farm fields and lawns. They include fertilizers, herbicides, and pesticides. Many of these chemicals are toxic to people and other animals. "
pollution of the land,T_0366,The greatest source of hazardous waste is industry. Agriculture is another major source. Even households produce a lot of hazardous waste. 
pollution of the land,T_0367,"Thanks to the lessons of Love Canal, the U.S. now has laws requiring the safe disposal of hazardous waste. Companies must ensure that hazardous waste is not allowed to enter the environment in dangerous amounts. They must also protect their workers from hazardous materials. For example, they must provide employees with the proper safety gear and training (see Figure 19.10). "
pollution of the land,T_0368,"Cleaning products, lawn chemicals, paints, batteries, motor oil these are just some of the many hazardous materials that may be found in households. You might think that a household doesnt produce enough hazardous waste to worry about. But when you add up all the waste from all the households in a community, its a different story. A city of just 50,000 people might produce more than 40 tons of hazardous waste each year! Clearly, how households deal with hazardous waste matters. What can your family do? Reduce, reuse, recycle, or properly dispose of the wastes. 1. Reduce the amount of hazardous products you buy. For example, if you only need a quart of paint for a job, dont buy a gallon. 2. Use less hazardous products if you can. For example, clean windows with vinegar and water instead of toxic window cleaners. 3. Reuse products if its safe to do so. For example, paint thinner that has been used to clean paint brushes can be strained and reused. 4. Recycle whenever possible. For example, some service stations allow you to drop off used motor oil, car batteries, or tires for recycling. 5. Always properly dispose of hazardous waste. For example, let liquid waste evaporate before placing the container in the trash. Proper disposal depends on the waste. Many hazardous products have disposal guidelines on the label. Thats one reason why you should keep the products in their original containers. The labels also explain how to use the products safely. Follow the instructions to protect yourself and the environment. Most communities have centers for disposing of household hazardous waste (see Figure 19.11). Do you know how to dispose of hazardous waste in your community? "
introduction to earths surface,T_0369,"To describe your location wherever you are on Earths surface, you could use a coordinate system. For example, you could say that you are at 1234 Main Street, Springfield, Ohio. Or you could use a point of reference. If you want to meet up with a friend, you could tell him the distance and direction you are from the reference point. An example is, I am at the corner of Maple Street and Main Street, about two blocks north of your apartment. When studying Earths surface, scientists must be able to pinpoint a feature they are interested in. Scientists and others have a system to describe the location of any feature. Usually they use latitude and longitude as a coordinate system. Lines of latitude and longitude form a grid. The grid is centered on a reference point. You will learn about this type of grid when we discuss maps later in this chapter. "
introduction to earths surface,T_0370,"When an object is moving, it is not enough to describe its location. We also need to know direction. Direction is important for describing moving objects. For example, a wind blows a storm over your school. Where is that storm coming from? Where is it going? The most common way to describe direction is by using a compass. A compass is a device with a floating needle (Figure 2.1). The needle is a small magnet that aligns itself with the Earths magnetic field. The compass needle always points to magnetic north. If you have a compass and you find north, you can then know any other direction. See the directions, such as east, south, west, etc., on a compass rose. A compass needle lines up with Earths magnetic north pole. This is different from Earths geographic north pole, or true north. The geographic north pole is the top of the imaginary axis around which Earth rotates. The geographic north pole is much like the spindle of a spinning top. The location of the geographic north pole does not change. However, the magnetic north pole shifts in location over time. Depending on where you live, you can correct for the difference between the two poles when you use a map and a compass (Figure 2.2). Some maps have a double compass rose. This allows users to make the corrections between magnetic north and true north. An example is a nautical chart that boaters use to chart their positions at sea (Figure 2.3). "
introduction to earths surface,T_0371,"As you know, the surface of Earth is not flat. Some places are high and some places are low. For example, mountain ranges like the Sierra Nevada in California or the Andes in South America are high above the surrounding areas. We can describe the topography of a region by measuring the height or depth of that feature relative to sea level (Figure mountains, while others are more like small hills! Relief, or terrain, includes all the landforms of a region. A topographic map shows the height, or elevation, of features in an area. This includes mountains, craters, valleys, and rivers. For example, Figure 2.5 shows the San Francisco Peaks in northern Arizona. Features on the map include mountains, hills and lava flows. You can recognize these features from the differences in elevation. We will talk about some different landforms in the next section. "
introduction to earths surface,T_0372,"If you take away the water in the oceans (Figure 2.6), Earth looks really different. You see that the surface has two main features: continents and ocean basins. Continents are large land areas. Ocean basins extend from the edges of continents to the ocean floor and into deep trenches. Continents are much older than ocean basins. Some rocks on the continents are billions of years old. Ocean basins are only millions of years old at their oldest. Because the continents are so old, a lot has happened to them! As we view the land around us we see landforms. Landforms are physical features on Earths surface. Landforms are introduced in this section but will be discussed more in later chapters. Constructive forces cause landforms to grow. Lava flowing into the ocean can build land outward. A volcano can be a constructive force. Destructive forces may blow landforms apart. A volcano blowing its top off is a destructive force. The destructive forces of weathering and erosion change landforms more slowly. Over millions of years, mountains are worn down by rivers and streams. Constructive and destructive forces work together to create landforms. Constructive forces create mountains and erosion may wear them away. Mountains are very large landforms. Mountains may wear away into a high flat area called a plateau, or a lower-lying plain. Interior plains are in the middle of continents. Coastal plains are on the edge of a continent, where it meets the ocean. Rivers and streams flow across continents. They cut away at rock, forming river valleys (Figure 2.8). These are "
introduction to earths surface,T_0373,"The ocean basin begins where the ocean meets the land. The continental margin begins at the shore and goes down to the ocean floor. It includes the continental shelf, slope, and rise. The continental shelf is part of the continent, but it is underwater today. It is about 100-200 meters deep, much shallower than the rest of the ocean. The continental shelf usually goes out about 100 to 200 kilometers from the shore (Figure 2.9). The continental slope is the slope that forms the edge of the continent. It is seaward of the continental shelf. In some places, a large pile of sediments brought from rivers creates the continental rise. The continental rise ends at the Besides seamounts, there are long, very tall (about 2 km) mountain ranges. These ranges are connected so that they form huge ridge systems called mid-ocean ridges (Figure 2.11). The mid-ocean ridges form from volcanic eruptions. Lava from inside Earth breaks through the crust and creates the mountains. The deepest places of the ocean are the ocean trenches. Many trenches line the edges of the Pacific Ocean. The Mariana Trench is the deepest place in the ocean. (Figure 2.12). At about 11 km deep, it is the deepest place on Earth! To compare, the tallest place on Earth, Mount Everest, is less than 9 km tall. "
modeling earths surface,T_0374,"Imagine you are going on a road trip. Perhaps you are going on vacation. How do you know where to go? Most likely, you will use a map. A map is a picture of specific parts of Earths surface. There are many types of maps. Each map gives us different information. Lets look at a road map, which is the probably the most common map that you use (Figure 2.13). "
modeling earths surface,T_0375,"Look for the legend on the top left side of the map. It explains how this map records different features. You can see the following: The boundaries of the state show its shape. Black dots represent the cities. Each city is named. The size of the dot represents the population of the city. Red and brown lines show major roads that connect the cities. Blue lines show rivers. Their names are written in blue. Blue areas show lakes and other waterways the Gulf of Mexico, Biscayne Bay, and Lake Okeechobee. Names for bodies of water are also written in blue. A line or scale of miles shows the distance represented on the map an inch or centimeter on the map represents a certain amount of distance (miles or kilometers). The legend explains other features and symbols on the map. It is the convention for north to be at the top of a map. For this reason, a compass rose is not needed on most maps. You can use this map to find your way around Florida and get from one place to another along roadways. "
modeling earths surface,T_0376,"There are many other types of maps besides road maps. Some examples include: Political or geographic maps show the outlines and borders of states and/or countries. Satellite view maps show terrains and vegetation forests, deserts, and mountains. Relief maps show elevations of areas, but usually on a larger scale, such as the whole Earth, rather than a local area. Topographic maps show detailed elevations of features on the map. Climate maps show average temperatures and rainfall. Precipitation maps show the amount of rainfall in different areas. Weather maps show storms, air masses, and fronts. Radar maps show storms and rainfall. Geologic maps detail the types and locations of rocks found in an area. These are but a few types of maps that various Earth scientists might use. You can easily carry a map around in your pocket or bag. Maps are easy to use because they are flat or two-dimensional. However, the world is three- dimensional. So, how do map makers represent a three-dimensional world on flat paper? "
modeling earths surface,T_0377,"Earth is a round, three-dimensional ball. In a small area, Earth looks flat, so it is not hard to make accurate maps of a small place. When map makers want to map the round Earth on flat paper, they use projections. What happens if you try to flatten out the skin of a peeled orange? Or if you try to gift wrap a soccer ball? To flatten out, the orange peel must rip and its shape must become distorted. To wrap around object with flat paper requires lots of extra cuts and folds. A projection is a way to represent Earths curved surface on flat paper (Figure 2.14). There are many types of projections. Each uses a different way to change three dimensions into two dimensions. There are two basic methods that the map maker uses in projections: The map maker slices the sphere in some way and unfolds it to make a flat map, like flattening out an orange peel. The map maker can look at the sphere from a certain point and then translate this view onto a flat paper. Lets look at a few commonly used projections. "
modeling earths surface,T_0378,"In 1569, Gerardus Mercator (1512-1594) (Figure 2.15) figured out a way to make a flat map of our round world, called the Mercator projection (Figure 2.16). Imagine wrapping the round, ball-shaped Earth with a big, flat piece of paper. First you make a tube or a cylinder. The cylinder will touch Earth at its fattest part, the equator. The equator is the imaginary line running horizontally around the middle of Earth. The poles are the farthest points from the cylinder. If you shine a light from the inside of your model Earth out to the cylinder, the image projected onto the paper is a Mercator projection. Where does the projection represent Earth best? Where is it worst? Your map would be most correct at the equator. The shapes and sizes of continents become more stretched out near the poles. Early sailors and navigators found the Mercator map useful because most explorations were located near the equator. Many world maps still use the Mercator projection. The Mercator projection is best within 15 degrees north or south of the equator. Landmasses or countries outside that zone get stretched out of shape. The further the feature is from the equator, the more out of shape it is stretched. For example, if you look at Greenland on a globe, you see it is a relatively small country near the North Pole. Yet, on a Mercator projection, Greenland looks almost as big the United States. Because Greenland is closer to the pole, the continents shape and size are greatly increased. The United States is closer to its true dimensions. In a Mercator projection, all compass directions are straight lines. This makes it a good type of map for navigation. The top of the map is north, the bottom is south, the left side is west and the right side is east. However, because it is a flat map of a curved surface, a straight line on the map is not the shortest distance between the two points it connects. "
modeling earths surface,T_0379,"Instead of a cylinder, you could wrap the flat paper into a cone. Conic map projections use a cone shape to better represent regions near the poles (Figure 2.17). Conic projections are best where the cone shape touches the globe. This is along a line of latitude, usually the equator. "
modeling earths surface,T_0380,What if want to wrap a different approach? Lets say you dont want to wrap a flat piece of paper around a round object? You could put a flat piece of paper right on the area that you want to map. This type of map is called a gnomonic map projection (Figure 2.18). The paper only touches Earth at one point. The sizes and shapes of countries near that point are good. The poles are often mapped this way to avoid distortion. A gnomic projection is best for use over a small area. 
modeling earths surface,T_0381,"In 1963, Arthur Robinson made a map with more accurate sizes and shapes of land areas. He did this using mathematical formulas. The formulas could directly translate coordinates onto the map. This type of projection is shaped like an oval rather than a rectangle (Figure 2.19). Robinsons map is more accurate than a Mercator projection. The shapes and sizes of continents are closer to true. Robinsons map is best within 45 degrees of the equator. Distances along the equator and the lines parallel to it are true. However, the scales along each line of latitude are different. In 1988, the National Geographic Society began to use Robinsons projection for its world maps. Whatever map projection is used, maps help us find places and to be able to get from one place to another. So how do you find your location on a map? "
modeling earths surface,T_0382,"Most maps use a grid of lines to help you to find your location. This grid system is called a geographic coordinate system. Using this system you can define your location by two numbers, latitude and longitude. Both numbers are angles between your location, the center of Earth, and a reference line (Figure 2.20). "
modeling earths surface,T_0383,"Lines of latitude circle around Earth. The equator is a line of latitude right in the middle of the planet. The equator is an equal distance from both the North and South Pole. If you know your latitude, you know how far you are north or south of the equator. "
modeling earths surface,T_0384,"Lines of longitude are circles that go around Earth from pole to pole, like the sections of an orange. Lines of longitude start at the Prime Meridian. The Prime Meridian is a circle that runs north to south and passes through Greenwich, England. Longitude tells you how far you are east or west from the Prime Meridian (Figure 2.21). You can remember latitude and longitude by doing jumping jacks. When your hands are above your head and your feet are together, say longitude (your body is long!). When you put your arms out to the side horizontally, say latitude (your head and arms make a cross, like the t in latitude). While you are jumping, your arms are going the same way as each of these grid lines: horizontal for latitude and vertical for longitude. "
modeling earths surface,T_0385,"If you know the latitude and longitude of a place, you can find it on a map. Simply place one finger on the latitude on the vertical axis of the map. Place your other finger on the longitude along the horizontal axis of the map. Move your fingers along the latitude and longitude lines until they meet. For example, say the location you want to find is at 30o N and 90o W. Place your right finger along 30o N at the right of the map. Place your left finger along the bottom at 90o W. Move your fingers along the lines until they meet. Your location should be near New Orleans, Louisiana, along the Gulf coast of the United States. What if you want to know the latitude and longitude of your location? If you know where you are on a map, point to the place with your fingers. Take one finger and move it along the latitude line to find your latitude. Then move another finger along the longitude line to find your and longitude. "
modeling earths surface,T_0386,"You can also use a polar coordinate system. Your location is marked by an angle and distance from some reference point. The angle is usually the angle between your location, the reference point, and a line pointing north. The distance is given in meters or kilometers. To find your location or to move from place to place, you need a map, a compass, and some way to measure your distance, such as a range finder. Suppose you need to go from your location to a marker that is 20o E and 500 m from your current position. You must do the following: Use the compass and compass rose on the map to orient your map with north. Use the compass to find which direction is 20o E. Walk 500 meters in that direction to reach your destination. Polar coordinates are used in a sport called orienteering. People who do orienteering use a compass and a map with polar coordinates. Participants find their way along a course across wilderness terrain (Figure 2.22). They move to various checkpoints along the course. The winner is the person who completes the course in the fastest time. "
modeling earths surface,T_0387,"Earth is a sphere and so is a globe. A globe is the best way to make a map of the whole Earth. Because both the planet and a globe have curved surfaces, the sizes and shapes of countries are not distorted. Distances are true to scale. (Figure 2.23). Globes usually have a geographic coordinate system and a scale. The shortest distance between two points on a globe is the length of the portion of a circle that connects them. Globes are difficult to make and carry around. They also cannot be enlarged to show the details of any particular area. Globes are best sitting on your desk for reference. Google Earth is a neat site to download to your computer. This is a link that you can follow to get there: http://w tilt your image and lots more. "
topographic maps,T_0388,"Mapping is an important part of Earth Science. Topographic maps use a line, called a contour line, to show different elevations on a map. Contour lines show the location of hills, mountains and valleys. A regular road map shows where a road goes. But a road map doesnt show if the road goes over a mountain pass or through a valley. A topographic map shows you the features the road is going through or past. Lets look at topographic maps. Look at this view of the Swamp Canyon Trail in Bryce Canyon National Park, Utah (Figure 2.25). You can see the rugged canyon walls and valley below. The terrain has many steep cliffs with high and low points between the cliffs. Now look at the same section of the visitors map (Figure 2.26). You can see a green line that is the main road. The black dotted lines are trails. You see some markers for campsites, a picnic area, and a shuttle bus stop. The map does not show the height of the terrain. Where are the hills and valleys located? What is Natural Bridge? How high are the canyon walls? Which way do streams flow? A topographic map represents the elevations in an area (Figure 2.27). We mentioned topographic maps in the section on orienteering above. "
topographic maps,T_0389,Contour lines connect all the points on the map that have the same elevation. Lets take a closer look at this (Figure Each contour line represents a specific elevation. The contour line connects all the points that are at the same elevation. Every fifth contour line is made bold. The bold contour lines have numbers to show elevation. Contour lines run next to each other and NEVER cross one another. If the lines crossed it would mean that one place had two different elevations. This cannot happen. 
topographic maps,T_0390,"Since each contour line represents a specific elevation, two different contour are separated by the same difference in elevation (e.g. 20 ft or 100 ft.). This difference between contour lines is called the contour interval. You can calculate the contour interval by following these steps: a. Take the difference in elevation between 2 bold lines. b. Divide that difference by the number of contour lines between them. Imagine that the difference between two bold lines is 100 feet and there are five lines between them. What is the contour interval? If you answered 20 feet, then you are correct (100 ft/5 lines = 20 ft between lines). The legend on the map also gives the contour interval. "
topographic maps,T_0391,"How does a topographic map tell you about the terrain? Lets consider the following principles: 1. The spacing of contour lines shows the slope of the land. Contour lines that are close together indicate a steep slope. This is because the elevation changes quickly in a small area. Contour lines that seem to touch indicate a very steep slope, like a cliff. When contour lines are spaced far apart the slope is gentle. So contour lines help us see the three-dimensional shape of the land. Look at the topographic map of Stowe, Vermont (Figure 2.28). There is a steep hill rising just to the right of the city of Stowe. You can tell this because the contour lines there are closely spaced. The contour lines also show that the hill has a sharp rise of about 200 feet. Then the slope becomes less steep toward the right. 2. Concentric circles indicate a hill. Figure 2.29 shows another side of the topographic map of Stowe, Vermont. When contour lines form closed loops, there is a hill. The smallest loops are the higher elevations on the hill. The larger loops encircling the smaller loops are downhill. If you look at the map, you can see Cady Hill in the lower left and another, smaller hill in the upper right. 3. Hatched concentric circles indicate a depression. The hatch marks are short, perpendicular lines inside the circle. The innermost hatched circle represents the deepest part of the depression. The outer hatched circles represent higher elevations (Figure 2.30). 4. V-shaped portions of contour lines indicate stream valleys. The V shape of the contour lines point uphill. There is a V shape because the stream channel passes through the point of the V. The open end of the V represents the downstream portion. A blue line indicates that there is water running through the valley. If there is not a blue line the V pattern indicates which way water flows. In Figure 2.31, you can see examples of V-shaped markings. Try to find the direction a stream flows. 5. Like other maps, topographic maps have a scale so that you can find the horizontal distance. You can use the horizontal scale to calculate the slope of the land (vertical height/horizontal distance). Common scales used in United States Geological Service (USGS) maps include the following: 1:24,000 scale - 1 inch = 2000 ft 1:100,000 scale - 1 inch = 1.6 miles 1:250,000 scale - 1 inch = 4 miles Including contour lines, contour intervals, circles, and V-shapes allows a topographic map to show three-dimensional information on a flat piece of paper. A topographic map gives us a good idea of the shape of the land. "
topographic maps,T_0392,"As we mentioned above, topographic maps show the shape of the land. You can determine a lot of information about the landscape using a topographic map. These maps are invaluable for Earth scientists. "
topographic maps,T_0393,"Earth scientists use topographic maps for many things: Describing and locating surface features, especially geologic features. Determining the slope of the Earths surface. Determining the direction of flow for surface water, groundwater, and mudslides. Hikers, campers, and even soldiers use topographic maps to locate their positions in the field. Civil engineers use topographic maps to determine where roads, tunnels, and bridges should go. Land use planners and architects use topographic maps when planning development projects, such as housing projects, shopping malls, and roads. "
topographic maps,T_0394,"Oceanographers use a type of topographic map that shows water depths (Figure 2.32). On this map, the contour lines represent depth below the surface. Therefore, high numbers are deeper depths and low numbers are shallow depths. These maps are made from depth soundings or sonar data. They help oceanographers understand the shape of bottoms of lakes, bays, and the ocean. This information also helps boaters navigate safely. "
topographic maps,T_0395,"A geologic map shows the different rocks that are exposed at the surface of a region. Rock units are shown in a color identified in a key. On the geologic map of the Grand Canyon, for example, different rock types are shown in different colors. Some people call the Grand Canyon layer cake geology because most of the rock units are in layers. Rock units show up on both sides of a stream valley. A geologic map looks very complicated in a region where rock layers have been folded, like the patterns in marble cake. Faults are seen on this geologic map cutting across rock layers. When rock layers are tilted, you will see stripes of each layer on the map. There are symbols on a geologic map that tell you which direction the rock layers slant, and often there is a cut away diagram, called a cross section, that shows what the rock layers look like below the surface. A large-scale geologic map will just show geologic provinces. They do not show the detail of individual rock layers. "
using satellites and computers,T_0396,"To understand what satellites can do, lets look at an example. One of the deadliest hurricanes in United States history hit Galveston, Texas in 1900. The storm was first spotted at sea on Monday, August 27th , 1900. It was a tropical storm when it hit Cuba on September 3rd . By September 8th , it had intensified to a hurricane over the Gulf of Mexico. It came ashore at Galveston (Figure 2.34). Because there was not advanced warning, more than 8000 people lost their lives. Today, we have satellites with many different types of instruments that orbit the Earth. With these satellites, satellites can see hurricanes form at sea. They can follow hurricanes as they move from far out in the oceans to shore. Weather forecasters can warn people who live along the coasts. These advanced warning give people time to prepare for the storm. They can find a safe place or even evacuate the area, which helps save lives. "
using satellites and computers,T_0397,Satellites orbit high above the Earth in several ways. Different orbits are important for viewing different things about the planet. 
using satellites and computers,T_0398,"A satellite in a geostationary orbit flies above the planet at a distance of 36,000 km. It takes 24 hours to complete one orbit. The satellite and the Earth both complete one rotation in 24 hours. This means that the satellite stays over the same spot. Weather satellites use this type of orbit to observe changing weather conditions over a region. Communications satellites, like satellite TV, use this type of orbit to keep communications going full time. "
using satellites and computers,T_0399,"Another useful orbit is the polar orbit (Figure 2.35). The satellite orbits at a distance of several hundred kilometers. It makes one complete orbit around the Earth from the North Pole to the South Pole about every 90 minutes. In this same amount of time, the Earth rotates only slightly underneath the satellite. So in less than a day, the satellite can see the entire surface of the Earth. Some weather satellites use a polar orbit to see how the weather is changing globally. Also, some satellites that observe the land and oceans use a polar orbit. "
using satellites and computers,T_0400,"The National Aeronautics and Space Administration (NASA) has launched a fleet of satellites to study the Earth (Figure 2.36). The satellites are operated by several government agencies, including NASA, the National Oceano- graphic and Atmospheric Administration (NOAA), and the United States Geological Survey (USGS). By using different types of scientific instruments, satellites make many kinds of measurements of the Earth. Some satellites measure the temperatures of the land and oceans. Some record amounts of gases in the atmosphere, such as water vapor and carbon dioxide. Some measure their height above the oceans very precisely. From this information, they can measure sea level. Some measure the ability of the surface to reflect various colors of light. This information tells us about plant life. Some examples of the images from these types of satellites are shown in Figure 2.37. "
using satellites and computers,T_0401,"In order to locate your position on a map, you must know your latitude and your longitude. But you need several instruments to measure latitude and longitude. What if you could do the same thing with only one instrument? Satellites can also help you locate your position on the Earths surface. By 1993, the United States military had launched 24 satellites to help soldiers locate their positions on battlefields. This system of satellites was called the Global Positioning System (GPS). Later, the United States government allowed the public to use this system. Heres how it works. You must have a GPS receiver to use the system (Figure 2.38). You can buy many types of these in stores. The "
using satellites and computers,T_0402,"Prior to the late 20th and early 21st centuries, mapmakers sent people out in the field to determine the boundaries and locations for various features for maps. State or county borders were used to mark geological features. Today, people in the field use GPS receivers to mark the locations of features. Map-makers also use various satellite images and computers to draw maps. Computers are able to break apart the fine details of a satellite image, store the pieces of information, and put them back together to make a map. In some instances, computers can make 3-D images of the map and even animate them. For example, scientists used computers and satellite images from Mars to create a 3-D image of Mars ice cap (Figure 2.39). The image makes you feel as if you are looking at the ice cap from the surface of Mars. When you link any type of information to a location, you can put together incredibly useful maps and images. The information could be numbers of people living in an area, types of plants or soil, locations of groundwater or levels of rainfall. As long as you can link the information to a position with a GPS receiver, you can store it in a computer for later processing and map-making. This type of mapping is called a Geographic Information System (GIS). Geologists can use GIS to make maps of natural resources. City leaders might link these resources to where people live and help plan the growth of cities or communities. Other types of data can be linked by GIS. For example, Figure 2.40 shows a map of the counties where farmers made insurance claims for crop damage in 2008. Computers have improved how maps are made. They have also increased the amount of information that can be displayed. During the 21st century, computers will be used more and more in mapping. "
using satellites and computers,T_0403,5. What would have happened if there had been satellites during the time of the 1900 Galveston earthquake? 6. What would have happened if there had been no satellites when hurricane Katrina struck the Gulf of Mexico coast in 2005? 
use and conservation of resources,T_0404,"We need natural resources for just about everything we do. We need them for food and clothing, for building materials and energy. We even need them to have fun. Table 20.1 gives examples of how we use natural resources. Can you think of other ways we use natural resources? Use Vehicles Resources Rubber for tires from rubber trees Steel frames and other metal parts from minerals such as iron Example iron ore Use Electronics Resources Plastic cases from petroleum prod- ucts Glass screens from minerals such as lead Example lead ore Homes Nails from minerals such as iron Timber from trees spruce timber Jewelry Gemstones such as diamonds Minerals such as silver silver ore Food Sunlight, water, and soil Minerals such as phosphorus corn seeds in soil Clothing Wool from sheep Cotton from cotton plants cotton plants Recreation Water for boating and swimming Forests for hiking and camping pine forest Some natural resources are renewable. Others are not. It depends in part on how we use them. "
use and conservation of resources,T_0405,"Renewable resources can be renewed as they are used. An example is timber, which comes from trees. New trees can be planted to replace those that are cut down. Sunlight is a renewable resource. It seems we will never run out of that! Just because a resource is renewable, it doesnt mean we should use it carelessly. If we arent careful, we can pollute resources. Then they may no longer be fit for use. Water is one example. If we pollute a water source it may not be usable for drinking, bathing or any other type of use. We can also overuse resources that should be renewable. In this case the resources may not be able to recover. For example, fish are renewable resources. Thats because they can reproduce and make more fish. But water pollution and overfishing can cause them to die out if their population becomes too low. Figure 20.1 shows another example. "
use and conservation of resources,T_0406,"Some resources cant be renewed. At least, they cant be renewed fast enough to keep up with use. Fossil fuels are examples. It takes millions of years for them to form. We are using them up much more quickly. Elements that are used to produce nuclear power are other examples. They include uranium. This element is already rare. Sooner or later, it will run out. Supplies of non-renewable resources are shrinking. This makes them harder to get. Oil is a good example. Oil reserves beneath land are running out. So oil companies have started to drill for oil far out in the ocean. This costs more money. Its also more dangerous. Figure 20.2 shows an oil rig that exploded in 2010. The explosion killed 11 people. Millions of barrels of oil spilled into the water. It took months to plug the leak. "
use and conservation of resources,T_0407,"Rich nations use more natural resources than poor nations. In fact, the richest 20 percent of people use 85 percent of the worlds resources. What about the poorest 20 percent of people? They use only 1 percent of the worlds resources. You can see this unequal distribution of oil resources in Figure 20.3. Imagine a world in which everybody had equal access to resources. Some people would have fewer resources than they do now. But many people would have more. In the real world, the difference between rich and poor just keeps growing. "
use and conservation of resources,T_0408,"Every 20 minutes, the human population adds 3,500 more people. More people need more resources. For example, we now use five times more fossil fuels than we did in 1970. The human population is expected to increase for at least 40 years. What will happen to resource use? "
use and conservation of resources,T_0409,"How can we protect Earths natural resources? One answer is conservation. This means saving resources. We need to save resources so some will be left for the future. We also need to protect resources from pollution and overuse. When we conserve resources, we also cut down on the trash we produce. Americans throw out 340 million tons of trash each year. We throw out 2.5 million plastic bottles alone every hour! Most of what we throw out ends up in landfills. You can see a landfill in Figure 20.4. In a landfill, all those plastic bottles take hundreds of years to break down. What are the problems caused by producing so much trash? Natural resources must be used to produce the materials. Land must be given over to dump the materials. If the materials are toxic, they may cause pollution. "
use and conservation of resources,T_0410,"You probably already know about the three Rs. They stand for reduce, reuse, and recycle. The third R recycle has caught on in a big way. Thats because its easy. There are thousands of places to drop off items such as aluminum cans for recycling. Many cities allow you to just put your recycling in a special can and put it at the curb. We havent done as well with the first two Rs reducing and reusing. But they arent always as easy as recycling. Recycling is better than making things from brand new materials. But it still takes some resources to turn recycled items into new ones. It takes no resources at all to reuse items or not buy them in the first place. "
use and conservation of resources,T_0411,"Reducing resource use means just what it says using fewer resources. There are lots of ways to reduce our use of resources. Buy durable goods. Choose items that are well made so they will last longer. Youll buy fewer items in the long run, so youll save money as well as resources. Thats a win-win! Repair rather than replace. Fix your bike rather than buying a new one. Sew on a button instead of buying a new shirt. Youll use fewer resources and save money. Buy only what you need. Dont buy a gallon of milk if you can only drink half of it before it spoils. Instead, buy a half gallon and drink all of it. You wont be wasting resources (or money!). Buy local. For example, buy local produce at a farmers market, like the one in Figure 20.5. A lot of resources are saved by not shipping goods long distances. Products bought at farmers markets use less packaging, too! About a third of what we throw out is packaging. Try to buy items with the least amount of packaging. For example, buy bulk items instead of those that are individually wrapped. Also, try to select items with packaging that can be reused or recycled. This is called precycling. Pop cans and plastic water bottles, for example, are fairly easy to recycle. Some types of packaging are harder to recycle. You can see examples in Figure 20.6. If it cant be reused or recycled, its a waste of resources. Many plastics: The recycling symbol on the bottom of plastic containers shows the type of plastic they contain. Numbers 1 and 2 are easier to recycle than higher numbers. Mixed materials: Packaging that contains more than one material may be hard to recycle. This carton is made mostly of cardboard. But it has plastic around the opening. "
use and conservation of resources,T_0412,"Reusing resources means using items again instead of throwing them away. A reused item can be used in the same way by someone else. Or it can be used in a new way. For example, Shana has a pair of jeans she has outgrown. She might give them to her younger sister to wear. Or she might use them to make something different for herself, say, a denim shoulder bag. Some other ideas for reusing resources are shown in Figure 20.7. "
use and conservation of resources,T_0413,"Many things can be recycled. The materials in them can be reused in new products. For example, plastic water bottles can be recycled. The recycled material can be made into t-shirts! Old phone books can also be recycled and made into textbooks. When you shop for new products, look for those that are made of recycled materials (see Figure 20.8). Even food scraps and lawn waste can be recycled. They can be composted and turned into humus for the garden. At most recycling centers, you can drop off metal cans, cardboard and paper products, glass containers, and plastic bottles. Recycling stations like the one in Figure 20.9 are common. Curbside recycling usually takes these items too. Do you know how to recycle in your community? Contact your local solid waste authority to find out. If you dont already recycle, start today. Its a big way you can help the planet! "
use and conservation of energy,T_0414,Think about your typical day. How do you use energy? Do you take a shower when you first get out of bed? What about taking a shower uses energy? It takes energy to heat the water and to pump the water to your home. Do you eat a hot breakfast? Energy is used to cook your food. Do you ride a bus or have someone drive you to school? Motor vehicles need energy from fossil fuels to run. 
use and conservation of energy,T_0415,"Figure 20.10 shows the major ways energy is used in the U.S. A lot of energy is used in homes. In fact, more energy is used in homes than in stores and businesses. Even more energy is used for transportation. A lot of fuel is necessary to move people and goods around the country. Industry uses the most energy. Industrial uses account for one-third of all the energy used in the U.S. "
use and conservation of energy,T_0416,"Figure 20.11 shows the energy resources used in the U.S. The U.S. depends mainly on fossil fuels. Petroleum is used more than any other resource. Renewable energy resources, such as solar and wind energy, could provide all the energy we need, but they are not yet widely used in the U.S. "
use and conservation of energy,T_0417,"We must use energy to get energy resources. This is true of non-renewable and renewable energy. Getting fossil fuels so that they can be used takes many steps. All of these steps use energy. 1. 2. 3. 4. 5. Fossil fuels must be found. The resources must be removed from the ground. These resources need to be refined, some more than others. Fossil fuels may need to be changed to a different form of energy. Energy resources must be transported from where they are produced to where they are sold or used. Consider petroleum as an example. Oil companies explore for petroleum in areas where they think it might be. When they find it, they must determine how much is there. They must also know how hard it will be to get. If theres enough to make it worthwhile, they will decide to go for it. To extract petroleum, companies they must build huge rigs, like the one in Figure 20.12. An oil rig drills deep into the ground and pumps the oil to the surface. The oil is then transported to a refinery. At the refinery, the oil is heated. It will then separate into different products, such as gasoline and motor oil. Finally, the oil products are transported to gas stations, stores, and industries. At every step, energy is used. For every five barrels of oil we use, it takes at least one barrel to get the oil. Less energy is needed to get renewable energy sources. Solar energy is a good example. Sunlight is everywhere, so no one needs to go out and find it. We dont have to drill for it or pump it to the surface. We just need to install solar panels like the ones in Figure 20.13 and let sunlight strike them. The energy from the sunlight is changed to electricity. The electricity is used to power lights and appliances in the house. So solar energy doesnt have to be transported. "
use and conservation of energy,T_0418,"Nonrenewable energy resources will run out before long. Using these energy resources also produces pollution and increases global warming. For all these reasons, we need to use less of these energy sources. We also need to use them more efficiently. "
use and conservation of energy,T_0419,"There are many ways to use less energy. Table 20.2 lists some of them. Can you think of other ways to use less energy? For example, how might schools use less energy? Use of Energy Transportation How to Use Less Plan ahead to reduce the number of trips you make. Take a bus or train instead of driving. Walk or bike rather than ride. Home Unplug appliances when not in use. Turn off lights when you leave a room. Put on a sweater instead of turning up the heat. Run the dishwasher and washing machine only when full. "
use and conservation of energy,T_0420,"We can get more work out of the energy we use. Table 20.3 show some ways to use energy more efficiently. By getting more bang for the buck, we wont need to use as much energy overall. Does your family use energy efficiently? How could you find out? Use of Energy More Efficient Use Another way to use energy more efficiently is with Energy Star appliances. They carry the Energy Star logo, shown in Figure 20.14. To be certified as Energy Star, the appliance must use less energy. Energy Star appliances save a lot of energy over their lifetime. What if millions of households used Energy Star appliances? How much energy would it save? "
humans and the water supply,T_0421,"Figure 21.1 shows how people use water worldwide. The greatest use is for agriculture and then industry. Municipal use is last, but is also important. Municipal use refers to water used by homes and businesses in communities. "
humans and the water supply,T_0422,"Many crops are grown where there isnt enough rainfall for plants to thrive. For example, crops are grown in deserts of the American southwest. How is this possible? The answer is irrigation. Irrigation is any way of providing extra water to plants. Most of the water used in agriculture is used for irrigation. Livestock also use water, but they use much less. Irrigation can waste a lot of water. The type of irrigation shown in Figure 21.2 is the most wasteful. The water is sprayed into the air and then falls to the ground. But much of the water never reaches the crops. Instead, it evaporates in the air or runs off the fields. Irrigation water may cause other problems. The water may dissolve agricultural chemicals such as pesticides. When the water soaks into the ground, the dissolved chemicals do, too. They may enter groundwater or run off into rivers or lakes. Salts in irrigation water can also collect in the soil. The soil may get too salty for plants to grow. "
humans and the water supply,T_0423,Almost a quarter of the water used worldwide is used in industry. Industries use water for many purposes. Chemical processes need a lot of water. Water is used to generate electricity. An important way that industries use water is to cool machines and power plants. 
humans and the water supply,T_0424,"Think about all the ways people use water at home. Besides drinking it, they use it for cooking, bathing, washing dishes, doing laundry, and flushing toilets. The water used inside homes goes down the drain. From there it usually ends up in a sewer system. At the sewage treatment plant, water can be is treated and prepared for reuse. Households may also use water outdoors. If your family has a lawn or garden, you may water them with a hose or sprinkler. You probably use water to wash the car, like the teen in Figure 21.3. Much of the water used outdoors evaporates or runs off into the gutter. The runoff water may end up in storm sewers that flow into a body of water, such as the ocean. "
humans and the water supply,T_0425,"There are many ways to use water for fun, from white water rafting to snorkeling. When you do these activities you dont actually use water. You are doing the activity on or in the water. What do you think is the single biggest use of water for fun? Believe it or not, its golf! Keeping golf courses green uses an incredible amount of water. Since many golf courses are in sunny areas, much of the water is irrigation water. Many golf courses, like the one in Figure 21.4, have sprinkler systems. Like any similar sprinkler system, much of this water is wasted. It evaporates or runs off the ground. "
humans and the water supply,T_0426,"Most Americans have plenty of fresh, clean water. But many people around the world do not. In fact, water scarcity is the worlds most serious resource problem. How can that be? Water is almost everywhere. More than 70 percent of Earths surface is covered by water. "
humans and the water supply,T_0427,"One problem is that only a tiny fraction of Earths water is fresh, liquid water that people can use. More than 97 percent of Earths water is salt water in the oceans. Just 3 percent is freshwater. Most of the freshwater is frozen in ice sheets, icebergs, and glaciers (see Figure 21.5). "
humans and the water supply,T_0428,"Rainfall varies around the globe. About 40 percent of the land gets very little rain. About the same percentage of the worlds people dont have enough water. You can compare global rainfall with the worldwide freshwater supply at the two URLs below. Drier climates generally have less water for people to use. In some places, people may have less water available to them for an entire year than many Americans use in a single day! How much water is there where you live? Global rainfall: http://commons.wikimedia.org/wiki/File:World_precip_annual.png Freshwater supply: http://commons.wikimedia.org/wiki/File:2006_Global_Water_Availability.svg "
humans and the water supply,T_0429,"Richer nations can drill deep wells, build large dams or supply people with water in other ways. In these countries, just about everyone has access to clean running water in their homes. Its no surprise that people in these countries also use the most water. In poorer nations, there is little money to develop water supplies. Look at the people in Figure 21.6. These people must carry water home in a bucket from a distant pump. "
humans and the water supply,T_0430,"Water shortages are common in much of the world. People are most likely to run short of water during droughts. A drought is a period of unusually low rainfall. Human actions have increased how often droughts occur. One way people can help to bring on drought is by cutting down trees. Trees add a lot of water vapor to the air. With fewer trees, the air is drier and droughts are more common. We already use six times as much water today as we did a hundred years ago. As the number of people rises, our need for water will grow. By the year 2025, only half the worlds people will have enough clean water. Water is such a vital resource that serious water shortages may cause other problems. Crops and livestock may die, so people will have less food available. Other uses of water, such as industry, may have to stop. This reduces the jobs people can get and the products they can buy. People and nations may fight over water resources. In extreme cases, people may die from lack of water. The Figure 21.7 shows the global water situation in the 2030s with water stress and water scarcity on the map. "
humans and the water supply,T_0431,"The water Americans get from their faucets is generally safe. This water has been treated and purified. But at least 20 percent of the worlds people do not have clean drinking water. Their only choice may be to drink water straight from a river (see Figure 21.8). If the river is polluted with wastes, it will contain bacteria and other organisms that cause disease. Almost 9 out of 10 cases of disease worldwide are caused by unsafe drinking water. Diseases from unsafe drinking water are the leading cause of death in young children. "
water pollution,T_0432,"Pollution that enters water at just one point is called point source pollution. For example, chemicals from a factory might empty into a stream through a pipe or set of pipes (see Figure 21.9). Pollution that enters in many places is called non-point source pollution. This means that the pollution is from multiple sources. With non-point source pollution, runoff may carry the pollution into a body of water. Which type of pollution do you think is harder to control? "
water pollution,T_0433,"There are three main sources of water pollution: 1. Agriculture. 2. Industry. 3. Municipal, or community, sources. "
water pollution,T_0434,"Huge amounts of chemicals, such as fertilizers and pesticides, are applied to farm fields (see Figure 21.10). Some of the chemicals are picked up by rainwater. Runoff then carries the chemicals to nearby rivers or lakes. Dissolved fertilizer causes too much growth of water plants and algae. This can lead to dead zones where nothing can live in lakes and at the mouths of rivers. Some of the chemicals can infiltrate into groundwater. The contaminated water comes up in water wells. If people drink the polluted water, they may get sick. Waste from livestock can also pollute water. The waste contains bacteria and other organisms that cause disease. In fact, more than 40 human diseases can be caused by water polluted with animal waste. Many farms in the U.S. have thousands of animals. These farms produce millions of gallons of waste. The waste is stored in huge lagoons, like the one in Figure 21.11. Unfortunately, many leaks from these lagoons have occurred. Two examples are described below. In North Carolina, 25 million gallons of hog manure spilled into a nearby river. The contaminated water killed "
water pollution,T_0435,"Factories and power plants may pollute water with harmful substances. Many industries produce toxic chemicals. Some of the worst are arsenic, lead, and mercury. Nuclear power plants produce radioactive chemicals. They cause cancer and other serious health problems. Oil tanks and pipelines can leak. Leaks may not be noticed until a lot of oil has soaked into the ground. The oil may pollute groundwater so it is no longer fit to drink. "
water pollution,T_0436,Municipal refers to the community. Households and businesses in a community are also responsible for polluting the water supply. For example: People apply chemicals to their lawns. The chemicals may be picked up by rainwater. The contaminated runoff enters storm sewers and ends up in nearby rivers or lakes. Underground septic tanks can develop leaks. This lets household sewage seep into groundwater. Municipal sewage treatment plants dump treated wastewater into rivers or lakes. Sometimes the wastewater is not treated enough and contains bacteria or toxic chemicals. 
water pollution,T_0437,The oceans are vast. You might think they are too big to be harmed by pollution. But thats not the case. Ocean water is becoming seriously polluted. 
water pollution,T_0438,"The oceans are most polluted along coasts. Why do you think thats the case? Of course, its because most pollution enters the oceans from the land. Runoff and rivers carry the majority of pollution into the ocean. Many cities dump their wastewater directly into coastal waters. In some parts of the world, raw sewage and trash may be thrown into the water (see Figure 21.12). Coastal water may become so polluted that people get sick if they swim in it or eat seafood from it. The polluted water may also kill fish and other ocean life. "
water pollution,T_0439,"Oil spills are another source of ocean pollution. To get at oil buried beneath the seafloor, oil rigs are built in the oceans. These rigs pump oil from beneath the ocean floor. Huge ocean tankers carry oil around the world. If something goes wrong with a rig on a tanker, millions of barrels of oil may end up in the water. The oil may coat and kill ocean animals. Some of the oil will wash ashore. This oil may destroy coastal wetlands and ruin beaches. Figure 21.13 shows an oil spill on a beach. The oil washed ashore after a deadly oil rig explosion in the Gulf of Mexico in 2010. "
water pollution,T_0440,"Thermal pollution is pollution that raises the temperature of water. This is caused by power plants and factories that use the water to cool their machines. The plants pump cold water from a lake or coastal area through giant cooling towers, like those in Figure 21.14. As it flows through the towers, the cold water absorbs heat. This warmed water is returned to the lake or sea. Thermal pollution can kill fish and other water life. Its not just the warm temperature that kills them. Warm water cant hold as much oxygen as cool water. If the water gets too warm, there may not be enough oxygen for living things. "
protecting the water supply,T_0441,"In the mid 1900s, people were startled to see the Cuyahoga River in Cleveland, Ohio, burst into flames! The river was so polluted with oil and other industrial wastes that it was flammable. Nothing could live in it. You can see the Cuyahoga River in Figure 21.16 "
protecting the water supply,T_0442,"Disasters such as rivers burning led to new U.S. laws to protect the water. For example, the Environmental Protection Agency (EPA) was established, and the Clean Water Act was passed. Now, water is routinely tested. Pollution is tracked to its source, and polluters are forced to fix the problem and clean up the pollution. They are also fined. These consequences have led industries, agriculture, and communities to pollute the water much less than before. "
protecting the water supply,T_0443,"Most water pollution comes from industry, agriculture, and municipal sources. Homes are part of the municipal source and the individuals and families that live in them can pollute the water supply. What can you do to reduce water pollution? Read the tips below. Properly dispose of motor oil and household chemicals. Never pour them down the drain. Also, dont let them spill on the ground. This keeps them out of storm sewers and bodies of water. Use fewer lawn and garden chemicals. Use natural products instead. For example, use compost instead of fertilizer. Or grow plants that can thrive on their own without any extra help. Repair engine oil leaks right away. A steady drip of oil from an engine can quickly add up to gallons. When the oil washes off driveways and streets it can end up in storm drains and pollute the water supply. Dont let pet litter or pet wastes get into the water supply (see Figure 21.17). The nitrogen they contain can cause overgrowth of algae. The wastes may also contain bacteria and other causes of disease. "
protecting the water supply,T_0444,"Water treatment is a series of processes that remove unwanted substances from water. The goal of water treatment is to make the water safe to return to the natural environment or to the human water supply. Treating water for other purposes may not include all the same steps. Thats because water used in agriculture or industry may not have to be as clean as drinking water. You can see how water for drinking is treated in Figure 21.18. Treating drinking water requires at least four processes: 1. Chemicals are added to untreated water. They cause solids in the water to clump together. This is called coagulation. 2. The water is moved to tanks. The clumped solids sink to the bottom of the water. This is called sedimentation. 3. The water is passed through filters that remove smaller particles from the water. This is called filtration. 4. Chlorine is added to the water to kill bacteria and other microbes. This is called disinfection. Finally, the water is pure enough to drink. "
protecting the water supply,T_0445,"Conserving water means using less of it. Of course, this mostly applies to people in the wealthy nations that have the most water and also waste the most. "
protecting the water supply,T_0446,Irrigation is the single biggest use of water. Overhead irrigation wastes a lot of water. Drip irrigation wastes a lot less. Figure 21.19 shows a drip irrigation system. Water pipes run over the surface of the ground. Tiny holes in the pipes are placed close to each plant. Water slowly drips out of the holes and soaks into the soil around the plants. Very little of the water evaporates or runs off the ground. 
protecting the water supply,T_0447,"Some communities save water with rationing. Much rationing takes place only during times of drought. During rationing, water may not be used for certain things. For example, communities may ban lawn watering and car washing. People may be fined if they use water in these ways. You can do your part. Follow any bans where you live. "
protecting the water supply,T_0448,"Its easy to save water at home. If you save even a few gallons a day you can make a big difference over the long run. The best place to start saving water is in the bathroom. Toilet flushing is the single biggest use of water in the home. Showers and baths are the next biggest use. Follow the tips below to save water at home. Install water-saving toilets. They use only about half as much water per flush. A single household can save up to 20,000 gallons a year with this change alone! Take shorter showers. You can get just as clean in 5 minutes as you can in 10. And youll save up to 50 gallons of water each time you shower. Thats thousands of gallons each year. Use low-flow shower heads. They use about half as much water as regular shower heads. They save thousands of gallons of water. Fix leaky shower heads and faucets. All those drips really add up. At one drip per second, more than 6,000 gallons go down the drain in a year per faucet! Dont leave the water running while you brush your teeth. You could save as much as 10 gallons each time you brush. That could add up to 10,000 gallons in a year. Landscape your home with plants that need little water. This could result in a huge savings in water use. Look at the garden in Figure 21.20. It shows that you dont have to sacrifice beauty to save water. "
air pollution,T_0452,"Air quality is a measure of the pollutants in the air. More pollutants mean poorer air quality. Air quality, in turn, depends on many factors. Some natural processes add pollutants to the air. For example, forest fires and volcanoes add carbon dioxide and soot. In dry areas, the air often contains dust. However, human actions cause the most air pollution. The single biggest cause is fossil fuel burning. "
air pollution,T_0453,"Poor air quality started to become a serious problem after the Industrial Revolution. The machines in factories burned coal. This released a lot of pollutants into the air. After 1900, motor vehicles became common. Cars and trucks burn gasoline, which adds greatly to air pollution. "
air pollution,T_0454,"By the mid-1900s, air quality in many big cities was very bad. The worst incident came in December 1952. A temperature inversion over London, England, kept cold air and pollutants near the ground. The air became so polluted that thousands of people died in just a few days. This event was called the Big Smoke. "
air pollution,T_0455,"At the same time, many U.S. cities had air pollution problems. Some of the worst were in California. Cars were becoming more popular. Oil refineries and power plants also polluted the air. Mountain ranges trapped polluted air over cities. The California sunshine caused chemical reactions among the pollutants. These reactions produced many more harmful compounds. "
air pollution,T_0456,"By 1970, it was clear that something needed to be done to protect air quality. In the U.S., the Clean Air Act was passed. It limits what can be released into the air. As a result, the air in the U.S. is much cleaner now than it was 50 years ago. But air pollution has not gone away. Vehicles, factories, and power plants still release more than 150 million tons of pollutants into the air each year. "
air pollution,T_0457,There are two basic types of pollutants in air. They are known as primary pollutants and secondary pollutants. 
air pollution,T_0458,"Primary pollutants enter the air directly. Some are released by natural processes, like ash from volcanoes. Most are released by human activities. They pour into the air from vehicles and smokestacks. Several of these pollutants are described below. Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO2 ). Carbon oxides are released when fossil fuels burn. Nitrogen oxides include nitric oxide (NO) and nitrogen dioxide (NO2 ). Nitrogen oxides form when nitrogen and oxygen combine at high temperatures. This occurs in hot exhausts from vehicles, factories, and power plants. Sulfur oxides include sulfur dioxide (SO2 ) and sulfur trioxide (SO3 ). Sulfur oxides are produced when sulfur and oxygen combine. This happens when coal burns. Coal can contain up to 10 percent sulfur. Toxic heavy metals include mercury and lead. Mercury is used in some industrial processes. It is also found in fluorescent light bulbs. Lead was once widely used in gasoline, paint, and pipes. It is still found in some products. Volatile organic compounds (VOCs) are carbon compounds such as methane. VOCs are released in many human activities, such as raising livestock. Livestock wastes produce a lot of methane. Particulates are solid particles. These particles may be ash, dust, or even animal wastes. Many are released when fossil fuels burn (see Figure 22.1). "
air pollution,T_0459,"Secondary pollutants form when primary pollutants undergo chemical reactions after they are released. Many occur as part of photochemical smog. This type of smog is seen as a brown haze in the air. Photochemical smog forms when certain pollutants react together in the presence of sunlight. You can see smog hanging in the air over San Francisco in Figure 22.2. Photochemical smog consists mainly of ozone (O3 ). The ozone in smog is the same compound as the ozone in the ozone layer,(O3 ). But ozone in smog is found near the ground. Figure 22.3 shows how it forms. When nitrogen oxides and VOCs are heated by the Sun, they lose oxygen atoms. The oxygen atoms combine with molecules of oxygen to form ozone. Smog ozone is harmful to humans and other living things. "
air pollution,T_0460,Most pollutants enter the air when fossil fuels burn. Some are released when forests burn. Others evaporate into the air. 
air pollution,T_0461,"Burning fossil fuels releases many pollutants into the air. These pollutants include carbon monoxide, carbon dioxide, nitrogen dioxide, and sulfur dioxide. Motor vehicles account for almost half of fossil fuel use. Most vehicles run on gasoline, which comes from petroleum. Power plants and factories account for more than a quarter of fossil fuel use. Power plants burn fossil fuels to generate electricity. Factories burn fossil fuels to power machines. Homes and other buildings also burn fossil fuels. The energy they release is used for heating, cooking, and other purposes. "
air pollution,T_0462,Millions of acres of forest have been cut and burned to make way for farming. Figure 22.4 shows an example. Burning trees produces most of the same pollutants as burning fossil fuels. 
air pollution,T_0463,"VOCs enter the air by evaporation. VOCs are found in many products, like paints and petroleum products. Methane is a VOC that evaporates from livestock waste and landfills. "
effects of air pollution,T_0464,All air pollutants are harmful. Thats why theyre called pollutants. Some air pollutants damage the environment as well as the health of living things. The type of damage depends on the pollutant. 
effects of air pollution,T_0465,Particulates cause lung diseases. They can also increase the risk of heart disease and the number of asthma attacks. Particulates block sunlight from reaching Earths surface. This means there is less energy for photosynthesis. Less photosynthesis means that plants and phytoplankton produce less food. This affects whole ecosystems. 
effects of air pollution,T_0466,"The ozone in smog may damage plants. The effects of ozone add up over time. Plants such as trees, which normally live a long time, are most affected. Entire forests may die out if ozone levels are very high. Other plants, including crop plants, may also be damaged by ozone. You can see evidence of ozone damage in Figure 22.5. The ozone in smog is also harmful to human health. Figure 22.6 shows the levels of ozone to watch out for. Some people are especially sensitive to ozone. They can be harmed by levels of ozone that would not affect most other people. These people include those with lung or heart problems. "
effects of air pollution,T_0467,"Both nitrogen and sulfur oxides are toxic to humans. These compounds can cause lung diseases or make them worse. Nitrogen and sulfur oxides form acid rain, which is described below. "
effects of air pollution,T_0468,"Carbon monoxide (CO) is toxic to both plants and animals. CO is deadly to people in a confined space, such as a closed home. Carbon monoxide is odorless and colorless, so people cant tell when they are breathing it. Thats why homes should have carbon monoxide detectors. You can see one in Figure 22.7. "
effects of air pollution,T_0469,"Heavy metals, such as mercury and lead, are toxic to living things. They can enter food chains from the atmosphere. The metals build up in the tissues of organisms by bioaccumulation. Bioaccumulation is illustrated in Figure 22.8. As heavy metals are passed up a food chain they accumulate. Imagine a low-level consumer eating a producer. That consumer takes in all of the heavy metals from all of the producers that it eats. Then a higher-level consumer eats it and accumulates all the heavy metals from all of the lower-level consumers that it eats. In this way, heavy metals may accumulate. At high levels in the food chain, the heavy metals may be quite become quite concentrated. The higher up a food chain that humans eat, the greater the levels of toxic metals they take in. Thats why people should avoid eating too much of large fish such as tuna. Tuna are predators near the top of their food chains. They have been shown to contain high levels of mercury. In people, heavy metals can damage the brain and other organs. Unborn babies and young children are most affected. Thats because their organs are still developing. "
effects of air pollution,T_0470,"VOCs are toxic to humans and other living things. In people, they can cause a wide range of problems, from eye and nose irritation to brain damage and cancer. Levels of VOCs are often higher indoors than out. Thats because they are released by products such as paints, cleaning solutions, and building materials. How might you reduce your exposure to VOCs? "
effects of air pollution,T_0471,"Acid rain is rain that has a pH less than 5 (see Figure 22.9). The pH of normal rain is 5.6. Its slightly acidic because carbon dioxide in the air dissolves in rain. This forms carbonic acid, a weak acid. "
effects of air pollution,T_0472,Acid rain forms when nitrogen and sulfur oxides in air dissolve in rain (see Figure 22.10). This forms nitric and sulfuric acids. Both are strong acids. Acid rain with a pH as low as 4.0 is now common in many areas. Acid fog may be even more acidic than acid rain. Fog with a pH as low as 1.7 has been recorded. Thats the same pH as toilet bowl cleaner! 
effects of air pollution,T_0473,"Figure 22.11 shows some of the damage done by acid rain. Acid rain ends up in soil and bodies of water. This can make them very acidic. The acid strips soil of its nutrients. These changes can kill trees, fish, and other living things. Acid rain also dissolves limestone and marble. This can damage buildings, monuments, and statues. "
effects of air pollution,T_0474,Ozone near the ground harms human health. But the ozone layer in the stratosphere protects us from solar rays. Thats why people were alarmed in the 1980s to learn that there was a hole in the ozone layer. 
effects of air pollution,T_0475,"Whats destroying the ozone layer? The chief cause is chlorofluorocarbons (CFCs). These are human-made chemicals that contain the element chlorine (Cl). In the past, CFCs were widely used in spray cans, refrigerators, and many other products. CFCs are stable compounds that can remain in the atmosphere for hundreds of years. Once CFCs are in the air, they float up into the stratosphere. What happens next is shown in Figure 22.12. Sunlight breaks apart the molecules. This releases their chlorine atoms (Cl). The free chlorine atoms may then combine with oxygen atoms in ozone. This breaks down the ozone molecules into an oxygen molecule and an oxygen atom. One CFC molecule can break down as many as 100,000 ozone molecules in this way! These forms of oxygen do not protect the planet from ultraviolet radiation. "
effects of air pollution,T_0476,"Most ozone loss it taking place over the South Pole and Antarctica. This is the location of the ozone hole. The ozone hole is also seasonal. The hole forms during the early part spring in the Southern Hemisphere and then grows northward. You can see the hole in Figure 22.13. Besides the ozone hole, the ozone layer is thinner over the Northern Hemisphere. "
effects of air pollution,T_0477,"With less ozone in the stratosphere, more UV rays reach the ground. More UV rays increase skin cancer rates. Just a 1 percent loss of ozone causes a 5 percent increase in skin cancer. More UV rays also harm plants and phytoplankton. As a result, they produce less food. This may affect entire ecosystems. "
reducing air pollution,T_0478,"There are two basic types of strategies for reducing pollution from fossil fuels: 1. Use less fossil fuel to begin with. 2. When fossil fuels must be used, prevent the pollution from entering the air. "
reducing air pollution,T_0479,"We can reduce our use of fossil fuels in several ways: Conserve fossil fuels. For example, turning out lights when we arent using them saves electricity. Why does this help? A lot of the electricity we use comes from coal-burning power plants. Use fossil fuels more efficiently. For example, driving a fuel-efficient car lets you go farther on each gallon of gas. This can add up to a big savings in fossil fuel use. Change to alternative energy sources that produce little or no air pollution. For example, hybrid cars run on electricity that would be wasted during braking. These cars use gas only as a backup fuel. As a result, they produce just 10 percent of the air pollution produced by cars that run only on gas. Cars that run on hydrogen and produce no pollution at all have also been developed (see Figure 22.14). "
reducing air pollution,T_0480,"Some of the pollutants from fossil fuels can be filtered out of exhaust before it is released into the air. Other pollutants can be changed to harmless compounds before they are released. Two widely used technologies are scrubbers and catalytic converters. Scrubbers are used in factories and power plants. They remove particulates and waste gases from exhaust before it is released to the air. You can see how a scrubber works in Figure 22.15. Catalytic converters are used on motor vehicles. They break down pollutants in exhaust to non-toxic com- pounds. For example, they change nitrogen oxides to harmless nitrogen and oxygen gasses. "
reducing air pollution,T_0481,"The problems of ozone loss and global warming were unknown in 1970. When they were discovered, worldwide efforts were made to reduce CFCs and carbon dioxide emissions. "
reducing air pollution,T_0482,"The Montreal Protocol is a worldwide agreement on air pollution. It focuses on CFCs. It was signed by many countries in 1987. It controls almost 100 chemicals that can damage the ozone layer. Its aim is to return the ozone layer to its normal state. The Montreal Protocol has been effective in controlling CFCs. By 1995, few CFCs were still being used. But the ozone hole kept growing for several years after that because of the CFCs already in the atmosphere. It peaked in 2006. Since then, it has been somewhat smaller. "
reducing air pollution,T_0483,"The Kyoto Protocol is another worldwide agreement on air pollution. It was passed in 1997. The Protocol focuses on controlling greenhouse gas emissions. Its aim is to control global warming. Carbon dioxide is the main greenhouse gas causing global warming. There are several possible ways to reduce carbon dioxide emissions. They include cap-and-trade systems, carbon taxes, and carbon sequestration In a cap-and-trade system, each nation is given a cap, or upper limit, on carbon dioxide emissions. If a nation needs to go over its cap, it can trade with another nation that is below its cap. Figure 22.16 shows how this works. Carbon taxes are taxes placed on gasoline and other products that produce carbon dioxide. The taxes encourage people to use less fossil fuel, which reduces carbon dioxide emissions. Carbon sequestration is any way of removing carbon dioxide from the atmosphere and storing it in another form. Carbon is sequestered naturally by forests. Trees take in carbon dioxide for photosynthesis. Artificial methods of sequestering carbon underground are being researched. The Kyoto Protocol has not been as successful as the Montreal Protocol. One reason is that the worlds biggest producer of greenhouse gases, the U.S., did not sign the Kyoto Protocol. Of the nations that signed it, few are "
telescopes,T_0484,Earth is just a tiny speck in the universe. Our planet is surrounded by lots of space. Light travels across empty space. Astronomers can study light from stars to learn about the universe. Light is the visible part of the electromagnetic spectrum. Astronomers use the light that comes to us to gather information about the universe. 
telescopes,T_0485,"In space, light travels at about 300,000,000 meters per second (670,000,000 miles per hour). How fast is that? A beam of light could travel from New York to Los Angeles and back again nearly 40 times in just one second. Even at that amazing rate, objects in space are so far away that it takes a lot of time for their light to reach us. Even light from the nearest star, our Sun, takes about 8 minutes to reach Earth. "
telescopes,T_0486,"We need a really big unit to measure distances out in space because distances between stars are so great. A light- year, 9.5 trillion kilometers (5.9 trillion miles), is the distance that light travels in one year. Thats a long way! Out in space, its actually a pretty short distance. Proxima Centauri is the closest star to us after the Sun. This near neighbor is 4.22 light-years away. That means the light from Proxima Centauri takes 4.22 years to reach us. Our galaxy, the Milky Way Galaxy, is about 100,000 light-years across. So it takes light 100,000 years to travel from one side of the galaxy to the other! It turns out that even 100,000 light years is a short distance. The most distant galaxies we have detected are more than 13 billion light-years away. Thats over a hundred-billion-trillion kilometers! "
telescopes,T_0487,"When we look at stars and galaxies, we are seeing over great distances. More importantly, we are also seeing back in time. When we see a distant galaxy, we are actually seeing how the galaxy used to look. For example, the Andromeda Galaxy, shown in Figure 23.1, is about 2.5 million light-years from Earth. When you see an image of the galaxy what are you seeing? You are seeing the galaxy as it was 2.5 million years ago! Since scientists can look back in time they can better understand the Universes history. Check out http://science.n "
telescopes,T_0488,"Light is one type of electromagnetic radiation. Light is energy that travels in the form of an electromagnetic wave. Figure 23.2 shows a diagram of an electromagnetic wave. An electromagnetic (EM) wave has two parts: an electric field and a magnetic field. The electric and magnetic fields vibrate up and down, which makes the wave. The wavelength is the horizontal distance between two of the same points on the wave, like wave crest to wave crest. A waves frequency measures the number of wavelengths that pass a given point every second. As wavelength increases, frequency decreases. This means that as wavelengths get shorter, more waves move past a particular spot in the same amount of time. "
telescopes,T_0489,"Visible light is the part of the electromagnetic spectrum (Figure 23.3) that humans can see. Visible light includes all the colors of the rainbow. Each color is determined by its wavelength. Visible light ranges from violet wavelengths of 400 nanometers (nm) through red at 700 nm. There are parts of the electromagnetic spectrum that humans cannot see. This radiation exists all around you. You just cant see it! Every star, including our Sun, emits radiation of many wavelengths. Astronomers can learn a lot from studying the details of the spectrum of radiation from a star. Many extremely interesting objects cant be seen with the unaided eye. Astronomers use telescopes to see objects at wavelengths all across the electromagnetic spectrum. Some very hot stars emit light primarily at ultraviolet wavelengths. There are extremely hot objects that emit X-rays and even gamma rays. Some very cool stars shine mostly in the infrared light wavelengths. Radio waves come from the faintest, most distant objects. To learn more about stars spectra, visit "
telescopes,T_0491,"Humans have been making and using magnifying lenses for thousands of years. The first telescope was built by Galileo in 1608. His telescope used two lenses to make distant objects appear both nearer and larger. Telescopes that use lenses to bend light are called refracting telescopes, or refractors (Figure 23.4). The earliest telescopes were all refractors. Many amateur astronomers still use refractors today. Refractors are good for viewing details within our solar system. Craters on the surface of Earths Moon or the rings around Saturn are two such details. Around 1670, Sir Isaac Newton built a different kind of telescope. Newtons telescope used curved mirrors instead of lenses to focus light. This type of telescope is called a reflecting telescope, or reflector (see Figure 23.5). The mirrors in a reflecting telescope are much lighter than the heavy glass lenses in a refractor. This is important because a refracting telescope must be much stronger to support the heavy glass. Its much easier to precisely make mirrors than to precisely make glass lenses. For that reason, reflectors can be made larger than refractors. Larger telescopes can collect more light. This means that they can study dimmer or more distant objects. The largest optical telescopes in the world today are reflectors. Telescopes can also be made to use both lenses and mirrors. For more on how telescopes were developed, visit http://galileo.rice.edu/sci/instruments/telescope.html . "
telescopes,T_0492,"Radio telescopes collect radio waves. These telescopes are even larger telescopes than reflectors. Radio telescopes look a lot like satellite dishes. In fact, both are designed to collect and focus radio waves or microwaves from space. The largest single radio telescope in the world is at the Arecibo Observatory in Puerto Rico (see Figure 23.6). This telescope is located in a natural sinkhole. The sinkhole formed when water flowing underground dissolved the limestone. This telescope would collapse under its own weight if it were not supported by the ground. There is a big disadvantage to this design. The telescope can only observe the part of the sky that happens to be overhead at a given time. A group of radio telescopes can be linked together with a computer. The telescopes observe the same object. The computer then combines the data from each telescope. This makes the group function like one single telescope. An example is shown in Figure 23.7. To learn more about radio telescopes and radio astronomy in general, go to "
telescopes,T_0493,"Telescopes on Earth all have one big problem: Incoming light must pass through the atmosphere. This blocks some wavelengths of radiation. Also, motion in the atmosphere distorts light. You see this when you see stars twinkling in the night sky. Many observatories are built on high mountains. There is less air above the telescope, so there is less interference from the atmosphere. Space telescopes avoid such problems completely since they orbit outside the atmosphere. The Hubble Space Telescope is the best known space telescope. Hubble is shown in Figure 23.8. Hubble began operations in 1994. Since then it has provided huge amounts of data. The telescope has helped astronomers answer many of the biggest questions in astronomy. The National Aeronautics and Space Administration (NASA) has placed three other major space telescopes in orbit. Each uses a different part of the electromagnetic spectrum. The James Webb Space Telescope will launch in 2014. The telescope will replace the aging Hubble. To learn more about NASAs great observatories, check out "
telescopes,T_0495,"Humans have been studying the night sky for thousands of years. Knowing the motions of stars helped people keep track of seasons. With this information they could know when to plant crops. Stars were so important that the patterns they made in the sky were named. These patterns are called constellations. Even now, constellations help astronomers know where they are looking in the night sky. The ancient Greeks carefully observed the locations of stars in the sky. They noticed that some of the stars moved across the background of other stars. They called these bright spots in the sky planets. The word in Greek means wanderers. Today we know that the planets are not stars. They are objects in the solar system that orbit the Sun. Ancient astronomers made all of their observations without the aid of a telescope. "
telescopes,T_0496,"In 1610, Galileo looked at the night sky through the first telescope. This tool allowed him to make the following discoveries (among others): There are more stars in the night sky than the unaided eye can see. The band of light called the Milky Way consists of many stars. The Moon has craters (see Figure 23.10). Venus has phases like the Moon. Jupiter has moons orbiting around it. There are dark spots that move across the surface of the Sun. Galileos observations made people think differently about the universe. They made them think about the solar system and Earths place in it. Until that time, people believed that the Sun and planets revolved around Earth. One hundred years before Galileo, Copernicus had said that the Earth and the other planets revolved around the Sun. No one would believe him. But Galileos observations through his telescope proved that Copernicus was right. "
telescopes,T_0497,"Galileos telescope got people to think about the solar system in the right way. Modern tools have also transformed our way of thinking about the universe. Imagine this: Today you can see all of the things Galileo saw using a good pair of binoculars. You can see sunspots if you have special filters on the lenses. (Never look directly at the Sun without using the proper filters!) With the most basic telescope, you can see polar caps on Mars, the rings of Saturn, and bands in the atmosphere of Jupiter. You can see many times more stars with a telescope than without a telescope. Still, stars seen in a telescope look like single points of light. They are so far away. Only the red supergiant star Betelgeuse is large enough to appear as a disk. Except for our Sun, of course. Today, astronomers attach special instruments to telescopes. This allows them to collect a wide variety of data. The data is fed into computers so that it can be studied. An astronomer may take weeks to analyze all of the data collected from just a single night! "
telescopes,T_0498,"A spectrometer is a special tool that astronomers commonly use. Spectrometers allow them to study the light from a star or galaxy. A spectrometer produces a spectrum, like the one shown in Figure 23.11. A prism breaks light into all its colors. Gases from the outer atmosphere of a star absorb light. This forms dark lines in the spectrum. These dark lines reveal what elements the star contains. Astronomers use the spectrum to learn even more about the star. One thing they learn is how hot the star is. They also learn the direction the star is going and how fast. By carefully studying light from many stars, astronomers know how stars evolve. They have learned about the distribution and kinds of matter found throughout the universe. They even know something about how the universe might have formed. To find out what you can expect to see when looking through a telescope, check out "
early space exploration,T_0499,Humans did not reach space until the second half of the 20th century. They needed somehow to break past Earths gravity. A rocket moves rapidly in one direction. The device is propelled by particles flying out of it at high speed in the other direction. There are records of the Chinese using rockets in war against the Mongols as early as the 13th century. The Mongols then used rockets to attack Eastern Europe. Early rockets were also used to launch fireworks. 
early space exploration,T_0500,"Rockets were used for centuries before anyone could explain how they worked. The theory came about in 1687. Isaac Newton (16431727) described three basic laws of motion, now referred to as Newtons Laws of Motion: 1. An object in motion will remain in motion unless acted upon by a force. 2. Force equals mass multiplied by acceleration. 3. To every action, there is an equal and opposite reaction. Which of these three best explains how a rocket works? Newtons third law of motion. When a rockets propulsion pushes in one direction, the rocket moves in the opposite direction, as seen in the Figure 23.12. For a long time, many people believed that a rocket wouldnt work in space. There would be nothing for the rocket to push against. But they do work! Fuel is ignited in a chamber. The gases in the chamber explode. The explosion creates pressure that forces the gases out of one side of the rocket. The rocket moves in the opposite direction, as shown in Figure 23.13. The force pushing the rocket is called thrust. "
early space exploration,T_0501,"For centuries, rockets were powered by gunpowder or other solid fuels. These rockets could travel only short distances. Around the turn of the 20th century, several breakthroughs took place. These breakthroughs led to rockets that could travel beyond Earth. Liquid fuel gave rockets enough power to escape Earths gravity (Figure 23.14). By using multiple stages, empty fuel containers could drop away. This reduced the mass of the rocket so that it could fly higher. Rockets were used during World War II. The V2 was the first human-made object to travel high enough to be considered in space (Figure 23.15). Its altitude was 176 km (109 miles) above Earths surface. Wernher von Braun was a German rocket scientist. After he fled Germany in WWII, he helped the United States develop missile weapons. After the war, von Braun worked for NASA. He designed the Saturn V rocket (Figure "
early space exploration,T_0502,One of the first uses of rockets in space was to launch satellites. A satellite is an object that orbits a larger object. An orbit is a circular or elliptical path around an object. Natural objects in orbit are called natural satellites. The Moon is a natural satellite. Human-made objects in orbit are called artificial satellites. There are more and more artificial satellites orbiting Earth all the time. They all get into space using some sort of rocket. 
early space exploration,T_0503,"Why do satellites stay in orbit? Why dont they crash into Earth due to the planets gravity? Newtons law of universal gravitation describes what happens. Every object in the universe is attracted to every other object. Gravity makes an apple fall to the ground. Gravity also keeps you from floating away into the sky. Gravity holds the Moon in orbit around Earth. It keeps Earth in orbit around the Sun. Newton used an example to explain how gravity makes orbiting possible. Imagine a cannonball launched from a high mountain, as shown in Figure 23.17. If the cannonball is launched at a slow speed, it will fall back to Earth. This is shown as paths (A) and (B). Something different happens if the cannonball is launched at a fast speed. The Earth below curves away at the same rate that the cannonball falls. The cannonball then goes into a circular orbit, as in path (C). If the cannonball is launched even faster, it could go into an elliptical orbit (D). It might even leave Earths gravity and go into space (E). Unfortunately, Newtons idea would not work in real life. A cannonball launched at a fast speed from Mt. Everest would not go into orbit. The cannonball would burn up in the atmosphere. However, a rocket can launch straight up, then steer into orbit. It wont burn up in the orbit. A rocket can carry a satellite above the atmosphere and then release the satellite into orbit. "
early space exploration,T_0504,"The first artificial satellite was launched just over 50 years ago. Thousands are now in orbit around Earth. Satellites have orbited other objects in the solar system. These include the Moon, the Sun, Venus, Mars, Jupiter, and Saturn. Satellites have many different purposes. Imaging satellites take pictures Earths surface. These images are used for military or scientific purposes. Astronomers use imaging satellites to study and make maps of the Moon and other planets. Communications satellites, such as the one in Figure 23.18, are now extremely common. These satellites receive and send signals for telephone, television, or other types of communications. Navigational satellites are used for navigation systems, such as the Global Positioning System (GPS). The largest artificial satellite is the International Space Station. The ISS is designed for humans to live in space while conducting scientific research. "
early space exploration,T_0505,"Dozens of satellites collect data about the Earth. One example is NASAs Landsat satellites. These satellites make detailed images of Earths continents and coastal areas. Other satellites study the oceans, atmosphere, polar ice sheets, and other Earth systems. This data helps us to monitor climate change. Other long-term changes in the planet are also best seen from space. Satellite images help scientists understand how Earths systems affect one another. Different satellites monitor different wavelengths of energy, as in Figure 23.19. "
early space exploration,T_0506,Satellites have different views depending on their orbit. Satellites may be put in a low orbit. These satellites orbit from north to south over the poles. These satellites view a different part of Earth each time they circle. Imaging and weather satellites need this type of view. Satellite may be placed so that they orbit at the same rate the Earth spins. The satellite then remains over the same location on the surface. Communications satellites are often placed in these orbits. 
early space exploration,T_0507,The Cold War was between the Soviet Union (USSR) and the United States. The war lasted from the end of World War II in 1945 to the breakup of the USSR in 1991. The hallmark of the Cold War was an arms race. The two nations spared no expense to create new and more powerful weapons. The development of better missiles fostered better rocket technologies. 
early space exploration,T_0508,"The USSR launched Sputnik 1 on October 4, 1957. This was the first artificial satellite ever put into orbit. Sputnik 1, shown in Figure 23.20, sent out radio signals, which were detected by scientists and amateur radio operators around the world. The satellite stayed in orbit for about 3 months, until it burned up as a result of friction with Earths atmosphere. The launch of Sputnik 1 started the Space Race between the USSR and the USA. Americans were shocked that the Soviets had the technology to put the satellite into orbit. They worried that the Soviets might also be winning the arms race. On November 3, 1957, the Soviets launched Sputnik 2. This satellite carried the first living creature into orbit, a dog named Laika. "
early space exploration,T_0509,"In response to Sputnik program, the U.S. launched two satellites. Explorer I was launched on January 31, 1958 and Vanguard 1 on March 17, 1958. National Aeronautics and Space Administration (NASA) was established that same year. The race was on! On April 12, 1961, a Soviet cosmonaut became the first human in space and in orbit. Less than one month later May 5, 1961 the U.S. sent its first astronaut into space: Alan Shepherd. The first American in orbit was John Glenn, in February 1962. And on it went. "
early space exploration,T_0510,"On May 25, 1961, President John F. Kennedy challenged the U.S. Congress: I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him back safely to the Earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish. The Soviets were also trying to reach the Moon. Who would win? The answer came eight years after Kennedys challenge, on July 20, 1969. NASAs Apollo 11 mission put astronauts Neil Armstrong and Buzz Aldrin on the Moon, as shown in Figure 23.21. A total of five American missions put astronauts on the Moon. The last was Apollo 17. This mission landed on December 11, 1972. No other country has yet put a person on the Moon. Today, most space missions are done by "
early space exploration,T_0511,"Both the United States and the Soviet Union sent space probes to other planets. A space probe is an unmanned spacecraft. The craft collects data by flying near or landing on an object in space. This could be a planet, moon, asteroid, or comet. The USSR sent several probes to Venus in the Venera missions. Some landed on the surface and sent back data. The U.S. sent probes to Mercury, Venus, and Mars in the Mariner missions. Two probes landed on Mars during the Viking missions. The U.S. also sent probes to the outer solar system. These probes conducted fly-bys of Jupiter, Saturn, Uranus, and Neptune. The Pioneer and Voyager probes are now out beyond the edges of our solar system. We have lost contact with the two Pioneer probes. We hope to maintain contact with the two Voyager probes until at least 2020. "
recent space exploration,T_0512,"While the United States continued missions to the Moon in the early 1970s, the Soviets worked to build a space station. A space station is a large spacecraft. People can live on this craft for a long period of time. "
recent space exploration,T_0513,"Between 1971 and 1982, the Soviets put a total of seven Salyut space stations into orbit. Figure 23.22 shows the last of these, Salyut 7. These were all temporary stations. They were launched and later inhabited by a human crew. Three of the Salyut stations were used for secret military purposes. The others were used to study the problems of living in space. Cosmonauts aboard the stations performed a variety of experiments in astronomy, biology, and Earth science. Salyut 6 and Salyut 7 each had two docking ports. One crew could dock a spacecraft to one end. A replacement crew could dock to the other end. The U.S. only launched one space station during this time. It was called Skylab. Skylab was launched in May 1973. Three crews visited Skylab, all within its first year in orbit. Skylab was used to study the effects of staying in space for long period. Devices on board were and for studying the Sun. Skylab reentered Earths atmosphere in 1979, sooner than expected. "
recent space exploration,T_0514,The first space station designed for long-term use was the Mir space station (Figure 23.23). Mir was launched in several separate pieces. These pieces were put together in space. Mir holds the current record for the longest continued presence in space. There were people living on Mir continuously for almost 10 years! Mir was the first major space project in which the United States and Russia worked together. American space shuttles transported supplies and people to and from Mir. American astronauts lived on Mir for many months. This cooperation allowed the two nations to learn from each other. The U.S. learned about Russias experiences with long-duration space flights. Mir was taken out of orbit in 2001. It fell into the Pacific Ocean. 
recent space exploration,T_0515,"The International Space Station, shown in Figure 23.24 is a joint project between the space agencies of many nations These include the United States (NASA), Russia (RKA), Japan (JAXA), Canada (CSA), several European countries (ESA) and the Brazilian Space Agency. The International Space Station is a very large station. It has many different sections and is still being assembled. The station has had people on board since 2000. American space shuttles deliver most of the supplies and equipment to the station. Russian Soyuz spacecraft carry people. The primary purpose of the station is scientific research. This is important because the station has a microgravity environment. Experiments are done in the fields of biology, chemistry, physics, physiology and medicine. "
recent space exploration,T_0516,"NASA wanted a new kind of space vehicle. This vehicle had to be reusable. It had to able to carry large pieces of equipment, such as satellites, space telescopes, or sections of a space station. The new vehicle was called a space shuttle, shown in Figure 23.25. There have been five space shuttles: Columbia, Challenger, Discovery, Atlantis, and Endeavor. A space shuttle has three main parts. You are probably most familiar with the orbiter. This part has wings like At the end of the mission, the orbiter re-enters Earths atmosphere. The outside heats up as it descends. Pilots have to steer the shuttle to the runway very precisely. Space shuttles usually land at Kennedy Space Center or at Edwards Air Force Base in California. The orbiter is later hauled back to Florida on the back of a jet airplane. "
recent space exploration,T_0517,"The space shuttle program has been very successful. Over 100 mission have been flown. Space shuttle missions have made many scientific discoveries. Crews have launched many satellites. There have been other great achievements in space. However, the program has also had two tragic disasters. The first came just 73 seconds after launch, on January 28, 1986. The space shuttle Challenger disintegrated in mid-air, as shown in Figure 23.27. On board were seven crew members. All of them died. One of them was Christa McAuliffe, who was to be the first teacher in space. The problem was later shown to be an O-ring. This small part was in one of the rocket boosters. Space shuttle missions were put on hold while NASA improved the safety of the shuttles. The second occurred during the takeoff of the Columbia on January 16, 2003. A small piece of insulating foam broke off the fuel tank. The foam smashed into a tile on the shuttles wing. The tile was part of the shuttles heat shield. The shield protects the shuttle from extremely high temperatures as it reenters the atmosphere. When Columbia returned to Earth on February 3, 2003, it could not withstand the high temperatures. The shuttle broke apart. Again, all seven crew members died. The space shuttle will be retired in 2011. All the remaining shuttle missions will be to the ISS. Orion will replace the shuttle. Known as a Crew Exploration Vehicle, Orion is expected to be ready by 2016. "
recent space exploration,T_0518,The disasters have caused NASA to focus on developing unmanned missions. Missions without a crew are less expensive and less dangerous. These missions still provide a great deal of valuable information. 
recent space exploration,T_0519,"Incredible images have come from the Hubble Space Telescope (HST). Even more incredible scientific discoveries have come from HST. The Hubble was the first telescope in space. It was put into orbit by the space shuttle Discovery in 1990. Since then, four shuttle missions have gone to the Hubble to make repairs and upgrades. The last repair mission to the Hubble happened in 2009. An example of a HST image is in Figure 23.28, "
recent space exploration,T_0520,"We continue to explore the solar system. A rover is like a spacecraft on wheels (Figure 23.29). It can wheel around on the surface. Scientists on Earth tell it where to go. The craft then collects and sends back data from that locations. The Mars Pathfinder studied the red planet for nearly three months in 1997. Two more rovers, Spirit and Opportunity, landed on Mars in 2004. Both were only designed to last 90 days, but have lasted many times longer. Spirit sent back data until it became stuck in January 2010. Opportunity continues to explore Mars. Several spacecraft are currently in orbit, studying the Martian surface and thin atmosphere. "
recent space exploration,T_0521,Budget concerns have impacted NASA in recent years. Many scientists have come together to discuss the goals of the U.S. space program. Some would like to further explore the Moon. Others are interested in landing on Mars. A variety of destinations in the inner solar system may also be visited. Private aerospace companies will play more of a role in the coming years. 
recent space exploration,T_0522,"How to Discover a New Planet Thousands of planets - ones that look totally different than what were used to, and possibly could support life, exist outside of our solar system. But were only just now starting to find them. In the video below, Ashley takes you behind the simple technique that astronomers have been using to discover these curious new planets. MEDIA Click image to the left or use the URL below. URL: "
planet earth,T_0523,"As you walk, the ground usually looks pretty flat, even though the Earth is round. How do we know this? We have pictures of Earth taken from space that show that Earth is round. Astronauts aboard the Apollo 17 shuttle took this one, called The Blue Marble (Figure 24.1). Earth looks like a giant blue and white ball. Long before spacecraft took photos of Earth from space, people knew that Earth was round. How? One way was to look at ships sailing off into the distance. What do you see when you watch a tall ship sail over the horizon of the Earth? The bottom part of the ship disappears faster than the top part. What would that ship look like if Earth was flat? No part of it would disappear before the other. It would all just get smaller as it moved further away. In the solar system, the planets orbit around the Sun. The Sun and each of the planets of our solar system are round. Earth is the third planet from the Sun. It is one of the inner planets. Jupiter is an outer planet. It is the largest planet in the solar system at about 1,000 times the size of Earth. The Sun is about 1,000 times bigger than Jupiter! (Figure The outer planets in the solar system are giant balls of swirling gas. Earth and the other inner planets are relatively small, dense, and rocky. Most of Earths surface is covered with water. As far as we know, Earth is also the only planet that has liquid water. Earths atmosphere has oxygen. The water and oxygen are crucial to life as we know it. Earth appears to be the only planet in the solar system with living creatures. You can learn more about the planets in the Our Solar System chapter. Some of the different parts of the Earth are our: Since Earth is round, the layers all have the word sphere at the end (Figure 24.3). All of Earths layers interact. Therefore, Earths surface is constantly undergoing change. "
planet earth,T_0524,"Earth and Moon orbit each other. This Earth-Moon system orbits the Sun in a regular path (Figure 24.4). Gravity is the force of attraction between all objects. Gravity keeps the Earth and Moon in their orbits. Earths gravity pulls the Moon toward Earths center. Without gravity, the Moon would continue moving in a straight line off into space. All objects in the universe have a gravitational attraction to each other (Figure 24.5). The strength of the force of gravity depends on two things. They are the mass of the objects and the distance between them. The greater the objects mass, the greater the force of attraction. As the distance between the objects increases, the force of attraction decreases. "
planet earth,T_0525,"Earth has a magnetic field (Figure 24.6). The magnetic field has north and south poles. The field extends several thousand kilometers into space. Earths magnetic field is created by the movements of molten metal in the outer core. Earths magnetic field shields us from harmful radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can hang it from a string. Then watch as it aligns itself in a north-south direction, in response to Earths magnetic field. A compass needle also aligns with Earths magnetic field. People can navigate by finding magnetic north (Figure 24.8). "
planet earth,T_0526,"Earths axis is an imaginary line passing through the North and South Poles. Earthsrotation is its spins on its axis. Rotation is what a top does around its spindle. As Earth spins on its axis, it also orbits around the Sun. This is called Earths revolution. These motions lead to the cycles we see. Day and night, seasons, and the tides are caused by Earths motions. "
planet earth,T_0527,"In 1851, Lon Foucault, a French scientist, hung a heavy iron weight from a long wire. He pulled the weight to one side and then released it. The weight swung back and forth in a straight line. If Earth did not rotate, the pendulum would not change direction as it was swinging. But it did, or at least it appeared to. The direction of the pendulum appeared to change because Earth rotated beneath it. Figure 24.9 shows how this might look. A Turn of the Earth In this video, MIT students demonstrate how a Foucault Pendulum is used to prove that the Earth is rotating. See the video at  . MEDIA Click image to the left or use the URL below. URL: "
planet earth,T_0528,"How long does it take Earth to spin once on its axis? One rotation is 24 hours. That rotation is the length of a day! Whatever time it is, the side of Earth facing the Sun has daylight. The side facing away from the Sun is dark. If you look at Earth from the North Pole, the planet spins counterclockwise. As the Earth rotates, you see the Sun moving across the sky from east to west. We often say that the Sun is rising or setting. The Sun rises in the east and sets in the west. Actually, it is the Earths rotation that makes it appear that way. The Moon and the stars at night also seem to rise in the east and set in the west. Earths rotation is also responsible for this too. As Earth turns, the Moon and stars change position in the sky. "
planet earth,T_0529,"The Earth is tilted 23 1/2 on its axis (Figure 24.10). This means that as the Earth rotates, one hemisphere has longer days with shorter nights. At the same time the other hemisphere has shorter days and longer nights. For example, in the Northern hemisphere summer begins on June 21. On this date, the North Pole is pointed directly toward the Sun. This is the longest day and shortest night of the year in the Northern Hemisphere. The South Pole is pointed away from the Sun. This means that the Southern Hemisphere experiences its longest night and shortest day (Figure 24.11). The hemisphere that is tilted away from the Sun is cooler because it receives less direct rays. As Earth orbits the Sun, the Northern Hemisphere goes from winter to spring, then summer and fall. The Southern Hemisphere does the opposite from summer to fall to winter to spring. When it is winter in the Northern hemisphere, it is summer in the Southern hemisphere, and vice versa. "
planet earth,T_0530,"Earths revolution around the Sun takes 365.24 days. That is equal to one year. The Earth stays in orbit around the Sun because of the Suns gravity (Figure 24.12). Earths orbit is not a circle. It is somewhat elliptical. So as we travel around the Sun, sometimes we are a little farther away from the Sun. Sometimes we are closer to the Sun. Students sometimes think the slightly oval shape of our orbit causes Earths seasons. Thats not true! The seasons are due to the tilt of Earths axis, as discussed above. "
earths moon,T_0531,"The Moon is Earths only natural satellite. The Moon is about one-fourth the size of Earth, 3,476 kilometers in diameter. Gravity on the Moon is only one-sixth as strong as it is on Earth. If you weigh 120 pounds on Earth, you would only weigh 20 pounds on the Moon. You can jump six times as high on the Moon as you can on Earth. The Moon makes no light of its own. Like every other body in the solar system, it only reflects light from the Sun. The Moon rotates on its axis once for every orbit it makes around the Earth. What does this mean? This means that the same side of the Moon always faces Earth. The side of the Moon that always faces Earth is called the near side. The side of the Moon that always faces away from Earth is called the far side (Figure 24.13). All people for all time have only seen the Moons near side. The far side has only been seen by spacecraft. The Moon has no atmosphere. With no atmosphere, the Moon is not protected from extreme temperatures. The average surface temperature during the day is approximately 107C (225F). Daytime temperatures can reach as high as 123C (253F). At night, the average temperature drops to -153C (-243F). The lowest temperatures measured are as low as -233C (-397F). "
earths moon,T_0532,"We all know what the Moon looks like. Its always looked the same during our lifetime. In fact, the Moon has looked the same to every person who has looked up at it for all time. Even the dinosaurs and trilobites, should they have looked up at it, would have seen the same thing. This is not true of Earth. Natural processes continually alter the Earths surface. Without these processes, would Earths surface resemble the Moons? Even though we cant see it from Earth, the Moon has changed recently too. Astronauts footprints are now on the Moon. They will remain unchanged for thousands of years, because there is no wind, rain, or living thing to disturb them. Only a falling meteorite could destroy them. "
earths moon,T_0533,"The landscape of the Moon - its surface features - is very different from Earth. The lunar landscape is covered by craters caused by asteroid impacts (Figure 24.14). The craters are bowl-shaped basins on the Moons surface. Because the Moon has no water, wind, or weather, the craters remain unchanged. The Moons coldest temperatures are found deep in the craters. The coldest craters are at the south pole on the Moons far side, where the Sun never shines. These temperatures are amongst the coldest in our entire solar system. "
earths moon,T_0534,"When you look at the Moon from Earth, you notice dark and light areas. The maria are dark, solid, flat areas of lava. Maria covers around 16% of the Moons surface, mostly on the near side. The maria formed about 3.0 to 3.5 billion years ago, when the Moon was continually bombarded by meteorites (Figure 24.15). Large meteorites broke through the Moons newly formed surface. This caused magma to flow out and fill the craters. Scientists estimate volcanic activity on the Moon ended about 1.2 billion years ago. The lighter parts on the Moon are called terrae, or highlands (Figure 24.15). They are higher than the maria and include several high mountain ranges. The rock that makes up the highlands is lighter in color and crystallized more slowly than the maria. The rock looks light because it reflects more of the Suns light. "
earths moon,T_0535,"There are no lakes, rivers, or even small puddles anywhere to be found on the Moons surface. So there is no running water and no atmosphere. This means that there is no erosion. Natural processes continually alter the Earths surface. Without these processes, our planets surface would be covered with meteorite craters just like the Moon. Many moons in our solar system have cratered surfaces. NASA scientists have discovered a large number of water molecules mixed in with lunar dirt. There is also surface water ice. Even though there is a very small amount of water, there is no atmosphere. Temperatures are extreme. So it comes as no surprise that there has not been evidence of life on the Moon. "
earths moon,T_0536,"Like Earth, the Moon has a distinct crust, mantle, and core. The crust is composed of igneous rock. This rock is rich in the elements oxygen, silicon, magnesium, and aluminum. On the near side, the Moons crust is about 60 kilometers thick. On the far side, the crust is about 100 kilometers thick. The mantle is made of rock like Earths mantle. The Moon has a small metallic core, perhaps 600 to 800 kilometers in diameter. The composition of the core is probably mostly iron with some sulfur and nickel. We learned this both from the rock samples gathered by astronauts and from spacecraft sent to the Moon. "
the sun,T_0537,"The Sun is made almost entirely of the elements hydrogen and helium. The Sun has no solid material. Most atoms in the Sun exist as plasma. Plasma is superheated gas with an electrical charge. Because the Sun is made of gases, it does not have a defined outer boundary. Like Earth, the Sun has an internal structure. The inner three layers make up what we would actually call the Sun. "
the sun,T_0538,"The core is the Suns innermost layer. The core is plasma. It has a temperature of around 15 million degrees Celsius (C). Nuclear fusion reactions create the immense temperature. In these reactions, hydrogen atoms fuse to form helium. This releases vast amounts of energy. The energy moves towards the outer layers of the Sun. Energy from the Suns core powers most of the solar system. "
the sun,T_0539,"The radiative zone is the next layer out. It has a temperature of about 4 million degrees C. Energy from the core travels through the radiative zone. The rate the energy travels is extremely slow. Light particles, called photons, can only travel a few millimeters before they hit another particle. The particles are absorbed and then released again. It may take 50 million years for a photon to travel all the way through the radiative zone. "
the sun,T_0540,"The convection zone surrounds the radiative zone. In the convection zone, hot material from near the Suns center rises. This material cools at the surface, and then plunges back downward. The material then receives more heat from the radiative zone. "
the sun,T_0541,The three outer layers of the Sun are its atmosphere. 
the sun,T_0542,"The photosphere is the visible surface of the Sun (Figure 24.17). Its the part that we see shining. Surprisingly, the photosphere is also one of the coolest layers of the Sun. It is only about 6000 degrees C. "
the sun,T_0543,"The chromosphere lies above the photosphere. It is about 2,000 km thick. The thin chromosphere is heated by energy from the photosphere. Temperatures range from about 4000 degrees C to about 10,000 degrees C. The chromosphere is not as hot as other parts of the Sun, and it glows red. Jets of gas sometimes fly up through the chromosphere. With speeds up to 72,000 km per hour, the jets can fly as high as 10,000 kilometers. "
the sun,T_0544,"The corona is the outermost part of the Suns atmosphere. It is the Suns halo, or crown. With a temperature of 1 to 3 million K, the corona is much hotter than the photosphere. The corona extends millions of kilometers into space. Sometime you should try to see a total solar eclipse. If you do you will see the Suns corona shining out into space. "
the sun,T_0545,The Sun has many incredible surface features. Dont try to look at them though! Looking directly at the Sun can cause blindness. Find the appropriate filters for a pair of binoculars or a telescope and enjoy! 
the sun,T_0546,"The most noticeable magnetic activity of the Sun is the appearance of sunspots. Sunspots are cooler, darker areas on the Suns surface (Figure 24.18). Sunspots occur in an 11 year cycle. The number of sunspots begins at a minimum. The number gradually increases to the maximum. Then the number returns to a minimum again. Sunspots form because loops of the Suns magnetic field break through the surface. Sunspots usually occur in pairs. The loop breaks through the surface where it comes out of the Sun. It breaks through again where it goes back into the Sun. Sunspots disrupt the transfer of heat from the Suns lower layers. "
the sun,T_0547,"A loop of the Suns magnetic field may break. This creates solar flares. Solar flares are violent explosions that release huge amounts of energy (Figure 24.19). The streams of high energy particles they emit make up the solar wind. Solar wind is dangerous to spacecraft and astronauts. Solar flares can even cause damage on Earth. They have knocked out entire power grids and can disturb radio, satellite, and cell phone communications. "
the sun,T_0548,"Another amazing feature on the Sun is solar prominences. Plasma flows along the loop that connects sunspots. This plasma forms a glowing arch. The arch is a solar prominence. Solar prominences can reach thousands of kilometers into the Suns atmosphere. Prominences can last for a day to several months. Prominences can be seen during a total solar eclipse. NASAs Solar Dynamics Observatory (SDO) was launched on February 11, 2010. SDO is studying the Suns magnetic field. This includes how the Sun affects Earths atmosphere and climate. SDO provides extremely high resolution images. The craft gathers data faster than anything that ever studied the Sun. To learn more about the SDO mission, visit: http://sdo.gsfc.nasa.gov To find these videos for download, check out:  There are other ways to connect with NASA. Subscribe to NASAs Goddard Shorts HD podcast (http://svs.gsfc.nasa "
the sun and the earthmoon system,T_0549,"When a new moon passes directly between the Earth and the Sun, it causes a solar eclipse (Figure 24.20). The Moon casts a shadow on the Earth and blocks our view of the Sun. This happens only all three are lined up and in the same plane. This plane is called the ecliptic. The ecliptic is the plane of Earths orbit around the Sun. The Moons shadow has two distinct parts. The umbra is the inner, cone-shaped part of the shadow. It is the part in which all of the light has been blocked. The penumbra is the outer part of Moons shadow. It is where the light is only partially blocked. When the Moons shadow completely blocks the Sun, it is a total solar eclipse (Figure 24.21). If only part of the Sun is out of view, it is a partial solar eclipse. Solar eclipses are rare events. They usually only last a few minutes. That is because the Moons shadow only covers a very small area on Earth and Earth is turning very rapidly. Solar eclipses are amazing to experience. It appears like night only strange. Birds may sing as they do at dusk. Stars become visible in the sky and it gets colder outside. Unlike at night, the Sun is out. So during a solar eclipse, its easy to see the Suns corona and solar prominences. This NASA page will inform you on when solar eclipses are expected: http://eclipse.gsfc.nasa.gov/solar.html "
the sun and the earthmoon system,T_0550,"Sometimes a full moon moves through Earths shadow. This is a lunar eclipse (Figure 24.22). During a total lunar eclipse, the Moon travels completely in Earths umbra. During a partial lunar eclipse, only a portion of the Moon enters Earths umbra. When the Moon passes through Earths penumbra, it is a penumbral eclipse. Since Earths shadow is large, a lunar eclipse lasts for hours. Anyone with a view of the Moon can see a lunar eclipse. Partial lunar eclipses occur at least twice a year, but total lunar eclipses are less common. The Moon glows with a dull red coloring during a total lunar eclipse. "
the sun and the earthmoon system,T_0551,"The Moon does not produce any light of its own. It only reflects light from the Sun. As the Moon moves around the Earth, we see different parts of the Moon lit up by the Sun. This causes the phases of the Moon. As the Moon revolves around Earth, it changes from fully lit to completely dark and back again. A full moon occurs when the whole side facing Earth is lit. This happens when Earth is between the Moon and the Sun. About one week later, the Moon enters the quarter-moon phase. Only half of the Moons lit surface is visible from Earth, so it appears as a half circle. When the Moon moves between Earth and the Sun, the side facing Earth is completely dark. This is called the new moon phase. Sometimes you can just barely make out the outline of the new moon in the sky. This is because some sunlight reflects off the Earth and hits the Moon. Before and after the quarter-moon phases are the gibbous and crescent phases. During the crescent moon phase, the Moon is less than half lit. It is seen as only a sliver or crescent shape. During the gibbous moon phase, the Moon is more than half lit. It is not full. The Moon undergoes a complete cycle of phases about every 29.5 days. "
introduction to the solar system,T_0553,"The Sun and all the objects that are held by the Suns gravity are known as the solar system. These objects all revolve around the Sun. The ancient Greeks recognized five planets. These lights in the night sky changed their position against the background of stars. They appeared to wander. In fact, the word planet comes from a Greek word meaning wanderer. These objects were thought to be important, so they named them after gods from their mythology. The names for the planets Mercury, Venus, Mars, Jupiter, and Saturn came from the names of gods and a goddess. "
introduction to the solar system,T_0554,"The ancient Greeks thought that Earth was at the center of the universe, as shown in Figure 25.1. The sky had a set of spheres layered on top of one another. Each object in the sky was attached to one of these spheres. The object moved around Earth as that sphere rotated. These spheres contained the Moon, the Sun, and the five planets they recognized: Mercury, Venus, Mars, Jupiter, and Saturn. An outer sphere contained all the stars. The planets appear to move much faster than the stars, so the Greeks placed them closer to Earth. Ptolemy published this model of the solar system around 150 AD. "
introduction to the solar system,T_0555,"About 1,500 years after Ptolemy, Copernicus proposed a startling idea. He suggested that the Sun is at the center of the universe. Copernicus developed his model because it better explained the motions of the planets. Figure 25.2 shows both the Earth-centered and Sun-centered models. Copernicus did not publish his new model until his death. He knew that it was heresy to say that Earth was not the center of the universe. It wasnt until Galileo developed his telescope that people would take the Copernican "
introduction to the solar system,T_0556,"Today we know that we have eight planets, five dwarf planets, over 165 moons, and many, many asteroids and other small objects in our solar system. We also know that the Sun is not the center of the universe. But it is the center of the solar system. Figure 25.3 shows our solar system. The planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Table 25.1 gives some data on the mass and diameter of the Sun and planets relative to Earth. Object Mass (Relative to Earth) Sun Mercury Venus Earth Mars Jupiter Saturn Uranus 333,000 Earths mass 0.06 Earths mass 0.82 Earths mass 1.00 Earths mass 0.11 Earths mass 317.8 Earths mass 95.2 Earths mass 14.6 Earths mass Diameter of Planet (Relative to Earth) 109.2 Earths diameter 0.39 Earths diameter 0.95 Earths diameter 1.00 Earths diameter 0.53 Earths diameter 11.21 Earths diameter 9.41 Earths diameter 3.98 Earths diameter Neptune 17.2 Earths mass "
introduction to the solar system,T_0557,"Youve probably heard about Pluto. When it was discovered in 1930, Pluto was called the ninth planet. Astronomers later found out that Pluto was not like other planets. For one thing, what they were calling Pluto was not a single object. They were actually seeing Pluto and its moon, Charon. In older telescopes, they looked like one object. This one object looked big enough to be a planet. Alone, Pluto was not very big. Astronomers also discovered many objects like Pluto. They were rocky and icy and there were a whole lot of them. Astronomers were faced with a problem. They needed to call these other objects planets. Or they needed to decide that Pluto was something else. In 2006, these scientists decided what a planet is. According to the new definition, a planet must: Orbit a star. Be big enough that its own gravity causes it to be round. Be small enough that it isnt a star itself. Have cleared the area of its orbit of smaller objects. If the first three are true but not the fourth, then that object is a dwarf planet. We now call Pluto a dwarf planet. There are other dwarf planets in the solar system. They are Eris, Ceres, Makemake and Haumea. There are many other reasons why Pluto does not fit with the other planets in our solar system. "
introduction to the solar system,T_0558,"Figure 25.4 shows the Sun and planets with the correct sizes. The distances between them are way too small. In general, the farther away from the Sun, the greater the distance from one planets orbit to the next. In Figure 25.5, you can see that the orbits of the planets are nearly circular. Plutos orbit is a much longer ellipse. Some astronomers think Pluto was dragged into its orbit by Neptune. Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles). Table 25.2 shows the distance from the Sun to each planet in AU. The table shows how long it takes each planet to spin on its axis. It also shows how long it takes each planet to complete an orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus! Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Average Distance from Sun (AU) 0.39 AU 0.72 1.00 1.52 5.20 9.54 19.22 30.06 Length of Day (In Earth Days) 56.84 days 243.02 1.00 1.03 0.41 0.43 0.72 0.67 Length of Year Earth Years) 0.24 years 0.62 1.00 1.88 11.86 29.46 84.01 164.8 (In "
introduction to the solar system,T_0559,"Planets are held in their orbits by the force of gravity. What would happen without gravity? Imagine that you are swinging a ball on a string in a circular motion. Now let go of the string. The ball will fly away from you in a straight line. It was the string pulling on the ball that kept the ball moving in a circle. The motion of a planet is very similar to the ball on a string. The force pulling the planet is the pull of gravity between the planet and the Sun. Every object is attracted to every other object by gravity. The force of gravity between two objects depends on the mass of the objects. It also depends on how far apart the objects are. When you are sitting next to your dog, there is a gravitational force between the two of you. That force is far too weak for you to notice. You can feel the force of gravity between you and Earth because Earth has a lot of mass. The force of gravity between the Sun and planets is also very large. This is because the Sun and the planets are very large objects. Gravity is great enough to hold the planets to the Sun even though the distances between them are enormous. Gravity also holds moons in orbit around planets. "
introduction to the solar system,T_0560,"Since the early 1990s, astronomers have discovered other solar systems. A solar system has one or more planets orbiting one or more stars. We call these planets extrasolar planets, or exoplanets. They are called exoplanets because they orbit a star other than the Sun. As of June 2013, 891 exoplanets have been found. More exoplanets are found all the time. You can check out how many we have found at http://planetquest.jpl.nasa.gov/. We have been able to take pictures of only a few exoplanets. Most are discovered because of some tell-tale signs. One sign is a very slight motion of a star that must be caused by the pull of a planet. Another sign is the partial dimming of a stars light as the planet passes in front of it. "
introduction to the solar system,T_0561,"To figure out how the solar system formed, we need to put together what we have learned. There are two other important features to consider. First, all the planets orbit in nearly the same flat, disk-like region. Second, all the planets orbit in the same direction around the Sun. These two features are clues to how the solar system formed. "
introduction to the solar system,T_0562,"Scientists think the solar system formed from a big cloud of gas and dust, called a nebula. This is the solar nebula hypothesis. The nebula was made mostly of hydrogen and helium. There were heavier elements too. Gravity caused the nebula to contract (Figure 25.6). As the nebula contracted, it started to spin. As it got smaller and smaller, it spun faster and faster. This is what happens when an ice skater pulls her arms to her sides during a spin move. She spins faster. The spinning caused the nebula to form into a disk shape. This model explains why all the planets are found in the flat, disk-shaped region. It also explains why all the planets revolve in the same direction. The solar system formed from the nebula about 4.6 billion years ago "
introduction to the solar system,T_0563,"The Sun was the first object to form in the solar system. Gravity pulled matter together to the center of the disk. Density and pressure increased tremendously. Nuclear fusion reactions begin. In these reactions, the nuclei of atoms come together to form new, heavier chemical elements. Fusion reactions release huge amounts of nuclear energy. From these reactions a star was born, the Sun. Meanwhile, the outer parts of the disk were cooling off. Small pieces of dust started clumping together. These clumps collided and combined with other clumps. Larger clumps attracted smaller clumps with their gravity. Eventually, all these pieces grew into the planets and moons that we find in our solar system today. The outer planets Jupiter, Saturn, Uranus, and Neptune condensed from lighter materials. Hydrogen, helium, water, ammonia, and methane were among them. Its so cold by Jupiter and beyond that these materials can form solid particles. Closer to the Sun, they are gases. Since the gases can escape, the inner planets Mercury, Venus, Earth, and Mars formed from denser elements. These elements are solid even when close to the Sun. "
inner planets,T_0564,"Mercury is the smallest planet. It has no moon. The planet is also closest to the Sun and appears in Figure 25.7. As Figure 25.8 shows, the surface of Mercury is covered with craters, like Earths Moon. The presence of impact craters that are so old means that Mercury hasnt changed much geologically for billions of years. With only a trace of an atmosphere, it has no weather to wear down the ancient craters. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. The Mariner 10 spacecraft did a flyby of Mercury in 19741975, which was the best data from the planet for decades. In 2004, the MESSENGER mission left Earth. On its way to Mercury it did one flyby of Earth, two of Venus and three of Mercury. In March 2011, MESSENGER became the first spacecraft to enter an orbit around Mercury. During its year-long mission, the craft will map the planets surface and conduct other studies. One of these images can be seen in Figure 25.9. "
inner planets,T_0565,"Mercury is named for the Roman messenger god. Mercury was a messenger because he could run extremely fast. The Greeks gave the planet this name because Mercury moves very quickly in its orbit around the Sun. Mercury orbits the Sun in just 88 Earth days. Mercury has a very short year, but it also has very long days. Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. Therefore, each day on Mercury is 58 Earth days long. "
inner planets,T_0566,"Mercury is very close to the Sun, so it can get very hot. Mercury also has virtually no atmosphere. As the planet rotates very slowly, the temperature varies tremendously. In direct sunlight, the surface can be as hot as 427C (801F). On the dark side, the surface can be as cold as 183C (297F)! The coldest temperatures may be on the insides of craters. Most of Mercury is extremely dry. Scientists think that there may be a small amount of water, in the form of ice, at the planets poles. The poles never receive direct sunlight. "
inner planets,T_0567,Figure 25.10 shows a diagram of Mercurys interior. Mercury is one of the densest planets. Scientists think that the interior contains a large core made mostly of melted iron. Mercurys core takes up about 42% of the planets volume. Mercurys highly cratered surface is evidence that Mercury is not geologically active. 
inner planets,T_0568,"Named after the Roman goddess of love, Venus is the only planet named after a female. Venus is sometimes called Earths sister planet. But just how similar is Venus to Earth? Venus is our nearest neighbor. Venus is most like Earth in size. "
inner planets,T_0569,"Viewed through a telescope, Venus looks smooth and featureless. The planet is covered by a thick layer of clouds. You can see the clouds in pictures of Venus, such as Figure 25.11. We make maps of the surface using radar, because the thick clouds wont allow us to take photographs of the surface of Venus. Figure 25.12 shows the topographical features of Venus. The image was produced by the Magellan probe on a flyby. Radar waves sent by the spacecraft reveal mountains, valleys, vast lava plains, and canyons. Like Mercury, Venus does not have a moon. Clouds on Earth are made of water vapor. Venuss clouds are a lot less pleasant. They are made of carbon dioxide, sulfur dioxide and large amounts of corrosive sulfuric acid! The atmosphere of Venus is so thick that the pressure on the surface of Venus is very high. In fact, it is 90 times greater than the pressure at Earths surface! The thick atmosphere causes a strong greenhouse effect. As a result, Venus is the hottest planet. Even though it is farther from the Sun, Venus is much hotter even than Mercury. Temperatures at the surface reach 465C (860F). Thats hot enough to melt lead! "
inner planets,T_0570,"Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means that Venus doesnt have tectonic plates. Plate tectonics on Earth erases features over time. Figure 25.13 is an image made using radar data. The volcano is Maat Mons. Lava beds are in the foreground. Scientists think the color of sunlight on Venus is "
inner planets,T_0571,"Venus is the only planet that rotates clockwise as viewed from its North Pole. All of the other planets rotate counterclockwise. Venus turns slowly, making only one turn every 243 days. This is longer than a year on Venus! It takes Venus only 225 days to orbit the Sun. Because the orbit of Venus is inside Earths orbit, Venus always appears close to the Sun. You can see Venus rising early in the morning, just before the Sun rises. For this reason, Venus is sometimes called the morning star. When it sets in the evening, just after the Sun sets, it may be called the evening star. Since planets only reflect the Suns light, Venus should not be called a star at all! Venus is very bright because its clouds reflect sunlight very well. Venus is the brightest object in the sky besides the Sun and the Moon. "
inner planets,T_0572,"Earth is the third planet out from the Sun, shown in Figure 25.14. Because it is our planet, we know a lot more about Earth than we do about any other planet. What are main features of Earth? "
inner planets,T_0573,"Earth is a very diverse planet, seen in Figure 25.14. Water appears as vast oceans of liquid. Water is also seen as ice at the poles or as clouds of vapor. Earth also has large masses of land. Earths average surface temperature is 14C (57F). At this temperature, water is a liquid. The oceans and the atmosphere help keep Earths surface temperatures fairly steady. Earth is the only planet known to have life. Conditions on Earth are ideal for life! The atmosphere filters out harmful radiation. Water is abundant. Carbon dioxide was available for early life forms. The evolution of plants introduced more oxygen for animals. "
inner planets,T_0574,"The Earth is divided into many plates. These plates move around on the surface. The plates collide or slide past each other. One may even plunge beneath another. Plate motions cause most geological activity. This activity includes earthquakes, volcanoes, and the buildup of mountains. The reason for plate movement is convection in the mantle. Earth is the only planet that we know has plate tectonics. "
inner planets,T_0575,"Earth rotates on its axis once every 24 hours. This is the length of an Earth day. Earth orbits the Sun once every 365.24 days. This is the length of an Earth year. Earth has one large moon. This satellite orbits Earth once every 29.5 days. This moon is covered with craters, and also has large plains of lava. The Moon came into being from material that flew into space after Earth and a giant asteroid collided. This moon is not a captured asteroid like other moons in the solar system. "
inner planets,T_0576,"Mars, shown in Figure 25.15, is the fourth planet from the Sun. The Red Planet is the first planet beyond Earths orbit. Mars atmosphere is thin compared to Earths. This means that there is much lower pressure at the surface. Mars also has a weak greenhouse effect, so temperatures are only slightly higher than they would be if the planet did not have an atmosphere. Mars is the easiest planet to observe. As a result, it has been studied more than any other planet besides Earth. People can stand on Earth and observe the planet through a telescope. We have also sent many space probes to Mars. In April 2011, there were three scientific satellites in orbit around Mars. The rover, Opportunity, was still moving around on the surface. No humans have ever set foot on Mars. NASA and the European Space Agency have plans to send people to Mars. The goal is to do it sometime between 2030 and 2040. The expense and danger of these missions are phenomenal. "
inner planets,T_0577,"Viewed from Earth, Mars is red. This is due to large amounts of iron in the soil. The ancient Greeks and Romans named the planet Mars after the god of war. The planets red color reminded them of blood. Mars has only a very thin atmosphere, made up mostly of carbon dioxide. "
inner planets,T_0578,"Mars is home to the largest volcano in the solar system. Olympus Mons is shown in Figure 25.16. Olympus Mons is a shield volcano. The volcano is similar to the volcanoes of the Hawaiian Islands. But Olympus Mons is a giant, about 27 km (16.7 miles/88,580 ft) tall. Thats three times taller than Mount Everest! At its base, Olympus Mons is about the size of the entire state of Arizona. Mars also has the largest canyon in the solar system, Valles Marineris (Figure 25.17). This canyon is 4,000 km (2,500 miles) long. Thats as long as Europe is wide! One-fifth of the circumference of Mars is covered by the canyon. Valles Marineris is 7 km (4.3 miles) deep. How about Earths Grand Canyon? Earths most famous canyon is only 446 km (277 miles) long and about 2 km (1.2 miles) deep. Mars has mountains, canyons, and other features similar to Earth. But it doesnt have as much geological activity as Earth. There is no evidence of plate tectonics on Mars. There are also more craters on Mars than on Earth. Buy there are fewer craters than on the Moon. What does this suggest to you regarding Mars plate tectonic history? "
inner planets,T_0579,"Water on Mars cant be a liquid. This is because the pressure of the atmosphere is too low. The planet does have a lot of water; it is in the form of ice. The south pole of Mars has a very visible ice cap. Scientists also have evidence that there is also a lot of ice just under the Martian surface. The ice melts when volcanoes erupt. At this times liquid water flows across the surface. Scientists think that there was once liquid water on the planet. There are many surface features that look like water- eroded canyons. The Mars rover collected round clumps of crystals that, on Earth, usually form in water. If there was liquid water on Mars, life might have existed there in the past. "
inner planets,T_0580,"Mars has two very small, irregular moons, Phobos (seen in Figure 25.18) and Deimos. These moons were discovered in 1877. They are named after the two sons of Ares, who followed their father into war. The moons were probably asteroids that were captured by Martian gravity. "
outer planets,T_0581,"Jupiter, shown in Figure 25.19, is the largest planet in our solar system. Jupiter is named for the king of the gods in Roman mythology. Jupiter is truly a giant! The planet has 318 times the mass of Earth, and over 1,300 times Earths volume. So Jupiter is much less dense than Earth. Because Jupiter is so large, it reflects a lot of sunlight. When it is visible, it is the brightest object in the night sky besides the Moon and Venus. Jupiter is quite far from the Earth. The planet is more than five times as far from Earth as the Sun. It takes Jupiter about 12 Earth years to orbit once around the Sun. "
outer planets,T_0582,"Since Jupiter is a gas giant, could a spacecraft land on its surface? The answer is no. There is no solid surface at all! Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements. The outer layers of the planet are gas. Deeper within the planet, the intense pressure condenses the gases into a liquid. Jupiter may have a small rocky core at its center. "
outer planets,T_0583,"Jupiters atmosphere is unlike any other in the solar system! The upper layer contains clouds of ammonia. The ammonia is different colored bands. These bands rotate around the planet. The ammonia also swirls around in tremendous storms. The Great Red Spot, shown in Figure 25.20, is Jupiters most noticeable feature. The spot is an enormous, oval-shaped storm. It is more than three times as wide as the entire Earth! Clouds in the storm rotate counterclockwise. They make one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least 300 years. It may have been observed as early as 1664. It is possible that this storm is a permanent feature on Jupiter. No one knows for sure. "
outer planets,T_0584,"Jupiter has lots of moons. As of 2011, we have discovered over 60 natural satellites of Jupiter. Four are big enough and bright enough to be seen from Earth using a pair of binoculars. These four moons were first discovered by Galileo in 1610. They are called the Galilean moons. Figure 25.21 shows the four Galilean moons and their sizes relative to Jupiters Great Red Spot. These moons are named Io, Europa, Ganymede, and Callisto. The Galilean moons are larger than even the biggest dwarf planets, Pluto and Eris. Ganymede is the biggest moon in the solar system. It is even larger than the planet Mercury! Scientists think that Europa is a good place to look for extraterrestrial life. Europa is the smallest of the Galilean moons. The moons surface is a smooth layer of ice. Scientists think that the ice may sit on top of an ocean of liquid water. How could Europa have liquid water when it is so far from the Sun? Europa is heated by Jupiter. Jupiters tidal forces are so great that they stretch and squash its moon. This could produce enough heat for there to be liquid water. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted. In 1979, two spacecrafts, Voyager 1 and Voyager 2, visited Jupiter and its moons. Photos from the Voyager missions "
outer planets,T_0585,"Saturn, shown in Figure 25.22, is famous for its beautiful rings. Saturn is the second largest planet in the solar system. Saturns mass is about 95 times Earths mass. The gas giant is 755 times Earths volume. Despite its large size, Saturn is the least dense planet in our solar system. Saturn is actually less dense than water. This means that if there were a bathtub big enough, Saturn would float! In Roman mythology, Saturn was the father of Jupiter. Saturn orbits the Sun once about every 30 Earth years. Saturns composition is similar to Jupiters. The planet is made mostly of hydrogen and helium. These elements are gases in the outer layers and liquids in the deeper layers. Saturn may also have a small solid core. Saturns upper atmosphere has clouds in bands of different colors. These clouds rotate rapidly around the planet. But Saturn has fewer storms than Jupiter. Thunder and lightning have been seen in the storms on Saturn (Figure 25.23). "
outer planets,T_0586,"There is a strange feature at Saturns north pole. The clouds form a hexagonal pattern, as shown in the infrared image in Figure 25.24. This hexagon was viewed by Voyager 1 in the 1980s. It was still there when the Cassini Orbiter visited in 2006. No one is sure why the clouds form this pattern. "
outer planets,T_0587,"Saturns rings were first observed by Galileo in 1610. He didnt know they were rings and thought that they were two large moons. One moon was on either side of the planet. In 1659, the Dutch astronomer Christiaan Huygens realized that they were rings circling Saturns equator. The rings appear tilted. This is because Saturn is tilted about 27 degrees to its side. The Voyager 1 spacecraft visited Saturn in 1980. Voyager 2 followed in 1981. These probes sent back detailed pictures of Saturn, its rings, and some of its moons. From the Voyager data, we learned that Saturns rings are made of particles of water and ice with a little bit of dust. There are several gaps in the rings. These gaps were cleared out by moons within the rings. Ring dust and gas are attracted to the moon by its gravity. This leaves a gap in the rings. Other gaps in the rings are caused by the competing forces of Saturn and its moons outside the rings. "
outer planets,T_0588,"As of 2011, over 60 moons have been identified around Saturn. Only seven of Saturns moons are round. All but one is smaller than Earths Moon. Some of the very small moons are found within the rings. All the particles in the rings are like little moons, because they orbit around Saturn. Someone must decide which ones are large enough to call moons. Saturns largest moon, Titan, is about one and a half times the size of Earths Moon. Titan is even larger than the planet Mercury. Figure 25.25 compares the size of Titan to Earth. Scientists are very interested in Titan. The moon has an atmosphere that is thought to be like Earths first atmosphere. This atmosphere was around before life developed on Earth. Like Jupiters moon, Europa, Titan may have a layer of liquid water under a layer of ice. Scientists now think that there are lakes on Titans surface. Dont take a dip, though. These lakes contain liquid methane and ethane instead of water! Methane and ethane are compounds found in natural gas. "
outer planets,T_0589,"Uranus, shown in Figure 25.26, is named for the Greek god of the sky, the father of Saturn. Astronomers pronounce the name YOOR-uh-nuhs. Uranus was not known to ancient observers. The planet was first discovered with a telescope by the astronomer William Herschel in 1781. Uranus is faint because it is very far away. Its distance from the Sun is 2.8 billion kilometers (1.8 billion miles). A photon from the Sun takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the Sun once about every 84 Earth years. "
outer planets,T_0590,"Uranus is a lot like Jupiter and Saturn. The planet is composed mainly of hydrogen and helium. There is a thick layer of gas on the outside. Further on the inside is liquid. But Uranus has a higher percentage of icy materials than Jupiter and Saturn. These materials include water, ammonia, and methane. Uranus is also different because of its blue-green color. Clouds of methane filter out red light. This leaves a blue-green color. The atmosphere of Uranus has bands of clouds. These clouds are hard to see in normal light. The result is that the planet looks like a plain blue ball. Uranus is the least massive outer planet. Its mass is only about 14 times the mass of Earth. Like all of the outer planets, Uranus is much less dense than Earth. Gravity is actually weaker than on Earths surface. If you were at the top of the clouds on Uranus, you would weigh about 10 percent less than what you weigh on Earth. "
outer planets,T_0591,All of the planets rotate on their axes in the same direction that they move around the Sun. Except for Uranus. Uranus is tilted on its side. Its axis is almost parallel to its orbit. So Uranus rolls along like a bowling ball as it revolves around the Sun. How did Uranus get this way? Scientists think that the planet was struck and knocked over by another planet-sized object. This collision probably took place billions of years ago. 
outer planets,T_0592,"Uranus has a faint system of rings, as shown in Figure 25.27. The rings circle the planets equator. However, Uranus is tilted on its side. So the rings are almost perpendicular to the planets orbit. We have discovered 27 moons around Uranus. All but a few are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus, Miranda, Ariel, Umbriel, Titania, and Oberon, are shown in Figure "
outer planets,T_0593,"Neptune is shown in Figure 25.29. It is the eighth planet from the Sun. Neptune is so far away you need a telescope to see it from Earth. Neptune is the most distant planet in our solar system. It is nearly 4.5 billion kilometers (2.8 billion miles) from the Sun. One orbit around the Sun takes Neptune 165 Earth years. Scientists guessed Neptunes existence before it was discovered. Uranus did not always appear exactly where it should. They said this was because a planet beyond Uranus was pulling on it. This gravitational pull was affecting its orbit. Neptune was discovered in 1846. It was just where scientists predicted it would be! Due to its blue color, the planet was named Neptune for the Roman god of the sea. Uranus and Neptune are often considered sister planets. They are very similar to each other. Neptune has slightly more mass than Uranus, but it is slightly smaller in size. "
outer planets,T_0594,"Like Uranus, Neptune is blue. The blue color is caused by gases in its atmosphere, including methane. Neptune is not a smooth looking ball like Uranus. The planet has a few darker and lighter spots. When Voyager 2 visited Neptune in 1986, there was a large dark-blue spot south of the equator. This spot was called the Great Dark Spot. When the Hubble Space Telescope photographed Neptune in 1994, the Great Dark Spot had disappeared. Another dark spot had appeared north of the equator. Astronomers believe that both of these spots represent gaps in the methane clouds on Neptune. Neptunes appearance changes due to its turbulent atmosphere. Winds are stronger than on any other planet in the solar system. Wind speeds can reach 1,100 km/h (700 mph). This is close to the speed of sound! The rapid winds surprised astronomers. This is because Neptune receives little energy from the Sun to power weather systems. It is not surprising that Neptune is one of the coldest places in the solar system. Temperatures at the top of the clouds are about 218C (360F). "
outer planets,T_0595,"Like the other outer planets, Neptune has rings of ice and dust. These rings are much thinner and fainter than Saturns. Neptunes rings may be unstable. They may change or disappear in a relatively short time. Neptune has 13 known moons. Only Triton, shown in Figure 25.30, has enough mass to be round. Triton orbits in the direction opposite to Neptunes orbit. Scientists think Triton did not form around Neptune. The satellite was captured by Neptunes gravity as it passed by. "
outer planets,T_0596,"Pluto was once considered one of the outer planets, but when the definition of a planet was changed in 2006, Pluto became one of the dwarf planets. It is one of the largest and brightest objects that make up this group. Look for Pluto in the next lesson, in the discussion of dwarf planets. "
other objects in the solar system,T_0597,"Asteroids are very small, irregularly shaped, rocky bodies. Asteroids orbit the Sun, but they are more like giant rocks than planets. Since they are small, they do not have enough gravity to become round. They are too small to have an atmosphere. With no internal heat, they are not geologically active. An asteroid can only change due to a collision. A collision may cause the asteroid to break up. It may create craters on the asteroids surface. An asteroid may strike a planet if it comes near enough to be pulled in by its gravity. Figure 25.31 shows a typical asteroid. "
other objects in the solar system,T_0598,"Hundreds of thousands of asteroids have been found in our solar system. They are still being discovered at a rate of about 5,000 new asteroids per month! The majority are located in between the orbits of Mars and Jupiter. This region is called the asteroid belt, as shown in Figure 25.32. There are many thousands of asteroids in the asteroid belt. Still, their total mass adds up to only about 4 percent of Earths Moon. Asteroids formed at the same time as the rest of the solar system. Although there are many in the asteroid belt, they were never were able to form into a planet. Jupiters gravity kept them apart. "
other objects in the solar system,T_0599,"Near-Earth asteroids have orbits that cross Earths orbit. This means that they can collide with Earth. There are over 4,500 known near-Earth asteroids. Small asteroids do sometimes collide with Earth. An asteroid about 510 m in diameter hits about once per year. Five hundred to a thousand of the known near-Earth asteroids are much bigger. They are over 1 kilometer in diameter. When large asteroids hit Earth in the past, many organisms died. At times, many species became extinct. Astronomers keep looking for near-Earth asteroids. They hope to predict a possible collision early so they can to try to stop it. "
other objects in the solar system,T_0600,"Scientists are very interested in asteroids. Most are composed of material that has not changed since early in the solar system. Scientists can learn a lot from them about how the solar system formed. Asteroids may be important for space travel. They could be mined for rare minerals or for construction projects in space. Scientists have sent spacecraft to study asteroids. In 1997, the NEAR Shoemaker probe orbited the asteroid 433 Eros. The craft finally landed on its surface in 2001. The Japanese Hayabusa probe returned to Earth with samples of a small near-earth asteroid in 2010. The U.S. Dawn mission will visit Vesta in 2011 and Ceres in 2015. "
other objects in the solar system,T_0601,"If you look at the sky on a dark night, you may see a meteor, like in Figure 25.33. A meteor forms a streak of light across the sky. People call them shooting stars because thats what they look like. But meteors are not stars at all. The light you see comes from a small piece of matter burning up as it flies through Earths atmosphere. "
other objects in the solar system,T_0602,"Before these small pieces of matter enter Earths atmosphere, they are called meteoroids. Meteoroids are as large as boulders or as small as tiny sand grains. Larger objects are called asteroids; smaller objects are interplanetary dust. Meteoroids sometimes cluster together in long trails. They are the debris left behind by comets. When Earth passes through a comet trail, there is a meteor shower. During a meteor shower, there are many more meteors than normal for a night or two. "
other objects in the solar system,T_0603,"A meteoroid is dragged towards Earth by gravity and enters the atmosphere. Friction with the atmosphere heats the object quickly, so it starts to vaporize. As it flies through the atmosphere, it leaves a trail of glowing gases. The object is now a meteor. Most meteors vaporize in the atmosphere. They never reach Earths surface. Large meteoroids may not burn up entirely in the atmosphere. A small core may remain and hit the Earths surface. This is called a meteorite. Meteorites provide clues about our solar system. Many were formed in the early solar system (Figure 25.34). Some are from asteroids that have split apart. A few are rocks from nearby bodies like Mars. For this to happen, an asteroid smashed into Mars and sent up debris. A bit of the debris entered Earths atmosphere as a meteor. "
other objects in the solar system,T_0604,"Comets are small, icy objects that orbit the Sun. Comets have highly elliptical orbits. Their orbits carry them from close to the Sun to the solar systems outer edges. When a comet gets close to the Sun, its outer layers of ice melt and evaporate. The vaporized gas and dust forms an atmosphere around the comet. This atmosphere is called a coma. Radiation and particles streaming from the Sun push some of this gas and dust into a long tail. A comets tail always points away from the Sun, no matter which way the comet is moving. Why do you think that is? Figure Gases in the coma and tail of a comet reflect light from the Sun. Comets are very hard to see except when they have comas and tails. That is why they appear only when they are near the Sun. They disappear again as they move back to the outer solar system. The time between one visit from a comet and the next is called the comets period. The first comet whose period was known was Halleys Comet. Its period is 75 years. Halleys Comet last traveled through the inner solar system in 1986. The comet will appear again in 2061. Who will look up at it? "
other objects in the solar system,T_0605,"Some comets have periods of 200 years or less. They are called short-period comets. Short-period comets are from a region beyond the orbit of Neptune called the Kuiper Belt. Kuiper is pronounced KI-per, rhyming with viper. The Kuiper Belt is home to comets, asteroids, and at least two dwarf planets. Some comets have periods of thousands or even millions of years. Most long-period comets come from a very distant region of the solar system. This region is called the Oort cloud. The Oort cloud is about 50,000100,000 times the distance from the Sun to Earth. Comets carry materials in from the outer solar system. Comets may have brought water into the early Earth. Other substances could also have come from comets. "
other objects in the solar system,T_0606,"For several decades, Pluto was a planet. But new solar system objects were discovered that were just as planet-like as Pluto. Astronomers figured out that they were like planets except for one thing. These objects had not cleared their orbits of smaller objects. They didnt have enough gravity to do so. Astronomers made a category called dwarf planets. There are five dwarf planets in our solar system: Ceres, Pluto, Makemake, Haumea and Eris. Figure 25.36 shows Ceres. Ceres is a rocky body that orbits the Sun and is not a star. It could be an asteroid or a planet. Before 2006, Ceres was thought to be the largest asteroid. Is it an asteroid? Ceres is in the asteroid belt. But it is by far the largest object in the belt. Ceres has such high gravity that it is spherical. Is it a planet? Ceres only has about 1.3% of the mass of the Earths Moon. Its orbit is full of other smaller bodies. Its gravity was not high enough to clear its orbit. Ceres fails the fourth criterion for being a planet. Ceres is now considered a dwarf planet along with Pluto. "
other objects in the solar system,T_0607,"For decades Pluto was a planet. But even then, scientists knew it was an unusual planet. The other outer planets are all gas giants. Pluto is small, icy and rocky. With a diameter of about 2400 kilometers, it has only about 1/5 the mass of Earths Moon. The other planets orbit in a plane. Plutos orbit is tilted. The shape of the orbit is like a long, narrow ellipse. Plutos orbit is so elliptical that sometimes it is inside the orbit of Neptune. Plutos orbit is in the Kuiper belt. We have discovered more than 200 million Kuiper belt objects. Pluto has 3 moons of its own. The largest, Charon, is big. Some scientists think that Pluto-Charon system is a double dwarf planet (Figure 25.37). Two smaller moons, Nix and Hydra, were discovered in 2005. "
other objects in the solar system,T_0608,"Haumea was named a dwarf planet in 2008. It is an unusual dwarf planet. The body is shaped like an oval! Haumeas longest axis is about the same as Plutos diameter, and its shortest axis is about half as long. The bodys orbit is tilted 28. Haumea is so far from the Sun that it takes 283 years to make one orbit (Figure 25.38). Haumea is the third-brightest Kuiper Belt object. It was named for the Hawaiian goddess of childbirth. Haumea has two moons, Hiiaka and Namaka, the names of the goddess Haumeas daughters. Haumeas odd oval shape is probably caused by its extremely rapid rotation. It rotates in just less than 4 hours! Like other Kuiper belt objects, Haumea is covered by ice. Its density is similar to Earths Moon, at 2.6 3.3 g/cm3 . This means that most of Haumea is rocky. Haumea is part of a collisional family. This is a group of astronomical objects that formed from an impact. This family has Haumea, its two moons, and five more objects. All of these objects are thought to have formed from a collision very early in the formation of the solar system. "
other objects in the solar system,T_0609,"Makemake is the third-largest and second-brightest dwarf planet we have discovered so far (Figure 25.39). Make- make is only 75 percent the size of Pluto. Its diameter is between 1300 and 1900 kilometers. The name comes from the mythology of the Eastern Islanders. Makemake was the god that created humanity. At a distance between 38.5 to 53 AU, this dwarf planet orbits the Sun in 310 years. Makemake is made of methane, ethane, and nitrogen ices. "
other objects in the solar system,T_0610,"Eris is the largest known dwarf planet in the solar system. It is 27 percent larger than Pluto (Figure 25.40). Like Pluto and Makemake, Eris is in the Kuiper belt. But Eris is about 3 times farther from the Sun than Pluto. Because of its distance, Eris was not discovered until 2005. Early on, it was thought that Eris might be the tenth planet. Its discovery helped astronomers realize that they needed a new definition of planet. Eris has a small moon, Dysnomia. Its moon orbits Eris once about every 16 days. Astronomers know there may be other dwarf planets far out in the solar system. Look for Quaoar, Varuna and Orcus to be possibly added to the list of dwarf planets in the future. We still have a lot to discover and explore! "
stars,T_0611,"The stars that make up a constellation appear close to each other from Earth. In reality, they may be very distant from one another. Constellations were important to people, like the Ancient Greeks. People who spent a lot of time outdoors at night, like shepherds, named them and told stories about them. Figure 26.1 shows one of the most easily recognized constellations. The ancient Greeks thought this group of stars looked like a hunter. They named it Orion, after a great hunter in Greek mythology. The constellations stay the same night after night. The patterns of the stars never change. However, each night the constellations move across the sky. They move because Earth is spinning on its axis. The constellations also move with the seasons. This is because Earth revolves around the Sun. Different constellations are up in the winter than in the summer. For example, Orion is high up in the winter sky. In the summer, its only up in the early morning. "
stars,T_0612,"Only a tiny bit of the Suns light reaches Earth. But that light supplies most of the energy at the surface. The Sun is just an ordinary star, but it appears much bigger and brighter than any of the other stars. Of course, this is just because it is very close. Some other stars produce much more energy than the Sun. How do stars generate so much energy? "
stars,T_0613,"Stars shine because of nuclear fusion. Fusion reactions in the Suns core keep our nearest star burning. Stars are made mostly of hydrogen and helium. Both are very lightweight gases. A star contains so much hydrogen and helium that the weight of these gases is enormous. The pressure at the center of a star is great enough to heat the gases. This causes nuclear fusion reactions. A nuclear fusion reaction is named that because the nuclei (center) of two atoms fuse (join) together. In stars like our Sun, two hydrogen atoms join together to create a helium atom. Nuclear fusion reactions need a lot of energy to get started. Once they begin, they produce even more energy. "
stars,T_0614,"Scientists have built machines called particle accelerators. These amazing tools smash particles that are smaller than atoms into each other head-on. This creates new particles. Scientists use particle accelerators to learn about nuclear fusion in stars. They can also learn about how atoms came together in the early universe. Two well-known accelerators are SLAC, in California, and CERN, in Switzerland. "
stars,T_0615,Stars shine in many different colors. The color relates to a stars temperature and often its size. 
stars,T_0616,"Think about the coil of an electric stove as it heats up. The coil changes in color as its temperature rises. When you first turn on the heat, the coil looks black. The air a few inches above the coil begins to feel warm. As the coil gets hotter, it starts to glow a dull red. As it gets even hotter, it becomes a brighter red. Next it turns orange. If it gets extremely hot, it might look yellow-white, or even blue-white. Like a coil on a stove, a stars color is determined by the temperature of the stars surface. Relatively cool stars are red. Warmer stars are orange or yellow. Extremely hot stars are blue or blue-white. "
stars,T_0617,"The most common way of classifying stars is by color as shown, in Table 26.1. Each class of star is given a letter, a color, and a range of temperatures. The letters dont match the color names because stars were first grouped as A through O. It wasnt until later that their order was corrected to go by increasing temperature. When you try to remember the order, you can use this phrase: Oh Be A Fine Good Kid, Man. Class O Color Blue Temperature range 30,000 K or more Sample Star An artists depiction of the O class star Zeta Pup- pis. B Blue-white 10,00030,000 K An artists depiction of Rigel, a Class B star. Class A Color White Temperature range 7,50010,000 K Sample Star Sirius A is the brightest star that we see in the night sky. The dot on the right, Sirius B, is a white dwarf. F Yellowish-white 6,0007,500 K There are two F class stars in this image, the super- giant Polaris A and Po- laris B. What we see in the night sky as the single star Polaris, we also known as the North Star. G Yellow 5,5006,000 K Our Sun: the most im- portant G class star in the Universe, at least for hu- mans. Class K M Color Orange Red Temperature range 3,5005,000 K 2,0003,500 K Sample Star Arcturus is a Class K star that looks like the Sun but is much larger. There are two types of Class M stars: red dwarfs and red giants. An artists concept of a red dwarf star. Most stars are red dwarfs. The red supergiant Betel- geuse is seen near Orions belt. The blue star in the lower right is the Class B star Rigel. The surface temperature of most stars is due to its size. Bigger stars produce more energy, so their surfaces are hotter. But some very small stars are very hot. Some very big stars are cool. "
stars,T_0618,"We could say that stars are born, change over time, and eventually die. Most stars change in size, color, and class at least once during their lifetime. "
stars,T_0619,"Stars are born in clouds of gas and dust called nebulas. Our Sun and solar system formed out of a nebula. A nebula is shown in Figure 26.2. In Figure 26.1, the fuzzy area beneath the central three stars contains the Orion nebula. For a star to form, gravity pulls gas and dust into the center of the nebula. As the material becomes denser, the pressure and the temperature increase. When the temperature of the center becomes hot enough, nuclear fusion begins. The ball of gas has become a star! "
stars,T_0620,"For most of a stars life, hydrogen atoms fuse to form helium atoms. A star like this is a main sequence star. The hotter a main sequence star is, the brighter it is. A star remains on the main sequence as long as it is fusing hydrogen to form helium. Our Sun has been a main sequence star for about 5 billion years. As a medium-sized star, it will continue to shine for about 5 billion more years. Large stars burn through their supply of hydrogen very quickly. These stars live fast and die young! A very large star may only be on the main sequence for 10 million years. A very small star may be on the main sequence for tens to hundreds of billions of years. "
stars,T_0621,"A star like our Sun will become a red giant in its next stage. When a star uses up its hydrogen, it begins to fuse helium atoms. Helium fuses into heavier atoms like carbon. At this time the stars core starts to collapse inward. The stars outer layers spread out and cool. The result is a larger star that is cooler on the surface, and red in color. Eventually a red giant burns up all of the helium in its core. What happens next depends on the stars mass. A star like the Sun stops fusion and shrinks into a white dwarf star. A white dwarf is a hot, white, glowing object about the size of Earth. Eventually, a white dwarf cools down and its light fades out. "
stars,T_0622,"A more massive star ends its life in a more dramatic way. Very massive stars become red supergiants, like Betelgeuse. In a red supergiant, fusion does not stop. Lighter atoms fuse into heavier atoms. Eventually iron atoms form. When there is nothing left to fuse, the stars iron core explodes violently. This is called a supernova explosion. The incredible energy released fuses heavy atoms together. Gold, silver, uranium and the other heavy elements can only form in a supernova explosion. A supernova can shine as brightly as an entire galaxy, but only for a short time, as shown in Figure 26.3. "
stars,T_0623,"After a supernova explosion, the stars core is left over. This material is extremely dense. If the core is less than about four times the mass of the Sun, the star will become a neutron star. A neutron star is shown in Figure 26.4. This type of star is made almost entirely of neutrons. A neutron star has more mass than the Sun, yet it is only a few kilometers in diameter. If the core remaining after a supernova is more than about 5 times the mass of the Sun, the core collapses to become a black hole. Black holes are so dense that not even light can escape their gravity. For that reason, we cant see black holes. How can we know something exists if radiation cant escape it? We know a black hole is there by the effect that it has on objects around it. Also, some radiation leaks out around its edges. A black hole isnt a hole at all. It is the tremendously dense core of a supermassive star. "
stars,T_0624,"Astronomers use light years as the unit to describe distances in space. Remember that a light year is the distance light travels in one year. How do astronomers measure the distance to stars? For stars that are close to us, they measure shifts in their position over time. This is called parallax. For distant stars, they use the stars brightness. For example, if a star is like the Sun, it should be about as bright as the Sun. They then figure out the stars distance from Earth by measuring how much less bright it is than expected. "
stars,T_0625,"Our solar system has only one star. But many stars are in systems of two or more stars. Two stars that orbit each other are called a binary star system. If more than two stars orbit each other, it is called a multiple star system. Figure 26.5 shows two binary star systems orbiting each other. This creates an unusual quadruple star system. "
galaxies,T_0626,"Star clusters are groups of stars smaller than a galaxy. There are two main types, open clusters and globular clusters. Open clusters are groups of up to a few thousand stars held together by gravity. The Jewel Box, shown in Figure an open cluster are young stars that all formed from the same nebula. Globular clusters are groups of tens to hundreds of thousands of stars held tightly together by gravity. Globular clusters have a definite, spherical shape. They contain mostly old, reddish stars. Near the center of a globular cluster, the stars are closer together. Figure 26.7 shows a globular cluster. The heart of the globular cluster M13 has hundreds of thousands of stars. M13 is 145 light years in diameter. The cluster contains red and blue giant stars. "
galaxies,T_0627,"The biggest groups of stars are called galaxies. A few million to many billions of stars may make up a galaxy. With the unaided eye, every star you can see is part of the Milky Way Galaxy. All the other galaxies are extremely far away. The closest spiral galaxy, the Andromeda Galaxy, shown in Figure 26.8, is 2,500,000 light years away and contains one trillion stars! "
galaxies,T_0628,"Galaxies are divided into three types, according to shape. There are spiral galaxies, elliptical galaxies, and irregular galaxies. Spiral galaxies are a rotating disk of stars and dust. In the center is a dense bulge of material. Several arms spiral out from the center. Spiral galaxies have lots of gas and dust and many young stars. Figure 26.9 shows a spiral galaxy from the side. You can see the disk and central bulge. "
galaxies,T_0629,Figure 26.10 shows a typical elliptical galaxy. Elliptical galaxies are oval in shape. The smallest are called dwarf elliptical galaxies. Look back at the image of the Andromeda Galaxy. It has two dwarf elliptical galaxies as its companions. Dwarf galaxies are often found near larger galaxies. They sometimes collide with and merge into their larger neighbors. Giant elliptical galaxies contain over a trillion stars. Elliptical galaxies are red to yellow in color because they contain mostly old stars. Most contain very little gas and dust because the material has already formed into stars. 
galaxies,T_0630,"Look at the galaxy in Figure 26.11. Do you think this is a spiral galaxy or an elliptical galaxy? It doesnt look like either! If a galaxy is not spiral or elliptical, it is an irregular galaxy. Most irregular galaxies have been deformed. This can occur either by the pull of a larger galaxy or by a collision with another galaxy. "
galaxies,T_0631,"If you get away from city lights and look up in the sky on a very clear night, you will see something spectacular. A band of milky light stretches across the sky, as in Figure 26.12. This band is the disk of the Milky Way Galaxy. This is the galaxy where we all live. The Milky Way Galaxy looks different to us than other galaxies because our view is from inside of it! "
galaxies,T_0632,"The Milky Way Galaxy is a spiral galaxy that contains about 400 billion stars. Like other spiral galaxies, it has a disk, a central bulge, and spiral arms. The disk is about 100,000 light-years across. It is about 3,000 light years thick. Most of the galaxys gas, dust, young stars, and open clusters are in the disk. Some astronomers think that there is a gigantic black hole at the center of the galaxy. Figure 26.13 shows what the Milky Way probably looks like from the outside. Our solar system is within one of the spiral arms. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. We are a little more than halfway out from the center of the Galaxy to the edge, as shown in Figure 26.13. Our solar system orbits the center of the galaxy as the galaxy spins. One orbit of the solar system takes about 225 to 250 million years. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. "
types of rocks,T_0685,"All rocks on Earth change, but these changes usually happen very slowly. Some changes happen below Earths surface. Some changes happen above ground. These changes are all part of the rock cycle. The rock cycle describes each of the main types of rocks, how they form and how they change. Figure 4.1 shows how the three main rock types are related to each other. The arrows within the circle show how one type of rock may change to rock of another type. For example, igneous rock may break down into small pieces of sediment and become sedimentary rock. Igneous rock may be buried within the Earth and become metamorphic rock. Igneous rock may also change back to molten material and re-cool into a new igneous rock. Rocks are made of minerals. The minerals may be so tiny that you can only see them with a microscope. The minerals may be really large. A rock may be made of only one type of mineral. More often rocks are made of a mixture of different minerals. Rocks are named for the combinations of minerals they are made of and the ways those minerals came together. Remember that different minerals form under different environmental conditions. So the minerals in a rock contain clues about the conditions in which the rock formed (Figure 4.2). "
types of rocks,T_0686,"Geologists group rocks based on how they were formed. The three main kinds of rocks are: 1. Igneous rocks form when magma cools below Earths surface or lava cools at the surface (Figure 4.3). 2. Sedimentary rocks form when sediments are compacted and cemented together (Figure 4.4). These sediments may be gravel, sand, silt or clay. Sedimentary rocks often have pieces of other rocks in them. Some sedimentary rocks form the solid minerals left behind after a liquid evaporates. 3. Metamorphic rocks form when an existing rock is changed by heat or pressure. The minerals in the rock change but do not melt (Figure 4.5). The rock experiences these changes within the Earth. Rocks can be changed from one type to another, and the rock cycle describes how this happens. "
types of rocks,T_0687,"Any type of rock can change and become a new type of rock. Magma can cool and crystallize. Existing rocks can be weathered and eroded to form sediments. Rock can change by heat or pressure deep in Earths crust. There are three main processes that can change rock: Cooling and forming crystals. Deep within the Earth, temperatures can get hot enough to melt rock. This molten material is called magma. As it cools, crystals grow, forming an igneous rock. The crystals will grow larger if the magma cools slowly, as it does if it remains deep within the Earth. If the magma cools quickly, the crystals will be very small. Weathering and erosion. Water, wind, ice, and even plants and animals all act to wear down rocks. Over time they can break larger rocks into smaller pieces called sediments. Moving water, wind, and glaciers then carry these pieces from one place to another. The sediments are eventually dropped, or deposited, somewhere. The sediments may then be compacted and cemented together. This forms a sedimentary rock. This whole process can take hundreds or thousands of years. Metamorphism. This long word means to change form. A rock undergoes metamorphism if it is exposed to extreme heat and pressure within the crust. With metamorphism, the rock does not melt all the way. The rock changes due to heat and pressure. A metamorphic rock may have a new mineral composition and/or texture. An interactive rock cycle diagram can be found here:  The rock cycle really has no beginning or end. It just continues. The processes involved in the rock cycle take place over hundreds, thousands, or even millions of years. Even though for us rocks are solid and unchanging, they slowly change all the time. "
igneous rocks,T_0688,"Igneous rocks form when magma cools and forms crystals. These rocks can form at Earths surface or deep underground. Figure 4.7 shows a landscape in Californias Sierra Nevada that consists entirely of granite. Intrusive igneous rocks cool and form into crystals beneath the surface. Deep in the Earth, magma cools slowly. Slow cooling gives large crystals a chance to form. Intrusive igneous rocks have relatively large crystals that are easy to see. Granite is the most common intrusive igneous rock. Figure 4.8 shows four types of intrusive rocks. Extrusive igneous rocks form above the surface. The lava cools quickly as it pours out onto the surface (Figure "
igneous rocks,T_0689,"Igneous rocks are grouped by the size of their crystals and the minerals they contain. The minerals in igneous rocks are grouped into families. Some contain mostly lighter colored minerals, some have a combination of light and dark minerals, and some have mostly darker minerals. The combination of minerals is determined by the composition of the magma. Magmas that produce lighter colored minerals are higher in silica. These create rocks such as granite and rhyolite. Darker colored minerals are found in rocks such as gabbro and basalt. There are actually more than 700 different types of igneous rocks. Diorite is extremely hard and is commonly used for art. It was used extensively by ancient civilizations for vases and other decorative art work (Figure 4.11). "
sedimentary rocks,T_0690,"Most sedimentary rocks form from sediments. Sediments are small pieces of other rocks, like pebbles, sand, silt, and clay. Sedimentary rocks may include fossils. Fossils are materials left behind by once-living organisms. Fossils can be pieces of the organism, like bones. They can also be traces of the organism, like footprints. Most often, sediments settle out of water (Figure 4.13). For example, rivers carry lots of sediment. Where the water slows, it dumps these sediments along its banks, into lakes and the ocean. When sediments settle out of water, they form horizontal layers. A layer of sediment is deposited. Then the next layer is deposited on top of that layer. So each layer in a sedimentary rock is younger than the layer under it. It is older than the layer over it. Sediments are deposited in many different types of environments. Beaches and deserts collect large deposits of sand. Sediments also continuously wind up at the bottom of the ocean and in lakes, ponds, rivers, marshes, and swamps. Avalanches produce large piles of sediment. The environment where the sediments are deposited determines the type of sedimentary rock that can form. "
sedimentary rocks,T_0691,Sedimentary rocks form in two ways. Particles may be cemented together. Chemicals may precipitate. 
sedimentary rocks,T_0692,"Over time, deposited sediments may harden into rock. First, the sediments are compacted. That is, they are squeezed together by the weight of sediments on top of them. Next, the sediments are cemented together. Minerals fill in the spaces between the loose sediment particles. These cementing minerals come from the water that moves through the sediments. These types of sedimentary rocks are called clastic rocks. Clastic rocks are rock fragments that are compacted and cemented together. Clastic sedimentary rocks are grouped by the size of the sediment they contain. Conglomerate and breccia are made of individual stones that have been cemented together. In conglomerate, the stones are rounded. In breccia, the stones are angular. Sandstone is made of sand-sized particles. Siltstone is made of smaller particles. Silt is smaller than sand but larger than clay. Shale has the smallest grain size. Shale is made mostly of clay-sized particles and hardened mud. "
sedimentary rocks,T_0693,"Chemical sedimentary rocks form when crystals precipitate out from a liquid. The mineral halite, also called rock salt, forms this way. You can make halite! Leave a shallow dish of salt water out in the Sun. As the water evaporates, salt crystals form in the dish. There are other chemical sedimentary rocks, like gypsum. Table 4.1 shows some common types of sedimentary rocks and the types of sediments that make them up. Picture Rock Name Conglomerate Type of Sedimentary Rock Clastic Breccia Clastic Sandstone Clastic Siltstone Clastic Limestone Bioclastic Coal Organic Picture Rock Name Rock Salt Type of Sedimentary Rock Chemical precipitate "
metamorphic rocks,T_0694,"Metamorphic rocks start off as some kind of rock. The starting rock can be igneous, sedimentary or even another metamorphic rock. Heat and/or pressure then change the rocks physical or chemical makeup. During metamorphism a rock may change chemically. Ions move and new minerals form. The new minerals are more stable in the new environment. Extreme pressure may lead to physical changes like foliation. Foliation forms as the rocks are squeezed. If pressure is exerted from one direction, the rock forms layers. This is foliation. If pressure is exerted from all directions, the rock usually does not show foliation. There are two main types of metamorphism: 1. Contact metamorphism results when magma contacts a rock, changing it by extreme heat (Figure 4.14). 2. Regional metamorphism occurs over a wide area. Great masses of rock are exposed to pressure from rock and sediment layers on top of it. The rock may also be compressed by other geological processes. Metamorphism does not cause a rock to melt completely. It only causes the minerals to change by heat or pressure. Hornfels is a rock with alternating bands of dark and light crystals. Hornfels is a good example of how minerals rearrange themselves during metamorphism (Figure 4.14). The minerals in hornfels separate by density. The result is that the rock becomes banded. Gneiss forms by regional metamorphism from extremely high temperature and pressure. "
metamorphic rocks,T_0695,Quartzite and marble are the most commonly used metamorphic rocks. They are frequently chosen for building materials and artwork. Marble is used for statues and decorative items like vases (Figure 4.16). Quartzite is very hard and is often crushed and used in building railroad tracks. Schist and slate are sometimes used as building and landscape materials. 
earths energy,T_0696,"Almost all energy comes from the Sun. Plants make food energy from sunlight. Fossil fuels are made of the remains of plants and animals that stored the Suns energy millions of years ago. The Sun heats some areas more than others, which causes wind. The Suns energy also drives the water cycle, which moves water over the surface of the Earth. Both wind and water power can be used as renewable resources. Earths internal heat does not depend on the Sun for energy. This heat comes from remnant heat when the planet formed. It also comes from the decay of radioactive elements. Radioactivity is an important source of energy. "
earths energy,T_0697,"Energy provides the ability to move or change matter from one state to another (for example, from solid to liquid). Every living thing needs energy to live and grow. Your body gets its energy from food, but that is only a small part of the energy you use every day. Cooking your food takes energy, and so does keeping it cold in the refrigerator or the freezer. The same is true for heating or cooling your home. Whether you are turning on a light in the kitchen or riding in a car to school, you are using energy. Billions of people all around the world use energy, so there is a huge demand for resources to provide all of this energy. Why do we need so much energy? The main reason is that almost everything that happens on Earth involves energy. "
earths energy,T_0698,Energy changes form when something happens. But the total amount of energy always stays the same. The Law of Conservation of Energy says that energy cannot be created or destroyed. Scientists observed that energy could change from one form to another. They also observed that the overall amount of energy did not change. 
earths energy,T_0699,"Here is an example of how energy changes form: kicking a soccer ball. Your body gets energy from food. Where does the food get its energy? If youre eating a plant, then the energy comes directly from the Sun. If youre eating an animal, then the energy comes from a plant that got its energy from the Sun. Your body breaks down the food. It converts the food to chemical energy and stores it. When you are about to kick the ball, the energy must be changed again. Potential energy has the potential to do work. When your leg is poised to kick the ball but is not yet moving, your leg has potential energy. A ball at the top of a hill has the potential energy of location. Kinetic energy is the energy of anything in motion. Your muscles move your leg, your foot kicks the ball, and the ball gains kinetic energy (Figure 5.1). The kinetic energy was converted from potential energy that was in your leg before the kick. The action of kicking the ball is energy changing forms. The same is true for anything that involves change. "
earths energy,T_0700,Energy is the ability to do work. Fuel stores energy and can be released to do work. Heat is given off when fuel is burned. 
earths energy,T_0701,"What makes energy available whenever you need it? If you unplug a lamp, the light goes off. The lamp does not have a supply of energy to keep itself lit. The lamp uses electricity that comes through the outlet as its source of energy. The electricity comes from a power plant. The power plant has a source of energy to produce this electricity. "
earths energy,T_0702,"The energy to make the electricity comes from fuel. Fuel stores the energy and releases it when it is needed. Fuel is any material that can release energy in a chemical change. The food you eat acts as a fuel for your body. Gasoline and diesel fuel are fuels that provide the energy for most cars, trucks, and buses. But there are many different kinds of fuel. For fuel to be useful, its energy must be released in a way that can be controlled. "
earths energy,T_0703,"When fuel is burned, most of the energy is released as heat. Some of this heat can be used to do work. Heat cooks food or warms your house. Sometimes the heat is just waste heat. It still heats the environment, though. Heat from a fire can boil a pot of water. If you put an egg in the pot, you can eat a hard boiled egg in 15 minutes (cool it down first!). The energy to cook the egg was stored in the wood. The wood got that energy from the Sun when it was part of a tree. The Sun generated the energy by nuclear fusion. You started the fire with a match. The head of the match stores energy as chemical energy. That energy lights the wood on fire. The fire burns as long as there is energy in the wood. Once the wood has burned up, there is no energy left in it. The fire goes out. "
earths energy,T_0704,"Energy resources can be put into two categories renewable or non-renewable. Nonrenewable resources are used faster than they can be replaced. Renewable resources can be replaced as quickly as they are used. Renewable resources may also be so abundant that running out is impossible. The difference between non-renewable and renewable resources is like the difference between ordinary batteries and rechargeable ones. If a flashlight when ordinary batteries goes dead, the batteries need to be replaced. But if the flashlight has rechargeable batteries, the batteries can be placed in a charger. The charger transfers energy from an outlet into the batteries. Once recharged, the batteries can be put back into the flashlight. Rechargeable batteries can be used again and again (Figure 5.2). In this way, the energy in the rechargeable batteries is renewable. "
earths energy,T_0705,"Fossil fuels include coal, oil, and natural gas. Fossil fuels are the greatest energy source for modern society. Millions of years ago, plants used energy from the Sun to form carbon compounds. These compounds were later transformed into coal, oil, or natural gas. Fossil fuels take millions of years to form. For this reason, they are non-renewable. We will use most fossil fuels up in a matter of decades. Burning fossil fuels releases large amounts of pollution. The most important of these may be the greenhouse gas carbon dioxide. "
earths energy,T_0706,"Renewable energy resources include solar, water, wind, biomass, and geothermal power. These resources are usually replaced at the same rate that we use them. Scientists know that the Sun will continue to shine for billions of years. So we can use the solar energy without it ever running out. Water flows from high places to lower ones. Wind blows from areas of high pressure to areas of low pressure. We can use the flow of wind and water to generate power. We can count on wind and water to continue to flow! Burning wood is an example of biomass energy. Changing grains into biofuels is biomass energy. Biomass is renewable because we can plant new trees or crops to replace the ones we use. Geothermal energy uses water that was heated by hot rocks. There are always more hot rocks available to heat more water. Even renewable resources can be used unsustainably. We can cut down too many trees without replanting. We might need grains for food rather than biofuels. Some renewable resources are too expensive to be widely used. As the technology improves and more people use renewable energy, the prices will come down. The cost of renewable resources will go down relative to fossil fuels as we use fossil fuels up. In the long run renewable resources will need to make up a large amount of what we use. "
earths energy,T_0707,"Before we put effort into increasing the use of an energy source, we should consider two things. Is there a practical way to turn the resource into useful form of energy? For example, it is not practical if we dont get much more energy from burning a fuel than we put into making it. What happens when we turn the resource into energy? What happens when we use that resource? Mining the resource may cause a lot of health problems or environmental damage. Using the resource may create a large amount of pollution. In this case, that fuel may also not be the best choice for an energy resource. "
earths energy,T_0708,"Today we rely on electricity more than ever, but the resources that currently supply our power are finite. The race is on to harness more renewable resources, but getting all that clean energy from production sites to homes and businesses is proving to be a major challenge. Learn more by watching the resource below:  MEDIA Click image to the left or use the URL below. URL: "
nonrenewable energy resources,T_0709,"Fossil fuels are made from plants and animals that lived hundreds of millions of years ago. The plants and animals died. Their remains settled onto the ground and at the bottom of the sea. Layer upon layer of organic material was laid down. Eventually, the layers were buried very deeply. They experienced intense heat and pressure. Over millions of years, the organic material turned into fossil fuels. Fossil fuels are compounds of carbon and hydrogen, called hydrocarbons. Hydrocarbons can be solid, liquid, or gas. The solid form is coal. The liquid form is petroleum, or crude oil. The gaseous form is natural gas. "
nonrenewable energy resources,T_0710,"Coal is a solid hydrocarbon. Coal is useful as a fuel, especially for generating electricity. "
nonrenewable energy resources,T_0711,"Coal forms from dead plants that settled at the bottom of swamps millions of years ago. Water and mud in the swamp kept oxygen away from the plant material. Sand and clay settled on top of the decaying plants. The weight of this material squeezed out the water and some other substances. Over time, the organic material became a carbon-rich rock. This rock is coal. "
nonrenewable energy resources,T_0712,"Coal is a black or brownish-black rock that burns easily (Figure 5.3). Most coal is sedimentary rock. The hardest type of coal, anthracite, is a metamorphic rock. That is because it is exposed to higher temperature and pressure as it forms. Coal is mostly carbon, but some other elements can be found in coal, including sulfur. "
nonrenewable energy resources,T_0713,"Around the world, coal is the largest source of energy for electricity. The United States is rich in coal. Pennsylvania and the region to the west of the Appalachian Mountains are some of the most coal-rich areas of the United States. Coal has to be mined to get it out of the ground. Coal mining affects the environment and human health. Coal mining can take place underground or at the surface. Each method has some advantages and disadvantages. Surface mining exposes minerals that were underground to air and water at the surface. These minerals contain the chemical element sulfur. Sulfur mixes with air and water to make sulfuric acid. This acid is a highly corrosive chemical. Sulfuric acid gets into nearby streams and can kill fish, plants, and animals. Surface mining is safer for the miners. Coal mining underground is dangerous for the coal miners. Miners are sometimes killed if there is an explosion or a mine collapse. Miners breathe in coal dust and can get terrible lung diseases after a number of years in the mines. "
nonrenewable energy resources,T_0714,"To prepare coal for use, the coal is first crushed into powder and burned in a furnace. Like other fuels, coal releases most of its energy as heat when it burns. The heat from the burning coal is used to boil water. This makes steam. The steam spins turbines, which creates electricity. "
nonrenewable energy resources,T_0715,"Oil is a thick, dark brown or black liquid. It is found in rock layers of the Earths crust. Oil is currently the most commonly used source of energy in the world. "
nonrenewable energy resources,T_0716,"The way oil forms is similar in many ways to coal. Tiny organisms like plankton and algae die and settle to the bottom of the sea. Sediments settle over the organic material. Oxygen is kept away by the sediments. When the material is buried deep enough, it is exposed to high heat and pressure. Over millions of years, the organic material transforms into liquid oil. "
nonrenewable energy resources,T_0717,"The United States produces only about one-quarter as much oil as it uses. The main oil producing regions in the U.S. are the Gulf of Mexico, Texas, Alaska, and California. Geologists look for oil in folded layers of rock called anticlines. Oil moves through permeable rock and is trapped by the impermeable cap rock. "
nonrenewable energy resources,T_0718,"Oil comes out of the ground as crude oil. Crude oil is a mixture of many different hydrocarbons. Oil is separated into different compounds at an oil refinery (Figure 5.4). This is done by heating the oil. Each hydrocarbon compound in crude oil boils at a different temperature. We get gasoline, diesel, and heating oil, plus waxes, plastics, and fertilizers from crude oil. These fuels are rich sources of energy. Since they are mostly liquids they can be easily transported. These fuels provide about 90% of the energy used for transportation around the world. "
nonrenewable energy resources,T_0719,"Gasoline is a concentrated resource. It contains a large amount of energy for its weight. This is important because the more something weighs, the more energy is needed to move it. If gasoline could only provide a little energy, a car would have to carry a lot of it to be able to travel very far. Or the car would need to be filled up frequently. So a highly concentrated energy resource is a practical fuel to power cars and other forms of transportation. Lets consider how gasoline powers a car. As gasoline burns, it releases most of its energy as heat. It also releases carbon dioxide gas and water vapor. The heat makes the gases expand. This forces the pistons inside the engine to move. The engine makes enough power to move the car. "
nonrenewable energy resources,T_0720,Using gasoline to power automobiles affects the environment. The exhaust fumes from burning gasoline cause air pollution. These pollutants include smog and ground-level ozone. Air pollution is a big problem for cities where large numbers of people drive every day. Burning gasoline also produces carbon dioxide. This is a greenhouse gas and is a cause of global warming. Similar pollutants come from other forms of oil. 
nonrenewable energy resources,T_0721,Natural gas is mostly methane. 
nonrenewable energy resources,T_0722,Natural gas is often found along with coal or oil in underground deposits. This is because natural gas forms with these other fossil fuels. One difference between natural gas and oil is that natural gas forms at higher temperatures. 
nonrenewable energy resources,T_0723,"The largest natural gas reserves in the United States are located in the Rocky Mountain states, Texas, and the Gulf of Mexico region. California also has natural gas, mostly in the northern Sacramento Valley and the Sacramento Delta. Natural gas must be processed before it can be used as a fuel. Poisonous chemicals and water must be removed. Natural gas is delivered to homes, where it is used for cooking and heating. Natural gas is also a major energy source for powering turbines to make electricity. Natural gas releases most of its energy as heat when it burns. The power plant is able to use this heat, either in the form of hot gases or steam, to spin turbines. The spinning turbines turn generators, and the generators create electricity. "
nonrenewable energy resources,T_0724,"Processing natural gas has harmful effects on the environment, just like oil. Natural gas burns cleaner than other fossil fuels. As a result, it causes less air pollution. It also produces less carbon dioxide than the other fossil fuels. Still, natural gas does emit pollutants. "
nonrenewable energy resources,T_0725,"Fossil fuels present many problems. These fuels are non-renewable resources, so our supplies of them will eventually run out. Safety can be a problem, too. Since these fuels burn so easily, a natural gas leak in a building or an underground pipe can lead to a deadly explosion. Using fossil fuels affects the environment in a variety of ways. There are impacts to the environment when we extract these resources. Burning these fuels causes air pollution. These fuels release carbon dioxide, which is a major factor in global warming (Figure 5.5). Many of the problems with fossil fuels are worse for coal than for oil or natural gas. Burning coal releases more carbon dioxide than either oil or natural gas. Yet coal is the most common fossil fuel, so we continue to burn large amounts of it. That makes coal the biggest contributor to global warming. Another problem with coal is that most coal contains sulfur. As it burns, the sulfur goes into the air as sulfur dioxide. Sulfur dioxide is the main cause of acid rain. Acid rain can be deadly to plants, animals, and whole ecosystems. Burning coal also puts a large number of small solid particulates into the air. These particles are dangerous to people, especially those who have asthma. People with asthma may end up in the hospital on days when particulate pollution is high. "
nonrenewable energy resources,T_0726,Nuclear energy is produced by splitting the nucleus of an atom. This releases a huge amount of energy. 
nonrenewable energy resources,T_0727,"Nuclear power plants use uranium that has been concentrated in fuel rods (Figure 5.6). The uranium atoms are split apart when they are hit by other extremely tiny particles. These particles must be controlled or they would cause a dangerous explosion. Nuclear power plants use the energy they produce to heat water. The water turns into steam, which causes a turbine to spin. This in turn produces electricity. "
nonrenewable energy resources,T_0728,"Many countries around the world use nuclear energy as a source of electricity. For example, France gets about 80% of its electricity from nuclear energy. In the United States, a little less than 20% of electricity comes from nuclear energy. Nuclear energy does not pollute. If there are no accidents, a nuclear power plant releases nothing but steam into the air. But nuclear energy does create other environmental problems. Splitting atoms creates dangerous radioactive waste. These wastes can remain dangerous for hundreds of thousands of years. Scientists and engineers are still looking for ways to keep this waste safely away from people. "
nonrenewable energy resources,T_0729,"Nuclear power is a controversial subject in California and most other places. Nuclear power has no pollutants including carbon emissions, but power plants are not always safe and the long-term disposal of wastes is a problem that has not yet been solved. The future of nuclear power is murky. Find out more at: http://science.kqed.org/ques MEDIA Click image to the left or use the URL below. URL: "
renewable energy resources,T_0731,"The Sun is Earths main source of energy. The Sun gives us both light and heat. The Sun changes hydrogen into helium through nuclear fusion. This releases huge amounts of energy. The energy travels to the Earth mostly as visible light. The energy is carried through the empty space by radiation. We can use sunlight as an energy resource, called solar energy (Figure 5.7). "
renewable energy resources,T_0732,"Solar energy has been used on a small scale for hundreds of years. Today we are using solar energy for more of our power demands. Solar power plants are being built in many locations around the world. In the United States, the southwestern deserts are well suited for solar plants. "
renewable energy resources,T_0733,"Sunlight is turned into electricity at a solar power plant. These power plants use a large group of mirrors to focus sunlight on one place. This place is called a receiver (Figure 5.8). At the receiver, a liquid such as oil or water is heated to a high temperature. The liquid transfers its heat by conduction. In conduction, energy moves between two objects that are in contact. The higher temperature object transfers heat to the lower temperature object. For example, when you heat a pot of water on a stove top, energy moves from the pot to its metal handle by conduction. At a solar power plant, the energy conducted by the heated liquid is used to make electricity. "
renewable energy resources,T_0734,"Solar energy is used to heat homes and water, and to make electricity. Scientists and engineers have many ways to get energy from the Sun (Figure 5.9). One is by using solar cells. Solar cells are devices that turn sunlight directly into electricity. Lots of solar cells make up an individual solar panel. You may have seen solar panels on roof tops. The Suns heat can also be trapped in your home by using south facing windows and good insulation. "
renewable energy resources,T_0735,"Solar energy has many benefits. It does not produce any pollution. There is plenty of it available, much more than we could possibly use. But solar energy has problems. The Sun doesnt shine at night. A special battery is needed to store extra energy during the day for use at night. The technology for most uses of solar energy is still expensive. Until solar technology becomes more affordable, most people will prefer to get their energy from other sources. "
renewable energy resources,T_0736,"Moving water has energy (Figure 5.10). That energy is used to make electricity. Hydroelectric power harnesses the energy of water moving down a stream. Hydropower is the most widely used form of renewable energy in the world. This abundant energy source provides almost one fifth of the worlds electricity. The energy of waves and tides can also be used to produce water power. At this time, wave and tidal power are rare. "
renewable energy resources,T_0737,"To harness water power, a stream must be dammed. Narrow valleys are the best for dams. While sitting in the reservoir behind the dam, the water has potential energy. Water is allowed to flow downhill into a large turbine. While flowing downhill, the water has kinetic energy. Kinetic energy makes the turbine spin. The turbine is connected to a generator, which makes electricity. "
renewable energy resources,T_0738,Many of the suitable streams in the United States have been developed for hydroelectric power. Many streams worldwide also have hydroelectric plants. Hydropower is a major source of Californias electricity. It accounts for about 14.5 percent of the total. Most of Californias nearly 400 hydroelectric power plants are located in the Sierra Nevada mountains. 
renewable energy resources,T_0739,"Water power does not burn a fuel. So it causes less pollution than many other kinds of energy. Water power is also a renewable resource. Water keeps flowing downhill. Although we use some of the energy from this movement, we are not using up the water. Water power does have problems. A large dam stops a streams flow, which floods the land upstream. A beautiful location may be lost. People may be displaced. The dams and turbines also change the downstream environment. Fish and other living things may not be able to survive. Dams slow the release of silt. Downstream deltas retreat and beaches may be starved of sand. Seaside cities may become exposed to storms and rising sea levels. Tidal power stations may need to close off a narrow bay or estuary. Wave power plants must withstand coastal storms and the corrosion of seawater. "
renewable energy resources,T_0740,"Although not yet widely used, many believe tidal power has more potential than wind or solar power for meeting alternative energy needs. Quest radio looks at plans for harnessing power from the sea by San Francisco and along the northern California coast. Learn more at: http://science.kqed.org/quest/audio/harnessing-power-from-the-sea/ MEDIA Click image to the left or use the URL below. URL: "
renewable energy resources,T_0741,"The energy from the Sun creates wind (Figure 5.11). Wind energy moves by convection. The Sun heats some locations more than others. Warm air rises, so other air rushes in to fill the hole left by the rising air. This horizontal movement of air is called wind. "
renewable energy resources,T_0742,"Wind power uses moving air as a source of energy. Some types of wind power have been around for a long time. People have used windmills to grind grain and pump water for hundreds of years. Sailing ships have depended on wind for millennia. Wind is now used to generate electricity. Moving air can make a turbine spin, just like moving water can. Moving air has kinetic energy. When wind hits the blades of the turbine, the kinetic energy makes the blades move. The turbine spins and creates electricity. "
renewable energy resources,T_0743,"Wind power has many advantages. It is clean: it does not release pollutants or carbon dioxide. It is plentiful almost everywhere. The technology to harness wind energy is being developed rapidly. Wind power also has problems. Wind does not blow all of the time, so wind energy must be stored for later use. Alternatively, another energy source needs to be available when the wind is not blowing. Wind turbines are expensive. They can wear out quickly. Finally, windmills are not welcomed by residents of some locations. They say that they are unattractive. Yet even with these problems, wind turbines are a competitive form of renewable energy. Many states are currently using wind power. Wind turbines are set up in mountain passes. This is common in California, where cool Pacific Ocean air is sucked across the passes and into the warmer inland valleys. "
renewable energy resources,T_0744,"Biomass is another renewable source of energy. Biomass includes wood, grains, and other plant materials or waste materials. People can burn wood directly for energy in the form of heat. Biomass can also be processed to make biofuel. Biofuel is a fairly new type of energy that is becoming more popular. Biomass is useful because it can be made liquid. This means that they can be used in cars and trucks. Some car engines can be powered by pure vegetable oil or even recycled vegetable oil. Sometimes the exhaust from these cars smells like French fries! By using biofuels, we can cut down on the amount of fossil fuel that we use. Because living plants take carbon dioxide out of the air, growing plants for biofuel can mean that we will put less of this gas into the air overall. This could help us do something about the problem of global warming. "
renewable energy resources,T_0745,"Geothermal energy comes the Earths internal heat. Hot springs and geysers are produced by water that is heated by magma or hot rock below the surface. At a geothermal power plant, engineers drill wells into the hot rocks. Hot water or steam may come up through the wells. Alternatively, water may be put down into the well to be heated. It then comes up. The hot water or steam makes a turbine spin. This makes electricity. "
renewable energy resources,T_0746,"Because the hot water or steam can be used directly to make a turbine spin, geothermal energy can be used without processing. Geothermal energy is clean and safe. It is renewable. There will always be hot rocks and water can be pumped down into a well. There, the water can be heated again to make more steam. Geothermal energy is an excellent resource in some parts of the world. Iceland is gets about one fourth of its electricity from geothermal sources. In the United States, California leads all states in producing geothermal energy. Geothermal energy in California is concentrated in the northern part of the state. The largest plant is in the Geysers Geothermal Resource Area. Geothermal energy is not economical everywhere. Many parts of the world do not have underground sources of heat that are close enough to the surface for building geothermal power plants. "
renewable energy resources,T_0747,"Where Earths internal heat gets close to the surface, geothermal power is a clean source of energy. In California, The Geysers supplies energy for many nearby homes and businesses. Learn more at: http://science.kqed.org/ques MEDIA Click image to the left or use the URL below. URL: "
continental drift,T_0757,"Alfred Wegener was an early 20th century German meteorologist. Wegener believed that the continents were once all joined together. He named the supercontinent Pangaea, meaning all earth. Wegener suggested that Pangaea broke up long ago. Since then, the continents have been moving to their current positions. He called his hypothesis continental drift. "
continental drift,T_0758,Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. Wegener found that this evidence was best explained if the continents had at one time been joined together. 
continental drift,T_0759,"Wegener found rocks of the same type and age on both sides of the Atlantic Ocean. He thought that the rocks formed side by side. These rocks then drifted apart on separate continents. Wegener also matched up mountain ranges across the Atlantic Ocean. The Appalachian Mountains were just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway. Wegener concluded that they formed as a single mountain range. This mountain range broke apart as the continents split up. The mountain range separated as the continents drifted. "
continental drift,T_0760,"Wegener also found evidence for continental drift from fossils (Figure 6.7). The same type of plant and animal fossils are found on continents that are now widely separated. These organisms would not have been able to travel across the oceans. Fossils of the seed fern Glossopteris are found across all of the southern continents. These seeds are too heavy to be carried across the ocean by wind. Mesosaurus fossils are found in South America and South Africa. Mesosaurus could swim, but only in fresh water. Cynognathus and Lystrosaurus were reptiles that lived on land. Both of these animals were unable to swim at all. Their fossils have been found across South America, Africa, India and Antarctica. Wegener thought that all of these organisms lived side by side. The lands later moved apart so that the fossils are separated. "
continental drift,T_0761,"Wegener also looked at evidence from ancient glaciers. Glaciers are found in very cold climates near the poles. The evidence left by some ancient glaciers is very close to the equator. Wegener knew that this was impossible! However, if the continents had moved, the glaciers would have been centered close to the South Pole. "
continental drift,T_0762,Coral reefs are found only in warm water. Coal swamps are also found in tropical and subtropical environments. Wegener discovered ancient coal seams and coral reef fossils in areas that are much too cold today. Wegener thought that the continents have moved since the time of Pangaea. 
continental drift,T_0763,"Some important evidence for continental drift came after Wegeners death. This is the magnetic evidence. Earths magnetic field surrounds the planet from pole to pole. If you have ever been hiking or camping, you may have used a compass to help you find your way. A compass points to the magnetic North Pole. The compass needle aligns with Earths magnetic field (Figure 6.8). Some rocks contain little compasses too! As lava cools, tiny iron-rich crystals line up with Earths magnetic field. "
stress in earths crust,T_0793,"Stress is the force applied to a rock. There are four types of stresses: Confining stress happens as weight of all the overlying rock pushes down on a deeply buried rock. The rock is being pushed in from all sides, which compresses it. The rock will not deform because there is no place for it to move. Compression stress squeezes rocks together. Compression causes rocks to fold or fracture (Figure 7.1). When two cars collide, compression causes them to crumple. Compression is the most common stress at convergent plate boundaries. Tension stress pulls rocks apart. Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries. Shear stress happens when forces slide past each other in opposite directions (Figure 7.2). This is the most common stress found at transform plate boundaries. The amount of stress on a rock may be greater than the rocks strength. In that case, the rock will change and deform (Figure 7.3). Deep within the Earth, the pressure is very great. A rock behaves like a stretched rubber band. When the stress stops, the rock goes back to its original shape. If more stress is applied to the rock, it bends and flows. It does not return to its original shape. Near the surface, if the stress continues, the rock will fracture and break. "
stress in earths crust,T_0794,"Sedimentary rocks are formed in horizontal layers. This is magnificently displayed around the southwestern United States. The arid climate allows rock layers to be well exposed (Figure 7.4). The lowest layers are the oldest and the higher layers are younger. Folds, joints and faults are caused by stresses. Figure 7.5 shows joints in a granite hillside. If a sedimentary rock is tilted or folded, we know that stresses have changed the rock (Figure 7.6). "
stress in earths crust,T_0795,"Deep within the Earth, as plates collide, rocks crumple into folds. You can model these folds by placing your hands on opposite edges of a piece of cloth and pushing your hands together. In sedimentary rocks, you can easily trace the folding of the layers. In the Figure 7.6, the rock layers are no longer horizontal. They tilt downhill from right to left in a monocline. Once rocks are folded, they do not return to their original shape. There are three types of folds: monoclines, anticlines, and synclines. A monocline is a simple one step bend in the rock layers (Figure 7.7). In a monocline, the oldest rocks are still at the bottom and the youngest are at the top. An anticline is a fold that arches upward. The rocks dip away from the center of the fold (Figure 7.8). The oldest rocks are found at the center of an anticline. The youngest rocks are draped over them at the top of the structure. When upward folding rocks form a circular structure, that structure is called a dome. If the top of the dome is eroded off, the oldest rocks are exposed at the center. A syncline is a fold that bends downward (Figure 7.9). In a syncline, the youngest rocks are at the center. The oldest rocks are at the outside edges. When rocks bend downward in a circular structure, it is called a basin. If the rocks are eroded, the youngest rocks are at the center. Basins can be enormous, like the Michigan Basin. "
stress in earths crust,T_0796,"With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). "
stress in earths crust,T_0797,"A strike-slip fault is a dip-slip fault where the dip of the fault plane is vertical. Strike-slip faults result from shear stresses. If you stand with one foot on each side of a strike-slip fault, one side will be moving toward you while the other side moves away from you. If your right foot moves toward you, the fault is known as a right-lateral strike-slip fault. If your left foot moves toward you, the fault is a left-lateral strike-slip fault (Figure 7.14). "
stress in earths crust,T_0798,"The San Andreas Fault in California is a right-lateral strike-slip fault (Figure 7.15). It is also a transform fault because the San Andreas is a plate boundary. As you can see, California will not fall into the ocean someday. The land west of the San Andreas Fault is moving northeastward, while the North American plate moves southwest. Someday, millions of years from now, Los Angeles will be a suburb of San Francisco! "
stress in earths crust,T_0799,"Many processes create mountains. Most mountains form along plate boundaries. A few mountains may form in the middle of a plate. For example, huge volcanoes are mountains formed at hotspots within the Pacific Plate. "
stress in earths crust,T_0800,"Most of the worlds largest mountains form as plates collide at convergent plate boundaries. Continents are too buoyant to get pushed down into the mantle. So when the plates smash together, the crust crumples upwards. This creates mountains. Folding and faulting in these collision zones makes the crust thicker. The worlds highest mountain range, the Himalayas, is growing as India collides with Eurasia. About 80 million years ago, India was separated from Eurasia by an ocean (Figure 7.16). As the plates collided, pieces of the old seafloor were forced over the Asian continent. This old seafloor is now found high in the Himalayas (Figure 7.17). "
stress in earths crust,T_0801,Volcanic mountain ranges form when oceanic crust is pushed down into the mantle at convergent plate boundaries. The Andes Mountains are a chain of coastal volcanic mountains. They are forming as the Nazca plate subducts beneath the South American plate (Figure 7.18). 
stress in earths crust,T_0802,"Mid-ocean ridges form at divergent plate boundaries. As the ocean floor separates an enormous line of volcanoes is created. When continental crust is pulled apart, it breaks into blocks. These blocks of crust are separated by normal faults. The blocks slide up or down. The result is alternating mountain ranges and valleys. This topography is known as basin-and-range (Figure 7.19). The area near Death Valley, California is the center of a classic basin-and-range province (Figure 7.20). "
igneous landforms and geothermal activ,T_0863,Extrusive igneous rocks cool at the surface. Volcanoes are one type of feature that forms from extrusive rocks. Several other interesting landforms are also extrusive features. Intrusive igneous rocks cool below the surface. These rocks do not always remain hidden. Rocks that formed in the crust are exposed when the rock and sediment that covers them is eroded away. 
igneous landforms and geothermal activ,T_0864,"When lava is thick, it flows slowly. If thick lava makes it to the surface, it cannot flow far from the vent. It often stays right in the middle of a crater at the top of a volcano. Here the lava creates a large, round lava dome (Figure "
igneous landforms and geothermal activ,T_0865,"A lava plateau is made of a large amount of fluid lava. The lava flows over a large area and cools. This creates a large, flat surface of igneous rock. Lava plateaus may be huge. The Columbia Plateau covers over 161,000 square kilometers (63,000 square miles). It makes up parts of the states of Washington, Oregon, and Idaho. Thin, fluid lava created the rock that makes up the entire ocean floor. This is from multiple eruptions from vents at the mid-ocean ridge. While not exactly a lava plateau, its interesting to think about so much lava! "
igneous landforms and geothermal activ,T_0866,New land is created in volcanic eruptions. The Hawaiian Islands are shield volcanoes. These volcanoes formed from fluid lava (Figure 8.21). The island grows as lava is added on the coast. New land may also emerge from lava that erupts from beneath the water. This is one way that new land is created. 
igneous landforms and geothermal activ,T_0867,Magma that cools underground forms intrusions (Figure 8.22). Intrusions become land formations if they are exposed at the surface by erosion. 
igneous landforms and geothermal activ,T_0868,"Water works its way through porous rocks or soil. Sometimes this water is heated by nearby magma. If the water makes its way to the surface, it forms a hot spring or a geyser. "
igneous landforms and geothermal activ,T_0869,"When hot water gently rises to the surface, it creates a hot spring. A hot spring forms where a crack in the Earth allows water to reach the surface after being heated underground. Many hot springs are used by people as natural hot tubs. Some people believe that hot springs can cure illnesses. Hot springs are found all over the world, even in Antarctica! "
igneous landforms and geothermal activ,T_0870,"Geysers are also created by water that is heated beneath the Earths surface. The water may become superheated by magma. It becomes trapped in a narrow passageway. The heat and pressure build as more water is added. When the pressure is too much, the superheated water bursts out onto the surface. This is a geyser. There are only a few areas in the world where the conditions are right for the formation of geysers. Only about 1,000 geysers exist worldwide. About half of them are in the United States. The most famous geyser is Old Faithful at Yellowstone National Park (Figure 8.23). It is rare for a geyser to erupt so regularly, which is why Old Faithful is famous. "
weathering,T_0871,"Weathering changes solid rock into sediments. Sediments are different sizes of rock particles. Boulders are sedi- ments; so is gravel. At the other end, silt and clay are also sediments. Weathering causes rocks at the Earths surface to change form. The new minerals that form are stable at the Earths surface. It takes a long time for a rock or mountain to weather. But a road can do so much more quickly. If you live in a part of the world that has cold winters, you may only have to wait one year to see a new road start to weather (Figure "
weathering,T_0872,"Mechanical weathering breaks rock into smaller pieces. These smaller pieces are just like the bigger rock; they are just smaller! The rock has broken without changing its composition. The smaller pieces have the same minerals in the same proportions. You could use the expression a chip off the old block to describe mechanical weathering! The main agents of mechanical weathering are water, ice, and wind. "
weathering,T_0873,"Rocks can break apart into smaller pieces in many ways. Ice wedging is common where water goes above and below its freezing point (Figure 9.2). This can happen in winter in the mid-latitudes or in colder climates in summer. Ice wedging is common in mountainous regions. This is how ice wedging works. When liquid water changes into solid ice, it increases in volume. You see this when you fill an ice cube tray with water and put it in the freezer. The ice cubes go to a higher level in the tray than the water. You also may have seen this if you put a can of soda into the freezer so that it cools down quickly. If you leave the can in the freezer too long, the liquid expands so much that it bends or pops the can. (For the record, water is very unusual. Most substances get smaller when they change from a liquid to a solid.) "
weathering,T_0874,"Abrasion is another type of mechanical weathering. With abrasion, one rock bumps against another rock. Gravity causes abrasion as a rock tumbles down a slope. Moving water causes abrasion it moves rocks so that they bump against one another (Figure 9.3). Strong winds cause abrasion by blasting sand against rock surfaces. Finally, the ice in glaciers cause abrasion. Pieces of rock embedded in ice at the bottom of a glacier scrape against the rock below. If you have ever collected beach glass or pebbles from a stream, you have witnessed the work of abrasion. "
weathering,T_0875,"Sometimes biological elements cause mechanical weathering. This can happen slowly. A plants roots grow into a crack in rock. As the roots grow larger, they wedge open the crack. Burrowing animals can also cause weathering. By digging for food or creating a hole to live in the animal may break apart rock. Today, human beings do a lot of mechanical weathering whenever we dig or blast into rock. This is common when we build homes, roads, and subways, or quarry stone for construction or other uses. "
weathering,T_0876,"Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more places for chemical weathering to occur. Lets say you wanted to make some hot chocolate on a cold day. It would be hard to get a big chunk of chocolate to dissolve in your milk or hot water. Maybe you could make hot chocolate from some smaller pieces like chocolate chips, but it is much easier to add a powder to your milk. This is because the smaller the pieces are, the more surface area they have. Smaller pieces dissolve more easily. "
weathering,T_0877,"Chemical weathering is different than mechanical weathering. The minerals in the rock change. The rock changes composition and becomes a different type of rock. Most minerals form at high pressure or high temperatures deep within Earth. But at Earths surface, temperatures and pressures are much lower. Minerals that were stable deeper in the crust are not stable at the surface. Thats why chemical weathering happens. Minerals that formed at higher temperature and pressure change into minerals that are stable at the surface. Chemical weathering is important. It starts the process of changing solid rock into soil. We need soil to grow food and create other materials we need. Chemical weathering works through chemical reactions that change the rock. There are many agents of chemical weathering. Remember that water was a main agent of mechanical weathering. Well, water is also an agent of chemical weathering. That makes it a double agent! Carbon dioxide and oxygen are also agents of chemical weathering. Each of these is discussed below. "
weathering,T_0878,"Water is an amazing molecule. It has a very simple chemical formula, H2 O. It is made of just two hydrogen atoms bonded to one oxygen atom. Water is remarkable in terms of all the things it can do. Lots of things dissolve easily in water. Some types of rock can even completely dissolve in water! Other minerals change by adding water into their structure. "
weathering,T_0879,"Carbon dioxide (CO2 ) combines with water as raindrops fall through the air. This makes a weak acid, called carbonic acid. This happens so often that carbonic acid is a common, weak acid found in nature. This acid works to dissolve rock. It eats away at sculptures and monuments. While this is normal, more acids are made when we add pollutants to the air. Any time we burn any fossil fuel, it adds nitrous oxide to the air. When we burn coal rich in sulfur, it adds sulfur dioxide to the air. As nitrous oxide and sulfur dioxide react with water, they form nitric acid and sulfuric acid. These are the two main components of acid rain. Acid rain accelerates chemical weathering. "
weathering,T_0880,"Oxygen strongly reacts with elements at the Earths surface. You are probably most familiar with the rust that forms when iron reacts with oxygen (Figure 9.4). Many minerals are rich in iron. They break down as the iron changes into iron oxide. This makes the red color in soils. Plants and animals also cause chemical weathering. As plant roots take in nutrients, elements are exchanged. "
weathering,T_0881,"Each type of rock weathers in its own way. Certain types of rock are very resistant to weathering. Igneous rocks tend to weather slowly because they are hard. Water cannot easily penetrate them. Granite is a very stable igneous rock. Other types of rock are easily weathered because they dissolve easily in weak acids. Limestone is a sedimentary rock that dissolves easily. When softer rocks wear away, the more resistant rocks form ridges or hills. Devils Tower in Wyoming shows how different types of rock weather at different rates (Figure 9.5). The softer materials of the surrounding rocks were worn away. The resistant center of the volcano remains behind. Minerals also weather differently. Some minerals completely dissolve in water. As less resistant minerals dissolve away, a rocks surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock. "
acid rain,T_0900,"Acid rain is caused by sulfur and nitrogen oxides emanating from power plants or metal refineries. The smokestacks have been built tall so that pollutants dont sit over cities (Figure 1.1). As they move, these pollutants combine with water vapor to form sulfuric and nitric acids. The acid droplets form acid fog, rain, snow, or they may be deposited dry. Most typical is acid rain (Figure 1.2). "
acid rain,T_0901,"Acid rain water is more acidic than normal rain water. Acidity is measured on the pH scale. Lower numbers are more acidic and higher numbers are less acidic (also called more alkaline) (Figure 1.3). Natural rain is somewhat acidic, with a pH of 5.6; acid rain must have a pH of less than 5.0. A small change in pH represents a large change in acidity: rain with a pH of 4.6 is 10 times more acidic than normal rain (with a pH of 5.6). Rain with a pH of 3.6 is 100 times more acidic. Regions with a lot of coal-burning power plants have the most acidic rain. The acidity of average rainwater in the northeastern United States has fallen to between 4.0 and 4.6. Acid fog has even lower pH with an average of around 3.4. One fog in Southern California in 1986 had a pH of 1.7, equal to toilet-bowl cleaner. In arid climates, such as in Southern California, acids deposit on the ground dry. Acid precipitation ends up on the land surface and in water bodies. Some forest soils in the northeast are five to ten times more acidic than they were two or three decades ago. Acid droplets move down through acidic soils to lower the pH of streams and lakes even more. Acids strip soil of metals and nutrients, which collect in streams and lakes. As a result, stripped soils may no longer provide the nutrients that native plants need. A pH scale goes from 1 to 14; numbers are shown with the pH of some common substances. A value of 7 is neutral. The strongest acids are at the low end of the scale and the strongest bases are at the high end. "
acid rain,T_0902,"Acid rain takes a toll on ecosystems (Figure 1.4). Plants that are exposed to acids become weak and are more likely to be damaged by bad weather, insect pests, or disease. Snails die in acid soils, so songbirds do not have as much food to eat. Young birds and mammals do not build bones as well and may not be as strong. Eggshells may also be weak and break more easily. As lakes become acidic, organisms die off. No fish can live if the pH drops below 4.5. Organic material cannot decay, and mosses take over the lake. Wildlife that depend on the lake for drinking water suffer population declines. Crops are damaged by acid rain. This is most noticeable in poor nations where people cant afford to fix the problems with fertilizers or other technology. Acid rain has killed trees in this forest in the Czech Republic. Acid rain damages cultural monuments like buildings and statues. These include the U.S. Capitol and many buildings in Europe, such as Westminster Abbey. Carbonate rocks neutralize acids and so some regions do not suffer the effects of acid rain nearly as much. Limestone in the midwestern United States protects the area. One reason that the northeastern United States is so vulnerable to acid rain damage is that the rocks are not carbonates. Because pollutants can travel so far, much of the acid rain that falls hurts states or nations other than ones where the pollutants were released. All the rain that falls in Sweden is acidic and fish in lakes all over the country are dying. The pollutants come from the United Kingdom and Western Europe, which are now working to decrease their emissions. Canada also suffers from acid rain that originates in the United States, a problem that is also improving. Southeast Asia is experiencing more acid rain between nations as the region industrializes. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
adaptation and evolution of populations,T_0903,"The characteristics of an organism that help it to survive in a given environment are called adaptations. Adaptations are traits that an organism inherits from its parents. Within a population of organisms are genes coding for a certain number of traits. For example, a human population may have genes for eyes that are blue, green, hazel, or brown, but as far as we know, not purple or lime green. Adaptations develop when certain variations or differences in a population help some members survive better than others (Figure 1.1). The variation may already exist within the population, but often the variation comes from a mutation, or a random change in an organisms genes. Some mutations are harmful and the organism dies; in that case, the variation will not remain in the population. Many mutations are neutral and remain in the population. If the environment changes, the mutation may be beneficial and it may help the organism adapt to the environment. The organisms that survive pass this favorable trait on to their offspring. "
adaptation and evolution of populations,T_0904,"Many changes in the genetic makeup of a species may accumulate over time, especially if the environment is changing. Eventually the descendants will be very different from their ancestors and may become a whole new species. Changes in the genetic makeup of a species over time are known as biological evolution. "
adaptation and evolution of populations,T_0905,"The mechanism for evolution is natural selection. Traits become more or less common in a population depending on whether they are beneficial or harmful. An example of evolution by natural selection can be found in the deer mouse, species Peromyscus maniculatus. In Nebraska this mouse is typically brown, but after glaciers carried lighter sand over the darker soil in the Sand Hills, predators could more easily spot the dark mice. Natural selection favored the light mice, and over time, the population became light colored. An explanation of how adaptations de- velop. Click image to the left or use the URL below. URL: "
age of earth,T_0906,"During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. What methods can you think of for doing this? One example is that by measuring how much sediment a stream deposited in a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer. Not surprisingly, these methods resulted in wildly different estimates. A relatively good estimate was produced by the British geologist Charles Lyell, who thought that 240 million years had passed since the appearance of the first animals with shells. Today scientists know that this event occurred about 530 million years ago. In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old, which he later lowered to 20 million years. He did this systematically assuming that the planet started off as a molten ball and calculating the time it would take for it to cool to its current temperature. This estimate was a blow to geologists and supporters of Charles Darwins theory of evolution, which required an older Earth to provide time for geological and evolutionary processes to take place. Kelvins calculations were soon shown to be flawed when radioactivity was discovered in 1896. What Kelvin didnt know is that radioactive decay of elements inside Earths interior provides a steady source of heat. He also didnt know that the mantle is able to flow and so convection moves heat from the interior to the surface of the planet. Thomson had grossly underestimated Earths age. "
age of earth,T_0907,"Radioactivity turned out to be useful for dating Earth materials and for coming up with a quantitative age for Earth. Scientists not only date ancient rocks from Earths crust, they also date meteorites that formed at the same time Earth and the rest of the solar system were forming. Moon rocks also have been radiometrically dated. Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well- defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossil ages have become increasingly accurate. This is the modern geologic time scale with all of the ages. Click image to the left or use the URL below. URL: "
agriculture and human population growth,T_0908,"Every major advance in agriculture has allowed global population to increase. Early farmers could settle down to a steady food supply. Irrigation, the ability to clear large swaths of land for farming efficiently, and the development of farm machines powered by fossil fuels allowed people to grow more food and transport it to where it was needed. "
agriculture and human population growth,T_0909,"What is Earths carrying capacity for humans? Are humans now exceeding Earths carrying capacity for our species? Many anthropologists say that the carrying capacity of humans on the planet without agriculture is about 10 million (Figure 1.1). This population was reached about 10,000 years ago. At the time, people lived together in small bands of hunters and gatherers. Typically men hunted and fished; women gathered nuts and vegetables. Obviously, human populations have blown past this hypothetical carrying capacity. By using our brains, our erect posture, and our hands, we have been able to manipulate our environment in ways that no other species has ever done. What have been the important developments that have allowed population to grow? "
agriculture and human population growth,T_0910,"About 10,000 years ago, we developed the ability to grow our own food. Farming increased the yield of food plants and allowed people to have food available year round. Animals were domesticated to provide meat. With agriculture, people could settle down, so that they no longer needed to carry all their possessions (Figure 1.2). They could develop better farming practices and store food for when it was difficult to grow. Agriculture allowed people to settle in towns and cities. More advanced farming practices allowed a single farmer to grow food for many more people. When advanced farming practices allowed farmers to grow more food than they needed for their families (Figure "
agriculture and human population growth,T_0911,"The next major stage in the growth of the human population was the Industrial Revolution, which started in the late 1700s (Figure 1.4). This major historical event marks when products were first mass-produced and when fossil fuels were first widely used for power. "
agriculture and human population growth,T_0912,"The Green Revolution has allowed the addition of billions of people to the population in the past few decades. The Green Revolution has improved agricultural productivity by: Improving crops by selecting for traits that promote productivity; recently, genetically engineered crops have been introduced. Increasing the use of artificial fertilizers and chemical pesticides. About 23 times more fertilizer and 50 times more pesticides are used around the world than were used just 50 years ago (Figure 1.5). Agricultural machinery: plowing, tilling, fertilizing, picking, and transporting are all done by machines. About 17% of the energy used each year in the United States is for agriculture. Increasing access to water. Many farming regions depend on groundwater, which is not a renewable resource. Some regions will eventually run out of this water source. Currently about 70% of the worlds fresh water is used for agriculture. Rows of a single crop and heavy ma- chinery are normal sights for modern day farms. The Green Revolution has increased the productivity of farms immensely. A century ago, a single farmer produced enough food for 2.5 people, but now a farmer can feed more than 130 people. The Green Revolution is credited for feeding 1 billion people that would not otherwise have been able to live. "
agriculture and human population growth,T_0913,"The flip side to this is that for the population to continue to grow, more advances in agriculture and an ever increasing supply of water will be needed. Weve increased the carrying capacity for humans by our genius: growing crops, trading for needed materials, and designing ways to exploit resources that are difficult to get at, such as groundwater. And most of these resources are limited. The question is, even though we have increased the carrying capacity of the planet, have we now exceeded it (Figure There is not yet an answer to that question, but there are many different opinions. In the eighteenth century, Thomas Malthus predicted that human population would continue to grow until we had exhausted our resources. At that point, humans would become victims of famine, disease, or war. This has not happened, at least not yet. Some scientists think that the carrying capacity of the planet is about 1 billion people, not the 7 billion people we have today. The limiting factors have changed as our intelligence has allowed us to expand our population. Can we continue to do this indefinitely into the future? "
air quality,T_0919,"Pollutants include materials that are naturally occurring but are added to the atmosphere so that they are there in larger quantities than normal. Pollutants may also be human-made compounds that have never before been found in the atmosphere. Pollutants dirty the air, change natural processes in the atmosphere, and harm living things. "
air quality,T_0920,Air pollution started to be a problem when early people burned wood for heat and cooking fires in enclosed spaces such as caves and small tents or houses. But the problems became more widespread as fossil fuels such as coal began to be burned during the Industrial Revolution. 
air quality,T_0921,Air pollution started to be a problem when early people burned wood for heat and cooking fires in enclosed spaces such as caves and small tents or houses. But the problems became more widespread as fossil fuels such as coal began to be burned during the Industrial Revolution (Figure 1.1). The 2012 Olympic Games in London opening ceremony contained a reen- actment of the Industrial Revolution - complete with pollution streaming from smokestacks. 
air quality,T_0922,"Photochemical smog, a different type of air pollution, first became a problem in Southern California after World War II. The abundance of cars and sunshine provided the perfect setting for a chemical reaction between some of the molecules in auto exhaust or oil refinery emissions and sunshine (Figure 1.2). Photochemical smog consists of more than 100 compounds, most importantly ozone. Smog over Los Angeles as viewed from the Hollywood Hills. "
air quality,T_0923,"Terrible air pollution events in Pennsylvania and London, in which many people died, plus the recognition of the hazards of photochemical smog, led to the passage of the Clean Air Act in 1970 in the United States. The act now regulates 189 pollutants. The six most important pollutants regulated by the Act are ozone, particulate matter, sulfur dioxide, nitrogen dioxide, carbon monoxide, and the heavy metal lead. Other important regulated pollutants include benzene, perchloroethylene, methylene chloride, dioxin, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds. What is the result of the Clean Air Act? In short, the air in the United States is much cleaner. Visibility is better and people are no longer incapacitated by industrial smog. However, despite the Act, industry, power plants, and vehicles put 160 million tons of pollutants into the air each year. Some of this smog is invisible and some contributes to the orange or blue haze that affects many cities. "
air quality,T_0924,"Air quality in a region is not just affected by the amount of pollutants released into the atmosphere in that location but by other geographical and atmospheric factors. Winds can move pollutants into or out of a region and a mountain range can trap pollutants on its leeward side. Inversions commonly trap pollutants within a cool air mass. If the inversion lasts long enough, pollution can reach dangerous levels. Pollutants remain over a region until they are transported out of the area by wind, diluted by air blown in from another region, transformed into other compounds, or carried to the ground when mixed with rain or snow. Table 1.1 lists the smoggiest cities in 2013: 7 of the 10 are in California. Why do you think California cities are among those with the worst air pollution? The state has the right conditions for collecting pollutants including mountain ranges that trap smoggy air, arid and sometimes windless conditions, agriculture, industry, and lots and lots of cars. Rank 1 2 3 4 5 6 7 8 9 10 City, State Los Angeles area, California Visalia-Porterville, California Bakersfield-Delano, California Fresno-Madera, California Hanford-Corcoran, California Sacramento area, California Houston area, Texas Dallas-Fort Worth, Texas Washington D.C. area El Centro, California "
asteroids,T_0925,"Asteroids are very small, rocky bodies that orbit the Sun. ""Asteroid"" means ""star-like,"" and in a telescope, asteroids look like points of light, just like stars. Asteroids are irregularly shaped because they do not have enough gravity to become round. They are also too small to maintain an atmosphere, and without internal heat they are not geologically active (Figure 1.1). Collisions with other bodies may break up the asteroid or create craters on its surface. Asteroid impacts have had dramatic impacts on the shaping of the planets, including Earth. Early impacts caused the planets to grow as they cleared their portions of space. An impact with an asteroid about the size of Mars caused fragments of Earth to fly into space and ultimately create the Moon. Asteroid impacts are linked to mass extinctions throughout Earths history. "
asteroids,T_0926,"Hundreds of thousands of asteroids have been discovered in our solar system. They are still being discovered at a rate of about 5,000 new asteroids per month. The majority of the asteroids are found in between the orbits of Mars In 1991, Asteroid 951 Gaspra was the first asteroid photographed at close range. Gaspra is a medium-sized asteroid, mea- suring about 19 by 12 by 11 km (12 by 7.5 by 7 mi). and Jupiter, in a region called the asteroid belt, as shown in Figure 1.2. Although there are many thousands of asteroids in the asteroid belt, their total mass adds up to only about 4% of Earths Moon. The white dots in the figure are asteroids in the main asteroid belt. Other groups of asteroids closer to Jupiter are called the Hildas (orange), the Trojans (green), and the Greeks (also green). Scientists think that the bodies in the asteroid belt formed during the formation of the solar system. The asteroids might have come together to make a single planet, but they were pulled apart by the intense gravity of Jupiter. "
asteroids,T_0927,"More than 4,500 asteroids cross Earths orbit; they are near-Earth asteroids. Between 500 and 1,000 of these are over 1 km in diameter. Any object whose orbit crosses Earths can collide with Earth, and many asteroids do. On average, each year a rock about 5-10 m in diameter hits Earth (Figure 1.3). Since past asteroid impacts have been implicated in mass extinctions, astronomers are always on the lookout for new asteroids, and follow the known near-Earth asteroids closely, so they can predict a possible collision as early as possible. A painting of what an asteroid a few kilometers across might look like as it strikes Earth. "
asteroids,T_0928,Scientists are interested in asteroids because they are representatives of the earliest solar system (Figure 1.4). Eventually asteroids could be mined for rare minerals or for construction projects in space. A few missions have studied asteroids directly. NASAs DAWN mission explored asteroid Vesta in 2011 and 2012 and will visit dwarf planet Ceres in 2015. Click image to the left or use the URL below. URL:  The NEAR Shoemaker probe took this photo as it was about to land on 433 Eros in 2001. 
asteroids,T_0929,"Thousands of objects, including comets and asteroids, are zooming around our solar system; some could be on a collision course with Earth. QUEST explores how these Near Earth Objects are being tracked and what scientists are saying should be done to prevent a deadly impact. Click image to the left or use the URL below. URL: "
availability of natural resources,T_0931,"From the table in the concept ""Materials Humans Use,"" you can see that many of the resources we depend on are non-renewable. Non-renewable resources vary in their availability; some are very abundant and others are rare. Materials, such as gravel or sand, are technically non-renewable, but they are so abundant that running out is no issue. Some resources are truly limited in quantity: when they are gone, they are gone, and something must be found that will replace them. There are even resources, such as diamonds and rubies, that are valuable in part because they are so rare. "
availability of natural resources,T_0932,"Besides abundance, a resources value is determined by how easy it is to locate and extract. If a resource is difficult to use, it will not be used until the price for that resource becomes so great that it is worth paying for. For example, the oceans are filled with an abundant supply of water, but desalination is costly, so it is used only where water is really limited (Figure 1.1). As the cost of desalination plants comes down, more will likely be built. Tampa Bay, Florida, has one of the few desalination plants in the United States. "
availability of natural resources,T_0933,"Politics is also part of determining resource availability and cost. Nations that have a desired resource in abundance will often export that resource to other countries, while countries that need that resource must import it from one of the countries that produces it. This situation is a potential source of economic and political trouble. Of course the greatest example of this is oil. Twelve countries have approximately 80% of all of the worlds oil (Figure 1.2). However, the biggest users of oil, the United States, China, and Japan, are all located outside this oil-rich region. This leads to a situation in which the availability and price of the oil is determined largely by one set of countries that have their own interests to look out for. The result has sometimes been war, which may have been attributed to all sorts of reasons, but at the bottom, the reason is oil. "
availability of natural resources,T_0934,"The topic of overconsumption was touched on in the chapter Life on Earth. Many people in developed countries, such as the United States and most of Europe, use many more natural resources than people in many other countries. We have many luxury and recreational items, and it is often cheaper for us to throw something away than to fix it or just hang on to it for a while longer. This consumerism leads to greater resource use, but it also leads to more waste. Pollution from discarded materials degrades the land, air, and water (Figure 1.3). Natural resource use is generally lower in developing countries because people cannot afford many products. Some of these nations export natural resources to the developed world since their deposits may be richer and the cost of labor lower. Environmental regulations are often more lax, further lowering the cost of resource extraction. Click image to the left or use the URL below. URL:  The nations in blue are the 12 biggest producers of oil; they are Algeria, Angola, Ecuador, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, the United Arab Emirates, and Venezuela. Pollution from discarded materials de- grades the environment and reduces the availability of natural resources. "
bathymetric evidence for seafloor spreading,T_0939,"Well go out on the research vessel (R/V in ship-speak) Atlantis, owned by the US Navy and operated by the Woods Hole Oceanographic Institution for the oceanographic community. The Atlantis has six science labs and storage spaces, precise navigation systems, seafloor-mapping sonar and satellite communications. Most importantly, the ship has all of the heavy equipment necessary to deploy and operate Alvin, the manned research submersible. The ship has 24 bunks available for scientists, including two for the chief scientists. The majority of these bunks are below waterline, which makes for good sleeping in the daytime. Ship time is really expensive research, so vessels operate all night and so do the scientists. Your watch, as your time on duty is called, may be 12-4, 4-8 or 8-12 - thats AM and PM. Alternately, if youre on the team doing a lot of diving in Alvin, you may just be up during the day. If youre mostly doing operations that dont involve Alvin, you may just be up at night. For safety reasons, Alvin is deployed and recovered only in daylight. Alvin is deployed from the stern of the R/V Atlantis. Scientists come from all over to meet a research ship in a port. An oceanographer these days doesnt need to be near the ocean, he or she just needs to have access to an airport! Lets begin this cruise in Woods Hole, Massachusetts, Atlantis home port. Our first voyage will be out to the Mid- Atlantic Ridge. Transit time to the research site can take days. By doing this virtually, we dont have to spend days in transit to our research site, and we dont have to get seasick! As we head to the site, we will run the echo sounder. Lets see what we can find! "
bathymetric evidence for seafloor spreading,T_0940,"The people who first mapped the seafloor were aboard military vessels during World War II. As stated in the Earth as a Planet chapter, echo sounders used sound waves to search for submarines, but also produced a map of seafloor depths. Depth sounding continued in earnest after the war. Scientists pieced together the ocean depths to produce bathymetric maps of the seafloor. During WWII and in the decade or so later, echo sounders had only one beam, so they just returned a line showing the depth beneath the ship. Later echo sounders sent out multiple beams and could create a bathymetric map of the seafloor below. We will run a multi-beam echo sounder as we go from Woods Hole out to the Mid-Atlantic Ridge. "
bathymetric evidence for seafloor spreading,T_0941,"Although they expected an expanse of flat, featureless plains, scientists were shocked to find tremendous features like mountain ranges, rifts, and trenches. This work continues on oceanographic research vessels as they sail across the seas today. The map in the Figure 1.2 is a modern map with data from several decades. The major features of the ocean basins and their colors on the map in Figure 1.2 include: mid-ocean ridges: these features rise up high above the deep seafloor as a long chain of mountains, e.g. the light blue gash in middle of Atlantic Ocean. rift zones: in the middle of the mid-ocean ridges is a rift zone that is lower in elevation than the mountains surrounding it. deep sea trenches: these features are found at the edges of continents or in the sea near chains of active volcanoes, e.g. the very deepest blue, off of western South America. abyssal plains: these features are flat areas, although many are dotted with volcanic mountains, e.g. consistent blue off of southeastern South America. See if you can identify each of these features in Figure 1.2. A modern map of the southeastern Pacific and Atlantic Oceans. When they first observed these bathymetric maps, scientists wondered what had formed these features. It turns out that they were crucial for fitting together ideas about seafloor spreading. "
bathymetric evidence for seafloor spreading,T_0942,"As we have seen, the ocean floor is not flat: mid-ocean ridges, deep sea trenches, and other features all rise sharply above or plunge deeply below the abyssal plains. In fact, Earths tallest mountain is Mauna Kea volcano, which rises 10,203 m (33,476 ft.)meters) from the Pacific Ocean floor to become one of the volcanic mountains of Hawaii. The deepest canyon is also on the ocean floor, the Challenger Deep in the Marianas Trench, 10,916 m (35,814 ft). The continental margin is the transition from the land to the deep sea or, geologically speaking, from continental crust to oceanic crust. More than one-quarter of the ocean basin is continental margin. (Figure 1.3). Click image to the left or use the URL below. URL: "
big bang,T_0943,"Timeline of the Big Bang and the expan- sion of the Universe. The Big Bang theory is the most widely accepted cosmological explanation of how the universe formed. If we start at the present and go back into the past, the universe is contracting  getting smaller and smaller. What is the end result of a contracting universe? According to the Big Bang theory, the universe began about 13.7 billion years ago. Everything that is now in the universe was squeezed into a very small volume. Imagine all of the known universe in a single, hot, chaotic mass. An enormous explosion  a big bang  caused the universe to start expanding rapidly. All the matter and energy in the universe, and even space itself, came out of this explosion. What came before the Big Bang? There is no way for scientists to know since there is no remaining evidence. "
big bang,T_0944,"In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. After only a few seconds, protons, neutrons, and electrons could form. After a few minutes, those subatomic particles came together to create hydrogen. Energy in the universe was great enough to initiate nuclear fusion, and hydrogen nuclei were fused into helium nuclei. The first neutral atoms that included electrons did not form until about 380,000 years later. The matter in the early universe was not smoothly distributed across space. Dense clumps of matter held close together by gravity were spread around. Eventually, these clumps formed countless trillions of stars, billions of galaxies, and other structures that now form most of the visible mass of the universe. If you look at an image of galaxies at the far edge of what we can see, you are looking at great distances. But you are also looking across a different type of distance. What do those far away galaxies represent? Because it takes so long for light from so far away to reach us, you are also looking back in time (Figure 1.2). "
big bang,T_0945,"After the origin of the Big Bang hypothesis, many astronomers still thought the universe was static. Nearly all came around when an important line of evidence for the Big Bang was discovered in 1964. In a static universe, the space between objects should have no heat at all; the temperature should measure 0 K (Kelvin is an absolute temperature scale). But two researchers at Bell Laboratories used a microwave receiver to learn that the background radiation in the universe is not 0 K, but 3 K (Figure 1.3). This tiny amount of heat is left over from the Big Bang. Since nearly Images from very far away show what the universe was like not too long after the Big Bang. all astronomers now accept the Big Bang hypothesis, what is it usually referred to as? Click image to the left or use the URL below. URL: "
carbon cycle and climate,T_0958,"Carbon is a very important element to living things. As the second most common element in the human body, we know that human life without carbon would not be possible. Protein, carbohydrates, and fats are all part of the body and all contain carbon. When your body breaks down food to produce energy, you break down protein, carbohydrates, and fat, and you breathe out carbon dioxide. Carbon occurs in many forms on Earth. The element moves through organisms and then returns to the environment. When all this happens in balance, the ecosystem remains in balance too. "
carbon cycle and climate,T_0959,The short term cycling of carbon begins with carbon dioxide (CO2 ) in the atmosphere. 
carbon cycle and climate,T_0960,"Through photosynthesis, the inorganic carbon in carbon dioxide plus water and energy from sunlight is transformed into organic carbon (food) with oxygen given off as a waste product. The chemical equation for photosynthesis is: "
carbon cycle and climate,T_0961,"Plants and animals engage in the reverse of photosynthesis, which is respiration. In respiration, animals use oxygen to convert the organic carbon in sugar into food energy they can use. Plants also go through respiration and consume some of the sugars they produce. The chemical reaction for respiration is: C6 H12 O6 + 6 O2  6 CO2 + 6 H2 O + useable energy Photosynthesis and respiration are a gas exchange process. In photosynthesis, CO2 is converted to O2 ; in respiration, O2 is converted to CO2 . Remember that plants do not create energy. They change the energy from sunlight into chemical energy that plants and animals can use as food (Figure 1.1). "
carbon cycle and climate,T_0963,"Places in the ecosystem that store carbon are reservoirs. Places that supply and remove carbon are carbon sources and carbon sinks, respectively. If more carbon is provided than stored, the place is a carbon source. If more carbon dioxide is absorbed than is emitted, the reservoir is a carbon sink. What are some examples of carbon sources and sinks? Carbon sinks are reservoirs where carbon is stored. Healthy living forests and the oceans act as carbon sinks. Carbon sources are reservoirs from which carbon can enter the environment. The mantle is a source of carbon from volcanic gases. A reservoir can change from a sink to a source and vice versa. A forest is a sink, but when the forest burns it becomes a source. The amount of time that carbon stays, on average, in a reservoir is the residence time of carbon in that reservoir. "
carbon cycle and climate,T_0964,"Remember that the amount of CO2 in the atmosphere is very low. This means that a small increase or decrease in the atmospheric CO2 can have a large effect. By measuring the composition of air bubbles trapped in glacial ice, scientists can learn the amount of atmospheric CO2 at times in the past. Of particular interest is the time just before the Industrial Revolution, when society began to use fossil fuels. That value is thought to be the natural content of CO2 for this time period; that number was 280 parts per million (ppm). By 1958, when scientists began to directly measure CO2 content from the atmosphere at Mauna Loa volcano in the Pacific Ocean, the amount was 316 ppm (Figure 1.2). In 2014, the atmospheric CO2 content had risen to around 400 ppm. The amount of CO2 in the atmosphere has been measured at Mauna Loa Obser- vatory since 1958. The blue line shows yearly averaged CO2 . The red line shows seasonal variations in CO2 . This is an increase in atmospheric CO2 of 40% since the before the Industrial Revolution. About 65% of that increase has occurred since the first CO2 measurements were made on Mauna Loa Volcano, Hawaii, in 1958. "
carbon cycle and climate,T_0965,"Humans have changed the natural balance of the carbon cycle because we use coal, oil, and natural gas to supply our energy demands. Fossil fuels are a sink for CO2 when they form, but they are a source for CO2 when they are burned. The equation for combustion of propane, which is a simple hydrocarbon looks like this: The equation shows that when propane burns, it uses oxygen and produces carbon dioxide and water. So when a car burns a tank of gas, the amount of CO2 in the atmosphere increases just a little. Added over millions of tanks of gas and coal burned for electricity in power plants and all of the other sources of CO2 , the result is the increase in atmospheric CO2 seen in the Figure 1.2. The second largest source of atmospheric CO2 is deforestation (Figure 1.3). Trees naturally absorb CO2 while they are alive. Trees that are cut down lose their ability to absorb CO2 . If the tree is burned or decomposes, it becomes a source of CO2 . A forest can go from being a carbon sink to being a carbon source. This forest in Mexico has been cut down and burned to clear forested land for agri- culture. "
carbon cycle and climate,T_0966,"Why is such a small amount of carbon dioxide in the atmosphere even important? Carbon dioxide is a greenhouse gas. Greenhouse gases trap heat energy that would otherwise radiate out into space, which warms Earth. These gases were discussed in the chapter Atmospheric Processes. "
causes of air pollution,T_0967,"Most air pollutants come from burning fossil fuels or plant material. Some are the result of evaporation from human- made materials. Nearly half (49%) of air pollution comes from transportation, 28% from factories and power plants, and the remaining pollution from a variety of other sources. "
causes of air pollution,T_0968,"Fossil fuels are burned in most motor vehicles and power plants. These non-renewable resources are the power for nearly all manufacturing and other industries. Pure coal and petroleum can burn cleanly and emit only carbon dioxide and water, but most of the time these fossil fuels do not burn completely and the incomplete chemical reactions produce pollutants. Few sources of these fossil fuels are pure, so other pollutants are usually released. These pollutants include carbon monoxide, nitrogen dioxide, sulfur dioxide, and hydrocarbons. In large car-dependent cities such as Los Angeles and Mexico City, 80% to 85% of air pollution is from motor vehicles (Figure 1.1). Ozone, carbon monoxide, and nitrous oxides come from vehicle exhaust. Auto exhaust like this means that the fuels is not burning efficiently. A few pollutants come primarily from power plants or industrial plants that burn coal or oil. Sulfur dioxide (SO2 ) is a major component of industrial air pollution that is released whenever coal and petroleum are burned. SO2 mixes with H2 O in the air to produce sulfuric acid (H2 SO4 ). Mercury is released when coal and some types of wastes are burned. Mercury is emitted as a gas, but as it cools, it becomes a droplet. Mercury droplets eventually fall to the ground. If they fall into sediments, bacteria convert them to the most dangerous form of mercury: methyl mercury. Highly toxic, methyl mercury is one of the metals organic forms. "
causes of air pollution,T_0969,"Fossil fuels are ancient plants and animals that have been converted into usable hydrocarbons. Burning plant and animal material directly also produces pollutants. Biomass is the total amount of living material found in an environment. The biomass of a rainforest is the amount of living material found in that rainforest. The primary way biomass is burned is for slash-and-burn agriculture (Figure 1.2). The rainforest is slashed down and then the waste is burned to clear the land for farming. Biomass from other biomes, such as the savannah, is also burned to clear farmland. The pollutants are much the same as from burning fossil fuels: CO2 , carbon monoxide, methane, particulates, nitrous oxide, hydrocarbons, and organic and elemental carbon. Burning forests increases greenhouse gases in the atmosphere by releasing the CO2 stored in the biomass and also by removing the forest so that it cannot store CO2 in the future. As with all forms of air pollution, the smoke from biomass burning often spreads far and pollutants can plague neighboring states or countries. Particulates result when anything is burned. About 40% of the particulates that enter the atmosphere above the United States are from industry and about 17% are from vehicles. Particulates also occur naturally from volcanic eruptions or windblown dust. Like other pollutants, they travel all around the world on atmospheric currents. "
causes of air pollution,T_0970,"Volatile organic compounds (VOCs) enter the atmosphere by evaporation. VOCs evaporate from human-made substances, such as paint thinners, dry cleaning solvents, petroleum, wood preservatives, and other liquids. Naturally occurring VOCs evaporate off of pine and citrus trees. The atmosphere contains tens of thousands of different VOCs, A forest that has been slash-and-burned to make new farmland. nearly 100 of which are monitored. The most common is methane, a greenhouse gas (Figure 1.3). Methane occurs naturally, but human agriculture is increasing the amount of methane in the atmosphere. Methane forms when organic material decomposes in an oxygen-poor environment. In the top image, surface methane production is shown. Stratospheric methane concentrations in the bottom image show that methane is carried up into the stratosphere by the upward flow of air in the tropics. "
characteristics and origins of life,T_0975,"No one knows how or when life first began on the turbulent early Earth. There is little hard evidence from so long ago. Scientists think that it is extremely likely that life began and was wiped out more than once; for example, by the impact that created the Moon. This issue of whats living and whats not becomes important when talking about the origin of life. If were going to know when a blob of organic material crossed over into being alive, we need to have a definition of life. "
characteristics and origins of life,T_0976,"To be considered alive a molecule must: be organic. The organic molecules needed are amino acids, the building blocks of life. have a metabolism. be capable of replication (be able to reproduce). "
characteristics and origins of life,T_0977,"To look for information regarding the origin of life, scientists: perform experiments to recreate the environmental conditions found at that time. study the living creatures that make their homes in the types of extreme environments that were typical in Earths early days. seek traces of life left by ancient microorganisms, also called microbes, such as microscopic features or isotopic ratios indicative of life. Any traces of life from this time period are so ancient it is difficult to be certain whether they originated by biological or non-biological means. Click image to the left or use the URL below. URL: "
characteristics and origins of life,T_0978,"Amino acids are the building blocks of life because they create proteins. To form proteins, the amino acids are linked together by covalent bonds to form polymers called polypeptide chains (Figure 1.1). These chains are arranged in a specific order to form each different type of protein. Proteins are the most abundant class of biological molecules. An important question facing scientists is where the first amino acids came from: did they originate on Earth or did they fly in from outer space? No matter where they originated, the creation of amino acids requires the right starting materials and some energy. "
characteristics and origins of life,T_0979,"To see if amino acids could originate in the environment thought to be present in the first years of Earths existence, Stanley Miller and Harold Urey performed a famous experiment in 1953. To simulate the early atmosphere they Amino acids form polypeptide chains. The setup of the Miller-Urey experiment. placed hydrogen, methane, and ammonia in a flask of heated water that created water vapor, which they called the primordial soup. Sparks simulated lightning, which the scientists thought could have been the energy that drove the chemical reactions that created the amino acids. It worked! The gases combined to form water-soluble organic compounds including amino acids. Amino acids might also have originated at hydrothermal vents or deep in the crust where Earths internal heat is the energy source. Meteorites containing amino acids currently enter the Earth system and so meteorites could have delivered amino acids to the planet from elsewhere in the solar system (where they would have formed by processes similar to those outlined here). "
chemical bonding,T_0980,"Ions come together to create a molecule so that electrical charges are balanced; the positive charges balance the negative charges and the molecule has no electrical charge. To balance electrical charge, an atom may share its electron with another atom, give it away, or receive an electron from another atom. The joining of ions to make molecules is called chemical bonding. There are three main types of chemical bonds that are important in our discussion of minerals and rocks: Ionic bond: Electrons are transferred between atoms. An ion will give one or more electrons to another ion. Table salt, sodium chloride (NaCl), is a common example of an ionic compound. Note that sodium is on the left side of the periodic table and that chlorine is on the right side of the periodic table. In the Figure 1.2, an atom of lithium donates an electron to an atom of fluorine to form an ionic compound. The transfer of the electron gives the lithium ion a net charge of +1, and the fluorine ion a net charge of -1. These ions bond because they experience an attractive force due to the difference in sign of their charges. Covalent bond : In a covalent bond, an atom shares one or more electrons with another atom. Periodic Table of the Elements. Lithium (left) and fluorine (right) form an ionic compound called lithium fluoride. In the picture of methane (CH4 ) below (Figure 1.3), the carbon ion (with a net charge of +4) shares a single electron from each of the the four hydrogens. Covalent bonding is prevalent in organic compounds. In fact, your body is held together by electrons shared by carbons and hydrogens! Covalent bonds are also very strong, meaning it takes a lot of energy to break them apart. Hydrogen bond: These weak, intermolecular bonds are formed when the positive side of one polar molecule is attracted to the negative side of another polar molecule. Water is a classic example of a polar molecule because it has a slightly positive side, and a slightly negative side. In fact, this property is why water is so good at dissolving things. The positive side of the molecule is attracted to Methane is formed when four hydrogens and one carbon covalently bond. negative ions and the negative side is attracted to positive ions. "
cleaning up groundwater,T_0989,"Preventing groundwater contamination is much easier and cheaper than cleaning it. To clean groundwater, the water, as well as the rock and soil through which it travels, must be cleansed. Thoroughly cleaning an aquifer would require cleansing each pore within the soil or rock unit. For this reason, cleaning polluted groundwater is very costly, takes years, and is sometimes not technically feasible. If the toxic materials can be removed from the aquifer, disposing of them is another challenge. "
cleaning up groundwater,T_0991,"If the source is an underground tank, the tank will be pumped dry and then dug out from the ground. If the source is a factory that is releasing toxic chemicals that are ending up in the groundwater, the factory may be required to stop the discharge. "
cleaning up groundwater,T_0992,"Hydrologists must determine how far, in what direction, and how rapidly the plume is moving. They must determine the concentration of the contaminant to determine how much it is being diluted. The scientists will use existing wells and may drill test wells to check for concentrations and monitor the movement of the plume. "
cleaning up groundwater,T_0993,"Using the well data, the hydrologist uses a computer program with information on the permeability of the aquifer and the direction and rate of groundwater flow, then models the plume to predict the dispersal of the contaminant through the aquifer. Drilling test wells to monitor pollution is expensive. "
cleaning up groundwater,T_0994,"First, an underground barrier is constructed to isolate the contaminated groundwater from the rest of the aquifer. Next, the contaminated groundwater may be treated in place. Bioremediation is relatively inexpensive. Bioengineered microorganisms are injected into the contaminant plume and allowed to consume the pollutant. Air may be pumped into the polluted region to encourage the growth and reproduction of the microbes. With chemical remediation, a chemical is pumped into the aquifer so the contaminant is destroyed. Acids or bases can neutralize contaminants or cause pollutants to precipitate from the water. The most difficult and expensive option is for reclamation teams to pump the water to the surface, cleanse it using chemical or biological methods, then re-inject it into the aquifer. The contaminated portions of the aquifer must be dug up and the pollutant destroyed by incinerating or chemically processing the soil, which is then returned to the ground. This technique is often prohibitively expensive and is done only in extreme cases. Click image to the left or use the URL below. URL: "
climate change in earth history,T_0995,"Climate has changed throughout Earth history. Much of the time Earths climate was hotter and more humid than it is today, but climate has also been colder, as when glaciers covered much more of the planet. The most recent ice ages were in the Pleistocene Epoch, between 1.8 million and 10,000 years ago (Figure 1.1). Glaciers advanced and retreated in cycles, known as glacial and interglacial periods. With so much of the worlds water bound into the ice, sea level was about 125 meters (395 feet) lower than it is today. Many scientists think that we are now in a warm, interglacial period that has lasted about 10,000 years. For the past 1500 years, climate has been relatively mild and stable when compared with much of Earths history. Why has climate stability been beneficial for human civilization? Stability has allowed the expansion of agriculture and the development of towns and cities. Fairly small temperature changes can have major effects on global climate. The average global temperature during glacial periods was only about 5.5o C (10o F) less than Earths current average temperature. Temperatures during the interglacial periods were about 1.1o C (2.0o F) higher than today (Figure 1.2). The maximum extent of Northern Hemi- sphere glaciers during the Pleistocene epoch. Since the end of the Pleistocene, the global average temperature has risen about 4o C (7o F). Glaciers are retreating and sea level is rising. While climate is getting steadily warmer, there have been a few more extreme warm and cool times in the last 10,000 years. Changes in climate have had effects on human civilization. The Medieval Warm Period from 900 to 1300 A.D. allowed Vikings to colonize Greenland and Great Britain to grow wine grapes. The Little Ice Age, from the 14th to 19th centuries, the Vikings were forced out of Greenland and humans had to plant crops further south. The graph is a compilation of 5 recon- structions (the green line is the mean of the five records) of mean temperature changes. This illustrates the high tem- peratures of the Medieval Warm Period, the lows of the Little Ice Age, and the very high (and climbing) temperature of this decade. Click image to the left or use the URL below. URL: "
climate zones and biomes,T_0996,"The major factors that influence climate determine the different climate zones. In general, the same type of climate zone will be found at similar latitudes and in similar positions on nearly all continents, both in the Northern and Southern Hemispheres. The exceptions to this pattern are the climate zones called the continental climates, which are not found at higher latitudes in the Southern Hemisphere. This is because the Southern Hemisphere land masses are not wide enough to produce a continental climate. "
climate zones and biomes,T_0997,"Climate zones are classified by the Kppen classification system. This system is based on the temperature, the amount of precipitation, and the times of year when precipitation occurs. Since climate determines the type of vegetation that grows in an area, vegetation is used as an indicator of climate type. "
climate zones and biomes,T_0998,"A climate type and its plants and animals make up a biome. The organisms of a biome share certain characteristics around the world, because their environment has similar advantages and challenges. The organisms have adapted to that environment in similar ways over time. For example, different species of cactus live on different continents, but they have adapted to the harsh desert in similar ways. Click image to the left or use the URL below. URL: "
climate zones and biomes,T_0999,"The Kppen classification system recognizes five major climate groups. Each group is divided into subcategories. Some of these subcategories are forest, monsoon, and wet/dry types, based on the amount of precipitation and season when that precipitation occurs (Figure 1.1). This world map of the Kppen classification system indicates where the climate zones and major biomes are located. "
climate zones and biomes,T_1000,"Tropical moist climates are found in a band about 15o to 25o N and S of the Equator (Figure 1.1). Temperature: Intense sunshine. Each month has an average temperature of at least 18o C (64o F). Rainfall: Abundant, at least 150 cm (59 inches) per year. The main vegetation for this climate is the tropical rainforest. "
climate zones and biomes,T_1001,Dry climates have less precipitation than evaporation. Temperature: Abundant sunshine. Summer temperatures are high; winters are cooler and longer than in tropical moist climates. Rainfall: Irregular; several years of drought are often followed by a single year of abundant rainfall. Dry climates cover about 26% of the worlds land area. Low latitude deserts are found at the Ferrell cell high pressure zone. Higher latitude deserts occur within continents or in rainshadows. Vegetation is sparse but well adapted to the dry conditions. 
climate zones and biomes,T_1002,"Moist subtropical mid-latitude climates are found along the coastal areas in the United States. Temperature: The coldest month ranges from just below freezing to almost balmy, between -3o C and 18o C (27o to 64o F). Summers are mild, with average temperatures above 10o C (50o F). Seasons are distinct. Rainfall: There is plentiful annual rainfall. "
climate zones and biomes,T_1003,"Continental climates are found in most of the North American interior from about 40 N to 70 N. Temperature: The average temperature of the warmest month is higher than 10 C (50 F) and the coldest month is below -3 C (27 F). Precipitation: Winters are cold and stormy (look at the latitude of this zone and see if you can figure out why). Snowfall is common and snow stays on the ground for long periods of time. Trees grow in continental climates, even though winters are extremely cold, because the average annual temperature is fairly mild. Continental climates are not found in the Southern Hemisphere because of the absence of a continent large enough to generate this effect. "
climate zones and biomes,T_1004,"Polar climates are found across the continents that border the Arctic Ocean, Greenland, and Antarctica. Temperature: Winters are entirely dark and bitterly cold. Summer days are long, but the Sun is low on the horizon so summers are cool. The average temperature of the warmest month is less than 10o C (50o F). The annual temperature range is large. Precipitation: The region is dry, with less than 25 cm (10 inches) of precipitation annually; most precipitation occurs during the summer. "
climate zones and biomes,T_1005,"When climate conditions in a small area are different from those of the surroundings, the climate of the small area is called a microclimate. The microclimate of a valley may be cool relative to its surroundings since cold air sinks. The ground surface may be hotter or colder than the air a few feet above it, because rock and soil gain and lose heat readily. Different sides of a mountain will have different microclimates. In the Northern Hemisphere, a south-facing slope receives more solar energy than a north-facing slope, so each side supports different amounts and types of vegetation. Altitude mimics latitude in climate zones. Climates and biomes typical of higher latitudes may be found in other areas of the world at high altitudes. Click image to the left or use the URL below. URL: "
coal power,T_1012,"Coal, a solid fossil fuel formed from the partially decomposed remains of ancient forests, is burned primarily to produce electricity. Coal use is undergoing enormous growth as the availability of oil and natural gas decreases and cost increases. This increase in coal use is happening particularly in developing nations, such as China, where coal is cheap and plentiful. Coal is black or brownish-black. The most common form of coal is bituminous, a sedimentary rock that contains impurities such as sulfur (Figure 1.1). Anthracite coal has been metamorphosed and is nearly all carbon. For this reason, anthracite coal burns more cleanly than bituminous coal. "
coal power,T_1013,"Coal forms from dead plants that settled at the bottom of ancient swamps. Lush coal swamps were common in the tropics during the Carboniferous period, which took place more than 300 million years ago (Figure 1.2). The climate was warmer then. Mud and other dead plants buried the organic material in the swamp, and burial kept oxygen away. When plants are buried without oxygen, the organic material can be preserved or fossilized. Sand and clay settling on top of the decaying plants squeezed out the water and other substances. Millions of years later, what remains is a carbon- containing rock that we know as coal. "
coal power,T_1014,"Around the world, coal is the largest source of energy for electricity. The United States is rich in coal (Figure 1.3). California once had a number of small coal mines, but the state no longer produces coal. To turn coal into electricity, the rock is crushed into powder, which is then burned in a furnace that has a boiler. Like other fuels, coal releases its energy as heat when it burns. Heat from the burning coal boils the water in the boiler to make steam. The steam spins turbines, which turn generators to create electricity. In this way, the energy stored in the coal is converted to useful energy like electricity. "
coal power,T_1015,"For coal to be used as an energy source, it must first be mined. Coal mining occurs at the surface or underground by methods that are described in the the chapter Materials of Earths Crust (Figure 1.4). Mining, especially underground The location of the continents during the Carboniferous period. Notice that quite a lot of land area is in the region of the tropics. mining, can be dangerous. In April 2010, 29 miners were killed at a West Virginia coal mine when gas that had accumulated in the mine tunnels exploded and started a fire. Coal mining exposes minerals and rocks from underground to air and water at the surface. Many of these minerals contain the element sulfur, which mixes with air and water to make sulfuric acid, a highly corrosive chemical. If the sulfuric acid gets into streams, it can kill fish, plants, and animals that live in or near the water. Click image to the left or use the URL below. URL: "
coastal pollution,T_1016,"Most ocean pollution comes as runoff from land and originates as agricultural, industrial, and municipal wastes (Figure 1.1). The remaining 20% of water pollution enters the ocean directly from oil spills and people dumping wastes directly into the water. Ships at sea empty their wastes directly into the ocean, for example. Coastal pollution can make coastal water unsafe for humans and wildlife. After rainfall, there can be enough runoff pollution that beaches must be closed to prevent the spread of disease from pollutants. A surprising number of beaches are closed because of possible health hazards each year. A large proportion of the fish we rely on for food live in the coastal wetlands or lay their eggs there. Coastal runoff from farm waste often carries water-borne organisms that cause lesions that kill fish. Humans who come in In some areas of the world, ocean pollution is all too obvious. contact with polluted waters and affected fish can also experience harmful symptoms. More than one-third of the shellfish-growing waters of the United States are adversely affected by coastal pollution. "
coastal pollution,T_1017,"Fertilizers that run off of lawns and farm fields are extremely harmful to the environment. Nutrients, such as nitrates, in the fertilizer promote algae growth in the water they flow into. With the excess nutrients, lakes, rivers, and bays become clogged with algae and aquatic plants. Eventually these organisms die and decompose. Decomposition uses up all the dissolved oxygen in the water. Without oxygen, large numbers of plants, fish, and bottom-dwelling animals die. Every year dead zones appear in lakes and nearshore waters. A dead zone is an area of hundreds of kilometers of ocean without fish or plant life. The Mississippi is not the only river that carries the nutrients necessary to cause a dead zone. Rivers that drain regions where human population density is high and where crops are grown create dead zones all over the world (Figure 1.2). "
comets,T_1025,"Comets are small, icy objects that have very elliptical orbits around the Sun. Their orbits carry them from the outer solar system to the inner solar system, close to the Sun. Early in Earths history, comets may have brought water and other substances to Earth during collisions. Comet tails form the outer layers of ice melt and evaporate as the comet flies close to the Sun. The ice from the comet vaporizes and forms a glowing coma, which reflects light from the Sun. Radiation and particles streaming from the Sun push this gas and dust into a long tail that always points away from the Sun (Figure 1.1). Comets appear for only a short time when they are near the Sun, then seem to disappear again as they move back to the outer solar system. Comet Hale-Bopp, also called the Great Comet of 1997, shone brightly for several months in 1997. The comet has two visible tails: a bright, curved dust tail and a fainter, straight tail of ions (charged atoms) pointing directly away from the Sun. The time between one appearance of a comet and the next is called the comets period. Halleys comet, with a period of 75 years, will next be seen in 2061. The first mention of the comet in historical records may go back as much as two millennia. "
comets,T_1026,"Short-period comets, with periods of about 200 years or less, come from a region beyond the orbit of Neptune called the Kuiper belt (pronounced KI-per). It contains not only comets, but also asteroids and at least two dwarf planets. Comets with periods as long as thousands or even millions of years come from a very distant region of the solar system called the Oort cloud, about 50,000  100,000 AU from the Sun (50,000 - 100,000 times the distance from the Sun to Earth). Click image to the left or use the URL below. URL: "
conserving water,T_1032,"Water consumption per person has been going down for the past few decades. There are many ways that water conservation can be encouraged. Charging more for water gives a financial incentive for careful water use. Water use may be restricted by time of day, season, or activity. Good behavior can be encouraged; for example, people can be given an incentive to replace grass with desert plants in arid regions. "
conserving water,T_1033,"As human population growth continues, water conservation will become increasingly important globally, especially in developed countries where people use an enormous amount of water. What are some of the ways you can conserve water in and around your home? Avoid polluting water so that less is needed. Convert to more efficient irrigation methods on farms and in gardens. Reduce household demand by installing water-saving devices such as low-flow shower heads and toilets. Reduce personal demand by turning off the tap when water is not being used and taking shorter showers. Engage in water-saving practices: for instance, water lawns less and sweep rather than hose down sidewalks. At Earth Summit 2002, many governments approved a Plan of Action to address the scarcity of water and safe drinking water in developing countries. One goal of this plan was to cut in half the number of people without access to safe drinking water by 2015. Although this is a very important goal, it will not be met. Goals like these are made more difficult as population continues to grow. This colorful adobe house in Tucson, Arizona is surrounded by native cactus, which needs little water to thrive. Click image to the left or use the URL below. URL: "
continental drift,T_1034,"Alfred Wegener, born in 1880, was a meteorologist and explorer. In 1911, Wegener found a scientific paper that listed identical plant and animal fossils on opposite sides of the Atlantic Ocean. Intrigued, he then searched for and found other cases of identical fossils on opposite sides of oceans. The explanation put out by the scientists of the day was that land bridges had once stretched between these continents. Instead, Wegener pondered the way Africa and South America appeared to fit together like puzzle pieces. Other scientists had suggested that Africa and South America had once been joined, but Wegener was the ideas most dogged supporter. Wegener amassed a tremendous amount of evidence to support his hypothesis that the continents had once been joined. Imagine that youre Wegeners colleague. What sort of evidence would you look for to see if the continents had actually been joined and had moved apart? "
continental drift,T_1035,"Here is the main evidence that Wegener and his supporters collected for the continental drift hypothesis: The continents appear to fit together. Ancient fossils of the same species of extinct plants and animals are found in rocks of the same age but are on continents that are now widely separated (Figure 1.1). Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. His critics suggested that the organisms moved over long-gone land bridges, but Wegener thought that the organisms could not have been able to travel across the oceans. Fossils of the seed fern Glossopteris were too heavy to be carried so far by wind. Mesosaurus was a swimming reptile, but could only swim in fresh water. Cynognathus and Lystrosaurus were land reptiles and were unable to swim. Wegener used fossil evidence to support his continental drift hypothesis. The fos- sils of these organisms are found on lands that are now far apart. Identical rocks, of the same type and age, are found on both sides of the Atlantic Ocean. Wegener said the rocks had formed side by side and that the land had since moved apart. Mountain ranges with the same rock types, structures, and ages are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway (Figure 1.2). Wegener concluded that they formed as a single mountain range that was separated as the continents drifted. Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the Equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today, glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on. The similarities between the Appalachian and the eastern Greenland mountain ranges are evidences for the continental drift hypothesis. Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later drifted to new locations on the continents. Wegener thought that mountains formed as continents ran into each other. This got around the problem of the leading hypothesis of the day, which was that Earth had been a molten ball that bulked up in spots as it cooled (the problem with this idea was that the mountains should all be the same age and they were known not to be). Click image to the left or use the URL below. URL: "
coriolis effect,T_1036,"The Coriolis effect describes how Earths rotation steers winds and surface ocean currents (Figure 1.1). Coriolis causes freely moving objects to appear to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The objects themselves are actually moving straight, but the Earth is rotating beneath them, so they seem to bend or curve. Thats why it is incorrect to call Coriolis a force. It is not forcing anything to happen! An example might make the Coriolis effect easier to visualize. If an airplane flies 500 miles due north, it will not arrive at the city that was due north of it when it began its journey. Over the time it takes for the airplane to fly 500 miles, that city moved, along with the Earth it sits on. The airplane will therefore arrive at a city to the west of the original city (in the Northern Hemisphere), unless the pilot has compensated for the change. So to reach his intended destination, the pilot must also veer right while flying north. As wind or an ocean current moves, the Earth spins underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve instead of in a straight line. Wind or water that travels toward the poles from the Equator is deflected to the east, while wind or water that travels toward the Equator from the poles gets bent to the west. The Coriolis effect bends the direction of surface currents to the right in the Northern Hemisphere and left in the Southern Hemisphere. The Coriolis effect causes winds and cur- rents to form circular patterns. The di- rection that they spin depends on the hemisphere that they are in. Coriolis effect is demonstrated using a metal ball and a rotating plate in this video. The ball moves in a circular path just like a freely moving particle of gas or liquid moves on the rotating Earth (5b). Click image to the left or use the URL below. URL: "
correlation using relative ages,T_1037,Superposition and cross-cutting are helpful when rocks are touching one another and lateral continuity helps match up rock layers that are nearby. To match up rocks that are further apart we need the process of correlation. How do geologists correlate rock layers that are separated by greater distances? There are three kinds of clues: 
correlation using relative ages,T_1038,1. Distinctive rock formations may be recognizable across large regions (Figure 1.1). 
correlation using relative ages,T_1039,"2. Two separated rock units with the same index fossil are of very similar age. What traits do you think an index fossil should have? To become an index fossil the organism must have (1) been widespread so that it is useful for identifying rock layers over large areas and (2) existed for a relatively brief period of time so that the approximate age of the rock layer is immediately known. Many fossils may qualify as index fossils (Figure below). Ammonites, trilobites, and graptolites are often used as index fossils. Microfossils, which are fossils of microscopic organisms, are also useful index fossils. Fossils of animals that drifted in the upper layers of the ocean are particularly useful as index fossils, since they may be distributed over very large areas. A biostratigraphic unit, or biozone, is a geological rock layer that is defined by a single index fossil or a fossil assemblage. A biozone can also be used to identify rock layers across distances. The famous White Cliffs of Dover in southwest England can be matched to similar white cliffs in Denmark and Germany. "
correlation using relative ages,T_1040,"3. A key bed can be used like an index fossil since a key bed is a distinctive layer of rock that can be recognized across a large area. A volcanic ash unit could be a good key bed. One famous key bed is the clay layer at the boundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct (Figure in asteroids. In 1980, the father-son team of Luis and Walter Alvarez proposed that a huge asteroid struck Earth 66 million years ago and caused the mass extinction. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
deep ocean currents,T_1044,"Thermohaline circulation drives deep ocean circulation. Thermo means heat and haline refers to salinity. Dif- ferences in temperature and in salinity change the density of seawater. So thermohaline circulation is the result of density differences in water masses because of their different temperature and salinity. What is the temperature and salinity of very dense water? Lower temperature and higher salinity yield the densest water. When a volume of water is cooled, the molecules move less vigorously, so same number of molecules takes up less space and the water is denser. If salt is added to a volume of water, there are more molecules in the same volume, so the water is denser. "
deep ocean currents,T_1045,"Changes in temperature and salinity of seawater take place at the surface. Water becomes dense near the poles. Cold polar air cools the water and lowers its temperature, increasing its salinity. Fresh water freezes out of seawater to become sea ice, which also increases the salinity of the remaining water. This very cold, very saline water is very dense and sinks. This sinking is called downwelling. This video lecture discusses the vertical distribution of life in the oceans. Seawater density creates currents, which provide different habitats for different creatures: Click image to the left or use the URL below. URL:  Two things then happen. The dense water pushes deeper water out of its way and that water moves along the bottom of the ocean. This deep water mixes with less dense water as it flows. Surface currents move water into the space vacated at the surface where the dense water sank (Figure 1.1). Water also sinks into the deep ocean off of Antarctica. Cold water (blue lines) sinks in the North Atlantic, flows along the bottom of the ocean and upwells in the Pacific or Indian. The water then travels in surface currents (red lines) back to the North Atlantic. Deep water also forms off of Antarctica. "
deep ocean currents,T_1046,"Since unlimited amounts of water cannot sink to the bottom of the ocean, water must rise from the deep ocean to the surface somewhere. This process is called upwelling (Figure 1.2). Upwelling forces denser water from below to take the place of less dense water at the surface that is pushed away by the wind. Generally, upwelling occurs along the coast when wind blows water strongly away from the shore. This leaves a void that is filled by deep water that rises to the surface. Upwelling is extremely important where it occurs. During its time on the bottom, the cold deep water has collected nutrients that have fallen down through the water column. Upwelling brings those nutrients to the surface. Those nutrients support the growth of plankton and form the base of a rich ecosystem. California, South America, South Africa, and the Arabian Sea all benefit from offshore upwelling. Upwelling also takes place along the Equator between the North and South Equatorial Currents. Winds blow the surface water north and south of the Equator, so deep water undergoes upwelling. The nutrients rise to the surface and support a great deal of life in the equatorial oceans. Click image to the left or use the URL below. URL: "
determining relative ages,T_1047,"Stenos and Smiths principles are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, (1) Do you know which is older and which is younger? (2) Do you know how old the layers are in years? In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. Can you figure out what happened in what order in (Figure 1.1)? Write it down and then check the following paragraphs. The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault cuts through all three sedimentary rock layers (A, B, and C) and also the intrusion (D). So the fault must be the youngest feature. The intrusion (D) cuts through the three sedimentary rock layers, so it must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is still younger. The full sequence of events is: 1. Layer C formed. 2. Layer B formed. A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), fault (E). 3. Layer A formed. 4. After layers A-B-C were present, intrusion D cut across all three. 5. Fault E formed, shifting rocks A through C and intrusion D. 6. Weathering and erosion created a layer of soil on top of layer A. Click image to the left or use the URL below. URL: "
development of hypotheses,T_1048,"Before we develop some hypotheses, lets find a new question that we want to answer. What we just learned that atmospheric CO2 has been increasing at least since 1958. This leads us to ask this question: Why is atmospheric CO2 increasing? "
development of hypotheses,T_1049,"We do some background research to find the possible sources of carbon dioxide into the atmosphere. We discover two things: Carbon dioxide is released into the atmosphere by volcanoes when they erupt. Carbon dioxide is released when fossil fuels are burned. A hypothesis is a reasonable explanation to explain a small range of phenomena. A hypothesis is limited in scope, explaining a single event or a fact. A hypothesis must be testable and falsifiable. We must be able to test it and it must be possible to show that it is wrong. From these two facts we can create two hypotheses. We will have multiple working hypotheses. We can test each of these hypotheses. "
development of hypotheses,T_1050,"Atmospheric CO2 has increased over the past five decades, because the amount of CO2 gas released by volcanoes has increased. "
development of hypotheses,T_1051,"The increase in atmospheric CO2 is due to the increase in the amount of fossil fuels that are being burned. Usually, testing a hypothesis requires making observations or performing experiments. In this case, we will look into the scientific literature to see if we can support or refute either or both of these hypotheses. Click image to the left or use the URL below. URL: "
distance between stars,T_1054,"Distances to stars that are relatively close to us can be measured using parallax. Parallax is an apparent shift in position that takes place when the position of the observer changes. To see an example of parallax, try holding your finger about 1 foot (30 cm) in front of your eyes. Now, while focusing on your finger, close one eye and then the other. Alternate back and forth between eyes, and pay attention to how your finger appears to move. The shift in position of your finger is an example of parallax. Now try moving your finger closer to your eyes, and repeat the experiment. Do you notice any difference? The closer your finger is to your eyes, the greater the position changes because of parallax. As Figure 1.1 shows, astronomers use this same principle to measure the distance to stars. Instead of a finger, they focus on a star, and instead of switching back and forth between eyes, they switch between the biggest possible differences in observing position. To do this, an astronomer first looks at the star from one position and notes where the star is relative to more distant stars. Now where will the astronomer go to make an observation the greatest possible distance from the first observation? In six months, after Earth moves from one side of its orbit around the Sun to the other side, the astronomer looks at the star again. This time parallax causes the star to appear in a different position relative to more distant stars. From the size of this shift, astronomers can calculate the distance to the star. "
distance between stars,T_1055,"Even with the most precise instruments available, parallax is too small to measure the distance to stars that are more than a few hundred light years away. For these more distant stars, astronomers must use more indirect methods of determining distance. Most of these methods involve determining how bright the star they are looking at really is. For example, if the star has properties similar to the Sun, then it should be about as bright as the Sun. The astronomer compares the observed brightness to the expected brightness. "
distribution of water on earth,T_1056,"Earths oceans contain 97% of the planets water. That leaves just 3% as fresh water, water with low concentrations of salts (Figure 1.1). Most fresh water is trapped as ice in the vast glaciers and ice sheets of Greenland and Antarctica. How is the 3% of fresh water divided into different reservoirs? How much of that water is useful for living creatures? How much for people? A storage location for water such as an ocean, glacier, pond, or even the atmosphere is known as a reservoir. A water molecule may pass through a reservoir very quickly or may remain for much longer. The amount of time a molecule stays in a reservoir is known as its residence time. The distribution of Earths water. Click image to the left or use the URL below. URL: "
dwarf planets,T_1062,"In 2006, the International Astronomical Union decided that there were too many questions surrounding what could be called a planet, and so refined the definition of a planet. According to the new definition, a planet must: Orbit a star. Be big enough that its own gravity causes it to be shaped as a sphere. Be small enough that it isnt a star itself. Have cleared the area of its orbit of smaller objects. "
dwarf planets,T_1063,"The dwarf planets of our solar system are exciting proof of how much we are learning about our solar system. With the discovery of many new objects in our solar system, astronomers refined the definition of a dwarf planet in 2006. According to the IAU, a dwarf planet must: Orbit a star. Have enough mass to be nearly spherical. Not have cleared the area around its orbit of smaller objects. Not be a moon. "
dwarf planets,T_1064,"The reclassification of Pluto to the new category dwarf planet stirred up a great deal of controversy. How the classification of Pluto has evolved is an interesting story in science. From the time it was discovered in 1930 until the early 2000s, Pluto was considered the ninth planet. When astronomers first located Pluto, the telescopes were not as good, so Pluto and its moon, Charon, were seen as one much larger object (Figure 1.1). With better telescopes, astronomers realized that Pluto was much smaller than they had thought. Pluto and its moon, Charon, are actually two objects. Better technology also allowed astronomers to discover many smaller objects like Pluto that orbit the Sun. One of them, Eris, discovered in 2005, is even larger than Pluto. Even when it was considered a planet, Pluto was an oddball. Unlike the other outer planets in the solar system, which are all gas giants, it is small, icy, and rocky. With a diameter of about 2,400 km, it is only about one-fifth the mass of Earths Moon. Plutos orbit is tilted relative to the other planets and is shaped like a long, narrow ellipse. Plutos orbit sometimes even passes inside Neptunes orbit. From what youve read above, do you think Pluto should be called a planet? Why are people hesitant to take away Plutos planetary status? Is Pluto a dwarf planet? Pluto has three moons of its own. The largest, Charon, is big enough that the Pluto-Charon system is sometimes considered to be a double dwarf planet (Figure 1.1). Two smaller moons, Nix and Hydra, were discovered in 2005. But having moons is not enough to make an object a planet. Pluto and the other dwarf planets, besides Ceres, are found orbiting out beyond Neptune. Click image to the left or use the URL below. URL: "
dwarf planets,T_1065,"Ceres is by far the closest dwarf planet to the Sun; it resides between Mars and Jupiter. Ceres is the largest object in the asteroid belt (Figure 1.2). Before 2006, Ceres was considered the largest of the asteroids, with only about 1.3% of the mass of the Earths Moon. But unlike the asteroids, Ceres has enough mass that its gravity causes it to be shaped like a sphere. Like Pluto, Ceres is rocky. Is Ceres a planet? How does it match the criteria above? Ceres orbits the Sun, is round, and is not a moon. As part of the asteroid belt, its orbit is full of other smaller bodies, so Ceres fails the fourth criterion for being a planet. "
dwarf planets,T_1066,"Makemake is the third largest and second brightest dwarf planet we have discovered so far (Figure 1.3). With a diameter estimated to be between 1,300 and 1,900 km, it is about three-quarters the size of Pluto. Makemake orbits the Sun in 310 years at a distance between 38.5 to 53 AU. It is thought to be made of methane, ethane, and nitrogen ices. Largest Known Trans-Neptunian Objects. Makemake is named after the deity that created humanity in the mythology of the people of Easter Island. "
dwarf planets,T_1067,"Eris is the largest known dwarf planet in the solar system  it has about 27% more mass than Pluto (Figure 1.3). The object was not discovered until 2003 because it is about three times farther from the Sun than Pluto, and almost 100 times farther from the Sun than Earth is. For a short time Eris was considered the tenth planet in the solar system, but its discovery helped to prompt astronomers to better define planets and dwarf planets in 2006. Eris also has a small moon, Dysnomia, that orbits it once about every 16 days. Astronomers know there may be other dwarf planets in the outer reaches of the solar system. Haumea was made a dwarf planet in 2008, so the total number of dwarf planets is now five. Quaoar, Varuna, and Orcus may be added to the list of dwarf planets in the future. We still have a lot to discover and explore. Click image to the left or use the URL below. URL: "
early atmosphere and oceans,T_1068,"Earths first atmosphere was made of hydrogen and helium, the gases that were common in this region of the solar system as it was forming. Most of these gases were drawn into the center of the solar nebula to form the Sun. When Earth was new and very small, the solar wind blew off atmospheric gases that collected. If gases did collect, they were vaporized by impacts, especially from the impact that brought about the formation of the Moon. Eventually things started to settle down and gases began to collect. High heat in Earths early days meant that there were constant volcanic eruptions, which released gases from the mantle into the atmosphere (see opening image). Just as today, volcanic outgassing was a source of water vapor, carbon dioxide, small amounts of nitrogen, and other gases. Scientists have calculated that the amount of gas that collected to form the early atmosphere could not have come entirely from volcanic eruptions. Frequent impacts by asteroids and comets brought in gases and ices, including water, carbon dioxide, methane, ammonia, nitrogen, and other volatiles from elsewhere in the solar system (Figure Calculations also show that asteroids and comets cannot be responsible for all of the gases of the early atmosphere, so both impacts and outgassing were needed. "
early atmosphere and oceans,T_1069,"The second atmosphere, which was the first to stay with the planet, formed from volcanic outgassing and comet ices. This atmosphere had lots of water vapor, carbon dioxide, nitrogen, and methane but almost no oxygen. Why was there so little oxygen? Plants produce oxygen when they photosynthesize but life had not yet begun or had not yet developed photosynthesis. In the early atmosphere, oxygen only appeared when sunlight split water molecules into hydrogen and oxygen and the oxygen accumulated in the atmosphere. Without oxygen, life was restricted to tiny simple organisms. Why is oxygen essential for most life on Earth? 1. Oxygen is needed to make ozone, a molecule made of three oxygen ions, O3 . Ozone collects in the atmospheric ozone layer and blocks harmful ultraviolet radiation from the Sun. Without an ozone layer, life in the early Earth was almost impossible. 2. Animals need oxygen to breathe. No animals would have been able to breathe in Earths early atmosphere. "
early atmosphere and oceans,T_1070,"The early atmosphere was rich in water vapor from volcanic eruptions and comets. When Earth was cool enough, water vapor condensed and rain began to fall. The water cycle began. Over millions of years enough precipitation collected that the first oceans could have formed as early as 4.2 to 4.4 billion years ago. Dissolved minerals carried by stream runoff made the early oceans salty. What geological evidence could there be for the presence of an early ocean? Marine sedimentary rocks can be dated back about 4 billion years. By the Archean, the planet was covered with oceans and the atmosphere was full of water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases. Click image to the left or use the URL below. URL: "
early atmosphere and oceans,T_1071,"When photosynthesis evolved and spread around the planet, oxygen was released in abundance. The addition of oxygen is what created Earths third atmosphere. This event, which occurred about 2.5 billion years ago, is sometimes called the oxygen catastrophe because so many organisms died. Although entire species died out and went extinct, this event is also called the Great Oxygenation Event because it was a great opportunity. The organisms that survived developed a use for oxygen through cellular respiration, the process by which cells can obtain energy from organic molecules. This opened up many opportunities for organisms to evolve to fill different niches and many new types of organisms first appeared on Earth. "
early atmosphere and oceans,T_1072,"What evidence do scientists have that large quantities of oxygen entered the atmosphere? The iron contained in the rocks combined with the oxygen to form reddish iron oxides. By the beginning of the Proterozoic, banded-iron formations (BIFs) were forming. Banded-iron formations display alternating bands of iron oxide and iron-poor chert that probably represent a seasonal cycle of an aerobic and an anaerobic environment. The oldest BIFs are 3.7 billion years old, but they are very common during the Great Oxygenation Event 2.4 billion years ago (Figure 1.2). By 1.8 billion years ago, the amount of BIF declined. In recent times, the iron in these formations has been mined, and that explains the location of the auto industry in the upper Midwest. "
early atmosphere and oceans,T_1073,"With more oxygen in the atmosphere, ultraviolet radiation could create ozone. With the formation of an ozone layer to protect the surface of the Earth from UV radiation, more complex life forms could evolve. Banded-iron formation. Click image to the left or use the URL below. URL: "
earth history and clues from fossils,T_1074,"Fossils are our best form of evidence about Earth history, including the history of life. Along with other geological evidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and other major geological events. Since the present is the key to the past, what we know about a type of organism that lives today can be applied to past environments. "
earth history and clues from fossils,T_1075,"That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocks resemble animals and plants that are living today. In general, fossils in older rocks are less similar to modern organisms. We would know very little about the organisms that came before us if there were no fossils. Modern technology has allowed scientists to reconstruct images and learn about the biology of extinct animals like dinosaurs! "
earth history and clues from fossils,T_1076,"By knowing something about the type of organism the fossil was, geologists can determine whether the region was terrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give clues to whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allows scientists to learn about what happened to the region after the organism died; for example, whether it was exposed to wave action. "
earth history and clues from fossils,T_1077,The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine. Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabed were uplifted. 
earth history and clues from fossils,T_1078,"By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher the climate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coal beds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was much warmer. Recall from the chapter Plate Tectonics that Wegener used the presence of coal beds in Antarctica as one of the lines of evidence for continental drift. "
earth history and clues from fossils,T_1079,"An index fossil can be used to identify a specific period of time. Organisms that make good index fossils are distinctive, widespread, and lived briefly. Their presence in a rock layer can be used to identify rocks that were deposited at that period of time over a large area. The fossil of a juvenile mammoth found near downtown San Jose California reveals an enormous amount about these majestic creatures: what they looked like, how they lived, and what the environment of the Bay Area was like so long ago. "
earths core,T_1099,"At the planets center lies a dense metallic core. Scientists know that the core is metal because: 1. The density of Earths surface layers is much less than the overall density of the planet, as calculated from the planets rotation. If the surface layers are less dense than average, then the interior must be denser than average. Calculations indicate that the core is about 85% iron metal with nickel metal making up much of the remaining 15%. 2. Metallic meteorites are thought to be representative of the core. The 85% iron/15% nickel calculation above is also seen in metallic meteorites (Figure 1.1). If Earths core were not metal, the planet would not have a magnetic field. Metals such as iron are magnetic, but rock, which makes up the mantle and crust, is not. Scientists know that the outer core is liquid and the inner core is solid because: 1. S-waves do not go through the outer core. 2. The strong magnetic field is caused by convection in the liquid outer core. Convection currents in the outer core are due to heat from the even hotter inner core. The heat that keeps the outer core from solidifying is produced by the breakdown of radioactive elements in the inner core. Click image to the left or use the URL below. URL: "
earths interior material,T_1103,"It wasnt always known that fossils were parts of living organisms. In 1666, a young doctor named Nicholas Steno dissected the head of an enormous great white shark that had been caught by fisherman near Florence, Italy. Steno was struck by the resemblance of the sharks teeth to fossils found in inland mountains and hills (Figure ??). Most people at the time did not believe that fossils were once part of living creatures. Authors in that day thought that the fossils of marine animals found in tall mountains, miles from any ocean could be explained in one of two ways: The shells were washed up during the Biblical flood. (This explanation could not account for the fact that fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earths surface.) The fossils formed within the rocks as a result of mysterious forces. But for Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were once parts of living creatures. "
earths interior material,T_1104,"A fossil is any remains or traces of an ancient organism. Fossils include body fossils, left behind when the soft parts have decayed away, and trace fossils, such as burrows, tracks, or fossilized coprolites (feces) as seen above. Collections of fossils are known as fossil assemblages. "
earths interior material,T_1105,"Becoming a fossil isnt easy. Only a tiny percentage of the organisms that have ever lived become fossils. Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelope that dies on the African plain (Figure ??). Most of its body is eaten by hyenas and other scavengers and the remaining flesh is devoured by insects and bacteria. Only bones are left behind. As the years go by, the bones are scattered and fragmented into small pieces, eventually turning into dust. The remaining nutrients return to the soil. This antelope will not be preserved as a fossil. Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the soft parts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces. The shells are also attacked by worms, sponges, and other animals (Figure ??). How about a soft bodied organism? Will a creature without hard shells or bones become a fossil? There is virtually no fossil record of soft bodied organisms such as jellyfish, worms, or slugs. Insects, which are by far the most common land animals, are only rarely found as fossils (Figure ??). "
earths interior material,T_1106,"Despite these problems, there is a rich fossil record. How does an organism become fossilized? "
earths interior material,T_1107,"Usually its only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine creatures are common also. "
earths interior material,T_1108,"Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near a river delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains (Figure ??). Quick burial is rare on land, so fossils of land animals and plants are less common than marine fossils. Land organisms can be buried by mudslides, volcanic ash, or covered by sand in a sandstorm (Figure ??). Skeletons can be covered by mud in lakes, swamps, or bogs. "
earths interior material,T_1109,"Unusual circumstances may lead to the preservation of a variety of fossils, as at the La Brea Tar Pits in Los Angeles, California. Although the animals trapped in the La Brea Tar Pits probably suffered a slow, miserable death, their bones were preserved perfectly by the sticky tar. (Figure ??). In spite of the difficulties of preservation, billions of fossils have been discovered, examined, and identified by thousands of scientists. The fossil record is our best clue to the history of life on Earth, and an important indicator of past climates and geological conditions as well. "
earths interior material,T_1110,Some rock beds contain exceptional fossils or fossil assemblages. Two of the most famous examples of soft organism preservation are from the 505 million-year-old Burgess Shale in Canada (Figure ??). The 145 million-year-old Solnhofen Limestone in Germany has fossils of soft body parts that are not normally preserved (Figure ??). 
earths interior material,T_1111,Use this resource to answer the questions that follow. Click image to the left for more content. 1. What are fossils? 2. What type of rocks are fossils found in? 3. What are sediments? 4. Explain how a fossil is created. 5. What factors have exposed sedimentary rock? 
earths magnetic field,T_1114,Earth is surrounded by a magnetic field (Figure 1.1) that behaves as if the planet had a gigantic bar magnet inside of it. Earths magnetic field also has a north and south pole. The magnetic field arises from the convection of molten iron and nickel metals in Earths liquid outer core. 
earths magnetic field,T_1115,"Many times during Earth history, even relatively recent Earth history, the planets magnetic field has flipped. That is, the north pole becomes the south pole and the south pole becomes the north pole. Scientists are not sure why this happens. One hypothesis is that the convection that drives the magnetic field becomes chaotic and then reverses itself. Another hypothesis is that an external event, such as an asteroid impact, disrupts motions in the core and causes the reversal. The first hypothesis is supported by computer models, but the second does not seem to be supported by much data. There is little correlation between impact events and magnetic reversals. Click image to the left or use the URL below. URL:  Earths magnetic field is like a bar magnet resides in the center of the planet. "
earths shape,T_1119,"Earth is a sphere or, more correctly, an oblate spheroid, which is a sphere that is a bit squished down at the poles and bulges a bit at the Equator. To be more technical, the minor axis (the diameter through the poles) is smaller than the major axis (the diameter through the Equator). Half of the sphere is a hemisphere. North of the Equator is the northern hemisphere and south of the Equator is the southern hemisphere. Eastern and western hemispheres are also designated. What evidence is there that Earth is spherical? What evidence was there before spaceships and satellites? Try to design an experiment involving a ship and the ocean to show Earth is round. If you are standing on the shore and a ship is going out to sea, the ship gets smaller as it moves further away from you. The ships bottom also starts to disappear as the vessel goes around the arc of the planet (Figure 1.1). There are many other ways that early scientists and mariners knew that Earth was not flat. The Sun and the other planets of the solar system are also spherical. Larger satellites, those that have enough mass for their gravitational attraction to have made them round, are spherical as well. Earths actual shape is not spherical but an oblate spheroid. The planet bulges around the equator due to mass collecting in the middle due to rotational momentum. "
eclipses,T_1123,"A solar eclipse occurs when the new Moon passes directly between the Earth and the Sun (Figure 1.1). This casts a shadow on the Earth and blocks Earths view of the Sun. A total solar eclipse occurs when the Moons shadow completely blocks the Sun (Figure 1.2). When only a portion of the Sun is out of view, it is called a partial solar eclipse. Solar eclipses are rare and usually only last a few minutes because the Moon casts only a small shadow (Figure 1.3). As the Sun is covered by the Moons shadow, it will actually get cooler outside. Birds may begin to sing, and stars will become visible in the sky. During a solar eclipse, the corona and solar prominences can be seen. A solar eclipse occurs when the Moon passes between Earth and the Sun in such a way that the Sun is either partially or totally hidden from view. Some people, including some scientists, chase eclipses all over the world to learn or just observe this amazing phenomenon. A solar eclipse shown as a series of pho- tos. Click image to the left or use the URL below. URL: "
eclipses,T_1124,"A lunar eclipse occurs when the full moon moves through Earths shadow, which only happens when Earth is between the Moon and the Sun and all three are lined up in the same plane, called the ecliptic (Figure 1.4). In an eclipse, Earths shadow has two distinct parts: the umbra and the penumbra. The umbra is the inner, cone-shaped part of the shadow, in which all of the light has been blocked. The penumbra is the outer part of Earths shadow where only part of the light is blocked. In the penumbra, the light is dimmed but not totally absent. A total lunar eclipse occurs when the Moon travels completely in Earths umbra. During a partial lunar eclipse, only a portion of the Moon enters Earths umbra. Earths shadow is large enough that a lunar eclipse lasts for hours and can be seen by any part of Earth with a view of the Moon at the time of the eclipse (Figure 1.5). A lunar eclipse does not occur every month because Moons orbit is inclined 5-degrees to Earths orbit, so the two bodies are not in the same plane every month. "
effect of latitude on climate,T_1127,"Many factors influence the climate of a region. The most important factor is latitude because different latitudes receive different amounts of solar radiation. The Equator receives the most solar radiation. Days are equally long year-round and the Sun is just about directly overhead at midday. The polar regions receive the least solar radiation. The night lasts six months during the winter. Even in summer, the Sun never rises very high in the sky. Sunlight filters through a thick wedge of atmosphere, making the sunlight much less intense. The high albedo, because of ice and snow, reflects a good portion of the Suns light. "
effect of latitude on climate,T_1128,"Its easy to see the difference in temperature at different latitudes in the Figure 1.1. But temperature is not completely correlated with latitude. There are many exceptions. For example, notice that the western portion of South America The maximum annual temperature of the Earth, showing a roughly gradual temperature gradient from the low to the high latitudes. has relatively low temperatures due to the Andes Mountains. The Rocky Mountains in the United States also have lower temperatures due to high altitudes. Western Europe is warmer than it should be due to the Gulf Stream. Click image to the left or use the URL below. URL: "
effects of air pollution on human health,T_1129,Human health suffers in locations with high levels of air pollution. 
effects of air pollution on human health,T_1130,"Different pollutants have different health effects: Lead is the most common toxic material and is responsible for lead poisoning. Carbon monoxide can kill people in poorly ventilated spaces, such as tunnels. Nitrogen and sulfur-oxides cause lung disease and increased rates of asthma, emphysema, and viral infections such as the flu. Ozone damages the human respiratory system, causing lung disease. High ozone levels are also associated with increased heart disease and cancer. Particulates enter the lungs and cause heart or lung disease. When particulate levels are high, asthma attacks are more common. By some estimates, 30,000 deaths a year in the United States are caused by fine particle pollution. "
effects of air pollution on human health,T_1131,"Many but not all cases of asthma can be linked to air pollution. During the 1996 Olympic Games, Atlanta, Georgia, closed off their downtown to private vehicles. This action decreased ozone levels by 28%. At the same time, there were 40% fewer hospital visits for asthma. Can scientists conclude without a shadow of a doubt that the reduction in ozone caused the reduction in hospital visits? What could they do to make that determination? Lung cancer among people who have never smoked is around 15% and is increasing. One study showed that the risk of being afflicted with lung cancer increases directly with a persons exposure to air pollution (Figure 1.1). The study concluded that no level of air pollution should be considered safe. Exposure to smog also increased the risk of dying from any cause, including heart disease. One study found that in the United States, children develop asthma at more than twice the rate of two decades ago and at four times the rate of children in Canada. Adults also suffer from air pollution-related illnesses that include lung disease, heart disease, lung cancer, and weakened immune systems. The asthma rate worldwide is rising 20% to 50% every decade. "
electromagnetic energy in the atmosphere,T_1139,"Energy travels through space or material. This is obvious when you stand near a fire and feel its warmth or when you pick up the handle of a metal pot even though the handle is not sitting directly on the hot stove. Invisible energy waves can travel through air, glass, and even the vacuum of outer space. These waves have electrical and magnetic properties, so they are called electromagnetic waves. The transfer of energy from one object to another through electromagnetic waves is known as radiation. Different wavelengths of energy create different types of electromagnetic waves (Figure 1.1). The wavelengths humans can see are known as visible light. When viewed together, all of the wavelengths of visible light appear white. But a prism or water droplets can break the white light into different wavelengths so that separate colors appear (Figure 1.2). What objects can you think of that radiate visible light? Two include the Sun and a light bulb. The longest wavelengths of visible light appear red. Infrared wavelengths are longer than visible red. Snakes can see infrared energy. We feel infrared energy as heat. Wavelengths that are shorter than violet are called ultraviolet. Can you think of some objects that appear to radiate visible light, but actually do not? The Moon and the planets do not emit light of their own; they reflect the light of the Sun. Reflection is when light (or another wave) bounces back from a surface. Albedo is a measure of how well a surface reflects light. A surface with high albedo reflects a large percentage of light. A snow field has high albedo. One important fact to remember is that energy cannot be created or destroyed  it can only be changed from one form to another. This is such a fundamental fact of nature that it is a law: the law of conservation of energy. In photosynthesis, for example, plants convert solar energy into chemical energy that they can use. They do not create new energy. When energy is transformed, some nearly always becomes heat. Heat transfers between materials easily, from warmer objects to cooler ones. If no more heat is added, eventually all of a material will reach the same temperature. "
energy conservation,T_1140,"Everyone can reduce their use of energy resources and the pollution the resources cause by conserving energy. Conservation means saving resources by using them more efficiently, using less of them, or not using them at all. You can read below about some of the ways you can conserve energy on the road and in the home. "
energy conservation,T_1141,"Much of the energy used in the U.S. is used for transportation. You can conserve transportation energy in several ways. For example, you can: plan ahead to avoid unnecessary trips. take public transit such as subways (see Figure 1.1) instead of driving. drive an energy-efficient vehicle when driving is the only way to get there. Q: What are some other ways you could save energy in transportation? A: You could carpool to save transportation energy. Even if you carpool with just one other person, thats one less vehicle on the road. For short trips, you could ride a bike or walk to you destination. The extra exercise is another benefit of using your own muscle power to get where you need to go. "
energy conservation,T_1142,"Many people waste energy at home, so a lot of energy can be saved there as well. What can you do to conserve energy? You can: turn off lights and unplug appliances and other electrical devices when not in use. use energy-efficient light bulbs and appliances. turn the thermostat down in winter and up in summer. Q: How can you tell which light bulbs and appliances use less energy? "
energy from biomass,T_1143,"Biomass is the material that comes from plants and animals that were recently living. Biomass can be burned directly, such as setting fire to wood. For as long as humans have had fire, people have used biomass for heating and cooking. People can also process biomass to make fuel, called biofuel. Biofuel can be created from crops, such as corn or Biofuels, such as ethanol, are added to gasoline to cut down the amount of fossil fuels that are used. algae, and processed for use in a car (Figure 1.1). The advantage to biofuels is that they burn more cleanly than fossil fuels. As a result, they create less pollution and less carbon dioxide. Organic material, like almond shells, can be made into electricity. Biomass power is a great use of wastes and is more reliable than other renewable energy sources, but harvesting biomass energy uses energy and biomass plants produce pollutants including greenhouse gases. Cow manure can have a second life as a source of methane gas, which can be converted to electricity. Not only that food scraps can also be converted into green energy. Food that is tossed out produces methane, a potent greenhouse gas. But that methane from leftovers can be harnessed and used as fuel. Sounds like a win-win situation. "
energy from biomass,T_1144,"In many instances, the amount of energy, fertilizer, and land needed to produce the crops used make biofuels mean that they often produce very little more energy than they consume. The fertilizers and pesticides used to grow the crops run off and become damaging pollutants in nearby water bodies or in the oceans. To generate biomass energy, break down the cell walls of plants to release the sugars and then ferment those sugars to create fuel. Corn is a very inefficient source; scientists are looking for much better sources of biomass energy. "
energy from biomass,T_1145,"Research is being done into alternative crops for biofuels. A very promising alternative is algae. Growing algae requires much less land and energy than crops. Algae can be grown in locations that are not used for other things, like in desert areas where other crops are not often grown. Algae can be fed agricultural and other waste so valuable resources are not used. Much research is being done to bring these alternative fuels to market. Many groups are researching the use of algae for fuel. Many people think that the best source of biomass energy for the future is algae. Compared to corn, algae is not a food crop, it can grow in many places, its much easier to convert to a usable fuel, and its carbon neutral. "
energy use,T_1146,"Look at the circle graph in the Figure 1.1. It shows that oil is the single most commonly used energy resource in the U.S., followed by natural gas, and then by coal. All of these energy resources are nonrenewable. Nonrenewable resources are resources that are limited in supply and cannot be replaced as quickly as they are used up. Renewable resources, in contrast, provide only 8 percent of all energy used in the U.S. Renewable resources are natural resources that can be replaced in a relatively short period of time or are virtually limitless in supply. They include solar energy from sunlight, geothermal energy from under Earths surface, wind, biomass (from once-living things or their wastes), and hydropower (from running water). "
energy use,T_1147,"People in the U.S. use far more energyespecially energy from oilthan people in any other nation. The bar graph in the Figure 1.2 compares the amount of oil used by the top ten oil-using nations. The U.S. uses more oil than several other top-ten countries combined. If you also consider the population size in these countries, the differences are even more stunning. The average person in the U.S. uses a whopping 23 barrels of oil a year! In comparison, the average person in India or China uses just 1 or 2 barrels of oil a year. Q: How does the use of oil and other fossil fuels relate to pollution? A: Greater use of oil and other fossil fuels causes more pollution. "
environmental impacts of mining,T_1148,"Although mining provides people with many needed resources, the environmental costs can be high. Surface mining clears the landscape of trees and soil, and nearby streams and lakes are inundated with sediment. Pollutants from the mined rock, such as heavy metals, enter the sediment and water system. Acids flow from some mine sites, changing the composition of nearby waterways (Figure 1.1). U.S. law has changed in recent decades so that a mine region must be restored to its natural state, a process called reclamation. This is not true of older mines. Pits may be refilled or reshaped and vegetation planted. Pits may be allowed to fill with water and become lakes or may be turned into landfills. Underground mines may be sealed off or left open as homes for bats. Click image to the left or use the URL below. URL:  Acid drainage from a surface coal mine in Missouri. "
exoplanets,T_1158,"Since the early 1990s, astronomers have discovered other solar systems, with planets orbiting stars other than our own Sun. These are called ""extrasolar planets"" or simply exoplanets (see Figure 1.1). Exoplanets are not in our solar system, but are found in other solar systems. Some extrasolar planets have been directly imaged, but most have been discovered by indirect methods. One technique involves detecting the very slight motion of a star periodically moving toward and away from us along our line-of-sight (also known as a stars ""radial velocity""). This periodic motion can be attributed to the gravitational pull of a planet or, sometimes, another star orbiting the star. A planet may also be identified by measuring a stars brightness over time. A temporary, periodic decrease in light emitted from a star can occur when a planet crosses in front of, or ""transits,"" the star it is orbiting, momentarily blocking out some of the starlight. More than 1800 extrasolar planets have been identified and confirmed and the rate of discovery is increasing rapidly. Click image to the left or use the URL below. URL: "
expansion of the universe,T_1159,"After discovering that there are galaxies beyond the Milky Way, Edwin Hubble went on to measure the distance to hundreds of other galaxies. His data would eventually show how the universe is changing, and would even yield clues as to how the universe formed. "
expansion of the universe,T_1160,"If you look at a star through a prism, you will see a spectrum, or a range of colors through the rainbow. The spectrum will have specific dark bands where elements in the star absorb light of certain energies. By examining the arrangement of these dark absorption lines, astronomers can determine the composition of elements that make up a distant star. In fact, the element helium was first discovered in our Sun  not on Earth  by analyzing the absorption lines in the spectrum of the Sun. While studying the spectrum of light from distant galaxies, astronomers noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, as shown in Figure 1.1. This shift of absorption bands toward the red end of the spectrum is known as redshift. Redshift is a shift in absorption bands toward the red end of the spectrum. What could make the absorption bands of a star shift toward the red? Redshift occurs when the light source is moving away from the observer or when the space between the observer and the source is stretched. What does it mean that stars and galaxies are redshifted? When astronomers see redshift in the light from a galaxy, they know that the galaxy is moving away from Earth. If galaxies were moving randomly, would some be redshifted but others be blueshifted? Of course. Since almost every galaxy in the universe has a redshift, almost every galaxy is moving away from Earth. Click image to the left or use the URL below. URL: "
expansion of the universe,T_1161,"Edwin Hubble combined his measurements of the distances to galaxies with other astronomers measurements of redshift. From this data, he noticed a relationship, which is now called Hubbles Law: the farther away a galaxy is, the faster it is moving away from us. What could this mean about the universe? It means that the universe is expanding. Figure 1.2 shows a simplified diagram of the expansion of the universe. One way to picture this is to imagine a balloon covered with tiny dots to represent the galaxies. When you inflate the balloon, the dots slowly move away from each other because the rubber stretches in the space between them. If you were standing on one of the dots, you would see the other dots moving away from you. Also, the dots farther away from you on the balloon would move away faster than dots nearby. In this diagram of the expansion of the universe over time, the distance between galaxies gets bigger over time, although the size of each galaxy stays the same. An inflating balloon is only a rough analogy to the expanding universe for several reasons. One important reason is that the surface of a balloon has only two dimensions, while space has three dimensions. But space itself is stretching out between galaxies, just as the rubber stretches when a balloon is inflated. This stretching of space, which increases the distance between galaxies, is what causes the expansion of the universe. One other difference between the universe and a balloon involves the actual size of the galaxies. On a balloon, the dots will become larger in size as you inflate it. In the universe, the galaxies stay the same size; only the space between the galaxies increases. "
faults,T_1169,A rock under enough stress will fracture. There may or may not be movement along the fracture. 
faults,T_1170,"If there is no movement on either side of a fracture, the fracture is called a joint. The rocks below show horizontal and vertical jointing. These joints formed when the confining stress was removed from the rocks as shown in (Figure "
faults,T_1171,"If the blocks of rock on one or both sides of a fracture move, the fracture is called a fault (Figure 1.2). Stresses along faults cause rocks to break and move suddenly. The energy released is an earthquake. How do you know theres a fault in this rock? Try to line up the same type of rock on either side of the lines that cut across them. One side moved relative to the other side, so you know the lines are a fault. Slip is the distance rocks move along a fault. Slip can be up or down the fault plane. Slip is relative, because there is usually no way to know whether both sides moved or only one. Faults lie at an angle to the horizontal surface of the Earth. That angle is called the faults dip. The dip defines which of two basic types a fault is. If the faults dip is inclined relative to the horizontal, the fault is a dip-slip fault (Figure 1.3). "
faults,T_1172,"There are two types of dip-slip faults. In a normal fault, the hanging wall drops down relative to the footwall. In a reverse fault, the footwall drops down relative to the hanging wall. This diagram illustrates the two types of dip-slip faults: normal faults and reverse faults. Imagine miners extracting a re- source along a fault. The hanging wall is where miners would have hung their lanterns. The footwall is where they would have walked. A thrust fault is a type of reverse fault in which the fault plane angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 1.4). At Chief Mountain in Montana, the upper rocks at the Lewis Overthrust are more than 1 billion years older than the lower rocks. How could this happen? Normal faults can be huge. They are responsible for uplifting mountain ranges in regions experiencing tensional stress. "
faults,T_1173,"A strike-slip fault is a dip-slip fault in which the dip of the fault plane is vertical. Strike-slip faults result from shear stresses. Imagine placing one foot on either side of a strike-slip fault. One block moves toward you. If that block moves toward your right foot, the fault is a right-lateral strike-slip fault; if that block moves toward your left foot, the fault is a left-lateral strike-slip fault (Figure 1.5). Californias San Andreas Fault is the worlds most famous strike-slip fault. It is a right-lateral strike slip fault (See opening image). People sometimes say that California will fall into the ocean someday, which is not true. Strike-slip faults. Click image to the left or use the URL below. URL: "
flooding,T_1179,"Floods usually occur when precipitation falls more quickly than water can be absorbed into the ground or carried away by rivers or streams. Waters may build up gradually over a period of weeks, when a long period of rainfall or snowmelt fills the ground with water and raises stream levels. Extremely heavy rains across the Midwestern U.S. in April 2011 led to flooding of the rivers in the Mississippi River basin in May 2011 (Figures 1.1 and 1.2). Click image to the left or use the URL below. URL:  This map shows the accumulated rainfall across the U.S. in the days from April 22 to April 29, 2011. Record flow in the Ohio and Mississippi Rivers has to go somewhere. Normal spring river levels are shown in 2010. The flooded region in the image from May 3, 2011 is the New Madrid Floodway, where overflow water is meant to go. 2011 is the first time since 1927 that this floodway was used. "
flooding,T_1180,"Flash floods are sudden and unexpected, taking place when very intense rains fall over a very brief period (Figure streambed. A 2004 flash flood in England devastated two villages when 3-1/2 inches of rain fell in 60 minutes. Pictured here is some of the damage from the flash flood. Click image to the left or use the URL below. URL: "
flooding,T_1181,"Heavily vegetated lands are less likely to experience flooding. Plants slow down water as it runs over the land, giving it time to enter the ground. Even if the ground is too wet to absorb more water, plants still slow the waters passage and increase the time between rainfall and the waters arrival in a stream; this could keep all the water falling over a region from hitting the stream at once. Wetlands act as a buffer between land and high water levels and play a key role in minimizing the impacts of floods. Flooding is often more severe in areas that have been recently logged. "
flooding,T_1182,"People try to protect areas that might flood with dams, and dams are usually very effective. But high water levels sometimes cause a dam to break and then flooding can be catastrophic. People may also line a river bank with levees, high walls that keep the stream within its banks during floods. A levee in one location may just force the high water up or downstream and cause flooding there. The New Madrid Overflow in the Figure 1.2 was created with the recognition that the Mississippi River sometimes simply cannot be contained by levees and must be allowed to flood. "
flooding,T_1183,"Within the floodplain of the Nile, soils are fertile enough for productive agriculture. Beyond this, infertile desert soils prevent viable farming. Not all the consequences of flooding are negative. Rivers deposit new nutrient-rich sediments when they flood, so floodplains have traditionally been good for farming. Flooding as a source of nutrients was important to Egyptians along the Nile River until the Aswan Dam was built in the 1960s. Although the dam protects crops and settlements from the annual floods, farmers must now use fertilizers to feed their cops. Floods are also responsible for moving large amounts of sediments about within streams. These sediments provide habitats for animals, and the periodic movement of sediment is crucial to the lives of several types of organisms. Plants and fish along the Colorado River, for example, depend on seasonal flooding to rearrange sand bars. "
folds,T_1186,"Rocks deforming plastically under compressive stresses crumple into folds. They do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture. You can see three types of folds. "
folds,T_1187,"A monocline is a simple bend in the rock layers so that they are no longer horizontal (see Figure 1.1 for an example). At Utahs Cockscomb, the rocks plunge downward in a monocline. What you see in the image appears to be a monocline. Are you certain it is a monocline? What else might it be? What would you have to do to figure it out? "
folds,T_1188,"Anticline: An anticline is a fold that arches upward. The rocks dip away from the center of the fold (Figure 1.2). The oldest rocks are at the center of an anticline and the youngest are draped over them. When rocks arch upward to form a circular structure, that structure is called a dome. If the top of the dome is sliced off, where are the oldest rocks located? "
folds,T_1189,"A syncline is a fold that bends downward. The youngest rocks are at the center and the oldest are at the outside (Figure 1.3). When rocks bend downward in a circular structure, that structure is called a basin (Figure 1.4). If the rocks are exposed at the surface, where are the oldest rocks located? Click image to the left or use the URL below. URL:  Anticlines are formations that have folded rocks upward. (a) Schematic of a syncline. (b) This syncline is in Rainbow Basin, California. Some folding can be fairly complicated. What do you see in the photo above? "
formation of earth,T_1190,Earth formed at the same time as the other planets. The history of Earth is part of the history of the Solar System. 
formation of earth,T_1191,"Earth came together (accreted) from the cloud of dust and gas known as the solar nebula nearly 4.6 billion years ago, the same time the Sun and the rest of the solar system formed. Gravity caused small bodies of rock and metal orbiting the proto-Sun to smash together to create larger bodies. Over time, the planetoids got larger and larger until they became planets. "
formation of earth,T_1192,"When Earth first came together it was really hot, hot enough to melt the metal elements that it contained. Earth was so hot for three reasons: Gravitational contraction: As small bodies of rock and metal accreted, the planet grew larger and more massive. Gravity within such an enormous body squeezes the material in its interior so hard that the pressure swells. As Earths internal pressure grew, its temperature also rose. Radioactive decay: Radioactive decay releases heat, and early in the planets history there were many ra- dioactive elements with short half lives. These elements long ago decayed into stable materials, but they were responsible for the release of enormous amounts of heat in the beginning. Bombardment: Ancient impact craters found on the Moon and inner planets indicate that asteroid impacts were common in the early solar system. Earth was struck so much in its first 500 million years that the heat was intense. Very few large objects have struck the planet in the past many hundreds of millions of year. "
formation of earth,T_1193,"When Earth was entirely molten, gravity drew denser elements to the center and lighter elements rose to the surface. The separation of Earth into layers based on density is known as differentiation. The densest material moved to the center to create the planets dense metallic core. Materials that are intermediate in density became part of the mantle (Figure 1.1). "
formation of earth,T_1194,"Lighter materials accumulated at the surface of the mantle to become the earliest crust. The first crust was probably basaltic, like the oceanic crust is today. Intense heat from the early core drove rapid and vigorous mantle convection so that crust quickly recycled into the mantle. The recycling of basaltic crust was so effective that no remnants of it are found today. "
formation of earth,T_1195,"There is not much material to let us know about the earliest days of our planet Earth. What there is comes from three sources: (1) zircon crystals, the oldest materials found on Earth, which show that the age of the earliest crust formed at least 4.4 billion years ago; (2) meteorites that date from the beginning of the solar system, to nearly 4.6 billion years ago (Figure 1.2); and (3) lunar rocks, which represent the early days of the Earth-Moon system as far back as 4.5 billion years ago. "
formation of the moon,T_1196,"One of the most unique features of planet Earth is its large Moon. Unlike the only other natural satellites orbiting an inner planet, those of Mars, the Moon is not a captured asteroid. Understanding the Moons birth and early history reveals a great deal about Earths early days. "
formation of the moon,T_1197,"To determine how the Moon formed, scientists had to account for several lines of evidence: The Moon is large; not much smaller than the smallest planet, Mercury. Earth and Moon are very similar in composition. Moons surface is 4.5 billion years old, about the same as the age of the solar system. For a body its size and distance from the Sun, the Moon has very little core; Earth has a fairly large core. The oxygen isotope ratios of Earth and Moon indicate that they originated in the same part of the solar system. Earth has a faster spin than it should have for a planet of its size and distance from the Sun. Can you devise a birth story for the Moon that takes all of these bits of data into account? "
formation of the moon,T_1198,"Astronomers have carried out computer simulations that are consistent with these facts and have detailed a birth story for the Moon. A little more than 4.5 billion years ago, roughly 70 million years after Earth formed, planetary bodies were being pummeled by asteroids and planetoids of all kinds. Earth was struck by a Mars-sized asteroid (Figure 1.1). An artists depiction of the impact that produced the Moon. The tremendous energy from the impact melted both bodies. The molten material mixed up. The dense metals remained on Earth but some of the molten, rocky material was flung into an orbit around Earth. It eventually accreted into a single body, the Moon. Since both planetary bodies were molten, material could differentiate out of the magma ocean into core, mantle, and crust as they cooled. Earths fast spin is from energy imparted to it by the impact. "
formation of the moon,T_1199,"Lunar rocks reveal an enormous amount about Earths early days. The Genesis Rock, with a date of 4.5 billion years, is only about 100 million years younger than the solar system (see opening image). The rock is a piece of the Moons anorthosite crust, which was the original crust. Why do you think Moon rocks contain information that is not available from Earths own materials? Can you find how all of the evidence presented in the bullet points above is present in the Moons birth story? "
formation of the sun and planets,T_1200,"The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According to this hypothesis, the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula. The nebula was drawn together by gravity, which released gravitational potential energy. As small particles of dust and gas smashed together to create larger ones, they released kinetic energy. As the nebula collapsed, the gravity at the center increased and the cloud started to spin because of its angular momentum. As it collapsed further, the spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin. Much of the clouds mass migrated to its center but the rest of the material flattened out in an enormous disk. The disk contained hydrogen and helium, along with heavier elements and even simple organic molecules. "
formation of the sun and planets,T_1201,"As gravity pulled matter into the center of the disk, the density and pressure at the center became intense. When the pressure in the center of the disk was high enough, nuclear fusion began. A star was bornthe Sun. The burning star stopped the disk from collapsing further. Meanwhile, the outer parts of the disk were cooling off. Matter condensed from the cloud and small pieces of dust started clumping together. These clumps collided and combined with other clumps. Larger clumps, called An artists painting of a protoplanetary disk. planetesimals, attracted smaller clumps with their gravity. Gravity at the center of the disk attracted heavier particles, such as rock and metal and lighter particles remained further out in the disk. Eventually, the planetesimals formed protoplanets, which grew to become the planets and moons that we find in our solar system today. Because of the gravitational sorting of material, the inner planets  Mercury, Venus, Earth, and Mars  formed from dense rock and metal. The outer planets  Jupiter, Saturn, Uranus and Neptune  condensed farther from the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond, where its very cold, these materials form solid particles. The nebular hypothesis was designed to explain some of the basic features of the solar system: The orbits of the planets lie in nearly the same plane with the Sun at the center The planets revolve in the same direction The planets mostly rotate in the same direction The axes of rotation of the planets are mostly nearly perpendicular to the orbital plane The oldest moon rocks are 4.5 billion years Click image to the left or use the URL below. URL: "
fossil fuel formation,T_1202,"Can you name some fossils? How about dinosaur bones or dinosaur footprints? Animal skeletons, teeth, shells, coprolites (otherwise known as feces), or any other remains or traces from a living creature that becomes rock is a fossil. The same processes that formed these fossils also created some of our most important energy resources, fossil fuels. Coal, oil, and natural gas are fossil fuels. Fossil fuels come from living matter starting about 500 million years ago. Millions of years ago, plants used energy from the Sun to form sugars, carbohydrates, and other energy-rich carbon compounds. As plants and animals died, their remains settled on the ground on land and in swamps, lakes, and seas (Figure 1.1). Over time, layer upon layer of these remains accumulated. Eventually, the layers were buried so deeply that they were crushed by an enormous mass of earth. The weight of this earth pressing down on these plant and animal remains created intense heat and pressure. After millions of years of heat and pressure, the material in these layers turned into chemicals called hydrocarbons (Figure 1.2). Hydrocarbons are made of carbon and hydrogen atoms. This molecule with one carbon and four hydrogen atoms is methane. Hydrocarbons can be solid, liquid, or gaseous. The solid form is what we know as coal. The liquid form is petroleum, or crude oil. Natural gas is the gaseous form. The solar energy stored in fossil fuels is a rich source of energy. Although fossil fuels provide very high quality energy, they are non-renewable. Click image to the left or use the URL below. URL: "
fossil fuel reserves,T_1203,"Fossil fuels provide about 85% of the worlds energy at this time. Worldwide fossil fuel usage has increased many times over in the past half century (coal - 2.6x, oil - 8x, natural gas - 14x) because of population increases, because of increases in the number of cars, televisions, and other fuel-consuming uses in the developed world, and because of lifestyle improvements in the developing world. The amount of fossil fuels that remain untapped is unknown, but can likely be measured in decades for oil and natural gas and in a few centuries for coal (Figure 1.1). "
fossil fuel reserves,T_1204,"As the easy-to-reach fossil fuel sources are depleted, alternative sources of fossil fuels are increasingly being exploited (Figure 1.2). These include oil shale and tar sands. Oil shale is rock that contains dispersed oil that has not collected in reservoirs. To extract the oil from the shale requires enormous amounts of hot water. Tar sands are rocky materials mixed with very thick oil. The tar is too thick to pump and so tar sands are strip-mined. Hot water and caustic soda are used to separate the oil from the rock. The environmental consequences of mining these fuels, and of fossil fuel use in general, along with the fact that these fuels do not have a limitless supply, are prompting the development of alternative energy sources in some regions. Click image to the left or use the URL below. URL:  A satellite image of an oil-sands mine in Canada. Click image to the left or use the URL below. URL: "
fresh water ecosystems,T_1205,"Organisms that live in lakes, ponds, streams, springs or wetlands are part of freshwater ecosystems. These ecosys- tems vary by temperature, pressure (in lakes), the amount of light that penetrates and the type of vegetation that lives there. "
fresh water ecosystems,T_1206,"Limnology is the study of bodies of fresh water and the organisms that live there. A lake has zones just like the ocean. The ecosystem of a lake is divided into three distinct zones (Figure 1.1): 1. The surface (littoral) zone is the sloped area closest to the edge of the water. 2. The open-water zone (also called the photic or limnetic zone) has abundant sunlight. 3. The deep-water zone (also called the aphotic or profundal zone) has little or no sunlight. There are several life zones found within a lake: In the littoral zone, sunlight promotes plant growth, which provides food and shelter to animals such as snails, insects, and fish. In the open-water zone, other plants and fish, such as bass and trout, live. The deep-water zone does not have photosynthesis since there is no sunlight. Most deep-water organisms are scavengers, such as crabs and catfish that feed on dead organisms that fall to the bottom of the lake. Fungi and bacteria aid in the decomposition in the deep zone. Though different creatures live in the oceans, ocean waters also have these same divisions based on sunlight with similar types of creatures that live in each of the zones. The three primary zones of a lake are the littoral, open-water, and deep-water zones. "
fresh water ecosystems,T_1207,Wetlands are lands that are wet for significant periods of time. They are common where water and land meet. Wetlands can be large flat areas or relatively small and steep areas. Wetlands are rich and unique ecosystems with many species that rely on both the land and the water for survival. Only specialized plants are able to grow in these conditions. Wetlands tend have a great deal of biological diversity. Wetland ecosystems can also be fragile systems that are sensitive to the amount and quality of water present within them. Click image to the left or use the URL below. URL: 
fresh water ecosystems,T_1208,"Marshes are shallow wetlands around lakes, streams, or the ocean where grasses and reeds are common, but trees are not (Figure 1.2). Frogs, turtles, muskrats, and many varieties of birds are at home in marshes. A salt marsh on Cape Cod in Mas- sachusetts. "
fresh water ecosystems,T_1209,"A swamp is a wetland with lush trees and vines found in low-lying areas beside slow-moving rivers (Figure 1.3). Like marshes, they are frequently or always inundated with water. Since the water in a swamp moves slowly, oxygen in the water is often scarce. Swamp plants and animals must be adapted for these low-oxygen conditions. Like marshes, swamps can be fresh water, salt water, or a mixture of both. "
fresh water ecosystems,T_1210,"As mentioned above, wetlands are home to many different species of organisms. Although they make up only 5% of the area of the United States, wetlands contain more than 30% of the plant types. Many endangered species live in wetlands, so wetlands are protected from human use. Wetlands also play a key biological role by removing pollutants from water. For example, they can trap and use fertilizer that has washed off a farmers field, and therefore they prevent that fertilizer from contaminating another body of water. Since wetlands naturally purify water, preserving wetlands also helps to maintain clean supplies of water. "
galaxies,T_1211,"Galaxies are the biggest groups of stars and can contain anywhere from a few million stars to many billions of stars. Every star that is visible in the night sky is part of the Milky Way Galaxy. To the naked eye, the closest major galaxy the Andromeda Galaxy, shown in Figure 1.1  looks like only a dim, fuzzy spot. But that fuzzy spot contains one trillion  1,000,000,000,000  stars! Galaxies are divided into three types according to shape: spiral galaxies, elliptical galaxies, and irregular galaxies. "
galaxies,T_1212,"Spiral galaxies spin, so they appear as a rotating disk of stars and dust, with a bulge in the middle, like the Sombrero Galaxy shown in Figure 1.2. Several arms spiral outward in the Pinwheel Galaxy (seen in Figure 1.2) and are appropriately called spiral arms. Spiral galaxies have lots of gas and dust and lots of young stars. The Andromeda Galaxy is a large spiral galaxy similar to the Milky Way. (a) The Sombrero Galaxy is a spiral galaxy that we see from the side so the disk and central bulge are visible. (b) The Pinwheel Galaxy is a spiral galaxy that we see face-on so we can see the spiral arms. Because they contain lots of young stars, spiral arms tend to be blue. "
galaxies,T_1213,"Figure 1.3 shows a typical egg-shaped elliptical galaxy. The smallest elliptical galaxies are as small as some globular clusters. Giant elliptical galaxies, on the other hand, can contain over a trillion stars. Elliptical galaxies are reddish to yellowish in color because they contain mostly old stars. Most elliptical galaxies contain very little gas and dust because the gas and dust have already formed into stars. However, some elliptical galaxies, such as the one shown in Figure 1.4, contain lots of dust. Why might some elliptical galaxies contain dust? "
galaxies,T_1214,"Is the galaxy in Figure 1.5 a spiral galaxy or an elliptical galaxy? It is neither one! Galaxies that are not clearly elliptical galaxies or spiral galaxies are irregular galaxies. How might an irregular galaxy form? Most irregular galaxies were once spiral or elliptical galaxies that were then deformed either by gravitational attraction to a larger galaxy or by a collision with another galaxy. This galaxy, called NGC 1427A, has nei- ther a spiral nor an elliptical shape. "
galaxies,T_1215,"Dwarf galaxies are small galaxies containing only a few million to a few billion stars. Dwarf galaxies are the most common type in the universe. However, because they are relatively small and dim, we dont see as many dwarf galaxies from Earth. Most dwarf galaxies are irregular in shape. However, there are also dwarf elliptical galaxies and dwarf spiral galaxies. Look back at the picture of the elliptical galaxy. In the figure, you can see two dwarf elliptical galaxies that are companions to the Andromeda Galaxy. One is a bright sphere to the left of center, and the other is a long ellipse below and to the right of center. Dwarf galaxies are often found near larger galaxies. They sometimes collide with and merge into their larger neighbors. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
geologic time scale,T_1216,"To be able to discuss Earth history, scientists needed some way to refer to the time periods in which events happened and organisms lived. With the information they collected from fossil evidence and using Stenos principles, they created a listing of rock layers from oldest to youngest. Then they divided Earths history into blocks of time with each block separated by important events, such as the disappearance of a species of fossil from the rock record. Since many of the scientists who first assigned names to times in Earths history were from Europe, they named the blocks of time from towns or other local places where the rock layers that represented that time were found. From these blocks of time the scientists created the geologic time scale (Figure 1.1). In the geologic time scale the youngest ages are on the top and the oldest on the bottom. Why do you think that the more recent time periods are divided more finely? Do you think the divisions in the scale below are proportional to the amount of time each time period represented in Earth history? In what eon, era, period and epoch do we now live? We live in the Holocene (sometimes called Recent) epoch, Quaternary period, Cenozoic era, and Phanerozoic eon. "
geologic time scale,T_1217,"Its always fun to think about geologic time in a framework that we can more readily understand. Here are when some major events in Earth history would have occurred if all of earth history was condensed down to one calendar year. January 1 12 am: Earth forms from the planetary nebula - 4600 million years ago February 25, 12:30 pm: The origin of life; the first cells - 3900 million years ago March 4, 3:39 pm: Oldest dated rocks - 3800 million years ago March 20, 1:33 pm: First stromatolite fossils - 3600 million years ago July 17, 9:54 pm: first fossil evidence of cells with nuclei - 2100 million years ago November 18, 5:11 pm: Cambrian Explosion - 544 million years ago December 1, 8:49 am: first insects - 385 million years ago December 2, 3:54 am: first land animals, amphibians - 375 million years ago December 5, 5:50 pm: first reptiles - 330 million years ago December 12, 12:09 pm: Permo-Triassic Extinction - 245 million years ago December 13, 8:37 pm: first dinosaurs - 228 million years ago December 14, 9:59 am: first mammals  220 million years ago December 22, 8:24 pm: first flowering plants - 115 million years ago December 26, 7:52 pm: Cretaceous-Tertiary Extinction - 66 million years ago December 26, 9:47 pm: first ancestors of dogs - 64 million years ago December 27, 5:25 am: widespread grasses - 60 million years ago December 27, 11:09 am: first ancestors of pigs and deer - 57 million years ago December 28, 9:31 pm: first monkeys - 39 million years ago December 31, 5:18 pm: oldest hominid - 4 million years ago December 31, 11:02 pm: oldest direct human ancestor - 1 million years ago December 31, 11:48 pm: first modern human - 200,000 years ago December 31, 11:59 pm: Revolutionary War - 235 years ago "
geological stresses,T_1218,"Stress is the force applied to an object. In geology, stress is the force per unit area that is placed on a rock. Four types of stresses act on materials. A deeply buried rock is pushed down by the weight of all the material above it. Since the rock cannot move, it cannot deform. This is called confining stress. Compression squeezes rocks together, causing rocks to fold or fracture (break) (Figure 1.1). Compression is the most common stress at convergent plate boundaries. Stress caused these rocks to fracture. Rocks that are pulled apart are under tension. Rocks under tension lengthen or break apart. Tension is the major type of stress at divergent plate boundaries. When forces are parallel but moving in opposite directions, the stress is called shear (Figure 1.2). Shear stress is the most common stress at transform plate boundaries. Shearing in rocks. The white quartz vein has been elongated by shear. When stress causes a material to change shape, it has undergone strain or deformation. Deformed rocks are common in geologically active areas. A rocks response to stress depends on the rock type, the surrounding temperature, the pressure conditions the rock is under, the length of time the rock is under stress, and the type of stress. "
geological stresses,T_1219,"Rocks have three possible responses to increasing stress (illustrated in Figure 1.3): elastic deformation: the rock returns to its original shape when the stress is removed. plastic deformation: the rock does not return to its original shape when the stress is removed. fracture: the rock breaks. Under what conditions do you think a rock is more likely to fracture? Is it more likely to break deep within Earths crust or at the surface? What if the stress applied is sharp rather than gradual? At the Earths surface, rocks usually break quite quickly, but deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically. Sudden stress, such as a hit with a hammer, is more likely to make a rock break. Stress applied over time often leads to plastic deformation. Click image to the left or use the URL below. URL: "
geothermal power,T_1220,"The heat that is used for geothermal power may come to the surface naturally as hot springs or geysers, like The Geysers in northern California. Where water does not naturally come to the surface, engineers may pump cool water into the ground. The water is heated by the hot rock and then pumped back to the surface for use. The hot water or steam from a geothermal well spins a turbine to make electricity. Geothermal energy is clean and safe. The energy source is renewable since hot rock is found everywhere in the Earth, although in many parts of the world the hot rock is not close enough to the surface for building geothermal power plants. In some areas, geothermal power is common (Figure 1.1). In the United States, California is a leader in producing geothermal energy. The largest geothermal power plant in the state is in the Geysers Geothermal Resource Area in Napa and Sonoma Counties. The source of heat is thought to be a large magma chamber lying beneath the area. Where Earths internal heat gets close to the surface, geothermal power is a clean source of energy. In California, The Geysers supplies energy for many nearby homes and businesses. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
glaciers,T_1221,"Nearly all glacial ice, 99%, is contained in ice sheets in the polar regions, particularly Antarctica and Greenland. Glaciers often form in the mountains because higher altitudes are colder and more likely to have snow that falls and collects. Every continent, except Australia, hosts at least some glaciers in the high mountains. "
glaciers,T_1222,"The types of glaciers are: Continental glaciers are large ice sheets that cover relatively flat ground. These glaciers flow outward from where the greatest amounts of snow and ice accumulate. Alpine (valley) glaciers flow downhill from where the snow and ice accumulates through mountains along existing valleys. Ice caps are large glaciers that cover a larger area than just a valley, possibly an entire mountain range or region. Glaciers come off of ice caps into valleys. The Greenland ice cap covers the entire landmass. "
glaciers,T_1224,"Glaciers grow when more snow falls near the top of the glacier, in the zone of accumulation, than is melted from lower down in the glacier, in the zone of ablation. These two zones are separated by the equilibrium line. Snow falls and over time converts to granular ice known as firn. Eventually, as more snow and ice collect, the firn becomes denser and converts to glacial ice. Water is too warm for a glacier to form, so they form only on land. A glacier may run out from land into water, but it usually breaks up into icebergs that eventually melt into the water. "
glaciers,T_1225,"Whether an ice field moves or not depends on the amount of ice in the field, the steepness of the slope and the roughness of the ground surface. Ice moves where the pressure is so great that it undergoes plastic flow. Ice also slides at the bottom, often lubricated by water that has melted and travels between the ground and the ice. The speed of a glacier ranges from extremely fast, where conditions are favorable, to nearly zero. Because the ice is moving, glaciers have crevasses, where cracks form in the ice as a result of movement. The large crevasse at the top of an alpine glacier where ice that is moving is separated from ice that is stuck to the mountain above is called a bergshrund. Crevasses in a glacier are the result of movement. "
glaciers,T_1226,"Glaciers are melting back in many locations around the world. When a glacier no longer moves, it is called an ice sheet. This usually happens when it is less than 0.1 km2 in area and 50 m thick. "
glaciers,T_1227,"Many of the glaciers in Glacier National Park have shrunk and are no longer active. Summer temperatures have risen rapidly in this part of the country and so the rate of melting has picked up. Whereas Glacier National Park had 150 glaciers in 1850, there are only about 25 today. Recent estimates are that the park will have no active glaciers as early as 2020. This satellite image shows Grinnell Glacier, Swiftcurrent Glacier, and Gem Glacier in 2003 with an outline of the extent of the glaciers as they were in 1950. Although it continues to be classified as a glacier, Gem Glacier is only 0.020 km2 (5 acres) in area, only one-fifth the size of the smallest active glaciers. "
glaciers,T_1228,"In regions where summers are long and dry, melting glaciers in mountain regions provide an important source of water for organisms and often for nearby human populations. Click image to the left or use the URL below. URL: "
global warming,T_1229,"With more greenhouse gases trapping heat, average annual global temperatures are rising. This is known as global warming. "
global warming,T_1230,"While temperatures have risen since the end of the Pleistocene, 10,000 years ago, this rate of increase has been more rapid in the past century, and has risen even faster since 1990. The 10 warmest years in the 134-year record have all occurred since in the 21st century, and only one year during the 20th century (1998) was warmer than 2013, the 4th warmest year on record (through 2013) (Figure 1.1). The 2000s were the warmest decade yet. Annual variations aside, the average global temperature increased about 0.8o C (1.5o F) between 1880 and 2010, according to the Goddard Institute for Space Studies, NOAA. This number doesnt seem very large. Why is it important? "
global warming,T_1231,"The United States has long been the largest emitter of greenhouse gases, with about 20% of total emissions in 2004. As a result of Chinas rapid economic growth, its emissions surpassed those of the United States in 2008. However, its also important to keep in mind that the United States has only about one-fifth the population of China. Whats the significance of this? The average United States citizen produces far more greenhouse gas emissions than the average Chinese person. "
global warming,T_1232,"The following images show changes in the Earth and organisms as a result of global warming: Figure 1.2, Figure (a) Breakup of the Larsen Ice Shelf in Antarctica in 2002 was related to climate warming in the region. (b) The Boulder Glacier has melted back tremendously since 1985. Other mountain glaciers around the world are also melting. The timing of events for species is changing. Mating and migrations take place earlier in the spring months. Species that can are moving their ranges uphill. Some regions that were already marginal for agriculture are no longer arable because they have become too warm or dry. What are the two major effects being seen in this animation? Glaciers are melting and vegetation zones are moving uphill. If fossil fuel use exploded in the 1950s, why do these changes begin early in the animation? Does this mean that the climate change we are seeing is caused by natural processes and not by fossil fuel use? Permafrost is melting and its extent de- creasing. There are now fewer summer lakes in Siberia. (a) Melting ice caps add water to the oceans, so sea level is rising. Remember that water slightly expands as it warms this expansion is also causing sea level to rise. (b) Weather is becoming more variable with more severe storms and droughts. Snow blanketed the west- ern United States in December 2009. (c) As surface seas warm, phytoplankton productivity has decreased. (d) Coral reefs are dying worldwide; corals that are stressed by high temperatures turn white. (e) Pine beetle infestations have killed trees in western North America The insects have expanded their ranges into areas that were once too cold. Warming temperatures are bringing changes to much of the planet, including California. Sea level is rising, snow pack is changing, and the ecology of the state is responding to these changes. Click image to the left or use the URL below. URL: "
gravity in the solar system,T_1238,"Isaac Newton first described gravity as the force that causes objects to fall to the ground and also the force that keeps the Moon circling Earth instead of flying off into space in a straight line. Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists between all objects in the universe (Figure from each other. The greater the objects mass, the greater the force of attraction; in addition, the greater the distance between objects, the smaller the force of attraction. The distance between the Sun and each of its planets is very large, but the Sun and each of the planets are also very large. Gravity keeps each planet orbiting the Sun because the star and its planets are very large objects. The force of gravity also holds moons in orbit around planets. The force of gravity exists between all objects in the universe; the strength of the force depends on the mass of the objects and the distance between them. Click image to the left or use the URL below. URL: "
greenhouse effect,T_1239,"The exception to Earths temperature being in balance is caused by greenhouse gases. But first the role of greenhouse gases in the atmosphere must be explained. Greenhouse gases warm the atmosphere by trapping heat. Some of the heat that radiates out from the ground is trapped by greenhouse gases in the troposphere. Like a blanket on a sleeping person, greenhouse gases act as insulation for the planet. The warming of the atmosphere because of insulation by greenhouse gases is called the greenhouse effect (Figure 1.1). Greenhouse gases are the component of the atmosphere that moderate Earths temperatures. "
greenhouse effect,T_1240,"Greenhouse gases include CO2 , H2 O, methane, O3 , nitrous oxides (NO and NO2 ), and chlorofluorocarbons (CFCs). All are a normal part of the atmosphere except CFCs. Table 1.1 shows how each greenhouse gas naturally enters the atmosphere. Greenhouse Gas Carbon dioxide Methane Nitrous oxide Ozone Chlorofluorocarbons Where It Comes From Respiration, volcanic eruptions, decomposition of plant material; burning of fossil fuels Decomposition of plant material under some condi- tions, biochemical reactions in stomachs Produced by bacteria Atmospheric processes Not naturally occurring; made by humans Different greenhouse gases have different abilities to trap heat. For example, one methane molecule traps 23 times as much heat as one CO2 molecule. One CFC-12 molecule (a type of CFC) traps 10,600 times as much heat as one CO2 . Still, CO2 is a very important greenhouse gas because it is much more abundant in the atmosphere. "
greenhouse effect,T_1241,Human activity has significantly raised the levels of many of greenhouse gases in the atmosphere. Methane levels are about 2 1/2 times higher as a result of human activity. Carbon dioxide has increased more than 35%. CFCs have only recently existed. What do you think happens as atmospheric greenhouse gas levels increase? More greenhouse gases trap more heat and warm the atmosphere. The increase or decrease of greenhouse gases in the atmosphere affect climate and weather the world over. Click image to the left or use the URL below. URL: 
groundwater aquifers,T_1242,"To be a good aquifer, the rock in the aquifer must have good: porosity: small spaces between grains permeability: connections between pores To reach an aquifer, surface water infiltrates downward into the ground through tiny spaces or pores in the rock. The water travels down through the permeable rock until it reaches a layer that does not have pores; this rock is impermeable (Figure 1.1). This impermeable rock layer forms the base of the aquifer. The upper surface where the groundwater reaches is the water table. Groundwater is found beneath the solid surface. Notice that the water table roughly mirrors the slope of the lands surface. A well penetrates the water table. "
groundwater aquifers,T_1243,"For a groundwater aquifer to contain the same amount of water, the amount of recharge must equal the amount of discharge. What are the likely sources of recharge? What are the likely sources of discharge? What happens to the water table when there is a lot of rainfall? What happens when there is a drought? Although groundwater levels do not rise and fall as rapidly as at the surface, over time the water table will rise during wet periods and fall during droughts. In wet regions, streams are fed by groundwater; the surface of the stream is the top of the water table (Figure 1.2). In dry regions, water seeps down from the stream into the aquifer. These streams are often dry much of the year. Water leaves a groundwater reservoir in streams or springs. People take water from aquifers, too. "
groundwater aquifers,T_1244,"Groundwater meets the surface in a stream (Figure 1.2) or a spring (Figure 1.3). A spring may be constant, or may only flow at certain times of year. Towns in many locations depend on water from springs. Springs can be an extremely important source of water in locations where surface water is scarce. "
groundwater aquifers,T_1245,"A well is created by digging or drilling to reach groundwater. It is important for anyone who intends to dig a well to know how deep beneath the surface the water table is. When the water table is close to the surface, wells are a convenient method for extracting water. When the water table is far below the surface, specialized equipment must The top of the stream is the top of the water table. The stream feeds the aquifer. A spring in Croatia bubbles to the surface and feeds the river Cetina. be used to dig a well. Most wells use motorized pumps to bring water to the surface, but some still require people to use a bucket to draw water up (Figure 1.4). An old-fashioned well that uses a bucket drawn up by hand. "
groundwater depletion,T_1246,"Some aquifers are overused; people pump out more water than is replaced. As the water is pumped out, the water table slowly falls, requiring wells to be dug deeper, which takes more money and energy. Wells may go completely dry if they are not deep enough to reach into the lowered water table. Other problems may stem from groundwater overuse. Subsidence and saltwater intrusion are two of them. "
groundwater depletion,T_1247,"The Ogallala Aquifer supplies about one-third of the irrigation water in the United States. The Ogallala Aquifer is widely used by people for municipal and agricultural needs. (Figure 1.2). The aquifer is found from 30 to 100 meters deep over an area of about 440,000 square kilometers! The water in the aquifer is mostly from the last ice age. About eight times more water is taken from the Ogallala Aquifer each year than is replenished. Much of the water is used for irrigation (Figure 1.3). Click image to the left or use the URL below. URL:  Intense drought has reduced groundwater levels in the southern U.S., particularly in Texas and New Mexico. "
groundwater depletion,T_1248,Lowering the water table may cause the ground surface to sink. Subsidence may occur beneath houses and other structures (Figure 1.4). 
groundwater depletion,T_1249,"When coastal aquifers are overused, salt water from the ocean may enter the aquifer, contaminating the aquifer and making it less useful for drinking and irrigation. Salt water incursion is a problem in developed coastal regions, such as on Hawaii. "
groundwater pollution,T_1250,"Groundwater pollutants are the same as surface water pollutants: municipal, agricultural, and industrial. Ground- water is more susceptible to some sources of pollution. For example, irrigation water infiltrates into the ground, bringing with it the pesticides, fertilizers, and herbicides that were sprayed on the fields. Water that seeps through landfills also carries toxins into the ground. Toxic substances and things like gasoline are kept in underground storage tanks; more than 100,000 of the tanks are currently leaking and many more may develop leaks. "
groundwater pollution,T_1251,"Groundwater is a bit safer from pollution than surface water from some types of pollution because some pollutants are filtered out by the rock and soil that water travels through as it travels through the ground or once it is in the aquifer. But rock and soil cant get out everything, depending on the type of rock and soil and on the types of pollutants. As it is, about 25% of the usable groundwater and 45% of the municipal groundwater supplies in the United States are polluted. "
groundwater pollution,T_1252,"When the pollutant enters the aquifer, contamination spreads in the water outward from the source and travels in the direction that the water is moving. This pollutant plume may travel very slowly, only a few inches a day, but over time can contaminate a large portion of the aquifer. Many wells that are currently in use are contaminated. In Florida, for example, more than 90% of wells have detectible contaminants and thousands have been closed. "
growth of human populations,T_1253,"Human population growth over the past 10,000 years has been tremendous (Figure 1.1). The entire human popula- tion was estimated to be 5 million in 8000 B.C. 300 million in A.D. 1 1 billion in 1802 3 billion in 1961 7 billion in 2011 As the human population continues to grow, different factors limit population in different parts of the world. What might be a limiting factor for human population in a particular location? Space, clean air, clean water, and food to feed everyone are limiting in some locations. "
growth of human populations,T_1254,"Not only has the population increased, but the rate of population growth has increased (Figure 1.2). The population was estimated to reach 7 billion in 2012, but it did so in 2011, just 12 years after reaching 6 billion. Human population from 10,000 BC through 2000 AD, showing the exponential increase in human population that has occurred in the last few centuries. The amount of time between the addition of each one billion people to the planets population, including speculation about the future. Although population continues to grow rapidly, the rate that the growth rate is increasing has declined. Still, a recent estimate by the United Nations estimates that 10.1 billion people will be sharing this planet by the end of the century. The total added will be about 3 billion people, which is more than were even in existence as recently as 1960. "
hazardous waste,T_1255,"Hazardous waste is any waste material that is dangerous to human health or that degrades the environment. Haz- ardous waste includes substances that are: 1. 2. 3. 4. Toxic: causes serious harm or death, or is poisonous. Chemically active: causes dangerous or unwanted chemical reactions, such as explosions. Corrosive: destroys other things by chemical reactions. Flammable: easily catches fire and may send dangerous smoke into the air. All sorts of materials are hazardous wastes and there are many sources. Many people have substances that could become hazardous wastes in their homes. Several cleaning and gardening chemicals are hazardous if not used properly. These include chemicals like drain cleaners and pesticides that are toxic to humans and many other creatures. While these chemicals are fine if they are stored and used properly, if they are used or disposed of improperly, they may become hazardous wastes. Others sources of hazardous waste are shown in Table 1.1. Type of Hazardous Waste Chemicals from the automobile in- dustry Example Gasoline, used motor oil, battery acid, brake fluid Batteries Car batteries, household batteries Medical wastes Dry cleaning chemicals Surgical gloves, wastes contami- nated with body fluids such as blood, x-ray equipment Paints, paint thinners, paint strip- pers, wood stains Many various chemicals Agricultural chemicals Pesticides, herbicides, fertilizers Paints Why it is Hazardous Toxic to humans and other organ- isms; often chemically active; often flammable. Contain toxic chemicals; are often corrosive. Toxic to humans and other organ- isms; may be chemically active. Toxic; flammable. Toxic; many cause cancer in hu- mans. Toxic to humans; can harm other organism; pollute soils and water. Click image to the left or use the URL below. URL: "
heat budget of planet earth,T_1256,"About half of the solar radiation that strikes the top of the atmosphere is filtered out before it reaches the ground. This energy can be absorbed by atmospheric gases, reflected by clouds, or scattered. Scattering occurs when a light wave strikes a particle and bounces off in some other direction. About 3% of the energy that strikes the ground is reflected back into the atmosphere. The rest is absorbed by rocks, soil, and water and then radiated back into the air as heat. These infrared wavelengths can only be seen by infrared sensors. Click image to the left or use the URL below. URL: "
heat budget of planet earth,T_1257,"Because solar energy continually enters Earths atmosphere and ground surface, is the planet getting hotter? The answer is no (although the next section contains an exception), because energy from Earth escapes into space through the top of the atmosphere. If the amount that exits is equal to the amount that comes in, then average global temperature stays the same. This means that the planets heat budget is in balance. What happens if more energy comes in than goes out? If more energy goes out than comes in? To say that the Earths heat budget is balanced ignores an important point. The amount of incoming solar energy is different at different latitudes. Where do you think the most solar energy ends up and why? Where does the least solar energy end up and why? See the Table 1.1. Equatorial Region Polar Regions Day Length Nearly the same all year Night 6 months Sun Angle High Solar Radiation High Albedo Low Low Low High Note: Colder temperatures mean more ice and snow cover the ground, making albedo relatively high. The difference in solar energy received at different latitudes drives atmospheric circulation. "
heat transfer in the atmosphere,T_1258,"Heat moves in the atmosphere the same way it moves through the solid Earth or another medium. What follows is a review of the way heat flows, but applied to the atmosphere. Radiation is the transfer of energy between two objects by electromagnetic waves. Heat radiates from the ground into the lower atmosphere. In conduction, heat moves from areas of more heat to areas of less heat by direct contact. Warmer molecules vibrate rapidly and collide with other nearby molecules, transferring their energy. In the atmosphere, conduction is more effective at lower altitudes, where air density is higher. This transfers heat upward to where the molecules are spread further apart or transfers heat laterally from a warmer to a cooler spot, where the molecules are moving less vigorously. Heat transfer by movement of heated materials is called convection. Heat that radiates from the ground initiates convection cells in the atmosphere (Figure 1.1). Click image to the left or use the URL below. URL: "
heat transfer in the atmosphere,T_1259,Different parts of the Earth receive different amounts of solar radiation. Which part of the planet receives the most solar radiation? The Suns rays strike the surface most directly at the Equator. The difference in solar energy received at different latitudes drives atmospheric circulation. 
heat waves and droughts,T_1260,"A heat wave is different depending on its location. According to the World Meteorological Organization, a region is in a heat wave if it has more than five consecutive days of temperatures that are more than 9 F (5 C) above average. Heat waves have increased in frequency and duration in recent years. The summer 2011 North American heat wave brought record temperatures across the Midwestern and Eastern United States. Many states and localities broke records for temperatures and for most days above 100 F. "
heat waves and droughts,T_1261,A high pressure cell sitting over a region with no movement is the likely cause of a heat wave. What do you think caused the heat wave in the image below (Figure 1.1)? A high pressure zone kept the jet stream further north than normal for August. A heat wave over the United States as in- dicated by heat radiated from the ground. The bright yellow areas are the hottest and the blue and white are coolest. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: 
heat waves and droughts,T_1262,"Droughts also depend on what is normal for a region. When a region gets significantly less precipitation than normal for an extended period of time, it is in drought. The Southern United States is experiencing an ongoing and prolonged drought. Drought has many consequences. When soil loses moisture it may blow away, as happened during the Dust Bowl in the United States in the 1930s. Forests may be lost, dust storms may become common, and wildlife are disturbed. Wildfires become much more common during times of drought. "
hot springs and geysers,T_1277,Water sometimes comes into contact with hot rock. The water may emerge at the surface as either a hot spring or a geyser. 
hot springs and geysers,T_1278,"Water heated below ground that rises through a crack to the surface creates a hot spring. The water in hot springs may reach temperatures in the hundreds of degrees Celsius beneath the surface, although most hot springs are much cooler. Click image to the left or use the URL below. URL: "
hot springs and geysers,T_1279,"Geysers are also created by water that is heated beneath the Earths surface, but geysers do not bubble to the surface they erupt. When water is both superheated by magma and flows through a narrow passageway underground, the environment is ideal for a geyser. The passageway traps the heated water underground, so that heat and pressure can build. Eventually, the pressure grows so great that the superheated water bursts out onto the surface to create a geyser. Figure 1.2. Conditions are right for the formation of geysers in only a few places on Earth. Of the roughly 1,000 geysers worldwide, about half are found in the United States. Yellowstone isnt the only place in the continental U.S. with hot springs and geysers. Hot Creek in California deserves its name; Like Yellowstone, it is above a supervolcano. Click image to the left or use the URL below. URL:  Castle Geyser is one of the many gey- sers at Yellowstone National Park. Castle erupts regularly, but not as frequently or predictably as Old Faithful. "
how fossilization creates fossils,T_1280,"It wasnt always known that fossils were parts of living organisms. In 1666, a young doctor named Nicholas Steno dissected the head of an enormous great white shark that had been caught by fisherman near Florence, Italy. Steno was struck by the resemblance of the sharks teeth to fossils found in inland mountains and hills (Figure 1.1). Most people at the time did not believe that fossils were once part of living creatures. Authors in that day thought that the fossils of marine animals found in tall mountains, miles from any ocean could be explained in one of two ways: The shells were washed up during the Biblical flood. (This explanation could not account for the fact that fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earths surface.) The fossils formed within the rocks as a result of mysterious forces. But for Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were once parts of living creatures. Fossil Shark Tooth (left) and Modern Shark Tooth (right). "
how fossilization creates fossils,T_1281,"A fossil is any remains or traces of an ancient organism. Fossils include body fossils, left behind when the soft parts have decayed away, and trace fossils, such as burrows, tracks, or fossilized coprolites (feces). Collections of fossils are known as fossil assemblages. Click image to the left or use the URL below. URL: "
how fossilization creates fossils,T_1282,"Becoming a fossil isnt easy. Only a tiny percentage of the organisms that have ever lived become fossils. Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelope that dies on the African plain (Figure 1.2). Most of its body is eaten by hyenas and other scavengers and the remaining flesh is devoured by insects and bacteria. Only bones are left behind. As the years go by, the bones are scattered and fragmented into small pieces, eventually turning into dust. The remaining nutrients return to the soil. This antelope will not be preserved as a fossil. Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the soft parts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces. The shells are also attacked by worms, sponges, and other animals (Figure 1.3). How about a soft bodied organism? Will a creature without hard shells or bones become a fossil? There is virtually no fossil record of soft bodied organisms such as jellyfish, worms, or slugs. Insects, which are by far the most common land animals, are only rarely found as fossils (Figure 1.4). "
how fossilization creates fossils,T_1283,"Despite these problems, there is a rich fossil record. How does an organism become fossilized? A rare insect fossil. "
how fossilization creates fossils,T_1284,"Usually its only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine creatures are common also. "
how fossilization creates fossils,T_1285,"Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near a river delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains (Figure 1.5). This fish was quickly buried in sediment to become a fossil. Quick burial is rare on land, so fossils of land animals and plants are less common than marine fossils. Land People buried by the extremely hot eruption of ash and gases at Mt. Vesuvius in 79 AD. "
how fossilization creates fossils,T_1286,"Unusual circumstances may lead to the preservation of a variety of fossils, as at the La Brea Tar Pits in Los Angeles, California. Although the animals trapped in the La Brea Tar Pits probably suffered a slow, miserable death, their bones were preserved perfectly by the sticky tar. (Figure 1.7). Artists concept of animals surrounding the La Brea Tar Pits. In spite of the difficulties of preservation, billions of fossils have been discovered, examined, and identified by thousands of scientists. The fossil record is our best clue to the history of life on Earth, and an important indicator "
how fossilization creates fossils,T_1287,"Some rock beds contain exceptional fossils or fossil assemblages. Two of the most famous examples of soft organism preservation are from the 505 million-year-old Burgess Shale in Canada (Figure 1.8). The 145 million-year-old Solnhofen Limestone in Germany has fossils of soft body parts that are not normally preserved (Figure 1.8). (a) The Burgess shale contains soft-bodied fossils. (b) Anomalocaris, meaning abnormal shrimp is now extinct. The image is of a fossil. (c) The famous Archeopteryx fossil from the Solnhofen Limestone has distinct feathers and was one of the earliest birds. Click image to the left or use the URL below. URL: "
how ocean currents moderate climate,T_1288,"Surface currents play an enormous role in Earths climate. Even though the Equator and poles have very different climates, these regions would have more extremely different climates if ocean currents did not transfer heat from the equatorial regions to the higher latitudes. The Gulf Stream is a river of warm water in the Atlantic Ocean, about 160 kilometers wide and about a kilometer deep. Water that enters the Gulf Stream is heated as it travels along the Equator. The warm water then flows up the east coast of North America and across the Atlantic Ocean to Europe (see opening image). The energy the Gulf Stream transfers is enormous: more than 100 times the worlds energy demand. The Gulf Streams warm waters raise temperatures in the North Sea, which raises the air temperatures over land between 3 to 6 C (5 to 11 F). London, U.K., for example, is at about six degrees further south than Quebec, Canada. However, Londons average January temperature is 3.8 C (38 F), while Quebecs is only -12 C (10 F). Because air traveling over the warm water in the Gulf Stream picks up a lot of water, London gets a lot of rain. In contrast, Quebec is much drier and receives its precipitation as snow. Quebec City, Quebec in winter. Click image to the left or use the URL below. URL: "
human evolution,T_1289,"Humans evolved during the later Cenozoic. New fossil discoveries alter the details of what we know about the evolution of modern humans, but the major evolutionary path is well understood. "
human evolution,T_1290,"Humans evolved from primates, and apes and humans have a primate common ancestor. About 7 million years ago, chimpanzees (our closest living relatives) and humans shared their last common ancestor. "
human evolution,T_1291,"Animals of the genus Ardipithecus, living roughly 4 to 6 million years ago, had brains roughly the size of a female chimp. Although they lived in trees, they were bipedal. Standing on two feet allows an organism to see and also to use its hands and arms for hunting. By the time of Australopithecus afarensis, between 3.9 and 2.9 million years ago, these human ancestors were completely bipedal and their brains were growing rapidly (Figure 1.1). Australopithecus afarensis is a human ancestor that lived about 3 million years ago. The genus Homo appeared about 2.5 million years ago. Humans developed the first stone tools. Homo erectus evolved in Africa about 1.8 million years ago. Fossils of these animals show a much more human-like body structure, which allowed them to travel long distances to hunt. Cultures begin and evolve. Homo sapiens, our species, originated about 200,000 years ago in Africa. Evidence of a spiritual life appears about 32,000 years ago with stone figurines that probably have religious significance (Figure 1.2). The ice ages allowed humans to migrate. During the ice ages, water was frozen in glaciers and so land bridges such as the Bering Strait allowed humans to walk from the old world to the new world. DNA evidence suggests that the humans who migrated out of Africa interbred with Neanderthal since these people contain some Neanderthal DNA. Click image to the left or use the URL below. URL:  Stone figurines likely indicate a spiritual life. "
igneous rocks,T_1298,Different factors play into the composition of a magma and the rock it produces. 
igneous rocks,T_1299,The rock beneath the Earths surface is sometimes heated to high enough temperatures that it melts to create magma. Different magmas have different composition and contain whatever elements were in the rock or rocks that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust. Table 1.1 lists the abundance of elements found in the Earths crust and in magma. The remaining 1.5% is made up of many other elements that are present in tiny quantities. Element Symbol Percent Element Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium Total Symbol O Si Al Fe Ca Na K Mg Percent 46.6% 27.7% 8.1% 5.0% 3.6% 2.8% 2.6% 2.1% 98.5% 
igneous rocks,T_1300,"Whether rock melts to create magma depends on: Temperature: Temperature increases with depth, so melting is more likely to occur at greater depths. Pressure: Pressure increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures. Water: The addition of water changes the melting point of rock. As the amount of water increases, the melting point decreases. Rock composition: Minerals melt at different temperatures, so the temperature must be high enough to melt at least some minerals in the rock. The first mineral to melt from a rock will be quartz (if present) and the last will be olivine (if present). The different geologic settings that produce varying conditions under which rocks melt will be discussed in the chapter Plate Tectonics. "
igneous rocks,T_1301,"As a rock heats up, the minerals that melt at the lowest temperatures melt first. Partial melting occurs when the temperature on a rock is high enough to melt only some of the minerals in the rock. The minerals that will melt will be those that melt at lower temperatures. Fractional crystallization is the opposite of partial melting. This process describes the crystallization of different minerals as magma cools. Bowens Reaction Series indicates the temperatures at which minerals melt or crystallize (Figure 1.1). An under- standing of the way atoms join together to form minerals leads to an understanding of how different igneous rocks form. Bowens Reaction Series also explains why some minerals are always found together and some are never found together. If the liquid separates from the solids at any time in partial melting or fractional crystallization, the chemical composition of the liquid and solid will be different. When that liquid crystallizes, the resulting igneous rock will have a different composition from the parent rock. Bowens Reaction Series. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
impact of continued global warming,T_1302,"The amount CO2 levels will rise in the next decades is unknown. What will this number depend on in the developed nations? What will it depend on in the developing nations? In the developed nations it will depend on technological advances or lifestyle changes that decrease emissions. In the developing nations, it will depend on how much their lifestyles improve and how these improvements are made. If nothing is done to decrease the rate of CO2 emissions, by 2030, CO2 emissions are projected to be 63% greater than they were in 2002. "
impact of continued global warming,T_1303,"Computer models are used to predict the effects of greenhouse gas increases on climate for the planet as a whole and also for specific regions. If nothing is done to control greenhouse gas emissions and they continue to increase at current rates, the surface temperature of the Earth can be expected to increase between 0.5o C and 2.0o C (0.9o F and 3.6o F) by 2050 and between 2o and 4.5o C (3.5o and 8o F) by 2100, with CO2 levels over 800 parts per million (ppm). Global CO2 emissions are rising rapidly. The industrial revolution began about 1850 and industrialization has been ac- celerating. On the other hand, if severe limits on CO2 emissions begin soon, temperatures could rise less than 1.1o C (2o F) by 2100. Click image to the left or use the URL below. URL:  Whatever the temperature increase, it will not be uniform around the globe. A rise of 2.8o C (5o F) would result in 0.6o to 1.2o C (1o to 2o F) at the Equator, but up to 6.7o C (12o F) at the poles. So far, global warming has affected the North Pole more than the South Pole, but temperatures are still increasing at Antarctica (Figure 1.2). "
impact of continued global warming,T_1304,"As greenhouse gases increase, changes will be more extreme. Oceans will become more acidic, making it more difficult for creatures with carbonate shells to grow, and that includes coral reefs. A study monitoring ocean acidity in the Pacific Northwest found ocean acidity increasing ten times faster than expected and 10% to 20% of shellfish (mussels) being replaced by acid-tolerant algae. Plant and animal species seeking cooler temperatures will need to move poleward 100 to 150 km (60 to 90 miles) or upward 150 m (500 feet) for each 1.0o C (8o F) rise in global temperature. There will be a tremendous loss of biodiversity because forest species cant migrate that rapidly. Biologists have already documented the extinction of high-altitude species that have nowhere higher to go. Decreased snow packs, shrinking glaciers, and the earlier arrival of spring will all lessen the amount of water available in some regions of the world, including the western United States and much of Asia. Ice will continue to melt and sea level is predicted to rise 18 to 97 cm (7 to 38 inches) by 2100 (Figure 1.3). An increase this large will gradually flood coastal regions, where about one-third of the worlds population lives, forcing billions of people to move inland. Sea ice thickness around the North Pole has been decreasing in recent decades and will continue to decrease in the com- ing decades. Weather will become more extreme, with more frequent and more intense heat waves and droughts. Some modelers predict that the midwestern United States will become too dry to support agriculture and that Canada will become the new breadbasket. In all, about 10% to 50% of current cropland worldwide may become unusable if CO2 doubles. You may notice that the numerical predictions above contain wide ranges. Sea level, for example, is expected to rise somewhere between 18 and 97 cm  quite a wide range. What is the reason for this uncertainty? It is partly because scientists cannot predict exactly how the Earth will respond to increased levels of greenhouses gases. How quickly greenhouse gases continue to build up in the atmosphere depends in part on the choices we make. An important question people ask is this: Are the increases in global temperature natural? In other words, can natural variations in temperature account for the increase in temperature that we see? The answer is no. Changes in the Suns irradiance, El Nio and La Nia cycles, natural changes in greenhouse gas, and other atmospheric gases cannot account for the increase in temperature that has already happened in the past decades. Along with the rest of the worlds oceans, San Francisco Bay is rising. Changes are happening slowly in the coastal arena of the San Francisco Bay Area and even the most optimistic estimates about how high and how quickly this rise will occur indicate potentially huge problems for the region. Click image to the left or use the URL below. URL: "
impacts of hazardous waste,T_1305,"The story of Love Canal, New York, begins in the 1950s, when a local chemical company placed hazardous wastes in 55-gallon steel drums and buried them. Love Canal was an abandoned waterway near Niagara Falls and was thought to be a safe site for hazardous waste disposal because the ground was fairly impermeable (Figure 1.1). After burial, the company covered the containers with soil and sold the land to the local school system for $1. The company warned the school district that the site had been used for toxic waste disposal. Steel drums were used to contain 21,000 tons of hazardous chemicals at Love Canal. Soon a school, a playground, and 100 homes were built on the site. The impermeable ground was breached when sewer systems were dug into the rock layer. Over time, the steel drums rusted and the chemicals were released into the ground. In the 1960s people began to notice bad odors. Children developed burns after playing in the soil, and they were often sick. In 1977 a swamp created by heavy rains was found to contain 82 toxic chemicals, including 11 suspected cancer-causing chemicals. A Love Canal resident, Lois Gibbs, organized a group of citizens called the Love Canal Homeowners Association to try to find out what was causing the problems (See opening image). When they discovered that toxic chemicals were buried beneath their homes and school, they demanded that the government take action to clean up the area and remove the chemicals. "
impacts of hazardous waste,T_1306,"In 1978, people were relocated to safe areas. The problem of Love Canal was instrumental in the passage of the the Superfund Act in 1980. This law requires companies to be responsible for hazardous chemicals that they put into the environment and to pay to clean up polluted sites, which can often cost hundreds of millions of dollars. Love Canal became a Superfund site in 1983 and as a result, several measures were taken to secure the toxic wastes. The land was capped so that water could not reach the waste, debris was cleaned from the nearby area, and contaminated soils were removed. "
impacts of hazardous waste,T_1307,"The pollution at Love Canal was not initially visible, but it became visible. The health effects from the waste were also not initially visible, but they became clearly visible. The effects of the contamination that were seen in human health included sickness in children and a higher than normal number of miscarriages in pregnant women. Toxic chemicals may cause cancer and birth defects. Why do you think children and fetuses are more susceptible? Because young organisms grow more rapidly, they take in more of the toxic chemicals and are more affected. "
impacts of hazardous waste,T_1308,"Sometimes the chemicals are not so easily seen as they were at Love Canal. But the impacts can be seen statistically. For example, contaminated drinking water may cause an increase in some types of cancer in a community. Why is one person with cancer not enough to suspect contamination by toxic waste? One is not a statistically valid number. A certain number of people get cancer all the time. To identify contamination, a number of cancers above the normal rate, called a cancer cluster, must be discovered. A case that was made into a book and movie called A Civil Action involved the community of Woburn, Massachusetts. Groundwater contamination was initially suspected because of an increase in childhood leukemia and other illnesses. As a result of concern by parents, the well water was analyzed and shown to have high levels of TCE (trichloroethylene). "
impacts of hazardous waste,T_1309,"Lead and mercury are two chemicals that are especially toxic to humans. Lead was once a common ingredient in gasoline and paint, but it was shown to damage human brains and nervous systems. Since young children are growing rapidly, lead is especially harmful in children under the age of six (Figure 1.2). In the 1970s and 1980s, the United States government passed laws completely banning lead in gasoline and paint. Homes built before the 1970s may contain lead paint. Paint so old is likely to be peeling and poses a great threat to human health. About 200 children die every year from lead poisoning. (a) Leaded gasoline. (b) Leaded paint. Mercury is a pollutant that can easily spread around the world. Sources of mercury include volcanic eruptions, coal burning, and wastes such as batteries, electronic switches, and electronic appliances such as television sets. Like lead, mercury damages the brain and impairs nervous system function. More about the hazards of mercury pollution can be found later in this concept. "
importance of the atmosphere,T_1310,"Earths atmosphere is a thin blanket of gases and tiny particles  together called air. We are most aware of air when it moves and creates wind. Earths atmosphere, along with the abundant liquid water at Earths surface, are the keys to our planets unique place in the solar system. Much of what makes Earth exceptional depends on the atmosphere. For example, all living things need some of the gases in air for life support. Without an atmosphere, Earth would likely be just another lifeless rock. Lets consider some of the reasons we are lucky to have an atmosphere. "
importance of the atmosphere,T_1311,"Without the atmosphere, Earth would look a lot more like the Moon. Atmospheric gases, especially carbon dioxide (CO2 ) and oxygen (O2 ), are extremely important for living organisms. How does the atmosphere make life possible? How does life alter the atmosphere? The composition of Earths atmosphere. "
importance of the atmosphere,T_1312,"In photosynthesis, plants use CO2 and create O2 . Photosynthesis is responsible for nearly all of the oxygen currently found in the atmosphere. The chemical reaction for photosynthesis is: 6CO2 + 6H2 O + solar energy  C6 H12 O6 (sugar) + 6O2 "
importance of the atmosphere,T_1313,"By creating oxygen and food, plants have made an environment that is favorable for animals. In respiration, animals use oxygen to convert sugar into food energy they can use. Plants also go through respiration and consume some of the sugars they produce. The chemical reaction for respiration is: C6 H12 O6 + 6O2  6CO2 + 6H2 O + useable energy How is respiration similar to and different from photosynthesis? They are approximately the reverse of each other. In photosynthesis, CO2 is converted to O2 and in respiration, O2 is converted to CO2 (Figure 1.2). "
importance of the atmosphere,T_1314,"As part of the hydrologic cycle, water spends a lot of time in the atmosphere, mostly as water vapor. The atmosphere is an important reservoir for water. Chlorophyll indicates the presence of photosynthesizing plants as does the veg- etation index. "
importance of the atmosphere,T_1315,"Ozone is a molecule composed of three oxygen atoms, (O3 ). Ozone in the upper atmosphere absorbs high-energy ultraviolet (UV) radiation coming from the Sun. This protects living things on Earths surface from the Suns most harmful rays. Without ozone for protection, only the simplest life forms would be able to live on Earth. The highest concentration of ozone is in the ozone layer in the lower stratosphere. "
importance of the atmosphere,T_1316,"Along with the oceans, the atmosphere keeps Earths temperatures within an acceptable range. Without an atmo- sphere, Earths temperatures would be frigid at night and scorching during the day. If the 12-year-old in the scenario above asked why, she would find out. Greenhouse gases trap heat in the atmosphere. Important greenhouse gases include carbon dioxide, methane, water vapor, and ozone. "
importance of the atmosphere,T_1317,"The atmosphere is made of gases that take up space and transmit energy. Sound waves are among the types of energy that travel though the atmosphere. Without an atmosphere, we could not hear a single sound. Earth would be as silent as outer space (explosions in movies about space should be silent). Of course, no insect, bird, or airplane would be able to fly, because there would be no atmosphere to hold it up. Click image to the left or use the URL below. URL: "
importance of the oceans,T_1318,"The oceans, along with the atmosphere, keep temperatures fairly constant worldwide. While some places on Earth get as cold as -70o C and others as hot as 55o C, the range is only 125o C. On Mercury temperatures go from -180o C to 430o C, a range of 610o C. The oceans, along with the atmosphere, distribute heat around the planet. The oceans absorb heat near the Equator and then move that solar energy to more polar regions. The oceans also moderate climate within a region. At the same latitude, the temperature range is smaller in lands nearer the oceans than away from the oceans. Summer temperatures are not as hot, and winter temperatures are not as cold, because water takes a long time to heat up or cool down. "
importance of the oceans,T_1319,"The oceans are an essential part of Earths water cycle. Since they cover so much of the planet, most evaporation comes from oceans and most precipitation falls on oceans. "
importance of the oceans,T_1320,"The oceans are home to an enormous amount of life. That is, they have tremendous biodiversity (Figure 1.1). Tiny ocean plants, called phytoplankton, create the base of a food web that supports all sorts of life forms. Marine life makes up the majority of all biomass on Earth. (Biomass is the total mass of living organisms in a given area.) These organisms supply us with food and even the oxygen created by marine plants. Polar bears are well adapted to frigid Arc- tic waters. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
influences on weathering,T_1321,"Different rock types weather at different rates. Certain types of rock are very resistant to weathering. Igneous rocks, especially intrusive igneous rocks such as granite, weather slowly because it is hard for water to penetrate them. Other types of rock, such as limestone, are easily weathered because they dissolve in weak acids. Rocks that resist weathering remain at the surface and form ridges or hills. Shiprock in New Mexico is the throat of a volcano thats left after the rest of the volcano eroded away. The rock thats left behind is magma that cooled relatively slowly and is harder than the rock that had surrounded it. Different minerals also weather at different rates. Some minerals in a rock might completely dissolve in water, but the more resistant minerals remain. In this case, the rocks surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock. A beautiful example of this effect is the ""Stone Forest"" in China, see the video below: The Shiprock formation in northwest New Mexico is the central plug of resistant lava from which the surrounding rock weath- ered and eroded away. Click image to the left or use the URL below. URL: "
influences on weathering,T_1322,"A regions climate strongly influences weathering. Climate is determined by the temperature of a region plus the amount of precipitation it receives. Climate is weather averaged over a long period of time. Chemical weathering increases as: Temperature increases: Chemical reactions proceed more rapidly at higher temperatures. For each 10o C increase in average temperature, the rate of chemical reactions doubles. Precipitation increases: More water allows more chemical reactions. Since water participates in both mechan- ical and chemical weathering, more water strongly increases weathering. So how do different climates influence weathering? A cold, dry climate will produce the lowest rate of weathering. A warm, wet climate will produce the highest rate of weathering. The warmer a climate is, the more types of vegetation it will have and the greater the rate of biological weathering (Figure 1.2). This happens because plants and bacteria grow and multiply faster in warmer temperatures. "
influences on weathering,T_1323,"Some resources are concentrated by weathering processes. In tropical climates, intense chemical weathering carries away all soluble minerals, leaving behind just the least soluble components. The aluminum oxide, bauxite, forms this way and is our main source of aluminum ore. "
inner vs. outer planets,T_1324,"The inner planets, or terrestrial planets, are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. Figure 1.1 shows the relative sizes of these four inner planets. Unlike the outer planets, which have many satellites, Mercury and Venus do not have moons, Earth has one, and Mars has two. Of course, the inner planets have shorter orbits around the Sun, and they all spin more slowly. Geologically, the inner planets are all made of cooled igneous rock with iron cores, and all have been geologically active, at least early in their history. None of the inner planets has rings. Click image to the left or use the URL below. URL:  This composite shows the relative sizes of the four inner planets. From left to right, they are Mercury, Venus, Earth, and Mars. "
inner vs. outer planets,T_1325,"The four planets farthest from the Sun are the outer planets. Figure 1.2 shows the relative sizes of the outer planets and the Sun. These planets are much larger than the inner planets and are made primarily of gases and liquids, so they are also called gas giants. The gas giants are made up primarily of hydrogen and helium, the same elements that make up most of the Sun. Astronomers think that hydrogen and helium gases comprised much of the solar system when it first formed. Since the inner planets didnt have enough mass to hold on to these light gases, their hydrogen and helium floated away into space. The Sun and the massive outer planets had enough gravity to keep hydrogen and helium from drifting away. All of the outer planets have numerous moons. They all also have planetary rings, composed of dust and other small particles that encircle the planet in a thin plane. Click image to the left or use the URL below. URL:  This image shows the four outer planets and the Sun, with sizes to scale. From left to right, the outer planets are Jupiter, Saturn, Uranus, and Neptune. "
interior of the sun,T_1326,"Fossils are our best form of evidence about Earth history, including the history of life. Along with other geological evidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and other major geological events. Since the present is the key to the past, what we know about a type of organism that lives today can be applied to past environments. "
interior of the sun,T_1327,"That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocks resemble animals and plants that are living today. In general, fossils in older rocks are less similar to modern organisms. We would know very little about the organisms that came before us if there were no fossils. Modern technology has allowed scientists to reconstruct images and learn about the biology of extinct animals like dinosaurs! Click image to the left for more content. "
interior of the sun,T_1328,"By knowing something about the type of organism the fossil was, geologists can determine whether the region was terrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give clues to whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allows scientists to learn about what happened to the region after the organism died; for example, whether it was exposed to wave action. "
interior of the sun,T_1329,The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine. Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabed were uplifted. 
interior of the sun,T_1330,"By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher the climate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coal beds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was much warmer. Recall from Concept Plate Tectonics that Wegener used the presence of coal beds in Antarctica as one of the lines of evidence for continental drift. "
interior of the sun,T_1331,"An index fossil can be used to identify a specific period of time. Organisms that make good index fossils are distinctive, widespread, and lived briefly. Their presence in a rock layer can be used to identify rocks that were deposited at that period of time over a large area. "
interior of the sun,T_1332,Use this resource to answer the questions that follow. Clues to the End - Permian Extinction Click image to the left for more content. 1. Why is the paleocologists collecting samples? 2. What does he want to create from the fossil evidence? 3. How is this similar to forensic science? 4. Why is it important to understand insect feeding? 5. What has been discovered from these fossils? 
introduction to groundwater,T_1339,"Groundwater resides in aquifers, porous rock and sediment with water in between. Water is attracted to the soil particles, and capillary action, which describes how water moves through porous media, moves water from wet soil to dry areas. Aquifers are found at different depths. Some are just below the surface and some are found much deeper below the land surface. A region may have more than one aquifer beneath it and even most deserts are above aquifers. The source region for an aquifer beneath a desert is likely to be far away, perhaps in a mountainous area. "
introduction to groundwater,T_1340,"The amount of water that is available to enter groundwater in a region, called recharge, is influenced by the local climate, the slope of the land, the type of rock found at the surface, the vegetation cover, land use in the area, and water retention, which is the amount of water that remains in the ground. More water goes into the ground where there is a lot of rain, flat land, porous rock, exposed soil, and where water is not already filling the soil and rock. "
introduction to groundwater,T_1341,"The residence time of water in a groundwater aquifer can be from minutes to thousands of years. Groundwater is often called fossil water because it has remained in the ground for so long, often since the end of the ice ages. A diagram of groundwater flow through aquifers showing residence times. Deeper aquifers typically contain older ""fossil water."" Click image to the left or use the URL below. URL: "
intrusive and extrusive igneous rocks,T_1342,The rate at which magma cools determines whether an igneous rock is intrusive or extrusive. The cooling rate is reflected in the rocks texture. 
intrusive and extrusive igneous rocks,T_1343,"Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see Figure 1.1 for an example). Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and bi- otite (black, platy). igneous rocks to the surface. Figure 1.2 shows a landscape in Californias Sierra Nevada Mountains made of granite that has been raised to create mountains. Californias Sierra Nevada Mountains are intrusive igneous rock exposed at Earths surface. "
intrusive and extrusive igneous rocks,T_1344,"Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks (Figure 1.3). Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (Figure 1.4). Some volcanic rocks have a different texture. The rock has large crystals set within a matrix of tiny crystals. In this Extrusive igneous rocks form after lava cools above the surface. Cooled lava forms basalt with no visible crystals. Why are there no visible crys- tals? Cooling rate and gas content create other textures (see Figure 1.5 for examples of different textures). Lavas that cool extremely rapidly may have a glassy texture. Those with many holes from gas bubbles have a vesicular texture. Different cooling rate and gas content resulted in these different textures. Click image to the left or use the URL below. URL: "
jupiter,T_1345,"Jupiter is enormous, the largest object in the solar system besides the Sun. Although Jupiter is over 1,300 times Earths volume, it has only 318 times the mass of Earth. Like the other gas giants, it is much less dense than Earth. Because Jupiter is so large, it reflects a lot of sunlight. Jupiter is extremely bright in the night sky; only the Moon and Venus are brighter (Figure 1.1). This brightness is all the more impressive because Jupiter is quite far from the Earth  5.20 AUs away. It takes Jupiter about 12 Earth years to orbit once around the Sun. "
jupiter,T_1346,"Astronauts trying to land a spaceship on the surface of Jupiter would find that there is no solid surface at all! Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements (Figure 1.2). Jupiters atmosphere is composed of hydrogen and helium. Deeper within the planet, pressure compresses the gases into a liquid. Some evidence suggests that Jupiter may have a small rocky core of heavier elements at its center. This image of Jupiter was taken by Voy- ager 2 in 1979. The colors were later enhanced to bring out more details. "
jupiter,T_1347,"The upper layer of Jupiters atmosphere contains clouds of ammonia (NH3 ) in bands of different colors. These bands rotate around the planet, but also swirl around in turbulent storms. The Great Red Spot (Figure 1.3) is an enormous, oval-shaped storm found south of Jupiters equator. This storm is more than three times as wide as the entire Earth. Clouds in the storm rotate in a counterclockwise direction, making one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least 300 years, since astronomers could first see the storm through telescopes. Do you think the Great Red Spot is a permanent feature on Jupiter? How could you know? This image of Jupiters Great Red Spot (upper right of image) was taken by the Voyager 1 spacecraft. The white storm just below the Great Red Spot is about the same diameter as Earth. "
jupiter,T_1348,"Jupiter has a very large number of moons  63 have been discovered so far. Four are big enough and bright enough to be seen from Earth, using no more than a pair of binoculars. These moons  Io, Europa, Ganymede, and Callisto were first discovered by Galileo in 1610, so they are sometimes referred to as the Galilean moons (Figure 1.4). The Galilean moons are larger than the dwarf planets Pluto, Ceres, and Eris. Ganymede is not only the biggest moon in the solar system; it is even larger than the planet Mercury! Scientists are particularly interested in Europa because it may be a place to find extraterrestrial life. What features might make a satellite so far from the Sun a candidate for life? Although the surface of Europa is a smooth layer of ice, there is evidence that there is an ocean of liquid water underneath (Figure 1.5). Europa also has a continual source of energy  it is heated as it is stretched and squashed by tidal forces from Jupiter. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted. In 1979, two spacecraft  Voyager 1 and Voyager 2  visited Jupiter and its moons. Photos from the Voyager missions showed that Jupiter has a ring system. This ring system is very faint, so it is difficult to observe from Earth. This composite image shows the four Galilean moons and their sizes relative to the Great Red Spot. From top to bottom, the moons are Io, Europa, Ganymede, and Callisto. Jupiters Great Red Spot is in the background. Sizes are to scale. Click image to the left or use the URL below. URL: "
landforms from glacial erosion and deposition,T_1360,"Glaciers erode the underlying rock by abrasion and plucking. Glacial meltwater seeps into cracks of the underlying rock. When the water freezes, it pushes pieces of rock outward. The rock is then plucked out and carried away by the flowing ice of the moving glacier (Figure 1.1). With the weight of the ice over them, these rocks can scratch deeply into the underlying bedrock, making long, parallel grooves in the bedrock, called glacial striations. Mountain glaciers leave behind unique erosional features. When a glacier cuts through a V-shaped river valley, the glacier plucks rocks from the sides and bottom. This widens the valley and steepens the walls, making a U-shaped valley (Figure 1.2). Smaller tributary glaciers, like tributary streams, flow into the main glacier in their own shallower U-shaped valleys. A hanging valley forms where the main glacier cuts off a tributary glacier and creates a cliff. Streams plunge over the cliff to create waterfalls (Figure 1.3). Up high on a mountain, where a glacier originates, rocks are pulled away from valley walls. Some of the resulting erosional features are shown in Figure 1.4 and Figure 1.5. Glacial striations point the direction a glacier has gone. A U-shaped valley in Glacier National Park. Click image to the left or use the URL below. URL:  Yosemite Valley is known for waterfalls that plunge from hanging valleys. (a) A bowl-shaped cirque in Glacier Na- tional Park was carved by glaciers. (b) A high altitude lake, called a tarn, forms from meltwater trapped in the cirque. (c) Several cirques from glaciers flowing in different directions from a mountain peak, leave behind a sharp sided horn, like the Matterhorn in Switzerland. (d) When glaciers move down opposite sides of a mountain, a sharp edged ridge, called an arte, forms between them. Snowmelt and melting glaciers combine to create a fast moving stream at Glacier National Park. "
landforms from glacial erosion and deposition,T_1361,"As glaciers flow, mechanical weathering loosens rock on the valley walls, which falls as debris on the glacier. Glaciers can carry rock of any size, from giant boulders to silt (Figure 1.6). These rocks can be carried for many kilometers for many years. "
landforms from glacial erosion and deposition,T_1362,Rocks carried by a glacier are eventually dropped. These glacial erratics are noticeable because they are a different rock type from the surrounding bedrock. 
landforms from glacial erosion and deposition,T_1363,Melting glaciers deposit all the big and small bits of rocky material they are carrying in a pile. These unsorted deposits of rock are called glacial till. Glacial till is found in different types of deposits. Linear rock deposits are called moraines. Geologists study moraines to figure out how far glaciers extended and how long it took them to melt away. Moraines are named by their location relative to the glacier: Lateral moraines form at the edges of the glacier as material drops onto the glacier from erosion of the valley walls. Medial moraines form where the lateral moraines of two tributary glaciers join together in the middle of a larger glacier (Figure 1.7). Ground moraines forms from sediments that were beneath the glacier and left behind after the glacier melts. Ground moraine sediments contribute to the fertile transported soils in many regions. Terminal moraines are long ridges of till left at the furthest point the glacier reached. End moraines are deposited where the glacier stopped for a long enough period to create a rocky ridge as it retreated. Long Island in New York is formed by two end moraines. 
landforms from glacial erosion and deposition,T_1364,"Several types of stratified deposits form in glacial regions but are not formed directly by the ice. Varves form where lakes are covered by ice in the winter. Dark, fine-grained clays sink to the bottom in winter, but melting ice in spring brings running water that deposits lighter colored sands. Each alternating dark/light layer represents one year of deposits. (a) An esker is a winding ridge of sand and gravel deposited under a glacier by a stream of meltwater. (b) A drumlin is an asymmetrical hill made of sediments that points in the direction the ice moved. Usually drumlins are found in groups called drumlin fields. Click image to the left or use the URL below. URL: "
landforms from groundwater erosion and deposition,T_1365,"Rainwater absorbs carbon dioxide (CO2 ) as it falls. The CO2 combines with water to form carbonic acid. The slightly acidic water sinks into the ground and moves through pore spaces in soil and cracks and fractures in rock. The flow of water underground is groundwater. Groundwater is described further in the chapter Water on Earth. Groundwater is a strong erosional force, as it works to dissolve away solid rock (Figure 1.1). Carbonic acid is especially good at dissolving the rock limestone. "
landforms from groundwater erosion and deposition,T_1366,"Working slowly over many years, groundwater travels along small cracks. The water dissolves and carries away the solid rock, gradually enlarging the cracks. Eventually, a cave may form (Figure 1.2). "
landforms from groundwater erosion and deposition,T_1367,"If the roof of a cave collapses, a sinkhole could form. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood (Figure 1.3). Water flows through Russell Cave Na- tional Monument in Alabama. "
landforms from groundwater erosion and deposition,T_1368,"Groundwater carries dissolved minerals in solution. The minerals may then be deposited, for example, as stalag- mites or stalactites (Figure 1.4). Stalactites form as calcium carbonate drips from the ceiling of a cave, forming beautiful icicle-like formations. The word stalactite has a c, and it forms from the ceiling. Stalagmites form as calcium carbonate drips from the ceiling to the floor of a cave and then grow upwards. The g in stalagmite means it forms on the ground. If a stalactite and stalagmite join together, they form a column. One of the wonders of visiting a cave is to witness the beauty of these amazing and strangely captivating structures. Some of the largest, and most beautiful, natural crystals can be found in the Naica mine, in Mexico. These gypsum crystals were formed over thousands of years as groundwater, rich in calcium and sulfur flowed through an underground cave. Check it out: A relatively small sinkhole in a Georgia parking lot. Stalactites hang from the ceiling and stalagmites rise from the floor of Carlsbad Caverns in New Mexico. The large stalagmite on the right is almost tall enough to reach the ceiling (or a stalactite) and form a column. Click image to the left or use the URL below. URL: "
lithification of sedimentary rocks,T_1369,"Accumulated sediments harden into rock by lithification, as illustrated in the Figure 1.1. Two important steps are needed for sediments to lithify. 1. Sediments are squeezed together by the weight of overlying sediments on top of them. This is called com- paction. Cemented, non-organic sediments become clastic rocks. If organic material is included, they are bioclastic rocks. 2. Fluids fill in the spaces between the loose particles of sediment and crystallize to create a rock by cementation. The sediment size in clastic sedimentary rocks varies greatly (see Table in Sedimentary Rocks Classification). This cliff is made of sandstone. Sands were deposited and then lithified. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
location and direction,T_1381,How would you find Old Faithful? One way is by using latitude and longitude. Any location on Earths surface or on a map  can be described using these coordinates. Latitude and longitude are expressed as degrees that are divided into 60 minutes. Each minute is divided into 60 seconds. 
location and direction,T_1382,"A look on a reliable website shows us that Old Faithful Geyser is located at N44o 27 43. What does this mean? Latitude tells the distance north or south of the Equator. Latitude lines start at the Equator and circle around the planet. The North Pole is 90o N, with 90 degree lines in the Northern Hemisphere. Old Faithful is at 44 degrees, 27 minutes and 43 seconds north of the Equator. Thats just about exactly half way between the Equator and the North Pole! "
location and direction,T_1383,"The latitude mentioned above does not locate Old Faithful exactly, since a circle could be drawn that latitude north of the Equator. To locate Old Faithful we need another point - longitude. At Old Faithful the longitude is W110o 4957. Longitude lines are circles that go around the Earth from north to south, like the sections of an orange. Longitude is measured perpendicular to the Equator. The Prime Meridian is 0o longitude and passes through Greenwich, England. The International Date Line is the 180o meridian. Old Faithful is in the Western Hemisphere, between the Prime Meridian in the east and the International Date Line in the west. "
location and direction,T_1384,"An accurate location must take into account the third dimension. Elevation is the height above or below sea level. Sea level is the average height of the oceans surface or the midpoint between high and low tide. Sea level is the same all around Earth. Old Faithful is higher above sea level than most locations at 7,349 ft (2240 m). Of course, the highest point on Earth, Mount Everest, is much higher at 29,029 ft (8848 m). "
location and direction,T_1385,Satellites continually orbit Earth and can be used to indicate location. A global positioning system receiver detects radio signals from at least four nearby GPS satellites. The receiver measures the time it takes for radio signals to travel from a satellite and then calculates its distance from the satellite using the speed of radio signals. By calculating distances from each of the four satellites the receiver can triangulate to determine its location. You can use a GPS meter to tell you how to get to Old Faithful. 
location and direction,T_1386,"Direction is important if you want to go between two places. Directions are expressed as north (N), east (E), south (S), and west (W), with gradations in between. The most common way to describe direction in relation to the Earths surface is with a compass, a device with a floating needle that is actually a small magnet. The compass needle aligns itself with the Earths magnetic north pole. Since the magnetic north pole is 11.5 degrees offset from its geographic north pole on the axis of rotation, you must correct for this discrepancy. Map of the Visitor Center at Old Faithful, Yellowstone National Park, Wyoming. Without using a compass, we can say that to get to Old Faithful, you enter Yellowstone National Park at the South Entrance, drive north-northeast to West Thumb, and then drive west-northwest to Old Faithful. Click image to the left or use the URL below. URL: "
long term climate change,T_1387,"Many processes can cause climate to change. These include changes: In the amount of energy the Sun produces over years. In the positions of the continents over millions of years. In the tilt of Earths axis and orbit over thousands of years. That are sudden and dramatic because of random catastrophic events, such as a large asteroid impact. In greenhouse gases in the atmosphere, caused naturally or by human activities. "
long term climate change,T_1388,"The amount of energy the Sun radiates is variable. Sunspots are magnetic storms on the Suns surface that increase and decrease over an 11-year cycle (Figure 1.1). When the number of sunspots is high, solar radiation is also relatively high. But the entire variation in solar radiation is tiny relative to the total amount of solar radiation that there is, and there is no known 11-year cycle in climate variability. The Little Ice Age corresponded to a time when there were no sunspots on the Sun. Sunspots on the face of the Sun. "
long term climate change,T_1389,"Plate tectonic movements can alter climate. Over millions of years as seas open and close, ocean currents may distribute heat differently. For example, when all the continents are joined into one supercontinent (such as Pangaea), nearly all locations experience a continental climate. When the continents separate, heat is more evenly distributed. Plate tectonic movements may help start an ice age. When continents are located near the poles, ice can accumulate, which may increase albedo and lower global temperature. Low enough temperatures may start a global ice age. Plate motions trigger volcanic eruptions, which release dust and CO2 into the atmosphere. Ordinary eruptions, even large ones, have only a short-term effect on weather (Figure 1.2). Massive eruptions of the fluid lavas that create lava plateaus release much more gas and dust, and can change climate for many years. This type of eruption is exceedingly rare; none has occurred since humans have lived on Earth. "
long term climate change,T_1390,"The most extreme climate of recent Earth history was the Pleistocene. Scientists attribute a series of ice ages to variation in the Earths position relative to the Sun, known as Milankovitch cycles. The Earth goes through regular variations in its position relative to the Sun: 1. The shape of the Earths orbit changes slightly as it goes around the Sun. The orbit varies from more circular to more elliptical in a cycle lasting between 90,000 and 100,000 years. When the orbit is more elliptical, there is a greater difference in solar radiation between winter and summer. 2. The planet wobbles on its axis of rotation. At one extreme of this 27,000 year cycle, the Northern Hemisphere points toward the Sun when the Earth is closest to the Sun. Summers are much warmer and winters are much colder than now. At the opposite extreme, the Northern Hemisphere points toward the Sun when it is farthest from the Sun. An eruption like Sarychev Volcano (Kuril Islands, northeast of Japan) in 2009 would have very little impact on weather. This results in chilly summers and warmer winters. 3. The planets tilt on its axis varies between 22.1o and 24.5o . Seasons are caused by the tilt of Earths axis of rotation, which is at a 23.5o angle now. When the tilt angle is smaller, summers and winters differ less in temperature. This cycle lasts 41,000 years. When these three variations are charted out, a climate pattern of about 100,000 years emerges. Ice ages correspond closely with Milankovitch cycles. Since glaciers can form only over land, ice ages only occur when landmasses cover the polar regions. Therefore, Milankovitch cycles are also connected to plate tectonics. "
long term climate change,T_1391,"Since greenhouse gases trap the heat that radiates off the planets surfaces, what would happen to global temperatures if atmospheric greenhouse gas levels decreased? What if greenhouse gases increased? A decrease in greenhouse gas levels decreases global temperature and an increase raises global temperature. Greenhouse gas levels have varied throughout Earth history. For example, CO2 has been present at concentrations less than 200 parts per million (ppm) and more than 5,000 ppm. But for at least 650,000 years, CO2 has never risen above 300 ppm, during either glacial or interglacial periods (Figure 1.3). Natural processes add and remove CO2 from the atmosphere. Processes that add CO2 : volcanic eruptions decay or burning of organic matter. Processes that remove CO2 : absorption by plant and animal tissue. When plants are turned into fossil fuels, the CO2 in their tissue is stored with them. So CO2 is removed from the atmosphere. What does this do to Earths average temperature? What happens to atmospheric CO2 when the fossil fuels are burned? What happens to global temperatures? CO2 levels during glacial (blue) and inter- glacial (yellow) periods. Are CO2 levels relatively high or relatively low during in- terglacial periods? Current carbon diox- ide levels are at around 400 ppm, the highest level for the last 650,000 years. BP means years before present. "
magnetic evidence for seafloor spreading,T_1393,"On our transit to the Mid-Atlantic ridge, we tow a magnetometer behind the ship. Shipboard magnetometers reveal the magnetic polarity of the rock beneath them. The practice of towing a magnetometer began during WWII when navy ships towed magnetometers to search for enemy submarines. When scientists plotted the points of normal and reversed polarity on a seafloor map they made an astonishing discovery: the normal and reversed magnetic polarity of seafloor basalts creates a pattern. Stripes of normal polarity and reversed polarity alternate across the ocean bottom. Stripes form mirror images on either side of the mid-ocean ridges (Figure 1.1). Stripes end abruptly at the edges of continents, sometimes at a deep sea trench (Figure 1.2). The magnetic stripes are what created the Figure 1.1. Research cruises today tow magnetometers to add detail to existing magnetic polarity data. "
magnetic evidence for seafloor spreading,T_1394,"By combining magnetic polarity data from rocks on land and on the seafloor with radiometric age dating and fossil ages, scientists came up with a time scale for the magnetic reversals. The first four magnetic periods are: Brunhes normal - present to 730,000 years ago. Matuyama reverse - 730,000 years ago to 2.48 million years ago. Gauss normal - 2.48 to 3.4 million years ago. Gilbert reverse - 3.4 to 5.3 million years ago. The scientists noticed that the rocks got older with distance from the mid-ocean ridges. The youngest rocks were located at the ridge crest and the oldest rocks were located the farthest away, abutting continents. Scientists also noticed that the characteristics of the rocks and sediments changed with distance from the ridge axis as seen in the Table 1.1. Rock ages At ridge axis With distance from axis youngest becomes older Sediment thickness none becomes thicker Crust thickness Heat flow thinnest becomes thicker hottest becomes cooler Away from the ridge crest, sediment becomes older and thicker, and the seafloor becomes thicker. Heat flow, which indicates the warmth of a region, is highest at the ridge crest. The oldest seafloor is near the edges of continents or deep sea trenches and is less than 180 million years old (Figure something was happening to the older seafloor. Seafloor is youngest at the mid-ocean ridges and becomes progressively older with distance from the ridge. How can you explain the observations that scientists have made in the oceans? Why is rock younger at the ridge and oldest at the farthest points from the ridge? The scientists suggested that seafloor was being created at the ridge. Since the planet is not getting larger, they suggested that it is destroyed in a relatively short amount of geologic time. Click image to the left or use the URL below. URL: "
magnetic polarity evidence for continental drift,T_1395,"The next breakthrough in the development of the theory of plate tectonics came two decades after Wegeners death. Magnetite crystals are shaped like a tiny bar magnet. As basalt lava cools, the magnetite crystals line up in the magnetic field like tiny magnets. When the lava is completely cooled, the crystals point in the direction of magnetic north pole at the time they form. How do you expect this would help scientists see whether continents had moved or not? As a Wegener supporter, (and someone who is omniscient), you have just learned of a new tool that may help you. A magnetometer is a device capable of measuring the magnetic field intensity. This allows you to look at the magnetic properties of rocks in many locations. First, youre going to look at rocks on land. Which rocks should you seek out for study? "
magnetic polarity evidence for continental drift,T_1396,"Geologists noted important things about the magnetic polarity of different aged rocks on the same continent: Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole (Figure 1.2) no matter what continent or where on the continent the rocks are located. Older rocks that are the same age and are located on the same continent point to the same location, but that location is not the current north magnetic pole. Older rocks that are of different ages do not point to the same locations or to the current magnetic north pole. In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. Scientists were amazed to find that the north magnetic pole changed location over time (Figure Can you figure out the three possible explanations for this? They are: The location of the north magnetic north pole 80 million years before present (mybp), then 60, 40, 20, and now. 1. The continents remained fixed and the north magnetic pole moved. 2. The north magnetic pole stood still and the continents moved. 3. Both the continents and the north pole moved. "
magnetic polarity evidence for continental drift,T_1397,"How do you figure out which of those three possibilities is correct? You decide to look at magnetic rocks on different continents. Geologists noted that for rocks of the same age but on different continents, the little magnets pointed to different magnetic north poles. 400 million-year-old magnetite in Europe pointed to a different north magnetic pole than magnetite of the same age in North America. 250 million years ago, the north poles were also different for the two continents. Now look again at the three possible explanations. Only one can be correct. If the continents had remained fixed while the north magnetic pole moved, there must have been two separate north poles. Since there is only one north pole today, what is the best explanation? The only reasonable explanation is that the magnetic north pole has remained fixed but that the continents have moved. "
magnetic polarity evidence for continental drift,T_1398,"How does this help you to provide evidence for continental drift? To test the idea that the pole remained fixed but the continents moved, geologists fitted the continents together as Wegener had done. It worked! There has only been one magnetic north pole and the continents have drifted (Figure 1.4). They named the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander. On the left: The apparent north pole for Europe and North America if the continents were always in their current locations. The two paths merge into one if the continents are allowed to drift. This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planets surface. "
maps,T_1399,"Topographic maps represent the locations of geographical features, such as hills and valleys. Topographic maps use contour lines to show different elevations. A contour line is a line of equal elevation. If you walk along a contour line you will not go uphill or downhill. Topographic maps are also called contour maps. The rules of topographic maps are: Each line connects all points of a specific elevation. Contour lines never cross since a single point can only have one elevation. Every fifth contour line is bolded and labeled. Adjacent contour lines are separated by a constant difference in elevation (such as 20 ft or 100 ft). The difference in elevation is the contour interval, which is indicated in the map legend. Scales indicate horizontal distance and are also found on the map legend. Old Faithful erupting, Yellowstone Na- tional Park. While the Figure 1.1 isnt exactly the same view as the map at the top of this concept, it is easy to see the main features. Hills, forests, development, and trees are all seen around Old Faithful. "
maps,T_1400,"A bathymetric map is like a topographic map with the contour lines representing depth below sea level, rather than height above. Numbers are low near sea level and become higher with depth. Kilauea is the youngest volcano found above sea level in Hawaii. On the flank of Kilauea is an even younger volcano called Loihi. The bathymetric map pictured in the Figure 1.2 shows the form of Loihi. Loihi volcano growing on the flank of Kilauea volcano in Hawaii. Black lines in the inset show the land surface above sea level and blue lines show the topography below sea level. A geologic map of the region around Old Faithful, Yellowstone National Park. "
maps,T_1401,A geologic map shows the geological features of a region (see Figure 1.3 for an example). Rock units are color- coded and identified in a key. Faults and folds are also shown on geologic maps. The geology is superimposed on a topographic map to give a more complete view of the geology of the region. Click image to the left or use the URL below. URL: 
mars,T_1402,"Mars is the fourth planet from the Sun, and the first planet beyond Earths orbit (Figure 1.1). Mars is a quite different from Earth and yet more similar than any other planet. Mars is smaller, colder, drier, and appears to have no life, but volcanoes are common to both planets and Mars has many. Mars is easy to observe, so Mars has been studied more thoroughly than any other extraterrestrial planet. Space probes, rovers, and orbiting satellites have all yielded information to planetary geologists. Although no humans have ever set foot on Mars, both NASA and the European Space Agency have set goals of sending people to Mars sometime between 2030 and 2040. This image of Mars, taken by the Hubble Space Telescope in October, 2005, shows the planets red color, a small ice cap on the south pole, and a dust storm. "
mars,T_1403,"Viewed from Earth, Mars is reddish in color. The ancient Greeks and Romans named the planet after the god of war. The surface is not red from blood but from large amounts of iron oxide in the soil. The Martian atmosphere is very thin relative to Earths and has much lower atmospheric pressure. Although the atmosphere is made up mostly of carbon dioxide, the planet has only a weak greenhouse effect, so temperatures are only slightly higher than if the planet had no atmosphere. "
mars,T_1404,"Mars has mountains, canyons, and other features similar to Earth. Some of these surface features are amazing for their size! Olympus Mons is a shield volcano, similar to the volcanoes that make up the Hawaiian Islands. But Olympus Mons is also the largest mountain in the solar system (Figure 1.2). Mars also has the largest canyon in the solar system, Valles Marineris (Figure 1.3). "
mars,T_1405,"It was previously believed that water cannot stay in liquid form on Mars because the atmospheric pressure is too low. However, there is a lot of water in the form of ice and even prominent ice caps (Figure 1.4). Scientists also think Olympus Mons is about 27 km (16.7 miles/88,580 ft) above the Martian sur- face, more than three times taller than Mount Everest. The volcanos base is about the size of the state of Arizona. Valles Marineris is 4,000 km (2,500 mi) long, as long as Europe is wide, and one-fifth the circumference of Mars. The canyon is 7 km (4.3 mi) deep. By comparison, the Grand Canyon on Earth is only 446 km (277 mi) long and about 2 km (1.2 mi) deep. that there is a lot of ice present just under the Martian surface. This ice can melt when volcanoes erupt, and water can flow across the surface. In late 2015, NASA confirmed the presence of water on Mars. Scientists think that water once flowed over the Martian surface because there are surface features that look like water-eroded canyons. The presence of water on Mars suggests that it might have been possible for life to exist on Mars in the past. "
mars,T_1406,"Mars has two very small moons that are irregular rocky bodies (Figure 1.5). Phobos and Deimos are named after characters in Greek mythology  the two sons of Ares, who followed their father into war. Ares is equivalent to the Roman god Mars. Mars has two small moons, Phobos (left) and Deimos (right). Both were discovered in 1877 and are thought to be captured asteroids. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
measuring earthquake magnitude,T_1408,"A seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram (Figure 1.1). Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was. Modern seismometers record ground motions using electronic motion detectors. The data are then kept digitally on a computer. If a seismogram records P-waves and surface waves but not S-waves, the seismograph was on the other side of the Earth from the earthquake. The amplitude of the waves can be used to determine the magnitude of the earthquake, which will be discussed in a later section. "
measuring earthquake magnitude,T_1409,The seismogram in the introduction shows: foreshocks. the arrival of the P-waves. the arrival of the S-waves. the arrival of the surface waves (very hard to pick out). aftershocks. the times when all of these things occur. These seismograms show the arrival of P- waves and S-waves. The surface waves arrive just after the S-waves and are diffi- cult to distinguish. Time is indicated on the horizontal portion (or x-axis) of the graph. Click image to the left or use the URL below. URL: 
mechanical weathering,T_1410,"Mechanical weathering (also called physical weathering) breaks rock into smaller pieces. These smaller pieces are just like the bigger rock, but smaller. That means the rock has changed physically without changing its composition. The smaller pieces have the same minerals, in just the same proportions as the original rock. "
mechanical weathering,T_1411,"There are many ways that rocks can be broken apart into smaller pieces. Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point (Figure 1.1). Ice wedging works quickly, breaking apart rocks in areas with temperatures that cycle above and below freezing in the day and night, and also that cycle above and below freezing with the seasons. Ice wedging breaks apart so much rock that large piles of broken rock are seen at the base of a hillside, as rock fragments separate and tumble down. Ice wedging is common in Earths polar regions and mid latitudes, and also at higher elevations, such as in the mountains. "
mechanical weathering,T_1412,"Abrasion is another form of mechanical weathering. In abrasion, one rock bumps against another rock. Gravity causes abrasion as a rock tumbles down a mountainside or cliff. Moving water causes abrasion as particles in the water collide and bump against one another. Strong winds carrying pieces of sand can sandblast surfaces. Ice in glaciers carries many bits and pieces of rock. Rocks embedded at the bottom of the glacier scrape against the rocks below. Abrasion makes rocks with sharp or jagged edges smooth and round. If you have ever collected beach glass or cobbles from a stream, you have witnessed the work of abrasion (Figure 1.2). "
mechanical weathering,T_1413,"Now that you know what mechanical weathering is, can you think of other ways it could happen? Plants and animals can do the work of mechanical weathering (Figure 1.3). This could happen slowly as a plants roots grow into a crack or fracture in rock and gradually grow larger, wedging open the crack. Burrowing animals can also break apart rock as they dig for food or to make living spaces for themselves. "
mechanical weathering,T_1414,"Human activities are responsible for enormous amounts of mechanical weathering, by digging or blasting into rock to build homes, roads, and subways, or to quarry stone. (a) Humans are tremendous agents of mechanical weathering. (b) Salt weathering of building stone on the island of Gozo, Malta. "
mercury,T_1415,"The smallest planet, Mercury, is the planet closest to the Sun. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. However, the Mariner 10 spacecraft, shown in Figure 1.1, visited Mercury from 1974 to 1975. The MESSENGER spacecraft has been studying Mercury in detail since 2005. The craft is currently in orbit around the planet, where it is creating detailed maps. MESSENGER stands for Mercury Surface, Space Environment, Geochemistry and Ranging. (a) Mariner 10 made three flybys of Mercury in 1974 and 1975. (b) A 2008 image of compiled from a flyby by MESSENGER. As Figure 1.2 shows, the surface of Mercury is covered with craters, like Earths Moon. Ancient impact craters means that for billions of years Mercury hasnt changed much geologically. Also, with very little atmosphere, the processes of weathering and erosion do not wear down structures on the planet. "
mercury,T_1416,"Mercury is named for the Roman messenger god, who could run extremely quickly, just as the planet moves very quickly in its orbit around the Sun. A year on Mercury  the length of time it takes to orbit the Sun  is just 88 Earth days. Despite its very short years, Mercury has very long days. A day is defined as the time it takes a planet to turn on its axis. Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. Therefore, each day on Mercury is 57 Earth days long. In other words, on Mercury, a year is only a Mercury day and a half long! "
mercury,T_1417,"Mercury is close to the Sun, so it can get very hot. However, Mercury has virtually no atmosphere, no water to insulate the surface, and it rotates very slowly. For these reasons, temperatures on the surface of Mercury vary widely. In direct sunlight, the surface can be as hot as 427 C (801 F). On the dark side, or in the shadows inside craters, the surface can be as cold as -183 C (-297 F)! Although most of Mercury is extremely dry, scientists think Mercury is covered with craters, like Earths Moon. MESSENGER has taken extremely detailed pictures of the planets surface. there may be a small amount of water in the form of ice at the poles of Mercury, in areas that never receive direct sunlight. "
mercury,T_1418,"Figure 1.3 shows a diagram of Mercurys interior. Mercury is one of the densest planets. Its relatively large, liquid core, made mostly of melted iron, takes up about 42% of the planets volume. "
mercury pollution,T_1419,"Mercury is released into the atmosphere when coal is burned (Figure 1.1). But breathing the mercury is not harmful. In the atmosphere, the mercury forms small droplets that are deposited in water or sediments. "
mercury pollution,T_1420,"Do you know why you are supposed to eat large predatory fish like tuna infrequently? It is because of the bioaccu- mulation of mercury in those species. Some pollutants remain in an organism throughout its life, a phenomenon called bioaccumulation. In this process, an organism accumulates the entire amount of a toxic compound that it consumes over its lifetime. Not all substances bioaccumulate. Can you name one that does not? Aspirin does not bioaccumulate; if it did, a person would quickly accumulate a toxic amount in her body. Compounds that bioaccumulate are usually stored in the organisms fat. In the sediments, bacteria convert the droplets to the hazardous compound methyl mercury. Bacteria and plankton store all of the mercury from all of the seawater they ingest (Figure 1.2). A small fish that eats bacteria and plankton accumulates all of the mercury from all of the tiny creatures it eats over its lifetime. A big fish accumulates all of the mercury from all of the small fish it eats over its lifetime. For a tuna at the top of the food chain, thats a lot of mercury. Historic increases of mercury in the atmo- sphere: blue is volcanic eruptions; brown, purple, and pink are human-caused. The red region shows the effect of industrial- ization on atmospheric mercury. So tuna pose a health hazard to anything that eats them because their bodies are so high in mercury. This is why the government recommends limits on the amount of tuna that people eat. Limiting intake of large predatory fish is especially important for children and pregnant women. If the mercury just stayed in a persons fat, it would not be harmful, but that fat is used when a woman is pregnant or nursing a baby. A person will also get the mercury into her system when she (or he) burns the fat while losing weight. "
mercury pollution,T_1421,"Methyl mercury poisoning can cause nervous system or brain damage, especially in infants and children. Children may experience brain damage or developmental delays. The phrase mad as a hatter was common when Lewis Carroll wrote his Alice in Wonderland stories. It was based on symptoms suffered by hatters who were exposed to mercury and experienced mercury poisoning while using the metal to make hats (Figure 1.3). Like mercury, other metals and VOCS can bioaccumulate, causing harm to animals and people high on the food chain. Mercury, a potent neurotoxin, has been flowing into the San Francisco Bay since the Gold Rush Era. It has settled in the bays mud and made its way up the food chain, endangering wildlife and making many fish unsafe to eat. Now a multi-billion-dollar plan aims to clean it up. Click image to the left or use the URL below. URL: "
mesosphere,T_1422,"Above the stratosphere is the mesosphere. Temperatures in the mesosphere decrease with altitude. Because there are few gas molecules in the mesosphere to absorb the Suns radiation, the heat source is the stratosphere below. The mesosphere is extremely cold, especially at its top, about -90o C (-130o F). "
mesosphere,T_1423,"The air in the mesosphere has extremely low density: 99.9% of the mass of the atmosphere is below the mesosphere. As a result, air pressure is very low (Figure 1.1). A person traveling through the mesosphere would experience severe burns from ultraviolet light since the ozone layer, which provides UV protection, is in the stratosphere below. There would be almost no oxygen for breathing. And, of course, your blood would boil at normal body temperature. Click image to the left or use the URL below. URL: "
metamorphic rock classification,T_1430,"Table 1.1 shows some common metamorphic rocks and their original parent rock. Picture Rock Name Slate Type of Rock Foliated Metamorphic Comments Phyllite Foliated Metamorphism of slate, but under greater heat and pressure than slate Schist Foliated Often derived from meta- morphism of claystone or shale; metamorphosed under more heat and pres- sure than phyllite Gneiss Foliated Metamorphism of various different rocks, under ex- treme conditions of heat and pressure Hornfels Non-foliated Contact metamorphism of various different rock types Metamorphism of shale Picture Rock Name Comments Quartzite Type of Metamorphic Rock Non-foliated Marble Non-foliated Metamorphism of lime- stone Metaconglomerate Non-foliated Metamorphism of con- glomerate Metamorphism of quartz sandstone Click image to the left or use the URL below. URL: "
metamorphic rocks,T_1431,"Any type of rock - igneous, sedimentary, or metamorphic  can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rocks physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of different chemical composition. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Hornfels is shown in the table for the ""Metamorphic Rock Classification"" concept. "
metamorphic rocks,T_1432,"Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (Figure 1.1). Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated. Quartzite and marble, shown in the concept ""Metamorphic Rock Classification,"" are non-foliated. A foliated metamorphic rock. "
metamorphic rocks,T_1433,The two main types of metamorphism are both related to heat within Earth: 1. Regional metamorphism: Changes in enormous quantities of rock over a wide area caused by the extreme pressure from overlying rock or from compression caused by geologic processes. Deep burial exposes the rock to high temperatures. 2. Contact metamorphism: Changes in a rock that is in contact with magma. The changes occur because of the magmas extreme heat. Click image to the left or use the URL below. URL: 
meteors,T_1434,"A meteor, such as in Figure 1.1, is a streak of light across the sky. People call them shooting stars but they are actually small pieces of matter burning up as they enter Earths atmosphere from space. Meteors are called meteoroids before they reach Earths atmosphere. Meteoroids are smaller than asteroids and range from the size of boulders down to the size of tiny sand grains. Still smaller objects are called interplanetary dust. When Earth passes through a cluster of meteoroids, there is a meteor shower. These clusters are often remnants left behind by comet tails. "
meteors,T_1435,"Although most meteors burn up in the atmosphere, larger meteoroids may strike the Earths surface to create a meteorite. Meteorites are valuable to scientists because they provide clues about our solar system. Many meteorites are from asteroids that formed when the solar system formed (Figure 1.2). A few meteorites are made of rocky material that is thought to have come from Mars when an asteroid impact shot material off the Martian surface and into space. Click image to the left or use the URL below. URL: "
milky way,T_1438,"The Milky Way Galaxy, which is our galaxy. The Milky Way is made of millions of stars along with a lot of gas and dust. It looks different from other galaxies because we are looking at the main disk from within the galaxy. Astronomers estimate that the Milky Way contains 200 to 400 billion stars. "
milky way,T_1439,"Although it is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it, astronomers have identified it as a typical spiral galaxy containing about 200 billion to 400 billion stars (Figure 1.1). An artists rendition of what astronomers think the Milky Way Galaxy would look like seen from above. The Sun is located approximately where the arrow points. Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 light- years across and 3,000 light-years thick. Most of the Galaxys gas, dust, young stars, and open clusters are in the disk. What evidence do astronomers find that lets them know that the Milky Way is a spiral galaxy? 1. The shape of the galaxy as we see it (Figure 1.2). 2. The velocities of stars and gas in the galaxy show a rotational motion. 3. The gases, color, and dust are typical of spiral galaxies. The central bulge is about 12,000 to 16,000 light-years wide and 6,000 to 10,000 light-years thick. The central bulge contains mostly older stars and globular clusters. Some recent evidence suggests the bulge might not be spherical, but is instead shaped like a bar. The bar might be as long as 27,000 light-years long. The disk and bulge are surrounded by a faint, spherical halo, which also contains old stars and globular clusters. Astronomers have discovered that there is a gigantic black hole at the center of the galaxy. The Milky Way Galaxy is a big place. If our solar system were the size of your fist, the Galaxys disk would still be An infrared image of the Milky Way shows the long thin line of stars and the central bulge typical of spiral galaxies. wider than the entire United States! "
milky way,T_1440,"Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the Milky Way Galaxy. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. We are about 26,000 light-years from the center of the galaxy, a little more than halfway out from the center of the galaxy to the edge. Just as Earth orbits the Sun, the Sun and solar system orbit the center of the Galaxy. One orbit of the solar system takes about 225 to 250 million years. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. Astronomers have recently discovered that at the center of the Milky Way, and most other galaxies, is a supermassive black hole, although a black hole cannot be seen. This video describes the solar system in which we live. It is located in an outer edge of the Milky Way galaxy, which spans 100,000 light years. Click image to the left or use the URL below. URL:  The Universe contains many billions of stars and there are many billions of galaxies. Our home, the Milky Way galaxy, is only one. Click image to the left or use the URL below. URL: "
moon,T_1473,"The Moon is Earths only natural satellite, a body that moves around a larger body in space. The Moon orbits Earth for the same reason Earth orbits the Sun  gravity. The Moon is 3,476 km in diameter, about one-fourth the size of Earth. The satellite is also not as dense as the Earth; gravity on the Moon is only one-sixth as strong as it is on Earth. An astronaut can jump six times as high on the Moon as on Earth! The Moon makes one complete orbit around the Earth every 27.3 days. The Moon also rotates on its axis once every 27.3 days. Do you know what this means? The same side of the Moon always faces Earth, so that side of the Moon is what we always see in the night sky (Figure 1.1). The Moon makes no light of its own, but instead only reflects light from the Sun. (a) The near side of the Moon faces Earth continually. It has a thinner crust with many more maria (flat areas of basaltic rock). (b) The far side of the Moon has only been seen by spacecraft. It has a thicker crust and far fewer maria (flat areas of basaltic rock). "
moon,T_1474,"The Moon has no atmosphere. Since an atmosphere moderates temperature, the Moons average surface temperature during the day is approximately 225 F, but drops to -243 F at night. The coldest temperatures, around -397 F, occur in craters in the permanently shaded south polar basin. These are among the coldest temperatures recorded in the entire solar system. Earths landscape is extremely varied, with mountains, valleys, plains and hills. This landscape is always changing as plate tectonics builds new features and weathering and erosion destroys them. The landscape of the Moon is very different. With no plate tectonics, features are not built. With no atmosphere, features are not destroyed. Still, the Moon has a unique surface. Lunar surface features include the bowl-shaped craters that are caused by meteorite impacts (Figure 1.2). If Earth did not have plate tectonics or erosion, its surface would also be covered with meteorite craters. Even from Earth, the Moon has visible dark areas and light areas. The dark areas are called maria, which means seas because thats what the ancients thought they were. In fact, the maria are not water but solid, flat areas of basaltic lava. From about 3.0 to 3.5 billion years ago the Moon was continually bombarded by meteorites. Some of these meteorites were so large that they broke through the Moons newly formed surface. Then, magma flowed out and filled the craters. Scientists estimate this meteorite-caused volcanic activity on the Moon ceased about 1.2 billion years ago, but most occurred long before that. The lighter parts of the Moon are called terrae or highlands (Figure 1.3). The terrae are higher than the maria and A crater on the surface of the Moon. include several high mountain ranges. The terrae are the light silicate minerals that precipitated out of the ancient magma ocean and formed the early lunar crust. There are no lakes, rivers, or even small puddles anywhere to be found on the Moons surface, but water in the form of ice has been found in the extremely cold craters and bound up in the lunar soil. Despite the possible presence of water, the lack of an atmosphere and the extreme temperatures make it no surprise to scientists that the Moon has absolutely no evidence of life. Life from Earth has visited the Moon and there are footprints of astronauts on the lunar surface. With no wind, rain, or living thing to disturb them, these footprints will remain as long as the Moon exists. Only an impact with a meteorite could destroy them. "
moon,T_1475,"Like Earth, the Moon has a distinct crust, mantle, and core. What is known about the Moons interior was determined from the analysis of rock samples gathered by astronauts and from unmanned spacecraft sent to the Moon (Figure The Moons small core, 600 to 800 kilometers in diameter, is mostly iron with some sulfur and nickel. The mantle is composed of the minerals olivine and orthopyroxene. Analysis of Moon rocks indicates that there may also be high levels of iron and titanium in the lunar mantle. A close-up of the Moon, showing maria (the dark areas) and terrae (the light areas); maria covers around 16% of the Moons surface, mostly on the side of the Moon we see. LCROSS crashed into the Moon in May 2009. This QUEST video describes the mission. After watching, look up the mission to see what they found! Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
natural gas power,T_1480,"Natural gas, often known simply as gas, is composed mostly of the hydrocarbon methane. The amount of natural gas being extracted and used in the Untied States is increasing rapidly. "
natural gas power,T_1481,"Natural gas forms under the same conditions that create oil. Organic material buried in the sediments harden to become a shale formation that is the source of the gas. Although natural gas forms at higher temperatures than crude oil, the two are often found together. The largest natural gas reserves in the United States are in the Appalachian Basin, North Dakota and Montana, Texas, and the Gulf of Mexico region (Figure 1.1). California also has natural gas, found mostly in the Central Valley. In the northern Sacramento Valley and the Sacramento Delta, a sediment-filled trough formed along a location where crust was pushed together (an ancient convergent margin). Gas production in the lower 48 United States. "
natural gas power,T_1482,"Like crude oil, natural gas must be processed before it can be used as a fuel. Some of the chemicals in unprocessed natural gas are poisonous to humans. Other chemicals, such as water, make the gas less useful as a fuel. Processing natural gas removes almost everything except the methane. Once the gas is processed, it is ready to be delivered and used. Natural gas is delivered to homes for uses such as cooking and heating. Like coal and oil, natural gas is also burned to generate heat for powering turbines. The spinning turbines turn generators, and the generators create electricity. Click image to the left or use the URL below. URL: "
natural gas power,T_1483,"Natural gas burns much cleaner than other fossil fuels, meaning that it causes less air pollution. Natural gas also produces less carbon dioxide than other fossil fuels do for the same amount of energy, so its global warming effects are less (Figure 1.2). Unfortunately, drilling for natural gas can be environmentally destructive. One technique used is hydraulic fractur- ing, also called fracking, which increases the rate of recovery of natural gas. Fluids are pumped through a borehole to create fractures in the reservoir rock that contains the natural gas. Material is added to the fluid to prevent the fractures from closing. The damage comes primarily from chemicals in the fracturing fluids. Chemicals that have been found in the fluids may be carcinogens (cancer-causing), radioactive materials, or endocrine disruptors, which interrupt hormones in the bodies of humans and animals. The fluids may get into groundwater or may runoff into streams and other surface waters. As noted above, fracking may cause earthquakes. Click image to the left or use the URL below. URL: "
natural resource conservation,T_1484,"So that people in developed nations maintain a good lifestyle and people in developing nations have the ability to improve their lifestyles, natural resources must be conserved and protected (Figure 1.1). People are researching ways to find renewable alternatives to non-renewable resources. Here is a checklist of ways to conserve resources: Buy less stuff (use items as long as you can, and ask yourself if you really need something new). Reduce excess packaging (drink tap water instead of water from plastic bottles). Recycle materials such as metal cans, old cell phones, and plastic bottles. Purchase products made from recycled materials. Reduce pollution so that resources are maintained. Prevent soil erosion. Plant new trees to replace those that are cut down. Drive cars less, take public transportation, bicycle, or walk. Conserve energy at home (turn out lights when they are not needed). Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
neptune,T_1485,"Neptune, shown in Figure 1.1, is the only major planet that cant be seen from Earth without a telescope. Scientists predicted the existence of Neptune before it was discovered because Uranus did not always appear exactly where it should appear. They knew that the gravitational pull of another planet beyond Uranus must be affecting Uranus orbit. Neptune was discovered in 1846, in the position that had been predicted, and it was named Neptune for the Roman god of the sea because of its bluish color. This image of Neptune was taken by Voy- ager 2 in 1989. The Great Dark Spot seen on the left center in the picture has since disappeared, but a similar dark spot has appeared on another part of the planet. In many respects, Neptune is similar to Uranus (Figure 1.2). Neptune has slightly more mass than Uranus, but it is slightly smaller in size. Neptune is much farther from the Sun, at nearly 4.5 billion km (2.8 billion mi). The planets slow orbit means that it takes 165 Earth years to go once around the Sun. "
neptune,T_1486,"Neptunes blue color is mostly because of frozen methane (CH4 ). When Voyager 2 visited Neptune in 1986, there was a large dark-blue spot, which scientists named the Great Dark Spot, south of the equator. When the Hubble Space Telescope took pictures of Neptune in 1994, the Great Dark Spot had disappeared, but another dark spot had appeared north of the equator. Astronomers think that both of these spots represent gaps in the methane clouds on Neptune. The changing appearance of Neptune is caused by its turbulent atmosphere. The winds on Neptune are stronger than on any other planet in the solar system, reaching speeds of 1,100 km/h (700 mi/h), close to the speed of sound. This extreme weather surprised astronomers, since the planet receives little energy from the Sun to power weather systems. Neptunes core is 7000 C (12,632 C) which means that it produces more energy than it receives from the Sun. Neptune is also one of the coldest places in the solar system. Temperatures at the top of the clouds are about -218 C (-360 F). Neptunes composition is that of a gas giant: (1) upper atmosphere, (2) atmo- sphere composed of hydrogen, helium and methane gas, (3) mantle of water, ammonia and methane ice, (4) core of rock and ice. "
neptune,T_1487,"Neptune has faint rings of ice and dust that may change or disappear in fairly short time frames. Neptune has 13 known moons. Triton, shown in Figure 1.3, is the only one of them that has enough mass to be spherical in shape. Triton orbits in the direction opposite to the orbit of Neptune. Scientists think Triton did not form around Neptune, but instead was captured by Neptunes gravity as it passed by. This image of Triton, Neptunes largest moon, was taken by Voyager 2 in 1989. "
nitrogen cycle in ecosystems,T_1488,"Nitrogen (N2 ) is vital for life on Earth as an essential component of organic materials, such as amino acids, chloro- phyll, and nucleic acids such as DNA and RNA (Figure 1.1). Chlorophyll molecules, essential for photosynthesis, contain nitrogen. "
nitrogen cycle in ecosystems,T_1489,"Although nitrogen is the most abundant gas in the atmosphere, it is not in a form that plants can use. To be useful, nitrogen must be fixed, or converted into a more useful form. Although some nitrogen is fixed by lightning or blue-green algae, much is modified by bacteria in the soil. These bacteria combine the nitrogen with oxygen or hydrogen to create nitrates or ammonia (Figure 1.2). (a) Nucleic acids contain nitrogen (b) Chlorophyll molecules contain nitrogen "
nitrogen cycle in ecosystems,T_1490,"Animals eat plant tissue and create animal tissue. After a plant or animal dies or an animal excretes waste, bacteria and some fungi in the soil fix the organic nitrogen and return it to the soil as ammonia. Nitrifying bacteria oxidize the ammonia to nitrites, while other bacteria oxidize the nitrites to nitrates, which can be used by the next generation of plants. In this way, nitrogen does not need to return to a gas. Under conditions when there is no oxygen, some bacteria can reduce nitrates to molecular nitrogen. Click image to the left or use the URL below. URL: "
non renewable energy resources,T_1491,Nonrenewable resources are natural resources that are limited in supply and cannot be replaced as quickly as they are used up. A natural resource is anything people can use that comes from nature. Energy resources are some of the most important natural resources because everything we do requires energy. Nonrenewable energy resources include fossil fuels such as oil and the radioactive element uranium. 
non renewable energy resources,T_1492,"Oil, or petroleum, is one of several fossil fuels. Fossil fuels are mixtures of hydrocarbons (compounds containing only hydrogen and carbon) that formed over millions of years from the remains of dead organisms. In addition to oil, they include coal and natural gas. Fossil fuels provide most of the energy used in the world today. They are burned in power plants to produce electrical energy, and they also fuel cars, heat homes, and supply energy for many other purposes. You can see some ways they are used in the Figure 1.1. Q: Why do fossil fuels have energy? A: Fossil fuels contain stored chemical energy that came originally from the sun. "
non renewable energy resources,T_1493,"When ancient plants underwent photosynthesis, they changed energy in sunlight to stored chemical energy in food. The plants used the food and so did the organisms that ate the plants. After the plants and other organisms died, their remains gradually changed to fossil fuels as they were covered and compressed by layers of sediments. Petroleum and natural gas formed from ocean organisms and are found together. Coal formed from giant tree ferns and other swamp plants. "
non renewable energy resources,T_1494,"When fossil fuels burn, they release thermal energy, water vapor, and carbon dioxide. The thermal energy can be used to generate electricity or do other work. The carbon dioxide is released into the atmosphere and is a major cause of global climate change. The burning of fossil fuels also releases many pollutants into the air. Pollutants such as sulfur dioxide form acid rain, which kills living things and damages metals, stonework, and other materials. Pollutants such as nitrogen oxides cause smog, which is harmful to human health. Tiny particles, or particulates, released when fossil fuels burn also harm human health. The Figure 1.2 shows the amounts of pollutants released by different fossil fuels. Natural gas releases the least pollution; coal releases the most. Petroleum has the additional risk of oil spills, which may seriously damage ecosystems. Q: Some newer models of cars and other motor vehicles can run on natural gas. Why would a natural gas vehicle be better for the environment than a vehicle that burns gasoline, which is made from oil? A: Natural gas produces much less pollution and carbon dioxide when it burns than gasoline does. So a natural gas vehicle would contribute less to global climate change, acid rain, and air pollution that harms health. Besides being better for the environment, burning natural gas instead of gasoline results in less engine wear and provides more energy for a given amount of fuel. "
non renewable energy resources,T_1495,"Like fossil fuels, the radioactive element uranium can be used to generate electrical energy in power plants. This source of energy is known as nuclear energy. In a nuclear power plant, the nuclei of uranium atoms are split apart into smaller nuclei in the process of nuclear fission. This process releases a tremendous amount of energy from just a small amount of uranium. The total supply of uranium in the world is quite limited, however, and cannot be replaced once it is used up. Thats why nuclear energy is a nonrenewable resource. The use of nuclear energy also produces dangerous radioactive wastes. In addition, accidents at nuclear power plants have the potential to release large amounts of harmful radiation into the environment. Q: Why is nuclear energy often considered to be greener than energy from fossil fuels? A: Unlike energy from fossil fuels, nuclear energy doesnt produce air pollution or carbon dioxide that contributes to global climate change. "
nuclear power,T_1496,"When the nucleus of an atom is split, it releases a huge amount of energy called nuclear energy. For nuclear energy to be used as a power source, scientists and engineers have learned to split nuclei and to control the release of energy (Figure 1.1). "
nuclear power,T_1497,"Nuclear power plants, such as the one seen in Figure 1.2, use uranium, which is mined, processed, and then concentrated into fuel rods. When the uranium atoms in the fuel rods are hit by other extremely tiny particles, they split apart. The number of tiny particles allowed to hit the fuel rods needs to be controlled, or they would cause a dangerous explosion. The energy from a nuclear power plant heats water, which creates steam and causes a turbine to spin. The spinning turbine turns a generator, which in turn produces electricity. Many countries around the world use nuclear energy as a source of electricity. In the United States, a little less than 20% of electricity comes from nuclear energy. "
nuclear power,T_1498,"Nuclear power is clean. It does not pollute the air. However, the use of nuclear energy does create other environ- mental problems. Uranium must be mined (Figure 1.3). The process of splitting atoms creates radioactive waste, which remains dangerous for thousands or hundreds of thousands of years. As yet, there is no long-term solution for storing this waste. The development of nuclear power plants has been on hold for three decades. Accidents at Three Mile Island and Chernobyl, Ukraine verified peoples worst fears about the dangers of harnessing nuclear power (Figure 1.4). Recently, nuclear power appeared to be making a comeback as society looked for alternatives to fossil fuels. After all, nuclear power emits no pollutants, including no greenhouse gases. But the 2011 disaster at the Fukushima Daiichi Nuclear Power Plant in Japan may have resulted in a new fear of nuclear power. The cause of the disaster was a 9.0 magnitude earthquake and subsequent tsunami, which compromised the plant. Although a total meltdown was averted, the plant experienced multiple partial meltdowns, core breaches, radiation releases, and cooling failures. The plant is scheduled for a complete cold shutdown before the end of 2011. Damaged building near the site of the Chernobyl disaster. Nuclear power is a controversial subject in California and most other places. Nuclear power has no pollutants including carbon emissions, but power plants are not always safe and the long-term disposal of wastes is a problem that has not yet been solved. The future of nuclear power is murky. "
obtaining energy resources,T_1503,"Net energy is the amount of useable energy available from a resource after subtracting the amount of energy needed to make the energy from that resource available. For example, every 5 barrels of oil that are made available for use require 1 barrel for extracting and refining the petroleum. What is the net energy from this process? About 4 barrels (5 barrels minus 1 barrel). What happens if the energy needed to extract and refine oil increases? Why might that happen? The energy cost of an energy resource increases when the easy deposits of that resource have already been consumed. For example, if all the nearshore petroleum in a region has been extracted, more costly drilling must take place further offshore (Figure 1.1). If the energy cost of obtaining energy increases, the resource will be used even faster. Offshore drilling is taking place in deeper water than before. It takes a lot of energy to build a deep drilling platform and to run it. "
obtaining energy resources,T_1504,"The net-energy ratio demonstrates the difference between the amount of energy available in a resource and the amount of energy used to get it. If it takes 8 units of energy to make available 10 units of energy, then the net-energy ratio is 10/8 or 1.25. What does a net-energy ratio larger than 1 mean? What if the net-energy ratio is less than 1? A net-energy ratio larger than 1 means that there is a net gain in usable energy; a net-energy ratio smaller than one means there is an overall energy loss. Table 1.1 shows the net-energy ratios for some common energy sources. Energy Source Solar Energy Natural Gas Petroleum Coal-fired Electricity Net-energy Ratio 5.8 4.9 4.5 2.5-5.1 Notice from the table that solar energy yields much more net energy than other sources. This is because it takes very little energy to get usable solar energy. Sunshine is abundant and does not need to be found, extracted, or transported very far. The range for coal-fired electricity is because of the differing costs of transporting the coal. What does this suggest about using coal to generate electricity? The efficiency is greater in areas where the coal is locally mined and does not have to be transported great distances (Figure 1.2). Obtaining coal for energy takes a lot of energy. The coal must be located, extracted, refined, and transported. Because so much of the energy we use is from fossil fuels, we need to be especially concerned about using them efficiently. Sometimes our choices affect energy efficiency. For example, transportation by cars and airplanes is less energy-efficient than transportation by boats and trains. "
ocean ecosystems,T_1505,"Conditions in the intertidal zone change rapidly as water covers and uncovers the region and waves pound on the rocks. A great abundance of life is found in the intertidal zone (Figure 1.1). High energy waves hit the organisms that live in this zone, so they must be adapted to pounding waves and exposure to air during low tides. Hard shells protect from waves and also protect against drying out when the animal is above water. Strong attachments keep the animals anchored to the rock. In a tide pool, as in the photo, what organisms are found where and what specific adaptations do they have to that zone? The mussels on the top left have hard shells for protection and to prevent drying because they are often not covered by water. The sea anemones in the lower right are more often submerged and have strong attachments but can close during low tides. Many young organisms get their start in estuaries and so they must be adapted to rapid shifts in salinity. Organisms in a tide pool include sea stars and sea urchins. Click image to the left or use the URL below. URL: "
ocean ecosystems,T_1506,"Corals and other animals deposit calcium carbonate to create rock reefs near the shore. Coral reefs are the rain- forests of the oceans, with a tremendous amount of species diversity (Figure 1.2). Reefs can form interesting shapes in the oceans. Remember that hot spots create volcanoes on the seafloor. If these volcanoes rise above sea level to become islands, and if they occur in tropical waters, coral reefs will form on them. Since the volcanoes are cones, the reef forms in a circle around the volcano. As the volcano comes off the hot spot, the crust cools. The volcano subsides and then begins to erode away (Figure 1.3). Eventually, all that is left is a reef island called an atoll. A lagoon is found inside the reef. "
ocean ecosystems,T_1507,"The open ocean is a vast area. Food either washes down from the land or is created by photosynthesizing plankton. Zooplankton and larger animals feed on the phytoplankton and on each other. Larger animals such as whales and giant groupers may live their entire lives in the open water. How do fish survive in the deepest ocean? The few species that live in the greatest depths are very specialized (Figure 1.4). Since its rare to find a meal, the fish use very little energy; they move very little, breathe slowly, have minimal bone structure and a slow metabolism. These fish are very small. To maximize the chance of getting a meal, some species may have jaws that unhinge to accept a larger fish or backward-folding teeth to keep prey from escaping. Coral reefs are among the most densely inhabited and diverse areas on the globe. In this image of Maupiti Island in the South Pacific, the remnants of the volcano are surrounded by the circular reef. An 1896 drawing of a deep sea angler fish with a bioluminescent lure to attract prey. "
ocean ecosystems,T_1508,"Hydrothermal vents are among the most unusual ecosystems on Earth since they are dependent on chemosynthetic organisms at the base of the food web. At mid-ocean ridges at hydrothermal vents, bacteria that use chemosyn- thesis for food energy are the base of a unique ecosystem (Figure 1.5). This ecosystem is entirely separate from the photosynthesis at the surface. Shrimp, clams, fish, and giant tube worms have been found in these extreme places. Giant tube worms found at hydrothermal vents get food from the chemosynthetic bacteria that live within them. The bacte- ria provide food; the worms provide shel- ter. A video explaining hydrothermal vents with good footage is seen here: "
ocean garbage patch,T_1509,"Trash from land may end up as trash in the ocean, sometimes extremely far from land. Some of it will eventually wash ashore, possibly far from where it originated (Figure 1.1). "
ocean garbage patch,T_1510,"Although people had once thought that the trash found everywhere at sea was from ships, it turns out that 80% is from land. Some of that is from runoff, some is blown from nearshore landfills, and some is dumped directly into the sea. The 20% that comes from ships at sea includes trash thrown overboard by large cruise ships and many other vessels. It also includes lines and nets from fishing vessels. Ghost nets, nets abandoned by fishermen intentionally or not, float the seas and entangle animals so that they cannot escape. Containers sometimes go overboard in storms. Some noteworthy events, like a container of rubber ducks that entered the sea in 1992, are used to better understand ocean currents. The ducks went everywhere! "
ocean garbage patch,T_1511,"About 80% of the trash that ends up in the oceans is plastic. This is because a large amount of the trash produced since World War II is plastic. Also many types of plastic do not biodegrade, so they simply accumulate. While many types of plastic photodegrade  that is, they break up in sunlight  this process only works when the plastics are dry. Plastic trash in the water does break down into smaller pieces, eventually becoming molecule-sized polymers. Other trash in the oceans includes chemical sludge and materials that do biodegrade, like wood. "
ocean garbage patch,T_1512,"Some plastics contain toxic chemicals, such as bisphenol A. Plastics can also absorb organic pollutants that may be floating in the water, such as the pesticide DDT (which is banned in the U.S. but not in other nations) and some endocrine disruptors. "
ocean garbage patch,T_1513,"Trash from the lands all around the North Pacific is caught up in currents. The currents bring the trash into the center of the North Pacific Gyre. Scientists estimate that it takes about six years for trash to move from west coast of North America to the center of the gyre. The concentration of trash increases toward the center of the gyre. While recognizable pieces of garbage are visible, much of the trash is tiny plastic polymers that are invisible but can be detected in water samples. The particles are at or just below the surface within the gyre. Plastic confetti-like pieces are visible beneath the surface at the gyres center. This albatross likely died from the plastic it had ingested. The size of the garbage patch is unknown, since it cant be seen from above. Some people estimate that its twice the size of continental U.S, with a mass of 100 million tons. "
ocean garbage patch,T_1514,"Marine birds, such as albatross, or animals like sea turtles, live most of their lives at sea and just come ashore to mate. These organisms cant break down the plastic and they may eventually die (Figure 1.2). Boats may be affected. Plastic waste is estimated to kill 100,000 sea turtles and marine mammals annually, but exact numbers are unknown. Plastic shopping bags are extremely abundant in the oceans. If an organism accidentally ingests one, it may clog digestion and cause starvation by stopping food from moving through or making the animal not feel hungry. In some areas, plastics have seven times the concentration of zooplankton. This means that filter feeders are ingesting a lot of plastics. This may kill the organisms or the plastics may remain in their bodies. They are then eaten by larger organisms that store the plastics and may eventually die. Fish may eat organisms that have eaten plastic and then be eaten by people. This also exposes humans to toxic chemicals that the fish may have ingested with the plastic. There are similar patches of trash in the gyres of the North Atlantic and Indian oceans. The Southern Hemisphere has less trash buildup because less of the region is continent. "
ocean zones,T_1515,Oceanographers divide the ocean into zones both vertically and horizontally. 
ocean zones,T_1516,"To better understand regions of the ocean, scientists define the water column by depth. They divide the entire ocean into two zones vertically, based on light level. Large lakes are divided into similar regions. Sunlight only penetrates the sea surface to a depth of about 200 m, creating the photic zone (""photic"" means light). Organisms that photosynthesize depend on sunlight for food and so are restricted to the photic zone. Since tiny photosynthetic organisms, known as phytoplankton, supply nearly all of the energy and nutrients to the rest of the marine food web, most other marine organisms live in or at least visit the photic zone. In the aphotic zone there is not enough light for photosynthesis. The aphotic zone makes up the majority of the ocean, but has a relatively small amount of its life, both in diversity of type and in numbers. The aphotic zone is subdivided based on depth (Figure 1.1). The average depth of the ocean is 3,790 m, a lot more shallow than the deep trenches but still an incredible depth for sea creatures to live in. What makes it so hard to live at the bottom of the ocean? The three major factors that make the deep ocean hard to inhabit are the absence of light, low temperature, and extremely high pressure. "
ocean zones,T_1517,"The seabed is divided into the zones described above, but ocean itself is also divided horizontally by distance from the shore. Nearest to the shore lies the intertidal zone (also called the littoral zone), the region between the high and low tidal marks. The hallmark of the intertidal is change: water is in constant motion in the form of waves, tides, and currents. The land is sometimes under water and sometimes exposed. The neritic zone is from low tide mark and slopes gradually downward to the edge of the seaward side of the continental shelf. Some sunlight penetrates to the seabed here. The oceanic zone is the entire rest of the ocean from the bottom edge of the neritic zone, where sunlight does not reach the bottom. The sea bed and water column are subdivided further, as seen in the Figure 1.1. Click image to the left or use the URL below. URL: "
oil spills,T_1518,"Large oil spills, like the Exxon Valdez in Alaska in 1989, get a lot of attention, as they should. Besides these large spills, though, much more oil enters the oceans from small leaks that are only a problem locally. In this concept, well take a look at a large recent oil spill in the Gulf of Mexico. "
oil spills,T_1519,"New drilling techniques have allowed oil companies to drill in deeper waters than ever before. This allows us to access oil deposits that were never before accessible, but only with great technological difficulty. The risks from deepwater drilling and the consequences when something goes wrong are greater than those associated with shallower wells. "
oil spills,T_1520,"Working on oil platforms is dangerous. Workers are exposed to harsh ocean conditions and gas explosions. The danger was never more obvious than on April 20, 2010, when 11 workers were killed and 17 injured in an explosion on a deepwater oil rig in the Gulf of Mexico (Figure 1.1). The drilling rig, operated by BP, was 77 km (48 miles) offshore and the depth to the well was more than 5,000 feet. The U.S. Coast Guard tries to put out the fire and search for missing workers after the explosion on the Deepwater Horizon drilling rig. Eleven workers were killed. "
oil spills,T_1521,"Two days after the explosion, the drill rig sank. The 5,000-foot pipe that connected the wellhead to the drilling platform bent. Oil was free to gush into the Gulf of Mexico from nearly a mile deep (Figure 1.2). Initial efforts to cap or contain the spill at or near its source all failed to stop the vast oil spill. It was not until July 15, nearly three months after the accident, that the well was successfully capped. Estimating the flow of oil into the Gulf from the well was extremely difficult because the leak was so far below the surface. The U.S. government estimates that about 4.9 million barrels entered the Gulf at a rate of 35,000 to 60,000 barrels a day. The largest previous oil spill in the United States was of 300,000 barrels by the Exxon Valdez in 1989 in Prince William Sound, Alaska. "
oil spills,T_1522,"Once the oil is in the water, there are three types of methods for dealing with it: 1. Removal: Oil is corralled and then burned; natural gas is flared off (Figure 1.3). Machines that can separate oil from the water are placed aboard ships stationed in the area. These ships cleaned tens of thousands of barrels of contaminated seawater each day. 2. Containment: Floating containment booms are placed on the surface offshore of the most sensitive coastal areas in an attempt to attempt to trap the oil. But the seas must be calm for the booms to be effective, and so were not very useful in the Gulf (Figure 1.4). Sand berms have been constructed off of the Louisiana coast to keep the oil from reaching shore. (a) On May 17, 2010, oil had been leaking into the Gulf for nearly one month. On that date government estimates put the maximum total oil leak at 1,600,000 barrels, according to the New York Times. (b) The BP oil spill on June 19, 2010. The government estimates for total oil leaked by this date was 3,200,000 barrels. 3. Dispersal: Oil disperses naturally over time because it mixes with the water. However, such large amounts of oil will take decades to disperse. To speed the process up, BP has sprayed unprecedented amounts of chemical dispersants on the spill. That action did not receive support from the scientific community since no one knows the risks to people and the environment from such a large amount of these harmful chemicals. Some workers may have become ill from exposure to the chemicals. "
oil spills,T_1523,"BP drilled two relief wells into the original well. When the relief wells entered the original borehole, specialized liquids were pumped into the original well to stop the flow. Operation of the relief wells began in August 2010. The original well was declared effectively dead on September 19, 2010. "
oil spills,T_1524,"The economic and environmental impact of this spill will be felt for many years. Many people rely on the Gulf for their livelihoods or for recreation. Commercial fishing, tourism, and oil-related jobs are the economic engines of the region. Fearing contamination, NOAA imposed a fishing ban on approximately one-third of the Gulf (Figure 1.5). Tourism is down in the region as beach goers find other ways to spend their time. Real estate prices along the Gulf have declined precipitously. This was the extent of the banned area on June 21, 2010. The Gulf of Mexico is one of only two places in the world where bluefin tuna spawn and they are also already endangered. Marine mammals in the Gulf may come up into the slick as they come to the surface to breathe. Eight national parks and seashores are found along the Gulf shores. Other locations may be ecologically sensitive habitats such as mangroves or marshlands. "
oil spills,T_1525,There is still oil on beaches and in sediment on the seafloor in the region. Chemicals from the oil dispersants are still in the water. In October 2011 a report was issued that showed that whales and dolphins are dying in the Gulf at twice their normal rate. The long-term effects will be with us for a long time. Click image to the left or use the URL below. URL: 
overpopulation and over consumption,T_1526,The Green Revolution has brought enormous impacts to the planet. 
overpopulation and over consumption,T_1527,"Natural landscapes have been altered to create farmland and cities. Already, half of the ice-free lands have been converted to human uses. Estimates are that by 2030, that number will be more than 70%. Forests and other landscapes have been cleared for farming or urban areas. Rivers have been dammed and the water is transported by canals for irrigation and domestic uses. Ecologically sensitive areas have been altered: wetlands are now drained and coastlines are developed. "
overpopulation and over consumption,T_1528,"Modern agricultural practices produce a lot of pollution (Figure 1.1). Some pesticides are toxic. Dead zones grow as fertilizers drain off farmland and introduce nutrients into lakes and coastal areas. Farm machines and vehicles used to transport crops produce air pollutants. Pollutants enter the air, water, or are spilled onto the land. Moreover, many types of pollution easily move between air, water, and land. As a result, no location or organism  not even polar bears in the remote Arctic  is free from pollution. "
overpopulation and over consumption,T_1529,"The increased numbers of people have other impacts on the planet. Humans do not just need food. They also need clean water, secure shelter, and a safe place for their wastes. These needs are met to different degrees in different nations and among different socioeconomic classes of people. For example, about 1.2 billion of the worlds people do not have enough clean water for drinking and washing each day (Figure 1.2). "
overpopulation and over consumption,T_1530,"The addition of more people has not just resulted in more poor people. A large percentage of people expect much more than to have their basic needs met. For about one-quarter of people there is an abundance of food, plenty of water, and a secure home. Comfortable temperatures are made possible by heating and cooling systems, rapid trans- portation is available by motor vehicles or a well-developed public transportation system, instant communication takes place by phones and email, and many other luxuries are available that were not even dreamed of only a few The percentage of people in the world that live in abject poverty is decreasing some- what globally, but increasing in some re- gions, such as Sub-Saharan Africa. decades ago. All of these require resources in order to be produced, and fossil fuels in order to be powered (Figure Many people refer to the abundance of luxury items in these peoples lives as over-consumption. People in developed nations use 32 times more resources than people in the developing countries of the world. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
ozone depletion,T_1531,"At this point you might be asking yourself, Is ozone bad or is ozone good? There is no simple answer to that question: It depends on where the ozone is located (Figure 1.1). In the troposphere, ozone is a pollutant. In the ozone layer in the stratosphere, ozone screens out high energy ultraviolet radiation and makes Earth habitable. "
ozone depletion,T_1532,"Human-made chemicals are breaking ozone molecules in the ozone layer. Chlorofluorocarbons (CFCs) are the most common, but there are others, including halons, methyl bromide, carbon tetrachloride, and methyl chloroform. CFCs were once widely used because they are cheap, nontoxic, nonflammable, and non-reactive. They were used as spray-can propellants, refrigerants, and in many other products. Once they are released into the air, CFCs float up to the stratosphere. Air currents move them toward the poles. In the winter, they freeze onto nitric acid molecules in polar stratospheric clouds (PSC) (Figure 1.2). In the spring, (1) Solar energy breaks apart oxygen molecules into two oxygen atoms. (2) Ozone forms when oxygen atoms bond together as O3 . UV rays break apart the ozone molecules into one oxygen molecule (O2 ) and one oxygen atom (O). These processes convert UV radiation into heat, which is how the Sun heats the stratosphere. (3) Under natural cir- cumstances, the amount of ozone cre- ated equals the amount destroyed. When O3 interacts with chlorine or some other gases the O3 breaks down into O2 and O and so the ozone layer loses its ability to filter out UV. the Suns warmth starts the air moving, and ultraviolet light breaks the CFCs apart. The chlorine atom floats away and attaches to one of the oxygen atoms on an ozone molecule. The chlorine pulls the oxygen atom away, leaving behind an O2 molecule, which provides no UV protection. The chlorine then releases the oxygen atom and moves on to destroy another ozone molecule. One CFC molecule can destroy as many as 100,000 ozone molecules. PSCs form only where the stratosphere is coldest, and are most common above Antarctica in the wintertime. PSCs are needed for stratospheric ozone to be de- stroyed. "
ozone depletion,T_1533,"Ozone destruction creates the ozone hole where the layer is dangerously thin (Figure 1.3). As air circulates over Antarctica in the spring, the ozone hole expands northward over the southern continents, including Australia, New Zealand, southern South America, and southern Africa. UV levels may rise as much as 20% beneath the ozone hole. The hole was first measured in 1981 when it was 2 million square km (900,000 square miles). The 2006 hole was the largest ever observed at 28 million square km (11.4 million square miles). The size of the ozone hole each year depends on many factors, including whether conditions are right for the formation of PSCs. The September 2006 ozone hole, the largest observed (through 2013). Blue and purple colors show particularly low levels of ozone. "
ozone depletion,T_1534,"Ozone loss also occurs over the North Polar Region, but it is not enough for scientists to call it a hole. Why do you think there is less ozone loss over the North Pole area? The region of low ozone levels is small because the atmosphere is not as cold and PSCs do not form as readily. Still, springtime ozone levels are relatively low. This low moves south over some of the worlds most populated areas in Europe, North America, and Asia. At 40o N, the latitude of New York City, UV-B has increased about 4% per decade since 1978. At 55o N, the approximate latitude of Moscow and Copenhagen, the increase has been 6.8% per decade since 1978. Click image to the left or use the URL below. URL: "
ozone depletion,T_1535,"Ozone losses on human health and environment include: Increases in sunburns, cataracts (clouding of the lens of the eye), and skin cancers. A loss of ozone of only 1% is estimated to increase skin cancer cases by 5% to 6%. Decreases in the human immune systems ability to fight off infectious diseases. Reduction in crop yields because many plants are sensitive to ultraviolet light. Decreases in phytoplankton productivity. A decrease of 6% to 12% has been measured around Antarctica, which may be at least partly related to the ozone hole. The effects of excess UV on other organisms is not known. Whales in the Gulf of California have been found to have sunburned cells in their lowest skin layers, indicating very severe sunburns. The problem is greatest with light colored species or species that spend more time near the sea surface. When the problem with ozone depletion was recognized, world leaders took action. CFCs were banned in spray cans in some nations in 1978. The greatest production of CFCs was in 1986, but it has declined since then. This will be discussed more in the next concept. "
paleozoic and mesozoic seas,T_1536,"Some of the most important events of the Paleozoic and Mesozoic were the rising and falling of sea level over the continents. Sea level rises over the land during a marine transgression. During a marine regression, sea level retreats. During the Paleozoic there were four complete cycles of marine transgressions and regressions. There were two additional cycles during the Mesozoic (Figure 1.1). One of two things must happen for sea level to change in a marine transgression: either the land must sink or the water level must rise. What could cause sea level to rise? When little or no fresh water is tied up in glaciers and ice caps, sea level is high. Sea level also appears to rise if land is down dropped. Sea level rises if an increase in seafloor spreading rate buoys up the ocean crust, causing the ocean basin to become smaller. What could cause sea level to fall in a marine regression? Six marine transgressions and regres- sions have occurred during the Phanero- zoic. Geologists think that the Paleozoic marine transgressions and regressions were the result of the decrease and increase in the size of glaciers covering the lands. Click image to the left or use the URL below. URL: "
paleozoic and mesozoic seas,T_1537,"Geologists know about marine transgressions and regressions from the sedimentary rock record. These events leave characteristic rock layers known as sedimentary facies. On a shoreline, sand and other coarse grained rock fragments are commonly found on the beach where the wave energy is high. Away from the shore in lower energy environments, fine-grained silt that later creates shale is deposited. In deeper, low-energy waters, carbonate mud that later hardens into limestone is deposited. "
paleozoic and mesozoic seas,T_1538,"The Paleozoic sedimentary rocks of the Grand Canyon contain evidence of marine transgressions and regressions, but even there the rock record is not complete. Look at the sequence in the Figure 1.2 and see if you can determine whether the sea was transgressing or regressing. At the bottom, the Tonto Group represents a marine transgression: sandstone (11), shale (10), and limestone (9) laid down during 30 million years of the Cambrian Period. The Ordovician and Silurian are unknown because of an unconformity. Above that is freshwater limestone (8), which is overlain by limestone (7) and then shale (6), indicating that the sea was regressing. After another unconformity, the rocks of the Supai Group (5) include limestone, siltstone, and sandstone indicative of a regressing sea. Above those rocks are shale (4), sandstone (3), a limestone and sandstone mix (2) showing that the sea regressed and transgressed and finally limestone (1) indicating that the sea had come back in. "
paleozoic plate tectonics,T_1539,"The Paleozoic is the furthest back era of the Phanerozoic and it lasted the longest. But the Paleozoic was relatively recent, beginning only 570 million years ago. Compared with the long expanse of the Precambrian, the Phanerozoic is recent history. Much more geological evidence is available for scientists to study so the Phanerozoic is much better known. The Paleozoic begins and ends with a supercontinent. At the beginning of the Paleozoic, the supercontinent Rodinia began to split up. At the end, Pangaea came together. "
paleozoic plate tectonics,T_1540,"A mountain-building event is called an orogeny. Orogenies take place over tens or hundreds of millions of years. As continents smash into microcontinents and island arcs collided, mountains rise. Geologists find evidence for the orogenies that took place while Pangaea was forming in many locations. For example, Laurentia collided with the Taconic Island Arc during the Taconic Orogeny (Figure 1.1). The remnants of this mountain range make up the Taconic Mountains in New York. The Taconic Orogeny is an example of a collision between a continent and a volcanic island arc. Laurentia experienced other orogenies as it merged with the northern continents. The southern continents came together to form Gondwana. When Laurentia and Gondwana collided to create Pangaea, the Appalachians rose. Geologists think they may once have been higher than the Himalayas are now. "
paleozoic plate tectonics,T_1541,"Pangaea was the last supercontinent on Earth. Evidence for the existence of Pangaea was what Alfred Wegener used to create his continental drift hypothesis, which was described in the chapter Plate Tectonics. As the continents move and the land masses change shape, the shape of the oceans changes too. During the time of Pangaea, about 250 million years ago, most of Earths water was collected in a huge ocean called Panthalassa (Figure 1.2). Click image to the left or use the URL below. URL: "
petroleum power,T_1542,Oil is a liquid fossil fuel that is extremely useful because it can be transported easily and can be used in cars and other vehicles. Oil is currently the single largest source of energy in the world. 
petroleum power,T_1543,"Oil from the ground is called crude oil, which is a mixture of many different hydrocarbons. Crude oil is a thick dark brown or black liquid hydrocarbon. Oil also forms from buried dead organisms, but these are tiny organisms that live on the sea surface and then sink to the seafloor when they die. The dead organisms are kept away from oxygen by layers of other dead creatures and sediments. As the layers pile up, heat and pressure increase. Over millions of years, the dead organisms turn into liquid oil. "
petroleum power,T_1544,"In order to be collected, the oil must be located between a porous rock layer and an impermeable layer (Figure 1.1). Trapped above the porous rock layer and beneath the impermeable layer, the oil will remain between these layers until it is extracted from the rock. Oil (red) is found in the porous rock layer (yellow) and trapped by the impermeable layer (brown). The folded structure has allowed the oil to pool so a well can be drilled into the reservoir. To separate the different types of hydrocarbons in crude oil for different uses, the crude oil must be refined in refineries like the one shown in Figure 1.2. Refining is possible because each hydrocarbon in crude oil boils at a different temperature. When the oil is boiled in the refinery, separate equipment collects the different compounds. "
petroleum power,T_1545,"Most of the compounds that come out of the refining process are fuels, such as gasoline, diesel, and heating oil. Because these fuels are rich sources of energy and can be easily transported, oil provides about 90% of the energy used for transportation around the world. The rest of the compounds from crude oil are used for waxes, plastics, fertilizers, and other products. Gasoline is in a convenient form for use in cars and other transportation vehicles. In a car engine, the burned gasoline mostly turns into carbon dioxide and water vapor. The fuel releases most of its energy as heat, which causes the gases to expand. This creates enough force to move the pistons inside the engine and to power the car. Refineries like this one separate crude oil into many useful fuels and other chemi- cals. Click image to the left or use the URL below. URL: "
petroleum power,T_1546,"The United States does produce oil, but the amount produced is only about one-quarter as much as the nation uses. The United States has only about 1.5% of the worlds proven oil reserves, so most of the oil used by Americans must be imported from other nations. The main oil-producing regions in the United States are the Gulf of Mexico, Texas, Alaska, and California (Figure As in every type of mining, mining for oil has environmental consequences. Oil rigs are unsightly (Figure 1.4), and spills are too common (Figure 1.5). Click image to the left or use the URL below. URL:  Offshore well locations in the Gulf of Mex- ico. Note that some wells are located in very deep water. Drill rigs at the San Ardo Oil Field in Monterey, California. "
planet orbits in the solar system,T_1547,"Figure 1.1 shows the relative sizes of the orbits of the planets, asteroid belt, and Kuiper belt. In general, the farther away from the Sun, the greater the distance from one planets orbit to the next. The orbits of the planets are not circular but slightly elliptical, with the Sun located at one of the foci (see opening image). While studying the solar system, Johannes Kepler discovered the relationship between the time it takes a planet to make one complete orbit around the Sun, its ""orbital period,"" and the distance from the Sun to the planet. If the orbital period of a planet is known, then it is possible to determine the planets distance from the Sun. This is how astronomers without modern telescopes could determine the distances to other planets within the solar system. How old are you on Earth? How old would you be if you lived on Jupiter? How many days is it until your birthday on Earth? How many days until your birthday if you lived on Saturn? Click image to the left or use the URL below. URL:  The relative sizes of the orbits of planets in the solar system. The inner solar sys- tem and asteroid belt is on the upper left. The upper right shows the outer planets and the Kuiper belt. "
planets of the solar system,T_1548,"Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about our solar system. Astronomers have discovered two more planets (Uranus and Neptune), five dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects. Although the Sun is just an average star compared to other stars, it is by far the largest object in the solar system. The Sun is more than 500 times the mass of everything else in the solar system combined! Table 1.1 gives data on the sizes of the Sun and planets relative to Earth. Object Mass (Relative to Earth) Sun Mercury Venus Earth 333,000 Earths mass 0.06 Earths mass 0.82 Earths mass 1.00 Earths mass Diameter of Planet (Relative to Earth) 109.2 Earths diameter 0.39 Earths diameter 0.95 Earths diameter 1.00 Earths diameter Object Mass (Relative to Earth) Mars Jupiter Saturn Uranus Neptune 0.11 Earths mass 317.8 Earths mass 95.2 Earths mass 14.6 Earths mass 17.2 Earths mass Diameter of Planet (Relative to Earth) 0.53 Earths diameter 11.21 Earths diameter 9.41 Earths diameter 3.98 Earths diameter 3.81 Earths diameter "
planets of the solar system,T_1549,"Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km, or 93 million miles. Table 1.2 shows the distances to the planets (the average radius of orbits) in AU. The table also shows how long it takes each planet to spin on its axis (the length of a day) and how long it takes each planet to complete an orbit (the length of a year); in particular, notice how slowly Venus rotates relative to Earth. Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Average Distance from Sun (AU) 0.39 AU 0.72 1.00 1.52 5.20 9.54 19.22 30.06 Length of Day (In Earth Days) 56.84 days 243.02 1.00 1.03 0.41 0.43 0.72 0.67 Length of Year (In Earth Years) 0.24 years 0.62 1.00 1.88 11.86 29.46 84.01 164.8 Click image to the left or use the URL below. URL: "
ponds and lakes,T_1552,"Ponds are small bodies of fresh water that usually have no outlet; ponds are often are fed by underground springs. Like lakes, ponds are bordered by hills or low rises so the water is blocked from flowing directly downhill. "
ponds and lakes,T_1553,"Lakes are larger bodies of water. Lakes are usually fresh water, although the Great Salt Lake in Utah is just one exception. Water usually drains out of a lake through a river or a stream and all lakes lose water to evaporation. Lakes form in a variety of different ways: in depressions carved by glaciers, in calderas (Figure 1.1), and along tectonic faults, to name a few. Subglacial lakes are even found below a frozen ice cap. As a result of geologic history and the arrangement of land masses, most lakes are in the Northern Hemisphere. In fact, more than 60% of all the worlds lakes are in Canada  most of these lakes were formed by the glaciers that covered most of Canada in the last Ice Age (Figure 1.2). Lakes are not permanent features of a landscape. Some come and go with the seasons, as water levels rise and fall. Over a longer time, lakes disappear when they fill with sediments, if the springs or streams that fill them diminish, (a) Crater Lake in Oregon is in a volcanic caldera. Lakes can also form in volcanic craters and impact craters. (b) The Great Lakes fill depressions eroded as glaciers scraped rock out from the landscape. (c) Lake Baikal, ice coated in winter in this image, formed as water filled up a tectonic faults. Lakes near Yellowknife were carved by glaciers during the last Ice Age. or if their outlets grow because of erosion. When the climate of an area changes, lakes can either expand or shrink (Figure 1.3). Lakes may disappear if precipitation significantly diminishes. Large lakes have tidal systems and currents, and can even affect weather patterns. The Great Lakes in the United States contain 22% of the worlds fresh surface water (Figure 1.1). The largest them, Lake Superior, has a tide that rises and falls several centimeters each day. The Great Lakes are large enough to alter the weather system in Northeastern United States by the lake effect, which is an increase in snow downwind of the relatively warm lakes. The Great Lakes are home to countless species of fish and wildlife. Many lakes are not natural, but are human-made. People dam a stream in a suitable spot and then let the water back up behind it, creating a lake. These lakes are called ""reservoirs."" Click image to the left or use the URL below. URL: "
population size,T_1554,"Biotic and abiotic factors determine the population size of a species in an ecosystem. What are some important biotic factors? Biotic factors include the amount of food that is available to that species and the number of organisms that also use that food source. What are some important abiotic factors? Space, water, and climate all help determine a species population. When does a population grow? A population grows when the number of births is greater than the number of deaths. When does a population shrink? When deaths exceed births. What causes a population to grow? For a population to grow there must be ample resources and no major problems. What causes a population to shrink? A population can shrink either because of biotic or abiotic limits. An increase in predators, the emergence of a new disease, or the loss of habitat are just three possible problems that will decrease a population. A population may also shrink if it grows too large for the resources required to support it. "
population size,T_1555,"When the number of births equals the number of deaths, the population is at its carrying capacity for that habitat. In a population at its carrying capacity, there are as many organisms of that species as the habitat can support. The carrying capacity depends on biotic and abiotic factors. If these factors improve, the carrying capacity increases. If the factors become less plentiful, the carrying capacity drops. If resources are being used faster than they are being replenished, then the species has exceeded its carrying capacity. If this occurs, the population will then decrease in size. "
population size,T_1556,"Every stable population has one or more factors that limit its growth. A limiting factor determines the carrying capacity for a species. A limiting factor can be any biotic or abiotic factor: nutrient, space, and water availability are examples (Figure 1.1). The size of a population is tied to its limiting factor. What happens if a limiting factor increases a lot? Is it still a limiting factor? If a limiting factor increases a lot, another factor will most likely become the new limiting factor. This may be a bit confusing, so lets look at an example of limiting factors. Say you want to make as many chocolate chip cookies as you can with the ingredients you have on hand. It turns out that you have plenty of flour and other ingredients, but only two eggs. You can make only one batch of cookies, because eggs are the limiting factor. But then your neighbor comes over with a dozen eggs. Now you have enough eggs for seven batches of cookies, but only two pounds of butter. You can make four batches of cookies, with butter as the limiting factor. If you get more butter, some other ingredient will be limiting. Species ordinarily produce more offspring than their habitat can support (Figure 1.2). If conditions improve, more young survive and the population grows. If conditions worsen, or if too many young are born, there is competition between individuals. As in any competition, there are some winners and some losers. Those individuals that survive to fill the available spots in the niche are those that are the most fit for their habitat. Click image to the left or use the URL below. URL:  A frog in frog spawn. An animal produces many more offspring than will survive. "
precambrian continents,T_1557,"The first crust was made of basaltic rock, like the current ocean crust. Partial melting of the lower portion of the basaltic crust began more than 4 billion years ago. This created the silica-rich crust that became the felsic continents. "
precambrian continents,T_1558,"The earliest felsic continental crust is now found in the ancient cores of continents, called the cratons. Rapid plate motions meant that cratons experienced many continental collisions. Little is known about the paleogeography, or the ancient geography, of the early planet, although smaller continents could have come together and broken up. Geologists can learn many things about the Pre-Archean by studying the rocks of the cratons. Cratons also contain felsic igneous rocks, which are remnants of the first continents. Cratonic rocks contain rounded sedimentary grains. Of what importance is this fact? Rounded grains indicate that the minerals eroded from an earlier rock type and that rivers or seas also existed. One common rock type in the cratons is greenstone, a metamorphosed volcanic rock (Figure 1.1). Since greenstones are found today in oceanic trenches, what does the presence of greenstones mean? These ancient greenstones indicate the presence of subduction zones. Ice age glaciers scraped the Canadian Shield down to the 4.28 billion year old greenstone in Northwestern Quebec. "
precambrian continents,T_1559,"Places the craton crops out at the surface is known as a shield. Cratons date from the Precambrian and are called Precambrian shields. Many Precambrian shields are about 570 million years old (Figure 1.2). The Canadian Shield is the ancient flat part of Canada that lies around Hudson Bay, the northern parts of Minnesota, Wisconsin and Michigan and much of Greenland. "
precambrian continents,T_1560,"In most places the cratons were covered by younger rocks, which together are called a platform. Sometimes the younger rocks eroded away to expose the Precambrian craton (Figure 1.3). "
precambrian continents,T_1561,"During the Pre-Archean and Archean, Earths interior was warmer than today. Mantle convection was faster and plate tectonics processes were more vigorous. Since subduction zones were more common, the early crustal plates were relatively small. Since the time that it was completely molten, Earth has been cooling. Still, about half the internal heat that was generated when Earth formed remains in the planet and is the source of the heat in the core and mantle today. "
precambrian plate tectonics,T_1562,"By the end of the Archean, about 2.5 billion years ago, plate tectonics processes were completely recognizable. Small Proterozoic continents known as microcontinents collided to create supercontinents, which resulted in the uplift of massive mountain ranges. The history of the North American craton is an example of what generally happened to the cratons during the Precambrian. As the craton drifted, it collided with microcontinents and oceanic island arcs, which were added to the continents. Convergence was especially active between 1.5 and 1.0 billion years ago. These lands came together to create the continent of Laurentia. About 1.1 billion years ago, Laurentia became part of the supercontinent Rodinia (Figure 1.1). Rodinia probably contained all of the landmass at the time, which was about 75% of the continental landmass present today. Rodinia broke up about 750 million years ago. The geological evidence for this breakup includes large lava flows that are found where continental rifting took place. Seafloor spreading eventually started and created the oceans between the continents. The breakup of Rodinia may have triggered Snowball Earth around 700 million years ago. "
preventing hazardous waste problems,T_1581,"Nations that have more industry produce more hazardous waste. Currently, the United States is the worlds largest producer of hazardous wastes, but China, which produces so many products for the developed world, may soon take over the number-one spot. Countries with more industry produce more hazardous wastes than those with little industry. Problems with haz- ardous wastes and their disposal became obvious sooner in the developed world than in the developing world. As a result, many developed nations, including the United States, have laws to help control hazardous waste disposal and to clean toxic sites. As mentioned in the ""Impacts of Hazardous Waste"" concept, the Superfund Act requires companies to clean up contaminated sites that are designated as Superfund sites (Figure 1.1). If a responsible party cannot be identified, because the company has gone out of business or its culpability cannot be proven, the federal government pays for the cleanup out of a trust fund with money put aside by the petroleum and chemical industries. As a result of the Superfund Act, companies today are more careful about how they deal with hazardous substances. Superfund sites are located all over the nation and many are waiting to be cleaned up. The Resource Conservation and Recovery Act of 1976 requires that companies keep track of any hazardous materials they produce. These materials must be disposed of using government guidelines and records must be kept to show the government that the wastes were disposed of safely. Workers must be protected from the hazardous materials. To some extent, individuals can control the production and disposal of hazardous wastes. We can choose to use materials that are not hazardous, such as using vinegar as a cleanser. At home, people can control the amount of pesticides that they use (or they can use organic methods of pest control). It is also necessary to dispose of hazardous materials properly by not pouring them over the land, down the drain or toilet, or into a sewer or trashcan. Click image to the left or use the URL below. URL: "
principle of horizontality,T_1582,"Sedimentary rocks follow certain rules. 1. Sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top. 2. Sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some vol- canic rocks, such as ash falls. 3. Sedimentary rock layers that are not horizontal are deformed. Since sedimentary rocks follow these rules, they are useful for seeing the effects of stress on rocks. Sedimentary rocks that are not horizontal must have been deformed. You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest- on-bottom position. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses. "
principle of horizontality,T_1583,"Youre standing in the Grand Canyon and you see rocks like those in the Figure 1.1. Using the rules listed above, try to figure out the geologic history of the geologic column. The Grand Canyon is full mostly of sedimentary rocks, which are important for deciphering the geologic history of a region. In the Grand Canyon, the rock layers are exposed like a layer cake. Each layer is made of sediments that were deposited in a particular environment - perhaps a lake bed, shallow offshore region, or a sand dune. (a) The rocks of the Grand Canyon are like a layer cake. (b) A geologic column showing the rocks of the Grand Canyon. In this geologic column of the Grand Canyon, the sedimentary rocks of groups 3 through 6 are still horizontal. Group 2 rocks have been tilted. Group 1 rocks are not sedimentary. The oldest layers are on the bottom and youngest are on the top. The ways geologists figure out the geological history of an area will be explored more in the chapter Earth History. Click image to the left or use the URL below. URL: "
principle of uniformitarianism,T_1584,"The outcrop in the Figure 1.1 is at Checkerboard Mesa in Zion National Park, Utah. It has a very interesting pattern on it. As a geology student you may ask: how did this rock form? If you poke at the rock and analyze its chemistry you will see that its made of sand. In fact, the rock formation is called the Navajo sandstone. But knowing that the rock is sandstone doesnt tell you how it formed. It would be hard to design an experiment to show how this rock formed. But we can make observations now and apply them to this rock that formed long ago. "
principle of uniformitarianism,T_1585,"James Hutton came up with this idea in the late 1700s. The present is the key to the past. He called this the principle of uniformitarianism. It is that if we can understand a geological process now and we find evidence of that same Checkerboard Mesa in Zion National Park, Utah. process in the past, then we can assume that the process operated the same way in the past. Hutton speculated that it has taken millions of years to shape the planet, and it is continuing to be changed. He said that there are slow, natural processes that changed, and continue to change, the planets landscape. For example, given enough time, a stream could erode a valley, or sediment could accumulate and form a new landform. Lets go back to that outcrop. What would cause sandstone to have layers that cross each other, a feature called cross-bedding? "
principle of uniformitarianism,T_1586,"In the photo of the Mesquite sand dune in Death Valley National Park, California (Figure 1.2), we see that wind can cause cross-bedding in sand. Cross-bedding is due to changes in wind direction. There are also ripples caused by the wind waving over the surface of the dune. Since we can observe wind forming sand dunes with these patterns now, we have a good explanation for how the Navajo sandstone formed. The Navajo sandstone is a rock formed from ancient sand dunes in which wind direction changed from time to time. This is just one example of how geologists use observations they make today to unravel what happened in Earths past. Rocks formed from volcanoes, oceans, rivers, and many other features are deciphered by looking at the geological work those features do today. Click image to the left or use the URL below. URL: "
principles of relative dating,T_1587,"Early geologists had no way to determine the absolute age of a geological material. If they didnt see it form, they couldnt know if a rock was one hundred years or 100 million years old. What they could do was determine the ages of materials relative to each other. Using sensible principles they could say whether one rock was older than another and when a process occurred relative to those rocks. "
principles of relative dating,T_1588,"Remember Nicholas Steno, who determined that fossils represented parts of once-living organisms? Steno also noticed that fossil seashells could be found in rocks and mountains far from any ocean. He wanted to explain how that could occur. Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor. These rocks were then uplifted to become mountains. This scenario led him to develop the principles that are discussed below. They are known as Stenos laws. Stenos laws are illustrated in Figure 1.1. Original horizontality: Sediments are deposited in fairly flat, horizontal layers. If a sedimentary rock is found tilted, the layer was tilted after it was formed. Lateral continuity: Sediments are deposited in continuous sheets that span the body of water that they are deposited in. When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous. Superposition: Sedimentary rocks are deposited one on top of another. The youngest layers are found at the top of the sequence, and the oldest layers are found at the bottom. (a) Original horizontality. (b) Lateral continuity. (c) Superposition. "
principles of relative dating,T_1589,"Other scientists observed rock layers and formulated other principles. Geologist William Smith (1769-1839) identified the principle of faunal succession, which recognizes that: Some fossil types are never found with certain other fossil types (e.g. human ancestors are never found with dinosaurs) meaning that fossils in a rock layer represent what lived during the period the rock was deposited. Older features are replaced by more modern features in fossil organisms as species change through time; e.g. feathered dinosaurs precede birds in the fossil record. Fossil species with features that change distinctly and quickly can be used to determine the age of rock layers quite precisely. Scottish geologist, James Hutton (1726-1797) recognized the principle of cross-cutting relationships. This helps geologists to determine the older and younger of two rock units (Figure 1.2). If an igneous dike (B) cuts a series of metamorphic rocks (A), which is older and which is younger? In this image, A must have existed first for B to cut across it. "
principles of relative dating,T_1590,"The Grand Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedi- mentary rock illustrate the principle of original horizontality (Figure 1.3). The youngest rock layers are at the top and the oldest are at the bottom, which is described by the law of superposition. Distinctive rock layers, such as the Kaibab Limestone, are matched across the broad expanse of the canyon. These rock layers were once connected, as stated by the rule of lateral continuity. The Colorado River cuts through all the layers of rock to form the canyon. Based on the principle of cross- cutting relationships, the river must be younger than all of the rock layers that it cuts through. "
processes of the water cycle,T_1591,"The movement of water around Earths surface is the hydrological (water) cycle (Figure 1.1). Water inhabits reservoirs within the cycle, such as ponds, oceans, or the atmosphere. The molecules move between these reservoirs by certain processes, including condensation and precipitation. There are only so many water molecules and these molecules cycle around. If climate cools and glaciers and ice caps grow, there is less water for the oceans and sea level will fall. The reverse can also happen. The following section looks at the reservoirs and the processes that move water between them. "
processes of the water cycle,T_1592,"The Sun, many millions of kilometers away, provides the energy that drives the water cycle. Our nearest star directly impacts the water cycle by supplying the energy needed for evaporation. "
processes of the water cycle,T_1593,"Most of Earths water is stored in the oceans, where it can remain for hundreds or thousands of years. "
processes of the water cycle,T_1594,"Water changes from a liquid to a gas by evaporation to become water vapor. The Suns energy can evaporate water from the ocean surface or from lakes, streams, or puddles on land. Only the water molecules evaporate; the salts remain in the ocean or a fresh water reservoir. The water vapor remains in the atmosphere until it undergoes condensation to become tiny droplets of liquid. The droplets gather in clouds, which are blown about the globe by wind. As the water droplets in the clouds collide and grow, they fall from the sky as precipitation. Precipitation can be rain, sleet, hail, or snow. Sometimes precipitation falls back into the ocean and sometimes it falls onto the land surface. "
processes of the water cycle,T_1595,"When water falls from the sky as rain it may enter streams and rivers that flow downward to oceans and lakes. Water that falls as snow may sit on a mountain for several months. Snow may become part of the ice in a glacier, where it may remain for hundreds or thousands of years. Snow and ice may go directly back into the air by sublimation, the process in which a solid changes directly into a gas without first becoming a liquid. Although you probably have not seen water vapor undergoing sublimation from a glacier, you may have seen dry ice sublimate in air. Snow and ice slowly melt over time to become liquid water, which provides a steady flow of fresh water to streams, rivers, and lakes below. A water droplet falling as rain could also become part of a stream or a lake. At the surface, the water may eventually evaporate and reenter the atmosphere. "
processes of the water cycle,T_1596,A significant amount of water infiltrates into the ground. Soil moisture is an important reservoir for water (Figure The moisture content of soil in the United States varies greatly. 
processes of the water cycle,T_1597,"Water may seep through dirt and rock below the soil and then through pores infiltrating the ground to go into Earths groundwater system. Groundwater enters aquifers that may store fresh water for centuries. Alternatively, the water may come to the surface through springs or find its way back to the oceans. "
processes of the water cycle,T_1598,"Plants and animals depend on water to live. They also play a role in the water cycle. Plants take up water from the soil and release large amounts of water vapor into the air through their leaves (Figure 1.3), a process known as transpiration. "
processes of the water cycle,T_1599,"People also depend on water as a natural resource. Not content to get water directly from streams or ponds, humans create canals, aqueducts, dams, and wells to collect water and direct it to where they want it (Figure 1.4). Clouds form above the Amazon Rainfor- est even in the dry season because of moisture from plant transpiration. Pont du Gard in France is an ancient aqueduct and bridge that was part of of a well-developed system that supplied wa- ter around the Roman empire. Click image to the left or use the URL below. URL: "
protecting water from pollution,T_1600,Water pollution can be reduced in two ways: Keep the water from becoming polluted. Clean water that is already polluted. 
protecting water from pollution,T_1601,"Keeping water from becoming polluted often requires laws to be sure that people and companies behave responsibly. In the United States, the Clean Water Act gives the Environmental Protection Agency (EPA) the authority to set standards for water quality for industry, agriculture, and domestic uses. The law gives the EPA the authority to reduce the discharge of pollution into waterways, finance wastewater treatment plants, and manage runoff. Since its passage in 1972, more wastewater treatment plants have been constructed and the release of industrial waste into the water supply is better controlled. Scientists control water pollution by sam- pling the water and studying the pollutants that are in the water. The United Nations and other international groups are working to improve global water quality standards by pro- viding the technology for treating water. These organizations also educate people in how to protect and improve the quality of the water they use (Figure 1.1). Click image to the left or use the URL below. URL: "
protecting water from pollution,T_1602,"The goal of water treatment is to make water suitable for such uses as drinking, medicine, agriculture, and industrial processes. People living in developed countries suffer from few waterborne diseases and illness, because they have extensive water treatment systems to collect, treat, and redeliver clean water. Many underdeveloped nations have few or no water treatment facilities. Wastewater contains hundreds of contaminants, such as suspended solids, oxygen-demanding materials, dissolved inorganic compounds, and harmful bacteria. In a wastewater treatment plant, multiple processes must be used to produce usable water: Sewage treatment removes contaminants, such as solids and particles, from sewage. Water purification produces drinking water by removing bacteria, algae, viruses, fungi, unpleasant elements such as iron and sulfur, and man-made chemical pollutants. The treatment method used depends on the kind of wastewater being treated and the desired end result. Wastewater is treated using a series of steps, each of which produces water with fewer contaminants. "
protecting water from pollution,T_1603,"What can individuals do to protect water quality? Find approved recycling or disposal facilities for motor oil and household chemicals. Use lawn, garden, and farm chemicals sparingly and wisely. Repair automobile or boat engine leaks immediately. Keep litter, pet waste, leaves, and grass clippings out of street gutters and storm drains. Click image to the left or use the URL below. URL: "
radioactive decay as a measure of age,T_1604,"Radioactivity is the tendency of certain atoms to decay into lighter atoms, a process that emits energy. Radioactivity also provides a way to find the absolute age of a rock. First, we need to know about radioactive decay. "
radioactive decay as a measure of age,T_1605,"Some isotopes are radioactive; radioactive isotopes are unstable and spontaneously change by gaining or losing particles. Two types of radioactive decay are relevant to dating Earth materials (Table 1.1): Particle Alpha Composition 2 protons, 2 neutrons Beta 1 electron Effect on Nucleus The nucleus contains two fewer protons and two fewer neutrons. One neutron decays to form a pro- ton and an electron. The electron is emitted. The radioactive decay of a parent isotope (the original element) leads to the formation of stable daughter product, also known as daughter isotope. As time passes, the number of parent isotopes decreases and the number of daughter isotopes increases (Figure 1.1). "
radioactive decay as a measure of age,T_1606,"Radioactive materials decay at known rates, measured as a unit called half-life. The half-life of a radioactive substance is the amount of time it takes for half of the parent atoms to decay. This is how the material decays over time (see Table 1.2). No. of half lives passed 0 1 2 3 4 5 6 7 8 Percent parent remaining 100 50 25 12.5 6.25 3.125 1.563 0.781 0.391 Percent daughter produced 0 50 75 87.5 93.75 96.875 98.437 99.219 99.609 Pretend you find a rock with 3.125% parent atoms and 96.875% daughter atoms. How many half lives have passed? If the half-life of the parent isotope is 1 year, then how old is the rock? The decay of radioactive materials can be shown with a graph (Figure 1.2). Notice how it doesnt take too many half lives before there is very little parent remaining and most of the isotopes are daughter isotopes. This limits how many half lives can pass before a radioactive element is no longer useful for Decay of an imaginary radioactive sub- stance with a half-life of one year. dating materials. Fortunately, different isotopes have very different half lives. Click image to the left or use the URL below. URL: "
radiometric dating,T_1607,Radiometric dating is the process of using the concentrations of radioactive substances and daughter products to estimate the age of a material. Different isotopes are used to date materials of different ages. Using more than one isotope helps scientists to check the accuracy of the ages that they calculate. 
radiometric dating,T_1608,"Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range is especially useful for determining ages of human fossils and habitation sites (Figure 1.1). The atmosphere contains three isotopes of carbon: carbon-12, carbon-13 and carbon-14. Only carbon-14 is radioac- tive; it has a half-life of 5,730 years. The amount of carbon-14 in the atmosphere is tiny and has been relatively stable through time. Plants remove all three isotopes of carbon from the atmosphere during photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. After the organisms death, the carbon-14 decays to stable nitrogen-14 by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon-14 that has decayed can be estimated by measuring the proportion of radioactive carbon-14 to stable carbon- 12. As time passes, the amount of carbon-14 decreases relative to the amount of carbon-12. Carbon isotopes from the black material in these cave paintings places their cre- ating at about 26,000 to 27,000 years BP (before present). "
radiometric dating,T_1609,"Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Argon is a gas so it can escape from molten magma, meaning that any argon that is found in an igneous crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of potassium-40 to argon-40 yields a good estimate of the age of that crystal. Potassium is common in many minerals, such as feldspar, mica, and amphibole. With its half-life, the technique is used to date rocks from 100,000 years to over a billion years old. The technique has been useful for dating fairly young geological materials and deposits containing the bones of human ancestors. "
radiometric dating,T_1610,"Two uranium isotopes are used for radiometric dating. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life of 704 million years. Uranium-lead dating is usually performed on zircon crystals (Figure 1.2). When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. If any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium. Uranium-lead dating is useful for dating igneous rocks from 1 million years to around 4.6 billion years old. Zircon crystals from Australia are 4.4 billion years old, among the oldest rocks on the planet. "
radiometric dating,T_1611,"Radiometric dating is a very useful tool for dating geological materials but it does have limits: 1. The material being dated must have measurable amounts of the parent and/or the daughter isotopes. Ideally, different radiometric techniques are used to date the same sample; if the calculated ages agree, they are thought to be accurate. 2. Radiometric dating is not very useful for determining the age of sedimentary rocks. To estimate the age of a sedimentary rock, geologists find nearby igneous rocks that can be dated and use relative dating to constrain the age of the sedimentary rock. "
radiometric dating,T_1612,"As youve learned, radiometric dating can only be done on certain materials. But these important numbers can still be used to get the ages of other materials! How would you do this? One way is to constrain a material that cannot be dated by one or more that can. For example, if sedimentary rock A is below volcanic rock B and the age of volcanic rock B is 2.0 million years, then you know that sedimentary rock A is older than 2.0 million years. If sedimentary rock A is above volcanic rock C and its age is 2.5 million years then you know that sedimentary rock A is between 2.0 and 2.5 million years. In this way, geologists can figure out the approximate ages of many different rock formations. "
reducing air pollution,T_1613,"The Clean Air Act of 1970 and the amendments since then have done a great job in requiring people to clean up the air over the United States. Emissions of the six major pollutants regulated by the Clean Air Act  carbon monoxide, lead, nitrous oxides, ozone, sulfur dioxide, and particulates  have decreased by more than 50%. Cars, power plants, and factories individually release less pollution than they did in the mid-20th century. But there are many more cars, power plants, and factories. Many pollutants are still being released and some substances have been found to be pollutants that were not known to be pollutants in the past. There is still much work to be done to continue to clean up the air. "
reducing air pollution,T_1614,"Reducing air pollution from vehicles can be done in a number of ways. Breaking down pollutants before they are released into the atmosphere. Motor vehicles emit less pollution than they once did because of catalytic converters (Figure 1.1). Catalytic converters contain a catalyst that speeds up chemical reactions and breaks down nitrous oxides, carbon monoxide, and VOCs. Catalytic converters only work when they are hot, so a lot of exhaust escapes as the car is warming up. Catalytic converters are placed on mod- ern cars in the United States. Making a vehicle more fuel efficient. Lighter, more streamlined vehicles need less energy. Hybrid vehicles have an electric motor and a rechargeable battery. The energy that would be lost during braking is funneled into charging the battery, which then can power the car. The internal combustion engine only takes over when power in the battery has run out. Hybrids can reduce auto emissions by 90% or more, but many models do not maximize the possible fuel efficiency of the vehicle. A plug-in hybrid is plugged into an electricity source when it is not in use, perhaps in a garage, to make sure that the battery is charged. Plug-in hybrids run for a longer time on electricity and so are less polluting than regular hybrids. Plug-in hybrids began to become available in 2010. Developing new technologies that do not use fossil fuels. Fueling a car with something other than a liquid organic-based fuel is difficult. A fuel cell converts chemical energy into electrical energy. Hydrogen fuel cells harness the energy released when hydrogen and oxygen come together to create water (Figure 1.2). Fuel cells are extremely efficient and they produce no pollutants. But developing fuel-cell technology has had many problems and no one knows when or if they will become practical. "
reducing air pollution,T_1615,"Pollutants are removed from the exhaust streams of power plants and industrial plants before they enter the atmo- sphere. Particulates can be filtered out, and sulfur and nitric oxides can be broken down by catalysts. Removing these oxides reduces the pollutants that cause acid rain. Particles are relatively easy to remove from emissions by using motion or electricity to separate particles from the gases. Scrubbers remove particles and waste gases from exhaust using liquids or neutralizing materials (Figure 1.3). Gases, such as nitrogen oxides, can be broken down at very high temperatures. A hydrogen fuel-cell car looks like a gasoline-powered car. Scrubbers remove particles and waste gases from exhaust. "
reducing air pollution,T_1616,"Gasification is a developing technology. In gasification, coal (rarely is another organic material used) is heated to extremely high temperatures to create syngas, which is then filtered. The energy goes on to drive a generator. Syngas releases about 80% less pollution than regular coal plants, and greenhouse gases are also lower. Clean coal plants do not need scrubbers or other pollution control devices. Although the technology is ready, clean coal plants are more expensive to construct and operate. Also, heating the coal to high enough temperatures uses a great deal of energy, so the technology is not energy efficient. In addition, large amounts of the greenhouse gas CO2 are still released with clean coal technology. Nonetheless, a few of these plants are operating in the United States and around the world. "
reducing air pollution,T_1617,"How can air pollution be reduced? Using less fossil fuel is one way to lessen pollution. Some examples of ways to conserve fossil fuels are: Riding a bike or walking instead of driving. Taking a bus or carpooling. Buying a car that has greater fuel efficiency. Turning off lights and appliances when they are not in use. Using energy efficient light bulbs and appliances. Buying fewer things that are manufactured using fossil fuels. All these actions reduce the amount of energy that power plants need to produce. Click image to the left or use the URL below. URL:  Developing alternative energy sources is important. What are some of the problems facing wider adoption of alternative energy sources? The technologies for several sources of alternative energy, including solar and wind, are still being developed. Solar and wind are still expensive relative to using fossil fuels. The technology needs to advance so that the price falls. Some areas get low amounts of sunlight and are not suited for solar. Others do not have much wind. It is important that regions develop what best suits them. While the desert Southwest will need to develop solar, the Great Plains can use wind energy as its energy source. Perhaps some locations will rely on nuclear power plants, although current nuclear power plants have major problems with safety and waste disposal. Sometimes technological approaches are what is needed. Click image to the left or use the URL below. URL: "
reducing ozone destruction,T_1618,"One success story in reducing pollutants that harm the atmosphere concerns ozone-destroying chemicals. In 1973, scientists calculated that CFCs could reach the stratosphere and break apart. This would release chlorine atoms, which would then destroy ozone. Based only on their calculations, the United States and most Scandinavian countries banned CFCs in spray cans in 1978. More confirmation that CFCs break down ozone was needed before more was done to reduce production of ozone- destroying chemicals. In 1985, members of the British Antarctic Survey reported that a 50% reduction in the ozone layer had been found over Antarctica in the previous three springs. "
reducing ozone destruction,T_1619,"Two years after the British Antarctic Survey report, the ""Montreal Protocol on Substances that Deplete the Ozone Layer"" was ratified by nations all over the world. The Montreal Protocol controls the production and consumption of 96 chemicals that damage the ozone layer (Figure 1.1). Hazardous substances are phased out first by developed nations and one decade later by developing nations. More hazardous substances are phased out more quickly. CFCs have been mostly phased out since 1995, although were used in developing nations until 2010. Some of the less hazardous substances will not be phased out until 2030. The Protocol also requires that wealthier nations donate money to develop technologies that will replace these chemicals. Ozone levels over North America decreased between 1974 and 2009. Models of the future predict what ozone levels would have been if CFCs were not being phased out. Warmer colors indicate more ozone. Since CFCs take many years to reach the stratosphere and can survive there a long time before they break down, the ozone hole will probably continue to grow for some time before it begins to shrink. The ozone layer will reach the same levels it had before 1980 around 2068 and 1950 levels in one or two centuries. "
revolutions of earth,T_1620,"Certainly no one today doubts that Earth orbits a star, the Sun. Photos taken from space, observations made by astronauts, and the fact that there has been so much successful space exploration that depends on understanding the structure of the solar system all confirm it. But in the early 17th century saying that Earth orbited the Sun rather than the reverse could get you tried for heresy, as it did Galileo. Lets explore the evolution of the idea that Earth orbits the Sun. "
revolutions of earth,T_1621,"To an observer, Earth appears to be the center of the universe. That is what the ancient Greeks believed. This view is called the geocentric model, or ""Earth-centered"" model, of the universe. In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth. The geocentric model explained why all the stars appear to rotate around Earth once per day. The model also explained why the planets move differently from the stars and from each other. One problem with the geocentric model is that some planets seem to move backwards (in retrograde) instead of in their usual forward motion around Earth. Around 150 A.D. the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion of planets (Figure 1.1). In Ptolemys system, a planet moves in a small circle, called an epicycle. This circle moves around Earth in a larger circle, called a deferent. Ptolemys version of the geocentric model worked so well that it remained the accepted model of the universe for more than a thousand years. "
revolutions of earth,T_1622,"Ptolemys geocentric model worked, but it was complicated and occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model, or ""sun-centered"" model. Although Copernicus model was simpler - it didnt need epicycles and deferents - it still did not perfectly describe the motion of the planets. Johannes Kepler solved the problem a short time later when he determined that the planets moved around the Sun in ellipses (ovals), not circles (Figure 1.2). Keplers model matched observations perfectly. The heliocentric model did not catch on right away. When Galileo Galilei first turned a telescope to the heavens in 1610, he made several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided the first evidence that objects could orbit something besides Earth. Galileo also discovered that Venus has phases like the Moon (Figure 1.3), which provides direct evidence that Venus orbits the Sun. Galileos discoveries caused many more people to accept the heliocentric model of the universe, although Galileo himself was found guilty of heresy. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution. In their elliptical orbits, each planet is sometimes farther away from the Sun than at other times. This movement is called revolution. At the same time, Earth spins on its axis. Earths axis is an imaginary line passing through the Keplers model showed the planets moving around the Sun in ellipses. The phases of Venus. planets center that goes through both the North Pole and the South Pole. This spinning movement is called Earths rotation. "
revolutions of earth,T_1623,"Copernicus, Galileo, and Kepler were all right: Earth and the other planets travel in an elliptical orbit around the Sun. The gravitational pull of the Sun keeps the planets in orbit. This ellipse is barely elliptical; its very close to being a circle. The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd, and the furthest is at aphelion (152 million km) on July 4th. The shape of Earths orbit has nothing to do with Earths seasons. Earth and the other planets in the solar system make elliptical orbits around the Sun. For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year. Earth has one large moon, which orbits Earth once every 29.5 days, a period known as a month. Click image to the left or use the URL below. URL: "
rocks,T_1624,"A rock is a naturally formed, non-living Earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass (Figure 1.1). How is a rock different from a mineral? Rocks are made of minerals. The mineral grains in a rock may be so tiny that you can only see them with a microscope, or they may be as big as your fingernail or even your finger (Figure Rocks are identified primarily by the minerals they contain and by their texture. Each type of rock has a distinctive set of minerals. A rock may be made of grains of all one mineral type, such as quartzite. Much more commonly, rocks are made of a mixture of different minerals. Texture is a description of the size, shape, and arrangement of mineral grains. Are the two samples in Figure 1.3 the same rock type? Do they have the same minerals? The same texture? The different colors and textures seen in this rock are caused by the presence of different minerals. A pegmatite from South Dakota with crystals of lepidolite, tourmaline, and quartz (1 cm scale on the upper left). Sample 2 Crystals are tiny or microscopic Magma erupted and cooled quickly Andesite As seen in Table 1.1, these two rocks have the same chemical composition and contain mostly the same minerals, but they do not have the same texture. Sample 1 has visible mineral grains, but Sample 2 has very tiny or invisible grains. The two different textures indicate different histories. Sample 1 is a diorite, a rock that cooled slowly from magma (molten rock) underground. Sample 2 is an andesite, a rock that cooled rapidly from a very similar magma that erupted onto Earths surface. A few rocks are not made of minerals because the material they are made of does not fit the definition of a mineral. Coal, for example, is made of organic material, which is not a mineral. Can you think of other rocks that are not made of minerals? Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
rocks and processes of the rock cycle,T_1625,"The rock cycle, illustrated in Figure 1.1, depicts how the three major rock types - igneous, sedimentary, and meta- morphic - convert from one to another. Arrows connecting the rock types represent the processes that accomplish these changes. Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly. Rocks deep within the Earth are right now becoming other types of rocks. Rocks at the surface are lying in place before they are next exposed to a process that will change them. Even at the surface, we may not notice the changes. The rock cycle has no beginning or end. "
rocks and processes of the rock cycle,T_1626,"Rocks are classified into three major groups according to how they form. These three types are described in more detail in other concepts in this chapter, but here is a summary. The Rock Cycle. Igneous rocks form from the cooling and hardening of molten magma in many different environments. The chemical composition of the magma and the rate at which it cools determine what rock forms. Igneous rocks can cool slowly beneath the surface or rapidly at the surface. These rocks are identified by their composition and texture. More than 700 different types of igneous rocks are known. Sedimentary rocks form by the compaction and cementing together of sediments, broken pieces of rock-like gravel, sand, silt, or clay. Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates. Metamorphic rocks form when the minerals in an existing rock are changed by heat or pressure below the surface. Click image to the left or use the URL below. URL: "
rocks and processes of the rock cycle,T_1627,"Several processes can turn one type of rock into another type of rock. The key processes of the rock cycle are crystallization, erosion and sedimentation, and metamorphism. "
rocks and processes of the rock cycle,T_1628,"Magma cools either underground or on the surface and hardens into an igneous rock. As the magma cools, different crystals form at different temperatures, undergoing crystallization. For example, the mineral olivine crystallizes out of magma at much higher temperatures than quartz. The rate of cooling determines how much time the crystals will have to form. Slow cooling produces larger crystals. "
rocks and processes of the rock cycle,T_1629,"Weathering wears rocks at the Earths surface down into smaller pieces. The small fragments are called sediments. Running water, ice, and gravity all transport these sediments from one place to another by erosion. During sedimen- tation, the sediments are laid down or deposited. In order to form a sedimentary rock, the accumulated sediment must become compacted and cemented together. "
rocks and processes of the rock cycle,T_1630,"When a rock is exposed to extreme heat and pressure within the Earth but does not melt, the rock becomes meta- morphosed. Metamorphism may change the mineral composition and the texture of the rock. For that reason, a metamorphic rock may have a new mineral composition and/or texture. "
rotation of earth,T_1635,"In 1851, a French scientist named Lon Foucault took an iron sphere and hung it from a wire. He pulled the sphere to one side and then released it, as a pendulum. Although a pendulum set in motion should not change its motion, Foucault observed that his pendulum did seem to change direction relative to the circle below. Foucault concluded that Earth was moving underneath the pendulum. People at that time already knew that Earth rotated on its axis, but Foucaults experiment was nice confirmation. "
rotation of earth,T_1636,"Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an axis. Earth spins around its axis, just as a top spins around its spindle. This spinning movement is called Earths rotation. An observer in space will see that Earth requires 23 hours, 59 minutes, and 4 seconds to make one complete rotation on its axis. But because Earth moves around the Sun at the same time that it is rotating, the planet must turn just a little bit more to reach the same place relative to the Sun. Hence the length of a day on Earth is actually 24 hours. At the Equator, the Earth rotates at a speed of about 1,700 km per hour, but at the poles the movement speed is nearly nothing. "
rotation of earth,T_1637,"Earth rotates once on its axis about every 24 hours. To an observer looking down at the North Pole, the rotation appears counterclockwise. From nearly all points on Earth, the Sun appears to move across the sky from east to west each day. Of course, the Sun is not moving from east to west at all; Earth is rotating. The Moon and stars also seem to rise in the east and set in the west. Earths rotation means that there is a cycle of daylight and darkness approximately every 24 hours, the length of a day. Different places experience sunset and sunrise at different times and the amount of daylight and darkness also differs by location. Shadows are areas where an object obstructs a light source so that darkness takes on the form of the object. On Earth, a shadow can be cast by the Sun, Moon, or (rarely) Mercury or Venus. Click image to the left or use the URL below. URL: "
safety of water,T_1638,The water that comes out of our faucets is safe because it has gone through a series of treatment and purification processes to remove contaminants. Those of us who are fortunate enough to always be able to get clean water from a tap in our home may have trouble imagining life in a country that cannot afford the technology to treat and purify water. 
safety of water,T_1639,"Many people in the world have no choice but to drink from the same polluted river where sewage is dumped. One- fifth of all people in the world, more than 1.1 billion people, do not have access to safe water for drinking, personal cleanliness, and domestic use. Unsafe drinking water carries many pathogens, or disease-causing biological agents such as infectious bacteria and parasites. Toxic chemicals and radiological hazards in water can also cause diseases. "
safety of water,T_1640,"Waterborne disease caused by unsafe drinking water is the leading cause of death for children under the age of five in many nations and a cause of death and illness for many adults. About 88% of all diseases are caused by drinking unsafe water (Figure 1.1). Throughout the world, more than 14,000 people die every day from waterborne diseases, such as cholera, and many of the worlds hospital beds are occupied by patients suffering from a waterborne disease. Guinea worm is a serious problem in parts of Africa that is being eradicated. Learn what is being done to decrease the number of people suffering from this parasite at the video below. Click image to the left or use the URL below. URL: "
satellites shuttles and space stations,T_1641,"A rocket is propelled into space by particles flying out of one end at high speed (see Figure 1.1). A rocket in space moves like a skater holding the fire extinguisher. Fuel is ignited in a chamber, which causes an explosion of gases. The explosion creates pressure that forces the gases out of the rocket. As these gases rush out the end, the rocket moves in the opposite direction, as predicted by Newtons Third Law of Motion. The reaction force of the gases on the rocket pushes the rocket forward. The force pushing the rocket is called thrust. Nothing would get into space without being thrust upward by a rocket. "
satellites shuttles and space stations,T_1642,"One of the first uses of rockets in space was to launch satellites. A satellite is an object that orbits a larger object. An orbit is a circular or elliptical path around an object. The Moon was Earths first satellite, but now many human- made ""artificial satellites"" orbit the planet. Thousands of artificial satellites have been put into orbit around Earth (Figure 1.2). We have even put satellites into orbit around the Moon, the Sun, Venus, Mars, Jupiter, and Saturn. There are four main types of satellites. Imaging satellites take pictures of Earths surface for military or scientific purposes. Imaging satellites study the Moon and other planets. Communications satellites receive and send signals for telephone, television, or other types of communica- tions. Navigational satellites are used for navigation systems, such as the Global Positioning System (GPS). The International Space Station, the largest artificial satellite, is designed for humans to live in space while conducting scientific research. "
satellites shuttles and space stations,T_1643,"Humans have a presence in space at the International Space Station (ISS) (pictured in Figure 1.3). Modern space stations are constructed piece by piece to create a modular system. The primary purpose of the ISS is scientific research, especially in medicine, biology, and physics. "
satellites shuttles and space stations,T_1644,"Craft designed for human spaceflight, like the Apollo missions, were very successful, but were also very expensive, could not carry much cargo, and could be used only once. To outfit the ISS, NASA needed a space vehicle that was reusable and able to carry large pieces of equipment, such as satellites, space telescopes, or sections of a space station. The resulting spacecraft was a space shuttle, shown in (Figure 1.4). Satellites operate with solar panels for energy. A photograph of the International Space Station was taken from the space shuttle Atlantis in June 2007. Construction of the station was completed in 2011, but new pieces and experiments continue to be added. A space shuttle has three main parts. The part you are probably most familiar with is the orbiter, with wings like an airplane. When a space shuttle launches, the orbiter is attached to a huge fuel tank that contains liquid fuel. On the sides of the fuel tank are two large ""booster rockets."" All of this is needed to get the orbiter out of Earths atmosphere. Once in space, the orbiter can be used to release equipment (such as a satellite or supplies for the International Space Station), to repair existing equipment such as the Hubble Space Telescope, or to do experiments directly on board the orbiter. When the mission is complete, the orbiter re-enters Earths atmosphere and flies back to Earth more like a glider than an airplane. The Space Shuttle program did 135 missions between 1981 and 2011, when the remaining shuttles were retired. The ISS is now serviced by Russian Soyuz spacecraft. Atlantis on the launch pad in 2006. Since 1981, the space shuttle has been the United States primary vehicle for carrying people and large equipment into space. "
saturn,T_1645,"Saturn, shown in Figure 1.1, is famous for its beautiful rings. Although all the gas giants have rings, only Saturns can be easily seen from Earth. In Roman mythology, Saturn was the father of Jupiter. Saturns mass is about 95 times the mass of Earth, and its volume is 755 times Earths volume, making it the second largest planet in the solar system. Saturn is also the least dense planet in the solar system. It is less dense than water. What would happen if you had a large enough bathtub to put Saturn in? Saturn would float! Saturn orbits the Sun once about every 30 Earth years. Like Jupiter, Saturn is made mostly of hydrogen and helium gases in the outer layers and liquids at greater depths. The upper atmosphere has clouds in bands of different colors. These rotate rapidly around the planet, but there seems to be less turbulence and fewer storms on Saturn than on Jupiter. One interesting phenomenon that has been observed in the storms on Saturn is the presence of thunder and lightning (see video, below). The planet likely has a small rocky and metallic core. This image of Saturn and its rings is a composite of pictures taken by the Cassini orbiter in 2008 "
saturn,T_1646,"In 1610, Galileo first observed Saturns rings with his telescope, but he thought they might be two large moons, one on either side of the planet. In 1659, the Dutch astronomer Christian Huygens realized that the features were rings (Figure 1.2). Saturns rings circle the planets equator and appear tilted because Saturn itself is tilted about 27 degrees. The rings do not touch the planet. The Voyager 1 and 2 spacecraft in 1980 and 1981 sent back detailed pictures of Saturn, its rings, and some of its moons. Saturns rings are made of particles of water and ice, with some dust and rocks (Figure 1.3). There are several gaps in the rings that scientists think have originated because the material was cleared out by the gravitational pull within the rings, or by the gravitational forces of Saturn and of moons outside the rings. The rings were likely formed by the breakup of one of Saturns moons or from material that never accreted into the planet when Saturn originally formed. "
saturn,T_1647,"Most of Saturns moons are very small, and only seven are large enough for gravity to have made them spherical. Only Titan is larger than Earths Moon at about 1.5 times its size. Titan is even larger than the planet Mercury. Scientists are interested in Titan because its atmosphere is similar to what Earths was like before life developed. Nitrogen is dominant and methane is the second most abundant gas. Titan may have a layer of liquid water and ammonia under a layer of surface ice. Lakes of liquid methane (CH4 ) and ethane (C2 H6 ) are found on Titans surface. Although conditions are similar enough to those of early Earth for scientists to speculate that extremely A color-exaggerated mosaic of Saturn and its rings taken by Cassini as Saturn eclipses the Sun. A close-up of Saturns outer C ring show- ing areas with higher particle concentra- tion and gaps. This composite image compares Saturns largest moon, Titan (right) to Earth (left). Click image to the left or use the URL below. URL: "
scientific models,T_1662,"Scientific models are useful tools in science. Earths climate is extremely complex, with many factors that are dependent on one another. Such a system is impossible for scientists to work with as a whole. To deal with such complexity, scientists may create models to represent the system that they are interested in studying. Scientists must validate their ideas by testing. A model can be manipulated and adjusted far more easily than a real system. Models help scientists understand, analyze, and make predictions about systems that would be impossible to study as a whole. If a scientist wants to understand how rising CO2 levels will affect climate, it will be easier to model a smaller portion of that system. For example, he may model how higher levels of CO2 affect plant growth and the effect that will have on climate. "
scientific models,T_1663,"How can scientists know if a model designed to predict the future is likely to be accurate, since it may not be possible to wait long enough to see if the prediction comes true? One way is to run the model using a time in the past as the starting point see if the model can accurately predict the present. A model that can successfully predict the present is more likely to be accurate when predicting the future. Many models are created on computers because only computers can handle and manipulate such enormous amounts of data. For example, climate models are very useful for trying to determine what types of changes we can expect as the composition of the atmosphere changes. A reasonably accurate climate model would be impossible on anything other than the most powerful computers. "
scientific models,T_1664,"Since models are simpler than real objects or systems, they have limitations. A model deals with only a portion of a system. It may not predict the behavior of the real system very accurately. But the more computing power that goes into the model and the care with which the scientists construct the model can increase the chances that a model will be accurate. "
scientific models,T_1665,Physical models are smaller and simpler representations of the thing being studied. A globe or a map is a physical model of a portion or all of Earth. Conceptual models tie together many ideas to explain a phenomenon or event. Mathematical models are sets of equations that take into account many factors to represent a phenomenon. Mathematical models are usually done on computers. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: 
seafloor spreading hypothesis,T_1666,"Harry Hess was a geology professor and a naval officer who commanded an attack transport ship during WWII. Like other ships, Hesss ship had echo sounders that mapped the seafloor. Hess discovered hundreds of flat-topped mountains in the Pacific that he gave the name guyot. He puzzled at what could have formed mountains that appeared to be eroded at the top but were more than a mile beneath the sea surface. Hess also noticed trenches that were as much as 7 miles deep. Meanwhile, other scientists like Bruce Heezen discovered the underwater mountain range they called the Great Global Rift. Although the rift was mostly in the deep sea, it occasionally came close to land. These scientists thought the rift was a set of breaks in Earths crust. The final piece that was needed was the work of Vine and Matthews, who had discovered the bands of alternating magnetic polarity in the seafloor symmetrically about the rift. "
seafloor spreading hypothesis,T_1667,"The features of the seafloor and the patterns of magnetic polarity symmetrically about the mid-ocean ridges were the pieces that Hess needed. He resurrected Wegeners continental drift hypothesis and also the mantle convection idea of Holmes. Hess wrote that hot magma rose up into the rift valley at the mid-ocean ridges. The lava oozed up and forced the existing seafloor away from the rift in opposite directions. Since magnetite crystals point in the direction of the magnetic north pole as the lava cools, the different stripes of magnetic polarity revealed the different ages of the seafloor. The seafloor at the ridge is from the Brunhes normal; beyond that is basalt from the Matuyama reverse; and beyond that from the Gauss normal. Hess called this idea seafloor spreading. As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis. If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle. The oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust. Hess could also use seafloor spreading to explain the flat topped guyots. He suggested that they were once active volcanoes that were exposed to erosion above sea level. As the seafloor they sat on moved away from the ridge, the crust on which they sat become less buoyant and the guyots moved deeper beneath sea level. "
seafloor spreading hypothesis,T_1668,"Seafloor spreading is the mechanism for Wegeners drifting continents. Convection currents within the mantle take the continents on a conveyor-belt ride of oceanic crust that, over millions of years, takes them around the planets surface. The spreading plate takes along any continent that rides on it. Click image to the left or use the URL below. URL: "
seasons,T_1669,"A common misconception is that the Sun is closer to Earth in the summer and farther away from it during the winter. Instead, the seasons are caused by the 23.5o tilt of Earths axis of rotation relative to its plane of orbit around the Sun (Figure 1.1). Solstice refers to the position of the Sun when it is closest to one of the poles. At summer solstice, June 21 or 22, Earths axis points toward the Sun and so the Sun is directly overhead at its furthest north point of the year, the Tropic of Cancer (23.5o N). During the summer, areas north of the Equator experience longer days and shorter nights. In the Southern Hemi- sphere, the Sun is as far away as it will be and so it is their winter. Locations will have longer nights and shorter days. The opposite occurs on winter solstice, which begins on December 21. More about seasons can be found in the Atmospheric Processes chapter. "
seasons,T_1670,"Different parts of the Earth receive different amounts of solar radiation. Which part of the planet receives the most solar radiation? The Suns rays strike the surface most directly at the Equator. Different areas also receive different amounts of sunlight in different seasons. What causes the seasons? The seasons are caused by the direction Earths axis is pointing relative to the Sun. The Earth revolves around the Sun once each year and spins on its axis of rotation once each day. This axis of rotation is tilted 23.5o relative to its plane of orbit around the Sun. The axis of rotation is pointed toward Polaris, the North Star. As the Earth orbits the Sun, the tilt of Earths axis stays lined up with the North Star. "
seasons,T_1671,"The North Pole is tilted towards the Sun and the Suns rays strike the Northern Hemisphere more directly in summer (Figure 1.2). At the summer solstice, June 21 or 22, the Suns rays hit the Earth most directly along the Tropic of Cancer (23.5o N); that is, the angle of incidence of the Suns rays there is zero (the angle of incidence is the deviation in the angle of an incoming ray from straight on). When it is summer solstice in the Northern Hemisphere, it is winter solstice in the Southern Hemisphere. "
seasons,T_1672,"Winter solstice for the Northern Hemisphere happens on December 21 or 22. The tilt of Earths axis points away from the Sun (Figure 1.3). Light from the Sun is spread out over a larger area, so that area isnt heated as much. With fewer daylight hours in winter, there is also less time for the Sun to warm the area. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere. "
seasons,T_1673,"Halfway between the two solstices, the Suns rays shine most directly at the Equator, called an equinox (Figure 1.4). The daylight and nighttime hours are exactly equal on an equinox. The autumnal equinox happens on September 22 or 23 and the vernal, or spring, equinox happens March 21 or 22 in the Northern Hemisphere. Summer solstice in the Northern Hemisphere. Click image to the left or use the URL below. URL: "
seawater chemistry,T_1674,"Remember that H2 O is a polar molecule, so it can dissolve many substances (Figure 1.1). Salts, sugars, acids, bases, and organic molecules can all dissolve in water. "
seawater chemistry,T_1675,"Where does the salt in seawater come from? As water moves through rock and soil on land it picks up ions. This is the flip side of weathering. Salts comprise about 3.5% of the mass of ocean water, but the salt content, or salinity, is different in different locations. What would the salinity be like in an estuary? Where seawater mixes with fresh water, salinity is lower than average. What would the salinity be like where there is lots of evaporation? Where there is lots of evaporation but little circulation of water, salinity can be much higher. The Dead Sea has 30% salinity  nearly nine times the average salinity of ocean water (Figure 1.2). Why do you think this water body is called the Dead Sea? In some areas, dense saltwater and less dense freshwater mix, and they form an immiscible layer, just like oil and water. One such place is a ""cenote"", or underground cave, very common in certain parts of Central America. Ocean water is composed of many sub- stances, many of them salts such as sodium, magnesium, and calcium chlo- ride. Because of the increased salinity, the wa- ter in the Dead Sea is very dense, it has such high salinity that people can easily float in it! "
seawater chemistry,T_1676,"With so many dissolved substances mixed in seawater, what is the density (mass per volume) of seawater relative to fresh water? Water density increases as: salinity increases temperature decreases pressure increases Differences in water density are responsible for deep ocean currents, as will be discussed in the ""Deep Ocean Currents"" concept. Click image to the left or use the URL below. URL: "
sedimentary rock classification,T_1677,"Rock Conglomerate Breccia Sandstone Siltstone Shale Sediment Size Large Large Sand-sized Silt-sized, smaller than sand Clay-sized, smallest Other Features Rounded Angular When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical sedimentary rocks. As discussed in the concepts on minerals, dissolved ions in fluids precipitate out of the fluid and settle out, just like the halite in Figure 1.1. The evaporite, halite, on a cobble from the Dead Sea, Israel. Biochemical sedimentary rocks form in the ocean or a salt lake. Living creatures remove ions, such as calcium, magnesium, and potassium, from the water to make shells or soft tissue. When the organism dies, it sinks to the ocean floor to become a biochemical sediment, which may then become compacted and cemented into solid rock (Figure 1.2). Table 1.2 shows some common types of sedimentary rocks. Breccia Clastic Sandstone Clastic Siltstone Clastic Shale Clastic Rock Salt Chemical precipitate Dolostone Chemical precipitate Limestone Bioclastic (sediments from organic materials, or plant or animal re- mains) Coal Organic Click image to the left or use the URL below. URL: "
sedimentary rocks,T_1678,"Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include: fragments of other rocks that often have been worn down into small pieces, such as sand, silt, or clay. organic materials, or the remains of once-living organisms. chemical precipitates, which are materials that get left behind after the water evaporates from a solution. Rocks at the surface undergo mechanical and chemical weathering. These physical and chemical processes break rock into smaller pieces. Mechanical weathering simply breaks the rocks apart. Chemical weathering dissolves the less stable minerals. These original elements of the minerals end up in solution and new minerals may form. Sediments are removed and transported by water, wind, ice, or gravity in a process called erosion (Figure 1.1). Much more information about weathering and erosion can be found in the chapter Surface Processes and Landforms. Streams carry huge amounts of sediment (Figure 1.2). The more energy the water has, the larger the particle it can carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles. Water erodes the land surface in Alaskas Valley of Ten Thousand Smokes. Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps. Landslides drop large piles of sediment. Glaciers leave large piles of sediments, too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present. Darker sediments form when the environment is oxygen poor. Click image to the left or use the URL below. URL: "
seismic waves,T_1679,Energy is transmitted in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The distance between waves from crest to crest (or trough to trough) is its wavelength. The parts of a wave are illustrated in Figure 1.1. 
seismic waves,T_1680,The energy from earthquakes travels in waves. The study of seismic waves is known as seismology. Seismologists use seismic waves to learn about earthquakes and also to learn about the Earths interior. One ingenious way scientists learn about Earths interior is by looking at earthquake waves. Seismic waves travel outward in all directions from where the ground breaks and are picked up by seismographs around the world. Two types of seismic waves are most useful for learning about Earths interior. 
seismic waves,T_1681,"P-waves and S-waves are known as body waves because they move through the solid body of the Earth. P-waves travel through solids, liquids, and gases. S-waves only move through solids (Figure 1.2). Surface waves only travel along Earths surface. In an earthquake, body waves produce sharp jolts. They do not do as much damage as surface waves. P-waves (primary waves) are fastest, traveling at about 6 to 7 kilometers (about 4 miles) per second, so they arrive first at the seismometer. P-waves move in a compression/expansion type motion, squeezing and S-waves (secondary waves) are about half as fast as P-waves, traveling at about 3.5 km (2 miles) per second, and arrive second at seismographs. S-waves move in an up and down motion perpendicular to the direction of wave travel. This produces a change in shape for the Earth materials they move through. Only solids resist a change in shape, so S-waves are only able to propagate through solids. S-waves cannot travel through liquid. "
seismic waves,T_1682,"By tracking seismic waves, scientists have learned what makes up the planets interior (Figure 1.4). P-waves slow down at the mantle core boundary, so we know the outer core is less rigid than the mantle. S-waves disappear at the mantle core boundary, so we know the outer core is liquid. "
seismic waves,T_1683,"Surface waves travel along the ground, outward from an earthquakes epicenter. Surface waves are the slowest of all seismic waves, traveling at 2.5 km (1.5 miles) per second. There are two types of surface waves. The rolling motions of surface waves do most of the damage in an earthquake. "
short term climate change,T_1684,Short-term changes in climate are common and they have many causes (Figure 1.1). The largest and most important of these is the oscillation between El Nio and La Nia conditions. This cycle is called the ENSO (El Nio Southern Oscillation). The ENSO drives changes in climate that are felt around the world about every two to seven years. 
short term climate change,T_1685,"In a normal year, the trade winds blow across the Pacific Ocean near the Equator from east to west (toward Asia). A low pressure cell rises above the western equatorial Pacific. Warm water in the western Pacific Ocean raises sea levels by half a meter. Along the western coast of South America, the Peru Current carries cold water northward, and then westward along the Equator with the trade winds. Upwelling brings cold, nutrient-rich waters from the deep sea. "
short term climate change,T_1686,"In an El Nio year, when water temperature reaches around 28o C (82o F), the trade winds weaken or reverse direction and blow east (toward South America) (Figure 1.2). Warm water is dragged back across the Pacific Ocean and piles up off the west coast of South America. With warm, low-density water at the surface, upwelling stops. Without upwelling, nutrients are scarce and plankton populations decline. Since plankton form the base of the food web, fish cannot find food, and fish numbers decrease as well. All the animals that eat fish, including birds and humans, are affected by the decline in fish. By altering atmospheric and oceanic circulation, El Nio events change global climate patterns. Some regions receive more than average rainfall, including the west coast of North and South America, the southern United States, and Western Europe. Drought occurs in other parts of South America, the western Pacific, southern and northern Africa, and southern Europe. An El Nio cycle lasts one to two years. Often, normal circulation patterns resume. Sometimes circulation patterns bounce back quickly and extremely (Figure 1.3). This is a La Nia. "
short term climate change,T_1687,"In a La Nia year, as in a normal year, trade winds moves from east to west and warm water piles up in the western Pacific Ocean. Ocean temperatures along coastal South America are colder than normal (instead of warmer, as in El Nio). Cold water reaches farther into the western Pacific than normal. Other important oscillations are smaller and have a local, rather than global, effect. The North Atlantic Oscillation mostly alters climate in Europe. The Mediterranean also goes through cycles, varying between being dry at some times and warm and wet at others. Click image to the left or use the URL below. URL: "
solar energy on earth,T_1708,"Most of the energy that reaches the Earths surface comes from the Sun (Figure 1.1). About 44% of solar radiation is in the visible light wavelengths, but the Sun also emits infrared, ultraviolet, and other wavelengths. "
solar energy on earth,T_1709,"Of the solar energy that reaches the outer atmosphere, ultraviolet (UV) wavelengths have the greatest energy. Only about 7% of solar radiation is in the UV wavelengths. The three types are: UVC: the highest energy ultraviolet, does not reach the planets surface at all. UVB: the second highest energy, is also mostly stopped in the atmosphere. UVA: the lowest energy, travels through the atmosphere to the ground. "
solar energy on earth,T_1710,"The remaining solar radiation is the longest wavelength, infrared. Most objects radiate infrared energy, which we feel as heat. Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed by various gases (Figure 1.2). Ozone completely removes UVC, most UVB, and some UVA from incoming sunlight. O2 , CO2 , and H2 O also filter out some wavelengths. An image of the Sun taken by the SOHO spacecraft. The sensor is picking up only the 17.1 nm wavelength, in the ultraviolet wavelengths. Atmospheric gases filter some wave- lengths of incoming solar energy. Yel- low shows the energy that reaches the top of the atmosphere. Red shows the wavelengths that reach sea level. Ozone filters out the shortest wavelength UV and oxygen filters out most infrared. Click image to the left or use the URL below. URL: "
solar power,T_1711,"Energy from the Sun comes from the lightest element, hydrogen, fusing together to create the second lightest element, helium. Nuclear fusion on the Sun releases tremendous amounts of solar energy. The energy travels to the Earth, mostly as visible light. The light carries the energy through the empty space between the Sun and the Earth as radiation. "
solar power,T_1712,"Solar energy has been used for power on a small scale for hundreds of years, and plants have used it for billions of years. Unlike energy from fossil fuels, which almost always come from a central power plant or refinery, solar power can be harnessed locally (Figure 1.1). A set of solar panels on a homes rooftop can be used to heat water for a swimming pool or can provide electricity to the house. Societys use of solar power on a larger scale is just starting to increase. Scientists and engineers have very active, ongoing research into new ways to harness energy from the Sun more efficiently. Because of the tremendous amount of incoming sunlight, solar power is being developed in the United States in southeastern California, Nevada, and Arizona. Solar panels supply power to the Interna- tional Space Station. Solar power plants turn sunlight into electricity using a large group of mirrors to focus sunlight on one place, called a receiver (Figure 1.2). A liquid, such as oil or water, flows through this receiver and is heated to a high temperature by the focused sunlight. The heated liquid transfers its heat to a nearby object that is at a lower temperature through a process called conduction. The energy conducted by the heated liquid is used to make electricity. This solar power plant uses mirrors to focus sunlight on the tower in the center. The sunlight heats a liquid inside the tower to a very high temperature, producing energy to make electricity. "
solar power,T_1713,"Solar energy has many benefits. It is extremely abundant, widespread, and will never run out. But there are problems with the widespread use of solar power. Sunlight must be present. Solar power is not useful in locations that are often cloudy or dark. However, storage technology is being developed. The technology needed for solar power is still expensive. An increase in interested customers will provide incentive for companies to research and develop new technologies and to figure out how to mass-produce existing technologies (Figure 1.3). Solar panels require a lot of space. Fortunately, solar panels can be placed on any rooftop to supply at least some of the power required for a home or business. This experimental car is one example of the many uses that engineers have found for solar energy. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
star classification,T_1714,"Think about how the color of a piece of metal changes with temperature. A coil of an electric stove will start out black, but with added heat will start to glow a dull red. With more heat, the coil turns a brighter red, then orange. At extremely high temperatures the coil will turn yellow-white, or even blue-white (its hard to imagine a stove coil getting that hot). A stars color is also determined by the temperature of the stars surface. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white (Figure 1.1). "
star classification,T_1715,"Color is the most common way to classify stars. Table 1.1 shows the classification system. The class of a star is given by a letter. Each letter corresponds to a color, and also to a range of temperatures. Note that these letters dont match the color names; they are left over from an older system that is no longer used. Class O B A F G K M Color Blue Blue-white White Yellowish-white Yellow Orange Red Temperature Range 30,000 K or more 10,000-30,000 K 7,500-10,000 K 6,000-7,500 K 5,500-6,000 K 3,500-5,000 K 2,000-3,500 K Sample Star Zeta Ophiuchi Rigel Altair Procyon A Sun Epsilon Indi Betelgeuse, Proxima Cen- tauri For most stars, surface temperature is also related to size. Bigger stars produce more energy, so their surfaces are hotter. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
star constellations,T_1716,"When you look at the sky on a clear night, you can see dozens, perhaps even hundreds, of tiny points of light. Almost every one of these points of light is a star, a giant ball of glowing gas at a very, very high temperature. Stars differ in size, temperature, and age, but they all appear to be made up of the same elements and to behave according to the same principles. "
star constellations,T_1717,"People of many different cultures, including the Greeks, identified patterns of stars in the sky. We call these patterns constellations. Figure 1.1 shows one of the most easily recognized constellations. Why do the patterns in constellations and in groups or clusters of stars, called asterisms, stay the same night after night? Although the stars move across the sky, they stay in the same patterns. This is because the apparent nightly motion of the stars is actually caused by the rotation of Earth on its axis. The patterns also shift in the sky with the seasons as Earth revolves around the Sun. As a result, people in a particular location can see different constellations in the winter than in the summer. For example, in the Northern Hemisphere Orion is a prominent constellation in the winter sky, but not in the summer sky. This is the annual traverse of the constellations. "
star constellations,T_1718,"Although the stars in a constellation appear close together as we see them in our night sky, they are not at all close together out in space. In the constellation Orion, the stars visible to the naked eye are at distances ranging from just 26 light-years (which is relatively close to Earth) to several thousand light-years away. Click image to the left or use the URL below. URL: "
star constellations,T_1719,"There is no reason to think that the alignment of the stars has anything to do with events that happen on Earth. The constellations were defined by people who noticed that patterns could be made from stars, but the patterns do not reflect any characteristics of the stars themselves. When scientific tests are done to provide evidence in support of astrological ideas, the tests fail. When a scientific idea fails, it is abandoned or modified. Astrologers do not change or abandon their ideas. Click image to the left or use the URL below. URL: "
star power,T_1720,"The Sun is Earths major source of energy, yet the planet only receives a small portion of its energy. The Sun is just an ordinary star. Many stars produce much more energy than the Sun. The energy source for all stars is nuclear fusion. "
star power,T_1721,"Stars are made mostly of hydrogen and helium, which are packed so densely in a star that in the stars center the pressure is great enough to initiate nuclear fusion reactions. In a nuclear fusion reaction, the nuclei of two atoms combine to create a new atom. Most commonly, in the core of a star, two hydrogen atoms fuse to become a helium atom. Although nuclear fusion reactions require a lot of energy to get started, once they are going they produce enormous amounts of energy (Figure 1.1). In a star, the energy from fusion reactions in the core pushes outward to balance the inward pull of gravity. This energy moves outward through the layers of the star until it finally reaches the stars outer surface. The outer layer of the star glows brightly, sending the energy out into space as electromagnetic radiation, including visible light, heat, ultraviolet light, and radio waves (Figure 1.2). "
star power,T_1722,"In particle accelerators, subatomic particles are propelled until they have attained almost the same amount of energy as found in the core of a star (Figure 1.3). When these particles collide head-on, new particles are created. This process simulates the nuclear fusion that takes place in the cores of stars. The process also simulates the conditions A diagram of a star like the Sun. that allowed for the first helium atom to be produced from the collision of two hydrogen atoms in the first few minutes of the universe. The SLAC National Accelerator Lab in California can propel particles a straight 2 mi (3.2 km). The CERN Particle Accelerator presented in this video is the worlds largest and most powerful particle accelerator. The accelerator can boost subatomic particles to energy levels that simulate conditions in the stars and in the early history of the universe before stars formed. Click image to the left or use the URL below. URL: "
states of water,T_1723,"Water is simply two atoms of hydrogen and one atom of oxygen bonded together (Figure 1.1). The hydrogen ions are on one side of the oxygen ion, making water a polar molecule. This means that one side, the side with the hydrogen ions, has a slightly positive electrical charge. The other side, the side without the hydrogen ions, has a slightly negative charge. Despite its simplicity, water has remarkable properties. Water expands when it freezes, has high surface tension (because of the polar nature of the molecules, they tend to stick together), and others. Without water, life might not be able to exist on Earth and it certainly would not have the tremendous complexity and diversity that we see. "
states of water,T_1724,"Water is the only substance on Earth that is present in all three states of matter - as a solid, liquid or gas. (And Earth is the only planet where water is abundantly present in all three states.) Because of the ranges in temperature in specific locations around the planet, all three phases may be present in a single location or in a region. The three phases are solid (ice or snow), liquid (water), and gas (water vapor). See ice, water, and clouds (Figure 1.2). (a) Ice floating in the sea. Can you find all three phases of water in this image? (b) Liquid water. (c) Water vapor is invisible, but clouds that form when water vapor condenses are not. Click image to the left or use the URL below. URL: "
stratosphere,T_1729,"There is little mixing between the stratosphere, the layer above the troposphere, and the troposphere below it. The two layers are quite separate. Sometimes ash and gas from a large volcanic eruption may burst into the stratosphere. Once in the stratosphere, it remains suspended there for many years because there is so little mixing between the two layers. "
stratosphere,T_1730,"In the stratosphere, temperature increases with altitude. What is the heat source for the stratosphere? The direct heat source for the stratosphere is the Sun. The ozone layer in the stratosphere absorbs high energy ultraviolet radiation, which breaks the ozone molecule (3-oxygens) apart into an oxygen molecule (2-oxygens) and an oxygen atom (1- oxygen). In the mid-stratosphere there is less UV light and so the oxygen atom and molecule recombine to from ozone. The creation of the ozone molecule releases heat. Because warmer, less dense air sits over cooler, denser air, air in the stratosphere is stable. As a result, there is little mixing of air within the layer. There is also little interaction between the troposphere and stratosphere for this reason. "
stratosphere,T_1731,"The ozone layer is found within the stratosphere between 15 to 30 km (9 to 19 miles) altitude. The ozone layer has a low concentration of ozone; its just higher than the concentration elsewhere. The thickness of the ozone layer varies by the season and also by latitude. Ozone is created in the stratosphere by solar energy. Ultraviolet radiation splits an oxygen molecule into two oxygen atoms. One oxygen atom combines with another oxygen molecule to create an ozone molecule, O3 . The ozone is unstable and is later split into an oxygen molecule and an oxygen atom. This is a natural cycle that leaves some ozone in the stratosphere. The ozone layer is extremely important because ozone gas in the stratosphere absorbs most of the Suns harmful ultraviolet (UV) radiation. Because of this, the ozone layer protects life on Earth. High-energy UV light penetrates cells and damages DNA, leading to cell death (which we know as a bad sunburn). Organisms on Earth are not adapted to heavy UV exposure, which kills or damages them. Without the ozone layer to absorb UVC and UVB radiation, most complex life on Earth would not survive long. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
streams and rivers,T_1732,"Streams are bodies of water that have a current; they are in constant motion. Geologists recognize many categories of streams depending on their size, depth, speed, and location. Creeks, brooks, tributaries, bayous, and rivers are all streams. In streams, water always flows downhill, but the form that downhill movement takes varies with rock type, topography, and many other factors. Stream erosion and deposition are extremely important creators and destroyers of landforms. Rivers are the largest streams. People have used rivers since the beginning of civilization as a source of water, food, transportation, defense, power, recreation, and waste disposal. With its high mountains, valleys and Pacific coastline, the western United States exhibits nearly all of the features common to rivers and streams. The photos below are from the western states of Montana, California and Colorado. "
streams and rivers,T_1733,"A stream originates at its source. A source is likely to be in the high mountains where snows collect in winter and melt in summer, or a source might be a spring. A stream may have more than one source. Two streams come together at a confluence. The smaller of the two streams is a tributary of the larger stream (Figure 1.1). The confluence between the Yellowstone River and one of its tributaries, the Gar- diner River, in Montana. The point at which a stream comes into a large body of water, like an ocean or a lake, is called the mouth. Where the stream meets the ocean or lake is an estuary (Figure 1.2). The mouth of the Klamath River creates an estuary where it flows into the Pacific Ocean in California. The mix of fresh and salt water where a river runs into the ocean creates a diversity of environments where many different types of organisms create unique ecosystems. "
streams and rivers,T_1734,"As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. At a streams headwaters, often high in the mountains, gradients are steep (Figure 1.3). The stream moves fast and does lots of work eroding the stream bed. Headwaters of the Roaring Fork River in Colorado. As a stream moves into lower areas, the gradient is not as steep. Now the stream does more work eroding the edges of its banks. Many streams develop curves in their channels called meanders (Figure 1.4). As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain, which is a flat, level area surrounding the stream channel (Figure 1.5). Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do. The ultimate base level is sea level. "
streams and rivers,T_1735,"A divide is a topographically high area that separates a landscape into different water basins (Figure 1.6). Rain that falls on the north side of a ridge flows into the northern drainage basin and rain that falls on the south side flows into the southern drainage basin. On a much grander scale, entire continents have divides, known as continental divides. A green floodplain surrounds the Red Rock River as it flows through Montana. (a) The divides of North America. In the Rocky Mountains in Colorado, where does a raindrop falling on the western slope end up? How about on the eastern slope? (b) At Triple Divide Peak in Montana water may flow to the Pacific, the Atlantic, or Hudson Bay depending on where it falls. Can you locate where in the map of North America this peak sits? "
supervolcanoes,T_1736,"Supervolcano eruptions are extremely rare in Earths history. Its a good thing because they are unimaginably large. A supervolcano must erupt more than 1,000 cubic km (240 cubic miles) of material, compared with 1.2 km3 for Mount St. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in 1991. Not surprisingly, supervolcanoes are the most dangerous type of volcano. "
supervolcanoes,T_1737,"The exact cause of supervolcano eruptions is still debated. However, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion. This creates a huge hole or caldera into which the surface collapses (Figure 1.1). The caldera at Santorini in Greece is so large that it can only be seen by satellite. "
supervolcanoes,T_1738,"The largest supervolcano in North America is beneath Yellowstone National Park in Wyoming. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2.1 million, 1.3 million, and 640,000 years ago. Yellowstone has produced many smaller (but still enormous) eruptions more recently (Figure 1.2). Fortunately, current activity at Yellowstone is limited to the regions famous geysers. Click image to the left or use the URL below. URL:  The Yellowstone hotspot has produced enormous felsic eruptions. The Yellowstone caldera collapsed in the most recent super eruption. "
supervolcanoes,T_1739,"A supervolcano could change life on Earth as we know it. Ash could block sunlight so much that photosynthesis would be reduced and global temperatures would plummet. Volcanic eruptions could have contributed to some of the mass extinctions in our planets history. No one knows when the next super eruption will be. Interesting volcano videos are seen on National Geographic Videos, Environment Video, Natural Disasters, Earth- quakes: One interesting one is Mammoth Mountain, which explores Hot Creek and the volcanic area it is a part of in California. Click image to the left or use the URL below. URL: "
surface features of the sun,T_1740,"The Suns surface features are quite visible, but only with special equipment. For example, sunspots are only visible with special light-filtering lenses. "
surface features of the sun,T_1741,"The most noticeable surface features of the Sun are cooler, darker areas known as sunspots (Figure 1.1). Sunspots are located where loops of the Suns magnetic field break through the surface and disrupt the smooth transfer of heat from lower layers of the Sun, making them cooler, darker, and marked by intense magnetic activity. Sunspots usually occur in pairs. When a loop of the Suns magnetic field breaks through the surface, a sunspot is created where the loop comes out and where it goes back in again. Sunspots usually occur in 11-year cycles, increasing from a minimum number to a maximum number and then gradually decreasing to a minimum number again. "
surface features of the sun,T_1742,"There are other types of interruptions of the Suns magnetic energy. If a loop of the Suns magnetic field snaps and breaks, it creates solar flares, which are violent explosions that release huge amounts of energy (Figure 1.2). A strong solar flare can turn into a coronal mass ejection. A solar flare or coronal mass ejection releases streams of highly energetic particles that make up the solar wind. The solar wind can be dangerous to spacecraft and astronauts because it sends out large amounts of radiation that can harm the human body. Solar flares have knocked out entire power grids and disturbed radio, satellite, and cell phone communications. (a) Sunspots. (b) A close-up of a sunspot taken in ultraviolet light. "
surface features of the sun,T_1743,"Another highly visible feature on the Sun are solar prominences. If plasma flows along a loop of the Suns magnetic field from sunspot to sunspot, it forms a glowing arch that reaches thousands of kilometers into the Suns atmosphere. Prominences can last lengths of time ranging from a day to several months. Prominences are also visible during a total solar eclipse. Most of the imagery comes from SDOs AIA instrument; different colors represent different temperatures, a common technique for observing solar features. SDO sees the entire disk of the Sun in extremely high spatial and temporal resolution, allowing scientists to zoom in on notable events such as flares, waves, and sunspots. "
surface features of the sun,T_1744,"The video above was taken from the SDO, the most advanced spacecraft ever designed to study the Sun. During its five-year mission, SDO will examine the Suns magnetic field and also provide a better understanding of the role the Sun plays in Earths atmospheric chemistry and climate. Since just after its launch on February 11, 2010, SDO is providing images with clarity 10 times better than high-definition television and will return more comprehensive science data faster than any other solar-observing spacecraft. The Solar Dynamics Observatory is a NASA spacecraft launched in early 2010 is obtaining IMAX-like images of the Sun every second of the day, generating more data than any NASA mission in history. The data will allow researchers to learn about solar storms and other phenomena that can cause blackouts and harm astronauts. Click image to the left or use the URL below. URL: "
sustainable development,T_1750,"Can society change and get on a sustain- able path? A topic generating a great deal of discussion these days is sustainable development. The goals of sustainable development are to: help people out of poverty. protect the environment. use resources no faster than the rate at which they are regenerated. Science can be an important part of sustainable development. When scientists understand how Earths natural systems work, they can recognize how people are impacting them. Scientists can work to develop technologies that can be used to solve problems wisely. An example of a practice that can aid sustainable development is fish farming, as long as it is done in environmentally sound ways. Engineers can develop cleaner energy sources to reduce pollution and greenhouse gas emissions. Citizens can change their behavior to reduce the impact they have on the planet by demanding products that are produced sustainably. When forests are logged, new trees should be planted. Mining should be done so that the landscape is not destroyed. People can consume less and think more about the impacts of what they do consume. And what of the waste products of society? Will producing all that we need to keep the population growing result in a planet so polluted that the quality of life will be greatly diminished? Will warming temperatures cause problems for human populations? The only answer to all of these questions is, time will tell. Click image to the left or use the URL below. URL: "
testing hypotheses,T_1757,"How do you test a hypothesis? In this example, we will look into the scientific literature to find data in studies that were done using scientific method. To test Hypothesis 1 from the concept ""Development of Hypotheses,"" we need to see if the amount of CO2 gas released by volcanoes over the past several decades has increased. There are two ways volcanoes could account for the increase in CO2 : There has been an increase in volcanic eruptions in that time. The CO2 content of volcanic gases has increased over time globally. To test the first hypothesis, we look at the scientific literature. We see that the number of volcanic eruptions is about constant. We also learn from the scientific literature that volcanic gas compositions have not changed over time. Different types of volcanoes have different gas compositions, but overall the gases are the same. Another journal article states that major volcanic eruptions for the past 30 years have caused short-term cooling, not warming! Hypothesis 1 is wrong! Volcanic activity is not able to account for the rise in atmospheric CO2 . Remember that science is falsifiable. We can discard Hypothesis 1. "
testing hypotheses,T_1758,"Hypothesis 2 states that the increase in atmospheric CO2 is due to the increase in the amount of fossil fuels that are being burned. We look into the scientific literature and find this graph in the Figure 1.1. Global carbon dioxide emissions from fos- sil fuel consumption and cement produc- tion. The black line represents all emis- sion types combined, and colored lines show emissions from individual fossil fu- els. Fossil fuels have added an increasing amount of carbon dioxide to the atmosphere since the beginning of the Industrial Revolution in the mid 19th century. Hypothesis 2 is true! Click image to the left or use the URL below. URL: "
the hertzsprung russell diagra,T_1759,"The Hertzsprung-Russell diagram (often referred to as the H-R diagram) is a scatter graph that shows various classes of stars in the context of properties such as their luminosity, absolute magnitude, color, and effective temperature. Created around 1910 by Ejnar Hertzsprung and Henry Norris Russell, the diagram provided a great help in understanding stellar evolution. There are several forms of the Hertzsprung-Russell diagram, and the nomenclature is not very well defined. The original diagram displayed the spectral type of stars on the horizontal axis and the absolute magnitude on the vertical axis. The form below shows Kelvin temperature along the horizontal axis going from high temperature on the left to low temperature on the right and luminosity on the vertical axis. We can think of the luminosity as brightness in multiples of the Sun. A luminosity of 100 on the axis would mean 100 times as bright as the Sun. Most of the stars occupy a region in the diagram along a line called the Main Sequence. During that stage, stars are fusing hydrogen into helium in their cores. The position of the Sun in the main sequence is shown in the diagram. You should note that the axial scales for this diagram are not linear. The vertical scale is logarithmic, each line is 100 times greater than the previous line. On the horizontal axis, as we move to the right, the temperature reduces by between 1,000 and 10,000 degrees K between each line. If all other factors were the same, the highest temperature stars would also be the most luminous (the brightest). In the main sequence of stars, we see that as the temperature increases to the left, the luminosity also increases, demonstrating that the hottest stars in this grouping are also the brightest. There are stars, however, that are less bright than their temperature would predict. This group of stars is called white dwarfs. These stars are less bright than expected because of their very small size. These dwarf stars are only one one-thousandth the size of stars in the main sequence. There are also stars that are much brighter than their temperature would predict. This group of stars are called red giants. They are brighter than their temperature would predict because they are much larger than stars in the main sequence. These stars have expanded to several thousand times the size of stars in the main sequence. Stars that are reddish in color are cooler than other stars while stars that are bluish in color are hotter than other stars. A white dwarf is a stellar remnant that is very dense. A white dwarfs mass is comparable to the Sun and its volume is comparable to that of Earth. The very low brightness of a white dwarf comes from the emission of stored heat energy. White dwarfs are thought to be the final evolutionary state of any star whose mass is not great enough to become a neutron star. Approximately 97% of the stars in our galaxy will become neutron stars. After the hydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After blowing off its outer layers to form a planetary nebula, the core will be left behind to form the remnant white dwarf. White dwarfs are composed of carbon and oxygen. A white dwarf is very hot when it is formed, but since it has no source of energy (no further fusion reactions), it will gradually radiate away its energy and cool down. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant light, and it will become a cold black dwarf. A red giant star is a star with a mass like the Sun that is in the last phase of its life, when Hydrogen fusion reactions in the core decrease due to the lack of fuel. With the gravitational collapse of the core, the fusion reactions now occur in a shell surrounding the core. The outer layer of the star expands enormously up to 1000 times the size of the Sun. When the Sun becomes a red giant, its volume will include the orbit of Mercury and Venus and maybe even Earth. The increased size increases the luminosity even though the outer layer cools to only 3000 K or so. The cooler outer layer causes it to be a red star. After a few more million years, the star evolves into a white dwarf-planetary nebula system. "
the history of astronom,T_1761,"The Astronomy of the ancient Greeks was linked to mathematics, and Greek astronomers sought to create geomet- rical models that could imitate the appearance of celestial motions. This tradition originated around the 6th century BCE, with the followers of the mathematician Pythagoras (~580 - 500 BCE). Pythagoras believed that everything was related to mathematics and that through mathematics everything could be predicted and measured in rhythmic patterns or cycles. He placed astronomy as one of the four mathematical arts, the others being arithmetic, geometry and music. While best known for the Pythagorean Theorem, Pythagoras did have some input into astronomy. By the time of Pythagoras, the five planets visible to the naked eye - Mercury, Venus, Mars, Jupiter and Saturn - had long been identified. The names of these planets were initially derived from Greek mythology before being given the equivalent Roman mythological names, which are the ones we still use today. The word planet is a Greek term meaning wanderer, as these bodies move across the sky at different speeds from the stars, which appear fixed in the same positions relative to each other. For part of the year Venus appears in the eastern sky as an early morning object before disappearing and reappearing a few weeks later in the evening western sky. Early Greek astronomers thought this was two different bodies and assigned the names Phosphorus and Hesperus to the morning and evening apparitions respectively. Pythagoras is given credit for being the first to realize that these two bodies were in fact the same planet, a notion he arrived at through observation and geometrical calculations. Pythagoras was also one of the first to think that the Earth was round, a theory that was finally proved around 330 BCE by Aristotle. (Although, as you are probably aware, many people in 1642 CE still believed the earth to be flat.) Aristotle (384 BCE - 322 BCE) demonstrates in his writings that he knew we see the moon by the light of the sun, how the phases of the moon occur, and understood how eclipses work. He also knew that the earth was a sphere. Philosophically, he argued that each part of the earth is trying to be pulled to the center of the earth, and so the earth would naturally take on a spherical shape. He then pointed out observations that support the idea of a spherical earth. First, the shadow of the earth on the moon during a lunar eclipse is always circular. The only shape that always casts a circular shadow is a sphere. Second, as one traveles more north or south, the positions of the stars in the sky change. There are constellations visible in the north that one cannot see in the south and vice versa. He related this to the curvature of the earth. Aristotle talked about the work of earlier Greeks, who had developed an earth centered model of the planets. In these models, the center of the earth is the center of all the other motions. While it is not sure if the earlier Greeks actually thought the planets moved in circles, it is clear that Aristotle did. Aristotle rejected a moving earth for two reasons. Most importantly he didnt understand inertia. To Aristotle, the natural state for an object was to be at rest. He believed that it takes a force in order for an object to move. Using Aristotles ideas, if the earth were moving through space, if you tripped, you would not be in contact with the earth, and so would get left behind in space. Since this obviously does not happen, the earth must not move. This misunderstanding of inertia confused scientists until the time of Galileo. A second, but not as important, reason Aristotle rejected a moving earth is that he recognized that if the earth moved and rotated around the sun, there would be an observable parallax of the stars. One cannot see stellar parallax with the naked-eye, so Aristotle concluded that the earth must be at rest. (The stars are so far away, that one needs a good telescope to measure stellar parallax, which was first measured in 1838.) Aristotle believed that the objects in the heavens are perfect and unchanging. Since he believed that the only eternal motion is circular with a constant speed, the motions of the planets must be circular. This came to be called The Principal of Uniform Circular Motion. Aristotle and his ideas became very important because they became incorporated into the Catholic Churchs theology in the twelfth century by Thomas Aquinas. In the early 16th century, the Church banned new interpretations of scripture and this included a ban on ideas of a moving earth. Claudius Ptolemy (90 - 168 CE) was a citizen of Egypt which was under Roman rule during Ptolemys lifetime. During his lifetime he was a mathematician, astronomer, and geographer. His theories dominated the worlds understanding of astronomy for over a thousand years. While it is known that many astronomers published works during this time, only Ptolemys work The Almagest survived. In it, he outlined his geometrical reasoning for a geocentric view of the Universe. As outlined in the Almagest, the Universe according to Ptolemy was based on five main points: 1) the celestial realm is spherical, 2) the celestial realm moves in a circle, 3) the earth is a sphere, 4) the celestial realm orbit is a circle centered on the earth, and 5) earth does not move. Ptolemy also identified eight circular orbits surrounding earth where the other planets existed. In order, they were the moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the sphere of fixed stars. A serious problem with the earth-centered system was the fact that at certain times in their orbits, some of the planets appeared to move in the opposite direction of their normal movement. This reverse direction movement is referred to as retrograde motion. If the earth was to remain motionless at the center of the system, some very intricate designs were necessary to explain the movement of the retrograde planets. In the Ptolemaic system, each retrograde planet moved by two spheres. The Ptolemaic system had circles within circles that produced epicycles. In the sketch above on the left, the red ball moved clockwise in its little circle while the entire orbit also orbited clockwise around the big circle. This process produced a path like that shown in the sketch above on the right. As the red ball moved around its path, at some times it would be moving clockwise and then for a short period, it would move counterclockwise. This motion was able to explain the retrograde motion noted for some planets. "
the history of astronom,T_1762,"It was not until 1543, when Copernicus (1473 - 1543) introduced a sun-centered design (heliocentric), that Ptolemys astronomy was seriously questioned and eventually overthrown. Copernicus studied at the University of Bologna, where he lived in the same house as the principal astronomer there. Copernicus assisted the astronomer in some of his observations and in the production of the annual astrological forecasts for the city. It is at Bologna that he probably first encountered a translation of Ptolemys Almagest that would later make it possible for Copernicus to successfully refute the ancient astronomer. Later, at the University of Padua, Copernicus studied medicine, which was closely associated with astrology at that time due to the belief that the stars influenced the dispositions of the body. Returning to Poland, Copernicus secured a teaching post at Wroclaw, where he primarily worked as a medical doctor and manager of Church affairs. In his spare time, he studied the stars and the planets (decades before the telescope was invented), and applied his mathematical understanding to the mysteries of the night sky. In so doing, he developed his theory of a system in which the Earth, like all the planets, revolved around the sun, and which simply and elegantly explained the curious retrograde movements of the planets. Copernicus wrote his theory in De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Orbs). The book was completed in 1530 or so, but it wasnt published until the year he died, 1543. It has been suggested that Copernicus knew the publication would incur the wrath of the Catholic church and he didnt want to deal with problems so he didnt publish his theory until he was on his death bed. Legend has it that a copy of the printers proof was placed in his hands as he lay in a coma, and he woke long enough to recognize what he was holding before he died. Tycho Brahe (1546 - 1601) was born in a part of southern Sweden that was part of Denmark at the time. While attending the university to study law and philosophy, he became interested in astronomy and spent most evenings observing the stars. One of Tycho Brahes first contributions to astronomy was the detection and correction of several serious errors in the standard astronomical tables. Then, in 1572, he discovered a supernova located in the constellation of Cassiopeia. Tycho built his own instruments and made the most complete and accurate observations available without the use of a telescope. Eventually, his fame led to an offer from King Frederick II of Denmark & Norway to fund the construction of an astronomical observatory. The island of Van was chosen and in 1576, construction began. Tycho Brahe spent twenty years there, making observations on celestial bodies. During his life, Tycho Brahe did not accept Copernicus model of the universe. He attempted to combine it with the Ptolemaic model. As a theoretician, Tycho was a failure but his observations and the data he collected was far superior to any others made prior to the invention of the telescope. After Tycho Brahes death, his assistant, Johannes Kepler used Tycho Brahes observations to calculate his own three laws of planetary motion. In 1600, Johannes Kepler (1571 - 1630) began working as Tychos assistant. They recognized that neither the Ptolemaic (geocentric) or Copernican (heliocentric) models could predict positions of Mars as accurately as they could measure them. Tycho died in 1601 and after that Kepler had full access to Tychos data. He analyzed the data for 8 years and tried to calculate an orbit that would fit the data, but was unable to do so. Kepler later determined that the orbits were not circular but elliptical. "
the history of astronom,T_1763,"1. The orbits of the planets are elliptical. 2. An imaginary line connecting a planet and the sun sweeps out equal areas during equal time intervals. (Therefore, the earths orbital speed varies at different times of the year. The earth moves fastest in its orbit when closest to the sun and slowest when farthest away.) Keplers Second Law of Planetary Motion was calculated for Earth, then the hypothesis was tested using data for Mars, and it worked! 3. Keplers Third Law of Planetary Motion showed the relationship between the size of a planets orbit radius, R ( 12 the major axis), and its orbital period, T . R2 = T 3 This law is true for all planets if you use astronomical units (that is, distance in multiples of earths orbital radium and time in multiples of earth years). Keplers three laws replaced the cumbersome epicycles to explain planetary motion with three mathematical laws that allowed the positions of the planets to be predicted with accuracies ten times better than Ptolemaic or Copernican models. "
the history of astronom,T_1764,"Galileo Galilei (1564-1642) was a very important person in the development of modern astronomy, both because of his contributions directly to astronomy, and because of his work in physics. He provided the crucial observations that proved the Copernican hypothesis, and also laid the foundations for a correct understanding of how objects moved on the surface of the earth and of gravity. One could, with considerable justification, view Galileo as the father both of modern astronomy and of modern physics. Galileo did not invent the telescope, but he was the first to turn his telescope toward the sky to study the heavens systematically. His telescope was poorer than even a cheap modern amateur telescope, but what he observed in the heavens showed errors in Aristotles opinion of the universe and the worldview that it supported. Observations through Galileos telescope made it clear that the earth-centered and earth doesnt move solar system of Aristotle was incorrect. Since church officials had made some of Aristotles opinions a part of the religious views of the church, proving Aristotles views to be incorrect also pointed out flaws in the church. Galileo observed four points of light that changed their positions around the planet Jupiter and he concluded that these were moons in orbit around Jupiter. These observations showed that there were new things in the heavens that Aristotle and Ptolemy had known nothing about. Furthermore, they demonstrated that a planet could have moons circling it that would not be left behind as the planet moved around its orbit. One of the arguments against the Copernican system had been that if the moon were in orbit around the Earth and the Earth in orbit around the Sun, the Earth would leave the Moon behind as it moved around its orbit. Galileo used his telescope to show that Venus, like the moon, went through a complete set of phases. This observation was extremely important because it was the first observation that was consistent with the Copernican system but not the Ptolemaic system. In the Ptolemaic system, Venus should always be in crescent phase as viewed from the Earth because the sun is beyond Venus, but in the Copernican system Venus should exhibit a complete set of phases over time as viewed from the Earth because it is illuminated from the center of its orbit. It is important to note that this was the first empirical evidence (coming almost a century after Copernicus) that allowed a definitive test of the two models. Until that point, both the Ptolemaic and Copernican models described the available data. The primary attraction of the Copernican system was that it described the data in a simpler fashion, but here finally was conclusive evidence that not only was the Ptolemaic universe more complicated, it also was incorrect. As each new observation was brought to light, increasing doubt was cast on the old views of the heavens. It also raised the credibility issue: could the authority of Aristotle and Ptolemy be trusted concerning the nature of the Universe if there were so many things in the Universe about which they had been unaware and/or incorrect? Galileos challenge of the Churchs authority through his refutation of the Aristotelian concept of the Universe eventually got him into deep trouble. Late in his life he was forced, under threat of torture, to publicly recant his Copernican views and spent his last years under house arrest. Galileos life is a sad example of the conflict between the scientific method and unquestioned authority. Sir Isaac Newton (1642-1727), who was born the same year that Galileo died, would build on Galileos ideas to demonstrate that the laws of motion in the heavens and the laws of motion on the earth were the same. Thus Galileo began, and Newton completed, a synthesis of astronomy and physics in which astronomy was recognized as but a part of physics, and that the opinions of Aristotle were almost completely eliminated from both. Many scientists consider Newton to be a peer of Einstein in scientific thinking. Newtons accomplishments had even greater scope than those of Einstein. The poet Alexander Pope wrote of Newton: Nature and Natures laws lay hid in night; God said, Let Newton be! and all was light. In terms of astronomy, Newton gave reasons for and corrections to Keplers Laws. Kepler had proposed three Laws of Planetary motion based on Tycho Brahes data. These Laws were supposed to apply only to the motions of the planets. Further, they were purely empirical, that is, they worked, but no one knew why they worked. Newton changed all of that. First, he demonstrated that the motion of objects on the Earth could be described by three new Laws of motion, and then he went on to show that Keplers three Laws of Planetary Motion were but special cases of Newtons three Laws when his gravitational force was postulated to exist between all masses in the Universe. In fact, Newton showed that Keplers Laws of planetary motion were only approximately correct, and supplied the quantitative corrections that with careful observations proved to be valid. "
the history of astronom,T_1765,"The Big Bang Theory is the dominant and highly supported theory of the origin of the universe. It states that the universe began from an initial point which has expanded over billions of years to form the universe as we now know it. In 1922, Alexander Friedman found that the solutions to Einsteins general relativity equations resulted in an expanding universe. Einstein, at that time, believed in a static, eternal universe so he added a constant to his equations to eliminate the expansion. Einstein would later call this the biggest blunder of his life. In 1924, Edwin Hubble was able to measure the distance to observed celestial objects that were thought to be nebula and discovered that they were so far away they were not actually part of the Milky Way (the galaxy containing our sun). He discovered that the Milky Way was only one of many galaxies. In 1927, Georges Lemaitre, a physicist, suggested that the universe must be expanding. Lemaitres theory was supported by Hubble in 1929 when he found that the galaxies most distant from us also had the greatest red shift (were moving away from us with the greatest speed). The idea that the most distance galaxies were moving away from us at the greatest speed was exactly what was predicted by Lemaitre. In 1931, Lemaitre went further with his predictions and by extrapolating backwards, found that the matter of the universe would reach an infinite density and temperature at a finite time in the past (around 15 billion years). This meant that the universe must have begun as a small, extremely dense point of matter. At the time, the only other theory that competed with Lemaitres theory was the Steady State Theory of Fred Hoyle. The steady state theory predicted that new matter was created which made it appear that the universe was expanding but that the universe was constant. It was Hoyle who coined the term Big Bang Theory which he used as a derisive name for Lemaitres theory. George Gamow (1904 - 1968) was the major advocate of the Big Bang theory. He predicted that cosmic microwave background radiation should exist throughout the universe as a remnant of the Big Bang. As atoms formed from sub-atomic particles shortly after the Big Bang, electromagnetic radiation would be emitted and this radiation would still be observable today. Gamow predicted that the expansion of the universe would cool the original radiation so that now the radiation would be in the microwave range. The debate continued until 1965 when two Bell Telephone scientists stumbled upon the microwave radiation with their radio telescope. "
thermosphere and beyond,T_1766,"The density of molecules is so low in the thermosphere that one gas molecule can go about 1 km before it collides with another molecule. Since so little energy is transferred, the air feels very cold (See opening image). "
thermosphere and beyond,T_1767,"Within the thermosphere is the ionosphere. The ionosphere gets its name from the solar radiation that ionizes gas molecules to create a positively charged ion and one or more negatively charged electrons. The freed electrons travel within the ionosphere as electric currents. Because of the free ions, the ionosphere has many interesting characteristics. At night, radio waves bounce off the ionosphere and back to Earth. This is why you can often pick up an AM radio station far from its source at night. "
thermosphere and beyond,T_1768,"The Van Allen radiation belts are two doughnut-shaped zones of highly charged particles that are located very high the atmosphere in the magnetosphere. The particles originate in solar flares and fly to Earth on the solar wind. Once trapped by Earths magnetic field, they follow along the fields magnetic lines of force. These lines extend from above the Equator to the North Pole and also to the South Pole, then return to the Equator. "
thermosphere and beyond,T_1769,"When massive solar storms cause the Van Allen belts to become overloaded with particles, the result is the most spectacular feature of the ionosphere  the nighttime aurora (Figure 1.1). The particles spiral along magnetic field lines toward the poles. The charged particles energize oxygen and nitrogen gas molecules, causing them to light up. Each gas emits a particular color of light. (a) Spectacular light displays are visible as the aurora borealis or northern lights in the Northern Hemisphere. (b) The aurora australis or southern lights encircles Antarctica. What would Earths magnetic field look like if it were painted in colors? It would look like the aurora! This QUEST video looks at the aurora, which provides clues about the solar wind, Earths magnetic field and Earths atmosphere. Click image to the left or use the URL below. URL: "
thermosphere and beyond,T_1770,"There is no real outer limit to the exosphere, the outermost layer of the atmosphere; the gas molecules finally become so scarce that at some point there are no more. Beyond the atmosphere is the solar wind. The solar wind is made of high-speed particles, mostly protons and electrons, traveling rapidly outward from the Sun. "
tides,T_1779,"Tides are the daily rise and fall of sea level at any given place. The pull of the Moons gravity on Earth is the primary cause of tides and the pull of the Suns gravity on Earth is the secondary cause (Figure 1.1). The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer. The Moons pull is about twice that of the Suns. To understand the tides it is easiest to start with the effect of the Moon on Earth. As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much, but the water is pulled by the gravity and a bulge is created. This bulge is the high tide beneath the Moon. On the other side of the Earth, a high tide is produced where the Moons pull is weakest. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides. The places directly in between the high tides are low tides. As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides approximately every day. High tides occur about every 12 hours and 25 minutes. The reason is that the Moon takes 24 hours and 50 minutes to rotate once around the Earth, so the Moon is over the same location every 24 hours and 50 minutes. Since high tides occur twice a day, one arrives each 12 hours and 25 minutes. What is the time between a high tide and the next low tide? The gravity of the Sun also pulls Earths water towards it and causes its own tides. Because the Sun is so far away, its pull is smaller than the Moons. Some coastal areas do not follow this pattern at all. These coastal areas may have one high and one low tide per day or a different amount of time between two high tides. These differences are often because of local conditions, such as the shape of the coastline that the tide is entering. The gravitational attraction of the Moon to ocean water creates the high and low tides. "
tides,T_1780,"The tidal range is the difference between the ocean level at high tide and the ocean level at low tide (Figure 1.2). The tidal range in a location depends on a number of factors, including the slope of the seafloor. Water appears to move a greater distance on a gentle slope than on a steep slope. "
tides,T_1781,"If you look at the diagram of high and low tides on a circular Earth above, youll see that tides are waves. So when the Sun and Moon are aligned, what do you expect the tides to look like? Waves are additive, so when the gravitational pull of both bodies is in the same direction, the high tides are higher and the low tides lower than at other times through the month (Figure 1.3). These more extreme tides, with a greater tidal range, are called spring tides. Spring tides dont just occur in the spring; they occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days. Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90o angle (Figure 1.4). They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. How do the tides add up to create neap tides? The Moons high tide occurs in the same place as the Suns low tide and the Moons low tide in the same place as the Suns high tide. At neap tides, the tidal range is relatively small. The tidal range is the difference between the ocean level at high tide and low tide. Studying ocean tides rhythmic movements helps scientists understand the ocean and the Sun/Moon/Earth system. This QUEST video explains how tides work, and visits the oldest continually operating tidal gauge in the Western Hemisphere. Click image to the left or use the URL below. URL: "
tree rings ice cores and varves,T_1789,"In locations where summers are warm and winters are cool, trees have a distinctive growth pattern. Tree trunks display alternating bands of light-colored, low density summer growth and dark, high density winter growth. Each light-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived (Figure 1.1). The width of these growth rings varies with the conditions present that year. A summer drought may make the tree grow more slowly than normal and so its light band will be relatively small. These tree-ring variations appear in all trees in a region. The same distinctive pattern can be found in all the trees in an area for the same time period. Scientists have created continuous records of tree rings going back over the past 2,000 years. Wood fragments from old buildings and ancient ruins can be age dated by matching up the pattern of tree rings in the wood fragment in Cross-section showing growth rings. question and the scale created by scientists. The outermost ring indicates when the tree stopped growing; that is, when it died. The tree-ring record is extremely useful for finding the age of ancient structures. "
tree rings ice cores and varves,T_1790,"Besides tree rings, other processes create distinct yearly layers that can be used for dating. On a glacier, snow falls in winter but in summer dust accumulates. This leads to a snow-dust annual pattern that goes down into the ice (Figure gather allows them to determine how the environment has changed as the glacier has stayed in its position. Analyses of the ice tell how concentrations of atmospheric gases changed, which can yield clues about climate. The longest cores allow scientists to create a record of polar climate stretching back hundreds of thousands of years. "
tree rings ice cores and varves,T_1791,"Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer, the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers deposited in the winter. The resulting layers, called varves, give scientists clues about past climate conditions (Figure 1.3). A warm summer might result in a very thick sediment layer while a cooler summer might yield a thinner layer. "
troposphere,T_1792,"The temperature of the troposphere is highest near the surface of the Earth and decreases with altitude. On average, the temperature gradient of the troposphere is 6.5o C per 1,000 m (3.6o F per 1,000 ft) of altitude. Earths surface is the source of heat for the troposphere. Rock, soil, and water on Earth absorb the Suns light and radiate it back into the atmosphere as heat, so there is more heat near the surface. The temperature is also higher near the surface because gravity pulls in more gases. The greater density of gases causes the temperature to rise. Notice that in the troposphere warmer air is beneath cooler air. This condition is unstable since warm air is less dense than cool air. The warm air near the surface rises and cool air higher in the troposphere sinks, so air in the troposphere does a lot of mixing. This mixing causes the temperature gradient to vary with time and place. The rising and sinking of air in the troposphere means that all of the planets weather takes place in the troposphere. "
troposphere,T_1793,"Sometimes there is a temperature inversion, in which air temperature in the troposphere increases with altitude and warm air sits over cold air. Inversions are very stable and may last for several days or even weeks. Inversions form: Over land at night or in winter when the ground is cold. The cold ground cools the air that sits above it, making this low layer of air denser than the air above it. Near the coast, where cold seawater cools the air above it. When that denser air moves inland, it slides beneath the warmer air over the land. Since temperature inversions are stable, they often trap pollutants and produce unhealthy air conditions in cities (Figure 1.1). Smoke makes a temperature inversion visible. The smoke is trapped in cold dense air that lies beneath a cap of warmer air. At the top of the troposphere is a thin layer in which the temperature does not change with height. This means that the cooler, denser air of the troposphere is trapped beneath the warmer, less dense air of the stratosphere. Air from the troposphere and stratosphere rarely mix. Click image to the left or use the URL below. URL: "
tsunami,T_1794,"Tsunami are deadly ocean waves from the sharp jolt of an undersea earthquake. Less frequently, these waves can be generated by other shocks to the sea, like a meteorite impact. Fortunately, few undersea earthquakes, and even fewer meteorite impacts, generate tsunami. "
tsunami,T_1795,"Tsunami waves have small wave heights relative to their long wavelengths, so they are usually unnoticed at sea. When traveling up a slope onto a shoreline, the wave is pushed upward. As with wind waves, the speed of the bottom of the wave is slowed by friction. This causes the wavelength to decrease and the wave to become unstable. These factors can create an enormous and deadly wave. Landslides, meteorite impacts, or any other jolt to ocean water may form a tsunami. Tsunami can travel at speeds of 800 kilometers per hour (500 miles per hour). "
tsunami,T_1796,"Since tsunami are long-wavelength waves, a long time can pass between crests or troughs. Any part of the wave can make landfall first. In 1755 in Lisbon, Portugal, a tsunami trough hit land first. A large offshore earthquake did a great deal of damage on land. People rushed out to the open space of the shore. Once there, they discovered that the water was flowing seaward fast and some of them went out to observe. What do you think happened next? The people on the open beach drowned when the crest of the wave came up the beach. Large tsunami in the Indian Ocean and more recently Japan have killed hundreds of thousands of people in recent years. The west coast is vulnerable to tsunami since it sits on the Pacific Ring of Fire. Scientists are trying to learn everything they can about predicting tsunamis before a massive one strikes a little closer to home. Although most places around the Indian Ocean did not have warning systems in 2005, there is a tsunami warning system in that region now. Tsunami warning systems have been placed in most locations where tsunami are possible. Click image to the left or use the URL below. URL: "
types of air pollution,T_1797,"The two types of air pollutants are primary pollutants, which enter the atmosphere directly, and secondary pollutants, which form from a chemical reaction. "
types of air pollution,T_1798,"Some primary pollutants are natural, such as volcanic ash. Dust is natural but exacerbated by human activities; for example, when the ground is torn up for agriculture or development. Most primary pollutants are the result of human activities, the direct emissions from vehicles and smokestacks. Primary pollutants include: Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO2 ) (Figure 1.1). Both are colorless, odorless gases. CO is toxic to both plants and animals. CO and CO2 are both greenhouse gases. Nitrogen oxides are produced when nitrogen and oxygen from the atmosphere come together at high temper- atures. This occurs in hot exhaust gas from vehicles, power plants, or factories. Nitrogen oxide (NO) and nitrogen dioxide (NO2 ) are greenhouse gases. Nitrogen oxides contribute to acid rain. Sulfur oxides include sulfur dioxide (SO2 ) and sulfur trioxide (SO3 ). These form when sulfur from burning coal reaches the air. Sulfur oxides are components of acid rain. Particulates are solid particles, such as ash, dust, and fecal matter (Figure 1.2). They are commonly formed from combustion of fossil fuels, and can produce smog. Particulates can contribute to asthma, heart disease, and some types of cancers. Lead was once widely used in automobile fuels, paint, and pipes. This heavy metal can cause brain damage or blood poisoning. High CO2 levels are found in major metropolitan areas and along the major interstate highways. Particulates from a brush fire give the sky a strange glow in Arizona. "
types of air pollution,T_1799,"Any city can have photochemical smog, but it is most common in sunny, dry locations. A rise in the number of vehicles in cities worldwide has increased photochemical smog. Nitrogen oxides, ozone, and several other compounds are some of the components of this type of air pollution. Photochemical smog forms when car exhaust is exposed to sunlight. Nitrogen oxide is created by gas combustion in cars and then into the air (Figure 1.3). In the presence of sunshine, the NO2 splits and releases an oxygen ion (O). The O then combines with an oxygen molecule (O2 ) to form ozone (O3 ). This reaction can also go in reverse: Nitric oxide (NO) removes an oxygen atom from ozone to make it O2 . The direction the reaction goes depends on how much NO2 and NO there is. If NO2 is three times more abundant than NO, ozone will be produced. If nitric oxide levels are high, ozone will not be created. The brown color of the air behind the Golden Gate Bridge is typical of California cities, because of nitrogen oxides. Ozone is one of the major secondary pollutants. It is created by a chemical reaction that takes place in exhaust and in the presence of sunlight. The gas is acrid-smelling and whitish. Warm, dry cities surrounded by mountains, such as Los Angeles, Phoenix, and Denver, are especially prone to photochemical smog. Photochemical smog peaks at midday on the hottest days of summer. Ozone is also a greenhouse gas. "
types of fossilization,T_1800,Most fossils are preserved by one of five processes outlined below (Figure 1.1): 
types of fossilization,T_1801,"Most uncommon is the preservation of soft-tissue original material. Insects have been preserved perfectly in amber, which is ancient tree sap. Mammoths and a Neanderthal hunter were frozen in glaciers, allowing scientists the rare opportunity to examine their skin, hair, and organs. Scientists collect DNA from these remains and compare the DNA sequences to those of modern counterparts. "
types of fossilization,T_1802,"The most common method of fossilization is permineralization. After a bone, wood fragment, or shell is buried in sediment, mineral-rich water moves through the sediment. This water deposits minerals into empty spaces and Five types of fossils: (a) insect preserved in amber, (b) petrified wood (permineralization), (c) cast and mold of a clam shell, (d) pyritized ammonite, and (e) compression fossil of a fern. produces a fossil. Fossil dinosaur bones, petrified wood, and many marine fossils were formed by permineralization. "
types of fossilization,T_1803,"When the original bone or shell dissolves and leaves behind an empty space in the shape of the material, the depression is called a mold. The space is later filled with other sediments to form a matching cast within the mold that is the shape of the original organism or part. Many mollusks (clams, snails, octopi, and squid) are found as molds and casts because their shells dissolve easily. "
types of fossilization,T_1804,"The original shell or bone dissolves and is replaced by a different mineral. For example, calcite shells may be replaced by dolomite, quartz, or pyrite. If a fossil that has been replace by quartz is surrounded by a calcite matrix, mildly acidic water may dissolve the calcite and leave behind an exquisitely preserved quartz fossil. "
types of fossilization,T_1805,"Some fossils form when their remains are compressed by high pressure, leaving behind a dark imprint. Compression is most common for fossils of leaves and ferns, but can occur with other organisms. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
universe,T_1825,"The study of the universe is called cosmology. Cosmologists study the structure and changes in the present universe. The universe contains all of the star systems, galaxies, gas, and dust, plus all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe includes all of space and time. "
universe,T_1826,"What did the ancient Greeks recognize as the universe? In their model, the universe contained Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. This idea held for many centuries until Galileos telescope helped people recognize that Earth is not the center of the universe. They also found out that there are many more stars than were visible to the naked eye. All of those stars were in the Milky Way Galaxy. In the early 20th century, an astronomer named Edwin Hubble (Figure 1.1) discovered that what scientists called the Andromeda Nebula was actually over 2 million light years away  many times farther than the farthest distances that had ever been measured. Hubble realized that many of the objects that astronomers called nebulas were not actually clouds of gas, but were collections of millions or billions of stars  what we now call galaxies. Hubble showed that the universe was much larger than our own galaxy. Today, we know that the universe contains about a hundred billion galaxies  about the same number of galaxies as there are stars in the Milky Way Galaxy. (a) Edwin Hubble used the 100-inch reflecting telescope at the Mount Wilson Observatory in California to show that some distant specks of light were galaxies. (b) Hubbles namesake space telescope spotted this six galaxy group. Edwin Hubble demonstrated the existence of galaxies. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
uranus,T_1827,"Uranus (YOOR-uh-nuhs) is named for the Greek god of the sky. From Earth, Uranus is so faint that it was unnoticed by ancient observers. William Herschel first discovered the planet in 1781. Although Uranus is very large, it is extremely far away, about 2.8 billion km (1.8 billion mi) from the Sun. Light from the Sun takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the Sun once about every 84 Earth years. Uranus has a mass about 14 times the mass of Earth, but it is much less dense than Earth. Gravity at the surface of Uranus is weaker than on Earths surface, so if you were at the top of the clouds on Uranus, you would weigh about 10% less than what you weigh on Earth. "
uranus,T_1828,"Like Jupiter and Saturn, Uranus is composed mainly of hydrogen and helium, with an outer gas layer that gives way to liquid on the inside. Uranus has a higher percentage of icy materials, such as water, ammonia (NH3 ), and methane (CH4 ), than Jupiter and Saturn. When sunlight reflects off Uranus, clouds of methane filter out red light, giving the planet a blue-green color. There are bands of clouds in the atmosphere of Uranus, but they are hard to see in normal light, so the planet looks like a plain blue ball. "
uranus,T_1829,"Most of the planets in the solar system rotate on their axes in the same direction that they move around the Sun. Uranus, though, is tilted on its side, so its axis is almost parallel to its orbit. In other words, it rotates like a top that was turned so that it was spinning parallel to the floor. Scientists think that Uranus was probably knocked over by a collision with another planet-sized object billions of years ago. "
uranus,T_1830,"Uranus has a faint system of rings (Figure 1.1). The rings circle the planets equator, but because Uranus is tilted on its side, the rings are almost perpendicular to the planets orbit. This image from the Hubble Space Tele- scope shows the faint rings of Uranus. The planet is tilted on its side, so the rings are nearly vertical. Uranus has 27 known moons and all but a few of them are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus  Miranda, Ariel, Umbriel, Titania, and Oberon  are shown in Figure 1.2. These Voyager 2 photos have been resized to show the relative sizes of the five main moons of Uranus. Click image to the left or use the URL below. URL: "
uses of water,T_1831,Humans use six times as much water today as they did 100 years ago. People living in developed countries use a far greater proportion of the worlds water than people in less developed countries. What do people use all of that water for? 
uses of water,T_1832,"Besides drinking and washing, people need water for agriculture, industry, household uses, and recreation (Figure Water use can be consumptive or non-consumptive, depending on whether the water is lost to the ecosystem. Non-consumptive water use includes water that can be recycled and reused. For example, the water that goes down the drain and enters the sewer system is purified and then redistributed for reuse. By recycling water, the overall water consumption is reduced. Consumptive water use takes the water out of the ecosystem. Can you name some examples of consumptive water use? "
uses of water,T_1833,"Some of the worlds farmers still farm without irrigation by choosing crops that match the amount of rain that falls in their area. But some years are wet and others are dry. For farmers to avoid years in which they produce little or no food, many of the worlds crops are produced using irrigation. Water used for home, industrial, and agricultural purposes in different regions. Globally more than two-thirds of water is for agriculture. "
uses of water,T_1834,Three popular irrigation methods are: Overhead sprinklers. Trench irrigation: canals carry water from a water source to the fields. Flood irrigation: fields are flooded with water. All of these methods waste water. Between 15% and 36% percent of the water never reaches the crops because it evaporates or leaves the fields as runoff. Water that runs off a field often takes valuable soil with it. 
uses of water,T_1835,"A much more efficient way to water crops is drip irrigation (Figure 1.2). With drip irrigation, pipes and tubes deliver small amounts of water directly to the soil at the roots of each plant or tree. The water is not sprayed into the air or over the ground, so nearly all of it goes directly into the soil and plant roots. "
uses of water,T_1836,"Why do farmers use wasteful irrigation methods when water-efficient methods are available? Many farmers and farming corporations have not switched to more efficient irrigation methods for two reasons: 1. Drip irrigation and other more efficient irrigation methods are more expensive than sprinklers, trenches, and flooding. 2. In the United States and some other countries, the government pays for much of the cost of the water that is used for agriculture. Because farmers do not pay the full cost of their water use, they do not have any financial incentive to use less water. What ideas can you come up with to encourage farmers to use more efficient irrigation systems? "
uses of water,T_1837,"Aquaculture is a different type of agriculture. Aquaculture is farming to raise fish, shellfish, algae, or aquatic plants (Figure 1.3). As the supplies of fish from lakes, rivers, and the oceans dwindle, people are getting more fish from aquaculture. Raising fish increases our food resources and is especially valuable where protein sources are limited. Farmed fish are becoming increasingly common in grocery stores all over the world. Workers at a fish farm harvest fish they will sell to stores. Growing fish in a large scale requires that the fish stocks are healthy and protected from predators. The species raised must be hearty, inexpensive to feed, and able to reproduce in captivity. Wastes must be flushed out to keep animals healthy. Raising shellfish at farms can also be successful. "
uses of water,T_1838,"For some species, aquaculture is very successful and environmental harm is minimal. But for other species, aqua- culture can cause problems. Natural landscapes, such as mangroves, which are rich ecosystems and also protect coastlines from storm damage, may be lost to fish farms (Figure 1.4). For fish farmers, keeping costs down may be a problem since coastal land may be expensive and labor costs may be high. Large predatory fish at the 4th or 5th trophic level must eat a lot, so feeding large numbers of these fish is expensive and environmentally costly. Farmed fish are genetically different from wild stocks, and if they escape into the wild they may cause problems for native fish. Because the organisms live so close together, parasites are common and may also escape into the wild. Shrimp farms on the coast of Ecuador are shown as blue rectangles. Mangrove forests, salt flats, and salt marshes have been converted to shrimp farms. "
uses of water,T_1839,"Industrial water use accounts for an estimated 15% of worldwide water use, with a much greater percentage in developed nations. Industrial uses of water include power plants that use water to cool their equipment and oil refineries that use water for chemical processes. Manufacturing is also water intensive. "
uses of water,T_1840,"Think about all the ways you use water in a day. You need to count the water you drink, cook with, bathe in, garden with, let run down the drain, or flush down the toilet. In developed countries, people use a lot of water, while in less developed countries people use much less. Globally, household or personal water use is estimated to account for 15% of world-wide water use. Some household water uses are non-consumptive, because water is recaptured in sewer systems, treated, and returned to surface water supplies for reuse. Many things can be done to lower water consumption at home. Convert lawns and gardens to drip-irrigation systems. Install low-flow shower heads and low-flow toilets. In what other ways can you use less water at home? "
uses of water,T_1841,"People love water for swimming, fishing, boating, river rafting, and other activates. Even activities such as golf, where there may not be any standing water, require plenty of water to make the grass on the course green. Despite its value, the amount of water that most recreational activities use is low: less than 1% of all the water we use. Many recreational water uses are non-consumptive including swimming, fishing, and boating. Golf courses are the biggest recreational water consumer since they require large amounts for irrigation, especially because many courses are located in warm, sunny, desert regions where water is scarce and evaporation is high. "
uses of water,T_1842,Environmental use of water includes creating wildlife habitat. Lakes are built to create places for fish and water birds (Figure 1.5). Most environmental uses are non-consumptive and account for an even smaller percentage of water use than recreational uses. A shortage of this water is a leading cause of global biodiversity loss. Click image to the left or use the URL below. URL: 
venus,T_1843,"Venus thick clouds reflect sunlight well, so Venus is very bright. When it is visible, Venus is the brightest object in the sky besides the Sun and the Moon. Because the orbit of Venus is inside Earths orbit, Venus always appears close to the Sun. When Venus rises just before the Sun rises, the bright object is called the morning star. When it sets just after the Sun sets, it is the evening star. Of the planets, Venus is most similar to Earth in size and density. Venus is also our nearest neighbor. The planets interior structure is similar to Earths, with a large iron core and a silicate mantle (Figure 1.1). But the resemblance between the two inner planets ends there. "
venus,T_1844,"Venus rotates in a direction opposite the other planets and opposite to the direction it orbits the Sun. This rotation is extremely slow, only one turn every 243 Earth days. This is longer than a year on Venus  it takes Venus only 224 days to orbit the Sun. Diagram of Venuss interior, which is simi- lar to Earths. "
venus,T_1845,"Venus is covered by a thick layer of clouds, as shown in pictures of Venus taken at ultraviolet wavelengths (Figure This ultraviolet image from the Pioneer Venus Orbiter shows thick layers of clouds in the atmosphere of Venus. Venus clouds are not made of water vapor like Earths clouds. Clouds on Venus are made mostly of carbon dioxide Click image to the left or use the URL below. URL:  The atmosphere of Venus is so thick that the atmospheric pressure on the planets surface is 90 times greater than the atmospheric pressure on Earths surface. The dense atmosphere totally obscures the surface of Venus, even from spacecraft orbiting the planet. "
venus,T_1846,"Since spacecraft cannot see through the thick atmosphere, radar is used to map Venus surface. Many features found on the surface are similar to Earth and yet are very different. Figure 1.3 shows a topographical map of Venus produced by the Magellan probe using radar. This false color image of Venus was made from radar data collected by the Magellan probe between 1990 and 1994. What features can you identify? Most of the volcanoes are no longer active, but scientists have found evidence that there is some active volcanism (Figure 1.4). Think about what you know about the geology of Earth and what produces volcanoes. What does the presence of volcanoes suggest about the geology of Venus? What evidence would you look for to find the causes of volcanism on Venus? This image of the Maat Mons volcano with lava beds in the foreground was gen- erated by a computer from radar data. The reddish-orange color is close to what scientists think the color of sunlight would look like on the surface of Venus. Venus also has very few impact craters compared with Mercury and the Moon. What is the significance of this? Earth has fewer impact craters than Mercury and the Moon, too. Is this for the same reason that Venus has fewer impact craters? Its difficult for scientists to figure out the geological history of Venus. The environment is too harsh for a rover to go there. It is even more difficult for students to figure out the geological history of a distant planet based on the information given here. Still, we can piece together a few things. On Earth, volcanism is generated because the planets interior is hot. Much of the volcanic activity is caused by plate tectonic activity. But on Venus, there is no evidence of plate boundaries and volcanic features do not line up the way they do at plate boundaries. Because the density of impact craters can be used to determine how old a planets surface is, the small number of impact craters means that Venus surface is young. Scientists think that there is frequent, planet-wide resurfacing of Venus with volcanism taking place in many locations. The cause is heat that builds up below the surface, which has no escape until finally it destroys the crust and results in volcanoes. Click image to the left or use the URL below. URL: "
water distribution,T_1867,"Water is unevenly distributed around the world. Large portions of the world, such as much of northern Africa, receive very little water relative to their population (Figure 1.1). The map shows the number of months in which there is little rainfall in each region. In developed nations, water is stored, but in underdeveloped nations, water storage may be minimal. Over time, as population grows, rainfall totals will change, resulting in less water per person in some regions. In 2025, many nations, even developed nations, are projected to have less water per person than now "
water distribution,T_1868,"Water scarcity is a problem now and will become an even larger problem in the future as water sources are reduced or polluted and population grows. In 1995, about 40% of the worlds population faced water scarcity. Scientists estimate that by the year 2025, nearly half of the worlds people wont have enough water to meet their daily needs. Nearly one-quarter of the worlds people will have less than 500 m3 of water to use in an entire year. That amount is less water in a year than some people in the United States use in one day. Some regions have very little rainfall per month. "
water distribution,T_1869,"Droughts occur when a region experiences unusually low precipitation for months or years (Figure 1.2). Periods of drought may create or worsen water shortages. Human activities can contribute to the frequency and duration of droughts. For example, deforestation keeps trees from returning water to the atmosphere by transpiration; part of the water cycle becomes broken. Because it is difficult to predict when droughts will happen, it is difficult for countries to predict how serious water shortages will be each year. Extended periods with lower than normal rainfall cause droughts. "
water distribution,T_1870,"Global warming will change patterns of rainfall and water distribution. As the Earth warms, regions that currently receive an adequate supply of rain may shift. Regions that rely on snowmelt may find that there is less snow and the melt comes earlier and faster in the spring, causing the water to run off and not be available through the dry summers. A change in temperature and precipitation would completely change the types of plants and animals that can live successfully in that region. "
water distribution,T_1871,"Water scarcity can have dire consequences for the people, the economy, and the environment. Without adequate water, crops and livestock dwindle and people go hungry. Industry, construction, and economic development is halted, causing a nation to sink further into poverty. The risk of regional conflicts over scarce water resources rises. People die from diseases, thirst, or even in war over scarce resources. Californias population is growing by hundreds of thousands of people a year, but much of the state receives as much annual rainfall as Morocco. With fish populations crashing, global warming, and the demands of the countrys largest agricultural industry, the pressures on our water supply are increasing. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
water distribution,T_1872,"As water supplies become scarce, conflicts will arise between the individuals or nations that have enough clean water and those that do not (Figure 1.3). Some of todays greatest tensions are happening in places where water is scarce. Water disputes may add to tensions between countries where differing national interests and withdrawal rights have been in conflict. Just as with energy resources today, wars may erupt over water. Water disputes are happening along 260 different river systems that cross national boundaries. Some of these disputes are potentially very serious. International water laws, such as the Helsinki Rules, help interpret water rights among countries. Many regions already experience water scarcity. This map shows the number of months in which the amount of water that is used exceeds the availability of water that can be used sustainably. This is projected to get worse as demand increases. "
water pollution,T_1873,"Water pollution contributes to water shortages by making some water sources unavailable for use. In underdeveloped countries, raw sewage is dumped into the same water that people drink and bathe in. Even in developed countries, water pollution affects human and environmental health. Water pollution includes any contaminant that gets into lakes, streams, and oceans. The most widespread source of water contamination in developing countries is raw sewage. In developed countries, the three main sources of water pollution are described below. "
water pollution,T_1874,"Wastewater from cities and towns contains many different contaminants from many different homes, businesses, and industries (Figure 1.1). Contaminants come from: Sewage disposal (some sewage is inadequately treated or untreated). Storm drains. Septic tanks (sewage from homes). Boats that dump sewage. Yard runoff (fertilizer and herbicide waste). Large numbers of sewage spills into San Francisco Bay are forcing cities, water agencies and the public to take a closer look at wastewater and its impacts on the health of the bay. QUEST investigates the causes of the spills and whats being done to prevent them. Click image to the left or use the URL below. URL: "
water pollution,T_1875,"Factories and hospitals spew pollutants into the air and waterways (Figure 1.2). Some of the most hazardous industrial pollutants include: Radioactive substances from nuclear power plants and medical and scientific sources. Heavy metals, organic toxins, oils, and solids in industrial waste. Chemicals, such as sulfur, from burning fossil fuels. Oil and other petroleum products from supertanker spills and offshore drilling accidents. Heated water from industrial processes, such as power stations. "
water pollution,T_1876,"Runoff from crops, livestock, and poultry farming carries contaminants such as fertilizers, pesticides, and animal waste into nearby waterways (Figure 1.3). Soil and silt also run off farms. Animal wastes may carry harmful diseases, particularly in the developing world. The high density of animals in a factory farm means that runoff from the area is full of pollutants. Fertilizers that run off of lawns and farm fields are extremely harmful to the environment. Nutrients, such as nitrates, in the fertilizer promote algae growth in the water they flow into. With the excess nutrients, lakes, rivers, and bays become clogged with algae and aquatic plants. Eventually these organisms die and decompose. Decomposition uses up all the dissolved oxygen in the water. Without oxygen, large numbers of plants, fish, and bottom-dwelling animals die. "
weathering and erosion,T_1885,"Weathering is the process that changes solid rock into sediments. Sediments were described in the chapter ""Ma- terials of Earths Crust."" With weathering, rock is disintegrated. It breaks into pieces. Once these sediments are separated from the rocks, erosion is the process that moves the sediments. While plate tectonics forces work to build huge mountains and other landscapes, the forces of weathering gradually wear those rocks and landscapes away. Together with erosion, tall mountains turn into hills and even plains. The Appalachian Mountains along the east coast of North America were once as tall as the Himalayas. "
weathering and erosion,T_1886,"No human being can watch for millions of years as mountains are built, nor can anyone watch as those same mountains gradually are worn away. But imagine a new sidewalk or road. The new road is smooth and even. Over hundreds of years, it will completely disappear, but what happens over one year? What changes would you see? (Figure 1.1). What forces of weathering wear down that road, or rocks or mountains over time? A once smooth road surface has cracks and fractures, plus a large pothole. Click image to the left or use the URL below. URL: "
wegener and the continental drift hypothesis,T_1887,"Wegener put his idea and his evidence together in his book The Origin of Continents and Oceans, first published in 1915. New editions with additional evidence were published later in the decade. In his book he said that around 300 million years ago the continents had all been joined into a single landmass he called Pangaea, meaning all earth in ancient Greek. The supercontinent later broke apart and the continents having been moving into their current positions ever since. He called his hypothesis continental drift. "
wegener and the continental drift hypothesis,T_1888,"Wegeners idea seemed so outlandish at the time that he was ridiculed by other scientists. What do you think the problem was? To his colleagues, his greatest problem was that he had no plausible mechanism for how the continents could move through the oceans. Based on his polar experiences, Wegener suggested that the continents were like icebreaking ships plowing through ice sheets. The continents moved by centrifugal and tidal forces. As Wegeners colleague, how would you go about showing whether these forces could move continents? What observations would you expect to see on these continents? Alfred Wegener suggested that continen- tal drift occurred as continents cut through the ocean floor, in the same way as this icebreaker plows through sea ice. Early hypotheses proposed that centrifu- gal forces moved continents. This is the same force that moves the swings out- ward on a spinning carnival ride. Scientists at the time calculated that centrifugal and tidal forces were too weak to move continents. When one scientist did calculations that assumed that these forces were strong enough to move continents, his result was that if Earth had such strong forces the planet would stop rotating in less than one year. In addition, scientists also thought that the continents that had been plowing through the ocean basins should be much more deformed than they are. Wegener answered his question of whether Africa and South America had once been joined. But a hypothesis is rarely accepted without a mechanism to drive it. Are you going to support Wegener? A very few scientists did, since his hypothesis elegantly explained the similar fossils and rocks on opposite sides of the ocean, but most did not. "
wegener and the continental drift hypothesis,T_1889,"Wegener had many thoughts regarding what could be the driving force behind continental drift. Another of We- geners colleagues, Arthur Holmes, elaborated on Wegeners idea that there is thermal convection in the mantle. In a convection cell, material deep beneath the surface is heated so that its density is lowered and it rises. Near the surface it becomes cooler and denser, so it sinks. Holmes thought this could be like a conveyor belt. Where two adjacent convection cells rise to the surface, a continent could break apart with pieces moving in opposite directions. Although this sounds like a great idea, there was no real evidence for it, either. Alfred Wegener died in 1930 on an expedition on the Greenland icecap. For the most part the continental drift idea was put to rest for a few decades, until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegeners drifting continents. Since youre on a virtual field trip, you get to go along with them as well. Click image to the left or use the URL below. URL: "
wind waves,T_1893,"Waves have been discussed in previous concepts in several contexts: seismic waves traveling through the planet, sound waves traveling through seawater, and ocean waves eroding beaches. Waves transfer energy, and the size of a wave and the distance it travels depends on the amount of energy that it carries. This concept studies the most familiar waves, those on the oceans surface. "
wind waves,T_1894,"Ocean waves originate from wind blowing - steady winds or high storm winds - over the water. Sometimes these winds are far from where the ocean waves are seen. What factors create the largest ocean waves? The largest wind waves form when the wind is very strong blows steadily for a long time blows over a long distance The wind could be strong, but if it gusts for just a short time, large waves wont form. Wind blowing across the water transfers energy to that water. The energy first creates tiny ripples, which make an uneven surface for the wind to catch so that it may create larger waves. These waves travel across the ocean out of the area where the wind is blowing. Remember that a wave is a transfer of energy. Do you think the same molecules of water that start out in a wave in the middle of the ocean later arrive at the shore? The molecules are not the same, but the energy is transferred across the ocean. "
wind waves,T_1895,"Water molecules in waves make circles or ellipses (Figure 1.1). Energy transfers between molecules, but the molecules themselves mostly bob up and down in place. The circles show the motion of a water molecule in a wind wave. Wave energy is greatest at the surface and decreases with depth. ""A"" shows that a water molecule travels in a circular motion in deep water. ""B"" shows that molecules in shallow water travel in an elliptical path because of the ocean bottom. "
wind waves,T_1896,"When does a wave break? Do waves only break when they reach shore? Waves break when they become too tall to be supported by their base. This can happen at sea but happens predictably as a wave moves up a shore. The energy at the bottom of the wave is lost by friction with the ground, so that the bottom of the wave slows down but the top of the wave continues at the same speed. The crest falls over and crashes down. "
wind waves,T_1897,"Some of the damage done by storms is from storm surge. Water piles up at a shoreline as storm winds push waves into the coast. Storm surge may raise sea level as much as 7.5 m (25 ft), which can be devastating in a shallow land area when winds, waves, and rain are intense. Maverick waves are massive. Learning how they are generated can tell scientists a great deal about how the ocean creates waves and especially large waves. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
the microscope,T_1911,"Many life science discoveries would not have been possible without the microscope. For example: Cells are the tiny building blocks of living things. They couldnt be discovered until the microscope was invented. The discovery of cells led to the cell theory. This is one of the most important theories in life science. Bacteria are among the most numerous living things on the planet. They also cause many diseases. However, no one knew bacteria even existed until they could be seen with a microscope. The invention of the microscope allowed scientists to see cells, bacteria, and many other structures that are too small to be seen with the unaided eye. It gave them a direct view into the unseen world of the extremely tiny. You can get a glimpse of that world in Figure 1.10. "
the microscope,T_1912,"The microscope was invented more than four centuries ago. In the late 1500s, two Dutch eyeglass makers, Zacharias Jansen and his father Hans, built the first microscope. They put several magnifying lenses in a tube. They discovered that using more than one lens magnified objects more than a single lens. Their simple microscope could make small objects appear nine times bigger than they really were. "
the microscope,T_1913,"In the mid-1600s, the English scientist Robert Hooke was one of the first scientists to observe living things with a microscope. He published the first book of microscopic studies, called Micrographia. It includes wonderful drawings of microscopic organisms and other objects. One of Hookes most important discoveries came when he viewed thin slices of cork under a microscope. Cork is made from the bark of a tree. When Hooke viewed it under a microscope, he saw many tiny compartments that he called cells. He made the drawing in Figure 1.11 to show what he observed. Hooke was the first person to observe the cells from a once-living organism. "
the microscope,T_1914,"In the late 1600s, Anton van Leeuwenhoek, a Dutch lens maker and scientist, started making much stronger microscopes. His microscopes could magnify objects as much as 270 times their actual size. Van Leeuwenhoek made many scientific discoveries using his microscopes. He was the first to see and describe bacteria. He observed them in a sample of plaque that he had scraped off his own teeth. He also saw yeast cells, human sperm cells, and the microscopic life teeming in a drop of pond water. He even saw blood cells circulating in tiny blood vessels called capillaries. The drawings in Figure 1.12 show some of tiny organisms and living cells that van Leeuwenhoek viewed with his microscopes. He called them animalcules. "
the microscope,T_1915,"These early microscopes used lenses to refract light and create magnified images. This type of microscope is called a light microscope. Light microscopes continued to improve and are still used today. The microscope you might use in science class is a light microscope. The most powerful light microscopes now available can make objects look up to 2000 times their actual size. You can learn how to use a light microscope by watching this short video: http MEDIA Click image to the left or use the URL below. URL:  To see what you might observe with a light microscope, watch the following video. It shows some amazing creatures in a drop of stagnant water from an old boat. What do you think the creatures might be? Do they look like any of van Leeuwenhoeks animalcules in Figure 1.12? MEDIA Click image to the left or use the URL below. URL:  For an object to be visible with a light microscope, it cant be smaller than the wavelength of visible light (about 550 nanometers). To view smaller objects, a different type of microscope, such as an electron microscope, must be used. Electron microscopes pass beams of electrons through or across an object. They can make a very clear image that is up to 2 million times bigger than the actual object. An electron microscope was used to make the image of the ant head in Figure 1.10. "
flatworms and roundworms,T_1993,"Flatworms are invertebrates that belong to Phylum Platyhelminthes. There are more than 25,000 species in the flatworm phylum. Not all flatworms are as long as tapeworms. Some are only about a millimeter in length. "
flatworms and roundworms,T_1994,"Flatworms have a flat body because they lack a fluid-filled body cavity. They also have an incomplete digestive system with a single opening. However, flatworms represent several evolutionary advances in invertebrates. They have the following adaptations: Flatworms have three embryonic cell layers. They have a mesoderm layer in addition to ectoderm and endoderm layers. The mesoderm layer allows flatworms to develop muscle tissues so they can move easily over solid surfaces. Flatworms have a concentration of nerve tissue in the head end. This was a major step in the evolution of a brain. It was also needed for bilateral symmetry. Flatworms have bilateral symmetry. This gives them a better sense of direction than radial symmetry would. Watch this amazing flatworm video to learn about some of the other firsts these simple animals achieved, including being the first hunters: http://shapeoflife.org/video/flatworms-first-hunter MEDIA Click image to the left or use the URL below. URL: "
flatworms and roundworms,T_1995,"Flatworms reproduce sexually. In most species, the same individuals produce both eggs and sperm. After fertilization occurs, the fertilized eggs pass out of the adults body and hatch into larvae. There may be several different larval stages. The final larval stage develops into the adult form. Then the life cycle repeats. "
flatworms and roundworms,T_1996,"Some flatworms live in water or moist soil. They eat invertebrates and decaying animals. Other flatworms, such as tapeworms, are parasites that live inside vertebrate hosts. Usually, more than one type of host is needed to complete the parasites life cycle, as shown in Figure 12.12. "
flatworms and roundworms,T_1997,"Roundworms are invertebrates in Phylum Nematoda. This is a very diverse phylum. It has more than 80,000 known species. Roundworms range in length from less than 1 millimeter to over 7 meters in length. You can see an example of a roundworm in Figure 12.13. "
flatworms and roundworms,T_1998,"Roundworms have a round body because they have a partial fluid-filled body cavity (pseudocoelom). This is one way that roundworms differ from flatworms. Another way is their complete digestive system. It allows them to eat, digest food, and eliminate wastes all at the same time. Roundworms have a tough covering of cuticle on the surface of their body. It prevents their body from expanding. This allows the buildup of fluid pressure in their partial body cavity. The fluid pressure adds stiffness to the body. This provides a counterforce for the contraction of muscles, allowing roundworms to move easily over surfaces. "
flatworms and roundworms,T_1999,"Roundworms reproduce sexually. Sperm and eggs are produced by separate male and female adults. Fertilization takes place inside the female organism. Females lay huge numbers of eggs, sometimes as many as 100,000 per day! The eggs hatch into larvae, which develop into adults. Then the life cycle repeats. "
flatworms and roundworms,T_2000,"Roundworms may be free-living or parasitic organisms. Free-living worms are found mainly in freshwater habitats. Some live in moist soil. They generally feed on bacteria, fungi, protozoa, or decaying organic matter. By breaking down organic matter, they play an important role in the carbon cycle. Parasitic roundworms may have plant, invertebrate, or vertebrate hosts. Several roundworm species infect humans. Besides ascaris, they include hookworms. Hookworms are named for the hooks they use to grab onto the hosts intestines. You can see the hooks in Figure 12.14. Hookworm larvae enter the host through the skin. They migrate to the intestine, where they mature into adults. Female adults lay large quantities of eggs. Eggs pass out of the host in feces. Eggs hatch into larvae in the feces or soil. Then the cycle repeats. You can learn more about parasitic roundworms in humans by watching this short video:  . MEDIA Click image to the left or use the URL below. URL: "
mollusks and annelids,T_2001,"Have you ever been to the ocean or eaten seafood? If you have, then youve probably encountered members of Phylum Mollusca. In addition to snails, mollusks include squids, slugs, scallops, and clams. You can see a clam in Figure 12.15. There are more than 100,000 known species of mollusks. Some mollusks are nearly microscopic. The largest mollusk, the colossal squid, may be as long as a school bus and weigh over half a ton! Watch this short video to see an amazing diversity of mollusks:  . MEDIA Click image to the left or use the URL below. URL: "
mollusks and annelids,T_2002,"Mollusks have a true coelom and complete digestive system. They also have circulatory and excretory systems. They have a heart that pumps blood, and organs that filter out wastes from the blood. You can see some other traits of mollusks in the garden snail in Figure 12.16. Like the snail, many other mollusks have a hard outer shell. It is secreted by special tissue called mantle on the outer surface of the body. The shell covers the top of the body and encloses the internal organs. Most mollusks have a distinct head region. The head may have tentacles for sensing the environment and grasping food. Mollusks generally have a muscular foot, which may be used for walking or other purposes. A unique feature of mollusks is the radula. This is a feeding organ with teeth made of chitin. It is located in front of the mouth in the head region. It can be used to scrape algae off rocks or drill holes in the shells of prey. You can see the radula of the sea slug in Figure 12.17. "
mollusks and annelids,T_2003,"Mollusks reproduce sexually. Most species have separate male and female sexes. Fertilization may be internal or external, depending on the species. Fertilized eggs develop into larvae. There may be one or more larval stages. Each one is different from the adult stage. "
mollusks and annelids,T_2004,"Mollusks live in most terrestrial, freshwater, and marine habitats. However, the majority of species live in the ocean. They can be found in both shallow and deep water and from tropical to polar latitudes. They have a variety of ways of getting food. Some are free-living heterotrophs. Others are internal parasites. Mollusks are also eaten by many other organisms, including humans. "
mollusks and annelids,T_2005,"Annelids are segmented worms in Phylum Annelida. There are about 15,000 species of annelids. They range in length from less than a millimeter to more than 3 meters. To learn more about the amazing diversity and adaptations of annelids, watch this excellent video: http://shapeoflife.org/video/annelids-powerful-and-capable-worms MEDIA Click image to the left or use the URL below. URL: "
mollusks and annelids,T_2006,"Annelids are divided into many repeating segments. The earthworm in Figure 12.18 is an annelid. You can clearly see its many segments. Segmentation of annelids is highly adaptive. Each segment has its own nerve and muscle tissues. This allows the animal to move very efficiently. Some segments can also be specialized to carry out particular functions. They may have special structures on them. For example, they might have tentacles for sensing or feeding, paddles for swimming, or suckers for clinging to surfaces. "
mollusks and annelids,T_2007,"Annelids have a large coelom. They also have several organ systems. These include a: circulatory system; excretory system; complete digestive system; and nervous system, with a brain and sensory organs. "
mollusks and annelids,T_2008,Most annelids can reproduce both asexually and sexually. Asexual reproduction may occur by budding or fission. Sexual reproduction varies by species. Some species go through a larval stage before developing into adults. Other species grow to adult size without going through a larval stage. 
mollusks and annelids,T_2009,"Annelids live in a diversity of freshwater, salt-water, and terrestrial habitats. They vary in what they eat and how they get their food. Some annelids, such as earthworms, eat soil and extract organic material from it. Annelids called leeches are either predators or parasites. Some leeches capture and eat other invertebrates. Others feed off the blood of vertebrate hosts. Annelids called polychaete worms live on the ocean floor. They may be filter feeders, predators, or scavengers. The amazing feather duster worm in Figure 12.19 is a polychaete that has a fan-like crown of tentacles for filter feeding. "
introduction to vertebrates,T_2028,"Like all chordates, vertebrates are animals with four defining traits, at least during the embryonic stage. The four traits are: a notochord; a dorsal hollow nerve cord; a post-anal tail; and pharyngeal slits. Some invertebrates also have these traits and are classified as chordates. What traits do vertebrates have that set them apart from invertebrate chordates? "
introduction to vertebrates,T_2029,"The main trait that sets vertebrates apart from invertebrate chordates is their vertebral column, or backbone. It develops from the notochord after the embryonic stage. As you can see in Figure 13.2 the vertebral column runs from head to tail along the dorsal (top) side of the body. The vertebral column is made up of repeating units of bone called vertebrae (vertebra, singular). The vertebral column helps the vertebrate body hold its shape. It also protects the spinal (nerve) cord that runs through it. "
introduction to vertebrates,T_2030,"The vertebral column is the core of the vertebrate endoskeleton, or internal skeleton. You can see a human skeleton as an example of the vertebrate endoskeleton in Figure 13.3. In addition to the vertebral column, the vertebrate endoskeleton includes: a cranium, or bony skull, that encloses and protects the brain; two pairs of limbs (in humans, arms and legs); limb girdles that connect the limbs to the rest of the endoskeleton (in humans, shoulders and hips). "
introduction to vertebrates,T_2031,"The vertebrate endoskeleton is made of bone and cartilage. Cartilage is a tough, flexible tissue that contains a protein called collagen. Bone is a hard tissue consisting of a collagen framework that is filled in with minerals such as calcium. Bone is less flexible than cartilage but stronger. A bony endoskeleton allows an animal to grow larger and heavier than a cartilage endoskeleton would. Bone also provides more protection for soft tissues and internal organs. "
introduction to vertebrates,T_2032,"Most vertebrates share several other traits. The majority of vertebrates have: scales, feathers, fur, or hair covering their skin; muscles attached to the endoskeleton to allow movement; a circulatory system with a heart that pumps blood through a closed network of blood vessels; an excretory system that includes a pair of kidneys for filtering wastes out of the blood; a central nervous system with a brain, spinal cord, and nerve fibers throughout the body; an adaptive immune system that learns to recognize specific pathogens and launch tailor-made attacks against them; and an endocrine system with glands that secrete chemical messenger molecules called hormones. "
introduction to vertebrates,T_2033,"Vertebrates reproduce sexually. Most have separate male and female sexes. Vertebrates have one of three reproduc- tive strategies: ovipary, ovovivipary, or vivipary. Ovipary refers to the development of an embryo within an egg outside the mothers body. This occurs in most fish, amphibians, and reptiles. It also occurs in all birds. Ovovivipary refers to the development of an embryo inside an egg within the mothers body. The egg remains inside the mothers body until it hatches, but the mother provides no nourishment to the developing embryo inside the egg. This occurs in some species of fish and reptiles. Vivipary refers to the development and nourishment of an embryo within the mothers body but not inside an egg. Birth may be followed by a period of parental care of the offspring. This reproductive strategy occurs in almost all mammals including humans. "
introduction to vertebrates,T_2034,"There are about 50,000 living species of vertebrates. They are placed in nine different classes. Table 13.1 lists these vertebrate classes and some of their traits. Five of the classes are fish. The other four classes are amphibians, reptiles, birds, and mammals. Class Hagfish Distinguishing Traits They have a cranium but no back- bone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic. Example hagfish Class Lampreys Distinguishing Traits They have a partial backbone; they do not have jaws; their endoskele- ton is made of cartilage; they are ectothermic. Example lamprey Cartilaginous Fish They have a complete backbone; they have jaws; their endoskeleton is made of cartilage; they are ec- tothermic. shark Ray-Finned Fish They have a backbone and jaws; their endoskeleton is made of bones; they have thin, bony fins; they are ectothermic. perch Lobe-Finned Fish They have a backbone and jaws; their endoskeleton is made of bones; they have thick, fleshy fins; they are ectothermic. coelacanth Amphibians They have a bony endoskeleton with a backbone and jaws; they have gills as larvae and lungs as adults; they have four limbs; they are ectothermic frog Reptiles They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with scales; they have amniotic eggs; they are ectothermic. alligator Class Birds Distinguishing Traits They have a bony endoskeleton with a backbone but no jaws; they breathe only with lungs; they have four limbs, with the two front limbs modified as wings; their skin is cov- ered with feathers; they have amni- otic eggs; they are endothermic. Example bird Mammals They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with hair or fur; they have am- niotic eggs; they have mammary (milk-producing) glands; they are endothermic. bear "
introduction to vertebrates,T_2035,The earliest vertebrates were jawless fish. They evolved about 550 million years ago. They were probably similar to modern hagfish (see Table 13.1). The tree diagram in Figure 13.4 summarizes how vertebrates evolved from that time forward. 
introduction to vertebrates,T_2036,"The earliest fish had an endoskeleton made of cartilage rather than bone. They also lacked a complete vertebral column. The first fish with a complete vertebral column evolved about 450 million years ago. These fish had jaws. They may have been similar to living sharks. About 400 million years ago, the first fish with a bony endoskeleton evolved. A bony skeleton could support a bigger body. Early bony fish evolved into modern ray-finned fish and lobe-finned fish. "
introduction to vertebrates,T_2037,"The earliest amphibians evolved from a lobe-finned fish ancestor. This occurred about 365 million years ago. Amphibians were the first terrestrial vertebrates. They lived on land as adults, but they had to return to the water to reproduce. The earliest reptiles evolved from an amphibian ancestor. This occurred at least 300 million years ago. Reptiles were the first vertebrates that did not need water to reproduce. Thats because they laid waterproof amniotic eggs. These eggs allowed the embryo inside to breathe without drying out. Mammals and birds both evolved from reptile-like ancestors. The first mammals appeared about 200 million years ago. The earliest birds evolved about 150 million years ago. "
introduction to vertebrates,T_2038,"Early vertebrates were ectothermic. Ectothermy means controlling body temperature to just a limited extent from the outside by changing behavior. For example, an ectotherm might stay in the shade to keep cool on a hot, sunny day. On a cold day, an ectotherm might bask in the sun to warm up, like the snake in Figure 13.5. Almost all living fish, amphibians, and reptiles are ectothermic. They can raise or lower their body temperature by their behavior but not by very much. In cold weather, an ectotherm cools down. As its body temperature drops, its metabolism slows down and it becomes inactive. Both mammals and birds evolved endothermy. Endothermy means controlling body temperature within a narrow range from the inside through biochemical or physical means. For example, on a cold day, an endotherm may produce more body heat by increasing its rate of metabolism. On a hot day, it may give off more heat by increasing blood flow to the surface of the body. That way, some of the heat can radiate into the air from the bodys surface. Endothermy requires more energy (and food) than ectothermy. However, it allows the animal to stay active regardless of the temperature outside. You can learn more about how vertebrates regulate their temperature by watching this video:  . "
fish,T_2039,Fish are aquatic vertebrates. They make up more than half of all living vertebrate species. Most fish are ectothermic. They share several adaptations that suit them for life in the water. 
fish,T_2040,"You can see some of the aquatic adaptations of fish in Figure 13.7. For a video introduction to aquatic adaptations of fish, go to this link:  . MEDIA Click image to the left or use the URL below. URL:  Fish are covered with scales. Scales are overlapping tissues, like shingles on a roof. They reduce friction with the water. They also provide a flexible covering that lets fish move their body to swim. Fish have gills. Gills are organs behind the head that absorb oxygen from water. Water enters through the mouth, passes over the gills, and then exits the body. Fish typically have a stream-lined body. This reduces water resistance. Most fish have fins. Fins function like paddles or rudders. They help fish swim and navigate in the water. Most fish have a swim bladder. This is a balloon-like organ containing gas. By inflating or deflating their swim bladder, fish can rise or sink in the water. "
fish,T_2041,"Fish have a circulatory system with a heart. They also have a complete digestive system. It includes several organs and other structures. Fish with jaws use their jaws and teeth to chew food before swallowing it. This allows them to eat larger prey animals. Fish have a nervous system with a brain. Fish brains are small compared with the brains of other vertebrates. However, they are large and complex compared with the brains of invertebrates. Fish also have highly developed sense organs. They include organs to see, hear, feel, smell, and taste. "
fish,T_2042,"Almost all fish have sexual reproduction, generally with separate sexes. Each fish typically produces large numbers of sperm or eggs. Fertilization takes place in the water outside the body in the majority of fish. Most fish are oviparous. The embryo develops in an egg outside the mothers body. "
fish,T_2043,"Many species of fish reproduce by spawning. Spawning occurs when many adult fish group together and release their sperm or eggs into the water at the same time. You can see fish spawning in Figure 13.8. Spawning increases the changes that fertilization will take place. It typically results in a large number of embryos forming at once. This makes it more likely that at least some of the embryos will avoid being eaten by predators. You can watch trout spawning in Yellowstone Park in this interesting video: http://video.nationalgeographic.com/video/trout_spawning MEDIA Click image to the left or use the URL below. URL:  With spawning, fish parents cant identify their own offspring. Therefore, in most species, there is no parental care of offspring. However, there are exceptions. Some species of fish carry their fertilized eggs in their mouth until they "
fish,T_2044,Fish eggs hatch into larvae. Each larva swims around attached to a yolk sac from the egg (see Figure 13.9). The yolk sac provides it with food. Fish larvae look different from adult fish of the same species. They must go through metamorphosis to change into the adult form. 
fish,T_2045,"There are about 28,000 living species of fish. They are placed in five different classes. The classes are commonly called hagfish, lampreys, cartilaginous fish, ray-finned fish, and lobe-finned fish. Table 13.2 shows pictures of fish in each class. It also provides additional information about the classes. Class Hagfish Lampreys Cartilaginous Fish Distinguishing Traits Hagfish are very primitive fish. They lack scales and fins. They even lack a backbone, but they do have a cranium. They secrete large amounts of thick, slimy mucus. This makes them slippery, so they can slip out of the jaws of predators. Lampreys lack scales but have fins and a partial backbone. Their mouth is surrounded by a large round sucker with teeth. They use the sucker to suck the blood of other fish. Example hagfish Cartilaginous fish include sharks, rays, and ratfish. Their endoskele- ton is made of cartilage instead of bone. They also lack a swim blad- der. However, they have a complete vertebral column and jaws. They also have a relatively big brain. shark lampreys Class Ray-Finned Fish Lobe-Finned Fish Distinguishing Traits Ray-finned fish make up the ma- jority of living fish species. They are a type of bony fish, with an en- doskeleton made of bone instead of cartilage. Their fins consist of webs of skin over flexible bony spines, called rays. They have a swim blad- der. Lobe-finned fish include only coelacanths and lungfish. They are bony fish with an endoskeleton made of bone. Their fleshy fins contain bone and muscle. Lungfish are named for a lung-like organ that they can use for breathing air. It evolved from the swim bladder. It allows them to survive for long periods of time out of water. Example puffer lungfish "
fish,T_2046,"Fish vary in the types of places they live and what they eat. Many fish live in the salt water of the ocean. Other fish live in freshwater lakes, ponds, rivers, or streams. Most fish are predators, but they may differ in their prey and how they get it. Hagfish are deep-ocean bottom dwellers. They feed on other fish, either living or dead. They enter the body of their prey through the mouth or anus. Then they literally eat their prey from the inside out. Lampreys generally live in shallow water, either salty or fresh. They eat small invertebrates or suck the blood of larger fish. Cartilaginous fish, such as sharks, mainly live in the ocean. They prey on other fish and aquatic mammals, or else they eat plankton. Their jaws and teeth allow them to eat large prey. Bony fish, such as ray-finned or lobe-finned fish, may live in salt water or fresh water. They may eat algae, smaller fish like the butterfly fish in Figure 13.10, or dead organisms. To see how one species of predatory bony fish catches its prey, watch this amazing video: http://video.nationalgeographic.com/video/stonefish- MEDIA Click image to the left or use the URL below. URL: "
introduction to the human body,T_2121,"The basic building blocks of the human body are cells. Human cells are organized into tissues, tissues are organized into organs, and organs are organized into organ systems. "
introduction to the human body,T_2122,"The average human adult consists of an incredible 100 trillion cells! Cells are the basic units of structure and function in the human body, as they are in all living things. Each cell must carry out basic life processes in order to survive and help keep the body alive. Most human cells also have characteristics for carrying out other, special functions. For example, muscle cells have extra mitochondria to provide the energy needed to move the body. You can see examples of these and some other specialized human cells in Figure 16.1. To learn more about specialized human cells and what they do, watch this video:  . MEDIA Click image to the left or use the URL below. URL: "
introduction to the human body,T_2123,"Specialized cells are organized into tissues. A tissue is a group of specialized cells of the same kind that perform the same function. There are four basic types of human tissues: connective, epithelial, muscle, and nervous tissues. The four types are shown in Figure 16.2. Connective tissue consists of cells that form the bodys structure. Examples include bone and cartilage, which protect and support the body. Blood is also a connective tissue. It circulates and connects cells throughout the body. Epithelial tissue consists of cells that cover inner and outer body surfaces. Examples include skin and the linings of internal organs. Epithelial tissue protects the body and its internal organs. It also secretes substances such as hormones and absorbs substances such as nutrients. Muscle tissue consists of cells that can contract, or shorten. Examples include skeletal muscle, which is attached to bones and makes them move. Other types of muscle include cardiac muscle, which makes the heart beat, and smooth muscle, which is found in other internal organs. Nervous tissue consists of nerve cells, or neurons, which can send and receive electrical messages. Nervous tissue makes up the brain, spinal cord, and other nerves that run throughout the body. "
introduction to the human body,T_2124,"The four types of tissues make up all the organs of the human body. An organ is a structure composed of two or more types of tissues that work together to perform the same function. Examples of human organs include the skin, brain, lungs, kidneys, and heart. Consider the heart as an example. Figure 16.3 shows how all four tissue types work together to make the heart pump blood. "
introduction to the human body,T_2125,"Human organs are organized into organ systems. An organ system is a group of organs that work together to carry out a complex function. Each organ of the system does part of the overall job. For example, the heart is an organ in the circulatory system. The circulatory system also includes the blood vessels and blood. There are many different human organ systems. Figure 16.4 shows six of them and gives their functions. "
introduction to the human body,T_2126,"The organ systems of the body work together to carry out life processes and maintain homeostasis. The body is in homeostasis when its internal environment is kept more-or-less constant. For example, levels of sugar, carbon dioxide, and water in the blood must be kept within narrow ranges. This requires continuous adjustments. For example: After you eat and digest a sugary snack, the level of sugar in your blood quickly rises. In response, the endocrine system secretes the hormone insulin. Insulin helps cells absorb sugar from the blood. This causes the level of sugar in the blood to fall back to its normal level. When you work out on a hot day, you lose a lot of water through your skin in sweat. The level of water in the blood may fall too low. In response, the excretory system excretes less water in urine. Instead, the water is returned to the blood to keep water levels from falling lower. What happens if homeostasis is not maintained? Cells may not get everything they need, or toxic wastes may build up in the body. If homeostasis is not restored, it may cause illness or even death. "
the integumentary system,T_2127,"From the outside, the skin looks plain and simple, as you can see in Figure 16.5. But at a cellular level, theres nothing plain or simple about it. A single square inch of skin contains about 20 blood vessels, hundreds of sweat glands, and more than a thousand nerve endings. It also contains tens of thousands of pigment-producing cells. Clearly, there is much more to skin than meets the eye! For a dramatic introduction to the skin, watch this video: MEDIA Click image to the left or use the URL below. URL:  The skin is only about 2 mm thick, or about as thick as the cover of a book. Although it is very thin, it consists of two distinct layers, called the epidermis and the dermis. You can see both layers and some of their structures in Figure 16.6. Refer to the figure as you read about the epidermis and dermis below. "
the integumentary system,T_2128,"The epidermis is the outer layer of skin. It consists almost entirely of epithelial cells. There are no blood vessels, nerve endings, or glands in this skin layer. Nonetheless, this layer of skin is very active. It is constantly being renewed. How does this happen? 1. The cells at the bottom of the epidermis are always dividing by mitosis to form new cells. 2. The new cells gradually move up through the epidermis toward the surface of the body. As they move, they produce the tough, fibrous protein called keratin. 3. By the time the cells reach the surface, they have filled with keratin and died. On the surface, the dead cells form a protective, waterproof layer. 4. Dead cells are gradually shed from the surface of the epidermis. As they are shed, they are replaced by other dead cells that move up from below. The epidermis also contains cells called melanocytes. You can see a melanocyte in Figure 16.7. Melanocytes produce melanin. Melanin is a brown pigment that gives skin much of its color. Everyones skin has about the same number of melanocytes per square inch. However, the melanocytes of people with darker skin produce more melanin. The amount of melanin that is produced depends partly on your genes and partly on how much ultraviolet light strikes your skin. The more light you get, the more melanin your melanocytes produce. This explains why skin tans when its exposed to sunlight. "
the integumentary system,T_2129,"The dermis is the inner layer of skin. It is made of tough connective tissue. The dermis is attached to the epidermis by fibers made of the protein collagen. The dermis is where most skin structures are located. Look again at Figure pain, pressure, and temperature. If you cut your skin and it bleeds, the cut has penetrated the dermis and damaged a blood vessel. The cut probably hurts as well because of the nerve endings in this skin layer. The dermis also contains hair follicles and two types of glands. You can see some of these structures in Figure 16.8. Hair follicles are structures where hairs originate. Each hair grows out of a follicle, passes up through the epidermis, and extends above the skin surface. Sebaceous glands are commonly called oil glands. They produce an oily substance called sebum. Sebum is secreted into hair follicles. Then it makes its way along the hair shaft to the surface of the skin. Sebum waterproofs the hair and skin and helps prevent them from drying out. Sweat glands produce the salty fluid known as sweat. Sweat contains excess water, salts, and other waste products. Each sweat gland has a duct that passes through the epidermis. Sweat travels from the gland through the duct and out through a pore on the surface of the skin. "
the integumentary system,T_2130,"You couldnt survive without your skin. It has many important functions. In several ways, it helps maintain homeostasis. The main function of the skin is controlling what enters and leaves the body. It prevents the loss of too much water from the body. It also prevents bacteria and other microorganisms from entering the body. Melanin in the epidermis absorbs ultraviolet light. This prevents the light from reaching and damaging the dermis. The skin helps maintain a constant body temperature. It keeps the body cool in two ways. Sweat from sweat glands in the skin evaporates to cool the body. Blood vessels in the skin dilate, or widen, increasing blood flow to the body surface. This allows more heat to reach the surface and radiate into the environment. The opposite happens to retain body heat. Blood vessels in the skin constrict, or narrow, decreasing blood flow to the body surface. This reduces the amount of heat that reaches the surface so less heat is lost to the environment. "
the integumentary system,T_2131,"What can you do to keep your skin healthy? The most important step you can take is to protect your skin from sun exposure. On sunny days, wear long sleeves and pants and a hat with a brim. Also apply sunscreen to exposed areas of skin. Protecting your skin in these ways will reduce damage to your skin by ultraviolet light. This is important because skin that has been damaged by ultraviolet light is at greater risk of developing skin cancer. This is true whether the damage is due to sunlight or the light in tanning beds. About 85 percent of teens develop acne, like the boy in Figure 16.9. Acne is a condition in which pimples form on the skin. It is caused by a bacterial infection. It happens when the sebaceous glands secrete too much sebum. The excess oil provides a good place for bacteria to grow. Keeping the skin clean helps prevent acne. Over-the-counter products or prescription drugs may be needed if the problem is serious or doesnt clear up on its own. "
the integumentary system,T_2132,"You may spend a lot of time and money on your hair and nails. You may think of them as accessories, like clothes or jewelry. However, like the skin, the hair and nails also play important roles in helping the body maintain homeostasis. "
the integumentary system,T_2133,"Only mammals have hair. Hair is a fiber made mainly of the tough protein keratin. The cells of each hair are filled with keratin and no longer alive. The dead cells overlap each other, almost like shingles on a roof. They work like shingles as well, by helping shed water from hair. Head hair helps protect the scalp from sun exposure. It also helps insulate the body. It traps air so heat cant escape from the head. Hair in eyelashes and eyebrows helps keep water and dust out of the eyes. Hairs inside the nostrils of the nose trap dust and germs in the air so they cant reach the lungs. "
the integumentary system,T_2134,Fingernails and toenails are made of specialized cells that grow out of the epidermis. They too are filled with keratin. The keratin makes them tough and hard. Their job is to protect the ends of the fingers and toes. They also make it easier to feel things with the sensitive fingertips by acting as a counterforce when things are handled. 
the skeletal system,T_2135,"Bones are the main organs of the skeletal system. In adults, the skeleton consists of a whopping 206 bones, many of them in the hands and feet. You can see many of the bones of the human skeleton in Figure 16.10. The skeletal system also includes cartilage and ligaments. Cartilage is a tough, flexible connective tissue that contains the protein collagen. It covers the ends of bones where they meet. The gray tissue in Figure 16.10 is cartilage. A ligament is a band of fibrous connective tissue. Ligaments connect bones of the skeleton and hold them together. "
the skeletal system,T_2136,"Your skeletal system supports your body and gives it shape. What else does it do? The skeletal system makes blood cells. Most blood cells are produced inside certain types of bones. The skeletal system stores calcium and helps maintain normal levels of calcium in the blood. Bones take up and store calcium when blood levels of calcium are high. They release some of the stored calcium when blood levels of calcium are low. The skeletal system works with muscles to move the body. Try to walk without bending your knees and youll see how important the skeletal system is for movement. The skeletal system protects the soft organs of the body. For example, the skull surrounds and protects the brain. The ribs protect the heart and lungs. "
the skeletal system,T_2137,"Some people think bones are like chalk: dead, dry, and brittle. In reality, bones are very much alive. They consist of living tissues and are supplied with blood and nerves. "
the skeletal system,T_2138,"Bones are organs. Like other organs, they are made up of more than one kind of tissue. There are four different kinds of tissues in bones, as shown in Figure 16.11. From the outside of the bone to the center, the tissues are periosteum, compact bone, spongy bone, and bone marrow. Periosteum is a tough, fibrous membrane that covers and protects the outer surfaces of bone. Compact bone lies below periosteum. It is very dense and hard. Compact bone gives bones their strength. Spongy bone lies below compact bone. It is less dense than compact bone. Spongy bone contains many tiny holes, or pores, which provide spaces for blood vessels and bone marrow. Bone marrow is a soft connective tissue inside pores and cavities in spongy bone. Bone marrow makes blood cells. "
the skeletal system,T_2139,"Early in the development of a human fetus, the skeleton is made entirely of cartilage. The relatively soft cartilage gradually changes to hard bone through ossification. This is a process in which mineral deposits replace cartilage in bone. At birth, several areas of cartilage remain, including the ends of the long bones in the arms and legs. This allows these bones to keep growing in length during childhood. By the late teens or early twenties, all of the cartilage has been replaced by bone. Bones cannot grow in length after this point has been reached. However, bones can continue to grow in width. They are stimulated to grow thicker when they are put under stress by muscles. Weight-bearing activities such as weight lifting can increase growth in bone width. "
the skeletal system,T_2140,"A joint is a place where two or more bones of the skeleton meet. There are three different types of joints based on the degree to which they allow movement of the bones: immovable, partly movable, and movable joints. Immovable joints do not allow the bones to move at all. In these joints, the bones are fused together by very tough collagen. Examples of immovable joints include the joints between bones of the skull. You can see them in Figure 16.12. Partly movable joints allow very limited movement. In these joints, the bones are held together by cartilage, which is more flexible than collagen. Examples of partly moveable joints include the bones of the rib cage. Movable joints allow the greatest movement and are the most common. In these joints, the bones are connected by ligaments. The surfaces of the bones at the joints are covered with a smooth layer of cartilage. It reduces friction between the bones when they move. The space between the bones is also filled with a liquid called synovial fluid. It helps to cushion the bones. There are several different types of movable joints. You can see three of them in Figure 16.13. Move these three joints in your own skeleton to experience the range of motion each allows. "
the skeletal system,T_2141,"What you eat as a teen can affect how healthy your skeletal system is not only now but also in the future. Eating a diet with plenty of calcium and vitamin D can help keep your bones strong. If you dont get enough calcium and vitamin D in your diet as a teen, you will be more likely to develop osteoporosis when you are older. "
the skeletal system,T_2142,"Osteoporosis is a disease in which the bones become porous and weak because they do not contain enough calcium. The graph in Figure 16.14 shows how the mass of calcium in bone peaks around age 30 and declines after that, especially in women. Maximizing the calcium in your bones while youre young will reduce your risk of developing osteoporosis later in of life. "
the skeletal system,T_2143,"People with osteoporosis have an increased risk of bone fractures. A bone fracture is a crack or break in bone. Even if you have healthy bones, you may fracture a bone if too much stress is placed on it. This could happen in a car crash or while playing a sport. Wearing a seatbelt when you ride in a motor vehicle and wearing safety gear when you play sports may help prevent bone fractures. Bone fractures heal naturally as new bone tissue forms at the site of the fracture. However, the bone may have to be placed in a cast or have rods or screws inserted into it to keep it correctly aligned until it heals. The healing process usually takes several weeks or even months. "
the skeletal system,T_2144,"Another type of skeletal system injury is a sprain. A sprain is a strain or tear in a ligament that has been twisted or stretched too far. Ankle sprains are a common type of sprain. Athletes often strain a ligament in the knee called the ACL. Warming up adequately and stretching before playing sports may reduce the risk of a sprain. Ligament injuries can take a long time to heal. Rest, ice, compression, and elevation of the sprained area may help the healing process. "
the muscular system,T_2145,"Muscles are the main organs of the muscular system. Muscles are composed primarily of cells called muscle fibers. A muscle fiber is a very long, thin cell, as you can see in Figure 16.16. It contains multiple nuclei and many mitochondria, which produce ATP for energy. It also contains many organelles called myofibrils. Myofibrils allow muscles to contract, or shorten. Muscle contractions are responsible for virtually all the movements of the body, both inside and out. "
the muscular system,T_2146,"To understand how a muscle contracts, you need to dive deeper into the structure of muscle fibers. You can see in Figure 16.16 that a muscle fiber is full of myofibrils. Each myofibril is made up of two types of proteins, called actin and myosin. These proteins form thread-like filaments. The myosin filaments use energy from ATP to pull on the actin filaments. This causes the actin filaments to slide over the myosin filaments and shorten a section of the myofibril. You can see a simple animation of the process at this link: http://commons.wikimedia.org/wiki/File:Actin_Myosin.gif The sliding-and-shortening process occurs all along many myofibrils and in many muscle fibers. It causes the muscle fibers to shorten and the muscle to contract. "
the muscular system,T_2147,"There are three different types of muscle tissue in the human body: cardiac, smooth, and skeletal muscle tissues. All three types consist mainly of muscle fibers, but the fibers have different arrangements. You can see how each type of muscle tissue looks in Figure 16.17. Cardiac muscle is found only in the walls of the heart. It is striated, or striped, because its muscle fibers are arranged in bundles. Contractions of cardiac muscle are involuntary. This means that they are not under conscious control. When cardiac muscle contracts, the heart beats and pumps blood. Smooth muscle is found in the walls of other internal organs such as the stomach. It isnt striated because its muscle fibers are arranged in sheets rather than bundles. Contractions of smooth muscle are involuntary. When smooth muscles in the stomach contract, they squeeze food inside the stomach. This helps break the food into smaller pieces. Skeletal muscle is attached to the bones of the skeleton. It is striated like cardiac muscle because its muscle fibers are arranged in bundles. Contractions of skeletal muscle are voluntary. This means that they are under conscious control. Whether you are doing pushups or pushing a pencil, you are using skeletal muscles. Skeletal muscles are the most common type of muscles in the body. You can read more about them below. "
the muscular system,T_2148,The human body has more than 600 skeletal muscles. You can see some of them in Figure 16.18. A few of the larger muscles are labeled in the figure. 
the muscular system,T_2149,"You can see the bundles of muscle fibers that make up a skeletal muscle in Figure 16.19. You can also see in the figure how the muscle is attached to a bone by a tendon. Tendons are tough connective tissues that anchor skeletal muscles to bones throughout the body. Many skeletal muscles are attached to the ends of bones where they meet at a joint. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. "
the muscular system,T_2150,"Muscles can only contract. They cant actively lengthen. Therefore, to move bones back and forth at a joint, skeletal muscles must work in pairs. For example, the bicep and triceps muscles of the upper arm work as a pair. You can see how this pair of muscles works in Figure 16.20. When the bicep muscle contracts, it bends the arm at the elbow. When the triceps muscle contracts, it straightens the arm. "
the muscular system,T_2151,"Did you ever hear the saying, Use it or lose it? Thats certainly true when it comes to muscles. If you dont exercise your muscles, they will actually shrink in size. They will also become weaker and more prone to injury. "
the muscular system,T_2152,"Exercising muscles increases their size, and bigger muscles have greater strength. What type of exercises should you do? For all-round muscular health, you should do two basic types of exercise. To increase the size and strength of skeletal muscles, you need to make these muscles contract against a resisting force. For example, you can do sit-ups or pushups, where the resisting force is your own body weight. You can see another way to do it in Figure 16.21. To exercise cardiac muscle and increase muscle endurance, you need to do aerobic exercise. Aerobic exercise increases the size and strength of muscles in the heart and helps all your muscles develop greater endurance. This means they can work longer without getting tired. Aerobic exercise is any exercise such as running, biking, or swimming that causes an increase in your heart rate. You can see another example of aerobic exercise in Figure 16.22. Lifting weights is one way to pit skeletal muscles against a resisting force. Snowshoeing is a fun way to get aerobic exercise. "
the muscular system,T_2153,You are less likely to have a muscle injury if you exercise regularly and have strong muscles. Stretching also helps prevent muscle injuries. Stretching improves the range of motion of muscles and tendons at joints. You should always warm up before stretching or doing any type of exercise. Warmed-up muscles and tendons are less likely to be injured. One way to warm up is to jog slowly for a few minutes. 
food and nutrients,T_2154,Your body needs food for three purposes: 1. Food gives the body energy. You need energy for everything you do. The energy in food is measured in a unit called the Calorie. 2. Food provides building materials for the body. The body needs building materials for growth and repair. 3. Food contains substances that help control body processes. Body processes must be kept in balance for good health. 
food and nutrients,T_2155,"There are a variety of substances in foods that the body needs. Any substance in food that the body needs is called a nutrient. There are six major types of nutrients: carbohydrates, proteins, lipids, water, minerals, and vitamins. Carbohydrates, proteins, and lipids can be used for energy. Proteins also provide building materials. Proteins, minerals, and vitamins help control body processes. Water is needed by all cells just to stay alive. The six types of nutrients can be divided into two major categories based on how much of them the body needs. The categories are macronutrients and micronutrients. "
food and nutrients,T_2156,"Macronutrients are nutrients the body needs in relatively large amounts. They include carbohydrates, proteins, lipids, and water. "
food and nutrients,T_2157,"Carbohydrates include sugars, starches, and fiber. Sugars and starches are used by the body for energy. One gram of sugar or starch provides 4 Calories of energy. Fiber doesnt provide energy, but it is needed for other uses. At age 13 years, you need about 130 grams of carbohydrates a day. Figure 17.2 shows good food sources of each type. Sugars are small, simple carbohydrates. They are found in foods such as milk and fruit. Sugars in foods such as these are broken down by your digestive system to glucose, the simplest of all sugars. Glucose is taken up by cells for energy. Starches are larger, complex carbohydrates. They are found in foods such as grains and vegetables. Starches are broken down by your digestive system to glucose, which is used for energy. Fiber is a complex carbohydrate that consists mainly of cellulose and comes only from plants. High-fiber foods include whole grains and legumes such as beans. Fiber cant be broken down by the digestive system, but it plays important roles in the body. It helps keep sugar and lipids at normal levels in the blood. It also helps keep food waste moist so it can pass easily out of the body. "
food and nutrients,T_2158,"Proteins are nutrients made up of smaller molecules called amino acids. The digestive system breaks down proteins in food to amino acids, which are used for protein synthesis. Proteins synthesized from the amino acids in food serve many vital functions. They make up muscles, control body processes, fight infections, and carry substances in the blood. If you eat more protein than you need for these functions, the extra protein is used for energy. One gram of protein provides 4 Calories of energy, the same as carbohydrates. A 13-year-old needs to eat about 34 grams of protein a day. Figure 17.3 shows good food sources of protein. "
food and nutrients,T_2159,"Lipids are nutrients such as fats. They are used for energy and other important purposes. One gram of lipids provides the body with 9 Calories of energy, more than twice as much as carbohydrates or proteins. Lipids also make up cell membranes, protect nerves, control blood pressure, and help blood clot. You must consume some lipids for these purposes. Good food sources of lipids are shown in Figure 17.4. Any extra lipids you consume are stored as fat. A certain amount of stored fat is needed to cushion and protect internal organs and insulate the body. However, too much stored fat can lead to obesity and cause significant health problems. A type of lipid called trans fat is found in many processed foods. Trans fat is rare in nature but is manufactured and added to foods to preserve freshness. Eating foods that contain trans fat increases the risk of heart disease. Trans fat may be found in such foods as cookies, doughnuts, crackers, fried foods, ground beef, and margarine. "
food and nutrients,T_2160,"Water is essential to life because chemical reactions within cells take place in water. Most people can survive only a few days without consuming water to replace their water losses. How do you lose water? You lose water in your breath each time you exhale. You lose water in urine. You lose water in sweat, especially if you are active in warm weather. The boy in Figure 17.5 is taking a water break while playing outside on a hot day. If he doesnt take in enough water to replace the water lost in sweat, he may become dehydrated. Symptoms of dehydration include dry mouth, headache, and dizziness. Dehydration can be very serious. It can even cause death. "
food and nutrients,T_2161,"Micronutrients are nutrients the body needs in relatively small amounts. They include minerals and vitamins. These nutrients dont provide the body with energy, but they are still essential for good health. "
food and nutrients,T_2162,"Minerals are chemical elements that dont come from living things or include the element carbon. Many minerals are needed in the diet for normal functioning of the body. Several minerals that are needed in relatively large amounts are listed in Table 17.1. As you can see from these examples, minerals have a diversity of important functions. Your body cant produce any of the minerals it needs, so you must get them from the food you eat. The table shows good food sources of the minerals. Mineral Calcium Chloride Magnesium Phosphorus Potassium Sodium Function strong bones and teeth salt-water balance strong bones strong bones and teeth muscle and nerve functions muscle and nerve functions Good Food Sources milk, green leafy vegetables table salt, most packaged foods whole grains, nuts poultry, whole grains meat, bananas table salt, most packaged foods Not getting enough minerals can cause health problems. For example, not getting enough calcium may cause osteoporosis. This is a disease in which the bones become porous so they break easily. Getting too much of some minerals can also cause health problems. Many people get too much sodium. Sodium is added to most packaged foods. People often add more sodium to their food by using table salt. Too much sodium has been linked to high blood pressure in some people. "
food and nutrients,T_2163,"The vitamins to watch out for are A, D, E, and K. These vitamins are stored by the body, so they can build up to high levels. "
the digestive system,T_2171,"The digestive system is the body system that breaks down food and absorbs nutrients. It also eliminates solid food wastes that remain after food is digested. The major organs of the digestive system are shown in Figure 17.10. For an entertaining overview of the digestive system and how it works, watch this video:  MEDIA Click image to the left or use the URL below. URL: "
the digestive system,T_2172,"The organs in Figure 17.10 make up the gastrointestinal (GI) tract. This is essentially a long tube that connects the mouth to the anus. Food enters the mouth and then passes through the rest of the GI tract. Food waste leaves the body through the anus. In adults, the GI tract is more than 9 meters (30 feet) long! Organs of the GI tract are covered by muscles that contract to keep food moving along. A series of involuntary muscle contractions moves rapidly along the tract, like a wave travelling through a spring toy. The muscle contractions are called peristalsis. The diagram in Figure 17.11 shows how peristalsis works. "
the digestive system,T_2173,"As food is pushed through the GI tract by peristalsis, it undergoes digestion. Digestion is the process of breaking down food into nutrients. There are two types of digestion: mechanical digestion and chemical digestion. Mechanical digestion occurs when large chunks of food are broken down into smaller pieces. This is a physical process that happens mainly in the mouth and stomach. Chemical digestion occurs when large food molecules are broken down into smaller nutrient molecules. This is a chemical process that begins in the mouth and stomach but occurs mainly in the small intestine. "
the digestive system,T_2174,"After food is broken down into nutrient molecules, the molecules are absorbed by the blood. Absorption is the process in which nutrients or other molecules are taken up by the blood. Once absorbed by the blood, nutrients can travel in the bloodstream to cells throughout the body. "
the digestive system,T_2175,Some substances in food cant be broken down into nutrients. They remain behind in the digestive system after the nutrients have been absorbed. Any substances in food that cant be digested pass out of the body as solid waste. This process is called elimination. 
the digestive system,T_2176,"Chemical digestion could not take place without the help of digestive enzymes and other substances secreted into the GI tract. An enzyme is a protein that speeds up a biochemical reaction. Digestive enzymes speed up the reactions of chemical digestion. Table 17.3 lists a few digestive enzymes, the organs that produce them, and their functions in digestion. Enzyme Amylase Pepsin Organ that Produces It mouth stomach Substance It Helps Digest starch protein Enzyme Lipase Ribonuclease Organ that Produces It pancreas pancreas Substance It Helps Digest fat RNA Most digestive enzymes are secreted into the GI tract by organs of the GI tract or from a nearby gland named the pancreas. Figure 17.12 shows where the pancreas is located. The figure also shows the locations of the liver and gall bladder. These organs produce or store other digestive secretions. The liver secretes bile acids. Bile acids help digest fat. Some liver bile is secreted directly into the small intestine. Some liver bile goes to the gall bladder. This sac-like organ stores and concentrates the liver bile before releasing it into the small intestine. "
the digestive system,T_2177,"Does the sight or smell of your favorite food make your mouth water? When this happens, you are getting ready for digestion. "
the digestive system,T_2178,"The mouth is the first digestive organ that food enters. The sight, smell, or taste of food stimulates the release of saliva and digestive enzymes by salivary glands inside the mouth. Saliva wets the food, which makes it easier to break up and swallow. The enzyme amylase in saliva begins the chemical digestion of starches to sugars. Your teeth help to mechanically digest food. Look at the different types of human teeth in Figure 17.13. Sharp teeth in the front of the mouth cut or tear food when you bite into it. Broad teeth in the back of the mouth grind food when you chew. Your tongue helps mix the food with saliva and enzymes and also helps you swallow. When you swallow, a lump of chewed food passes from the mouth into a tube in your throat called the pharynx. From the pharynx, the food passes into the esophagus. "
the digestive system,T_2179,"The esophagus is a long, narrow tube that carries food from the pharynx to the stomach. It has no other purpose. Food moves through the esophagus because of peristalsis. At the lower end of the esophagus, a circular muscle, called a sphincter, controls the opening to the stomach. The sphincter relaxes to let food pass into the stomach. Then the sphincter contracts to prevent food from passing back into the esophagus. "
the digestive system,T_2180,"The stomach is a sac-like organ at the end of the esophagus. It has thick muscular walls that contract and relax to squeeze and mix food. This helps break the food into smaller pieces. It also helps mix the food with enzymes and other secretions in the stomach. For example, the stomach secretes the enzyme pepsin, which helps digest proteins. Water, salt, and simple sugars can be absorbed into the blood from the lining of the stomach. However, most substances must undergo further digestion in the small intestine before they can be absorbed. The stomach stores the partly digested food until the small intestine is empty. Then a sphincter between the stomach and small intestine relaxes, allowing food to enter the small intestine. "
the digestive system,T_2181,"The small intestine is a narrow tube that starts at the stomach and ends at the large intestine. In adults, its about 7 meters (23 feet) long. Most chemical digestion and almost all nutrient absorption take place in the small intestine. The small intestine is made up of three parts: 1. The duodenum is the first part of the small intestine. It is also the shortest part. This is where most chemical digestion takes place. Many enzymes and other substances involved in digestion are secreted into the duodenum 2. The jejunum is the second part of the small intestine. This is where most nutrients are absorbed into the blood. The inside surface of the jejunum is covered with tiny projections called villi (villus, singular). The villi make the inner surface of the small intestine 1000 times greater than it would be without them. You can read in Figure 17.14 how villi are involved in absorption. 3. The ileum is the last part of the small intestine. It is covered with villi like the jejunum. A few remaining nutrients are absorbed in the ileum. From the ileum, any remaining food waste passes into the large intestine. "
the digestive system,T_2182,"The large intestine is a wide tube that connects the small intestine with the anus. In adults, the large intestine is about 1.5 meters (5 feet) long. It is larger in width but shorter in length than the small intestine. "
the digestive system,T_2183,"Food waste enters the large intestine from the small intestine in a liquid state. As the waste moves through the large intestine, excess water is absorbed from it. The remaining solid waste is called feces. After a certain amount of feces have collected, a sphincter relaxes to let the feces pass out of the body through the anus. This is elimination. "
the digestive system,T_2184,"Trillions of bacteria normally live in the large intestine. Dont worrymost of them are helpful. They have several important roles. For example, intestinal bacteria: produce vitamins B12 and K. control the growth of harmful bacteria. break down toxins in the large intestine. break down fiber and some other substances in food that cant be digested. "
the digestive system,T_2185,"Much of the time, you probably arent aware of your digestive system. It works well without causing any problems. But most people have problems with their digestive system at least once in a while. Did you ever eat something that didnt agree with you? Maybe you had a stomachache or felt sick to your stomach. Perhaps you had diarrhea. These can be symptoms of food poisoning. "
the digestive system,T_2186,"Food poisoning is the common term for foodborne illness. This type of illness occurs when harmful bacteria enter your digestive system in food and make you sick. The bacteriaor toxins they producemay cause cramping, vomiting, or other GI tract symptoms. Following these healthy practices may decrease your risk of foodborne illness: Wash your hands after handling raw foods such as meats, poultry, fish, or eggs. These foods often contain bacteria that your hands could transfer to your mouth. Cook meats, poultry, fish, or eggs thoroughly before eating them. The heat of cooking kills any bacteria the foods may contain so they cant make you sick. Keep hot foods hot and cold foods cold. This is especially important when food is packed for lunch or a picnic (see Figure 17.15). Maintaining the proper temperature slows the growth of bacteria in the food. "
the digestive system,T_2187,"Food allergies occur when the immune system reacts to harmless substances in food as though they were harmful germs. Food allergies are relatively common. Almost 10 percent of children have them. Some of the foods most likely to cause allergies include milk, shellfish, nuts, grains, and eggs. If you eat foods to which you are allergic, you may experience vomiting, diarrhea, or a rash. In some people, eating even tiny amounts of certain foods causes them to have serious symptoms, such as difficulty breathing. They need immediate medical attention. The best way to prevent food allergy symptoms is to avoid eating the offending food. This may require careful reading of food labels. "
overview of the cardiovascular system,T_2188,The organs that make up the cardiovascular system are the heart and a network of blood vessels that run throughout the body. The blood in the cardiovascular system is a liquid connective tissue. Figure 18.1 shows the heart and major vessels through which blood flows in the system. The heart is basically a pump that keeps blood moving through the blood vessels. 
overview of the cardiovascular system,T_2189,"The main function of the cardiovascular system is transporting substances around the body. Figure 18.1 shows some of the substances that are transported in the blood. They include hormones, oxygen, nutrients from digested food, and cellular wastes. Transport of all these materials is necessary to maintain homeostasis of the body and life itself. The cardiovascular system also helps regulate body temperature by controlling where blood moves around the body. Blood is warm, so when more blood flows to the surface of the body, it warms the surface. This allows the body to lose excess heat from the surface. When less blood flows to the surface, it cools the surface. This allows the body to conserve heat and stay warm. You can see the role of blood vessels in the regulation of body temperature in this video:  . MEDIA Click image to the left or use the URL below. URL: "
overview of the cardiovascular system,T_2190,"The heart and blood vessels form a closed system through which blood keeps circulating. However, blood actually circulates in two different loops within this closed system. The two loops are called pulmonary circulation and systemic circulation. In both loops, blood passes through the heart. You can see a simple model of each circulation loop in Figure 18.2. As blood circulates through the body, it travels first through one loop and then the other loop, over and over again. "
overview of the cardiovascular system,T_2191,"Pulmonary circulation is the shorter loop of the cardiovascular system. It carries blood between the heart and lungs. Oxygen-poor blood flows from the heart to the lungs. In the lungs, the blood absorbs oxygen and releases carbon dioxide. Then the oxygen-rich blood returns to the heart. "
overview of the cardiovascular system,T_2192,"Systemic circulation is the longer loop of the cardiovascular system. It carries blood between the heart and the rest of the body. Oxygen-rich blood flows from the heart to cells throughout the body. As it passes cells, the blood releases oxygen and absorbs carbon dioxide. Then the oxygen-poor blood returns to the heart. "
heart and blood vessels,T_2193,"The heart is a muscular organ in the chest. It consists mainly of cardiac muscle tissue. It pumps blood by repeated, rhythmic contractions. This produces the familiar lub-dub sound of each heartbeat. For a good video introduction to the heart and how it works, watch this entertaining Bill Nye video:  MEDIA Click image to the left or use the URL below. URL: "
heart and blood vessels,T_2194,"The heart has four chambers, or rooms, which you can see in Figure 18.3. Each chamber is an empty space with muscular walls through which blood can flow. The top two chambers of the heart are called the left and right atria (atrium, singular). The atria of the heart receive blood from the body or lungs and pump it into the bottom chambers of the heart. The bottom two chambers of the heart are called the left and right ventricles. The ventricles receive blood from the atria and pump it out of the heart, either to the lungs or to the rest of the body. Flaps of tissue called valves separate the hearts chambers. Valves keep blood flowing in just one direction through the heart. For example, a valve at the bottom of the right atrium opens to let blood flow from the right atrium to the right ventricle. Then the valve closes so the blood cant flow back into the right atrium. "
heart and blood vessels,T_2195,Blood flows through the heart in two paths. Trace these two paths in Figure 18.4 as you read about them below. You can also learn about how blood flows through the heart with this rap:  MEDIA Click image to the left or use the URL below. URL:  1. One path of blood in the heart is through the right atrium and right ventricle. The right atrium receives oxygen- poor blood from the body. It pumps the blood into the right ventricle. Then the right ventricle pumps the blood out of the heart to the lungs. This path through the heart is part of the pulmonary circulation. 2. The other path of blood in the heart is through the left atrium and left ventricle. The left atrium receives oxygen-rich blood from the lungs. It pumps the blood into the left ventricle. Then the left ventricle pumps the blood out of the heart to the rest of the body. This path through the heart is part of the systemic circulation. 
heart and blood vessels,T_2196,"To move blood through the heart, cardiac muscles must contract in a certain sequence. First the atria must contract, followed quickly by the ventricles contracting. This series of contractions keeps blood moving continuously through the heart. Contractions of cardiac muscles arent under voluntary control. They are controlled by a cluster of special cells within the heart, commonly called the pacemaker. These cells send electrical signals to cardiac muscles so they contract in the correct sequence and with just the right timing. "
heart and blood vessels,T_2197,"Blood vessels are long, tube-like organs that consist mainly of muscle, connective, and epithelial tissues. They branch to form a complex network of vessels that run throughout the body. This network transports blood to all the bodys cells. "
heart and blood vessels,T_2198,"There are three major types of blood vessels: arteries, veins, and capillaries. You can see each type in Figure 18.5. You can watch a good video introduction to the three types at this link:  MEDIA Click image to the left or use the URL below. URL:  Arteries are muscular blood vessels that carry blood away from the heart. They have thick walls that can withstand the pressure of blood pumped by the heart. Arteries generally carry oxygen-rich blood. The largest artery is the aorta, which receives blood directly from the heart. It branches to form smaller and smaller arteries throughout the body. The smallest arteries are called arterioles. Veins are blood vessels that carry blood toward the heart. This blood is no longer under pressure, so veins have thinner walls. To keep the blood moving, many veins have valves that prevent the backflow of blood. Veins generally carry oxygen-poor blood. The smallest veins are called venules. They merge to form larger and larger veins. The largest vein is the inferior vena cava, which carries blood from the lower body directly to the heart. Capillaries are the smallest type of blood vessels. They connect the smallest arteries (arterioles) and veins (venules). Exchange of substances between cells and the blood takes place across the walls of capillaries, which may be only one cell thick. "
heart and blood vessels,T_2199,"Blood vessels help regulate body processes by either dilating (widening) or constricting (narrowing). This changes the amount of blood flowing to particular organs. For example, dilation of blood vessels in the skin allows more blood to flow to the surface of the body. This helps the body lose excess heat. Constriction of these blood vessels has the opposite effect and helps the body conserve heat. "
heart and blood vessels,T_2200,Diseases of the cardiovascular system are common and may be life threatening. A healthy lifestyle can reduce the risk of such diseases developing. 
heart and blood vessels,T_2201,"Diseases of the heart and blood vessels are called cardiovascular diseases. The leading cause of cardiovascular disease is atherosclerosis. Atherosclerosis is a condition in which a material called plaque builds up inside arteries. Plaque consists of cell debris, cholesterol, and other substances. As plaque builds up in an artery, the artery narrows, as shown in Figure If plaque blocks coronary arteries that supply blood to the heart, coronary heart disease results. Poor blood flow to the heart may cause chest pain or a heart attack. A heart attack occurs when the blood supply to part of the heart muscle is completely blocked so that cardiac muscle cells die. Coronary heart disease is the leading cause of death in U.S adults. "
heart and blood vessels,T_2202,"Many factors influence your risk of developing cardiovascular diseases. Some of these factors you cant control. Older age, male gender, and a family history of cardiovascular disease all increase the risk and cant be controlled. However, you can control many other factors. To reduce the risk of cardiovascular disease, you can: avoid smoking. get regular physical activity. maintain a healthy percent of body fat. eat a healthy, low-fat diet. get regular checkups to detect and manage problems such as high blood pressure and high blood cholesterol. "
blood,T_2203,Blood is a liquid connective tissue. It circulates throughout the body via blood vessels due to the pumping action of the heart. You couldnt survive without the approximately 4.5 to 5 liters of blood that are constantly being pumped through your blood vessels. 
blood,T_2204,"Blood consists of both liquid and cells. The liquid part of blood is called plasma. Plasma is a watery, golden-yellow fluid that contains many dissolved substances. Substances dissolved in plasma include glucose, proteins, and gases. Plasma also contains blood cells. There are three types of blood cells: red blood cells, white blood cells, and platelets. You can see all three types in Figure 18.8. 1. Red blood cells are shaped like flattened disks. There are trillions of red blood cells in your blood. Each red blood cell has millions of molecules of hemoglobin. Hemoglobin is a protein that contains iron. The iron in hemoglobin gives red blood cells their red color. It also explains how hemoglobin carries oxygen. The iron in hemoglobin binds with oxygen molecules so they can be carried by red blood cells. 2. White blood cells are larger than red blood cells, but there are far fewer of them. Their role is to defend the body in various ways. For example, white blood cells called phagocytes engulf and destroy microorganisms and debris in the blood. 3. Platelets are small, sticky cell fragments that help blood clot. A blood clot is a solid mass of cell fragments and other substances that plugs a leak in a damaged blood vessel. Platelets stick to tears in blood vessels and to each other, helping to form a clot at the site of injury. Platelets also release chemicals that are needed for clotting to occur. "
blood,T_2205,"The main function of blood is transport. Blood in arteries carries oxygen and nutrients to all the bodys cells. Blood in veins carries carbon dioxide and other wastes away from cells to be excreted. Blood also transports the chemical messengers called hormones to cells throughout the body where they are needed to regulate body functions. Blood has several other functions as well. For example, blood: defends the body against infections. repairs body tissues. controls the bodys pH. helps regulate body temperature. "
blood,T_2206,"Red blood cells carry proteins called antigens on their surface. People may vary in the exact antigens their red blood cells carry. The specific proteins are controlled by the genes they inherit from their parents. The particular antigens you inherit determine your blood type. Why does your blood type matter? Blood type is important for medical reasons. A patient cant safely receive a transfusion of blood containing antigens not found in the patients own blood. With foreign antigens, the transfused blood will be rejected by the persons immune system. This causes a reaction in the patients bloodstream, called agglutination. The transfused red blood cells clump together, as shown in Figure 18.9. The clumped cells block blood vessels and cause other life-threatening problems. There are many sets of antigens that determine different blood types. Two of the best known are the ABO and Rhesus antigens. Both are described below. You can also learn more about them by watching this video: "
blood,T_2207,"ABO blood type is determined by two common antigens, often called antigen A and antigen B. If your red blood cells carry only antigen A, you have blood type A. If your red blood cells carry only antigen B, you have blood type B. If your red blood cells carry both antigen A and antigen B, you have blood type AB. If your red blood cells carry neither antigen A nor antigen B, you have blood type O. "
blood,T_2208,"Another red blood cell antigen determines a persons Rhesus blood type. This blood type depends on a single common antigen, typically referred to as the Rhesus (Rh) antigen. If your red blood cells carry the Rhesus antigen, you have Rhesus-positive blood, or blood type Rh+. If your red blood cells lack the Rhesus antigen, you have Rhesus-negative blood, or blood type Rh-. "
blood,T_2209,"Some diseases affect mainly the blood or its components. They include anemia, leukemia, hemophilia, and sickle- cell disease. "
blood,T_2210,Anemia is a disease that occurs when there is not enough hemoglobin (or iron) in the blood so it cant carry adequate oxygen to the cells. There are many possible causes of anemia. One possible cause is excessive blood loss due to an injury or surgery. Not getting enough iron in the diet is another possible cause. 
blood,T_2211,"Leukemia is a type of cancer in which bone marrow produces abnormal white blood cells. The abnormal cells cant do their job of fighting infections. Like most cancers, leukemia is thought to be caused by a combination of genetic and environmental factors. It is the most common cancer in children. "
blood,T_2212,"Hemophilia is a genetic disorder in which blood fails to clot properly because a normal clotting factor in the blood is lacking. In people with hemophilia, even a minor injury can cause a life-threatening loss of blood. Most cases of hemophilia are caused by a recessive gene on the X chromosome. The disorder is expressed much more commonly in males because they have just one X chromosome. "
blood,T_2213,"Sickle-Cell Disease is another genetic disorder of the blood. It is more common in people with African origins because it helps protect against malaria. Sickle-cell disease occurs in people who inherit two copies of the recessive mutant gene for hemoglobin. The abnormal hemoglobin that results causes red blood cells to take on a characteristic sickle shape under certain conditions. You can compare sickle-shaped and normal red blood cells in Figure 18.10. The sickle-shaped cells get stuck in tiny capillaries and block blood flow. This causes serious, painful symptoms. Watch this video animation to learn more about the genetic basis of sickle-cell disease: "
the respiratory system,T_2214,"The bodys exchange of oxygen and carbon dioxide with the air is called respiration. Respiration actually consists of two stages. In one stage, air is taken into the body and carbon dioxide is released to the outside air. In the other stage, oxygen is delivered to all the cells of the body and carbon dioxide is carried away from the cells. Another kind of respiration takes place within body cells. This kind of respiration is called cellular respiration. Its the process in which cells obtain energy by burning glucose. Both types of respiration are connected. Cellular respiration uses oxygen and produces carbon dioxide. Respiration by the respiratory system supplies the oxygen needed for cellular respiration. It also removes the carbon dioxide produced by cellular respiration. "
the respiratory system,T_2215,"You can see the main structures of the respiratory system in Figure 19.1. They include the nose, trachea, lungs, and diaphragm. Use the figure to trace how air moves through the respiratory system when you read about it below. You can also use this interactive to explore the respiratory system and see how it functions: http://science.nationalgeogr "
the respiratory system,T_2216,"Take in a big breath of air through your nose. As you breathe in, you may feel the air pass down through your throat and notice your chest expand. Now breathe out and observe the opposite events occurring. Breathing in and out may seem like simple actions, but they are just part of the complex process of respiration. Respiration actually occurs in four steps: 1. 2. 3. 4. breathing (inhaling and exhaling) gas exchange between the air and blood gas transport by the blood gas exchange between the blood and cells "
the respiratory system,T_2217,"Breathing is the process of moving air into and out of the lungs. The process depends on a muscle called the diaphragm. This is a large, sheet-like muscle below the lungs. You can see it in Figure 19.2. Inhaling, or breathing in, occurs when the diaphragm contracts. This increases the size of the chest, which decreases air pressure inside the lungs. The difference in air pressure between the lungs and outside air causes air to rush into the lungs. Exhaling, or breathing out, occurs when the diaphragm relaxes. This decreases the size of the chest, which increases air pressure inside the lungs. The difference in air pressure between the lungs and outside air causes air to rush out of the lungs. When you inhale, air enters the respiratory system through your nose and ends up in your lungs, where gas exchange with the blood takes place. What happens to the air along the way? In the nose, mucus and hairs trap any dust or other particles in the air. The air is also warmed and moistened so it wont harm delicate tissues of the lungs. Next, air passes through the pharynx, a passageway that is shared with the digestive system. From the pharynx, the air passes next through the larynx, or voice box. After the larynx, air moves into the trachea, or wind pipe. This is a long tube that leads down to the lungs in the chest. In the chest, the trachea divides as it enters the lungs to form the right and left bronchi (bronchus, singular). These passages are covered with mucus and tiny hairs called cilia. The mucus traps any remaining particles in the air. The cilia move and sweep the particles and mucus toward the throat so they can be expelled from the body. Air passes from the bronchi into smaller passages called bronchioles. The bronchioles end in clusters of tiny air sacs called alveoli (alveolus, singular). "
the respiratory system,T_2218,"The alveoli in the lungs are where gas exchange between the air and blood takes place. Each alveolus is surrounded by a network of capillaries. When you inhale, air in the alveoli has a greater concentration of oxygen than does blood in the capillaries. The difference in oxygen concentration causes oxygen to diffuse from the air into the blood. You can see how this occurs in Figure 19.3. Unlike oxygen, carbon dioxide is more concentrated in the blood in the capillaries surrounding the alveoli than it is in the air inside the alveoli. Therefore, carbon dioxide diffuses in the opposite direction. It moves out of the blood and into the air. "
the respiratory system,T_2219,"After the blood in the capillaries in the lungs picks up oxygen, it leaves the lungs and travels to the heart. The heart pumps the oxygen-rich blood into arteries, which carry it throughout the body. The blood passes eventually into capillaries that supply body cells. "
the respiratory system,T_2220,"The cells of the body have a lower concentration of oxygen that does blood in the capillaries that supply body cells. Therefore, oxygen diffuses from the blood into the cells. Carbon dioxide, which cells produce in cellular respiration, is more concentrated in the cells. Therefore, carbon dioxide diffuses out of the cells and into the blood. The carbon dioxide travels in capillaries to veins and then to the heart. The heart pumps the blood to the lungs, where the carbon dioxide diffuses into the alveoli. It passes out of the body during exhalation. This brings the process of respiration full circle. "
the respiratory system,T_2221,"No doubt youve had the common cold. When you did, you probably had respiratory system symptoms. For example, you may have had a stuffy nose that made it hard to breathe. While you may feel miserable when you have a cold, it is generally a relatively mild disease. Many other respiratory system diseases are more serious. "
the respiratory system,T_2222,"Common diseases of the respiratory system include asthma, pneumonia, and emphysema. All of them are diseases of the lungs. You can see some of the changes in the lungs that occur in each of these diseases in Figure 19.4. Asthma is a disease in which bronchioles in the lungs periodically swell and fill with mucus. Symptoms of asthma may include difficulty breathing, wheezing, coughing, and chest tightness. An asthma attack may be triggered by allergies, strenuous exercise, stress, or another respiratory illness such as a cold. Pneumonia is a disease in which some of the alveoli of the lungs fill with fluid so they can no longer exchange gas. Symptoms of pneumonia typically include coughing, chest pain, difficulty breathing, and fatigue. Pneumonia may be caused by an infection or an injury to the lungs. Emphysema is a disease in which the walls of the alveoli break down so less gas can be exchanged by the lungs. The main symptom of emphysema is shortness of breath. The damage to the alveoli is usually caused by smoking and is permanent. "
the respiratory system,T_2223,"The main way to keep your respiratory system healthy is to avoid smoking or breathing in the smoke of others. Smoking causes, or makes you more susceptible to, many respiratory diseases, including asthma, bronchitis, em- physema, and lung cancer. Other steps you can take to keep your respiratory system healthy are listed below. Eat well, get enough sleep, and be active every day. These healthy lifestyle choices will help keep your immune system healthy so it can fight off respiratory infections and other diseases. Wash your hands often. This will reduce your risk of picking up viruses or bacteria that could make you sick with colds or other respiratory infections. Avoid contact with other people when they are sick and stay home when you are sick. These steps will help reduce the spread of infectious diseases. "
the excretory system,T_2224,"Excretion is any process in which excess water or wastes are removed from the body. Excretion is the job of the excretory system. Besides the kidneys, other organs of excretion include the large intestine, liver, skin and lungs. The large intestine eliminates food wastes that remain after digestion takes place. The liver removes excess amino acids and toxins from the blood. Sweat glands in the skin excrete excess water and salts in sweat. The lungs exhale carbon dioxide and also excess water as water vapor. Each of the above organs of excretion is also part of another body system. For example, the large intestine and liver are part of the digestive system, and the lungs are part of the respiratory system. The kidneys are the main organs of excretion. They are part of the urinary system. "
the excretory system,T_2225,"The urinary system is shown in Figure 19.6. It includes two kidneys, two ureters, the urinary bladder, and the urethra. The main function of the urinary system is to filter waste products and excess water from the blood and excrete them from the body as urine. For a visual presentation on the urinary system and how it works, watch this video:  . MEDIA Click image to the left or use the URL below. URL: "
the excretory system,T_2226,"The kidneys are a pair of bean-shaped organs at each side of the body just above the waist. You can see a diagram of a kidney in Figure 19.7. The function of the kidneys is to filter blood and form urine. Tiny structures in the kidneys, called nephrons, perform this function. Each kidney contains more than a million nephrons. "
the excretory system,T_2227,"Blood with wastes enters each kidney through an artery, which branches into many capillaries. After passing through capillaries and being filtered, the clean blood leaves the kidney through a vein. The part of each nephron called the glomerulus is where blood in the capillaries is filtered. Excess water and wastes are filtered out of the blood. The tubule of the nephron collects these substances. Some of the water is reabsorbed. The remaining fluid is urine. "
the excretory system,T_2228,"From the kidneys, urine enters the ureters. These are two muscular tubes that carry urine to the urinary bladder. Contractions of the muscles of the ureters move the urine along by peristalsis. The urinary bladder is a sac-like organ that stores urine. When the bladder is about half full, a sphincter relaxes to let urine flow out of the bladder and into the urethra. The urethra is a muscular tube that carries urine out of the body through another sphincter. The process of urine leaving the body is called urination. The second sphincter and the process of urination are normally under conscious control. "
the excretory system,T_2229,"The kidneys help the body maintain homeostasis in several ways. They filter all the blood in the body many times each day and produce urine. They control the amount of water and dissolved substances in the blood by excreting more or less of them in urine. The kidneys also secrete hormones that help maintain homeostasis. For example, they produce a hormone that stimulates bone marrow to produce red blood cells when more are needed. They also secrete a hormone that regulates blood pressure and keeps it in a normal range. "
the excretory system,T_2230,"You need only one kidney to live a normal, healthy life. A single kidney can do all the work of filtering the blood and maintaining homeostasis. However, at least one kidney must function properly to maintain life. Diseases that threaten the health and functioning of the kidneys include kidney stones, infections, and diabetes. You can learn more about kidney diseases in this video:  . MEDIA Click image to the left or use the URL below. URL:  Kidney stones are mineral crystals that form in urine inside a kidney, as shown in Figure 19.8. The stones may be extremely painful. If a kidney stone blocks a ureter, it must be removed so urine can leave the kidney and be excreted. Bacterial infections of urinary organs, especially the urinary bladder, are common. They are called urinary tract infections. Generally, they can be cured with antibiotic drugs. However, if they arent treated, they can lead to more serious infections and damage to the kidneys. Untreated diabetes may damage capillaries in the kidneys so the nephrons can no longer filter blood. This is called kidney failure. The only cure for kidney failure is to receive a healthy transplanted kidney from a donor. Until that happens, a patient with kidney failure can be kept alive by artificially filtering the blood through a machine. This is called hemodialysis. You can see how it works in Figure 19.9. "
chemistry of living things,T_2237,All known matter can be divided into a little more than 100 different substances called elements. 
chemistry of living things,T_2238,"An element is pure substance that cannot be broken down into other substances. Each element has a particular set of properties that, taken together, distinguish it from all other elements. Table 2.1 lists the major elements in the human body. As you can see, you consist mainly of the elements oxygen, carbon, and hydrogen. Element Oxygen Carbon Hydrogen Nitrogen Calcium Phosphorus Potassium Sulfur Percent of Body Mass 65 18 10 3 1.5 1.0 0.35 0.25 In your body, most elements are combined with other elements to form chemical compounds. A compound is a unique type of matter in which two or more elements are combined chemically in a certain ratio. For example, much of the oxygen and hydrogen in your body are combined in the chemical compound water, or H2O. "
chemistry of living things,T_2239,"The smallest particle of an element that still has the properties of that element is an atom. Atoms are extremely tiny. They can be observed only with an electron microscope. They are commonly represented by models, like the one Figure 2.6. An atom has a central nucleus that is positive in charge. The nucleus is surrounded by negatively charged particles called electrons. The smallest particle of a compound that still has the properties of that compound is a molecule. A molecule consists of two or more atoms. For example, a molecule of water consists of two atoms of hydrogen and one atom of oxygen. Thats why the chemical formula for water is H2 O. You can see a simple model of a water molecule in Figure 2.7. "
chemistry of living things,T_2240,"Besides water, most of the compounds in living things are biochemical compounds. A biochemical compound is a carbon-based compound that is found in living organisms. Carbon is an element that has a tremendous ability to form large compounds. Each atom of carbon can form four chemical bonds with other atoms. A chemical bond is the sharing of electrons between atoms. Bonds hold the atoms together in chemical compounds. A carbon atom can form bonds with other carbon atoms or with atoms of other elements. "
chemistry of living things,T_2241,"Biochemical compounds make up the cells and tissues of living things. They are also involved in all life processes. Given their diversity of functions, its not surprising that there are millions of different biochemical compounds. Even so, all biochemical compounds can be grouped into just four main classes: carbohydrates, proteins, lipids, and nucleic acids. The classes are summarized in Table 2.1. Class Elements Examples Functions Class Carbohydrates Elements carbon hydrogen oxygen Examples sugars starch glycogen cellulose Proteins carbon hydrogen oxygen nitrogen sulfur carbon hydrogen oxygen carbon hydrogen oxygen nitrogen phosphorus enzymes hormones Lipids Nucleic acids fats oils phospholipids DNA RNA Functions provide energy to cells stores energy in plants stores energy in animals makes up the cell walls of plants speed up biochemical re- actions regulate life processes store energy in animals store energy in plants make up cell membranes stores genetic information in cells helps cells make proteins "
chemistry of living things,T_2242,"You can see from Table 2.1 that all biochemical compounds contain hydrogen and oxygen as well as carbon. They may also contain nitrogen, phosphorus, and/or sulfur. Almost all biochemical compounds are polymers. Polymers are large molecules that consist of many smaller, repeating molecules, called monomers. Most biochemical molecules are macromolecules. The prefix macro- means large, and many biochemical molecules are very large indeed. They may contain thousands of monomer molecules. The largest known biochemical molecule contains more than 34,000 monomers! "
chemistry of living things,T_2243,"Carbohydrates are biochemical compounds that include sugar, starch, glycogen, and cellulose. Sugars are simple carbohydrates with relatively small molecules. Glucose is the smallest of all the sugar molecules with its chemical formula of C6 H12 O6 . This means that a molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen. Plants and some other organisms make glucose in the process of photosynthesis. Living things that cannot make glucose can obtain it by consuming plants or organisms that consume plants. Starches are complex carbohydrates. They are polymers of glucose. Starches contain hundreds of glucose monomers. Plants make starches to store extra glucose. Consumers can get starches by eating plants. Common sources of starches in the human diet are pictured in the Figure 2.8. Our digestive system breaks down starches to sugar, which our cells use for energy. Like other animals, we store any extra glucose as the complex carbohydrate called glycogen. Glycogen is also a polymer of glucose. Cellulose is another complex carbohydrate found in plants that is a polymer of glucose. Cellulose molecules bundle together to form long, tough fibers. Cellulose is the most abundant biochemical compound. It makes up the cell walls of plants and gives support to stems and tree trunks. "
chemistry of living things,T_2244,"Proteins are biochemical compounds that consist of one or more chains of small molecules called amino acids. Amino acids are the monomers of proteins. There are only about 20 different amino acids. The sequence of amino acids in chains and the number of chains in a protein determine the proteins shape. Shapes may be very complex. You can learn more about the shapes of proteins at this link: MEDIA Click image to the left or use the URL below. URL:  The shape of a protein determines its function. Proteins have many different functions. For example, proteins: make up muscle tissues. speed up chemical reactions in cells. regulate life processes. help defend against infections. 2.2. Chemistry of Living Things transport materials around the body in the blood. blood How hemoglobin transports oxygen in the "
chemistry of living things,T_2245,"Lipids are biochemical compounds that living things use to store energy and make cell membranes. Types of lipids include fats, oils, and phospholipids. Fats are solid lipids that animals use to store energy. Examples of fats include butter and the fat in meat. Oils are liquid lipids that plants use to store energy. Examples of oils include olive oil and corn oil. Phospholipids contain the element phosphorus. They make up the cell membranes of living things. Lipids are made of long chains consisting almost solely of carbon and hydrogen. These long chains are called fatty acids. Fatty acids may be saturated or unsaturated. The Figure 2.10 shows an example of each type of fatty acid. "
chemistry of living things,T_2246,"Nucleic acids are biochemical compounds that include RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). Nucleic acids consist of chains of small molecules called nucleotides. Nucleotides are the monomers of nucleic 40 acids. A nucleotide is shown in Figure 2.11. Each nucleotide consists of: 1. a phosphate group, which contains phosphorus and oxygen. 2. a sugar, which is deoxyribose in DNA and ribose in RNA. 3. one of four nitrogen-containing bases. (A base is a compound that is not neither acidic nor neutral.) In DNA, the bases are adenine, thymine, guanine, and cytosine. RNA has the base uracil instead of thymine, but the other three bases are the same. RNA consists of just one chain of nucleotides. DNA consists of two chains. Nitrogen bases on the two chains of DNA form bonds with each other. The bonded bases are called base pairs. Bonds form only between adenine and thymine, and between guanine and cytosine. They hold together the two chains of DNA and give it its characteristic double helix, or spiral, shape. You can see the shape of the DNA molecule in Figure 2.12. Sugars and phosphate groups form the backbone of each chain of DNA. Determining the structure of DNA was a huge scientific breakthrough. You can read the interesting story of its discovery and why it was so important at this link: DNA stores genetic information in the cells of all living things. It contains the genetic code. This is the code that instructs cells how to make proteins. The instructions are encoded in the sequence of nitrogen bases in DNAs nucleotide chains. RNA copies and interprets the genetic code in DNA. RNA is also involved in the synthesis of proteins based on the code. You can watch these events unfolding at this link: MEDIA Click image to the left or use the URL below. URL: "
chemistry of living things,T_2247,"The student athlete in Figure 2.13 is practically flying down the track! Running takes a lot of energy. But you dont have to run a race to use energy. All living things need energy all the time just to stay alive. The energy is produced in chemical reactions. A chemical reaction is a process in which some substances, called reactants, change chemically into different substances, called products. Reactants and products may be elements or compounds. Chemical reactions that take place inside living things are called biochemical reactions. Living things depend on biochemical reactions for more than just energy. Every function and structure of a living organism depends on thousands of biochemical reactions taking place in each cell. "
chemistry of living things,T_2248,"The sum of all of an organisms biochemical reactions is called metabolism. Biochemical reactions of metabolism can be divided into two general categories: catabolic reactions and anabolic reactions. You can watch an animation showing how the two categories of reactions are related at this link: Anabolic reactions involve forming bonds. Smaller molecules combine to form larger ones. These reactions require energy. For example, it takes energy to build starches from sugars. Catabolic reactions involve breaking bonds. Larger molecules break down to form smaller ones. These reactions release energy. For example, energy is released when starches break down to sugars. "
chemistry of living things,T_2249,"Each of the trillions of cells in your body is continuously performing thousands of anabolic and catabolic reactions. Thats an amazing number of biochemical reactionsfar more than the number of chemical reactions that might take place in a science lab or chemical plant. So many biochemical reactions can take place simultaneously in our cells because biochemical reactions occur very quickly. Thats because of enzymes. Enzymes are proteins that increase the rate of biochemical reactions. Enzymes arent changed or used up in the reactions, so they can be used to speed up the same reaction over and over again. Enzymes are highly specific for certain chemical reactions, so they are very effective. A reaction that would take years to occur without its enzyme might occur in a split second with the enzyme. "
chemistry of living things,T_2250,Some of the most important biochemical reactions are the reactions involved in photosynthesis and cellular respira- tion. Photosynthesis is the process in which producers capture light energy from the sun and use it to make glucose. This involves anabolic reactions. Cellular respiration is the process in which energy is released from glucose and stored in smaller amounts in other molecules that cells can use for energy. This involves catabolic reactions. Photosynthesis and cellular respiration together provide energy to almost all living cells. Figure 2.14 shows how photosynthesis and cellular respiration are related. You can read more about both processes in the chapter Cell Functions. 
the nervous system,T_2257,"Controlling muscles and maintaining balance are just two of the functions of the human nervous system. What else does the nervous system do? It senses the surrounding environment with sense organs that include the eyes and ears. It senses the bodys own internal environment, including its temperature. It controls internal body systems to make sure the body maintains homeostasis. It prepares the body to fight or flee in the case of an emergency. It allows thinking, learning, memory, and language. Remember Hakeem the skater from the first page of the chapter? When Hakeem started to fall off the railing, his nervous system sensed that he was losing his balance. It responded by sending messages to his muscles. Some muscles contracted while other relaxed. As a result, Hakeem gained his balance again. How did his nervous system accomplish all of this in just a split second? You need to know how the nervous system transmits messages to answer that question. "
the nervous system,T_2258,"The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages is called a neuron. The messages carried by neurons are called nerve impulses. A nerve impulse can travel very quickly because it is an electrical signal. Think about flipping on a light switch when you enter a room. When you flip the switch, electricity flows to the light through wires inside the walls. The electricity may have to travel many meters to reach the light. Nonetheless, the light still comes on as soon as you flip the switch. Nerve impulses travel just as quickly through the network of nerves inside the body. "
the nervous system,T_2259,"The structure of a neuron suits it for its function of transmitting nerve impulses. You can see what a neuron looks like in Figure 20.2. It has a special shape that lets it pass electrical signals to and from other cells. A neuron has three main parts: cell body, dendrites, and axon. 1. The cell body contains the nucleus and other organelles. 2. Dendrites receive nerve impulses from other cells. A single neuron may have thousands of dendrites. 3. The axon passes on the nerve impulses to other cells. It branches at the end into multiple nerve endings so it can transmit impulses to many other cells. "
the nervous system,T_2260,"There are three basic types of neurons: sensory neurons, motor neurons, and interneurons. All three types must work together to receive and respond to information. 1. Sensory neurons transmit nerve impulses from sense organs and internal organs to the brain via the spinal cord. In other words, they carry information about the inside and outside environment to the brain. 2. Motor neurons transmit nerve impulses from the brain via the spinal cord to internal organs, glands, and muscles. In other words, they carry information from the brain to the body, telling the body how to respond. 3. Interneurons carry nerve impulses back and forth between sensory and motor neurons. "
the nervous system,T_2261,"The nerve endings of an axon dont actually touch the dendrites of other neurons. The messages must cross a tiny gap between the two neurons, called the synapse. Chemicals called neurotransmitters carry the message across this gap. When a nerve impulse arrives at the end of an axon, neurotransmitters are released. They travel across the synaptic gap to a dendrite of another neuron. The neurotransmitters bind to the membrane of the dendrite, triggering a nerve impulse in the next neuron. You can see how this works in Figure 20.3 and in this animation:  The transmission of nerve impulses between neurons is like the passing of a baton between runners in a relay race. After the first runner races, she passes the baton to the second runner. Then the second runner takes over. Instead of a baton, a neuron passes neurotransmitters to the next neuron. "
the nervous system,T_2262,"The nervous system has two main parts, called the central nervous system and the peripheral nervous system. The peripheral nervous system is described later in this lesson. The central nervous system is shown in Figure 20.4. It includes the brain and spinal cord. "
the nervous system,T_2263,"The human brain is an amazing organ. It is the most complex organ in the human body. By adulthood, the brain weighs about 3 pounds and consists of billions of neurons. All those cells need a lot of energy. In fact, the adult brain uses almost a quarter of the total energy used by the body! The brain serves as the control center of the nervous system and the body as a whole. It lets us understand what we see, hear, or sense in other ways. It allows us to learn, think, remember, and use language. It controls all the organs and muscles in our body. "
the nervous system,T_2264,"The brain consists of three major parts, called the cerebrum, cerebellum, and brain stem. You can see these three parts of the brain in Figure 20.5. You can use this interactive animation to explore these parts of the brain: http://s 1. The cerebrum is the largest part of the brain. It controls conscious functions, such as thinking, sensing, speaking, and voluntary muscle movements. Whether you are chatting with a friend or playing a video game, you are using your cerebrum. 2. The cerebellum is the next largest part of the brain. It controls body position, coordination, and balance. Hakeems cerebellum kicked in when he started to lose his balance on the railing in the opening photo. It allowed him to regain his balance. 3. The brain stem (also called the medulla) is the smallest part of the brain. It controls involuntary body functions such as breathing, heartbeat, and digestion. It also carries nerve impulses back and forth between the rest of the brain and the spinal cord. "
the nervous system,T_2265,"The cerebrum is divided down the middle from the front to the back of the head. The two halves of the cerebrum are called the right and left hemispheres. The two hemispheres are very similar but not identical. They are connected to each other by a thick bundle of axons deep within the brain. These axons allow the two hemispheres to communicate with each other. Did you know that the right hemisphere of the cerebrum controls the left side of the body, and vice versa? This can happen because of the connections between the two hemispheres. Each hemisphere is further divided into four parts, called lobes, as you can see in Figure 20.6. Each lobe has different functions. One function of each lobe is listed in the figure. "
the nervous system,T_2266,"The spinal cord is a long, tube-shaped bundle of neurons. It runs from the brain stem to the lower back. The main job of the spinal cord is to carry nerve impulses back and forth between the body and brain. The spinal cord is like a two-way road. Messages about the body, both inside and out, pass through the spinal cord to the brain. Messages from the brain pass in the other direction through the spinal cord to tell the body what to do. "
the nervous system,T_2267,"All the other nervous tissues in the body are part of the peripheral nervous system. If you look again at Figure 20.1, you can see the major nerves of the peripheral nervous system. They include nerves that run through virtually every part of the body, both inside and out, except for the brain and spinal cord. The peripheral nervous system has two main divisions: the sensory division and the motor division. The divisions carry messages in opposite directions. Figure 20.7 shows these divisions of the peripheral nervous system. "
the nervous system,T_2268,"The sensory division of the peripheral nervous system carries messages from sense organs and internal organs to the central nervous system. For example, it carries messages about images from the eyes to the brain. Once the messages reach the brain, the brain interprets the information. "
the nervous system,T_2269,"The motor division of the peripheral nervous system carries messages from the central nervous system to muscles, internal organs, and glands throughout the body. The brain sends commands to these tissues, telling them how to respond. As you can see in Figure 20.7, the motor division is divided into additional parts. The autonomic part of the motor division controls involuntary responses. It sends messages to organs and glands. These messages control the body both during emergencies (sympathetic division) and during none- mergencies (parasympathetic division). The somatic part of the motor division controls voluntary responses. It sends messages to the skeletal muscles for movements that are under conscious control. "
the nervous system,T_2270,Nervous system problems include diseases and injuries. Most nervous system diseases cant be prevented. But you can take steps to decrease your risk of nervous system injuries. 
the nervous system,T_2271,"Bacteria and viruses can infect the brain or spinal cord. An infection of the brain is called encephalitis. An infection of the membranes that cover the brain and spinal cord is called meningitis. A vaccine is available to prevent meningitis caused by viruses (see Figure 20.8). Encephalitis and meningitis arent very common, but they can be extremely serious. They may cause swelling of the brain, which can be fatal. Thats why its important to know the symptoms of these diseases. Both encephalitis and meningitis typically cause a severe headache and a fever. Meningitis also causes a stiff neck. Both require emergency medical treatment. "
the nervous system,T_2272,"Epilepsy is a disease in which seizures occur. A seizure is a period of lost consciousness that may include violent muscle contractions. It is caused by abnormal electrical activity in the brain. Epilepsy may result from an infection, injury, or tumor. In many cases, however, the cause cant be identified. There is no known cure for epilepsy, but the seizures often can be prevented with medicine. Sometimes children with epilepsy outgrow it by adulthood. "
the nervous system,T_2273,"A stroke occurs when a blood clot blocks blood flow to part of the brain. Brain cells die quickly when their oxygen supply is cut off. Therefore, a stroke may cause permanent loss of normal mental functions. Many stroke patients suffer some degree of paralysis, or loss of the ability to feel or move certain parts of the body. If medical treatment is given very soon after a stroke occurs, some of the damage may be reversed. Strokes occur mainly in older adults. "
the nervous system,T_2274,"Alzheimers disease is another disease that occurs mainly in older adults. In Alzheimers disease, a person gradually loses most normal mental functions. The patient typically suffers from increasing memory loss, confusion, and mood swings. The cause of Alzheimers isnt known for certain, but it appears to be associated with certain abnormal changes in the brain. There is no known cure for this devastating disease, but medicines may be able to slow its progression. "
the nervous system,T_2275,"The brain and spinal cord are protected within bones of the skeletal system, but injuries to these organs still occur. With mild injuries, there may be no lasting effects. With severe injuries, there may be permanent disability or even death. Brain and spinal cord injuries most commonly occur because of car crashes or athletic activities. Fortunately, many injuries can be prevented by wearing seat belts and safety helmets (see Figure 20.9). Avoiding unnecessary risks, such as doing stunts on a bike or diving into shallow water, can also reduce the chances of brain and spinal cord injuries. The most common type of brain injury is a concussion. This is a bruise on the surface of the brain. It may cause temporary symptoms such as headache and confusion. Most concussions heal on their own in a few days or weeks. However, repeated concussions can lead to permanent changes in the brain. More serious brain injuries also often cause permanent brain damage. Spinal cord injuries may cause paralysis. Some people recover from spinal cord injuries. However, many people remain paralyzed for life. This happens when the spinal cord can no longer transmit nerve impulses between the body and brain. "
the nervous system,T_2276,"A drug is any chemical substance that affects the body or brain. Some drugs are medicines. Although these drugs are helpful when used properly, they can be misused like any other drug. Drugs that arent medicines include both legal and illegal drugs. Both can do harm. "
the nervous system,T_2277,"Many drugs affect the brain and influence how a person feels, thinks, or acts. Such drugs are called psychoactive drugs. They include legal drugs such as caffeine and alcohol, as well as illegal drugs such as cocaine and heroin. They also include certain medicines, such as antidepressant drugs and medical marijuana. Some psychoactive drugs, such as caffeine, stimulate the central nervous system. They may make the user feel more alert. Some psychoactive drugs, such as alcohol, depress the central nervous system. They may make the user feel more relaxed. Still other psychoactive drugs, such as marijuana, are hallucinogenic drugs. They may make the user have altered sensations, perceptions, or thoughts. "
the nervous system,T_2278,"Psychoactive drugs may bring about changes in mood that users find desirable. These drugs may be abused. Drug abuse is use of a drug without the advice of a medical professional and for reasons not originally intended. Continued use of a psychoactive drug may lead to drug addiction. This occurs when a drug user is unable to stop using the drug. Over time, a drug user may need more of the drug to get the desired effect. This can lead to drug overdose and death. "
the senses,T_2279,"The ability to see is called vision. It depends on both the eyes and the brain. The eyes sense light and form images. The brain interprets the images formed by the eyes and tells us what we are seeing. For a fascinating account of how the brain helps us see, watch this short video:  . MEDIA Click image to the left or use the URL below. URL: "
the senses,T_2280,"Did you ever use 3-D glasses to watch a movie, like the teens in Figure 20.11? If you did, then you know that the glasses make images on the flat screen seem more realistic by giving them depth. The images seem to jump right out of the screen toward you. Unlike many other animals, human beings and other primates normally see the world around them in three dimen- sions. Thats because we have two eyes that face the same direction but are a few inches apart. Both eyes focus on the same object at the same time but from slightly different angles. The brain uses the different images from the two eyes to determine the distance to the object. Human beings and other primates also have the ability to see in color. We have special cells inside our eyes that can distinguish different wavelengths of visible light. Visible light is light in the range of wavelengths that the human eye can sense. The exact wavelength of visible light determines its color. "
the senses,T_2281,"The function of the eye is to focus light and form images. We see some objects, such as stars and light bulbs, because they give off their own light. However, we see most objects because they reflect light from another source such as the sun. We form images of the objects when some of the reflected light enters our eyes. Look at the parts of the eye in Figure 20.12. Follow the path of light through the eye as you read about it below. 1. Light from an object passes first through the cornea. This is a clear, protective covering on the outside of the eye. 2. Then light passes through the pupil, an opening in the center of the eye. The pupil, which looks black, is surrounded by the colored part of the eye, called the iris. 3. Light entering through the pupil next passes through the lens. The lens is a clear, curved structure, like the lens of a magnifying glass. Along with the cornea, the lens focuses the light on the back of the eye. 4. The back of the eye is covered by a thin layer called the retina. This is where the image of the object normally forms. The retina consists of special light-sensing cells called rods and cones. Rods sense dim light. Cones sense different colors of light. 5. Nerve impulses from rods and cones travel to the optic nerve. It carries the nerve impulses to the brain. "
the senses,T_2282,"You probably know people who need eyeglasses or contact lenses to see clearly. Maybe you need them yourself. Lenses are used to correct vision problems. Two of the most common vision problems in young people are myopia and hyperopia. You can compare myopia and hyperopia in Figure 20.13. To learn about astigmatism, another common vision problem, watch this very short video:  . MEDIA Click image to the left or use the URL below. URL:  Myopia is commonly called nearsightedness. People with myopia can see nearby objects clearly, but distant objects appear blurry. Myopia occurs when images focus in front of the retina because the eyeball is too long. This vision problem can be corrected with concave lenses, which curve inward. The lenses focus images correctly on the retina. Hyperopia is commonly called farsightedness. People with hyperopia can see distant objects clearly, but nearby objects appear blurry. Hyperopia occurs when images focus in back of the retina because the eyeball is too short. This vision problem can be corrected with convex lenses, which curve outward. The lenses focus images correctly on the retina. "
the senses,T_2283,"Vision is just one of several human senses. Other human senses include hearing, touch, taste, and smell. Imagine shopping at the fruit market in Figure 20.14. It would stimulate all of these senses. You would hear the noisy bustle of the market. You could feel the smooth skin of the fruit. If you tried a sample, you could smell the fruity aroma and taste its sweet flavor. "
the senses,T_2284,"What do listening to music and riding a bike have in common? Both activities depend on the ears. The ears are organs that sense sound. They also sense the position of the body and help maintain balance. Hearing is the ability to sense sound. Sound travels through the air in waves. Suppose a car horn blows in the distance. Sound waves spread through the air from the horn. Some of the sound waves enter your ears and cause vibrations. The vibrations trigger nerve impulses that travel to the brain through the auditory nerve. You can learn how this happens in Figure 20.15. The brain then interprets the impulses and tells you what you are hearing. To find out how the brain determines where a sound is coming from, watch this amusing video:  MEDIA Click image to the left or use the URL below. URL:  The parts of the ears involved in balance are the semicircular canals. These are the curved structures above the cochlea in the inner ear in Figure 20.15. Like the cochlea, the semicircular canals contain liquid and are lined with tiny hair cells. As the head changes position, the liquid moves. This causes the hair cells to bend. The bending of the hair cells triggers nerve impulses that travel to the cerebellum in the brain. The cerebellum uses the information to maintain balance. "
the senses,T_2285,"Touch is the ability to sense pain, pressure, or temperature. Nerve cells that sense touch are found mainly in the skin. The skin on the palms, soles, face, and lips has the most neurons. Neurons that sense pain are also found inside the body inside the body in the tongue, joints, muscles, and other organs. Suppose you wanted to test the temperature of bath water before getting into the tub. You might stick one toe in the water. Neurons in the skin on your toe would sense the temperature of the water and send a message about it to the brain through the spinal cord. The brain would process the information. It might decide that the water is too hot and send a message to your muscles to pull your toe out of the water. "
the senses,T_2286,"The sense of taste is controlled by sensory neurons on the tongue. They are grouped in bundles called taste buds. You can see taste buds on the tongue in Figure 20.16. Taste neurons sense chemicals in food. They can detect five different tastes: sweet, salty, sour, bitter, and umami, which is a meaty taste. When taste neurons sense chemicals, they send messages to the brain about them. The brain then decides what you are tasting. The sense of smell also involves sensory neurons that sense chemicals. These neurons are found in the nose, and they sense chemicals in the air. Unlike taste neurons, smell neurons can detect thousands of different odors. Your sense of smell plays a big role in your sense of taste. You can use your sense of taste alone to learn that a food is sweet. However, you have to use your sense of smell as well to learn that the food tastes like apple pie. "
the endocrine system,T_2287,"The endocrine system is a system of glands that release chemical messenger molecules into the blood stream. The messenger molecules are called hormones. Hormones act slowly compared with the rapid transmission of electrical impulses of the nervous system. Endocrine hormones must travel through the bloodstream to the cells they control, and this takes time. On the other hand, because endocrine hormones are released into the bloodstream, they travel to cells everywhere in the body. For a good visual introduction to the endocrine system, watch this short video: http MEDIA Click image to the left or use the URL below. URL: "
the endocrine system,T_2288,"An endocrine gland is a gland that secretes hormones into the bloodstream for transport around the body (instead of secreting hormones locally, like sweat glands in the skin). Major glands of the endocrine system are shown in Figure 20.17. The glands are the same in males and females except for the ovaries and testes. "
the endocrine system,T_2289,"The hypothalamus is actually part of the brain, but it also secretes hormones. Some of its hormones go directly to the pituitary gland in the endocrine system. These hypothalamus hormones tell the pituitary to either secrete or stop secreting its hormones. In this way, the hypothalamus provides a link between the nervous and endocrine systems. The hypothalamus also produces hormones that directly regulate body processes. For example, it produces antid- iuretic hormone. This hormone travels to the kidneys and stimulates them to conserve water by producing more concentrated urine. "
the endocrine system,T_2290,The pea-sized pituitary gland is just below the hypothalamus and attached directly to it. The pituitary receives hormones from the hypothalamus. It also secretes its own hormones. Most pituitary hormones control other endocrine glands. Thats why the pituitary gland is called the master gland of the endocrine system. Table Pituitary Hormone Adrenocorticotropic (ACTH) hormone Target Glands/Cells adrenal glands Thyroid-stimulating (TSH) Growth hormone (GH) hormone thyroid gland Follicle-stimulating (FSH) hormone body cells ovaries or testes Luteinizing hormone (LH) ovaries or testes Prolactin (PRL) mammary glands Effects(s) Stimulates the cortex (outer layer) of the adrenal glands to secrete their hormones Stimulates the thyroid gland to se- crete its hormones Stimulates body cells to make pro- teins and grow Stimulates the ovaries to develop mature eggs; stimulates the testes to produce sperm Stimulates the ovaries or testes to secrete sex hormones; stimulates the ovaries to release eggs Stimulates the mammary glands to produce milk 
the endocrine system,T_2291,"There are several other endocrine glands. Find them in Figure 20.17 as you read about them below. The thyroid gland is a relatively large gland in the neck. Hormones secreted by the thyroid gland include thyroxin. Thyroxin increases the rate of metabolism in cells throughout the body. The pancreas is a large gland located near the stomach. Hormones secreted by the pancreas include insulin. Insulin helps cells absorb glucose from the blood. It also stimulates the liver to take up and store excess glucose. The two adrenal glands are glands located just above the kidneys. Each adrenal gland has an outer layer (cortex) and inner layer (medulla) that secrete different hormones. The hormone adrenaline is secreted by the inner layer. It prepares the body to respond to emergencies. For example, it increases the amount of oxygen and glucose going to the muscles. The gonads are glands that secrete sex hormones. Male gonads are called testes. They secrete the male sex hormone testosterone. The female gonads are called ovaries. They secrete the female sex hormone estrogen. Sex hormones stimulate the changes of puberty. They also control the production of sperm or eggs by the gonads. "
the endocrine system,T_2292,"Endocrine hormones travel throughout the body in the blood. However, each endocrine hormone affects only certain cells, called target cells. "
the endocrine system,T_2293,"A target cell is the type of cell on which a given endocrine hormone has an effect. A target cell is affected by a given hormone because it has proteins on its surface to which the hormone can bind. When the hormone binds to target cell proteins, it causes changes inside the cell. For example, binding of the hormone might cause the release of enzymes inside the cell. The enzymes then influence cell processes. "
the endocrine system,T_2294,"Endocrine hormones control many cell activities, so they are very important for homeostasis. But what controls the hormones? Most endocrine hormones are controlled by feedback loops. In a feedback loop, the hormone produced by a gland feeds back to control its own production by the gland. A feedback loop can be negative or positive. Most endocrine hormones are controlled by negative feedback loops. .Negative feedback occurs when rising levels of a hormone feed back to decrease secretion of the hormone or when falling levels of the hormone feed back to increase its secretion. You can see an example of a negative feedback loop in Figure 20.18. It shows how levels of thyroid hormones regulate the thyroid gland. This loop involves the hypothalamus and pituitary gland as well as the thyroid gland. Low levels of thyroid hormones in the blood cause the release of hormones by the hypothalamus and pituitary gland. These hormones stimulate the thyroid gland to secrete more hormones. The opposite happens with high levels of thyroid hormones in the blood. The hypothalamus and pituitary gland stop releasing hormones that stimulate the thyroid. "
the endocrine system,T_2295,"Diseases of the endocrine system are fairly common. An endocrine disease usually involves the secretion of too much or not enough hormone by an endocrine gland. This may happen because the gland develops an abnormal lump of cells called a tumor. For example, a tumor of the pituitary gland can cause secretion of too much growth hormone. If this occurs in a child, it may result in very rapid growth and unusual tallness by adulthood. This is called gigantism. Type 1 diabetes is another endocrine system disease. In this disease, the bodys own immune system attacks insulin- secreting cells of the pancreas. As a result, not enough insulin is secreted to maintain normal levels of glucose in the blood. Patients with type 1 diabetes must regularly check the level of glucose in their blood. When it gets too high, they must give themselves an injection of insulin to bring it under control. You can learn more about glucose, insulin, and type 1 diabetes by watching this video:  . MEDIA Click image to the left or use the URL below. URL: "
infectious diseases,T_2296,An infectious disease is a disease that is caused by a pathogen. A pathogen is an organism or virus that causes disease in another living thing. Pathogens are commonly called germs. Watch this dramatic video for an historic perspective on infectious diseases and their causes:  . MEDIA Click image to the left or use the URL below. URL: 
infectious diseases,T_2297,"There are several types of pathogens that cause diseases in human beings. They include bacteria, viruses, fungi, and protozoa. The different types are described in Table 21.1. The table also lists several diseases caused by each type of pathogen. Many infectious diseases caused by these pathogens can be cured with medicines. For example, antibiotic drugs can cure most diseases caused by bacteria. "
infectious diseases,T_2298,"Different pathogens spread in different ways. Some are easy to catch. Others are much less contagious. Some pathogens spread through food or water. When harmful bacteria contaminate food, they cause foodborne illness, commonly called food poisoning. An example of a pathogen that spreads through water is the protozoan named Giardia lamblia, described in Table 21.1. It causes a disease called giardiasis. Some pathogens spread through sexual contact. In the U.S., the pathogen most commonly spread this way is HPV, or human papillomavirus. It may cause genital warts and certain types of cancer. A vaccine can prevent the spread of this pathogen. Many pathogens spread by droplets in the air. Droplets are released when a person coughs or sneezes, as you can see in Figure 21.2. The droplets may be loaded with pathogens. Other people may get sick if they breathe in the pathogens on the droplets. Viruses that cause colds and flu can spread this way. Other pathogens spread when they are deposited on objects or surfaces. The fungus that causes athletes food spreads this way. For example, you might pick up the fungus from the floor of a public shower. You can also pick up viruses for colds and flu from doorknobs and other commonly touched surfaces. Still other pathogens are spread by vectors. A vector is an organism that carries pathogens from one person or animal to another. Most vectors are insects such as ticks or mosquitoes. They pick up pathogens when they bite an infected animal and then transmit the pathogens to the next animal they bite. Ticks spread the bacteria that cause Lyme disease. Mosquitoes spread the protozoa that cause malaria. "
infectious diseases,T_2299,"What can you do to avoid infectious diseases? Eating well and getting plenty of sleep are a good start. These habits will help keep your immune system healthy. With a healthy immune system, you will be able to fight off many pathogens. Vaccines are available for some infectious diseases. For example, there are vaccines to prevent measles, mumps, whooping cough, and chicken pox. These vaccines are recommended for infants and young children. You can also take the following steps to avoid picking up pathogens or spreading them to others. Watch this video for additional information on preventing the spread of infectious diseases:  MEDIA Click image to the left or use the URL below. URL:  Wash your hands often with soap and water. Spend at least 20 seconds scrubbing with soap. See Figure 21.3 for effective hand washing tips. Avoid touching your eyes, nose, or mouth with unwashed hands. Avoid close contact with people who are sick. This includes kissing, hugging, shaking hands, and sharing cups or eating utensils. Cover your coughs and sneezes with a tissue or shirt sleeve, not your hands. Disinfect frequently touched surfaces, such as keyboards and doorknobs, especially if someone is sick. Stay home when you are sick. The best way to prevent diseases spread by vectors is to avoid contact with the vectors. For example, you can wear long sleeves and long pants to avoid tick and mosquito bites. Using insect repellent can also reduce your risk of insect bites. "
noninfectious diseases,T_2300,"Cancer is a disease in which cells divide out of control. Normally, the body has ways to prevent cells from dividing out of control. However, in the case of cancer, these ways fail. The rapidly dividing cells may form a mass of abnormal tissue called a tumor. This is illustrated in Figure 21.4. Watch this video for an animated introduction to cancer:  . MEDIA Click image to the left or use the URL below. URL:  As a tumor increases in size, it may harm normal tissues around it. Sometimes cancer cells break away from a tumor. If they enter the bloodstream, they are carried throughout the body. Then the cells may start growing in other tissues. This is usually how cancer spreads from one part of the body to another. Once this happens, cancer is very hard to stop. "
noninfectious diseases,T_2301,"Most cancers are caused by mutations. Mutations are random errors in genes. Mutations that lead to cancer usually occur in genes that control the cell cycle. Because of the mutations, abnormal cells are allowed to divide. Some mutations that lead to cancer may be inherited. However, most of the mutations are caused by environmental factors. Anything in the environment that can cause cancer is called a carcinogen. Common carcinogens include certain chemicals and some types of radiation. Many different chemicals can cause cancer. For example, tobacco contains dozens of chemicals, including nicotine, that have been shown to cause cancer. Figure 21.5 shows some of these chemicals. Smoking tobacco or using smokeless tobacco increases the risk of cancer of the lung, mouth, throat, and urinary bladder. Types of radiation that cause cancer include ultraviolet (UV) radiation and radon. UV radiation is part of sunlight. It is the leading cause of skin cancer. Radon is a naturally occurring radioactive gas that escapes from underground rocks. It may seep into the basements of buildings. It can cause lung cancer. "
noninfectious diseases,T_2302,"Cancer occurs most often in adults, especially adults over the age of 50. The most common types of cancer in adults differ between males and females. The most common type of cancer in adult males is cancer of the prostate gland. The prostate gland is part of the male reproductive system. About one third of all cancers in men are prostate cancers. The most common type of cancer in adult females is cancer of the breast. About one third of all cancers in women are breast cancers. In both men and women, the second most common type of cancer is lung cancer. Most cases of lung cancer develop in people who smoke. Childhood cancer is rare. The main type of cancer in children is leukemia. It makes up about one third of all childhood cancers. It occurs when the body makes abnormal white blood cells. "
noninfectious diseases,T_2303,"Many cases of cancer can be cured if the cancer is diagnosed and treated early. Treatment often involves removing a tumor with surgery. This may be followed by other types of treatments. These treatments may include drugs and radiation, both of which target and kill cancer cells. Its important to know the warning signs of cancer so it can be diagnosed as early as possible. Having warning signs doesnt mean that you have cancer, but you should check with a doctor to be sure. Warning signs of cancer include: a change in bowel or bladder habits. a sore that doesnt heal. unusual bleeding or discharge. a lump in the breast or elsewhere. frequent, long-term indigestion. difficulty swallowing. obvious changes in a wart or mole. persistent cough or hoarseness. "
noninfectious diseases,T_2304,"Making healthy lifestyle choices can help prevent some types of cancer. For example, you can reduce your risk of lung cancer by not smoking. You can reduce your risk of skin cancer by using sunscreen (see Figure 21.6). "
noninfectious diseases,T_2305,"Diabetes is another type of noninfectious disease. Diabetes occurs when the pancreas doesnt make enough insulin or else the bodys cells are resistant to the effects of insulin. Insulin is a hormone that helps cells absorb glucose from the blood. When there is too little insulin or cells do not respond to it, the blood contains too much glucose. High glucose levels in the blood can damage blood vessels and other cells in the body. The kidneys work harder to filter the extra glucose from the blood and excrete it in urine. This leads to frequent urination, which in turn causes excessive thirst. Watch this short video for an animated introduction to diabetes, its causes, and its consequences:  MEDIA Click image to the left or use the URL below. URL:  There are two main types of diabetes: type 1 diabetes and type 2 diabetes. The two types of diabetes have different causes. "
noninfectious diseases,T_2306,"Type 1 diabetes is caused by the immune system attacking and destroying normal cells of the pancreas. As a result, the cells can no longer produce insulin. Why the immune system acts this way is not known for certain. Its possible that a virus may trigger the attack. This type of diabetes usually develops in childhood or adolescence. At present, there is no known way to prevent the development of type 1 diabetes. However, it is a treatable disease. Treatment of type 1 diabetes includes: taking several insulin injections every day or using an insulin pump (see Figure 21.7). monitoring blood glucose levels several times a day. eating a healthy diet that spreads out carbohydrate intake throughout the day. regular physical activity, which helps the body use insulin more efficiently. regular medical checkups. "
noninfectious diseases,T_2307,"Type 2 diabetes is much more common than type 1 diabetes. Type 2 diabetes occurs when body cells no longer respond normally to insulin. The pancreas still makes insulin, but the cells of the body cant use it. Being overweight and having high blood pressure increase the chances of developing type 2 diabetes. This type of diabetes usually develops in adulthood. However, it is becoming more common in teens and children because more young people are overweight now than ever before. You can greatly reduce your risk of developing type 2 diabetes by maintaining a healthy body weight. Some cases of type 2 diabetes can be cured with weight loss. However, most people with the disease need to take medicine to control their blood glucose. Regular exercise and balanced eating also help. Like people with type 1 diabetes, people with type 2 diabetes must frequently check their blood glucose. "
noninfectious diseases,T_2308,"The immune system is the body system that normally fights infections and defends against other causes of disease. When the immune system is working well, it usually keeps you from getting sick. But like any other body system, the immune system can have problems and develop diseases. Two types of immune system diseases are autoimmune diseases and allergies. "
noninfectious diseases,T_2309,"An autoimmune disease is a disease in which the immune system attacks the bodys own cells. Why this happens is not known for certain, but a combination of genetic and environmental factors are likely to be responsible. Type 1 diabetes is an example of an autoimmune disease. In this case, the immune system attacks cells of the pancreas. Two other examples are multiple sclerosis and rheumatoid arthritis. In multiple sclerosis, the immune system attacks nerve cells. This causes weakness and pain that gradually get worse over time. In rheumatoid arthritis, the immune system attacks joints. This causes joint damage and pain. These diseases cant be prevented and have no known cure. However, they can be treated with medicines that weaken the immune systems attack on normal cells. "
noninfectious diseases,T_2310,"An allergy is a disorder in which the immune system responds to a harmless substance as though it was a pathogen. Any substance that causes an allergy is called an allergen. The most common allergens are pollen, dust mites, mold, animal dander, insect stings, latex, and certain foods and medications. To see in greater detail how allergies occur, watch this animated video:  . MEDIA Click image to the left or use the URL below. URL:  Did you ever hear of hay fever? Its not really a fever, and it may have nothing to do with hay. Its actually an allergy to plant pollens. People with this type of allergy generally have seasonal allergies that come back year after year. Symptoms commonly include watery eyes and nasal congestion. Ragweed, shown blooming in Figure 21.8, causes more pollen allergies than any other plant. Allergy symptoms can range from mild to severe. Mild symptoms might include itchy eyes, sneezing, and a runny nose. Severe symptoms can cause difficulty breathing, which may be life threatening. Keep in mind that it is the immune system and not the allergen that causes the allergy symptoms. Allergy symptoms can be treated with medications such as antihistamines. Severe allergic reactions may require an injection of the hormone epinephrine. These treatments lessen or counter the immune systems response. Often, allergy symptoms can be prevented. One way is to avoid exposure to the allergens that cause your symptoms. If you are allergic to pollen, for example, you can reduce your exposure by staying inside when pollen levels are highest. Some people receive allergy shots to help prevent allergic reactions. The shots contain tiny amounts of allergens. After many months or years of shots, the immune system gets used to the allergens and no longer reacts to them. "
male reproductive system,T_2327,"The male reproductive system has two main functions: producing sperm and releasing testosterone. Sperm are male gametes, or reproductive cells. Sperm form when certain cells in the male reproductive system divide by meiosis to form haploid cells. Being haploid means they have half the number of chromosomes of other cells in the body. An adult male may produce millions of sperm each day! Testosterone is the major sex hormone in males. Testosterone has two primary roles: 1. During adolescence, testosterone causes most of the changes associated with puberty. It causes the reproduc- tive organs to mature. It also causes other adult male traits to develop. For example, it causes the voice to deepen and facial hair to start growing. 2. During adulthood, testosterone is needed for the production of sperm. "
male reproductive system,T_2328,"The male reproductive organs include the penis, testes, epididymis, vas deferens, and prostate gland. These organs are shown in Figure 22.1. The figure also shows some other parts of the male reproductive system. Find each organ in the drawing as you read about it below. For a cartoon about the male reproductive system, watch this video: http MEDIA Click image to the left or use the URL below. URL:  The penis is an external, cylinder-shaped organ that contains the urethra. The urethra is the tube that carries urine out of the body. It also carries sperm out of the body. The two testes (testis, singular) are oval organs that produce sperm and secrete testosterone. They are located inside a sac called the scrotum that hangs down outside the body. The scrotum also contains the epididymis. "
male reproductive system,T_2329,"Sperm are tiny cells. In fact, they are the smallest of all human cells. They have a structure that suits them well to perform their function. "
male reproductive system,T_2330,"As you can see in Figure 22.2, a sperm has three main parts: the head, connecting piece (or midpiece), and tail. 1. The head of the sperm contains the nucleus. The nucleus holds the chromosomes. In humans, the nucleus of a sperm cell contains 23 chromosomes. The acrosome on the head contains enzymes that help the sperm penetrate an egg. 2. The connecting piece of the sperm is packed with mitochondria. Mitochondria are organelles in cells that produce energy. Sperm use the energy to move. 3. The tail of the sperm moves like a propeller. It spins around and around and pushes the sperm forward. Sperm can travel about 30 inches per hour. "
male reproductive system,T_2331,"It takes up to two months for mature sperm to form. The process occurs in several steps: 1. Special cells in the testes go through mitosis to make identical copies of themselves. 2. The copies of the original cells divide by meiosis. This results in haploid cells called spermatids. These cells lack tails and cannot yet swim. 3. Spermatids move from the testes to the epididymis, where they slowly mature. For example, they grow a tail and lose some of the cytoplasm from the head. 4. Once sperm are mature, they can swim. The mature sperm remain in the epididymis until it is time for them to leave the body. Sperm leave the epididymis through the vas deferens. As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with secretions from these glands, forming semen. Semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as half a billion sperm! "
female reproductive system,T_2332,"Two functions of the female reproductive system are similar to the functions of the male reproductive system: producing gametes and secreting a major sex hormone. In the case of females, however, the gametes are eggs, and they are produced by the ovaries. The hormone is estrogen, which is the main sex hormone in females. Estrogen has two major roles: During adolescence, estrogen causes the changes of puberty. It causes the reproductive organs to mature. It also causes other female traits to develop. For example, it causes the breasts to grow and the hips to widen. During adulthood, estrogen is needed for a woman to release eggs from the ovaries. The female reproductive system has another important function, which is not found in males. It supports a baby as it develops before birth. It also gives birth to the baby at the end of pregnancy. "
female reproductive system,T_2333,"The female reproductive organs include the ovaries, fallopian tubes, uterus, and vagina. These organs are shown in Figure 22.3, along with some other structures of the female reproductive system. Find each organ in the drawing as you read about it below. For a cartoon about the female reproductive system, watch this video: http://education-por The two ovaries are small, oval organs on either side of the abdomen. Each ovary contains thousands of eggs. However, the eggs do not develop fully until a female has gone through puberty. Then, about once a month, an egg is released by one of the ovaries. The ovaries also secrete estrogen. The two fallopian tubes are thin tubes that are connected to the uterus and extend almost to the ovaries. The upper end of each fallopian tube has fingers (called fimbriae) that sweep an egg into the fallopian tube when it is released by the ovary. The egg then passes through the fallopian tube to the uterus. If an egg is fertilized, this occurs in the fallopian tube. The uterus is a hollow organ with muscular walls. The uterus is where a baby develops until birth. The walls of the uterus stretch to accommodate the growing fetus. The muscles in the walls contract to push the baby out during birth. The uterus is connected to the vagina by a small opening called the cervix. The vagina is a cylinder-shaped organ that opens to the outside of the body. The other end joins with the uterus. Sperm deposited in the vagina swim up through the cervix, into the uterus, and from there into a "
female reproductive system,T_2334,"When a baby girl is born, her ovaries contain all of the eggs they will ever produce. But these eggs are not fully developed. They develop only after the female reaches puberty at about age 12 or 13. Then, just one egg develops each month until she reaches her 40s or early 50s. "
female reproductive system,T_2335,"Human eggs are very large cells. In fact, they are the largest of all human cells. You can even see an egg without a microscope. Its almost as big as the period at the end of this sentence. Like a sperm cell, an egg cell is a haploid cell with half the number of chromosomes of other cells in the body. Unlike a sperm cell, the egg lacks a tail and contains a lot of cytoplasm. "
female reproductive system,T_2336,"Egg production takes place in the ovaries. It occurs in several steps: 1. Before birth, special cells in the ovaries go through mitosis to make identical daughter cells. 2. The daughter cells then start to divide by meiosis. However, they go though only the first of the two cell divisions of meiosis at this time. They remain in that stage until the girl goes through puberty. 3. After puberty, an egg develops in an ovary about once a month. As you can see in Figure 22.4, the egg rests in a nest of cells called a follicle. The follicle and egg grow larger and go through other changes. 4. After a couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This is called ovulation. After ovulation occurs, the moving fingers of the nearby fallopian tube sweep the egg into the tube. Fertilization may occur if sperm reach the egg while it is passing through the fallopian tube. If this happens, the egg finally completes meiosis. This results in two daughter cells that differ in size. The smaller cell is called a polar body. It soon breaks down and disappears. The larger cell is the fertilized egg, which will develop into a new human being. "
female reproductive system,T_2337,"Egg production in the ovary is part of the menstrual cycle. The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month on average. These changes include the development of an egg and follicle in the ovary. While the egg is developing, other changes are taking place in the uterus. It develops a thick lining that is full of tiny blood vessels. The lining prepares the uterus to receive a fertilized egg if fertilization actually takes place. If fertilization doesnt occur, the egg passes through the uterus and vagina and out of the body. The lining of the uterus also breaks down. Blood and other tissues from the lining pass through the vagina and leave the body. This is called menstruation. Menstruation is also called a menstrual period. It typically lasts about 4 days. When the menstrual period ends, the cycle begins repeats. "
reproduction and life stages,T_2338,"When a sperm penetrates the cell membrane of an egg, it triggers the egg to complete meiosis. The sperm also undergoes changes. Its tail falls off, and its nucleus fuses with the nucleus of the egg. The resulting cell, called a zygote, contains the diploid number of chromosomes. Half of the chromosomes come from the egg, and half come from the sperm. You can watch the process of fertilization and the development of a baby until birth in this amazing video:  MEDIA Click image to the left or use the URL below. URL: "
reproduction and life stages,T_2339,"The zygote spends the next few days traveling down the fallopian tube toward the uterus, where it will take up residence. As it travels, it divides many times by mitosis. It soon forms a tiny, fluid-filled ball of cells called a blastocyst. The blastocyst has an inner and outer layer of cells, as you can see in Figure 22.5. The inner layer, called the embryoblast, will develop into the new human being. The outer layer, called the trophoblast, will develop into other structures needed to support the new organism. "
reproduction and life stages,T_2340,"The blastocyst continues down the fallopian tube until it reaches the uterus, about 4 or 5 days after fertilization. When the outer cells of the blastocyst contact cells lining the uterus (the endometrium in Figure 22.5), the blastocyst embeds itself in the uterine lining. This process is called implantation. It generally occurs about a week after fertilization. "
reproduction and life stages,T_2341,"After implantation occurs, the blastocyst is called an embryo. The embryonic stage lasts from the end of the first week following fertilization through the end of the eighth week. During this time, the embryo grows in size and becomes more complex. It develops specialized cells and tissues. Most organs also start to form. You can see some of the specific changes that take place during weeks four to eight of the embryonic period in Figure 22.6. By the end of week eight, the embryo is about 30 millimeters (just over 1 inch) in length. It may also have begun to move. "
reproduction and life stages,T_2342,"From the eighth week following fertilization until birth, the developing human being is called a fetus. Birth typically occurs at about 38 weeks after fertilization, so the fetal period generally lasts about 30 weeks. During this time, the organs complete their development. The fetus also grows rapidly in length and weight. Some of the specific changes that occur during the fetal stage are listed in Figure 22.7. By the 38th week, the fetus is fully developed and ready to be born. A 38-week fetus normally ranges from about 36 to 51 centimeters (1420 inches) in length and weighs between 2.7 and 4.6 kilograms (about 610 pounds). "
reproduction and life stages,T_2343,"The fetus could not grow and develop without oxygen and nutrients from the mother. Wastes from the fetus also must be removed in order for it to survive. The exchange of these substances between the mother and fetus occurs through the placenta. The placenta is a temporary organ that starts to form shortly after implantation. It forms from the trophoblast layer of cells in the blastocyst and from maternal cells in the uterus. The placenta continues to develop and grow to meet the needs of the growing fetus. A fully developed placenta, like the one in Figure 22.8, is made up of a large mass of blood vessels from both mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces. This allows the mothers and fetuss blood to exchange substances across their capillary walls without the blood actually mixing. The fetus is connected to the placenta through the umbilical cord. This is a long tube that contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries. It exchanges gases and other substances with the mothers blood. Then it travels back to the fetus through the umbilical vein. Another structure that supports the fetus is the amniotic sac. This is a membrane that surrounds and protects the fetus. It contains amniotic fluid, which consists of water and dissolved substances. The fluid allows the fetus to move freely until it grows to fill most of the available space. The fluid also cushions the fetus and helps protect it from injury. "
reproduction and life stages,T_2344,Pregnancy is the carrying of one or more offspring from the time of implantation until birth. It is the development of an embryo and fetus from the expectant mothers point of view. 
reproduction and life stages,T_2345,"The pregnant mother plays a critical role in the development of the embryo and fetus. She must avoid toxic substances such as alcohol, which can damage the developing offspring. She also must provide all the nutrients and other substances needed for normal growth and development. Most nutrients are needed in greater amounts by a pregnant woman because she is literally eating for two people. Thats why its important for a woman to eat plenty of nutritious foods during pregnancy. The pregnant woman in Figure 22.9 is eating a variety of fresh fruits, which provide energy, vitamins, and other nutrients. "
reproduction and life stages,T_2346,"Near the time of birth, the amniotic sac breaks in a gush of liquid. Labor usually begins within a day of this event. Labor involves contractions of the muscular walls of the uterus. With the mothers help, the contractions eventually push the fetus out of the uterus and through the vagina. Within seconds of birth, the umbilical cord is cut. Without this connection to the placenta, the baby cant exchange gases, so carbon dioxide quickly builds up in the babys blood. This stimulates the babys brain to trigger breathing, and the newborn takes her first breath. "
reproduction and life stages,T_2347,"For the first year after birth, a baby is referred to as an infant. Childhood begins at the age of two years and continues until puberty. Adolescence begins with puberty and lasts until adulthood. "
reproduction and life stages,T_2348,"The first year of life after birth is called infancy. During infancy, a baby grows very quickly. The babys length typically doubles and her weight triples by her first birthday. Many other important changes also occur during infancy: The baby starts smiling, usually by about 6 weeks of age (see Figure 22.10). The baby starts noticing people and grabbing toys and other objects The baby teeth start to come in, usually by 6 months of age. The baby begins making babbling sounds. By the end of the first year, the baby may be saying a few words, such as Mama and Dada. The baby learns to sit, crawl, and stand. By the end of the first year, the baby may be starting to walk. "
reproduction and life stages,T_2349,"Childhood begins after the babys first birthday and continues until puberty. Between 1 and 3 years of age, a child is called a toddler. During the toddler stage, growth is still very rapid, but not as rapid as it was during infancy. Toddlers learn many new words and starts putting them together in simple sentences. Motor skills also develop quickly during the toddler stage. By the age of 3 years, most children can run and climb steps. They can hold crayons and scribble with them. They can also feed themselves, and most can use the toilet. From age 3 until puberty, growth slows down. The body also changes shape. The arms and legs grow longer relative to the trunk. Children continue to develop new motor skills. For example, many young children learn how to ride a tricycle and then a bicycle. Most learn how to play games and sports. By the age of 6 years, children start losing their baby teeth. Permanent teeth come in to replace them. Most children have started school by this age. They typically start learning to read and write around age 6 or 7 (see Figure 22.11). During the later years of childhood, children also start to develop friendships and become less dependent on their parents. "
reproduction and life stages,T_2350,"Puberty is the stage of life when a child becomes sexually mature. Puberty lasts from about 10 to 16 years of age in girls and from about 12 to 18 years of age in boys. In both girls and boys, puberty begins when the pituitary gland signals the gonads (ovaries or testes) to start secreting sex hormones (estrogen in girls, testosterone in boys). Sex hormones, in turn, cause many other changes to take place. In girls, estrogen causes the following changes to occur: The uterus and ovaries grow. The ovaries start releasing eggs. The menstrual cycle begins. Pubic hair grows. The hips widen and the breasts develop. In boys, testosterone causes these changes to take place: The penis and testes grow. The testes start producing sperm. Pubic and facial hair grow. The shoulders broaden. The voice becomes deeper as the larynx in the throat grows larger (see Figure 22.12). Girls and boys of the same age are similar in height during childhood. In both girls and boys, growth in height and weight is very fast during puberty. But boys grow more quickly that girls do, and their period of rapid growth also lasts longer. In addition, boys generally start puberty later than girls, so they have a longer period of childhood growth. For all these reasons, by the end of puberty, the average height of boys is 10 centimeters (about 4 inches) greater than the average height of girls. "
reproduction and life stages,T_2351,"Adolescence is the stage of life between the start of puberty and the beginning of adulthood. Adolescence begins with the physical changes of puberty. It also includes many other changes, including mental, emotional, and social changes. During adolescence: Teens develop new thinking abilities. For example, they develop the ability to understand abstract ideas, such as honesty and freedom. Their ability to think logically also improves. They usually get better at problem solving as well. Teens try to establish a sense of identity. They typically become increasingly independent from their parents. Many teens have emotional ups and downs. This is at least partly due to their changing hormone levels. Teens usually start spending more time with their peers, like the girls in Figure 22.13. Adolescents usually spend much more time with their friends and classmates than they do with family members. "
reproduction and life stages,T_2352,"Adulthood doesnt have a definite starting point. Teens may become physically mature by the age 16 years, but they are not adults in a legal sense until they are older. For example, in the U.S., you must be 18 years old to vote or serve in the armed forces. You must be 21 years old before you can take on many legal and financial responsibilities. Once adulthood begins, it can be divided into three stages: early, middle, and late adulthood. "
reproduction and life stages,T_2353,"Early adulthood refers to the 20s and early 30s. During early adulthood, most people are at their physical peak, and they are usually in good health. Often, they are completing their education and getting established in the workforce. Many people become engaged or marry during this time. "
reproduction and life stages,T_2354,"Middle adulthood is the period from the mid-30s to the mid-60s. During this stage of life, people start showing signs of aging. Their hair may thin and slowly turn gray. Their skin develops wrinkles. The risk of serious health problems increases. For example, cardiovascular diseases, cancer, and type 2 diabetes become more common in people of middle age. This is also the stage when many people raise a family and strive to attain career goals. "
reproduction and life stages,T_2355,"Late adulthood begins in the mid-60s and continues until death. This is the stage of life when most people retire from work. This frees up their time for hobbies, grandchildren, or other interests. For example, the man in Figure During late adulthood, the risk of developing diseases such as cardiovascular diseases and cancer continues to rise. Most people also have a decline in strength and stamina. Their senses may start failing, and their reflex time typically increases. Their immune system also doesnt work as well as it used to. As a result, common diseases like the flu may become more serious and even lead to death. The majority of late adults develop arthritis, and as many as one in four develop Alzheimers disease. Despite problems such as these, many people remain healthy and active into their 80s and even 90s. Do you want "
reproductive system health,T_2356,"A sexually transmitted infection (STI) is a disease that spreads mainly through sexual contact. STIs are caused by pathogens that enter the body through the reproductive organs. Many STIs also spread through body fluids such as blood. For example, a shared tattoo needle is one way that some STIs can spread. Some STIs can also spread from a mother to her infant during birth. "
reproductive system health,T_2357,"STIs are more common in teens and young adults than in older people. One reason is that young people are more likely to engage in risky behaviors. They also may not know how STIs spread. Instead, they may believe myths about STIs, like those in Table 22.1. Knowing the facts is important to prevent the spread of STIs. Myth If you are sexually active with just one person, then you cant get STIs. If you dont have any symptoms, then you dont have an STI. Getting STIs is no big deal, because they can be cured with medicines. Fact The only sure way to avoid getting STIs is to practice abstinence from sexual activity. Many STIs do not cause symptoms, especially in fe- males. Only some STIs can be cured with medicines; others cannot be cured. "
reproductive system health,T_2358,"A number of STIs are caused by bacteria. Bacterial STIs can usually be cured with antibiotics. However, some people with bacterial STIs may not have symptoms so they fail to get treatment. Left untreated, these infections may damage reproductive organs and lead to an inability to have children. Three bacterial STIs are chlamydia, gonorrhea, and syphilis. Chlamydia is the most common bacterial STI in the U.S. Females are more likely to develop it than males. Symptoms may include burning during urination and a discharge from the vagina or penis. Gonorrhea is another common bacterial STI. Symptoms may include painful urination and a discharge from the vagina or penis. Syphilis is a very serious STI but somewhat less common than chlamydia or gonorrhea. It usually begins with a small sore on the genitals. This is followed a few months later by a rash and flu-like symptoms. If syphilis isnt treated, it can eventually damage the heart, brain, and other organs and even cause death. "
reproductive system health,T_2359,"Several STIs are caused by viruses. Viral STIs cant be cured with antibiotics. Other drugs may help control the symptoms of viral STIs, but the infections usually last for life. Three viral STIs are genital warts, genital herpes, and AIDS. Genital herpes is a common STI caused by a herpes virus. The virus causes painful blisters on the penis or near the vaginal opening. The blisters generally go away on their own, but they may return repeatedly throughout life. There is no cure for genital herpes, but medicines can help prevent or shorten outbreaks. Acquired Immunodeficiency Syndrome (AIDS) is caused by human immunodeficiency virus (HIV). HIV destroys lymphocytes that normally fight infections. AIDS develops if the number of lymphocytes drops to a very low level. People with AIDS come down with diseasessuch as certain rare cancersthat almost never occur in people with a healthy immune system. Medicines can delay the progression of an HIV infection and may prevent AIDS from developing. Genital warts is an STI caused by human papilloma virus (HPV), which is pictured in Figure 22.15. This is one of the most common STIs in U.S. teens. Genital warts cant be cured, but a vaccine can prevent most HPV infections. The vaccine is recommended for boys and girls starting at 11 or 12 years of age. Its important to prevent HPV infections because they may lead to cancer later in life. "
reproductive system health,T_2360,Other reproductive system disorders include injuries and noninfectious diseases. These are different in males and females. 
reproductive system health,T_2361,"Most common disorders of the male reproductive system involve the testes. They include injuries and cancer. Injuries to the testes are very common. In teens, such injuries occur most often while playing sports. Injuries to the testes are likely to be very painful and cause bruising and swelling. However, they generally subside fairly quickly. Cancer of the testes is most common in males aged 15 to 35. It occurs when cells in the testes grow out of control and form a tumor. If found early, cancer of the testes usually can be cured with surgery. "
reproductive system health,T_2362,"Disorders of the female reproductive system may involve the vagina, uterus, or ovaries. They may also affect the breasts. Vaginitis is a very common disorder. Symptoms include redness and itching of the vagina. It may be caused by soap or bubble bath. Another possible cause is a yeast infection. Yeast normally grow in the vagina. If they multiple too quickly, they may cause irritation. A yeast infection can be treated with medication. Cysts may develop in the ovaries. A cyst is a sac filled with fluid or other material. Ovarian cysts are usually harmless and often disappear on their own. However, some cysts may be painful and require surgery. Many females experience abdominal cramps during menstruation. This is usually normal and not a cause for concern. Exercise, heat, or medication may help relieve the pain. In severe cases, prescription medicine may be needed. Breast cancer is the most common type of cancer in females. It occurs when cells in the breast grow out of control and form a tumor. Breast cancer is rare in teens but becomes more common as females get older. Regular screening is recommended for most women starting around age 40. If found early, breast cancer usually can be cured with surgery. "
reproductive system health,T_2363,"Maintaining overall good health will help keep your reproductive system healthy. You should eat right, get regular exercise, and follow other healthy lifestyle behaviors. In addition, the following practices will help keep the reproductive system healthy: Keep the genitals clean. A daily shower or bath is all thats needed. Avoid harsh soaps or other personal hygiene products that may be irritating. Avoid risky behaviors. This includes contact with blood or dirty needles as well as sexual activity. If you are a girl and use tampons, be sure to change them every 4 to 6 hours. This will reduce your risk of toxic shock syndrome. This is a very dangerous condition that may occur if tampons are left in too long. If you are a boy, wear a protective cup if you play a contact sport. This will help protect the testes from injury. You should also learn how to check yourself for testicular cancer (see Figure 22.16). You can learn how by watching this video:  MEDIA Click image to the left or use the URL below. URL: "
what is ecology,T_2364,"Organisms are individual living things. They range from microscopic bacteria to gigantic blue whales (see Figure must be obtained from the environment. Biotic factors are all of the living or once-living aspects of the environment. They include all the organisms that live there as well as the remains of dead organisms. Abiotic factors are all of the aspects of the environment that have never been alive. They include factors such as sunlight, minerals in soil, temperature, and moisture. "
what is ecology,T_2365,"Ecologists study organisms and environments at several different levels, from the individual to the biosphere. The levels are depicted in Figure 23.2 and described below. For a video introduction to the levels of organization in ecology, click on this link:  . MEDIA Click image to the left or use the URL below. URL:  An individual is an organism, or single living thing. A population is a group of individuals of the same species that live in the same area. Members of the same population generally interact with each other. A community is made up of all the populations of all the species that live in the same area. Populations in a community also generally interact with each other. "
populations,T_2366,"Population size is the number of individuals in a population. Population size influences the chances of a species surviving or going extinct. If a species populations become very small, the species may be at risk of going extinct. "
populations,T_2367,"Another sign of a species state of health is the density of its populations. Population density is the average number of individuals in a population for a given area. Density is a measure of how crowded or spread out the individuals in a population are on average. For example, a population of 100 deer that live in an area of 10 square kilometers has a population density of 10 deer per square kilometer. Population density is an average measure. Often, individuals in a population are not spread out evenly. Instead, they may live in clumps or some other pattern. How individuals in a population are distributed, or spread throughout their area, is called population distribution. You can see different patterns of population distribution in Figure 23.3. Different patterns characterize different species and types of environments, as you can read in the figure. "
populations,T_2368,"Whether its populations are growing or shrinking in size may be another indicator of a species health. Individuals may be added to a population through births and the migration of individuals into the population. Individuals may be lost from a population through deaths and the migration of individuals out of the population. The population growth rate is how quickly a population changes in size over time. The rate of growth of a population may be positive or negative. A positive growth rate means that the population is increasing in size because more people are being added than lost. A negative growth rate means that the population is decreasing in size because more people are being lost than added. Populations may show different patterns of growth. The growth pattern depends partly on the conditions under which a population lives. Two common growth patterns are exponential growth and logistic growth. Both are represented in Figure 23.4. With exponential growth, the population starts out growing slowly. As population size increases, the growth rate also increases. The larger the population becomes, the more quickly it grows. This type of growth generally occurs only when a population is living under ideal conditions. However, it cant continue for very long. With logistic growth, the population starts out growing slowly, and then the rate of growth increasesbut only to a point. The rate of growth tapers off as the population size approaches its carrying capacity. Carrying capacity is the largest population size that can be supported in an area without harming the environment. This type of growth characterizes many populations. "
populations,T_2369,"Another way of describing a population is its age-sex structure. This refers to the numbers of individuals of each sex and age in the population. The age-sex structure of a population may influence the population growth rate. This is because only individuals of certain ages are able to reproduce, and because individuals of certain ages may be more likely to die. For example, if there are many individuals of reproductive age, there are likely to be many births, causing the population to grow rapidly. The age-sex structure of a population is often represented with a special bar graph called a population pyramid. You can see an example of a population pyramid in Figure 23.5. The graph in the figure actually has a pyramid shape because the bars become narrower from younger to older ages. However, this is not always the case. In some populations, for example, there may be more people at older than younger ages, resulting in a top-heavy population pyramid. Learn more about population pyramids and what you can learn from them, watch this TED video: http://w MEDIA Click image to the left or use the URL below. URL: "
populations,T_2370,"Human beings have been called the most successful weed species on Earth. Like garden weeds, populations of human beings grow quickly and disperse rapidly. Human beings have colonized almost every terrestrial part of the planet. Overall, the human population has had a pattern of exponential growth, as you can see in Figure 23.6. The early human population grew very slowly. However, as the population grew larger, it started to grow more rapidly. "
populations,T_2371,"The earliest members of the human species evolved around 200,000 years ago in Africa. Early humans lived in small populations of nomadic hunters and gatherers. Human beings remained in Africa until about 40,000 years ago. After that, they spread throughout Europe, Asia, and Australia. By 10,000 years ago, the first human beings colonized the Americas. During this long period of time, the total number of human beings increased very slowly. Birth rates were fairly high but so were death rates, producing low rates of population growth. Human beings invented agriculture about 10,000 years ago. This provided a bigger, more dependable food supply. It also allowed people to settle down in villages and cities for the first time. Birth rates went up because there was more food and settled life had other advantages. Death rates also rose because of crowded living conditions and diseases that spread from domestic animals. Because the higher birth rates were matched by higher death rates, the human population continued to grow very slowly. "
populations,T_2372,"Major changes in the human population first began in the 1700s. These changes occurred mainly in Europe, North America, and a few other places that became industrialized. First death rates fell. Then, somewhat later, birth rates also fell. These changes in death and birth rates affected the rate of population growth and are referred to as the demographic transition. The graph in Figure 23.7 shows the stages in which the demographic transition occurred. You can learn more about the stages by watching this video: http://education-portal.com/academy/lesson/what-is-d In Stage 1, both birth and death rates were high so population growth was slow. In Stage 2, death rates fell while birth rates remained high. Why did death rates fall? There were several reasons, including new scientific knowledge of the causes of disease. Water supplies were cleaned up and sewage was disposed of more safely. Better farming techniques and machines increased the food supply and the distribution of food. For all these reasons, death rates fell, especially in children. Birth rates, on the other hand, remained high. This resulted in faster population growth. Before long, birth rates also started to fall. People started having fewer children because large families became too expensive. For example, with better farming machines, farm families no longer needed as many children to work in the fields. Laws were also passed that required children to go to school. They could no longer work and help support the family. Having many children became too costly. Eventually, birth rates fell to match death rates (Stage 4). As a result, population growth slowed down. "
populations,T_2373,"Just as they did in Europe and North America, death rates have fallen throughout the world. No country today remains in Stage 1 of the demographic transition. However, birth rates are still high in many of the poorest countries of the world. These populations seem to be stuck in Stage 2 or 3 of the demographic transition. They have high population growth rates because low death rates are not matched by equally low birth rates. Whether these populations will ever enter Stage 4 and attain very low rates of population growth is uncertain. "
populations,T_2374,"As of 2014, there were more than 7 billion human beings on planet Earth. That number is increasing rapidly. More than 200,000 people are added to the human population each day! At this rate, the human population will pass 9 billion by 2050. Many experts think that the human population has reached its carrying capacity. It has already harmed the environ- ment. An even larger human population may cause severe environmental problems. It could also lead to devastating outbreaks of disease, starvation, and war. To solve these problems, two approaches may be needed: Slow down human population growth so there are fewer people. Distribute Earths resources more fairly so that everyone has enough. Hopefully, we will act before its too late. Otherwise, the planet may be ruined for future generations of human beings and other species. "
biomes,T_2389,Terrestrial biomes are land-based biomes. They range from arctic tundra to tropical rainforests. Figure 23.18 shows the locations of the worlds major terrestrial biomes. 
biomes,T_2390,"Plants are the primary producers in terrestrial biomes. They make food for themselves and other organisms by photosynthesis. The major plants in a given biome, in turn, help determine the types of animals and other organisms that can live there. Which plants grow in a given biome depends mainly on climate. Climate is the average weather in a place over a long period of time. The major climatic factors affecting plant growth are temperature and moisture. "
biomes,T_2391,"You can read about three different terrestrial biomes in Figure 23.19: tropical rainforest, temperate grassland, and tundra. You can learn more about these and other terrestrial biomes by watching this video:  MEDIA Click image to the left or use the URL below. URL: "
biomes,T_2392,"Aquatic biomes are water-based biomes. They include both freshwater biomes, such as rivers and lakes, and marine biomes, which are salt-water biomes in the ocean. The primary producers in most aquatic biomes are phytoplankton. Phytoplankton consist of microscopic bacteria and tiny algae that make food by photosynthesis. Unlike terrestrial biomes, which are determined mainly by temperature and moisture, aquatic biomes are determined mainly by sunlight and dissolved substances in the water. These factors, in turn, depend mainly on depth of water and distance from shore. "
biomes,T_2393,"Only the top 200 meters or so of water receive enough sunlight for photosynthesis. This part of the water is called the photic zone. Below 200 meters, there is too little sunlight for photosynthesis to take place. This part of the water is called the aphotic zone. In this zone, food must come from other sources. It may be made by chemosynthesis, in which microorganisms use energy in chemicals instead of sunlight to make food. Or, food may drift down from the water above. "
biomes,T_2394,"In addition to sunlight, aquatic producers also need dissolved oxygen and nutrients. Water near the surface generally contains more dissolved oxygen than deeper water. Many nutrients enter the water from the land. Therefore, water nearer shore usually contains more dissolved nutrients than water farther from shore. "
biomes,T_2395,"A lake is an example of a freshwater biome. Water in a lake generally forms three different zones based on water depth and distance from shore. The shallow water near the shore is called the littoral zone. It has diverse community of organisms. There is adequate light for photosynthesis and plenty of dissolved oxygen and nutrients. Producers include algae and aquatic plants (see Figure 23.20). Animals in this zone may include insects, crustaceans, fish, and turtles. The top layer of water farther from shore is called the limnetic zone. There is enough light for photosynthesis and plenty of dissolved oxygen. However, dissolved nutrients tend not to be as plentiful as they are in the littoral zone. Producers here are mainly phytoplankton. A variety of zooplankton and fish also occupy this zone. The deeper water of a lake makes up the profundal zone. There isnt enough light for photosynthesis in this zone, so most organisms here eat dead organisms that drift down from the water above. Organisms in the profundal zone may include clams, snails, and some species of fish. "
biomes,T_2396,"Zones in the oceans include the intertidal, pelagic, and benthic zones. The types of organisms found in these ocean zones are also determined by such factors as depth of water and distance from shore, among other factors. One of the most familiar ocean zones is the intertidal zone. This is the narrow strip along a coastline that is covered by water at high tide and exposed to air at low tide. You can see an example of an intertidal zone in Figure 23.21. There are plenty of nutrients and sunlight in the intertidal zone. Producers here include phytoplankton and algae. Other organisms include barnacles, snails, crabs, and mussels. They must have adaptations for the constantly changing conditions in this zone. Other ocean zones are farther from shore in the open ocean. All the water in the open ocean is called the pelagic zone. It is further divided by depth: The top 200 meters of water is the photic zone. Producers here include seaweeds and phytoplankton. Other organisms are plentiful. They include zooplankton and animals such as fish, whales, and dolphins. "
cycles of matter,T_2407,"The chemical elements and water that are needed by living things keep recycling on Earth. They pass back and forth through biotic and abiotic components of ecosystems. Thats why their cycles are called biogeochemical cycles. For example, a chemical element or water might move from organisms (bio) to the atmosphere or ocean (geo) and back to organisms again. Elements or water may be held for various periods of time in different parts of a biogeochemical cycle. An exchange pool is part of a cycle that holds a substance for a short period of time. For example, the atmosphere is an exchange pool for water. It usually holds water (as water vapor) for just a few days. A reservoir is part of a cycle that holds a substance for a long period of time. For example, the ocean is a reservoir for water. It may hold water for thousands of years. The rest of this lesson describes three biogeochemical cycles: water cycle, carbon cycle, and nitrogen cycle. "
cycles of matter,T_2408,"Water is an extremely important aspect of every ecosystem. Life cant exist without water. Most organisms contain a large amount of water, and many live in water. Therefore, the water cycle is essential to life on Earth. Water on Earth is billions of years old. However, individual water molecules keep moving through the water cycle. The water cycle is a global cycle. It takes place on, above, and below Earths surface, as shown in Figure 24.7. During the water cycle, water occurs in three different states: gas (water vapor), liquid (water), and solid (ice). Many processes are involved as water changes state to move through the cycle. Watch this video for an excellent visual introduction to the water cycle:  . MEDIA Click image to the left or use the URL below. URL: "
cycles of matter,T_2409,"Water changes to a gas by three different processes called evaporation, sublimation, and transpiration. Evaporation takes place when water on Earths surface changes to water vapor. The sun heats the water and gives water molecules enough energy to escape into the atmosphere. Most evaporation occurs from the surface of the ocean. Sublimation takes place when snow and ice on Earths surface change directly to water vapor without first melting to form liquid water. This also happens because of heat from the sun. Transpiration takes place when plants release water vapor through pores in their leaves called stomata. "
cycles of matter,T_2410,"Rising air currents carry water vapor into the atmosphere. As the water vapor rises in the atmosphere, it cools and condenses. Condensation is the process in which water vapor changes to tiny droplets of liquid water. The water droplets may form clouds. If the droplets get big enough, they fall as precipitation. Precipitation is any form of water that falls from the atmosphere. It includes rain, snow, sleet, hail, and freezing rain. Most precipitation falls into the ocean. Eventually, this water evaporates again and repeats the water cycle. Some frozen precipitation becomes part of ice caps and glaciers. These masses of ice can store frozen water for hundreds of years or even longer. Condensation may also form fog or dew. Some living things, like the lizard in Figure 24.8, depend directly on these sources of liquid water. "
cycles of matter,T_2411,Precipitation that falls on land may flow over the surface of the ground. This water is called runoff. It may eventually flow into a body of water. Some precipitation that falls on land soaks into the ground. This water becomes groundwater. Groundwater may seep out of the ground at a spring or into a body of water such as the ocean. Some groundwater is taken up by plant roots. Some may flow deeper underground to an aquifer. An aquifer is an underground layer of rock that stores water. Water may be stored in an aquifer for thousands of years. 
cycles of matter,T_2412,"The element carbon is the basis of all life on Earth. Biochemical compounds consist of chains of carbon atoms and just a few other elements. Like water, carbon is constantly recycled through the biotic and abiotic factors of ecosystems. The carbon cycle includes carbon in sedimentary rocks and fossil fuels under the ground, the ocean, the atmosphere, and living things. The diagram in Figure 24.9 represents the carbon cycle. It shows some of the ways that carbon moves between the different parts of the cycle. You can see an animated carbon cycle at this link: http://commons.w "
cycles of matter,T_2413,"Major reservoirs of carbon include sedimentary rocks, fossil fuels, and the ocean. Sediments from dead organisms may form carbon-containing sedimentary rocks. Alternatively, the sediments may form carbon-rich fossil fuels, which include oil, natural gas, and coal. Carbon can be stored in these reservoirs for millions of years. However, if fossil fuels are extracted and burned, the stored carbon enters the atmosphere as carbon dioxide. Natural processes, such as volcanic eruptions, can also release underground carbon from rocks into the atmosphere. Water erosion by runoff, rivers, and streams dissolves carbon in rocks and carries it to the ocean. Ocean water near the surface dissolves carbon dioxide from the atmosphere. Dissolved carbon may be stored in the deep ocean for thousands of years. "
cycles of matter,T_2414,"Major exchange pools of carbon include organisms and the atmosphere. Carbon cycles more quickly between these components of the carbon cycle. Photosynthesis by plants and other producers removes carbon dioxide from the atmosphere to make organic compounds for living things. Cellular respiration by living things releases carbon into the atmosphere or ocean as carbon dioxide. Decomposition of dead organisms and organic wastes releases carbon back to the atmosphere, soil, or ocean. "
cycles of matter,T_2415,"Nitrogen is another common element found in living things. It is needed to form both proteins and nucleic acids such as DNA. Nitrogen gas makes up 78 percent of Earths atmosphere. In the nitrogen cycle, nitrogen flows back and forth between the atmosphere and living things. You can see how it happens in Figure 24.10. Several different types of bacteria play major roles in the cycle. Animals get nitrogen by eating plants or other organisms that eat plants. Where do plants get nitrogen? They cant use nitrogen gas in the air. The only form of nitrogen that plants can use is in chemical compounds called nitrates. Plants absorb nitrates through their roots. This is called assimilation. Most of the nitrates are produced by bacteria that live in soil or in the roots of plants called legumes. Nitrogen-fixing bacteria change nitrogen gas from the atmosphere to nitrates in soil. When organisms die and decompose, their nitrogen is returned to the soil as ammonium ions. Nitrifying bacteria change some of the ammonium ions into nitrates. The other ammonium ions are changed into nitrogen gas by denitrifying bacteria. "
air pollution,T_2421,"The major cause of outdoor air pollution is the burning of fossil fuels. Fossil fuels are burned in power plants, factories, motor vehicles, and home heating systems. Ranching and using chemicals such as fertilizers also cause outdoor air pollution. Erosion of soil in farm fields, mining activities, and construction sites adds dust particles to the air as well. Some specific outdoor air pollutants are described in Table 25.1. Air Pollutant Sulfur oxides Nitrogen oxides Carbon monoxide Carbon dioxide Particles (dust, smoke) Mercury Smog Ground-level ozone Source coal burning motor vehicle exhaust motor vehicle exhaust all fossil fuel burning wood and coal burning coal burning coal burning motor vehicle exhaust Problem acid rain acid rain poisoning global climate change respiratory problems nerve poisoning respiratory problems respiratory problems "
air pollution,T_2422,"Outdoor air pollution causes serious human health problems. For example, pollutants in the air are major contributors to respiratory and cardiovascular diseases. Air pollution may trigger asthma attacks and heart attacks in people with underlying health problems. In fact, more people die each year from air pollution than automobile accidents. "
air pollution,T_2423,"Air pollution may also cause acid rain. This is rain that is more acidic (has a lower pH) than normal rain. Acids form in the atmosphere when nitrogen and sulfur oxides mix with water in air. Nitrogen and sulfur oxides come mainly from motor vehicle exhaust and coal burning. If acid rain falls into lakes, it lowers the pH of the water and may kill aquatic organisms. If it falls on the ground, it may damage soil and soil organisms. If it falls on plants, it may make them sick or even kill them. Acid rain also damages stone buildings, bridges, and statues, like the one in Figure 25.1. "
air pollution,T_2424,"Another major problem caused by air pollution is global climate change. Gases such as carbon dioxide from the burning of fossil fuels increase the greenhouse effect and raise Earths temperature. The greenhouse effect is a natural feature of Earths atmosphere. It occurs when certain gases in the atmosphere, including carbon dioxide, radiate the suns heat back down to Earths surface. Figure 25.2 shows how this happens. Without greenhouse gases in the atmosphere, the heat would escape into space. The natural greenhouse effect of Earths atmosphere keeps the planets temperature within a range that can support life. The rise in greenhouse gases due to human actions is too much of a good thing. It increases the greenhouse effect and causes Earths average temperature to rise. Rising global temperatures, in turn, are melting polar ice caps and glaciers. Figure 25.3 shows how much smaller the Arctic ice cap was in 2012 than it was in 1984. With more liquid water on Earths surface, sea levels are rising. Adding more heat energy to Earths atmosphere also causes more extreme weather and changes in precipitation patterns. Global warming is already causing food and water shortages and species extinctions. These problems will only grow worse unless steps are taken to curb greenhouse gases and global climate change. "
air pollution,T_2425,"You may be able to avoid some of the health effects of outdoor air pollution by staying indoors on high-pollution days. However, some indoor air is just as polluted as outdoor air. "
air pollution,T_2426,"One source of indoor air pollution is radon gas. Radon is a radioactive gas that may seep into buildings from rocks underground. Exposure to radon gas may cause lung cancer. Another potential poison in indoor air is carbon monoxide. It may be released by faulty or poorly vented furnaces or other fuel-burning appliances. Indoor furniture, carpets, and paints may release toxic compounds into the air as well. Other possible sources of indoor air pollution include dust, mold, and pet dander. "
air pollution,T_2427,"Its easier to control the quality of indoor air than outdoor air. Steps home owners can take to improve indoor air quality include: keeping the home clean so it is as free as possible from dust, mold, and pet dander. choosing indoor furniture, flooring, and paints that are low in toxic compounds such as VOCs (volatile organic compounds). making sure that fuel-burning appliances are working correctly and venting properly. installing carbon monoxide alarms like the one in Figure 25.4 at every level of the home. "
water pollution,T_2428,"Water pollution has many causes. One of the biggest causes is fertilizer in runoff. Runoff dissolves fertilizer as it flows over farm fields, lawns, and golf courses. It carries the dissolved fertilizer into bodies of water. More dissolved fertilizer may enter a body of water at the mouth of a river, but there is generally no single point where this type of pollution enters the water. Thats why this type of water pollution is called nonpoint-source pollution. "
water pollution,T_2429,"When fertilizer ends up in bodies of water, the added nutrients cause excessive growth of algae. This is called an algal bloom. You can see one in Figure 25.5. The algae out-compete other water organisms. They may make the water unfit for human consumption or recreation. "
water pollution,T_2430,"Eventually, the algae in an algal bloom die and decompose. Their decomposition uses up oxygen in the water so that the water becomes hypoxic (without oxygen). This has occurred in many bodies of fresh water and large areas of the ocean, creating dead zones. Dead zones are areas where the hypoxic water cant support life. A very large dead zone exists in the Gulf of Mexico (see Figure 25.6). Nutrients carried into the Gulf by the Mississippi River caused this dead zone. Cutting down on the use of chemical fertilizers is one way to prevent dead zones in bodies of water. Preserving wetlands is also important. Wetlands are habitats such as swamps, marshes, and bogs where the ground is soggy or covered with water much of the year. Wetlands slow down and filter runoff before it reaches bodies of water. Wetlands also provide breeding grounds for many different species of organisms. "
water pollution,T_2431,"Unlike runoff, which enters bodies of water everywhere, some sources of pollution enter the water at a single point. This type of water pollution is called point-source pollution. "
water pollution,T_2432,"An example of point-source pollution is the release of pollution into a body of water through a pipe from a factory or sewage treatment plant. Waste water from a factory might contain dangerous chemicals such as strong acids, mercury, or lead. Water from a sewage treatment plant might contain untreated or partially treated sewage. Such pollution can make water dangerous for drinking or other uses. You can learn more about the problem of sewage contaminating the water in U.S. coastal communities by watching this video:  MEDIA Click image to the left or use the URL below. URL:  In poor nations, many people have no choice but to drink water from polluted sources. Drinking sewage-contaminated water causes waterborne diseases, due to pathogens such as protozoa, viruses, or bacteria. Most waterborne diseases cause diarrhea. "
water pollution,T_2433,"If heated water is released into a body of water, it may cause thermal pollution. Thermal pollution is a reduction in the quality of water because of an increase in water temperature. A common cause of thermal pollution is the use of water as a coolant by power plants and factories. This water is heated and then returned to the natural environment at a higher temperature. Warm water cant hold as much dissolved oxygen as cool water, so an increase in the temperature of water decreases the amount of oxygen it contains. Fish and other organisms adapted to a particular temperature range and oxygen concentration may be killed by the change in water temperature. "
water pollution,T_2434,The ocean is huge but even this body of water is becoming seriously polluted. Climate change also affects the quality of ocean water for living things. 
water pollution,T_2435,"One way that the ocean is becoming polluted is with trash, mainly plastics. The waste comes from shipping accidents, landfill erosion, and the dumping of trash. Plastics may take hundreds or even thousands of years to break down. In the meantime, the waste can be very dangerous to aquatic organisms. Some organisms may swallow plastic bags, for example, and others may be strangled by plastic six-pack rings. You can see some of the trash that routinely washes up on coastlines in Figure 25.7. There are five massive garbage patches floating on the Pacific Ocean. Watch this video to learn more about them:  . MEDIA Click image to the left or use the URL below. URL: "
water pollution,T_2436,"Ocean water normally dissolves some of the carbon dioxide in the atmosphere. The burning of fossil fuels has increased the amount of carbon dioxide in the atmosphere. As a result, ocean water is also dissolving more carbon dioxide. When carbon dioxide dissolves in water, it forms a weak acid. With higher levels of dissolved carbon dioxide in ocean water, the water becomes more acidic. This process is called ocean acidification. Ocean acidification can kill some aquatic organisms, including corals and shellfish. It may make it more difficult for other aquatic organisms to reproduce. Both effects of acidification interfere with marine food webs, threatening the survival of many aquatic organisms. "
natural resources,T_2437,"From a human point of view, natural resources can be classified as either renewable or nonrenewable. "
natural resources,T_2438,"Renewable resources are natural resources that are remade by natural processes as quickly as people use them. Examples of renewable resources include sunlight and wind. They are in no danger of being used up. Metals and some other minerals are considered renewable as well because they are not destroyed when they are used. Instead, they can be recycled and used over and over again. Living things are also renewable resources. They can reproduce to replace themselves. However, living things can be over-used or misused to the point of extinction. For example, over-fishing has caused some of the best fishing spots in the ocean to be nearly depleted, threatening entire fish species with extinction. To be truly renewable, living things must be used wisely. They must be used in a way that meets the needs of the present generation but also preserves them for future generations. Using resources in this way is called sustainable use. "
natural resources,T_2439,"Nonrenewable resources are natural resources that cant be remade or else take too long to remake to keep up with human use. Examples of nonrenewable resources are coal, oil, and natural gas, all of which are fossil fuels. Fossil fuels form from the remains of plants and animals over hundreds of millions of years. We are using them up far faster than they can be replaced. At current rates of use, oil and natural gas will be used up in just a few decades, and coal will be used up in a couple of centuries. Uranium is another nonrenewable resource. It is used to produce nuclear power. Uranium is a naturally occurring chemical element that cant be remade. It will run out sooner or later if nuclear energy continues to be used. Soil is a very important natural resource. Plants need soil to grow, and plants are the basis of terrestrial ecosystems. Theoretically, soil can be remade. However, it takes millions of years for soil to form, so from a human point of view, it is a nonrenewable resource. Soil can be misused and eroded (see Figure 25.9). It must be used wisely to preserve it for the future. This means taking steps to avoid soil erosion and contamination of soil by toxins such as oil spills. "
natural resources,T_2440,"Some of the resources we depend on the most are energy resources. Whether its powering our lights and computers, heating our homes, or providing energy for cars and other vehicles, its hard to imagine what our lives would be like without a constant supply of energy. "
natural resources,T_2441,"Fossil fuels and nuclear energy are nonrenewable energy resources. People worldwide depend far more on these energy sources than any others. Figure 25.10 shows the worldwide consumption of energy sources by type in 2010. Nonrenewable energy sources accounted for 83 percent of the total energy used. Fossil fuels and the uranium needed for nuclear power will soon be used up if we continue to consume them at these rates. Using fossil fuels and nuclear energy creates other problems as well. The burning of fossil fuels releases carbon dioxide into the atmosphere. This is one of the major greenhouse gases causing global climate change. Nuclear power creates another set of problems, including the disposal of radioactive waste. "
natural resources,T_2442,"Switching to renewable energy sources solves many of the problems associated with nonrenewable energy. While it may be expensive to develop renewable energy sources, they are clearly the way of the future. Figure 25.11 represents three different renewable energy sources: solar, wind, and biomass energy. The three types are described below. You can watch Bill Nyes introduction to renewable energy resources in this video:  MEDIA Click image to the left or use the URL below. URL:  Solar energy is energy provided by sunlight. Solar cells can turn sunlight into electricity. The energy in sunlight is virtually limitless and free and creates no pollution to use. Wind energy is energy provided by the blowing wind. Wind turbines, like those in Figure 25.11, can turn wind energy into electricity. The wind blows because of differences in heating of Earths atmosphere by the sun. There will never be a shortage of wind. Biomass energy is energy provided by burning or decomposing organic matter. For example, when garbage decomposes in a landfill, it releases methane gas. This gas can be captured and burned to produce electricity. Crops such as corn can also be converted into a liquid fuel and added to gasoline. Although biomass is renewable, burning it produces carbon dioxide, similar to fossil fuels. "
natural resources,T_2443,"Especially when it comes to nonrenewable resources, conserving natural resources is important. Using less of them means that they will last longer. It also means they will impact the environment less. Everyone can help make a difference. There are three basic ways that all of us can conserve natural resources. They are referred to as the three Rs: reduce, reuse, and recycle. "
natural resources,T_2444,"Reducing the amount of natural resources you use is the best way to conserve resources. It takes energy to make new items, and even reusing or recycling items takes energy. You can reduce the amount of natural resources you use by not using the resources in the first place. Often, this involves just being less wasteful. Follow these tips to reduce your use of natural resources: Walk, bike, or use public transit instead of driving. If you must drive, a fuel-efficient vehicle will reduce energy use. Plan ahead to avoid making extra trips. Dont buy more than you need. For example, dont buy more fresh food than you can use without it going to waste. You will not only reduce your use of food. You will also reduce your use of energy resources. It takes a lot of energy to grow, process, and ship many of the foods we buy. When you shop, keep packaging in mind. ""Precycle"" by buying items with the least amount of wasted packaging. Use energy-efficient appliances and LED light bulbs. Also, turn off appliances and lights when you arent using them. Both steps will reduce the amount of energy resources you use. Keep the thermostat set low in the winter and high in the summer (see Figure 25.12). Instead of turning up the heat in cold weather, put on an extra layer of clothes to save energy resources. Open windows and use fans in hot weather rather than turning on the air conditioning. "
natural resources,T_2445,"Reusing means to use an item again rather than throwing it away and replacing it. Items can be reused for the same purpose or for a different purpose. Generally, it takes less energy to reuse an item than to recycle it, so choose this option over recycling when you can. Here are some specific tips for reusing natural resources: Consider mending or repairing worn or broken items rather than throwing them out and replacing them. Shop with reuse in mind. You can find great buys at flea markets and resale shops. You may be able to get free items online at free-cycle sites. Youll save money as well as natural resources. You can also sell (or give away) your own reusable items. Reuse cloth shopping bags. Instead of getting new plastic or paper bags for your purchases each time you shop, take your own reusable bag to the store each time. Even little steps can add up and help save natural resources. For example, unwrap gifts carefully and youll be able to reuse the gift wrap on a package for someone else. You can also reuse writing paper that has only been used on one side. Its great for notes and shopping lists. "
natural resources,T_2446,"If an item can no longer be used or reused, try to recycle it. Recycling means taking a used item, breaking it down, and reusing the components. It generally takes less energy to recycle materials than obtain new ones. Recycling also keeps waste out of landfills. Some of the items that can be recycled include: glass, paper, cardboard, plastic, aluminum, iron, steel, batteries, electronics, tires, and concrete. You can learn how some of these materials are recycled by watching this video:  . MEDIA Click image to the left or use the URL below. URL:  Even kitchen scraps and garden wastes can be recycled. They can be tossed into a compost bin, like the one in Figure 25.13. The recycled compost gradually breaks down to form rich humus that can be added to lawns and gardens to improve the soil. Encourage your family to recycle if they dont already. Even if you dont have curbside recycling where you live, there are likely to be recycling drop boxes or centers available for recycling many items. If you have recycling bins at school, be sure to use them. If not, raise the issue with your teacher or principal. You can also write a letter to the editor of your local newspaper encouraging everyone in your community to recycle. "
photosynthesis,T_2492,"Chemical energy that organisms need comes from food. The nearly universal food for life is the sugar glucose. Glucose is a simple carbohydrate with the chemical formula C6 H12 O6 . The glucose molecule stores chemical energy in a concentrated, stable form. In your body, glucose is the form of energy that is carried in your blood and taken up by each of your trillions of cells. "
photosynthesis,T_2493,"What is the source of glucose for living things? It is made by plants and certain other organisms. The process in which glucose is made using energy in light is photosynthesis. This process requires carbon dioxide and water. It produces oxygen in addition to glucose. Photosynthesis consists of many chemical reactions. Overall, the reactions of photosynthesis can be summed up by this chemical equation: 6CO2 + 6H2 O + light energy ! C6 H12 O6 + 6O2 In words, this means that six molecules of carbon dioxide (CO2 ) combine with six molecules of water (H2 O) in the presence of light energy. This produces one molecule of glucose (C6 H12 O6 ) and six molecules of oxygen (O2 ). Use this interactive animation to learn more about photosynthesis: Click on this link for a song about photosynthesis to reinforce the basic ideas:  MEDIA Click image to the left or use the URL below. URL: "
photosynthesis,T_2494,"Types of organisms that make glucose by photosynthesis are pictured in Figure 4.7. They include plants, plant-like protists such as algae, and some kinds of bacteria. Living things that make glucose are called autotrophs (""self feeders""). All other living things obtain glucose by eating autotrophs (or organisms that eat autotrophs). These living things are called heterotrophs (""other feeders""). "
photosynthesis,T_2495,"In plants and algae, photosynthesis takes place in chloroplasts. (Photosynthetic bacteria have other structures for this purpose.) A chloroplast is a type of plastid, or plant organelle. It contains the green pigment known as chlorophyll. The presence of chloroplasts in plant cells is one of the major ways they differ from animal cells. You can see chloroplasts in plant cells Figure 4.8. "
photosynthesis,T_2496,"The structure of a chloroplast is shown in Figure 4.9. The chloroplast is surrounded by two membranes. Inside the chloroplast are stacks of flattened sacs of membrane, called thylakoids. The thylakoids contain chlorophyll. Surrounding the thylakoids is a space called the stroma. The stroma is filled with watery (""aqueous"") fluid. "
photosynthesis,T_2497,"In plants, most chloroplasts are found in the leaves. Therefore, all the raw materials needed for photosynthesis must be present in the leaves. These materials include light, water, and carbon dioxide. The shape of the leaves gives them a lot of surface area to absorb light for photosynthesis. Roots take up water from the soil. Stems carry the water from the roots to the leaves. Carbon dioxide enters the leaves through tiny openings called stomata. (The oxygen released during photosynthesis also exits the leaves through the stomata.) "
photosynthesis,T_2498,"Photosynthesis occurs in two stages, called the light reactions and the Calvin cycle. Figure 4.10 sums up what happens in these two stages. Both stages are described below. "
photosynthesis,T_2499,"The light reactions occur in the first stage of photosynthesis. This stage takes place in the thylakoid membranes of the chloroplast. In the light reactions, energy from sunlight is absorbed by chlorophyll. This energy is temporarily transferred to two molecules: ATP and NADPH. These molecules are used to store the energy for the second stage of photosynthesis. The light reactions use water and produce oxygen. "
photosynthesis,T_2500,"The Calvin cycle occurs in the second stage of photosynthesis. This stage takes place in the stroma of the chloroplast. In the Calvin cycle, carbon dioxide is used to produce glucose (sugar) using the energy stored in ATP and NADPH. The energy is released from these molecules when ATP loses phosphate (Pi ) to become ADP and NADPH loses hydrogen (H) to become NADP+ . "
cellular respiration,T_2501,"Cellular respiration is the process in which cells break down glucose, release the stored energy, and use the energy to make ATP. For each glucose molecule that undergoes this process, up to 38 molecules of ATP are produced. Each ATP molecules forms when a phosphate is added to ADP, or adenosine diphosphate. This requires energy, which is stored in the ATP molecule. When cells need energy, a phosphate can be removed from ATP. This releases the energy and forms ADP again. "
cellular respiration,T_2502,"Cellular respiration involves many biochemical reactions. However, the overall process can be summed up in a single chemical equation: C6 H12 O6 + 6O2 ! 6CO2 + 6H2 O + energy (stored in ATP) Cellular respiration uses oxygen in addition to glucose. It releases carbon dioxide and water as waste products. Cellular respiration actually ""burns"" glucose for energy. However, it doesnt produce light or intense heat like burning a candle or log. Instead, it releases the energy slowly, in many small steps. The energy is used to form dozens of molecules of ATP. "
cellular respiration,T_2503,"Cellular respiration takes place in the cells of all organisms. It occurs in autotrophs such as plants as well as heterotrophs such as animals. Cellular respiration begins in the cytoplasm of cells. It is completed in mitochondria. The mitochondrion is a membrane-enclosed organelle in the cytoplasm. Its sometimes called the ""powerhouse"" of the cell because of its role in cellular respiration. Figure 4.12 shows the parts of the mitochondrion involved in cellular respiration. "
cellular respiration,T_2504,"Cellular respiration occurs in three stages. The flow chart in Figure dont purge me shows the order in which the stages occur and how much ATP forms in each stage. The names of the stages are glycolysis, the Krebs cycle, and electron transport. Each stage is described below. "
cellular respiration,T_2505,"Glycolysis is the first stage of cellular respiration. It takes place in the cytoplasm of the cell. The world glycolysis means ""glucose splitting"". Thats exactly what happens in this stage. Enzymes split a molecule of glucose into two smaller molecules called pyruvate. This results in a net gain of two molecules of ATP. Other energy-storing molecules are also produced. (Their energy will be used in stage 3 to make more ATP.) Glycolysis does not require oxygen. Anything that doesnt need oxygen is described as anaerobic. "
cellular respiration,T_2506,"The pyruvate molecules from glycolysis next enter the matrix of a mitochondrion. Thats where the second stage of cellular respiration takes place. This stage is called the Krebs cycle. During this stage, two more molecules of ATP are produced. Other energy-storing molecules are also produced (to be used to make more ATP in stage 3). The Krebs cycle requires oxygen. Anything that needs oxygen is described as aerobic. The oxygen combines with the carbon from the pyruvate molecules. This forms carbon dioxide, a waste product. "
cellular respiration,T_2507,"The third and final stage of cellular respiration is called electron transport. Remember the other energy-storing molecules from glycolysis and the Krebs cycle? Their energy is used in this stage to make many more molecules of ATP. In fact, during this stage, as many as 34 molecules of ATP are produced. Electron transport requires oxygen, so this stage is also aerobic. The oxygen combines with hydrogen from the energy-storing molecules. This forms water, another waste product. "
cellular respiration,T_2508,"Cellular respiration and photosynthesis are like two sides of the same coin. This is clear from the diagram in Figure needed for photosynthesis. Together, the two processes store and release energy in virtually all living things. "
cellular respiration,T_2509,"Some organisms can produce ATP from glucose anaerobically. One way this happens is called fermentation. Fermentation includes the glycolysis step of cellular respiration. However, it doesnt include the other, aerobic steps. There are two types of fermentation: lactic acid fermentation and alcoholic fermentation. "
cellular respiration,T_2510,"In lactic acid fermentation, glycolysis is followed by a step that produces lactic acid. This step forms additional molecules of ATP. Lactic acid fermentation occurs in some bacteria, including the bacteria in yogurt. The lactic acid gives unsweetened yogurt its sour taste. Your own muscle cells can also undertake lactic acid fermentation. This occurs when the cells are working very hard. They use fermentation because they cant get oxygen fast enough for aerobic respiration to supply them with all the energy they need. The muscle cells of the hurdlers in Figure 4.15 are using lactic acid fermentation by the time the athletes reach finish line. "
cellular respiration,T_2511,"In alcoholic fermentation, glycolysis is followed by a step that produces alcohol and carbon dioxide. This step also forms additional molecules of ATP. It occurs in yeast, such as the yeast in bread. Carbon dioxide from alcoholic fermentation creates gas bubbles in bread dough. The bubbles leave little holes in the bread after it bakes. You can see them in the bread in Figure 4.16. The holes make the bread light and fluffy. "
cellular respiration,T_2512,"Both aerobic and anaerobic respiration have certain advantages. Aerobic respiration releases far more energy than anaerobic respiration does. It results in the formation of many more molecules of ATP. Anaerobic respiration is much quicker than aerobic respiration. It also allows organisms to live in places where there is little or no oxygen, such as deep under water or soil. For an entertaining review of aerobic and anaerobic respiration, watch this creative music video:  MEDIA Click image to the left or use the URL below. URL: "
protein synthesis,T_2537,"DNA and RNA are nucleic acids. DNA stores genetic information. RNA helps build proteins. Proteins, in turn, determine the structure and function of all your cells. Proteins consist of chains of amino acids. A proteins structure and function depends on the sequence of its amino acids. Instructions for this sequence are encoded in DNA. In eukaryotic cells, chromosomes are contained within the nucleus. But proteins are made in the cytoplasm at structures called ribosomes. How do the instructions in DNA reach the ribosomes in the cytoplasm? RNA is needed for this task. "
protein synthesis,T_2538,"RNA stands for ribonucleic acid. RNA is smaller than DNA. It can squeeze through pores in the membrane that encloses the nucleus. It copies instructions in DNA and carries them to a ribosome in the cytoplasm. Then it helps build the protein. RNA is not only smaller than DNA. It differs from DNA in other ways as well. It consists of one nucleotide chain rather than two chains as in DNA. It also contains the nitrogen base uracil (U) instead of thymine (T). In addition, it contains the sugar ribose instead of deoxyribose. You can see these differences in Figure 5.16. "
protein synthesis,T_2539,There are three different types of RNA. All three types are needed to make proteins. Messenger RNA (mRNA) copies genetic instructions from DNA in the nucleus. Then it carries the instructions to a ribosome in the cytoplasm. Ribosomal RNA (rRNA) helps form a ribosome. This is where the protein is made. Transfer RNA (tRNA) brings amino acids to the ribosome. The amino acids are then joined together to make the protein. 
protein synthesis,T_2540,"How is the information for making proteins encoded in DNA? The answer is the genetic code. The genetic code is based on the sequence of nitrogen bases in DNA. The four bases make up the letters of the code. Groups of three bases each make up code words. These three-letter code words are called codons. Each codon stands for one amino acid or else for a start or stop signal. There are 20 amino acids that make up proteins. With three bases per codon, there are 64 possible codons. This is more than enough to code for the 20 amino acids plus start and stop signals. You can see how to translate the genetic code in Figure 5.17. Start at the center of the chart for the first base of each three-base codon. Then work your way out from the center for the second and third bases. Find the codon AUG in Figure 5.17. It codes for the amino acid methionine. It also codes for the start signal. After an AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. You can see how this works in Figure 5.18. The figure shows the bases in a molecule "
protein synthesis,T_2541,The genetic code has three other important characteristics. The genetic code is the same in all living things. This shows that all organisms are related by descent from a common ancestor. Each codon codes for just one amino acid (or start or stop). This is necessary so the correct amino acid is always selected. Most amino acids are encoded by more than one codon. This is helpful. It reduces the risk of the wrong amino acid being selected if there is a mistake in the code. 
protein synthesis,T_2542,The process in which proteins are made is called protein synthesis. It occurs in two main steps. The steps are transcription and translation. Watch this video for a good introduction to both steps of protein synthesis: http://w MEDIA Click image to the left or use the URL below. URL: 
protein synthesis,T_2543,"Transcription is the first step in protein synthesis. It takes place in the nucleus. During transcription, a strand of DNA is copied to make a strand of mRNA. How does this happen? It occurs by the following steps, as shown in Figure 5.19. 1. An enzyme binds to the DNA. It signals the DNA to unwind. 2. After the DNA unwinds, the enzyme can read the bases in one of the DNA strands. 3. Using this strand of DNA as a template, nucleotides are joined together to make a complementary strand of mRNA. The mRNA contains bases that are complementary to the bases in the DNA strand. Translation is the second step in protein synthesis. It is shown in Figure 5.20. Translation takes place at a ribosome in the cytoplasm. During translation, the genetic code in mRNA is read to make a protein. Heres how it works: 1. 2. 3. 4. 5. The molecule of mRNA leaves the nucleus and moves to a ribosome. The ribosome consists of rRNA and proteins. It reads the sequence of codons in mRNA. Molecules of tRNA bring amino acids to the ribosome in the correct sequence. At the ribosome, the amino acids are joined together to form a chain of amino acids. The chain of amino acids keeps growing until a stop codon is reached. Then the chain is released from the ribosome. "
protein synthesis,T_2544,"Mutations have many possible causes. Some mutations occur when a mistake is made during DNA replication or transcription. Other mutations occur because of environmental factors. Anything in the environment that causes a mutation is known as a mutagen. Examples of mutagens are shown in Figure 5.21. They include ultraviolet rays in sunlight, chemicals in cigarette smoke, and certain viruses and bacteria. "
protein synthesis,T_2545,"Many mutations have no effect on the proteins they encode. These mutations are considered neutral. Occasionally, a mutation may make a protein even better than it was before. Or the protein might help the organism adapt to a new environment. These mutations are considered beneficial. An example is a mutation that helps bacteria resist antibiotics. Bacteria with the mutation increase in numbers, so the mutation becomes more common. Other mutations are harmful. They may even be deadly. Harmful mutations often result in a protein that no longer can do its job. Some harmful mutations cause cancer or other genetic disorders. Mutations also vary in their effects depending on whether they occur in gametes or in other cells of the body. Mutations that occur in gametes can be passed on to offspring. An offspring that inherits a mutation in a gamete will have the mutation in all of its cells. Mutations that occur in body cells cannot be passed on to offspring. They are confined to just one cell and its daughter cells. These mutations may have little effect on an organism. "
protein synthesis,T_2546,"The effect of a mutation is likely to depend as well on the type of mutation that occurs. A mutation that changes all or a large part of a chromosome is called a chromosomal mutation. This type of mutation tends to be very serious. Sometimes chromosomes are missing or extra copies are present. An example is the mutation that causes Down syndrome. In this case, there is an extra copy of one of the chromosomes. Deleting or inserting a nitrogen base causes a frameshift mutation. All of the codons following the mutation are misread. This may be disastrous. To see why, consider this English-language analogy. Take the sentence The big dog ate the red cat. If the second letter of big is deleted, then the sentence becomes: The bgd oga tet her edc at. Deleting a single letter makes the rest of the sentence impossible to read. Some mutations change just one or a few bases in DNA. A change in just one base is called a point mutation. Table 5.1 compares different types of point mutations and their effects. Type Silent Missense Nonsense Description mutated codon codes for the same amino acid mutated codon codes for a different amino acid mutated codon is a prema- ture stop codon Example CAA (glutamine) ! CAG (glutamine) CAA (glutamine) ! CCA (proline) CAA (glutamine) ! UAA (stop) Effect none variable serious "
darwins theory of evolution,T_2583,"Darwins theory of evolution by natural selection contains two major ideas: One idea is that evolution happens. Evolution is a change in the inherited traits of organisms over time. Living things have changed as descendants diverged from common ancestors in the past. The other idea is that evolution occurs by natural selection. Natural selection is the process in which living things with beneficial traits produce more offspring. As a result, their traits increase in the population over time. "
darwins theory of evolution,T_2584,"How did Darwin come up with the theory of evolution by natural selection? A major influence was an amazing scientific expedition he took on a ship called the Beagle. Darwin was only 22 years old when the ship set sail. The trip lasted for almost five years and circled the globe. Figure 7.2 shows the route the ship took. It set off from Plymouth, England in 1831. It wouldnt return to Plymouth until 1836. Imagine setting out for such an incredible adventure at age 22, and youll understand why the trip had such a big influence on Darwin. Darwins job on the voyage was to observe and collect specimens whenever the ship went ashore. This included plants, animals, rocks, and fossils. Darwin loved nature, so the job was ideal for him. During the long voyage, he made many observations that helped him form his theory of evolution. Some of his most important observations were made on the Galpagos Islands. The 16 Galpagos Islands lie 966 kilometers (about 600 miles) off the west coast of South America. (You can see their location on the map in Figure 7.2.) Some of the animals Darwin observed on the islands were giant tortoises and birds called finches. Watch this video for an excellent introduction to Darwin, his voyage, and the Galpagos: "
darwins theory of evolution,T_2585,"The Galpagos Islands are still famous for their giant tortoises. These gentle giants are found almost nowhere else in the world. Darwin was amazed by their huge size. He was also struck by the variety of shapes of their shells. You can see two examples in Figure 7.3. Each island had tortoises with a different shell shape. The local people even could tell which island a tortoise came from based on the shape of its shell. Darwin wondered how each island came to have its own type of tortoise. He found out that tortoises with dome- shaped shells lived on islands where the plants they ate were abundant and easy to reach. Tortoises with saddle- shaped shells, in contrast, lived on islands that were drier. On those islands, food was often scarce. The saddle shape of their shells allowed tortoises on those islands to reach up and graze on vegetation high above them. This made sense, but how had it happened? "
darwins theory of evolution,T_2586,"Darwin also observed that each of the Galpagos Islands had its own species of finches. The finches on different islands had beaks that differed in size and shape. You can see four examples in Figure 7.4. Darwin investigated further. He found that the different beaks seemed to suit the birds for the food available on their island. For example, finch number 1 in Figure 7.4 used its large, strong beak to crack open and eat big, tough seeds. Finch number 4 had a long, pointed beak that was ideal for eating insects. This seemed reasonable, but how had it come about? "
darwins theory of evolution,T_2587,"Besides his observations on the Beagle, other influences helped Darwin develop his theory of evolution by natural selection. These included his knowledge of plant and animal breeding and the ideas of other scientists. "
darwins theory of evolution,T_2588,"Darwin knew that people could breed plants and animals to have useful traits. By selecting which individuals were allowed to reproduce, they could change an organisms traits over several generations. Darwin called this type of change in organisms artificial selection. You can see an example in Figure 7.5. Keeping and breeding pigeons was a popular hobby in Darwins day. Both types of pigeons in the bottom row were bred from the common rock pigeon at the top of the figure. "
darwins theory of evolution,T_2589,"There were three other scientists in particular that influenced Darwin. Their names are Lamarck, Lyell, and Malthus. All three were somewhat older than Darwin, and he was familiar with their writings. Jean Baptiste Lamarck was a French naturalist. He was one of the first scientists to propose that species change over time. In other words, he proposed that evolution occurs. Lamarck also tried to explain how it happens, but he got that part wrong. Lamarck thought that the traits an organism developed during its life time could be passed on to its offspring. He called this the inheritance of acquired characteristics. Charles Lyell was an English geologist. He wrote a famous book called Principles of Geology. Darwin took the book with him on the Beagle. Lyell argued that geological processes such as erosion change Earths surface very gradually. To account for all the changes that had occurred on the planet, Earth must be a lot older than most people believed. Thomas Malthus was an English economist. He wrote a popular essay called On Population. He argued that human populations have the potential to grow faster than the resources they need. When populations get too big, disease and famine occur. These calamities control population size by killing off the weakest people. "
darwins theory of evolution,T_2590,"Darwin spent many years thinking about his own observations and the writings of Lamarck, Lyell, and Malthus. What did it all mean? How did it all fit together? The answer, of course, is the theory of evolution by natural selection. "
darwins theory of evolution,T_2591,"Heres how Darwin thought through his theory: Like Lamarck, Darwin assumed that species evolve, or change their traits over time. Fossils Darwin found on his voyage helped convince him that evolution occurs. From Lyell, Darwin realized that Earth is very old. This meant that living things had a long time in which to evolve. There was enough time to produce the great diversity of living things that Darwin had observed. From Malthus, Darwin saw that populations could grow faster than their resources. This overproduction of offspring led to a struggle for existence, in Darwins words. In this struggle, only the fittest survive. From Darwins knowledge of artificial selection, he knew how traits can change over time. Breeders artificially select the traits that they find beneficial. These traits become more common over many generations. In nature, Darwin reasoned, individuals with certain traits might be more likely to survive the struggle for existence and have offspring. Their traits would become more common over time. In this case, nature selects the traits that are beneficial. Thats why Darwin called this process natural selection. Darwin used the word fitness to refer to the ability to reproduce and pass traits to the next generation "
darwins theory of evolution,T_2592,Darwin finally published his theory of evolution by natural selection in 1859. He presented it in his book On the Origin of Species. The book is very detailed and includes a lot of evidence for the theory. Darwins book changed science forever. The theory of evolution by natural selection became the unifying theory of all life science. 
evidence for evolution,T_2593,"Fossils are the preserved remains or traces of organisms that lived during earlier ages. Remains that become fossils are generally the hard parts of organismsmainly bones, teeth, or shells. Traces include any evidence of life, such as footprints like the dinosaur footprint in Figure 7.7. Fossils are like a window into the past. They provide direct evidence of what life was like long ago. A scientist who studies fossils to learn about the evolution of living things is called a paleontologist. "
evidence for evolution,T_2594,"The soft parts of organisms almost always decompose quickly after death. Thats why most fossils consist of hard parts such as bones. Its rare even for hard parts to remain intact long enough to become fossils. Fossils form when water seeps through the remains and deposits minerals in them. The remains literally turn to stone. Remains are more likely to form fossils if they are covered quickly by sediments. Once in a while, remains are preserved almost unchanged. For example, they may be frozen in glaciers. Or they may be trapped in tree resin that hardens to form amber. Thats what happened to the wasp in Figure 7.8. The wasp lived about 20 million years ago, but even its fragile wings have been preserved by the amber. "
evidence for evolution,T_2595,"Fossils are useful for reconstructing the past only if they can be dated. Scientists need to determine when the organisms lived who left behind the fossils. Fossils can be dated in two different ways: absolute dating and relative dating. Absolute dating determines about how long ago a fossil organism lived. This gives the fossil an approximate age in years. Absolute dating is often based on the amount of carbon-14 or other radioactive element that remains in a fossil. You can learn how carbon-14 dating works by watching this short video: Relative dating determines which of two fossils is older or younger than the other but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower rock layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in Figure 7.9. "
evidence for evolution,T_2596,"The evolution of whales is a good example of how fossils can help us understand evolution. Scientists have long known that mammals first evolved on land about 200 million years ago. Its been a mystery, however, how whales evolved. Whales are mammals that live completely in the water. Did they evolve from earlier land mammals? Or did they evolve from animals that already lived in the water? Starting in the late 1970s, a growing number of fossils have allowed scientists to piece together the story of whale evolution. The fossils represent ancient, whale-like animals. They show that an ancient land mammal made its way back to the sea more than 50 million years ago. It became the ancestor of modern whales. In doing so, it lost its legs and became adapted to life in the water. In Figure 7.10 you can see an artists rendition of such a whale ancestor. It had legs and could walk on land, but it was also a good swimmer. Watch this short video to learn more about the amazing story of whale evolution based on the fossils: "
evidence for evolution,T_2597,"Scientists have learned a lot about evolution by comparing living organisms. They have compared body parts, embryos, and molecules such as DNA and proteins. "
evidence for evolution,T_2598,"Comparing body parts of different species may reveal evidence for evolution. For example, all mammals have front limbs that look quite different and are used for different purposes. Bats use their front limbs to fly, whales use them to swim, and cats use them to run and climb. However, the front limbs of all three animalsas well as humanshave the same basic underlying bone structure. You can see this in Figure 7.11. The similar bones provide evidence that all four animals evolved from a common ancestor. "
evidence for evolution,T_2599,"Some of the most interesting evidence for evolution comes from vestigial structures. These are body parts that are no longer used but are still present in modern organisms. Examples in humans include tail bones and the appendix. Human beings obviously dont have tails, but our ancestors did. We still have bones at the base of our spine that form a tail in other, related animals, such as monkeys. The appendix is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food. If your appendix becomes infected, a surgeon can remove it. You wont miss it because it no longer has any purpose in the human body. "
evidence for evolution,T_2600,"An embryo is an organism in the earliest stages of development. Embryos of different species may look quite similar, even when the adult forms look very different. Look at the drawings of embryos in Figure 7.12. They represent very early life stages of a chicken, turtle, pig, and human being. The embryos look so similar that its hard to tell them apart. Such similarities provide evidence that all four types of animals are related. They help document that evolution has occurred. "
evidence for evolution,T_2601,"Scientists can compare the DNA or proteins of different species. If the molecules are similar, this shows that the species are related. The more similar the molecules are, the closer the relationship is likely to be. When molecules are used in this way, they are called molecular clocks. This method assumes that random mutations occur at a constant rate for a given protein or segment of DNA. Over time, the mutations add up. The longer the amount of time since species diverged, the more differences there will be in their DNA or proteins. Table 7.1 compares the DNA of four different organisms with modern human DNA. The DNA of chimpanzees is almost 99 percent the same as the DNA of modern humans. This shows that chimpanzees are very closely related to us. We are less closely related to the other organisms in the table. Its no surprise that grapes, which are plants, are less like us than the animals in the table. Organism Chimpanzee Cow Chicken Honeybee Grape Similarity with Human DNA (percent the same) 98.8 85 65 44 24 "
evidence for evolution,T_2602,"The best evidence for evolution comes from actually observing changes in organisms through time. In the 1970s, biologists Peter and Rosemary Grant went to the Galpagos Islands to do fieldwork. They wanted to re-study Darwins finches. They spent the next 40 years on the project. Their hard work paid off. They were able to document evolution by natural selection taking place in the finches. A period of very low rainfall occurred while the Grants were on the islands. The drought resulted in fewer seeds for the finches to eat. Birds with smaller beaks could eat only the smaller seeds. Birds with bigger beaks were better off. They could eat seeds of all sizes. Therefore, there was more food available to them. Many of the small-beaked birds died in the drought. More of the big-beaked birds survived and reproduced. Within just a couple of years, the average beak size in the finches increased. This was clearly evolution by natural selection. "
the scale of evolution,T_2603,"We now know how variation in traits is inherited. Variation in traits is controlled by different alleles for genes. Alleles, in turn, are passed to gametes and then to offspring. Evolution occurs because of changes in alleles over time. How long a time? That depends on the time scale of evolution you consider. Evolution that occurs over a short period of time is known as microevolution. It might take place in just a couple of generations. This scale of evolution occurs at the level of the population. The Grants observed evolution at this scale in populations of Darwins finches. Beak size in finch populations changed in just two years because of a serious drought. Evolution that occurs over a long period of time is called macroevolution. It might take place over millions of years. This scale of evolution occurs above the level of the species. Fossils provide evidence for evolution at this scale. The evolution of the horse family, shown in Figure 7.13, is an example of macroevolution. "
the scale of evolution,T_2604,Individuals dont evolve. Their alleles dont change over time. The unit of microevolution is the population. 
the scale of evolution,T_2605,"A population is a group of organisms of the same species that live in the same area. All the genes in all the members of a population make up the populations gene pool. For each gene, the gene pool includes all the different alleles in the population. The gene pool can be described by its allele frequencies for specific genes. The frequency of an allele is the number of copies of that allele divided by the total number of alleles for the gene in the gene pool. A simple example will help you understand these concepts. The data in Table 7.2 represent a population of 100 individuals. For each gene, the gene pool has a total of 200 alleles (2 per individual x 100 individuals). The gene in question exists as two different alleles, A and a. The number of A alleles in the gene pool is 140. Of these, 100 are in the 50 AA homozygotes. Another 40 are in the 40 Aa heterozygotes. The number of a alleles in the gene pool is 60. Of these, 40 are in the 40 Aa heterozygotes. Another 20 are in the 10 aa homozygotes. The frequency of the A allele is 140/200 = 0.7. The frequency of the a allele is 60/200 = 0.3. Genotype AA Aa aa Totals Number of Individuals 50 40 10 100 Number of A Alleles 100 (50 x 2) 40 (40 x 1) 0 (10 x 0) 140 Number of a Alleles 0 (50 x 0) 40 (40 x 1) 20 (10 x 2) 60 Evolution occurs in a population when its allele frequencies change over time. For example, the frequency of the A allele might change from 0.7 to 0.8. If that happens, evolution has occurred. What causes allele frequencies to change? The answer is forces of evolution. "
the scale of evolution,T_2606,"There are four major forces of evolution that cause allele frequencies to change. They are mutation, gene flow, genetic drift, and natural selection. Mutation creates new genetic variation in a gene pool This is how all new alleles first arise. Its the ultimate source of new genetic variation, so it is essential for evolution. However, for any given gene, the chance of a mutation occurring is very small. Therefore, mutation alone does not have much effect on allele frequencies. Gene flow is the movement of genes into or out of a gene pool It occurs when individuals migrate into or out of the population. How much gene flow changes allele frequencies depends on how many migrants there are and their genotypes. Genetic drift is a random change in allele frequencies. It occurs in small populations. Allele frequencies in the offspring may differ by chance from those in the parents. This is like tossing a coin just a few times. You may, by chance, get more or less than the expected 50 percent heads or tails. In the same way, you may get more or less than the expected allele frequencies in the small number of individuals in the next generation. The smaller the population is, the more allele frequencies may drift. Natural selection is a change in allele frequencies that occurs because some genotypes are more fit than others. Genotypes with greater fitness produce more offspring and pass more copies of their alleles to the next generation. This is the force of evolution that Darwin identified. Figure 23.12 shows how Darwin thought natural selection led to variation in finches on the Galpagos Islands. "
the scale of evolution,T_2607,What happens when forces of evolution work over a long period of time? The answer is macroevolution. An example is the evolution of a new species. 
the scale of evolution,T_2608,"The evolution of a new species is called speciation. A species is a group of organisms that can mate and produce fertile offspring together but not with members of other such groups. What must happen for a new species to arise? Some members of an existing species must change so they can no produce fertile offspring with the rest of the species. Speciation often occurs when some members of a species break off from the rest. The splinter group evolves in isolation from the original species. The original species also continues to evolve. Sooner or later, the splinter group becomes too different to breed with members of the original species. At that point, a new species has formed. A good example of speciation involves anole lizards, like the one pictured in Figure 7.15. There are about 150 different species of anole lizards in the Caribbean Islands. Scientists think that a single species of lizard first colonized one of the islands about 50 million years ago. A few lizards from this original species eventually reached each of the other islands, where they evolved in isolation. Anoles in different habitats evolved traits that affected mating. For example, they evolved skin flaps of different colors. Females didnt respond to male anoles with the wrong color skin flap. This prevented them from mating. Eventually, all the different species of anoles known today evolved. Watch this interesting video to learn more about anole speciation in the Caribbean: "
the scale of evolution,T_2609,"Sometimes two species evolve the same traits. It happens because they live in similar habitats. This is called convergent evolution. Caribbean Anoles demonstrate this as well. On each Caribbean island, anoles in similar habitats evolved the same traits. For example, anoles that lived on the forest floor evolved long legs for leaping and running on the ground. Anoles that lived on tree branches evolved short legs that helped them cling to small branches and twigs. Anoles that lived at the tops of trees evolved large toe pads that allowed them to walk on leaves without falling. On each of the islands, there were anole species that evolved in each of these same ways. "
the scale of evolution,T_2610,"Two species may often interact with each other and have a close relationship. Examples include flowers and the animals that pollinate them. When one of the two species evolves new traits, the other species may evolve matching traits. This is called coevolution. You can see an example of this in Figure 7.16. The very long beak of this hummingbird co-evolved with the tubular flowers it pollinates. Only this species of hummingbird can reach nectar deep in the flowers. "
the scale of evolution,T_2611,"Darwin thought that evolution occurs very slowly. This is likely if conditions are stable. But what if conditions are changing rapidly? Evolution is likely to occur more rapidly as well. For example, the Grants showed that evolution occurred in just a couple of years in Darwins finches. This happened when a severe drought killed off a lot of the plants that the birds needed for food. Millions of fossils have been found since Darwins time. They show that evolution may occur in fits and starts. Long period of little or gradual change may be interrupted by bursts of rapid change. The rate of evolution is influenced by how the environment is changing. Today, Earths climate is changing rapidly. How do you think this might affect the rate of evolution? "
history of life on earth,T_2612,Its hard to grasp the vast amounts of time since Earth formed and life first appeared. It may help to think of Earths history as a 24-hour day. 
history of life on earth,T_2613,"Figure 7.17 shows the history of Earth in a day. In this model, the planet forms at midnight. The first prokaryotes evolve around 3:00 am. Eukaryotes evolve at about 1:00 pm. Animals dont evolve until almost 8:00 pm. Humans appear only in the last minute of the day. Relating these major events in Earths history to a 24-hour day helps to put them in perspective. "
history of life on earth,T_2614,"Another tool for understanding the history of Earth and its life is the geologic time scale. You can see this time scale in Figure 7.18. It divides Earths history into eons, eras, and periods. These divisions are based on major changes in geology, climate, and the evolution of life. The geologic time scale organizes Earths history on the basis of important events instead of time alone. It also puts more focus on recent events, about which we know the most. "
history of life on earth,T_2615,"The Precambrian Supereon is the first major division of Earths history (see Figure 7.18). It covers the time from Earths formation 4.6 billion years ago to 544 million years ago. To see how life evolved during the Precambrian and beyond, watch this wonderful video. Its a good introduction to the rest of the lesson. MEDIA Click image to the left or use the URL below. URL: "
history of life on earth,T_2616,"When Earth first formed, it was a fiery hot, barren ball. It had no oceans or atmosphere. Rivers of melted rock flowed over its surface. Gradually, the planet cooled and formed a solid crust. Gases from volcanoes formed an atmosphere, although it contained only a trace of oxygen. As the planet continued to cool, clouds formed and rain fell. Rainwater helped form oceans. The ancient atmosphere and oceans would be toxic to modern life, but they set the stage for life to begin. "
history of life on earth,T_2617,"All living things consist of organic molecules. Many scientists think that organic molecules evolved before cells, perhaps as early as 4 billion years ago. Its possible that lightning sparked chemical reactions in Earths early atmosphere. This could have created a soup of organic molecules from inorganic chemicals. Some scientists think that RNA was the first organic molecule to evolve. RNA can not only encode genetic instructions. Some RNA molecules can carry out chemical reactions. All living things are made of one or more cells. How the first cells evolved is not known for certain. Scientists speculate that lipid membranes grew around RNA molecules. The earliest cells may have consisted of little more than RNA inside a lipid membrane. You can see a model of such a cell in Figure 7.19. The first cells probably evolved between 3.8 and 4 billion years ago. Scientists think that one cell, called the Last Universal Common Ancestor (LUCA), gave rise to all of the following life on Earth. LUCA may have existed around 3.5 billion years ago. "
history of life on earth,T_2618,"The earliest cells were heterotrophs. They were unable to make food. Instead, they got energy by ""eating"" organic molecules in the soup around them. The earliest cells were also prokaryotes. They lacked a nucleus and other organelles. Gradually, these and other traits evolved. Photosynthesis evolved about 3 billion years ago. After that, certain cells could use sunlight to make food. These were the first autotrophs. They made food for themselves and other cells. They also added oxygen to the atmosphere. The oxygen was a waste product of photosynthesis. Oxygen was toxic to many cells. They had evolved in its absence. Many of them died out. The few that survived evolved a new way to use oxygen. They used it to get energy from food. This is the process of cellular respiration. The first eukaryotic cells probably evolved about 2 billion years ago. Thats when cells evolved organelles and a nucleus. Figure 7.20 shows one theory about the origin of organelles. According to this theory, a large cell engulfed small cells. The small cells took on special roles that helped the large cell function. In return, the small cells got nutrients from the large cell. Eventually, the large and small cells could no longer live apart. With their specialized organelles, eukaryotic cells were powerful and efficient. Eukaryotes would go on to evolve sexual reproduction. They would also evolve into multicellular organisms. The first multicellular organisms evolved about 1 billion years ago. "
history of life on earth,T_2619,"At the end of the Precambrian, a mass extinction occurred. In a mass extinction, the majority of species die out. The Precambrian mass extinction was the first of six mass extinctions that occurred on Earth. Its not certain what caused this first mass extinction. Changes in Earths geology and climate were no doubt involved. "
history of life on earth,T_2620,The Paleozoic Era lasted from 544 to 245 million years ago. It is divided into six periods. 
history of life on earth,T_2621,"The Precambrian mass extinction opened up many niches for new organisms to fill. As a result, the Cambrian Period began with an explosion of new kinds of living things. For example, many types of simple animals called sponges evolved. Trilobites were also very common. Sponges and trilobites were small ocean invertebrates. These are animals without a backbone. You can see examples of them in Figure 7.21. "
history of life on earth,T_2622,"During the Ordovician Period, the oceans became filled with many kinds of invertebrates. The first fish also evolved. Plants colonized the land for the first time. However, animals remained in the water. "
history of life on earth,T_2623,"Corals appeared in the oceans during the Silurian period. Fish continued to evolve. On land, vascular plants appeared. These are plants that have special tissues to circulate water and other substances. This allowed plants to become larger and colonize drier habitats. "
history of life on earth,T_2624,"During the Devonian Period, the first seed plants evolved. Seeds have a protective coat and contain stored food. This was a big advantage over other types of plant reproduction. Seed plants eventually became the most common type of plants on land. In the oceans, fish with lobe fins evolved. These fish could breathe air when they raised their head above water. This was a step in the evolution of animals that could live on land. "
history of life on earth,T_2625,"In the Carboniferous Period, forests of huge ferns and trees were widespread. You can see how these first forests might have looked in Figure 7.22. After the ferns and trees died, their remains eventually turned to coal. The first amphibians also evolved during this period. They could live on land but had to return to the water to lay their eggs. After amphibians, the earliest reptiles appeared. They were the first animals that could reproduce on land and move away from the water. "
history of life on earth,T_2626,"During the Permian Period, all the major landmasses moved together to form one supercontinent. The supercontinent has been named Pangaea. You can see how it looked in Figure 7.23. At this time, temperatures were extreme and the climate became very dry. As a result, plants and animals evolved ways to cope with dryness. For example, reptiles evolved leathery skin. This helped prevent water loss. Plants evolved waxy leaves for the same purpose. The Permian Period ended with Earths second mass extinction. During this event, most of Earths species went extinct. It was the most massive extinction ever recorded. Its not clear why it happened. One possible reason is that a very large meteorite struck Earth. Another possibility is the eruption of enormous volcanoes. Either event could create a huge amount of dust. The dust might block out sunlight for months. This would cool the planet and prevent photosynthesis. "
history of life on earth,T_2627,The Permian mass extinction paved the way for another burst of new life at the start of the Mesozoic Era. This era is known as the age of dinosaurs. It is divided into three periods. 
history of life on earth,T_2628,"During the Triassic Period, the first dinosaurs evolved from reptile ancestors. They eventually colonized the air and water in addition to the land. There were also forests of huge seed ferns and cone-bearing conifer trees in the Triassic Period. Modern corals, fish, and insects all evolved in this period as well. The supercontinent of Pangea started to break up. The Triassic Period ended in a mass extinction. The majority of species died out, but dinosaurs were spared. "
history of life on earth,T_2629,"The Triassic mass extinction gave dinosaurs the opportunity to really flourish during the Jurassic Period. Thats why this period is called the golden age of dinosaurs. The earliest birds also evolved during the Jurassic from dinosaur ancestors. In addition, all the major groups of mammals appeared. Flowering plants also appeared for the first time. New insects evolved to pollinate them. The continents continued to move apart. "
history of life on earth,T_2630,"During the Cretaceous Period, the dinosaurs reached their maximum size and distribution. For example, the well- known Tyrannosaurus rex weighed at least 7 tons! You can get an idea of how big it was from the T. rex skeleton in Figure 7.24. (Notice how small the person looks in the bottom left of the photo.) By the end of the Cretaceous, the continents were close to their present locations. The period ended with another mass extinction. This time, the dinosaurs went extinct. What happened to the dinosaurs? Some scientists think that a comet or asteroid may have crashed into Earth. This could darken the sky, shut down photosynthesis, and cause climate change. Other factors probably contributed to the mass extinction as well. "
history of life on earth,T_2631,The extinction of the dinosaurs at the end of the Mesozoic Era paved the way for mammals to take over. Thats why the Cenozoic Era is called the age of mammals. They soon became the dominant land animals on Earth. The Cenozoic is divided into two periods. 
history of life on earth,T_2632,"During the Tertiary Period, many new kinds of mammals evolved. For example, primates and human ancestors first appeared during this period. Many mammals also increased in size. Modern rain forests and grasslands appeared. Flowering plants and insects increased in numbers. "
history of life on earth,T_2633,"During the Quaternary Period, the climate cooled. This caused a series of ice ages. Glaciers advanced southward from the North Pole. They reached as far south as Chicago and New York City. Sea levels fell because so much water was frozen in glaciers. This exposed land bridges between continents. The land bridges allowed land animals to move to new areas. Some mammals adapted to the cold by evolving very large size and thick fur. An example is the woolly mammoth, shown in Figure 7.25. Other mammals moved closer to the equator. Those that couldnt adapt or move went extinct, along with many plants. The last ice age ended about 12,000 years ago. By then, our own species, Homo sapiens, had evolved. After that, we were eyewitnesses to the story of life. As a result, the recent past is less of a mystery than the billions of years before it. "
bacteria,T_2649,"Bacteria are the most abundant living things on Earth. They live in almost all environments. They are found in the air, ocean, soil, and intestines of animals. They are even found in rocks deep below Earths surface. Any surface that has not been sterilized is likely to be covered with bacteria. The total number of bacteria in the world is amazing. Its estimated to be about 5 million trillion trillion. If you write that number in digits, it has 30 zeroes! "
bacteria,T_2650,"Bacteria are the most diverse organisms on Earth. Thousands of species of bacteria have been discovered. Many more are thought to exist. The known species are classified on the basis of various traits. For example, they may be classified by the shape of their cells. They may also be classified by how they react to a dye called Gram stain. "
bacteria,T_2651,"Bacteria come in several different shapes. The different shapes can be seen by examining bacteria under a light microscope. Therefore, its relatively easy to classify them by shape. There are three types of bacteria based on shape: bacilli (bacillus, singular), or rod shaped. cocci (coccus, singular), or sphere shaped. spirilli (spirillus, singular), or spiral shaped. You can see a common example of each type of bacteria in Figure 8.10. "
bacteria,T_2652,"Different types of bacteria stain a different color when Gram stain is applied to them. This makes them easy to identify. Some stain purple and some stain red, as you can see in Figure 8.11. The two types differ in their outer layers. This explains why they stain differently. Bacteria that stain purple are called gram-positive bacteria. They have a thick cell wall without an outer membrane. Bacteria that stain red are called gram-negative bacteria. They have a thin cell wall with an outer membrane. "
bacteria,T_2653,"Bacteria and people have many important relationships. Bacteria make our lives easier in a variety of ways. In fact, we could not survive without them. On the other hand, many bacteria can make us sick. Some of them are even deadly. For a dramatic overview of the many roles of bacteria, watch this stunning video:  MEDIA Click image to the left or use the URL below. URL: "
bacteria,T_2654,"Bacteria help usand all other living thingsby decomposing wastes. In this way, they recycle carbon and nitrogen in ecosystems. In addition, photosynthetic cyanobacteria are important producers. On ancient Earth, they added oxygen to the atmosphere and changed the course of evolution forever. There are billions of bacteria inside the human digestive tract. They help us digest food. They also make vitamins and play other important roles. We use bacteria in many other ways as well. For example, we use them to: create medical products such as vaccines. transfer genes in gene therapy. make fuels such as ethanol. clean up oil spills. kill plant pests. ferment foods. Do you eat any of the fermented foods pictured in Figure 8.12? If so, you are eating bacteria and their wastes. Yum! "
bacteria,T_2655,"You have ten times as many bacterial cells as human cells in your body. Luckily for you, most of these bacteria are harmless. However, some of them can cause disease. Any organism that causes disease is called a pathogen. Diseases caused by bacterial pathogens include food poisoning, strep throat, and Lyme disease. Bacteria that cause disease may spread directly from person to person. For example, they may spread when people shake hands with, or sneeze on, other people. Bacteria may also spread through food, water, or objects that have become contaminated with them. Some bacteria are spread by vectors. A vector is an organism that spreads bacteria or other pathogens. Most vectors are animals, commonly insects. For example, deer ticks like the one in Figure 8.13 spread Lyme disease. Ticks carry Lyme disease bacteria from deer to people when they bite them. "
bacteria,T_2656,"Bacteria in food or water usually can be killed by heating it to a high temperature. Generally, this temperature is at least 71 C (160 F). Bacteria on surfaces such as countertops and floors can be killed with disinfectants, such as chlorine bleach. Bacterial infections in people can be treated with antibiotic drugs. These drugs kill bacteria and may quickly cure the disease. If youve ever had strep throat, you were probably prescribed an antibiotic to treat it. Some bacteria have developed antibiotic resistance. They have evolved traits that make them resistant to one or more antibiotic drugs. You can see how this happens in Figure 8.14. Its an example of natural selection. Some bacteria are now resistant to most common antibiotic drugs. These infections are very hard to treat. "
adulthood and aging,T_2690,"When is a person considered an adult? That depends. Most teens become physically mature by the age of 16 or so. But they are not adults in a legal sense until they are older. For example, in the U.S., you must be 18 to vote. Once adulthood begins, it can be divided into three stages: (1) early, (2) middle, and (3) late adulthood. "
adulthood and aging,T_2691,"Early adulthood starts at age 18 or 21. It continues until the mid-30s. During early adulthood, people are at their physical peak. They are also usually in good health. The ability to have children is greatest during early adulthood, as well. This is the stage of life when most people complete their education. They are likely to begin a career or take a full-time job. Many people also marry and start a family during early adulthood. "
adulthood and aging,T_2692,"Middle adulthood begins in the mid-30s. It continues until the mid-60s. During middle adulthood, people start to show signs of aging. Their hair slowly turns gray. Their skin develops wrinkles. The risk of health problems also increases during middle adulthood. For example, heart disease, cancer, and diabetes become more common during this time. This is the stage of life when people are most likely to achieve career goals. Their children also grow up and may leave home during this stage. "
adulthood and aging,T_2693,"Late adulthood begins in the mid-60s. It continues until death. This is the stage of life when most people retire from work. They are also likely to reflect on their life. They may focus on their grandchildren. During late adulthood, people are not as physically able. For example, they usually have less muscle and slower reflexes. Their immune system also doesnt work as well as it used to. As a result, they have a harder time fighting diseases like the flu. The risk of developing diseases such as heart disease and cancer continues to rise. Arthritis is also common. In arthritis, joints wear out and become stiff and painful. As many as one in four late adults may develop Alzheimers disease. In this disease, brain changes cause mental abilities to decrease. This family picture shows females in each of the three stages of life. Which stage does each represent? Despite problems such as these, many people remain healthy and active into their 80s or even 90s. Do you want to be one of them? Then adopt a healthy lifestyle now and follow it for life. Doing so will increase your chances of staying healthy and active to an old age. Exercising the body and brain help prevent the physical and mental effects of aging. "
aquatic biomes,T_2716,"Recall that terrestrial biomes are defined by their climate. Thats because plants and animals are adapted for certain amounts of temperature and moisture. However, would aquatic biomes be classified in the same way? No, that wouldnt make much senseall parts of an aquatic environment have plenty of water. Aquatic biomes can be generally classified based on the amount of salt in the water. Freshwater biomes have less than 1% salt and are typical of ponds and lakes, streams and rivers, and wetlands. Marine biomes have more salt and are characteristic of the oceans, coral reefs, and estuaries. Most aquatic organisms do not have to deal with extremes of temperature or moisture. Instead, their main limiting factors are the availability of sunlight and the concentration of dissolved oxygen and nutrients in the water. "
aquatic biomes,T_2717,"Aquatic biomes in the ocean are called marine biomes. Organisms that live in marine biomes must be adapted to the salt in the water. For example, many have organs for excreting excess salt. Marine biomes include the oceans, coral reefs, and estuaries ( Figure 1.1). The oceans are the largest of all the ecosystems. They can be divided into four separate zones based on the amount of sunlight. Ocean zones are also divided based on their depth and their distance from land. Each zone has a great diversity of species. Within a coral reef, the dominant organisms are corals. Corals consist partially of algae, which provide nutrients via photosynthesis. Corals also extend tentacles to obtain plankton from the water. Coral reefs include several species of microorganisms, invertebrates, fishes, sea urchins, octopuses, and sea stars. Estuaries are areas where freshwater streams or rivers merge with the ocean. An example of a marine biome, a kelp for- est, from Anacapa Island in the Channel Islands National Marine Sanctuary. "
aquatic biomes,T_2718,"Freshwater biomes are defined by their low salt concentration, usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration, such as the ocean. There are different types of freshwater biomes: ponds and lakes ( Figure 1.2), streams and rivers, and wetlands. Ponds and lakes range in size from just a few square meters to thousands of square kilometers. Streams and rivers are bodies of flowing water moving in one direction. They can be found everywhere. They get their starts at headwaters, which may be springs, melting snow, or even lakes, and then travel all the way to their mouths, emptying into another water channel or the ocean. Wetlands are areas of standing water that support aquatic plants. Wetlands include marshes, swamps, and bogs. Lake Tahoe in Northern California is a freshwater biome. "
aquatic biomes,T_2719,"In large bodies of water, such as the ocean and lakes, the water can be divided into zones based on the amount of sunlight it receives: 1. The photic zone extends to a maximum depth of 200 meters (656 feet) below the surface of the water. This is where enough sunlight penetrates for photosynthesis to occur. Algae and other photosynthetic organisms can make food and support food webs. 2. The aphotic zone is water deeper than 200 meters. This is where too little sunlight penetrates for photosyn- thesis to occur. As a result, producers must make ""food"" by chemosynthesis, or the food must drift down from the water above. "
aquatic biomes,T_2720,"Water in lakes and the ocean also varies in the amount of dissolved oxygen and nutrients it contains: 1. Water near the surface of lakes and the ocean usually has more dissolved oxygen than does deeper water. This is because surface water absorbs oxygen from the air above it. 2. Water near shore generally has more dissolved nutrients than water farther from shore. This is because most nutrients enter the water from land. They are carried by runoff, streams, and rivers that empty into a body of water. 3. Water near the bottom of lakes and the ocean may contain more nutrients than water closer to the surface. When aquatic organisms die, they sink to the bottom. Decomposers near the bottom of the water break down the dead organisms and release their nutrients back into the water. "
autoimmune diseases,T_2733,"The immune system usually protects you from pathogens and other causes of disease. When the immune system is working properly, it keeps you from getting sick. But the immune system is like any other system of the body. It can break down or develop diseases. AIDS is an infectious disease of the immune system caused by a virus. Some diseases of the immune system are noninfectious. They include autoimmune diseases and allergies. "
autoimmune diseases,T_2734,"Does it make sense for an immune system to attack the cells it is meant to protect? No, but an immune system that does not function properly will attack its own cells. An autoimmune disease is a disease in which the immune system attacks the bodys own cells. One example is type 1 diabetes. In this disease, the immune system attacks cells of the pancreas. Other examples are multiple sclerosis and rheumatoid arthritis. In multiple sclerosis, the immune system attacks nerve cells. This causes weakness and pain. In rheumatoid arthritis, the immune system attacks the cells of joints. This causes joint damage and pain. Autoimmune diseases cannot be cured. But they can be helped with medicines that weaken the immune systems attack on normal cells. Other autoimmune diseases include celiac disease (damages to the small intestine), inflam- matory bowel disease (damage to the digestive tract), psoriasis (damage to the skin), and lupus (damage to the joints, skin, kidneys, heart, and lungs). "
autoimmune diseases,T_2735,"An allergy occurs when the immune system attacks a harmless substance that enters the body from the outside. A substance that causes an allergy is called an allergen. It is the immune system, not the allergen, that causes the symptoms of an allergy. Did you ever hear of hay fever? Its not really a fever at all. Its an allergy to plant pollens. People with this type of allergy have symptoms such as watery eyes, sneezing, and a runny nose. A common cause of hay fever is the pollen of ragweed. Many people are also allergic to poison ivy ( Figure 1.2). Skin contact with poison ivy leads to an itchy rash in people who are allergic to the plant. Ragweed is a common roadside weed found throughout the United States. Many people are allergic to its pollen. Some people are allergic to certain foods. Nuts and shellfish are common causes of food allergies. Other common causes of allergies include: Drugs, such as penicillin. Mold. Dust. The dead skin cells of dogs and cats, called dander. Stings of wasps and bees. Most allergies can be treated with medicines. Medicines used to treat allergies include antihistamines and corticos- teroids. These medicines help control the immune system when it attacks an allergen. Sometimes, allergies cause severe symptoms, a condition known as anaphylaxis. For example, they may cause the throat to swell so it is hard to breathe. Severe allergies may be life threatening. They require emergency medical care. "
bacteria in the digestive system,T_2745,"Your large intestine is not just made up of cells. It is also an ecosystem, home to trillions of bacteria known as the ""gut flora"" ( Figure 1.1). But dont worry, most of these bacteria are helpful. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth. Gut bacteria have several roles in the body. For example, intestinal bacteria: Produce vitamin B12 and vitamin K. Control the growth of harmful bacteria. Break down poisons in the large intestine. Break down some substances in food that cannot be digested, such as fiber and some starches and sugars. Bacteria produce enzymes that digest carbohydrates in plant cell walls. Most of the nutritional value of plant material would be wasted without these bacteria. These help us digest plant foods like spinach. Your intestines are home to trillions of bacteria. A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria. About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food. It is estimated that 100 trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet. There are over 7 billion people on the planet. The bacteria in your digestive system are from anywhere between 300 and 1000 species. As these bacteria are helpful, your body does not attack them. They actually appear to the bodys immune system as cells of the digestive system, not foreign invaders. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system. As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship. Lastly, keep in mind the small size of bacteria. Together, all the bacteria in your gut may weight just about 2 pounds. "
bacteria nutrition,T_2746,"Like all organisms, bacteria need energy, and they can acquire this energy through a number of different ways. "
bacteria nutrition,T_2747,"Photosynthetic bacteria use the energy of the sun to make their own food. In the presence of sunlight, carbon dioxide and water are turned into glucose and oxygen. The glucose is then turned into usable energy. Glucose is like the ""food"" for the bacteria. An example of photosynthetic bacteria is cyanobacteria, as seen in the opening image. These bacteria are sometimes called blue-green algae, although they are not algae, due to their numerous chlorophyll molecules. "
bacteria nutrition,T_2748,"Bacteria known as decomposers break down wastes and dead organisms into smaller molecules. These bacteria use the organic substrates they break down to get their energy, carbon, and nutrients they need for survival. "
bacteria nutrition,T_2749,"Bacteria can also be chemotrophs. Chemosynthetic bacteria, or chemotrophs, obtain energy by breaking down chemical compounds in their environment. An example of one of these chemicals broken down by bacteria is nitrogen-containing ammonia. These bacteria are important because they help cycle nitrogen through the environ- ment for other living things to use. Nitrogen cannot be made by living organisms, so it must be continually recycled. Organisms need nitrogen to make organic compounds, such as DNA. "
bacteria nutrition,T_2750,"Some bacteria depend on other organisms for survival. For example, some bacteria live in the roots of legumes, such as pea plants ( Figure 1.1). The bacteria turn nitrogen-containing molecules into nitrogen that the plant can use. Meanwhile, the root provides nutrients to the bacteria. In this relationship, both the bacteria and the plant benefit, so it is known as a mutualism. Other mutualistic bacteria include gut microbes. These are bacteria that live in the intestines of animals. They are usually beneficial bacteria, needed by the host organism. These microbes obviously dont kill their host, as that would kill the bacteria as well. "
bacteria nutrition,T_2751,"Other bacteria are parasitic and can cause illness. In parasitism, the bacteria benefit, and the other organism is harmed. Harmful bacteria will be discussed in another concept. "
barriers to pathogens,T_2755,"It is the immune systems job to protect the body. Your body has many ways to protect you from pathogens. Your bodys defenses are like a castle. The outside of a castle was protected by a moat and high walls. Inside the castle, soldiers were ready to fight off any enemies that made it across the moat and over the walls. Like a castle, your body has a series of defenses. Only pathogens that get through all the defenses can harm you. The first line of defence includes both physical and chemical barriers that are always ready and prepared to defend the body from infection. Pathogens must make it past this first line of defense to cause harm. If this defense is broken, the second line of defense within your body is activated. Your bodys first line of defense is like a castles moat and walls. It keeps most pathogens out of your body. This is a non-specific type of defense, in that it tries to keep all pathogens out. The first line of defense includes different types of barriers. Being the ""first line"", it starts with the skin. The first line also includes tears, mucus, cilia, stomach acid, urine flow, and friendly bacteria. "
barriers to pathogens,T_2756,"The skin is a very important barrier to pathogens. The skin is the bodys largest organ. In adults, it covers an area of about 16 to 22 square feet! The skin is also the bodys most important defense against disease. It forms a physical barrier between the body and the outside world. The skin has several layers that stack on top of each other ( Figure The mouth and nose are not lined with skin. Instead, they are lined with mucous membranes. Other organs that are exposed to the outside world, including the lungs and stomach, are also lined with mucous membranes. Mucous membranes are not tough like skin, but they have other defenses. One defense of mucous membranes is the mucus they release. Mucus is a sticky, moist substance that covers mucous membranes. Most pathogens get stuck in the mucus before they can do harm to the body. Many mucous membranes also have cilia. Cilia in the lungs are pictured below ( Figure 1.2). Cilia are tiny finger-like projections. They move in waves and sweep mucus and trapped pathogens toward body openings. When you clear your throat or blow your nose, you remove mucus and pathogens from your body. "
barriers to pathogens,T_2757,"Most body fluids that you release from your body contain chemicals that kill pathogens. For example, mucus, sweat, tears, and saliva contain enzymes called lysozymes that kill pathogens. These enzymes can break down the cell walls of bacteria to kill them. The stomach also releases a very strong acid, called hydrochloric acid. This acid kills most pathogens that enter the stomach in food or water. Urine is also acidic, so few pathogens can grow in it. This is what the cilia lining the lungs look like when they are magnified. Their movements constantly sweep mucus and pathogens out of the lungs. Do they remind you of brushes? "
barriers to pathogens,T_2758,"You are not aware of them, but your skin is covered by millions (or more!) of bacteria. Millions more live inside your body. Most of these bacteria help defend your body from pathogens. How do they do it? They compete with harmful bacteria for food and space. This prevents the harmful bacteria from multiplying and making you sick. "
blood types,T_2774,"Do you know what your blood type is? Maybe you have heard people say that they have type A or type O blood. Blood type is a way to describe the type of antigens, or proteins, on the surface of red blood cells (RBCs). There are four blood types; A, B, AB, and O. 1. Type A blood has type A antigens on the RBCs in the blood. 2. Type AB blood has A and B antigens on the RBCs. 3. Type B has B antigens on the RBCs. 4. Type O does not have either A or B antigens. The ABO blood group system is important if a person needs a blood transfusion. A blood transfusion is the process of putting blood or blood products from one person into the circulatory system of another person. The blood type of the recipient needs to be carefully matched to the blood type of the donor. Thats because different blood types have different types of antibodies, or proteins, released by the blood cells. Antibodies attack strange substances in the body. This is a normal part of your immune response, which is your defense against disease. For example, imagine a person with type O blood was given type A blood. First, what type of antibodies do people with type O blood produce? They produce anti-A and anti-B antibodies. This means, if a person with type O blood received type A blood, the anti-A antibodies in the persons blood would attack the A antigens on the RBCs in the donor blood ( Figure 1.1). The antibodies would cause the RBCs to clump together, and the clumps could block a blood vessel. This clumping of blood cells could cause death. A person with type O blood has A and B antibodies in his/her plasma; if the person was to get type A blood instead of type O, his/her A antibodies would attach to the antigens on the RBCs and cause them to clump together. People with type A blood produce anti-B antibodies, and people with type B blood produce anti-A antibodies. People with type AB blood do not produce either antibody. "
blood types,T_2775,"The second most important blood group system in human blood is the Rhesus (Rh) factor. A person either has, or does not have, the Rh antigen on the surface of their RBCs. If they do have it, then the person is positive. If the person does not have the antigen, they are considered negative. "
blood types,T_2776,"Recall that people with type O blood do not have any antigens on their RBCs. As a result, type O blood can be given to people with blood types A, B, or AB. If there are no antigens on the RBCs, there cannot be an antibody reaction in the blood. People with type O blood are often called universal donors. The blood plasma of AB blood does not contain any anti-A or anti-B antibodies. People with type AB blood can receive any ABO blood type. People with type AB blood are called universal recipients because they can receive any blood type. The antigens and antibodies that define blood type are listed as follows ( Table 1.1). Blood Type Antigen Type Plasma Antibodies A B AB O A B A and B none anti-B anti-A none anti-A, anti-B Can Receive Blood from Types A,O B,O AB, A, B, O O Can Donate Blood to Types A, AB B, AB AB AB, A, B, O "
blood vessels,T_2777,"The blood vessels are an important part of the cardiovascular system. They connect the heart to every cell in the body. Arteries carry blood away from the heart, while veins return blood to the heart ( Figure 1.1). The right side of the heart pumps de- oxygenated blood into pulmonary circula- tion, while the left side pumps oxygenated blood into systemic circulation. "
blood vessels,T_2778,"There are specific veins and arteries that are more significant than others. The pulmonary arteries carry oxygen- poor blood away from the heart to the lungs. These are the only arteries that carry oxygen-poor blood. The aorta is the largest artery in the body. It carries oxygen-rich blood away from the heart. Further away from the heart, the aorta branches into smaller arteries, which eventually branch into capillaries. Capillaries are the smallest type of blood vessel; they connect very small arteries and veins. Gases and other substances are exchanged between cells and the blood across the very thin walls of capillaries. The veins that return oxygen-poor blood to the heart are the superior vena cava and the inferior vena cava. The pulmonary veins return oxygen-rich blood from the lungs to the heart. The pulmonary veins are the only veins that carry oxygen-rich blood. "
blood vessels,T_2779,"Pulmonary circulation is the part of the cardiovascular system that carries oxygen-poor blood away from the heart and brings it to the lungs. Oxygen-poor blood returns to the heart from the body and leaves the right ventricle through the pulmonary arteries, which carry the blood to each lung. Once at the lungs, the red blood cells release carbon dioxide and pick up oxygen when you breathe. The oxygen-rich blood then leaves the lungs through the pulmonary veins, which return it to the left side of the heart. This completes the pulmonary cycle. The oxygenated blood is then pumped to the body through systemic circulation, before returning again to pulmonary circulation. "
blood vessels,T_2780,"Systemic circulation is the part of the cardiovascular system that carries oxygen-rich blood away from the heart, to the body, and returns oxygen-poor blood back to the heart. Oxygen-rich blood leaves the left ventricle through the aorta. Then it travels to the bodys organs and tissues. The tissues and organs absorb the oxygen through the capillaries. Oxygen-poor blood is collected from the tissues and organs by tiny veins, which then flow into bigger veins, and, eventually, into the inferior vena cava and superior vena cava. This completes systemic circulation. The blood releases carbon dioxide and gets more oxygen in pulmonary circulation before returning to systemic circulation. The inferior vena cava returns blood from the body. The superior vena cava returns blood from the head. "
bony fish,T_2781,"There are about 27,000 species of bony fish ( Figure 1.1), which are divided into two classes: ray-finned fish and lobe-finned fish. Most bony fish are ray-finned. These thin fins consist of webs of skin over flexible spines. Lobe- finned fish, on the other hand, have fins that resemble stump-like appendages. Fins of bony fish: ray fin (left) and lobe fin (right). "
bony fish,T_2782,"Most fish are bony fish, making them the largest group of vertebrates in existence today. They are characterized by: 1. A head and pectoral girdles (arches supporting the forelimbs) that are covered with bones derived from the skin. 2. A lung or swim bladder, which helps the body create a balance between sinking and floating by either filling up with or emitting gases such as oxygen. Controlling the volume of this organ helps fish control their depth. 3. Jointed, segmented rods supporting the fins. 4. A cover over the gill called the operculum, which helps them breathe without having to swim. 5. The ability to see in color, unlike most other fish. "
bony fish,T_2783,"Most vertebrates are ray-finned fish, with close to 27,000 known species. By comparison, there are ""only"" about 10,000 species of birds. The ray-finned fish have fin rays, with fins supported by bony spines known as rays. The ray-finned fish are the dominant class of vertebrates, with nearly 99% of fish falling into this category. They live in all aquatic environments, from freshwater and marine environments from the deep sea to the highest mountain streams. "
bony fish,T_2784,"The lobe-finned fish are characterized by fleshy lobed fins, as opposed to the bony fins of the ray-finned fish. There are two types of living lobe-finned fish: the coelacanths and the lungfish. The pectoral and pelvic fins have joints resembling those of tetrapod (four-limbed land vertebrates) limbs. These fins evolved into legs of amphibians, the first tetrapod land vertebrates. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of ray-finned fish. All lobe-finned fishes possess teeth covered with true enamel. The lungfish also possess both gills and lungs, solidifying this class as the ancestors of amphibians. "
bony fish,T_2785,"The ocean sunfish is the most massive bony fish in the world, up to 11 feet long and weighing up to 5,070 pounds ( Figure 1.2). Other very large bony fish include the Atlantic blue marlin, the black marlin, some sturgeon species, the giant grouper, and the goliath grouper. The long-bodied oarfish can easily be over 30 feet long, but is not nearly as massive as the ocean sunfish. In contrast, the dwarf pygmy goby measures only 0.6 inches. Fish can also be quite valuable. In January 2013, at an auction in Tokyos Tsukiji fish market, a 222-kilogram (489-pound) tuna caught off northeastern Japan sold for 155.4 million yen, which is $1,760,000. An ocean sunfish, the most massive bony fish in the world, can reach up to 11 feet long and weigh up to 5,070 pounds! "
cancer,T_2786,"Cancer is a disease that causes cells to divide out of control. Normally, the body has systems that prevent cells from dividing out of control. But in the case of cancer, these systems fail. Cancer is usually caused by mutations. Mutations are random errors in genes. Mutations that lead to cancer usually happen to genes that control the cell cycle. Because of the mutations, abnormal cells divide uncontrollably. This often leads to the development of a tumor. A tumor is a mass of abnormal tissue. As a tumor grows, it may harm normal tissues around it. Anything that can cause cancer is called a carcinogen. Carcinogens may be pathogens, chemicals, or radiation. "
cancer,T_2787,Pathogens that cause cancer include the human papilloma virus (HPV) ( Figure 1.1) and the hepatitis B virus. HPV is spread through sexual contact. It can cause cancer of the reproductive system in females. The hepatitis B virus is spread through sexual contact or contact with blood containing the virus. It can cause cancer of the liver. 
cancer,T_2788,"Many different chemical substances cause cancer. Dozens of chemicals in tobacco smoke, including nicotine, have been shown to cause cancer ( Figure 1.2). In fact, tobacco smoke is one of the main sources of chemical carcinogens. Smoking tobacco increases the risk of cancer of the lung, mouth, throat, and bladder. Using smokeless tobacco can also cause cancer. Other chemicals that cause cancer include asbestos, formaldehyde, benzene, cadmium, and nickel. "
cancer,T_2789,Forms of radiation that cause cancer include ultraviolet (UV) radiation and radon ( Figure 1.3). UV radiation is part of sunlight. It is the leading cause of skin cancer. Radon is a natural radioactive gas that seeps into buildings from the ground. It can cause lung cancer. 
cancer,T_2790,"Cancer is usually found in adults, especially in adults over the age of 50. The most common type of cancer in adult males is cancer of the prostate gland. The prostate gland is part of the male reproductive system. Prostate cancer makes up about one third of all cancers in men. The most common type of cancer in adult females is breast cancer. It makes up about one third of all cancers in women. In both men and women, lung cancer is the second most common type of cancer. Most cases of lung cancer happen in people who smoke. Cancer can also be found in children. But childhood cancer is rare. Leukemia is the main type of cancer in children. It makes up about one third of all childhood cancers. It happens when the body makes abnormal white blood cells. Sometimes cancer cells break away from a tumor. If they enter the bloodstream, they are carried throughout the body. Then, the cells may start growing in other tissues. This is usually how cancer spreads from one part of the body to another. Once this happens, cancer is very hard to stop or control. "
cancer,T_2791,"If leukemia is treated early, it usually can be cured. In fact, many cancers can be cured, which is known as remission, if treated early. Treatment of cancer often involves removing a tumor with surgery. This may be followed by other types of treatments. These treatments may include drugs (known as chemotherapy) and radiation therapy, which kill cancer cells. The sooner cancer is treated, the greater the chances of a cure. This is why it is important to know the warning signs of cancer. Having warning signs does not mean that you have cancer. However, you should see a doctor to be sure. Everyone should know the warning signs of cancer. Detecting and treating cancer early can often lead to a cure. Some warning signs of cancer include: Change in bowel or bladder habits. Sores that do not heal. Unusual bleeding or discharge. Lump in the breast or elsewhere. Chronic indigestion. Difficulty swallowing. Obvious changes in a wart or mole. Persistent cough or hoarseness. "
cardiovascular diseases,T_2792,"A cardiovascular disease (CVD) is any disease that affects the cardiovascular system. But the term is usually used to describe diseases that are linked to atherosclerosis. Atherosclerosis ( Figure 1.1) is an inflammation of the walls of arteries that causes swelling and a buildup of material called plaque. Plaque is made of cell pieces, fatty substances, calcium, and connective tissue that builds up around the area of inflammation. As a plaque grows, it stiffens and narrows the artery, which decreases the flow of blood through the artery. Atherosclerosis normally begins in late childhood and is typically found in most major arteries. It does not usually have any early symptoms. Causes of atherosclerosis include a high-fat diet, high cholesterol, smoking, obesity, and diabetes. Atherosclerosis becomes a threat to health when the plaque buildup prevents blood circulation in the heart or the brain. A blocked blood vessel in the heart can cause a heart attack. Blockage of the circulation in the brain can cause a stroke. Ways to prevent atherosclerosis include eating healthy foods, getting plenty of exercise and not smoking. These three factors are not as hard to control as you may think. If you smoke, STOP. Start a regular exercise program and watch what you eat. A diet high in saturated fat and cholesterol can raise your cholesterol levels, which makes more plaque available to line artery walls and narrow your arteries. Cholesterol and saturated fats are found mostly in animal products such Atherosclerosis is sometimes referred to as hardening of the arteries; plaque build- up decreases the blood flow through the artery. as meat, eggs, milk, and other dairy products. Check food labels to find the amount of saturated fat in a product. Also, avoid large amounts of salt and sugar. Be careful with processed foods, such as frozen dinners, as they can be high in fat, sugar, salt and cholesterol. Eat lots of fresh or frozen fruits and vegetables, smaller portions of lean meats and fish, and whole grains such as oats and whole wheat. Limit saturated fats like butter, instead choose unsaturated vegetable oils such as canola oil. "
cardiovascular diseases,T_2793,"Like any other muscle, your heart needs oxygen. Hearts have arteries that provide oxygen through the blood. They are known as coronary arteries. Coronary heart disease is the end result of the buildup of plaque within the walls of the coronary arteries. Coronary heart disease often does not have any symptoms. A symptom of coronary heart disease is chest pain. Occasional chest pain can happen during times of stress or physical activity. The pain of angina means the heart muscle fibers need more oxygen than they are getting. Most people with coronary heart disease often have no symptoms for many years until they have a heart attack. A heart attack happens when the blood cannot reach the heart because a blood vessel is blocked. If cardiac muscle is starved of oxygen for more than roughly five minutes, it will die. Cardiac muscle cells cannot be replaced, so once they die, they are dead forever. Coronary heart disease is the leading cause of death of adults in the United States. The image below shows the way in which a blocked coronary artery can cause a heart attack and cause part of the heart muscle to die ( Figure 1.2). Maybe one day stem cells will be used to replace dead cardiac muscle cells. "
cardiovascular diseases,T_2794,"Atherosclerosis in the arteries of the brain can also lead to a stroke. A stroke is a loss of brain function due to a blockage of the blood supply to the brain. Risk factors for stroke include old age, high blood pressure, having a previous stroke, diabetes, high cholesterol, and smoking. The best way to reduce the risk of stroke is to have low blood pressure. "
cardiovascular system,T_2795,"Your cardiovascular system has many jobs. At times the cardiovascular system can work like a pump, a heating system, or even a postal carrier. To do these tasks, your cardiovascular system works with other organ systems, such as the respiratory, endocrine, and nervous systems. The cardiovascular system (Figure 1.1) is made up of the heart, the blood vessels, and the blood. It moves nutrients, gases (like oxygen), and wastes to and from your cells. Every cell in your body depends on your cardiovascular system. If your cells dont receive nutrients, they cannot survive. The main function of the cardiovascular system is to deliver oxygen to each of your cells. Blood receives oxygen in your lungs (the main organs of the respiratory system) and then is pumped, by your heart, throughout your body. The oxygen then diffuses into your cells, and carbon dioxide, a waste product of cellular respiration, moves from your cells into your blood to be delivered back to your lungs and exhaled. Each cell in your body needs oxygen, as oxygen is used in cellular respiration to produce energy in the form of ATP. Without oxygen, lactic acid fermentation would occur in your cells, which can only be maintained for a brief period of time. Arteries carry blood full of oxygen (""oxygen-rich"") away from the heart and veins return oxygen-poor blood back to the heart. The cardiovascular system also plays a role in maintaining body temperature. It helps to keep you warm by moving warm blood around your body. Your blood vessels also control your body temperature to keep you from getting too hot or too cold. When your brain senses that your body temperature is increasing, it sends messages to the blood vessels in the skin to increase in diameter. Increasing the diameter of the blood vessels increases the amount of blood and heat that moves near the skins surface. The heat is then released from the skin. This helps you cool down. What do you think your blood vessels do when your body temperature is decreasing? The blood also carries hormones, which are chemical messenger molecules produced by organs of the endocrine system, through your body. Hormones are produced in one area of your body and have an effect on another area. To get to that other area, they must travel through your blood. An example is the hormone adrenaline, produced by the adrenal glands on top of the kidneys. Adrenaline has multiple effects on the heart (it quickens the heart rate), on muscles and on the airway. "
cardiovascular system health,T_2796,"There are many risk factors that can cause a person to develop cardiovascular disease. A risk factor is anything that is linked to an increased chance of developing a disease. Some of the risk factors for cardiovascular disease you cannot control, but there are many risk factors you can control. Risk factors you cannot control include: Age: The older a person is, the greater their chance of developing a cardiovascular disease. Gender: Men under age 64 are much more likely to die of coronary heart disease than women, although the gender difference decreases with age. Genetics: Family history of cardiovascular disease increases a persons chance of developing heart disease. Risk factors you can control include many lifestyle factors: Tobacco smoking: Giving up smoking or never starting to smoke is the best way to reduce the risk of heart disease. Diabetes: Diabetes can cause bodily changes, such as high cholesterol levels, which are are risk factors for cardiovascular disease. High cholesterol levels: High amounts of ""bad cholesterol,"" increase the risk of cardiovascular disease. Obesity: Having a very high percentage of body fat, especially if the fat is mostly found in the upper body, rather than the hips and thighs, increases risk significantly. High blood pressure: If the heart and blood vessels have to work harder than normal, this puts the cardiovas- cular system under a strain. Lack of physical activity: Aerobic activities, such as the one pictured below ( Figure 1.1), help keep your heart healthy. To reduce the risk of disease, you should be active for at least 60 minutes a day, five days a week. Poor eating habits: Eating mostly foods that do not have many nutrients other than fat or carbohydrate leads to high cholesterol levels, obesity, and cardiovascular disease ( Figure 1.2). 60 minutes a day of vigorous aerobic activity, such as basketball, is enough to help keep your cardiovascular system healthy. "
cardiovascular system health,T_2797,"Cholesterol cant dissolve in the blood. It has to be transported to and from the cells by carriers called lipoproteins. Low-density lipoprotein, or LDL, is known as ""bad"" cholesterol. High-density lipoprotein (HDL) is known as good cholesterol. When too much LDL cholesterol circulates in the blood, it can slowly build up in the inner walls of the The USDAs MyPyramid recommends that you limit the amount of such foods in your diet to occasional treats. arteries that feed the heart and brain. Together with other substances, it can form plaque, and lead to atherosclerosis. If a clot forms and blocks a narrowed artery, a heart attack or stroke can result. Cholesterol comes from the food you eat as well as being made by the body. To lower bad cholesterol, a diet low in saturated fat and dietary cholesterol should be followed. Regular aerobic exercise also lowers LDL cholesterol and increases HDL cholesterol. "
cartilaginous fish,T_2798,"The 1,000 or so species of cartilaginous fish are subdivided into two subclasses: the first includes sharks, rays, and skates; the second includes chimaera, sometimes called ghost sharks. Fish from this group range in size from the dwarf lanternshark, at 6.3 inches, to the over 50-foot whale shark. Sharks obviously have jaws, as do the other cartilaginous fish. These fish evolved from the jawless fish. So why did fish eventually evolve to have jaws? Such an adaptation would allow fish to eat a much wider variety of food, including plants and other organisms. Other characteristics of cartilaginous fish include: Paired fins. Paired nostrils. Scales. Two-chambered hearts. Skeletons made of cartilage rather than bone. Cartilage is supportive tissue that does not have as much calcium as bones, which makes bones rigid. Cartilage is softer and more flexible than bone. "
cartilaginous fish,T_2799,"Since they do not have bone marrow (as they have no bones), red blood cells are produced in the spleen, in special tissue around the reproductive organs, and in an organ called Leydigs organ, only found in cartilaginous fishes. The tough skin of this group of fish is covered with placoid scales, which are hard scales formed from modified teeth. The scales are covered with a hard enamel. The hard covering and the way the scales are arranged, gives the fish skin rough, sandpaper-like feel. The function of these scales is for protection against predators. The shape of sharks teeth differ according to their diet. Species that feed on mollusks and crustaceans have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey, such as mammals, have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. Sharks continually shed and replace their teeth, with some shedding as much as 35,000 teeth in a lifetime. "
cartilaginous fish,T_2800,"The sharks, rays, and skates (which are similar to stingrays) are further broken into two superorders: 1. Rays and skates. 2. Sharks. Sharks are some of the most frequently studied cartilaginous fish. Sharks are distinguished by such features as: The number of gill slits. The number and type of fins. The type of teeth. The size of their jaws. Body shape. Their activity at night. An elongated, toothed snout used for slashing the fish that they eat, as seen in sawsharks. Teeth used for grasping and crushing shellfish, a characteristic of bullhead sharks. A whisker-like organ named barbels that help sharks find food, a characteristic of carpet sharks. A long snout (or nose-like area), characteristic of groundsharks. Ovoviviparous reproduction, where the eggs develop inside the mothers body after internal fertilization, and the young are born alive. This trait is characteristic of mackerel sharks. All sharks mate by internal fertilization. Some sharks then lay their eggs, others allow internal development. "
cellular respiration,T_2817,"How does the food you eat provide energy? When you need a quick boost of energy, you might reach for an apple or a candy bar. But cells do not ""eat"" apples or candy bars; these foods need to be broken down so that cells can use them. Through the process of cellular respiration, the energy in food is changed into energy that can be used by the bodys cells. Initially, the sugars in the food you eat are digested into the simple sugar glucose, a monosaccharide. Recall that glucose is the sugar produced by the plant during photosynthesis. The glucose, or the polysaccharide made from many glucose molecules, such as starch, is then passed to the organism that eats the plant. This organism could be you, or it could be the organism that you eat. Either way, it is the glucose molecules that holds the energy. "
cellular respiration,T_2818,"Specifically, during cellular respiration, the energy stored in glucose is transferred to ATP ( Figure 1.1). ATP, or adenosine triphosphate, is chemical energy the cell can use. It is the molecule that provides energy for your cells to perform work, such as moving your muscles as you walk down the street. But cellular respiration is slightly more complicated than just converting the energy from glucose into ATP. Cellular respiration can be described as the reverse or opposite of photosynthesis. During cellular respiration, glucose, in the presence of oxygen, is converted into carbon dioxide and water. Recall that carbon dioxide and water are the starting products of photosynthesis. What are the products of photosynthesis? The process can be summarized as: glucose + oxygen  carbon dioxide + water. During this process, the energy stored in glucose is transferred to ATP. Energy is stored in the bonds between the phosphate groups (PO4  ) of the ATP molecule. When ATP is broken down into ADP (adenosine diphosphate) and inorganic phosphate, energy is released. When ADP and inorganic phosphate are joined to form ATP, energy is stored. During cellular respiration, about 36 to 38 ATP molecules are produced for every glucose molecule. The structural formula for adenosine triphosphate (ATP). During cellular respi- ration, energy from the chemical bonds of the food you eat must be transferred to ATP. "
central nervous system,T_2825,"The central nervous system (CNS) ( Figure 1.1) is the largest part of the nervous system. It includes the brain and the spinal cord. The bony skull protects the brain. The spinal cord is protected within the bones of the spine, which are called vertebrae. "
central nervous system,T_2826,"What weighs about three pounds and contains up to 100 billion cells? The answer is the human brain. The brain is the control center of the nervous system. Its like the pilot of a plane. It tells other parts of the nervous system what to do. The brain is also the most complex organ in the body. Each of its 100 billion neurons has synapses connecting it with thousands of other neurons. All those neurons use a lot of energy. In fact, the adult brain uses almost a quarter of the total energy used by the body. The developing brain of a baby uses an even greater amount of the bodys total energy. The brain is the organ that lets us understand what we see, hear, or sense in other ways. It also allows us to use language, learn, think, and remember. The brain controls the organs in our body and our movements as well. The brain consists of three main parts, the cerebrum, the cerebellum, and the brain stem ( Figure 1.2). 1. The cerebrum is the largest part of the brain. It sits on top of the brain stem. The cerebrum controls functions that we are aware of, such as problem-solving and speech. It also controls voluntary movements, like waving to a friend. Whether you are doing your homework or jumping hurdles, you are using your cerebrum. 2. The cerebellum is the next largest part of the brain. It lies under the cerebrum and behind the brain stem. The cerebellum controls body position, coordination, and balance. Whether you are riding a bicycle or writing with a pen, you are using your cerebellum. 3. The brain stem is the smallest of the three main parts of the brain. It lies directly under the cerebrum. The brain stem controls basic body functions, such as breathing, heartbeat, and digestion. The brain stem also carries information back and forth between the cerebrum and spinal cord. The cerebrum is divided into a right and left half ( Figure 1.2). Each half of the cerebrum is called a hemisphere. The two hemispheres are connected by a thick bundle of axons called the corpus callosum. It lies deep inside the brain and carries messages back and forth between the two hemispheres. Did you know that the right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body? By connecting the two hemispheres, the corpus callosum allows this to happen. Each hemisphere of the cerebrum is divided into four parts, called lobes. The four lobes are the: 1. 2. 3. 4. Frontal. Parietal. Temporal. Occipital. Each lobe has different jobs. Some of the functions are listed below ( Table 1.1). Side view of the brain (right). Can you find the locations of the three major parts of the brain? Top view of the brain (left). Lobe Frontal Parietal Temporal Occipital Main Function(s) Speech, thinking, touch Speech, taste, reading Hearing, smell Sight "
central nervous system,T_2827,"The spinal cord is a long, tube-shaped bundle of neurons, protected by the vertebrae. It runs from the brain stem to the lower back. The main job of the spinal cord is to carry nerve impulses back and forth between the body and brain. The spinal cord is like a two-way highway. Messages about the body, both inside and out, pass through the spinal cord to the brain. Messages from the brain pass in the other direction through the spinal cord to tell the body what to do. "
chemistry of life,T_2835,"If you pull a flower petal from a plant and break it in half, and then take that piece and break it in half again, and take the next piece and break it half, and so on, and so on, until you cannot even see the flower anymore, what do you think you will find? We know that the flower petal is made of cells, but what are cells made of? Scientists have broken down matter, or anything that takes up space and has masslike a cellinto the smallest pieces that cannot be broken down anymore. Every physical object, including rocks, animals, flowers, and your body, are all made up of matter. Matter is made up of a mixture of things called elements. Elements are substances that cannot be broken down into simpler substances. There are more than 100 known elements, and 92 occur naturally around us. The others have been made only in the laboratory. Inside of elements, you will find identical atoms. An atom is the simplest and smallest particle of matter that still has chemical properties of the element. Atoms are the building block of all of the elements that make up the matter in your body or any other living or non-living thing. Atoms are so small that only the most powerful microscopes can see them. Atoms themselves are composed of even smaller particles, including positively charged protons, uncharged neu- trons, and negatively charged electrons. Protons and neutrons are located in the center of the atom, or the nucleus, and the electrons move around the nucleus. How many protons an atom has determines what element it is. For example, hydrogen (H) has just one proton, helium (He) always has two protons ( Figure 1.1), while sodium (Na) always has 11. All the atoms of a particular element have the exact same number of protons, and the number of protons is that elements atomic number. An atom usually has the same number of protons and electrons, but sometimes an atom may gain or lose an electron, giving the atom a positive or negative charge. These atoms are known as ions and are depicted with a ""+"" or ""-"" sign. Ions, such as H+ , Na+ , K+ , or Cl have significant biological roles. An atom of Helium (He) contains two positively charged protons (red), two uncharged neutrons (blue), and two negatively charged electrons (yellow). "
chemistry of life,T_2836,"In 1869, a Russian scientist named Dmitri Mendeleev created the periodic table, which is a way of organizing elements according to their unique characteristics, like atomic number, density, boiling point, and other values ( Figure 1.2). Each element is represented by a one or two letter symbol. For example, H stands for hydrogen, and Au stands for gold. The vertical columns in the periodic table are known as groups, and elements in groups tend to have very similar properties. The table is also divided into rows, known as periods. "
chemistry of life,T_2837,"A molecule is any combination of two or more atoms. The oxygen in the air we breathe is two oxygen atoms connected by a chemical bond to form O2 , or molecular oxygen. A carbon dioxide molecule is a combination of one carbon atom and two oxygen atoms, CO2 . Because carbon dioxide includes two different elements, it is a compound as well as a molecule. A compound is any combination of two or more different elements. A compound has different properties from the elements that it contains. Elements and combinations of elements (compounds) make up all the many types of matter in the Universe. A chemical reaction is a process that breaks or forms the bonds between atoms of molecules and compounds. For example, two hydrogens and one oxygen bind together to form water, H2 O. The molecules that come together to start a chemical reaction are the reactants. So hydrogen and oxygen are the reactants. The product is the end result of a reaction. In this example, water is the product. Atoms also come together to form compounds much larger than water. It is some of these large compounds that come together to form the basis of the cell. So essentially, your cells are made out of compounds, which are made out of atoms. "
chromosomal disorders,T_2848,"Some children are born with genetic defects that are not carried by a single gene. Instead, an error in a larger part of the chromosome or even in an entire chromosome causes the disorder. Usually the error happens when the egg or sperm is forming. Having extra chromosomes or damaged chromosomes can cause disorders. "
chromosomal disorders,T_2849,"One common example of an extra-chromosome disorder is Down syndrome ( Figure 1.1). Children with Down syndrome are mentally disabled and also have physical deformities. Down syndrome occurs when a baby receives an extra chromosome 21 from one of his or her parents. Usually, a child will receive one chromosome 21 from the mother and one chromosome 21 from the father. In an individual with Down syndrome, however, there are three Chromosomes of a person with Down Syndrome. Notice the extra chromosome 21. copies of chromosome 21 ( Figure 1.2). Therefore, Down syndrome is also known as Trisomy 21. These people have 47 total chromosomes. Another example of a chromosomal disorder is Klinefelter syndrome, in which a male inherits an extra X chromosome. These individuals have an XXY genotype. They have underdeveloped sex organs and elongated limbs. They also have difficulty learning new things. "
chromosomal disorders,T_2850,"Chromosomal disorders also occur when part of a chromosome becomes damaged. For example, if a tiny portion of chromosome 5 is missing, the individual will have cri du chat (cats cry) syndrome. These individuals have misshapen facial features, and the infants cry resembles a cats cry. "
circulation and the lymphatic system,T_2851,"The lymphatic system is a network of vessels and tissues that carry a clear fluid called lymph. The lymphatic system ( Figure 1.1) spreads all around the body and filters and cleans the lymph of any debris, abnormal cells, or pathogens. Lymph vessels are tube-shaped, just like blood vessels, with about 500-600 lymph nodes (in an adult) attached. The lymphatic system works with the cardiovascular system to return body fluids to the blood. The lymphatic system and the cardiovascular system are often called the bodys two ""circulatory systems."" Organs of the lymphatic system include the tonsils, thymus gland and spleen. The thymus gland produces T cells or T-lymphocytes (see below) and the spleen and tonsils help in fighting infections. The spleens main function is to filter the blood, removing unwanted red blood cells. The spleen also detects viruses and bacteria and triggers the release of pathogen fighting cells. The lymphatic system helps return fluid that leaks from the blood vessels back to the cardiovascular system. "
circulation and the lymphatic system,T_2852,"You may think that your blood vessels have thick walls without any leaks, but thats not true. Blood vessels can leak just like any other pipe. The lymphatic system makes sure leaked blood returns back to the bloodstream. When a small amount of fluid leaks out from the blood vessels, it collects in the spaces between cells and tissues. Some of the fluid returns to the cardiovascular system, and the rest is collected by the lymph vessels of the lymphatic system ( Figure 1.2). The fluid that collects in the lymph vessels is called lymph. The lymphatic system then returns the lymph to the cardiovascular system. Unlike the cardiovascular system, the lymphatic system is not closed (meaning it is an open circulatory system that releases and collects fluid) and has no central pump (or heart). Lymph moves slowly in lymph vessels. It is moved along in the lymph vessels by the squeezing action of smooth muscles and skeletal muscles. Lymph capillaries collect fluid that leaks out from blood capillaries. The lymphatic vessels return the fluid to the cardiovas- cular system. "
circulation and the lymphatic system,T_2853,"The lymphatic system also plays an important role in the immune system. For example, the lymphatic system makes white blood cells that protect the body from diseases. Cells of the lymphatic system produce two types of white blood cells, T cells and B cells, that are involved in fighting specific pathogens. Lymph nodes, which are scattered throughout the lymphatic system, act as filters or traps for foreign particles and are important in the proper functioning of the immune system. The role of the lymphatic system in the immune response is discussed in additional concepts. "
connecting cellular respiration and photosynthesis,T_2865,"Photosynthesis and cellular respiration are connected through an important relationship. This relationship enables life to survive as we know it. The products of one process are the reactants of the other. Notice that the equation for cellular respiration is the direct opposite of photosynthesis: Cellular Respiration: C6 H12 O6 + 6O2  6CO2 + 6H2 O Photosynthesis: 6CO2 + 6H2 O  C6 H12 O6 + 6O2 Photosynthesis makes the glucose that is used in cellular respiration to make ATP. The glucose is then turned back into carbon dioxide, which is used in photosynthesis. While water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water. While photosynthesis requires carbon dioxide and releases oxygen, cellular respiration requires oxygen and releases carbon dioxide. It is the released oxygen that is used by us and most other organisms for cellular respiration. We breathe in that oxygen, which is carried through our blood to all our cells. In our cells, oxygen allows cellular respiration to proceed. Cellular respiration works best in the presence of oxygen. Without oxygen, much less ATP would be produced. Cellular respiration and photosynthesis are important parts of the carbon cycle. The carbon cycle is the pathways through which carbon is recycled in the biosphere. While cellular respiration releases carbon dioxide into the environment, photosynthesis pulls carbon dioxide out of the atmosphere. The exchange of carbon dioxide and oxygen during photosynthesis ( Figure 1.1) and cellular respiration worldwide helps to keep atmospheric oxygen and carbon dioxide at stable levels. Cellular respiration and photosynthesis are direct opposite reactions. Energy from the sun enters a plant and is con- verted into glucose during photosynthe- sis. Some of the energy is used to make ATP in the mitochondria during cellular respiration, and some is lost to the envi- ronment as heat. "
diabetes,T_2879,"Diabetes is a non-infectious disease in which the body is unable to control the amount of sugar in the blood. People with diabetes have high blood sugar, either because their bodies do not produce enough insulin, or because their cells do not respond to insulin. Insulin is a hormone that helps cells take up sugar from the blood. Without enough insulin, the blood contains too much sugar. This can damage blood vessels and other cells throughout the body. The kidneys work hard to filter out and remove some of the extra sugar. This leads to frequent urination and excessive thirst. There are two main types of diabetes, type 1 diabetes and type 2 diabetes. Type 1 diabetes makes up about 5-10% of all cases of diabetes in the United States. Type 2 diabetes accounts for most of the other cases. Both types of diabetes are more likely in people that have certain genes. Having a family member with diabetes increases the risk of developing the disease. Either type of diabetes can increase the chances of having other health problems. For example, people with diabetes are more likely to develop heart disease and kidney disease. Type 1 and type 2 diabetes are similar in these ways. However, the two types of diabetes have different causes. "
diabetes,T_2880,"Type 1 diabetes occurs when the immune system attacks normal cells of the pancreas. Since the cells in the pancreas are damaged, the pancreas cannot make insulin. Type 1 diabetes usually develops in childhood or adolescence. People with type 1 diabetes must frequently check the sugar in their blood. They use a meter to monitor their blood sugar ( Figure 1.1). Whenever their blood sugar starts to get too high, they need a shot of insulin. The insulin brings their blood sugar back to normal. There is no cure for type 1 diabetes. Therefore, insulin shots must be taken for life. Most people with this type of diabetes learn how to give themselves insulin shots. This is one type of meter used by people with diabetes to measure their blood sugar. Modern meters like this one need only a drop of blood and take less than a minute to use. "
diabetes,T_2881,"Type 2 diabetes occurs when body cells are no longer sensitive to insulin. The pancreas may still make insulin, but the cells of the body cannot use it efficiently. Being overweight and having high blood pressure increase the chances of developing type 2 diabetes. Type 2 diabetes usually develops in adulthood, but it is becoming more common in teens and children. This is because more young people are overweight, due to a high sugar and fat diet, now than ever before. Some cases of type 2 diabetes can be cured with weight loss. However, most people with the disease need to take medicine to control their blood sugar. Regular exercise and balanced eating also help, and should be a regular part of the treatment for these people. Like people with type 1 diabetes, people with type 2 diabetes must frequently check their blood sugar. "
diabetes,T_2882,"Common symptoms of diabetes include the following: frequent urination feeling very thirsty feeling very hungry, even though you are eating extreme fatigue blurry vision cuts or bruises that are slow to heal weight loss, even though you are eating more (type 1) tingling, pain, or numbness in the hands or feet (type 2) "
diabetes,T_2883,Complications of diabetes can include the following: eye complications foot complications skin complications high blood pressure hearing issues nerve damage kidney disease artery disease stroke stress 
digestive system organs,T_2887,"The mouth and stomach are just two of the organs of the digestive system. Other digestive system organs are the esophagus, small intestine, and large intestine. Below, you can see that the digestive organs form a long tube ( Figure 1.1). In adults, this tube is about 30 feet long! At one end of the tube is the mouth. At the other end is the anus. Food enters the mouth and then passes through the rest of the digestive system. Food waste leaves the body through the anus. The organs of the digestive system are lined with muscles. The muscles contract, or tighten, to push food through the system ( Figure 1.2). The muscles contract in waves. The waves pass through the digestive system like waves through a slinky. This movement of muscle contractions is called peristalsis. Without peristalsis, food would not be able to move through the digestive system. Peristalsis is an involuntary process, which means that it occurs without your conscious control. The liver, gallbladder, and pancreas are also organs of the digestive system ( Figure 1.1). Food does not pass through these three organs. However, these organs are important for digestion. They secrete or store enzymes or other chemicals that are needed to help digest food chemically. "
digestive system organs,T_2888,"The mouth is the first organ that food enters. But digestion may start even before you put the first bite of food into your mouth. Just seeing or smelling food can cause the release of saliva and digestive enzymes in your mouth. This diagram shows how muscles push food through the digestive system. Muscle contractions travel through the system in waves, pushing the food ahead of them. This is called peristalsis. Once you start eating, saliva wets the food, which makes it easier to break up and swallow. Digestive enzymes, including the enzyme amylase, start breaking down starches into sugars. Your tongue helps mix the food with the saliva and enzymes. Your teeth also help digest food. Your front teeth are sharp. They cut and tear food when you bite into it. Your back teeth are broad and flat. They grind food into smaller pieces when you chew. Chewing is part of mechanical digestion. Your tongue pushes the food to the back of your mouth so you can swallow it. When you swallow, the lump of chewed food passes down your throat to your esophagus. The esophagus is a narrow tube that carries food from the throat to the stomach. Food moves through the esophagus because of peristalsis. At the lower end of the esophagus, a circular muscle controls the opening to the stomach. The muscle relaxes to let food pass into the stomach. Then the muscle contracts again to prevent food from passing back into the esophagus. Some people think that gravity moves food through the esophagus. If that were true, food would move through the esophagus only when you are sitting or standing upright. In fact, because of peristalsis, food can move through the esophagus no matter what position you are ineven upside down! Just dont try to swallow food when you are upside downyou could choke! The stomach is a sac-like organ at the end of the esophagus. It has thick muscular walls. The muscles contract and relax. This moves the food around and helps break it into smaller pieces. Mixing the food around with the enzyme pepsin and other chemicals helps digest proteins. Water, salt, and simple sugars can be absorbed into the blood from the stomach. Most other substances are broken down further in the small intestine before they are absorbed. The stomach stores food until the small intestine is ready to receive it. A circular muscle controls the opening between the stomach and small intestine. When the small intestine is empty, the muscle relaxes. This lets food pass from the stomach into the small intestine. "
digestive system organs,T_2889,"The small intestine a is narrow tube that starts at the stomach and ends at the large intestine ( Figure 1.1). In adults, the small intestine is about 23 feet long. Chemical digestion takes place in the first part of the small intestine. Many enzymes and other chemicals are secreted here. The small intestine is also where most nutrients are absorbed into the blood. The later sections of the small intestines are covered with tiny projections called villi ( Figure 1.3). Villi contain very tiny blood vessels. Nutrients are absorbed into the blood through these tiny vessels. There are millions of villi, so, altogether, there is a very large area for absorption to take place. In fact, villi make the inner surface area of the small intestine 1,000 times larger than it would be without them. The entire inner surface area of the small intestine is about as big as a basketball court! The small intestine is much longer than the large intestine. So why is it called small? If you compare small and large intestines ( Figure 1.1), you will see the small intestine is smaller in width than the large intestine. "
digestive system organs,T_2890,"The large intestine is a wide tube that connects the small intestine with the anus. In adults, it is about five feet long. Waste enters the large intestine from the small intestine in a liquid state. As the waste moves through the large intestine, excess water is absorbed from it. After the excess water is absorbed, the remaining solid waste is called feces. Circular muscles control the anus. They relax to let the feces pass out of the body through the anus. After feces pass out of the body, they are called stool. Releasing the stool from the body is referred to as a bowel movement. "
diseases of the nervous system,T_2891,"The nervous system controls sensing, feeling, and thinking. It also controls movement and just about every other body function. Thats why problems with the nervous system can affect the entire body. Diseases of the nervous system include brain and spinal cord infections. Other problems of the nervous system range from very serious diseases, such as tumors, to less serious problems, such as tension headaches. Some of these diseases are present at birth. Others begin during childhood or adulthood. "
diseases of the nervous system,T_2892,"When you think of infections, you probably think of an ear infection or strep throat. You probably dont think of a brain or spinal cord infection. But bacteria and viruses can infect these organs as well as other parts of the body. Infections of the brain and spinal cord are not very common. But when they happen, they can be very serious. Thats why its important to know their symptoms. "
diseases of the nervous system,T_2893,"Encephalitis is a brain infection ( Figure 1.1). If you have encephalitis, you are likely to have a fever and headache or feel drowsy and confused. The disease is most often caused by viruses. The immune system tries to fight off a brain infection, just as it tries to fight off other infections. But sometimes this can do more harm than good. The immune systems response may cause swelling in the brain. With no room to expand, the brain pushes against the skull. This may injure the brain and even cause death. Medicines can help fight some viral infections of the brain, but not all infections. "
diseases of the nervous system,T_2894,"Meningitis is an infection of the membranes that cover the brain and spinal cord. If you have meningitis, you are likely to have a fever and a headache. Another telltale symptom is a stiff neck. Meningitis can be caused by viruses or bacteria. Viral meningitis often clears up on its own after a few days. Bacterial meningitis is much more serious ( Figure 1.2). It may cause brain damage and death. People with bacterial meningitis need emergency medical treatment. They are usually given antibiotics to kill the bacteria. A vaccine to prevent meningitis recently became available. It can be given to children as young as two years old. Many doctors recommend that children receive the vaccine no later than age 12 or 13, or before they begin high school. "
diseases of the nervous system,T_2895,"A condition called Reyes syndrome can occur in young people that take aspirin when they have a viral infection. The syndrome causes swelling of the brain and may be fatal. Fortunately, Reyes syndrome is very rare. The best way to prevent it is by not taking aspirin when you have a viral infection. Products like cold medicines often contain aspirin. So, read labels carefully when taking any medicines ( Figure 1.3). Since 1988, the U.S. Food and Drug Ad- ministration has required that all aspirin and aspirin-containing products carry a warning about Reyes syndrome. "
diseases of the nervous system,T_2896,"Like other parts of the body, the nervous system may develop tumors. A tumor is a mass of cells that grows out of control. A tumor in the brain may press on normal brain tissues. This can cause headaches, difficulty speaking, or other problems, depending on where the tumor is located. Pressure from a tumor can even cause permanent brain damage. In many cases, brain tumors can be removed with surgery. In other cases, tumors cant be removed without damaging the brain even more. In those cases, other types of treatments may be needed. Cerebral palsy is a disease caused by injury to the developing brain. The injury occurs before, during, or shortly after birth. Cerebral palsy is more common in babies that have a low weight at birth. But the cause of the brain injury is not often known. The disease usually affects the parts of the brain that control body movements. Symptoms range from weak muscles in mild cases to trouble walking and talking in more severe cases. There is no known cure for cerebral palsy. Epilepsy is a disease that causes seizures. A seizure is a period of lost consciousness that may include violent muscle contractions. It is caused by abnormal electrical activity in the brain. The cause of epilepsy may be an infection, a brain injury, or a tumor. The seizures of epilepsy can often be controlled with medicine. There is no known cure for the disease, but children with epilepsy may outgrow it by adulthood. A headache is a very common nervous system problem. Headaches may be a symptom of serious diseases, but they are more commonly due to muscle tension. A tension headache occurs when muscles in the shoulders, neck, and head become too tense. This often happens when people are stressed out. Just trying to relax may help relieve this type of headache. Mild pain relievers such as ibuprofen may also help. Sometimes relaxation is the best medicine for a tension headache and to help muscles get rid of pain. A migraine is a more severe type of headache. It occurs when blood vessels in the head dilate, or expand. This may be triggered by certain foods, bright lights, weather changes, or other factors. People with migraines may also have nausea or other symptoms. Fortunately, migraines can often be relieved with prescription drugs. There are many other nervous system diseases. They include multiple sclerosis, Huntingtons disease, Parkinsons disease, and Alzheimers disease. However, these diseases rarely, if ever, occur in young people. Their causes and symptoms are listed below ( Table 1.1). The diseases have no known cure, but medicines may help control their symptoms. Disease Multiple sclerosis Cause The immune system attacks and damages the central nervous sys- tem so neurons cannot function nor- mally. Symptoms Muscle weakness, difficulty mov- ing, problems with coordination, difficulty keeping the body bal- anced Parkinsons disease Alzheimers disease "
dna the genetic material,T_2901,"DNA is the material that makes up our chromosomes and stores our genetic information. When you build a house, you need a blueprint, a set of instructions that tells you how to build. The DNA is like the blueprint for living organisms. The genetic information is a set of instructions that tell your cells what to do. DNA is an abbreviation for deoxyribonucleic acid. As you may recall, nucleic acids are a type of macromolecule that store information. The deoxyribo part of the name refers to the name of the sugar that is contained in DNA, deoxyribose. DNA may provide the instructions to make up all living things, but it is actually a very simple molecule. DNA is made of a very long chain of nucleotides. In fact, in you, the smallest DNA molecule has well over 20 million nucleotides. "
dna the genetic material,T_2902,"Nucleotides are composed of three main parts: 1. a phosphate group. 2. a 5-carbon sugar (deoxyribose in DNA). 3. a nitrogen-containing base. The only difference between each nucleotide is the identity of the base. There are only four possible bases that make up each DNA nucleotide: adenine (A), guanine (G), thymine (T), and cytosine (C). "
dna the genetic material,T_2903,"The various sequences of the four nucleotide bases make up the genetic code of your cells. It may seem strange that there are only four letters in the alphabet of DNA. But since your chromosomes contain millions of nucleotides, there are many, many different combinations possible with those four letters. But how do all these pieces fit together? James Watson and Francis Crick won the Nobel Prize in 1962 for piecing together the structure of DNA. Together with the work of Rosalind Franklin and Maurice Wilkins, they determined that DNA is made of two strands of nucleotides formed into a double helix, or a two-stranded spiral, with the sugar and phosphate groups on the outside, and the paired bases connecting the two strands on the inside of the helix (Figure 1.1). "
dna the genetic material,T_2904,"The bases in DNA do not pair randomly. When Erwin Chargaff looked closely at the bases in DNA, he noticed that the percentage of adenine (A) in the DNA always equaled the percentage of thymine (T), and the percentage of guanine (G) always equaled the percentage of cytosine (C). Watson and Cricks model explained this result by suggesting that A always pairs with T, and G always pairs with C in the DNA helix. Therefore A and T, and G and C, are ""complementary bases,"" or bases that always pair together, known as a base-pair. The base-pairing rules state that A will always bind to T, and G will always bind to C (Figure 1.2). For example, if one DNA strand reads ATGCCAGT, the other strand will be made up of the complementary bases: TACGGTCA. Hydrogen bonds hold the complementary bases together, with two bonds forming between an A and a T, and three bonds between a G and a C. The chemical structure of DNA includes a chain of nucleotides consisting of a 5- carbon sugar, a phosphate group, and a nitrogen base. Notice how the sugar and phosphate group form the backbone of DNA (strands highlighted in pink), with the hydrogen bonds between the bases joining the two strands. "
echinoderms,T_2908,"Youre probably familiar with starfish and sand dollars ( Figure 1.1). They are both echinoderms. Sea urchins and sea cucumbers are also echinoderms. Whats similar between these three organisms? They all have radial symmetry. This means that the body is arranged around a central point. Echinoderms belong to the phylum Echinodermata. This phylum includes 7,000 living species. It is the largest animal phylum without freshwater or land-living members. "
echinoderms,T_2909,"As mentioned earlier, echinoderms show radial symmetry. Other key echinoderm features include an internal skeleton and spines, as well as a few organs and organ systems. Although echinoderms look like they have a hard exterior, they do not have an external skeleton. Instead, a thin outer skin covers an internal skeleton made of tiny plates and spines. This provides rigid support. Some groups of echinoderms, such as sea urchins ( Figure 1.2), have spines that protect the organism. Sea cucumbers use these spines to help them move. A starfish (left) and a keyhole sand dollar (right), showing the radial symmetry char- acteristic of the echinoderms. Starfish are also known as sea stars. Another echinoderm, a sea urchin (Echi- nus esculentus), showing its spines. Echinoderms have a unique water vascular system. This network of fluid-filled tubes helps them to breathe, eat, and move. Therefore, they can function without gill slits. Echinoderms also have a very simple digestive system, circulatory system, and nervous system. The digestive system often leads directly from the mouth to the anus. The echinoderms have an open circulatory system, meaning that fluid moves freely in the body cavity. But echinoderms have no heart. This may be due to their simple radial symmetry - a heart is not needed to pump the freely moving fluid. The echinoderm nervous system is a nerve net, or interconnected neurons with no central brain. Many echinoderms have amazing powers of regeneration. For example, some sea stars (starfish) are capable of regenerating lost arms. In some cases, lost arms have been observed to regenerate a second complete sea star! Sea cucumbers often release parts of their internal organs if they perceive danger. The released organs and tissues are then quickly regenerated. "
echinoderms,T_2910,"Feeding strategies vary greatly among the different groups of echinoderms. Theres no one food or technique thats shared by all echinoderms. Different eating-methods include: 1. Passive filter-feeders, which are organisms that absorb suspended nutrients from passing water. Some echino- derms use their long arms to capture food particles floating past in the currents. 2. Grazers, such as sea urchins, are organisms that feed on available plants. Sea urchins are omnivorous, eating both plant and animals. The sea urchin mainly feeds on algae on the coral and rocks, along with decomposing matter such as dead fish, mussels, sponges, and barnacles. 3. Deposit feeders, which are organisms that feed on small pieces of organic matter, usually in the top layer of soil. Sea cucumbers are deposit feeders, living on the ocean floor. They eat the tiny scrap particles that are usually abundant in the environments that they inhabit. 4. Active hunters, which are organisms that actively hunt their prey. Many sea stars are predators, feeding on mollusks like clams by prying apart their shells and actually placing their stomach inside the mollusk shell to digest the meat. "
echinoderms,T_2911,"Echinoderms reproduce sexually. In most echinoderms, eggs and sperm cells are released into open water, and fertilization takes place when the eggs and sperm meet. This is called external fertilization, and is typical of many marine animals. The release of sperm and eggs often occurs when organisms are in the same place at the same time. Internal fertilization takes place in only a few species. Some species even take care of their offspring, like parents! "
effects of water pollution,T_2913,"Water pollutants can have an effect on both the ecology of ecosystems and on humans. As a result of water pollution, humans may not be able to use a waterway for recreation and fishing. Drinking water can also be affected if a toxin enters the groundwater. "
effects of water pollution,T_2914,"In a marine ecosystem, algae are the producers. Through photosynthesis, they provide glucose for the ecosystem. So, can too much algae be a bad thing? Eutrophication is an over-enrichment of chemical nutrients in a body of water. Usually these nutrients are the nitrogen and phosphorous found in fertilizers. Run-off from lawns or farms can wash fertilizers into rivers or coastal waters. Plants are not the only things that grow more quickly with added fertilizers. Algae like the excess nutrients in fertilizers too. When there are high levels of nutrients in the water, algae populations will grow large very quickly. This leads to overgrowths of algae called algal blooms. However, these algae do not live very long. They die and begin to decompose. This process uses oxygen, removing the oxygen from the water. Without oxygen, fish and shellfish cannot live, and this results in the death of these organisms ( Figure 1.1). Certain types of algal blooms can also create toxins. These toxins can enter shellfish. If humans eat these shellfish, then they can get very sick. These toxins cause neurological problems in humans. "
effects of water pollution,T_2915,"Ocean acidification occurs when excess carbon dioxide in the atmosphere causes the oceans to become acidic. Burning fossil fuels has led to an increase in carbon dioxide in the atmosphere. This carbon dioxide is then absorbed by the oceans, which lowers the pH of the water. Ocean acidification can kill corals and shellfish. It may also cause marine organisms to reproduce less, which could harm other organisms in the food chain. As a result, there also may be fewer marine organisms for humans to consume. "
effects of water pollution,T_2916,"Aquatic debris is trash that gets into fresh- and saltwater waterways. It comes from shipping accidents, landfill erosion, or the direct dumping of trash. Debris can be very dangerous to aquatic wildlife. Some animals may swallow plastic bags, mistaking them for food. Other animals can be strangled by floating trash like plastic six-pack rings. Wildlife can easily get tangled in nets ( Figure 1.2). Marine trash can harm different types of aquatic life. Pictured here is a marine turtle entangled in a net. How can you keep this from happening? "
effects of water pollution,T_2917,"Unsafe water supplies have drastic effects on human health. Waterborne diseases are diseases due to microscopic pathogens in fresh water. These diseases can be caused by protozoa, viruses, bacteria, and intestinal parasites. In many parts of the world there are no water treatment plants. If sewage or animal manure gets into a river, then people downstream will get sick when they drink the water. According to the World Health Organization (WHO), diarrheal disease is responsible for the deaths of 1.8 million people every year. It was estimated that 88% of the cases of diarrheal disease are caused by unsafe water supplies. "
energy pyramids,T_2918,"When an herbivore eats a plant, the energy in the plant tissues is used by the herbivore. But how much of that energy is transferred to the herbivore? Remember that plants are producers, bringing the energy into the ecosystem by converting sunlight into glucose. Does the plant use some of the energy for its own needs? Recall the energy is the ability to do work, and the plant has plenty or ""work"" to do. So of course it needs and uses energy. It converts the glucose it makes into ATP through cellular respiration just like other organisms. After the plant uses the energy from glucose for its own needs, the excess energy is available to the organism that eats the plant. The herbivore uses the energy from the plant to power its own life processes and to build more body tissues. However, only about 10% of the total energy from the plant gets stored in the herbivores body as extra body tissue. The rest of the energy is used by the herbivore and released as heat. The next consumer on the food chain that eats the herbivore will only store about 10% of the total energy from the herbivore in its own body. This means the carnivore will store only about 1% of the total energy that was originally in the plant. In other words, only about 10% of energy of one step in a food chain is stored in the next step in the food chain. The majority of the energy is used by the organism or released to the environment. Every time energy is transferred from one organism to another, there is a loss of energy. This loss of energy can be shown in an energy pyramid. An example of an energy pyramid is pictured below ( Figure 1.1). Since there is energy loss at each step in a food chain, it takes many producers to support just a few carnivores in a community. Each step of the food chain in the energy pyramid is called a trophic level. Plants or other photosynthetic organisms ( autotrophs) are found on the first trophic level, at the bottom of the pyramid. The next level will be the herbivores, and then the carnivores that eat the herbivores. The energy pyramid ( Figure 1.1) shows four levels of a food chain, from producers to carnivores. Because of the high rate of energy loss in food chains, there are usually only 4 or 5 trophic levels in the food chain or energy pyramid. There just is not enough energy to support any additional trophic levels. Heterotrophs are found in all levels of an energy pyramid other than the first level. "
enzymes in the digestive system,T_2919,"Chemical digestion could not take place without the help of digestive enzymes. An enzyme is a protein that speeds up chemical reactions in the body. Digestive enzymes speed up chemical reactions that break down large food molecules into small molecules. Did you ever use a wrench to tighten a bolt? You could tighten a bolt with your fingers, but it would be difficult and slow. If you use a wrench, you can tighten a bolt much more easily and quickly. Enzymes are like wrenches. They make it much easier and quicker for chemical reactions to take place. Like a wrench, enzymes can also be used over and over again. But you need the appropriate size and shape of the wrench to efficiently tighten the bolt, just like each enzyme is specific for the reaction it helps. Digestive enzymes are released, or secreted, by the organs of the digestive system. These enzymes include proteases that digest proteins, and nucleases that digest nucleic acids. Examples of digestive enzymes are: Amylase, produced in the mouth. It helps break down large starch molecules into smaller sugar molecules. Pepsin, produced in the stomach. Pepsin helps break down proteins into amino acids. Trypsin, produced in the pancreas. Trypsin also breaks down proteins. Pancreatic lipase, produced in the pancreas. It is used to break apart fats. Deoxyribonuclease and ribonuclease, produced in the pancreas. They are enzymes that break bonds in nucleic acids like DNA and RNA. Bile salts are bile acids that help to break down fat. Bile acids are made in the liver. When you eat a meal, bile is secreted into the intestine, where it breaks down the fats ( Figure 1.1). "
enzymes in the digestive system,T_2920,"If you are a typical teenager, you like to eat. For your body to break down, absorb and spread the nutrients from your food throughout your body, your digestive system and endocrine system need to work together. The endocrine system sends hormones around your body to communicate between cells. Essentially, hormones are chemical messenger molecules. Digestive hormones are made by cells lining the stomach and small intestine. These hormones cross into the blood where they can affect other parts of the digestive system. Some of these hormones are listed below. Gastrin, which signals the secretion of gastric acid. Cholecystokinin, which signals the secretion of pancreatic enzymes. Secretin, which signals secretion of water and bicarbonate from the pancreas. Ghrelin, which signals when you are hungry. Gastric inhibitory polypeptide, which stops or decreases gastric secretion. It also causes the release of insulin in response to high blood glucose levels. "
evolution acts on the phenotype,T_2921,"Natural selection acts on the phenotype (the traits or characteristics) of an individual. On the other hand, natural selection does not act on the underlying genotype (the genetic makeup) of an individual. For many traits, the homozygous genotype, AA, for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot select for a homozygous individual over a heterozygous individual. Even if the ""aa"" phenotype is lethal, the recessive a allele, will be maintained in the population through heterozygous Aa individuals. Furthermore, the mating of two heterozygous individuals can produce homozygous recessive (aa) individuals. However, natural selection can and does differentiate between dominant and recessive phenotypes. "
evolution acts on the phenotype,T_2922,"Since natural selection acts on the phenotype, if an allele causes death in a homozygous individual, aa, for example, it will not cause death in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers of the a allele, meaning that the a allele could be passed down to offspring. People who are carriers do not express the recessive phenotype, as they have a dominant allele. This allele is said to be kept in the populations gene pool. The gene pool is the complete set of genes and alleles within a population. For example, Tay-Sachs disease is a recessive human genetic disorder. That means only individuals with the homozygous recessive genotype, rr will be affected. Affected individuals usually die from complications of the disease in early childhood, at an age too young to reproduce. The two parents are each heterozygous (Rr) for the Tay-Sachs gene; they will not die in childhood and will be carriers of the disease gene. This deadly allele is kept in the gene pool even though it does not help humans adapt to their environment. This happens because evolution acts on the phenotype, not the genotype ( Figure 1.1). Tay-Sachs disease is inherited in the au- tosomal recessive pattern. Each parent is an unaffected carrier of the lethal allele. "
excretion,T_2923,"So what happens to your bodys wastes? Obviously, you must get rid of them. This is the job of the excretory system. You remove waste as a gas (carbon dioxide), as a liquid (urine and sweat), and as a solid. Excretion is the process of removing wastes and excess water from the body. Recall that carbon dioxide travels through the blood and is transferred to the lungs where it is exhaled. In the large intestine, the remains of food are turned into solid waste for excretion. How is waste other than carbon dioxide removed from the blood? That is the role of the kidneys. Urine is a liquid waste formed by the kidneys as they filter the blood. If you are getting plenty of fluids, your urine should be almost clear. But you might have noticed that sometimes your urine is darker than usual. Do you know why this happens? Sometimes your body is low on water and trying to reduce the amount of water lost in urine. Therefore, your urine gets darker than usual. Your body is striving to maintain homeostasis through the process of excretion. Urine helps remove excess water, salts, and nitrogen from your body. Your body also needs to remove the wastes that build up from cell activity and from digestion. If these wastes are not removed, your cells can stop working, and you can get very sick. The organs of your excretory system help to release wastes from the body. The organs of the excretory system are also parts of other organ systems. For example, your lungs are part of the respiratory system. Your lungs remove carbon dioxide from your body, so they are also part of the excretory system. More organs of the excretory system are listed below ( Table 1.1). Organ(s) Function Lungs Skin Remove carbon dioxide. Sweat glands remove water, salts, and other wastes. Removes solid waste and some wa- ter in the form of feces. Remove urea, salts, and excess wa- ter from the blood. Large intestine Kidneys Component of Other Organ Sys- tem Respiratory system Integumentary system Digestive system Urinary system "
excretory system problems,T_2924,The urinary system controls the amount of water in the body and removes wastes. Any problem with the urinary system can also affect many other body systems. 
excretory system problems,T_2925,"In some cases, certain mineral wastes can form kidney stones ( Figure 1.1). Stones form in the kidneys and may be found anywhere in the urinary system. Often, stones form when the urine becomes concentrated, allowing minerals to crystallize and stick together. They can vary in size, from small stones that can flow through your urinary system, to larger stones that cannot. Some stones cause great pain, while others cause very little pain. Some stones may need to be removed by surgery or ultrasound treatments. What are the symptoms of kidney stones? You may have a kidney stone if you have pain while urinating, see blood in your urine, and/or feel a sharp pain in your back or lower abdomen (the area between your chest and hips). The pain may last for a long or short time. You may also have nausea and vomiting with the pain. If you have a small stone that passes on its own easily, you may not experience any symptoms. If you have some of these symptoms, you should see your doctor. A kidney stone. The stones can form anywhere in the urinary system. "
excretory system problems,T_2926,"Kidney failure happens when the kidneys cannot remove wastes from the blood. If the kidneys are unable to filter wastes from the blood, the wastes build up in the body. Kidney failure can be caused by an accident that injures the kidneys, the loss of a lot of blood, or by some drugs and poisons. Kidney failure may lead to permanent loss of kidney function. But if the kidneys are not seriously damaged, they may recover. Chronic kidney disease is the slow decrease in kidney function that may lead to permanent kidney failure. A person who has lost kidney function may need to get kidney dialysis. Kidney dialysis is the process of filtering the blood of wastes using a machine. A dialysis machine ( Figure 1.2) filters waste from the blood by pumping the blood through a fake kidney. The filtered blood is then returned to the patients body. "
excretory system problems,T_2927,"Urinary tract infections (UTIs) are bacterial infections of any part of the urinary tract. When bacteria get into the bladder or kidney and produce more bacteria in the urine, they cause a UTI. The most common type of UTI is a bladder infection. Women get UTIs more often than men. UTIs are often treated with antibiotics. Most UTIs are not serious, but some infections can lead to serious problems. Long lasting kidney infections can cause permanent damage, including kidney scars, poor kidney function, high blood pressure, and other problems. Some sudden kidney infections can be life threatening, especially if the bacteria enter the bloodstream, a condition called septicemia. What are the signs and symptoms of a UTI? a burning feeling when you urinate, frequent or intense urges to urinate, even when you have little urine to pass, pain in your back or side below the ribs, cloudy, dark, bloody, or foul-smelling urine, fever or chills. You should see your doctor if you have signs of a UTI. Your doctor will diagnose a UTIs by asking about your symptoms and then testing a sample of your urine. "
features of populations,T_2928,"A population is a group of organisms of the same species, all living in the same area and interacting with each other. Since they live together in one area, members of the same species reproduce together. Ecologists who study populations determine how healthy or stable the populations are. They also study how the individuals of a species interact with each other and how populations interact with the environment. If a group of similar organisms in the same area cannot reproduce with members of the other group, then they are members of two distinct species and form two populations. Ecologists look at many factors that help to describe a population. First, ecologists can measure the number of individuals that make up the population, known as population size. They can then determine the population density, which is the number of individuals of the same species in an area. Population density can be expressed as number per area, such as 20 mice/acre, or 50 rabbits/square mile. Ecologists also study how individuals in a population are spread across an environment. This spacing of individuals within a population is called dispersion. Some species may be clumped or clustered ( Figure 1.1) in an area. Others may be evenly spaced ( Figure 1.2). Still others may be spaced randomly within an area. The population density and dispersion have an effect on reproduction and population size. What do you think the relationship is between population density, dispersion and size? Clumped species are closer together. This may allow for easier reproduction. A population of cacti in the Sonoran Desert generally shows even dispersion due to competition for water. Ecologists also study the birth and death rates of the population. Together these give the growth rate (the birth rate minus the death rate), which tells how fast (or slow) the population size is changing. The birth rate is the number of births within a population during a specific time period. The death rate is the number of deaths within a population during a specific time period. Knowing the birth and death rates of populations gives you information about a populations health. For example, when a population is made up of mostly young organisms and the birth rate is high, the population is growing. A population with equal birth and death rates will remain the same size. Populations that are decreasing in size have a higher death rate than birth rate. "
female reproductive structures,T_2929,"The female reproductive organs include the vagina, uterus, fallopian tubes, and ovaries ( Figure 1.1). The breasts are not shown in this figure. They are not considered reproductive organs, even though they are involved in reproduction. They contain mammary glands that give milk to feed a baby. The milk leaves the breast through the nipple when the baby sucks on it. The vagina is a cylinder-shaped organ found inside of the female body. One end of the vagina opens at the outside of the body. The other end joins with the uterus. During sexual intercourse, sperm may be released into the vagina. If this occurs, the sperm will move through the vagina and into the uterus. During birth, a baby passes from the uterus to the vagina to leave the body. The uterus is a hollow organ with muscular walls. The part that connects the vagina with the uterus is called the cervix. The uterus is where a baby develops until birth. The walls of the uterus grow bigger as the baby grows. The muscular walls of the uterus push the baby out during birth. This drawing shows the organs of the female reproductive system. It shows the organs from the side. Find each organ in the drawing as you read about it in the text. The two ovaries are small, oval organs on either side of the uterus. Each ovary contains thousands of eggs, with about 1-2 million immature eggs present at birth and 40,000 immature eggs present at puberty, as most of the eggs die off. The eggs do not fully develop until a female has gone through puberty. About once a month, on average one egg completes development and is released by the ovary. The ovaries also secrete estrogen, the main female sex hormone. The two fallopian tubes are narrow tubes that open off from the uterus. Each tube reaches for one of the ovaries, but the tubes are not attached to the ovaries. The end of each fallopian tube by the ovary has fingers ( Figure 1.1). They sweep an egg into the fallopian tube. Then the egg passes through the fallopian tube to the uterus. If an egg is to be fertilized, this will occur in the fallopian tube. A fertilized egg then implants into the wall of the uterus, where it begins to develop. An unfertilized egg will flow through the uterus and be excreted from the body. "
female reproductive system,T_2930,"Most of the male reproductive organs are outside of the body. But female reproductive organs are inside of the body. The male and female organs also look very different and have different jobs. Two of the functions of the female reproductive system are similar to the functions of the male reproductive system. The female system: 1. Produces gametes, the reproductive cells, which are called eggs in females. 2. Secretes a major sex hormone, estrogen. One of the main roles of the female reproductive system is to produce eggs. Eggs ( Figure 1.1) are female gametes, and they are made in the ovaries. After puberty, females release only one egg at a time. Eggs are actually made in the body before birth, but they do not fully develop until later in life. Like sperm, eggs are produced by meiosis, so they contain half the number of chromosomes as the original cell. Another role of the female system is to secrete estrogen. Estrogen is the main sex hormone in females. Estrogen has two major roles: 1. During the teen years, estrogen causes the reproductive organs to develop. It also causes other female traits to develop. For example, it causes the breasts to grow. 2. During adulthood, estrogen is needed for a woman to release eggs. On average, a woman releases one egg each month from her ovaries. The female reproductive system has another important function. After puberty, the female reproductive system must prepare itself to accept a fertilized egg each cycle (about every month). This cycle is controlled by a well-planned and very complex interplay of hormones. If an egg is not fertilized, the system must prepare itself again the next cycle. The female reproductive system also supports a baby as it develops before birth, and it facilitates the babys birth at the end of pregnancy. "
fermentation,T_2931,"Sometimes cells need to obtain energy from sugar, but there is no oxygen present to complete cellular respiration. In this situation, cellular respiration can be anaerobic, occurring in the absence of oxygen. In this process, called fermentation, only the first step of respiration, glycolysis, occurs, producing two ATP; no additional ATP is produced. Therefore, the organism only obtains the two ATP molecules per glucose molecule from glycolysis. Compared to the 36-38 ATP produced under aerobic conditions, anaerobic respiration is not a very efficient process. Fermentation allows the first step of cellular respiration to continue and produce some ATP, even without oxygen. Yeast (single-celled eukaryotic organisms) perform alcoholic fermentation in the absence of oxygen. The products of alcoholic fermentation are ethyl alcohol (drinking alcohol) and carbon dioxide gas. This process is used to make common food and drinks. For example, alcoholic fermentation is used to bake bread. The carbon dioxide bubbles allow the bread to rise and become fluffy. Meanwhile, the alcohol evaporates. In wine making, the sugars of grapes are fermented to produce wine. The sugars are the starting materials for glycolysis. Animals and some bacteria and fungi carry out lactic acid fermentation. Lactic acid is a waste product of this process. Our muscles perform lactic acid fermentation during strenuous exercise, since oxygen cannot be delivered to the muscles quickly enough. The buildup of lactic acid is believed to make your muscles sore after exercise. Bacteria that produce lactic acid are used to make cheese and yogurt. The lactic acid causes the proteins in milk to thicken. Lactic acid also causes tooth decay, because bacteria use the sugars in your mouth for energy. Pictured below are some products of fermentation ( Figure 1.1). Products of fermentation include cheese (lactic acid fermentation) and wine (alco- holic fermentation). "
fermentation,T_2932,"Behind every fart is an army of gut bacteria undergoing some crazy biochemistry. These bacteria break down the remains of digested food through fermentation, creating gas in the process. Learn what these bacteria have in common with beer brewing at http://youtu.be/R1kxajH629A?list=PLzMhsCgGKd1hoofiKuifwy6qRXZs7NG6a . Click image to the left or use the URL below. URL: "
fish,T_2936,"What exactly is a fish? You probably think the answer is obvious. You may say that a fish is an animal that swims in the ocean or a lake, using fins. But as we saw with the mudskipper, not all fish spend all their time in water. So how do scientists define fish? Some characteristics of fish include: 1. They are ectothermic, meaning their temperature depends on the temperature of their environment. Ectother- mic animals are cold-blooded in that they cannot raise their body temperature on their own. This is unlike humans, whose temperature is controlled from inside the body. 2. They are covered with scales. 3. They have two sets of paired fins and several unpaired fins. 4. They also have a streamlined body that allows them to swim rapidly. Fish are aquatic vertebrates, meaning they have backbones. They became a dominant form of sea life and eventually evolved into land vertebrates. There are three classes of fish: Class Agnatha (the jawless fish), Class Chondrichthyes (the cartilaginous fish), and Class Osteichthyes (the bony fish). All have the characteristics of fish in common, though there are differences unique to each class. "
fish,T_2937,"In order to absorb oxygen from the water, fish use gills ( Figure 1.2). Gills take dissolved oxygen from water as the water flows over the surface of the gill. Gills help a fish breathe. "
fish,T_2938,"Fish reproduce sexually. They lay eggs that can be fertilized either inside or outside of the body. In most fish, the eggs develop outside of the mothers body. In the majority of these species, fertilization also takes place outside the mothers body. The male and female fish release their gametes into the surrounding water, where fertilization occurs. Female fish release very high numbers of eggs to increase the chances of fertilization. "
fish,T_2939,"Fish range in size from the 65-foot, 75,000 pound whale shark ( Figure 1.3) to the stout infantfish, which is about 0.33 inches (8.4 mm), and the Paedocypris progenetica carp species of the Indonesian island of Sumatra, which is about 0.31 inches (7.9 mm) long, making it also the smallest known vertebrate animal. The second-largest fish is the basking shark, which grows to about 40 feet and 8,000 pounds. Both of the large sharks may look ferocious, and would probably scare anyone who comes across one in the water, but both species are filter-feeders, and feed on tiny fish and plankton. The tiny carp species is unique in that it has the appearance of larvae, with a reduced skeleton lacking a cranium, which leaves the brain unprotected by bone. The fish lives in dark acidic waters, having a pH of 3. Keep in mind that whales are not fish, they are mammals. "
fish,T_2940,"There are exceptions to many of these fish traits. For example, tuna, swordfish, and some species of shark show some warm-blooded adaptations and are able to raise their body temperature significantly above that of the water around them. Some species of fish have a slower, more maneuverable swimming style, like eels and rays ( Figure 1.4). Body shape and the arrangement of fins are highly variable, and the surface of the skin may be naked, as in moray eels, or covered with scales. Scales can be of a variety of different types. "
fish,T_2941,"How are fish important? Of course, they are used as food ( Figure 1.5). In fact, people all over the world either catch fish in the wild or farm them in much the same way as cattle or chickens. Farming fish is known as aquaculture. Fish are also caught for recreation to display in the home or in a public aquarium. "
flatworms,T_2942,"The word ""worm"" is not very scientific. But it is a word that informally describes animals (usually invertebrates) that have long bodies with no arms or legs. (Snakes are vertebrates, so they are not usually described as worms.) Worms are the first significant group of animals with bilateral symmetry, meaning that the right side of their bodies is a mirror of the left. One type of worm is the flatworm. Worms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. There are more than 18,500 known species of flatworms. "
flatworms,T_2943,"The main characteristics of flatworms ( Figure 1.1) include: 1. Flatworms have no true body cavity, but they do have bilateral symmetry. Due to the lack of a body cav- ity,flatworms are known as acoelomates. 2. Flatworms have an incomplete digestive system. This means that the digestive tract has only one opening. Digestion takes place in the gastrovascular cavity. 3. Flatworms do not have a respiratory system. Instead, they have pores that allow oxygen to enter through their body. Oxygen enters the pores by diffusion. 4. There are no blood vessels in the flatworms. Their gastrovascular cavity helps distribute nutrients throughout the body. 5. Flatworms have a ladder-like nervous system; two interconnected parallel nerve cords run the length of the body. 6. Most flatworms have a distinct head region that includes nerve cells and sensory organs, such as eyespots. The development of a head region, called cephalization, evolved at the same time as bilateral symmetry in animals. This process does not occur in cnidarians, which evolved prior to flatworms and have radial symmetry. Marine flatworms can be brightly colored, such as this one from the class Turbel- laria. These worms are mostly carnivores or scavengers. "
flatworms,T_2944,"Flatworms live in a variety of environments. Some species of flatworms are free-living organisms that feed on small organisms and rotting matter. These types of flatworms include marine flatworms and freshwater flatworms, such as Dugesia. Other types of flatworms are parasitic. That means they live inside another organism, called a host, in order to get the food and energy they need. For example, tapeworms have a head-like area with tiny hooks and suckers (known as the scolex) that help the worm attach to the intestines of an animal host ( Figure 1.2). There are over 11,000 species of parasitic flatworms. "
food and nutrients,T_2945,"Did you ever hear the old saying, An apple a day keeps the doctor away? Do apples really prevent you from getting sick? Probably not, but eating apples and other fresh fruits can help keep you healthy. Do you eat your vegetables? Maybe you do, but you may have friends who wont touch a piece of broccoli or asparagus. Should you eat these foods and food like them? The girls pictured in the Figure 1.1 are eating salads. Why do you need foods like these for good health? What role does food play in the body? Your body needs food for three reasons: 1. Food gives your body energy. You need energy for everything you do. Remember that cellular respiration converts the glucose in the food you eat into ATP, or cellular energy. Which has more glucose, a salad or a piece of meat? Do you remember what types of foods produce glucose? Recall that glucose is the product of photosynthesis. These girls are eating leafy green vegetables. Fresh vegetables such as these are excellent food choices for good health. 2. Food provides building materials for your body. Your body needs building materials so it can grow and repair itself. Specifically, it needs these materials to produce more cells and its components. 3. Food contains substances that help control body processes. Your body processes must be kept in balance for good health. For all these reasons, you must have a regular supply of nutrients. Nutrients are chemicals in food that your body needs. There are five types of nutrients. 1. 2. 3. 4. 5. Carbohydrates Proteins Lipids Vitamins Minerals Carbohydrates, proteins, and lipids are categories of organic compounds. They give your body energy, though carbohydrates are the main source of energy. Proteins provide building materials, such as amino acids to build your own proteins. Proteins, vitamins, and minerals also help control body processes. Carbohydrates include sugars such as the glucose made by photosynthesis. Often glucose is stored in large molecules such as starch. Proteins are found in foods like meats and nuts. Lipids includes fats and oils. Though you should stay away from many types of fats, others are needed by your body. Important vitamins include vitamins A, B (multiple types) C, D, and E. Important minerals include calcium and potassium. What should you drink to get calcium? Milk is a good source. "
fossils,T_2947,"Fossils are the preserved remains of animals, plants, and other organisms from the distant past. Examples of fossils include bones, teeth, and impressions. By studying fossils, evidence for evolution is revealed. Paleontologists are scientists who study fossils to learn about life in the past. Fossils allow these scientists to determine the features of extinct species. Paleontologists compare the features of species from different periods in history. With this information, they try to understand how species have evolved over millions of years ( Figure below). Until recently, fossils were the main source of evidence for evolution ( Figure below). Through studying fossils, we now know that todays organisms look much different in many cases than those that were alive in the past. Scientists have also shown that organisms were spread out differently across the planet. Earthquakes, volcanoes, shifting seas, and other movements of the continents have all affected where organisms live and how they adapted to their changing environments. "
fossils,T_2948,"There are many layers of rock in the Earths surface. Newer layers form on top of the older layers; the deepest rock layers are the oldest. Therefore, you can tell how old a fossil is by observing in which layer of rock it was found. Evolution of the horse. Fossil evi- dence, depicted by the skeletal frag- ments, demonstrates evolutionary mile- stones in this process. Notice the 57 million year evolution of the horse leg bones and teeth. Especially obvious is the transformation of the leg bones from having four distinct digits to that of todays horse. The fossils and the order in which fossils appear is called the fossil record. The fossil record provides evidence for when organisms lived on Earth, how species evolved, and how some species have gone extinct. Geologists use a method called radiometric dating to determine the exact age of rocks and fossils in each layer of rock. This technique, which is possible because radioactive materials decay at a known rate, measures how much of the radioactive materials in each rock layer have broken down ( Figure 1.3). Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4 and 5 billion years old. The oldest fossils are between 3 and 4 billion years old. Remember that during Darwins time, people believed the Earth was just about 6,000 years old. The fossil record proves that Earth is much older than people once thought. "
genetic disorders,T_2968,"Many genetic disorders are caused by mutations in one or a few genes. Others are caused by chromosomal mutations. Some human genetic disorders are X-linked or Y-linked, which means the faulty gene is carried on these sex chromosomes. Other genetic disorders are carried on one of the other 22 pairs of chromosomes; these chromosomes are known as autosomes or autosomal (non-sex) chromosomes. Some genetic disorders are due to new mutations, others can be inherited from your parents. "
genetic disorders,T_2969,"Some genetic disorders are caused by recessive alleles of a single gene on an autosome. An example of autosomal recessive genetic disorders are Tay-Sachs disease and cystic fibrosis. Children with cystic fibrosis have excessively thick mucus in their lungs, which makes it difficult for them to breathe. The inheritance of this recessive allele is the same as any other recessive allele, so a Punnett square can be used to predict the probability that two carriers of the disease will have a child with cystic fibrosis. Recall that carriers have the recessive allele for a trait but do not express the trait. What are the possible genotypes of the offspring in the following table ( Table 1.1)? What are the possible phenotypes? F FF (normal) Ff (carrier) F f f Ff (carrier) ff (affected) According to this Punnett square, two parents that are carriers (Ff ) of the cystic fibrosis gene have a 25% chance of having a child with cystic fibrosis (ff ). The affected child must inherit two recessive alleles. The carrier parents are not affected. Tay-Sachs disease is a severe genetic disorder in which affected children do not live to adulthood, so the gene is not passed from an affected individual. Carriers of the Tay-Sachs gene are not affected. How does a child become affected with Tay-Sachs? "
genetic disorders,T_2970,"Huntingtons disease is an example of an autosomal dominant disorder. This means that if the dominant allele is present, then the person will express the disease. A child only has to inherit one dominant allele to have the disease. The disease causes the brains cells to break down, leading to muscle spasms and personality changes. Unlike most other genetic disorders, the symptoms usually do not become apparent until middle age. You can use a simple Punnett square to predict the inheritance of a dominant autosomal disorder, like Huntingtons disease. If one parent has Huntingtons disease, what is the chance of passing it on to the children? If you draw the Punnett square, you will find that there is a 50 percent chance of the disorder being passed on to the children. "
hardy weinberg theorem,T_2985,"Sometimes understanding how common a gene is within a population is necessary. Or, more specifically, you may want to know how common a certain form of that gene is within the population, such as a recessive form. This can be done using the Hardy-Weinberg model, but it can only be done if the frequencies of the genes are not changing. The Hardy-Weinberg model describes how a population can remain at genetic equilibrium, referred to as the Hardy-Weinberg equilibrium. Genetic equilibrium occurs when there is no evolution within the population. In other words, the frequency of alleles (variants of a gene) will be the same from one generation to another. At genetic equilibrium, the gene or allele frequencies are stablethey do not change. For example, lets assume that red hair is determined by the inheritance of a gene with two allelesR and r. The dominant allele, R, encodes for non-red hair, while the recessive allele, r, encodes for red hair. If a populations gene pool contains 90% R and 10% r alleles, then the next generation would also have 90% R and 10% r alleles. However, this only works under a strict set of conditions. The five conditions that must be met for genetic equilibrium to occur include: 1. 2. 3. 4. 5. No mutation (change) in the DNA sequence. No migration (moving into or out of a population). A very large population size. Random mating. No natural selection. These five conditions rarely occur in nature. When one or more of the conditions does not exist, then evolution can occur. As a result, allele frequencies are constantly changing, and populations are constantly evolving. As mutations and natural selection occur frequently in nature, it is difficult for a population to be at genetic equilibrium. The Hardy-Weinberg model also serves a mathematical formula used to predict allele frequencies in a population at genetic equilibrium. If you know the allele frequencies of one generation, you can use this formula to predict the next generation. Again, this only works if all five conditions are being met in a population. "
harmful bacteria,T_2986,"With so many species of bacteria, some are bound to be harmful. Harmful bacteria can make you sick. They can also ruin food and be used to hurt people. "
harmful bacteria,T_2987,"There are also ways that bacteria can be harmful to humans and other animals. Bacteria are responsible for many types of human illness ( Figure 1.1), including: Strep throat Tuberculosis Pneumonia Leprosy Lyme disease Luckily most of these can be treated with antibiotics, which kill the bacteria. It is important that when a medical doctor prescribes antibiotics for you, you take the medicine exactly as the doctor tells you. You need to make sure the bacteria is killed. "
harmful bacteria,T_2988,"Bacterial contamination of foods can lead to digestive problems, an illness known as food poisoning. Raw eggs and undercooked meats commonly carry the bacteria that can cause food poisoning. Food poisoning can be prevented by cooking meat thoroughly, which kills most microbes, and washing surfaces that have been in contact with raw meat. Washing your hands before and after handling food also helps prevent contamination. "
harmful bacteria,T_2989,"Some bacteria also have the potential to be used as biological weapons by terrorists. An example is anthrax, a disease caused by the bacterium Bacillus anthracis. Inhaling the spores of this bacterium can lead to a deadly infection, and, therefore, it is a dangerous weapon. In 2001, an act of terrorism in the United States involved B. anthracis spores sent in letters through the mail. "
harmful bacteria,T_2990,KidsHealth Food Poisoning at http://kidshealth.org/kid/ill_injure/sick/food_poisoning.html . 1. What are the common bacteria that cause food poisoning? 2. What steps can you take to keep your food safe? 
health hazards of air pollution,T_2991,"The World Health Organization (WHO) reports that 2.4 million people die each year from causes directly related to air pollution. This includes both outdoor and indoor air pollution. Worldwide, there are more deaths linked to air pollution each year than to car accidents. Research by the WHO also shows that the worst air quality is in countries with high poverty and population rates, such as Egypt, Sudan, Mongolia, and Indonesia. Respiratory system disorders are directly related to air pollution. These disorders have severe effects on human health, some leading to death directly related to air pollution. Air pollution related respiratory disorders include asthma, bronchitis, and emphysema. Asthma is a respiratory disorder characterized by wheezing, coughing, and a feeling of constriction in the chest. Bronchitis is inflammation of the membrane lining of the bronchial tubes of the lungs. Emphysema is a deadly lung disease characterized by abnormal enlargement of air spaces in the lungs and destruction of the lung tissue. Additional lung and heart diseases are also related to air pollution, as are respiratory allergies. Air pollution can also indirectly cause other health issues and even deaths. Air pollutants can cause an increase in cancer including lung cancer, eye problems, and other conditions. For example, using certain chemicals on farms, such as the insecticide DDT (dichlorodiphenyltrichloroethane) and toxic PCBs (polychlorinated biphenyl), can cause cancer. Indoors, pollutants such as radon or asbestos can also increase your cancer risk. Lastly, air pollution can lead to heart disease, including heart attack and stroke. "
health hazards of air pollution,T_2992,"Certain respiratory conditions can be made worse in people who live closer to or in large cites. Some studies have shown that people in urban areas suffer lower levels of lung function and more chronic bronchitis and emphysema. If you live in a city, you have seen smog. It is a low-hanging, fog-like cloud that seems to never leave the city ( Figure 1.1). Smog is caused by coal burning and by ozone produced by motor vehicle exhaust. Smog can cause eye irritation and respiratory problems. A layer of smog is typical for Cairo, Egypt. "
health hazards of air pollution,T_2993,"After reading about the effects of air pollution, both indoors and outdoors, you may wonder how you can avoid it. As for outdoor air pollution, if you hear in the news that the outdoor air quality is particularly bad, then it might make sense to wear a mask outdoors or to stay indoors. Because you have more control over your indoor air quality than the outdoor air quality, there are some simple steps you can take indoors to make sure the air quality is less polluted. These include: 1. 2. 3. 4. Make sure that vents and chimneys are working properly, and never burn charcoal indoors. Place carbon monoxide detectors in the home. Keep your home as clean as possible from pet dander, dust, dust mites, and mold. Make sure air conditioning systems are working properly. Are there any other ways you can think of to protect yourself from air pollution? "
health of the digestive system,T_2994,"Most of the time, you probably arent aware of your digestive system. It works well without causing any problems. But most people have problems with their digestive system at least once in a while. Did you ever eat something that didnt agree with you? Maybe you had a stomachache or felt sick to your stomach? Maybe you had diarrhea? These could be symptoms of foodborne illness, food allergies, or a food intolerance. "
health of the digestive system,T_2995,"Harmful bacteria can enter your digestive system in food and make you sick. This is called foodborne illness or food poisoning. The bacteria, or the toxins they produce, may cause vomiting or cramping, in addition to the symptoms mentioned above. Foodborne illnesses can also be caused by viruses and parasites. The most common foodborne illnesses happen within a few minutes to a few hours, and make you feel really sick, but last for only about a day or so. Others can take longer for the illness to appear. Some people believe that the taste of food will tell you if it is bad. As a rule, you probably should not eat bad tasting food, but many contaminated foods can still taste good. You can help prevent foodborne illness by following a few simple rules. Keep hot foods hot and cold foods cold. This helps prevent any bacteria in the foods from multiplying. Wash your hands before you prepare or eat food. This helps prevent bacteria on your hands from getting on the food. This is the easiest way to prevent foodborne illnesses. Wash your hands after you touch raw foods, such as meats, poultry, fish, or eggs. These foods often contain bacteria that your hands could transfer to your mouth. Cook meats, poultry, fish, and eggs thoroughly before eating them. The heat of cooking kills any bacteria the foods may contain, so they cannot make you sick. Refrigerate cooked food soon after a meal. Cooked food can be left out for up to two hours before they need to be placed in the cold. This will prevent the spread of bacteria. Cooked foods should not be left out all day. Bacteria that cause foodborne illnesses include Salmonella, a bacterium found in many foods, including raw and undercooked meat, poultry, dairy products, and seafood. Campylobacter jejuni is found in raw or undercooked chicken and unpasteurized milk. Several strains of E. coli can cause illnesses, and are found in raw or undercooked hamburger, unpasteurized fruit juices and milk, and even fresh produce. Vibrio is a bacterium that may contaminate fish or shellfish. Listeria has been found in raw and undercooked meats, unpasteurized milk, soft cheeses, and ready- to-eat deli meats and hot dogs. Most of these bacterial illnesses can be prevented with proper cooking of food and washing of hands. Common foodborne viruses include norovirus and hepatitis A virus. Norovirus, which causes inflammation of the stomach and intestines, has been a recent issue on cruise ships, infecting hundreds of passengers and crew on certain voyages. Hepatitis A causes inflammation of the liver, which is treated with rest and diet changes. Parasites are tiny organisms that live inside another organism. Giardia is a parasite spread through water contaminated with the stools of people or animals who are infected. Food preparers who are infected with parasites can also contaminate food if they do not thoroughly wash their hands after using the bathroom and before handling food. Trichinella is a type of roundworm parasite. People may be infected with this parasite by consuming raw or undercooked pork or wild game. "
health of the digestive system,T_2996,"Food allergies are like other allergies. They occur when the immune system reacts to harmless substances as though they were harmful. Almost ten percent of children have food allergies. Some of the foods most likely to cause allergies are shown below ( Figure 1.1). Eating foods you are allergic to may cause vomiting, diarrhea, or skin rashes. Some people are very allergic to certain foods. Eating even tiny amounts of the foods causes them to have serious symptoms, such as difficulty breathing. If they eat the foods by accident, they may need emergency medical treatment. Some of the foods that commonly cause allergies are shown here. They include nuts, eggs, grains, milk, and shellfish. Are you allergic to any of these foods? The most common food allergy symptoms include: tingling or itching in the mouth hives, itching or eczema, swelling of the lips, face, tongue and throat, or other parts of the body, wheezing, nasal congestion or trouble breathing, abdominal pain, diarrhea, nausea or vomiting, dizziness, lightheadedness or fainting. In some people, a food allergy can trigger a severe allergic reaction called anaphylaxis. Emergency treatment is critical for anaphylaxis. Untreated, anaphylaxis can cause a coma or death. Anaphylaxis is vary rare. The vast majority of people will never have an anaphylactic reaction. The life-threatening symptoms of anaphylaxis include: constriction and tightening of the airway, a swollen throat or the sensation of a lump in your throat that makes it difficult to breathe, shock, with a severe drop in blood pressure, a rapid pulse, dizziness, lightheadedness or loss of consciousness. "
health of the digestive system,T_2997,"A food intolerance, or food sensitivity, is different from a food allergy. A food intolerance happens when the digestive system is unable to break down a certain type of food. This can result in stomach cramping, diarrhea, tiredness, and weight loss. Food intolerances are often mistakenly called allergies. Lactose intolerance is a food intolerance. A person who is lactose intolerant does not make enough lactase, the enzyme that breaks down the milk sugar, lactose. Lactose intolerance may be as high as 75% in some populations, but overall the percentage of affected individuals is much less. Still, well over 10% of the worlds population is lactose intolerant. "
hearing and balance,T_2998,What do listening to music and riding a bike have in common? It might surprise you to learn that both activities depend on your ears. The ears do more than just detect sound. They also sense the position of the body and help maintain balance. 
hearing and balance,T_2999,"Hearing is the ability to sense sound. Sound travels through the air in waves, much like the waves you see in the water pictured below ( Figure 1.1). Sound waves in air cause vibrations inside the ears. The ears sense the vibrations. The human ear is pictured below ( Figure 1.2). As you read about it, trace the path of sound waves through the ear. Assume a car horn blows in the distance. Sound waves spread through the air from the horn. Some of the sound waves reach your ear. The steps below show what happens next. They explain how your ears sense the sound. 1. The sound waves travel to the ear canal (external auditory canal in the figure). This is a tube-shaped opening in the ear. Sound waves travel through the air in all directions away from a sound, like waves traveling through water away from where a pebble was dropped. Read the names of the parts of the ear in the text; then find each of the parts in the diagram. Note that the round window is distinct from the oval window. 2. At the end of the ear canal, the sound waves hit the eardrum (tympanic membrane). This is a thin membrane that vibrates like the head of a drum when sound waves hit it. 3. The vibrations pass from the eardrum to the hammer (malleus). This is the first of three tiny bones that pass vibrations through the ear. 4. The hammer passes the vibrations to the anvil (incus), the second tiny bone that passes vibrations through the ear. 5. The anvil passes the vibrations to the stirrup (stapes), the third tiny bone that passes vibrations through the ear. 6. From the stirrup, the vibrations pass to the oval window. This is another membrane like the eardrum. 7. The oval window passes the vibrations to the cochlea. The cochlea is filled with liquid that moves when the vibrations pass through, like the waves in water when you drop a pebble into a pond. Tiny hair cells line the cochlea and bend when the liquid moves. When the hair cells bend, they release neurotransmitters. 8. The neurotransmitters trigger nerve impulses that travel to the brain through the auditory nerve (cochlear No doubt youve been warned that listening to loud music or other loud sounds can damage your hearing. Its true. In fact, repeated exposure to loud sounds is the most common cause of hearing loss. The reason? Very loud sounds can kill the tiny hair cells lining the cochlea. The hair cells do not generally grow back once they are destroyed, so this type of hearing loss is permanent. You can protect your hearing by avoiding loud sounds or wearing earplugs or other ear protectors. "
hearing and balance,T_3000,"Did you ever try to stand on one foot with your eyes closed? Try it and see what happens, but be careful! Its harder to keep your balance when you cant see. Your eyes obviously play a role in balance. But your ears play an even bigger role. The gymnast pictured below ( Figure 1.3) may not realize it, but her earsalong with her cerebellumare mostly responsible for her ability to perform on the balance beam. The parts of the ears involved in balance are the semicircular canals. Above, the semicircular canals are colored purple ( Figure 1.2). The canals contain liquid and are like the bottle of water pictured below ( Figure 1.4). When the bottle tips, the water surface moves up and down the sides of the bottle. When the body tips, the liquid in the semicircular canals moves up and down the sides of the canals. Tiny hair cells line the semicircular canals. Movement of the liquid inside the canals causes the hair cells to send nerve impulses. The nerve impulses travel to the cerebellum in the brain along the vestibular nerve. In response, the cerebellum sends commands to muscles to contract or relax so that the body stays balanced. "
heart,T_3001,"What does the heart look like? How does it pump blood? The heart is divided into four chambers ( Figure 1.1), or spaces: the left and right atria, and the left and right ventricles. An atrium (singular for atria) is one of the two small, thin-walled chambers on the top of the heart where the blood first enters. A ventricle is one of the two muscular V-shaped chambers that pump blood out of the heart. You can remember they are called ventricles because they are shaped like a ""V."" The atria receive the blood, and the ventricles pump the blood out of the heart. Each of the four chambers of the heart has a specific job. The right atrium receives oxygen-poor blood from the body. The right ventricle pumps oxygen-poor blood toward the lungs, where it receives oxygen. The left atrium receives oxygen-rich blood from the lungs. The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body. "
heart,T_3002,"Blood flows through the heart in two separate loops. You can think of them as a left side loop and a right side loop. The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it releases carbon dioxide and picks up oxygen. (Recall that carbon dioxide is a waste product that must be removed. It is removed when we exhale.) The left side carries the oxygen-rich blood back from the lungs into the left side of the heart, which then pumps the oxygen-rich blood to the rest of the body. The blood delivers oxygen to the cells of the body, where it is needed for cellular respiration, and returns to the heart oxygen-poor. To move blood through the heart, the cardiac muscle needs to contract in an organized way. Blood first enters the atria ( Figure 1.2). When the atria contract, blood is pushed into the ventricles. After the ventricles fill with blood, they contract, and blood is pushed out of the heart. The heart is mainly composed of cardiac muscle. These muscle cells contract in unison, causing the heart itself to contract and generating enough force to push the blood out. So how is the blood kept from flowing back on itself? Valves ( Figure 1.2) in the heart keep the blood flowing in one direction. The valves do this by opening and closing in one direction only. Blood only moves forward through the heart. The valves stop the blood from flowing backward. There are four valves of the heart. The two atrioventricular (AV) valves stop blood from moving from the ventricles to the atria. The two semilunar (SL) valves are found in the arteries leaving the heart, and they prevent blood from flowing back from the arteries into the ventricles. Why does a heart beat? The lub-dub sound of the heartbeat is caused by the closing of the AV valves (""lub"") and SL valves (""dub"") after blood has passed through them. "
helpful bacteria,T_3003,"Can we survive without bacteria? Could bacteria survive without us? No and yes. No, we could not survive without bacteria. And yes, bacteria could survive without us. "
helpful bacteria,T_3004,"Bacteria can be used to make cheese from milk. The bacteria turn the milk sugars into lactic acid. The acid is what causes the milk to curdle to form cheese. Bacteria are also involved in producing other foods. Yogurt is made by using bacteria to ferment milk ( Figure 1.1). Fermenting cabbage with bacteria produces sauerkraut. Yogurt is made from milk fermented with bacteria. The bacteria ingest natural milk sugars and release lactic acid as a waste product, which causes proteins in the milk to form into a solid mass, which becomes the yogurt. "
helpful bacteria,T_3005,"In the laboratory, bacteria can be changed to provide us with a variety of useful materials. Bacteria can be used as tiny factories to produce desired chemicals and medicines. For example, insulin, which is necessary to treat people with diabetes, can be produced using bacteria. Through the process of transformation, the human gene for insulin is placed into bacteria. The bacteria then use that gene to make a protein. The protein can be separated from the bacteria and then used to treat patients. The mass production of insulin by bacteria made this medicine much more affordable. During transformation, bacteria can take up any DNA from the environment. Therefore, transformation allows scientists to insert any DNA into a bacteria, potentially producing many different proteins. This makes the bacteria greatly useful to people. "
helpful bacteria,T_3006,"Bacteria also help you digest your food. Several species of bacteria, such as E. coli, are found in your digestive tract. In fact, in your gut, bacteria cells greatly outnumber your own cells! "
helpful bacteria,T_3007,"Bacteria are important in practically all ecosystems because many bacteria are decomposers. They break down dead materials and waste products and recycle nutrients back into the environment. The recycling of nutrients, such as nitrogen, by bacteria, is essential for living organisms. Organisms cannot produce nutrients, so they must come from other sources. We get nutrients from the food we eat; plants get them from the soil. How do these nutrients get into the soil? One way is from the actions of decomposers. Without decomposers, we would eventually run out of the materials we need to survive. We also depend on bacteria to decompose our wastes in sewage treatment plants. "
hiv and aids,T_3008,"HIV, or human immunodeficiency virus, causes AIDS. AIDS stands for ""acquired immune deficiency syndrome."" It is a condition that causes death and does not have a known cure. AIDS usually develops 10 to 15 years after a person is first infected with HIV. The development of AIDS can be delayed with proper medicines. The delay can be well over 20 years with the right medicines. Today, individuals who acquire HIV after 50 years of age can expect to reach an average human life span. "
hiv and aids,T_3009,"HIV spreads through contact between an infected persons body fluids and another persons bloodstream or mucus membranes, which are found in the mouth, nose, and genital areas. Body fluids that may contain HIV are blood, semen, vaginal fluid, and breast milk. The virus can spread through sexual contact or shared drug needles. It can also spread from an infected mother to her baby during childbirth or breastfeeding. Saliva can carry the HIV virus, but it wont spread it, unless the saliva gets into the bloodstream. Other body fluids such as urine and sweat do not contain the virus. HIV does not spread in any fluid in which the host cells cannot survive. Some people think they can become infected with HIV by donating blood or receiving donated blood. This is not true. The needles used to draw blood for donations are always new. Therefore, they cannot spread the virus. Donated blood is also tested to make sure it is does not contain HIV. HIV is not transmitted by day-to-day contact in the workplace, schools, or social settings. HIV is not transmitted through shaking hands, hugging, or a casual kiss. You cannot become infected from a toilet seat, a drinking fountain, a door knob, dishes, drinking glasses, food, or pets. "
hiv and aids,T_3010,"How does an HIV infection develop into AIDS? HIV destroys white blood cells called helper T cells. The cells are produced by the immune system. This is the body system that fights infections and other diseases. HIV invades helper T cells and uses them to produce more virus particles ( Figure 1.1). Then, the virus kills the helper T cells. As the number of viruses in the blood rises, the number of helper T cells falls. Without helper T cells, the immune system is unable to protect the body. The infected person cannot fight infections and other diseases because they do not have T cells. This is why people do not die from HIV. Instead, they die from another illness, like the common cold, that they cannot fight because they do not have helper T cells. Medications can slow down the increase of viruses in the blood. But the medications cannot remove the viruses from the body. At present, there is no cure for HIV infection. A vaccine against HIV could stop this disease, and such a vaccine is in development, though it could take many years before it can be given to prevent this virus. "
hiv and aids,T_3011,"AIDS is not really a single disease. It is a set of symptoms and other diseases. It results from years of damage to the immune system by HIV. AIDS occurs when helper T cells fall to a very low level, making it difficult for the affected person to fight various diseases and other infections. These people develop infections or cancers that people with a healthy immune systems can easily resist. These diseases are usually the cause of death of people with AIDS. The first known cases of AIDS occurred in 1981. Since then, AIDS has led to the deaths of more than 35 million people worldwide. Many of them were children. The greatest number of deaths occurred in Africa. It is also where medications to control HIV are least available. There are currently more people infected with HIV in Africa than any other part of the world. Well over 30 million people are living with HIV worldwide. "
homeostasis,T_3012,"When you walk outside on a cool day, does your body temperature drop? No, your body temperature stays stable at around 98.6 degrees Fahrenheit. Even when the temperature around you changes, your internal temperature stays the same. This ability of the body to maintain a stable internal environment despite a changing environment is called home- ostasis. Homeostasis doesnt just protect against temperature changes. Other aspects of your internal environment also stay stable. For example, your body closely regulates your fluid balance. You may have noticed that if you are slightly dehydrated, your urine is darker. Thats because the urine is more concentrated and less water is mixed in with it. "
homeostasis,T_3013,"So how does your body maintain homeostasis? The regulation of your internal environment is done primarily through negative feedback. Negative feedback is a response to a stimulus that keeps a variable close to a set value ( Figure For example, your body has an internal thermostat. During a winter day, in your house a thermostat senses the temperature in a room and responds by turning on or off the heater. Your body acts in much the same way. When body temperature rises, receptors in the skin and the brain sense the temperature change. The temperature change triggers a command from the brain. This command can cause several responses. If you are too hot, the skin makes sweat and blood vessels near the skin surface dilate. This response helps decrease body temperature. Another example of negative feedback has to do with blood glucose levels. When glucose (sugar) levels in the blood are too high, the pancreas secretes insulin to stimulate the absorption of glucose and the conversion of glucose into glycogen, which is stored in the liver. As blood glucose levels decrease, less insulin is produced. When glucose levels are too low, another hormone called glucagon is produced, which causes the liver to convert glycogen back to glucose. For additional information, see Homeostasis at  . Feedback Regulation. If a raise in body temperature (stimulus) is detected (recep- tor), a signal will cause the brain to main- tain homeostasis (response). Once the body temperature returns to normal, neg- ative feedback will cause the response to end. This sequence of stimulus-receptor- signal-response is used throughout the body to maintain homeostasis. "
homeostasis,T_3014,"Some processes in the body are regulated by positive feedback. Positive feedback is when a response to an event increases the likelihood of the event to continue. An example of positive feedback is milk production in nursing mothers. As the baby drinks her mothers milk, the hormone prolactin, a chemical signal, is released. The more the baby suckles, the more prolactin is released, which causes more milk to be produced. Other examples of positive feedback include contractions during childbirth. When constrictions in the uterus push a baby into the birth canal, additional contractions occur. "
how the eye works,T_3015,"Carbon is one of the most common elements found in living organisms. Chains of carbon molecules form the backbones of many organic molecules, such as carbohydrates, proteins, and lipids. Carbon is constantly cycling between living organisms and the atmosphere ( Figure 1.1). The cycling of carbon occurs through the carbon cycle. Living organisms cannot make their own carbon, so how is carbon incorporated into living organisms? In the atmosphere, carbon is in the form of carbon dioxide gas (CO2 ). Recall that plants and other producers capture the carbon dioxide and convert it to glucose (C6 H12 O6 ) through the process of photosynthesis. Then as animals eat plants or other animals, they gain the carbon from those organisms. The chemical equation of photosynthesis is 6CO2 + 6H2 O  C6 H12 O6 + 6O2 . How does this carbon in living things end up back in the atmosphere? Remember that we breathe out carbon dioxide. This carbon dioxide is generated through the process of cellular respiration, which has the reverse chemical reaction as photosynthesis. That means when our cells burn food (glucose) for energy, carbon dioxide is released. We, like all animals, exhale this carbon dioxide and return it back to the atmosphere. Also, carbon is released to the atmosphere as an organism dies and decomposes. Cellular respiration and photosynthesis can be described as a cycle, as one uses carbon dioxide (and water) and makes oxygen (and glucose), and the other uses oxygen (and glucose) and makes carbon dioxide (and water). The carbon cycle. The cycling of carbon dioxide in photosynthesis and cellular res- piration are main components of the car- bon cycle. Carbon is also returned to the atmosphere by the burning of fossil fuels and decomposition of organic matter. "
how the eye works,T_3016,"Millions of years ago, there were so many dead plants and animals that they could not completely decompose before they were buried. They were covered over by soil or sand, tar or ice. These dead plants and animals are organic matter made out of cells full of carbon-containing organic compounds (carbohydrates, lipids, proteins and nucleic acids). What happened to all this carbon? When organic matter is under pressure for millions of years, it forms fossil fuels. Fossil fuels are coal, oil, and natural gas. When humans dig up and use fossil fuels, we have an impact on the carbon cycle ( Figure 1.2). This carbon is not recycled until it is used by humans. The burning of fossil fuels releases more carbon dioxide into the atmosphere than is used by photosynthesis. So, there is more carbon dioxide entering the atmosphere than is coming out of it. Carbon dioxide is known as a greenhouse gas, since it lets in light energy but does not let heat escape, much like the panes of a greenhouse. The increase of greenhouse gasses in the atmosphere is contributing to a global rise in Earths temperature, known as global warming or global climate change. "
human causes of extinction,T_3017,"In addition to habitat destruction, other human-caused problems are also threatening many species. These include issues associated with climate change, pollution, and over-population. "
human causes of extinction,T_3018,"Another major cause of extinction is global warming, which is also known as global climate change. During the past century, the Earths average temperature has risen by almost 1C (about 1.3F). You may not think that is significant, but to organisms that live in the wild and are constantly adapting to their environments, any climate change can be hazardous. Recall that burning fossil fuels releases gasses into the atmosphere that warm the Earth. Our increased use of fossil fuels, such as coal and oil, is changing the Earths climate. Any long-term change in the climate can destroy the habitat of a species. Even a brief change in climate may be too stressful for an organism to survive. For example, if the seas increase in temperature, even briefly, it may be too warm for certain types of fish to reproduce. "
human causes of extinction,T_3019,"Pollution adds chemicals, noise, heat, or even light to an environment. This can have many different harmful effects on all kinds of organisms. For example, the pesticide DDT nearly eliminated the peregrine falcon in some parts of the world. This pesticide caused falcons to lay eggs with thinner shells. As a result, fewer falcon eggs survived to hatching. Populations of peregrine falcons declined rapidly. DDT was then banned in the U.S. and peregrine falcon populations have recovered. Water pollution threatens vital freshwater and marine resources throughout the world ( Figure 1.1). Specifically, industrial and agricultural chemicals, waste, and acid rain threaten water. As water is essential for all ecosystems, water pollution can result in the extinction of species. A bird that was the victim of an oil spill. About 58,000 gallons of oil spilled from a South Korea-bound container ship when it struck a tower supporting the San Francisco-Oakland Bay Bridge in dense fog in November, 2007. Finally, soil contamination can also result in extinction. Soil contamination can come from toxic industrial and municipal wastes ( Figure 1.2), salts from irrigation, and pesticides from agriculture. These all degrade the soil as well. As soil is the foundation of terrestrial ecosystems, this can result in extinction. "
human causes of extinction,T_3020,"Human populations are on the rise. The human population passed the 7 billion mark in October of 2011, and will pass 8 and 9 billion probably before the middle of the century. All these people will need resources such as places to live, food to eat, and water to drink, and they will use energy and create waste. Essentially, human population growth can effect all other causes of extinction. For example, more people on the Earth means more people contributing to global warming and pollution. More people also means more clearing of land for agriculture and development. Recall that development by humans often causes habitats to be destroyed. This destruction can force species to go extinct, or move somewhere else. "
human digestive system,T_3021,"Nutrients in the foods you eat are needed by the cells of your body. How do the nutrients in foods get to your body cells? What organs and processes break down the foods and make the nutrients available to cells? The organs are those of the digestive system. The processes are digestion and absorption. The digestive system is the body system that breaks down food and absorbs nutrients. It also gets rid of solid food waste. The digestive system is mainly one long tube from the mouth to the anus, known as the gastrointestinal tract (GI tract). The main organs of the digestive system include the esophagus, stomach and the intestine, and are pictured below ( Figure 1.1). The intestine is divided into the small and large intestine. The small intestine has three segments. The ileum is the longest segment of the small intestine, which is well over 10 feet long. The large intestine is about 5 feet long. This drawing shows the major organs of the digestive system. The liver, pancreas and gallbladder are also organs of the digestive system. Digestion is the process of breaking down food into nutrients. There are two types of digestion, mechanical and chemical. In mechanical digestion, large chunks of food are broken down into small pieces. Mechanical digestion begins in the mouth and involves physical processes, such as chewing. This process continues in the stomach as the food is mixed with digestive juices. In chemical digestion, large food molecules are broken down into small nutrient molecules. This is a chemical process which also begins in the mouth as saliva begins to break down food and continues in the stomach as stomach enzymes further digest the food. Absorption is the process that allows substances you eat to be taken up by the blood. After food is broken down into small nutrient molecules, the molecules are absorbed by the blood. After absorption, the nutrient molecules travel in the bloodstream to cells throughout the body. This happens mostly in the small intestine. Some substances in food cannot be broken down into nutrients. They remain behind in the digestive system after the nutrients are absorbed. Any substances in food that cannot be digested and absorbed pass out of the body as solid waste. The process of passing solid food waste out of the body is called elimination. "
human genome project,T_3025,"A persons genome is all of his or her genetic information. In other words, the human genome is all the information that makes us human. And unless you have an identical twin, your genome is unique. No one else has a genome just like yours, though all our genomes are similar. The Human Genome Project ( Figure 1.1) was an international effort to sequence all 3 billion bases that make up our DNA and to identify within this code more than 20,000 human genes. Scientists also completed a chromosome map, identifying where the genes are located on each of the chromosomes. The Human Genome Project was completed in 2003. Though the Human Genome Project is finished, analysis of the data will continue for many years. To say the Human Genome Project has been beneficial to mankind would be an understatement. Exciting applications of the Human Genome Project include the following: The genetic basis for many diseases can be more easily determined. Now there are tests for over 1,000 genetic disorders. The technologies developed during this effort, and since the completion of this project, will reduce the cost of sequencing a persons genome. This may eventually allow many people to sequence their individual genome. Analysis of your own genome could determine if you are at risk for specific diseases. Knowing you might be genetically prone to a certain disease would allow you to make preventive lifestyle changes or have medical screenings. To complete the Human Genome Project, all 23 pairs of chromosomes in the human body were sequenced. Each chromo- some contains thousands of genes. This is a karyotype, a visual representation of an individuals chromosomes lined up by size. The video Our Molecular Selves discusses the human genome, and is available at  or  . Genome, Unlocking Lifes Code is the Smithsonian National Museum of Natural Historys exhibit on the human genome. See http://unlockinglifescode.org to visit the exhibit. Click image to the left or use the URL below. URL: "
human population,T_3026,"How quickly is the human population growing? If we look at worldwide human population growth from 10,000 BCE through today, our growth looks like exponential growth. It increased very slowly at first, but later grew faster and faster as the population increased in size ( Figure 1.1). And recently, the human population has increased at a faster pace than ever before. It has taken only 12 years for the worlds population to increase from six billion to seven billion. Considering that in the year 1804, there were just one billion people, and in 1927, there were just two billion people (thats 123 years to increase from 1 to 2 billion), the recent increase in the human population growth rate is characteristic of exponential growth. Does this mean there are unlimited resources? Worldwide human population growth from 10,000 BCE through today. "
human population,T_3027,"On the other hand, if you look at human population growth in specific countries, you may see a different pattern. On the level of a country, the history of human population growth can be divided into five stages, as described in Table 1.1. Some countries have very high birth rates, in some countries the growth rate has stabilized, and in some countries the growth rate is in decline. Stage 1 2 3 4 5 Description Birth and death rates are high and population growth is stable. This occurred in early human history. Significant drop in death rate, resulting in exponential growth. This occurred in 18th- and 19th-century Eu- rope. Population size continues to grow. Birth rates equal death rates and populations become stable. Total population size may level off. The United Nations and the U.S. Census Bureau predict that by 2050, the Earth will be populated by 9.4 billion people. Other estimates predict 10 to 11 billion. "
human population,T_3028,"There are two different beliefs about what type of growth the human population will undergo in the future: 1. Neo-Malthusians believe that human population growth cannot continue without destroying the environment, and maybe humans themselves. 2. Cornucopians believe that the Earth can give humans a limitless amount of resources. They also believe that technology can solve problems caused by limited resources, such as lack of food. The Cornucopians believe that a larger population is good for technology and innovation. The 5-stage model above predicts that when all countries are industrialized, the human population will eventually level out. But many scientists and other Neo-Malthusians believe that humans have already gone over the Earths carrying capacity. That means, we may have already reached the maximum population size that can be supported, without destroying our resources and habitat. If this is true, then human overpopulation will lead to a lack of food and other resources. Overpopulation may also lead to increased disease, and/or war. These problems may cause the population of humans to crash. If these issues are not controlled, could the human population go extinct? Which of the above theories makes sense to you? Why? "
human skeletal system,T_3029,"How important is your skeleton? Can you imagine your body without it? You would be a wobbly pile of muscle and internal organs, and you would not be able to move. The adult human skeleton has 206 bones, some of which are named below ( Figure 1.1). Bones are made up of living tissue. They contain many different types of tissues. Cartilage, a dense connective tissue, is found at the end of bones and is made of tough protein fibers. Cartilage creates smooth surfaces for the movement of bones that are next to each other, like the bones of the knee. Ligaments are made of tough protein fibers and connect bones to each other. Your bones, cartilage, and ligaments make up your skeletal system. "
human skeletal system,T_3030,"Your skeletal system gives shape and form to your body, but it also plays other important roles. The main functions of the skeletal system include: The skeletal system is made up of bones, cartilage, and ligaments. The skeletal system has many important functions in your body. What bones protect the heart and lungs? What protects the brain? Support. The skeleton supports the body against the pull of gravity, meaning you dont fall over when you stand up. The large bones of the lower limbs support the rest of the body when standing. Protection. The skeleton supports and protects the soft organs of the body. For example, the skull surrounds the brain to protect it from injury. The bones of the rib cage help protect the heart and lungs. Movement. Bones work together with muscles to move the body. Making blood cells. Blood cells are mostly made inside certain types of bones. "
human skeletal system,T_3031,"Bones come in many different shapes and sizes, but they are all made of the same materials. Bones are organs, and recall that organs are made up of two or more types of tissues. The two main types of bone tissue are compact bone and spongy bone ( Figure 1.2). Compact bone makes up the dense outer layer of bones. Spongy bone is found at the center of the bone and is lighter and more porous than compact bone. Bones look tough, shiny, and white because they are covered by a layer called the periosteum. Many bones also contain a soft connective tissue called bone marrow in the pores of the spongy bone. Bone marrow is where blood cells are made. Bones are made up of different types of tissues. "
human skeletal system,T_3032,"Early in human development, the skeleton consists of only cartilage and other connective tissues. At this point, the skeleton is very flexible. As the fetus develops, hard bone begins to replace the cartilage, and the skeleton begins to harden. Not all of the cartilage, however, is replaced by bone. Cartilage remains in many places in your body, including your joints, your rib cage, your ears, and the tip of your nose. A baby is born with zones of cartilage in its bones that allow growth of the bones. These areas, called growth plates, allow the bones to grow longer as the child grows. By the time the child reaches an age of about 18 to 25 years, all of the cartilage in the growth plate has been replaced by bone. This stops the bone from growing any longer. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness throughout life. This thickening occurs in response to strain from increased muscle activity and from weight-lifting exercises. "
indoor air pollution,T_3086,"Recall that air pollution is due to chemical substances and particles released into the air mainly by human actions. When most people think of air pollution, they think of the pollution outdoors. But it is just as easy to have indoor air pollution. Your home or school classroom probably doesnt get much fresh air. Sealing up your home reduces heating and cooling costs. But this also causes air pollution to stay trapped indoors. And people today usually spend a majority of their time indoors. So exposure to indoor air pollution can become a significant health risk. Indoor air pollutants include both chemical and biological pollutants. Chemical pollutants include the following: Radon, a radioactive gas released from the Earth in certain locations. It can become trapped inside buildings and increase your risk of cancer. Formaldehyde, a toxic gas emitted from building materials, such as carpeting and plywood. Volatile organic compounds (VOCs), which are given off by paint and solvents as they dry. They can cause cause long-term health effects. Secondhand smoke, which comes from breathing the smoke release from tobacco products. Secondhand smoke is also the smoke exhaled by a cigarette smoker. This smoke is extremely dangerous to human health. Carbon monoxide (CO), a toxic gas released by burning fossil fuels. It is often released indoors by faulty chimneys, gas-powered generators, or burning charcoal; it can be extremely dangerous. Dry cleaning fluids, such as tetrachloroethylene, which can be released from clothing days after dry cleaning. The past use of asbestos in factories and in homes. Asbestos is a very dangerous material, and it was used in many buildings ( Figure 1.1). Asbestos can cause cancer and other lung diseases. The use of asbestos is not allowed today. The use of asbestos in industry and do- mestic environments in the past, as in the asbestos-covered pipes in the oil-refining plant pictured here, has left a potentially very dangerous material in many busi- nesses. Biological sources of air pollution are also found indoors. These are produced from: Pet dander. Dust from tiny skin flakes and decomposed hair. Dust mites. Mold from walls, ceilings, and other structures. Air conditioning systems that can incubate certain bacteria and mold. Pollen, dust, and mold from houseplants, soil, and surrounding gardens. "
indoor air pollution,T_3087,"Can you avoid indoor air pollution? You cant go to school outside. But it is possible to reduce your exposure to air pollution. Some tips to decrease your exposure to indoor air pollution include: Using less toxic chemicals when possible. Limiting your exposure to pesticides and cleaning fluids by keeping them in a garage or shed. When using toxic chemicals, allowing fresh air to circulate through open windows and doors. Having detectors for radon and carbon monoxide in your home. What else could you do to reduce your exposure to air pollution? "
infancy and childhood,T_3088,"The first year after birth is called infancy. Infancy is a period when the baby grows very fast. During infancy, the baby doubles in length and triples in weight. Other important changes also happen during infancy: The babys teeth start to come in, usually at about six months of age ( Figure 1.1). The baby starts smiling, paying attention to other people, and grabbing toys. The baby begins making babbling sounds. By the end of the first year, the baby is starting to say a few words, such as mama and dada. The baby learns to sit, crawl, and stand. By the end of the first year, the baby may be starting to walk. Childhood begins after the babys first birthday and continues until the teen years. Between one and three years of age, a child is called a toddler. During the toddler stage, growth is still fast, but not as fast as it was during infancy. A toddler learns many new words. The child even starts putting together words in simple sentences. Motor skills also develop quickly during this stage. By age three, most children can run and climb steps. They can hold crayons and scribble with them. They can also feed themselves and use the toilet. From age three until the teens, growth is slower. The body also changes shape. The arms and legs get longer compared to the trunk. Children continue to develop new motor skills. For example, many young children learn how to ride a tricycle and then a bicycle. Most also learn how to play games and sports ( Figure 1.2). By age six, children start losing their baby teeth. Their permanent teeth begin coming in to replace them. They also start school and learn how to read and write. They develop friendships and become less dependent on their parents. "
infancy and childhood,T_3089,"There are numerous milestones that occur during the first few years of childhood. These include the use of language, walking and running, understanding simple concepts, pretend play, the development of fine motor skills, the development of independence, Children develop better motor skills as they get older. having temper tantrums, demonstrating separation anxiety, becoming fully potty-trained, showing natural curiosity. "
influences on darwin,T_3093,"When Darwin returned to England five years later, in 1836, at the end of his voyage, he did not rush to announce his discoveries. Unlike other naturalists before him, Darwin did not want to present any ideas unless he had strong evidence supporting them. Instead, once Darwin returned to England, he spent over twenty years examining specimens, talking with other scientists and collecting more information before he presented his theories. Some of Darwins ideas conflicted with widely held beliefs, including those from religious leaders. At that time, many people believed that organisms never change and never go extinct, and that the world was only about 6,000 years old, always existing in the same way, never changing. These beliefs delayed Darwin in presenting his findings. How did Darwin come up with his theories? Charles Darwin was influenced by the ideas of several people. 1. Before the voyage of the Beagle, Jean-Baptiste Lamarck proposed the idea that species change over time. However, Darwin differed with Lamarck on several key points. Lamarck proposed that traits acquired during ones lifetime could be passed to the next generation. Darwin did not agree with this. 2. The findings of Charles Lyell, a well-known geologist, also influenced Darwin. Lyells writings taught Darwin about geology, paleontology, and the changing Earth. Lyells findings suggested the Earth must be much older than 6,000 years. And the evolution of life, as Darwin was developing his ideas, would definitely take much longer than just 6,000 years. During the Voyage of the Beagle, Darwin observed fossils of sea life high up in the mountains. What must happen to the Earth for this to occur? Darwin, using the readings of Lyell, took this as evidence of a constantly changing Earth. 3. After the Voyage of the Beagle, another naturalist, Alfred Russel Wallace ( Figure 1.1), developed a similar theory of evolution by natural selection. Wallace toured South America and made similar observations to Darwins. Darwin and Wallace presented their theories and evidence in public together. Due to the large number of observations and conclusions he made, Darwin is mostly credited and associated with this theory. Alfred Wallace developed a similar theory of evolution by natural selection. Imagine developing a theory that conflicted with widely held beliefs of the time, as Darwin did. Imagine pulling together material from all these different people, adding his own findings, and turning it all into his theory. Imag- ine the torment Darwin must have endured during this time, knowing the skepticism that would follow the release of his findings. But, upon his death, Darwin was given one of the highest honors in England. Darwin is buried in Westminster Abbey, the final resting place of many of Englands kings and queens. Why was he buried in such an important spot? "
injuries of the nervous system,T_3094,"Injuries to the central nervous system may damage tissues of the brain or spinal cord. If an injury is mild, a person may have a full recovery. If an injury is severe, it may cause permanent disability or even death. Brain and spinal cord injuries most commonly occur because of car crashes or sports accidents. The best way to deal with such injuries is to try to prevent them. "
injuries of the nervous system,T_3095,"Brain injuries can range from mild to extremely severe, but even mild injuries need medical attention. Brain injuries can result from falls, car accidents, violence, sports injuries, and war and combat. Falls are the most common cause of brain injuries, particularly in older adults and young children. The mildest and most common type of brain injury is a concussion. This is a bruise on the surface of the brain. It may cause temporary problems such as headache, drowsiness, and confusion. Most concussions in young people occur when they are playing sports, especially contact sports like football. Other sports, like soccer, boxing, baseball, lacrosse, skateboarding, and hockey can also result in concussions. A concussion normally heals on its own in a few days. A single concussion is unlikely to cause permanent damage. But repeated concussions may lead to lasting problems. People who have had two or more concussions may have life-long difficulties with memory, learning, speech, or balance. For this reason, concussions are treated very seriously among athletes and in professional sports. You can see an animation of how a concussion occurs by visiting  A person with a serious brain injury usually suffers permanent brain damage. These brain injuries usually occur when an external mechanical force, such as a violent blow or jolt to the head or body, causes brain dysfunction. An object penetrating the skull, such as a bullet or a shattered piece of the skull, also can cause traumatic brain injury. As a result, the person may have trouble talking or controlling body movements. Symptoms depend on what part of the brain was injured. Serious brain injuries can also cause personality changes and problems with mental abilities such as memory. Medicines, counseling, and other treatments may help people with serious brain injuries recover from, or at least learn to cope with, their disabilities. Symptoms of severe brain injuries include the loss of consciousness from several minutes to hours, profound confusion, slurred speech, the inability to awaken from sleep, seizures, loss of coordination, persistent headache or headache that worsens. "
injuries of the nervous system,T_3096,"A spinal cord injury is damage to any part of the spinal cord or nerves at the end of the spinal canal. This injury often causes permanent changes in strength, sensation and other body functions below the site of the injury. Spinal cord injuries make it difficult for messages to travel between the brain and body. They may cause a person to lose the ability to feel or move parts of the body. This is called paralysis. Whether paralysis occursand what parts of the body are affected if it doesdepends on the location and seriousness of the injury. In addition to car crashes and sports injuries, diving accidents are a common cause of spinal cord injuries. Quadriplegia means your arms, hands, trunk, legs and pelvic organs are all affected by your spinal cord injury. Paraplegia means the paralysis affects all or part of the trunk, legs and pelvic organs. These people can still use their arms and hands. Some people recover from spinal cord injuries. But many people are paralyzed for life. Thanks to the work of Christopher Reeve ( Figure 1.1), more research is being done on spinal cord injuries now than ever before. For example, scientists are trying to discover ways to regrow damaged spinal cord neurons. If you suspect that someone has a back or neck injury: dont move the injured person as permanent paralysis and other serious complications may result, call 911 or your local emergency medical assistance number, keep the person very still, place heavy towels on both sides of the neck or hold the head and neck to prevent them from moving, until emergency care arrives, provide basic first aid, such as stopping any bleeding and making the person comfortable, without moving the head or neck. Former ""man of steel"" Superman star Christopher Reeve (September 25, 1952 October 10, 2004) was paralyzed from the neck down in a fall from a horse. The injury crushed his spinal cord so his brain could no longer communicate with his body. "
jawless fish,T_3111,"What defines a jawless fish? You can probably guess. A jawless fish is a fish without a jaw. But there are other features that are shared by this class of organisms. Why would such an organism evolve? These fish were the first vertebrates to evolve. Logically, this makes sense, in that the vertebral column would evolve first, with the more complex jaw bones evolving later. The early jawless fish are thought to have relied on filter feeding to capture their food, and most likely would have sucked water and debris from the seafloor into their mouth, releasing water and waste out of their gills. As other sea life evolved, these jawless fish began to feed on other fish species, and are now considered a pest in their habitat. Lampreys have no natural predators. "
jawless fish,T_3112,"Jawless fish are missing the following parts: 1. Jaws. 2. Paired fins. 3. A stomach. Characteristics they do have include: 1. A notochord, both in larvae and adults. Recall a notochord is a support rod that runs along the back of the fish. 2. Seven or more paired gill pouches. These organs take dissolved oxygen from water. 3. The branchial arches, a series of arches that support the gills of aquatic amphibians and fishes. They lie close to the bodys surface. 4. A light sensitive pineal eye, an eye-like structure that can detect light. 5. A cartilaginous skeleton, a skeleton made of a flexible rubber-like supportive material called cartilage. This is similar to the skeleton of cartilaginous fish, which includes sharks and rays. 6. A heart with two chambers. 7. Reproduction using external fertilization. 8. They are ectothermic. This means that their internal temperature depends on the temperature of their envi- ronment. "
jawless fish,T_3113,"Most scientists agree that the jawless fish are part of the the superclass Agnatha. They belong to the phylum Chordata, subphylum Vertebrata. There are two living groups of jawless fish, with about 100 species in total: lampreys and hagfish ( Figure 1.1). Although hagfish belong to the subphylum Vertebrata, they do not technically have vertebrae (though they do have a skull), whereas lampreys do have vertebrae. For this reason, scientists still disagree on the classification of jawless fish. A hagfish. "
keeping bones and joints healthy,T_3114,"You can help keep your bones and skeletal system healthy by eating well and getting enough exercise. Weight- bearing exercises help keep bones strong. Weight-bearing exercises and activities work against gravity. Such activities include basketball, tennis, gymnastics, karate, running, and walking. When the body is exercised regularly by performing weight-bearing activity, bones respond by adding more bone cells to increase their bone density. "
keeping bones and joints healthy,T_3115,"Did you know that what you eat as a teenager can affect how healthy your skeletal system will be in 30, 40, and even 50 years? Calcium and vitamin D are two of the most important nutrients for a healthy skeletal system. Your bones need calcium to grow properly. If you do not get enough calcium in your diet as a teenager, your bones may become weak and break easily later in life. Osteoporosis is a disease in which bones lose mass and become more fragile than they should be. Osteoporosis also makes bones more likely to break. Two of the easiest ways to prevent osteoporosis are eating a healthy diet that has the right amount of calcium and vitamin D and to do some sort of weight-bearing exercise every day. Foods that are a good source of calcium include milk, yogurt, and cheese. Non-dairy sources of calcium include Chinese cabbage, kale, and broccoli. Many fruit juices, fruit drinks, tofu, and cereals have calcium added to them. It is recommended that teenagers get 1300 mg of calcium every day. For example, one cup (8 fl. oz.) of milk provides about 300 mg of calcium, or about 30% of the daily requirement. Other sources of calcium are pictured in the Figure 1.1. There are many different sources of cal- cium. Getting enough calcium in your daily diet is important for good bone health. Vitamin D is unusual since you dont have to rely on your diet alone to get enough of this vitamin. Your skin makes vitamin D when exposed to sunlight. Pigments in the skin act like a filter that can prevent the skin from making vitamin D. As a result, people with darker skin need more time in the sun than people with lighter skin to make the same amount of vitamin D. You can also get vitamin D from foods. Fish is naturally rich in vitamin D. In the United States, vitamin D is added to other foods, including milk, soy milk, and breakfast cereals. Teenagers are recommended to get 5 micrograms (200 IU) of vitamin D every day. A 3 12 -ounce portion of cooked salmon provides 360 IU of vitamin D. A 8-ounce glass of milk is fortified with about 100 IU of vitamin D. "
keeping bones and joints healthy,T_3116,"Even though they are very strong, bones can fracture, or break. Fractures can happen at different places on a bone. They are usually caused by excess bending stress on the bone. Bending stress is what causes a pencil to break if you bend it too far. Soon after a fracture, the body begins to repair the break. The area becomes swollen and sore. Within a few days, bone cells travel to the break site and begin to rebuild the bone. It takes about two to three months before compact and spongy bone form at the break site. Sometimes the body needs extra help in repairing a broken bone. In such a case, a surgeon will piece a broken bone together with metal pins. Moving the broken pieces together will help keep the bone from moving and give the body a chance to repair the break. Below, a broken ulna has been repaired with pins ( Figure 1.2). The upper part of the ulna, just above the elbow joint, is broken, as you can see in the X-ray to the left. The x-ray to the right was taken after a surgeon inserted a system of pins and wires across the fracture to bring the two pieces of the ulna into close proximity. "
keeping bones and joints healthy,T_3117,"Osteoarthritis occurs when the cartilage at the ends of the bones breaks down. The break down of the cartilage leads to pain and stiffness in the joint. Decreased movement of the joint because of the pain may lead to weakening of the muscles that normally move the joint, and the ligaments surrounding the joint may become loose. Osteoarthritis is the most common form of arthritis. It has many contributing factors, including aging, sport injuries, fractures, and obesity. "
keeping bones and joints healthy,T_3118,"Recall that a ligament is a short band of tough connective tissue that connects bones together to form a joint. Ligaments can get injured when a joint gets twisted or bends too far. The protein fibers that make up a ligament can get strained or torn, causing swelling and pain. Injuries to ligaments are called sprains. Ankle sprains are a common type of sprain. "
keeping bones and joints healthy,T_3119,"Preventing injuries to your bones and ligaments is easier and much less painful than treating an injury. Wearing the correct safety equipment when performing activities that require such equipment can help prevent many common injuries. For example, wearing a bicycle helmet can help prevent a skull injury if you fall. Warming up and cooling down properly can help prevent ligament and muscle injuries. Stretching before and after activity also helps prevent injuries. "
keeping skin healthy,T_3120,"Your skin is your largest organ and constantly protects you from infections, so keeping your skin healthy is a good idea. "
keeping skin healthy,T_3121,"Some sunlight is good for your health. Vitamin D is made in the skin when it is exposed to sunlight. But getting too much sun can be unhealthy. A sunburn is a burn to the skin that is caused by overexposure to UV radiation from the suns rays or tanning beds. Light-skinned people, like the man pictured below ( Figure 1.1), get sunburned more quickly than people with darker skin. This is because pigments (melanin) in the skin act as a natural sunblock that help to protect the body from UV radiation. With over one million new cases each year, skin cancer, which is cancer that forms in the tissues of the skin, is the most common form of human cancer. Children and teens who have been sunburned are at a greater risk of developing skin cancer later in life. Long-term exposure to UV radiation is the leading cause of skin cancer. About 90 percent of skin cancers are linked to sun exposure. UV radiation damages the genetic material (DNA) of skin cells. This damage can cause the skin cells to grow out of control and form a tumor. Some of these tumors are very difficult to cure. For this reason you should always wear sunscreen with a high sun protection factor (SPF), a hat, and clothing when out in the sun. Sunburn is caused by overexposure to UV rays. Getting sunburned as a child or a teen, especially sunburn that causes blistering, increases the risk of developing skin cancer later in life. "
keeping skin healthy,T_3122,"Keeping your skin clean is important because dirty skin is more prone to infection. Bathing every day helps to keep your skin clean and healthy. Also, you know that taking a bath or shower helps prevent body odor. But where does body odor come from? During the day, sweat, oil, dirt, dust, and dead skin cells can build up on the skin surface. If not washed away, the mix of these materials can encourage the excess growth of bacteria. These bacteria feed on these substances and cause a smell that is commonly called body odor. "
keeping skin healthy,T_3123,"Conditions that irritate, clog or inflame your skin can cause symptoms such as redness, swelling, burning and itching. Allergies, irritants, your genetic background and certain diseases and immune system problems can cause numerous skin conditions. Many skin problems, such as acne, also affect your appearance. Acne Your skin has tiny holes called pores that that can become blocked by oil, bacteria, dead skin and dirt. When this occurs, you may develop a pimple. Acne is a skin condition that causes pimples, and is one of the more common skin problem among teenagers. A diet high in refined sugars or carbohydrates such as bread and chips can also lead to acne. Each pore on your skin is the opening to a follicle, which is made of a hair and sebaceous gland that releases sebum. Acne may result from too much sebum produced by the follicle, dead skin cells accumulating in the pore, or bacteria built up in the pore. Cleaning your skin daily with a mild soap to remove excess oil and dirt can help prevent acne. Cold Sores Cold sores are red, fluid-filled blisters that appear near the mouth or on other areas of the face, usually caused by herpes simplex virus type 1. Visible sores are contagious, but herpes may be spread even when sores cant be seen. You can catch the herpes simplex virus through kissing, sharing cosmetics, or sharing food with infected individuals. Once you catch herpes simplex virus, it cant be cured. Even after sores have healed, the virus remains in your body, and new cold sores can appear at any time. This is not to be confused with genital herpes, which is caused by herpes simplex virus type 2. Canker Sore A canker sore is a mouth ulcer or sore that is open and painful. They may be on the lips or inside of the lip or cheek. Canker sores are usually white or yellowish, surrounded by red, inflamed soft tissue. A canker sore can be either a simple canker or a complex canker. A simple canker sore reemerges about three to four times every year, and is the common type in people between the ages of 10 and 20. Canker sores are not contagious and usually heal on their own within a week or two. Causes of canker sores include a viral infection, stress, hormonal fluctuations, food allergies, immune system problems, or mouth injuries. "
keeping the nervous system healthy,T_3124,"The nervous system is such an important part of your body. You want it to work at its best so that you can be at your best. Your nervous system contains what is probably the most important part of your body, which, of course, is your brain. Your brain allows you to learn. It allows you to feel emotions like love, anger, and sadness. Your brain gives you the ability to see, hear, taste, touch, and smell. It works together with the nerves and spinal cord to send the signals that make your body move. Your nervous system lets you do things like run, jump, play sports, and do your homework. There are many choices you can make to keep your nervous system healthy. One obvious choice is to avoid using alcohol or other drugs. Not only will you avoid the injury that drugs themselves can cause, but you will also be less likely to get involved in other risky behaviors that could harm your nervous system. Another way to keep the nervous system healthy is to eat a variety of healthy foods. The minerals sodium, calcium, and potassium, and vitamins B1 and B12 are important for a healthy nervous system. Some foods that are good sources for these minerals and vitamins include milk, whole grains, beef steak, and kidney beans (shown in Figure 1.1). Your brain also needs healthy fats like those in nuts and fish. Recall that fats insulate the axons of neurons. These fats help build new connections between nerves and brain cells. These fats may improve memory and increase learning and intelligence. Water is also important for the nervous system, so drink plenty of water and other fluids. This helps prevent dehydration, which can cause confusion and memory problems. And get plenty of rest. Your brain requires plenty of rest so it can strengthen circuits that help with memory. A good nights sleep will help keep your brain functioning at its best. These foods are sources of nutrients needed for a healthy nervous system. Daily physical activity is also important for nervous system health. Regular exercise makes your heart more efficient at pumping blood to your brain. As a result, your brain gets more oxygen, which it needs to function normally. The saying use it or lose it applies to your brain as well as your body. This means that mental activity, not just physical activity, is important for nervous system health. Doing crossword puzzles, reading, and playing a musical instrument are just a few ways you can keep your brain active. You can also choose to practice safe behaviors to protect your nervous system from injury. To keep your nervous system safe, choose to: Bicycle helmets help protect from head injuries. Making healthy choices like this can help prevent nervous system injuries that could cause lifelong disability. Furthermore, make sure to exercise your nervous system on a daily basis. The simple act of writing requires that you use all the major components of your motor and sensory pathways. These include a number of different sensory receptors, peripheral nerves, synaptic connections within your spinal cord, major tracts within your spinal cord, and nerve tissue throughout your brain. All these components need to be utilized with great precision and coordination to produce neatly written words. What should you do? Spend a few minutes each day writing on paper as neatly as you can. This takes a lot more effort on the part of the nervous system than typing on a keyboard, as typing on a keyboard doesnt require as much fine motor control as writing on paper. If you dont want to write, then draw. Drawing with precision also requires use of all the major components of the sensory and motor divisions of the nervous system. "
kidneys,T_3125,"The kidneys ( Figure 1.1) are important organs in maintaining homeostasis, the ability of the body to maintain a stable internal environment despite a changing environment. Kidneys perform a number of homeostatic functions. They maintain the volume of body fluids. They maintain the balance of salt ions in body fluids. They excrete harmful nitrogen-containing molecules, such as urea, ammonia, and uric acid. There are many blood vessels in the kidneys ( Figure 1.1). The kidneys remove urea and other wastes from the blood through tiny filtering units called nephrons. Nephrons ( Figure 1.2) are tiny, tube-shaped structures found inside each kidney. Each kidney has up to a million nephrons. Each nephron collects a small amount of fluid and waste from a small group of capillaries. Structures of the kidney; fluid leaks from the capillaries and into the nephrons where the fluid forms urine then moves to the ureter and on to the bladder. Nitrogen-containing wastes, together with water and other wastes, form the urine as it passes through the nephrons and the kidney. The fluid within nephrons is carried out into a larger tube in the kidney called a ureter, which carries it to the bladder ( Figure 1.2). The kidneys never stop filtering waste products from the blood, so they are always producing urine. The amount of urine your kidneys produce is dependent on the amount of fluid in your body. Your body loses water through sweating, breathing, and urination. The water and other fluids you drink every day help to replace the lost water. This water ends up circulating in the blood because blood plasma is mostly water. "
kidneys,T_3126,"The process of urine formation is as follows: 1. Blood flows into the kidney through the renal artery. The renal artery connects to capillaries inside the kidney. Capillaries and nephrons lie very close to each other in the kidney. 2. The blood pressure within the capillaries causes water, salts, sugars, and urea to leave the capillaries and move into the nephron. 3. The water and salts move along through the tube-shaped nephron to a lower part of the nephron. 4. The fluid that remains in the nephron at this point is called urine. 5. The blood that leaves the kidney in the renal vein has much less waste than the blood that entered the kidney. 6. The urine is collected in the ureters and is moved to the urinary bladder, where it is stored. Nephrons filter about 14 cup of body fluid per minute. In a 24-hour period, nephrons filter 180 liters of fluid, and 1.5 liters of the fluid is released as urine. Urine enters the bladder through the ureters. Similar to a balloon, the walls of the bladder are stretchy. The stretchy walls allow the bladder to hold a large amount of urine. The bladder can hold about 1 12 to 2 21 cups of urine but may also hold more if the urine cannot be released immediately. How do you know when you have to urinate? Urination is the process of releasing urine from the body. Urine leaves the body through the urethra. Nerves in the bladder tell you when it is time to urinate. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The urge to urinate becomes stronger as the bladder continues to fill up. The location of nephrons in the kidney. The fluid collects in the nephron tubules and moves to the bladder through the ureter. "
kidneys,T_3127,"The filtering action of the kidneys is controlled by the pituitary gland. The pituitary gland is about the size of a pea and is found below the brain ( Figure 1.3). The pituitary gland releases hormones that help the kidneys to filter water from the blood. The movement of water back into blood is controlled by a hormone called antidiuretic hormone (ADH). ADH is one of the hormones released from the pituitary gland in the brain. One of the most important roles of ADH is to control the bodys ability to hold onto water. If a person does not drink enough water, ADH is released. It causes the blood to reabsorb water from the kidneys. If the kidneys remove less water from the blood, what will the urine look like? It will look darker, because there is less water in it. When a person drinks a lot of water, then there will be a lot of water in the blood. The pituitary gland will then release a lower amount of ADH into the blood. This means less water will be reabsorbed by the blood. The kidneys then produce a large volume of urine. What color will this urine be? "
light reactions of photosynthesis,T_3136,"Photosynthesis takes place in the organelle of the plant cell known as the chloroplasts. Chloroplasts are one of the main differences between plant and animal cells. Animal cells do not have chloroplasts, so they cannot photosynthesize. Photosynthesis occurs in two stages. During the first stage, the energy from sunlight is absorbed by the chloroplast. Water is used, and oxygen is produced during this part of the process. During the second stage, carbon dioxide is used, and glucose is produced. Chloroplasts contain stacks of thylakoids, which are flattened sacs of membrane. Energy from sunlight is absorbed by the pigment chlorophyll in the thylakoid membrane. There are two separate parts of a chloroplast: the space inside the chloroplast itself, and the space inside the thylakoids ( Figure 1.1). The inner compartments inside the thylakoids are called the thylakoid space (or lumen). This is the site of the first part of photosynthesis. The interior space that surrounds the thylakoids is filled with a fluid called stroma. This is where carbon dioxide is used to produce glucose, the second part of photosynthesis. The chloroplast is the photosynthesis fac- tory of the plant. "
light reactions of photosynthesis,T_3137,"What goes into the plant cell to start photosynthesis? The reactants of photosynthesis are carbon dioxide and water. These are the molecules necessary to begin the process. But one more item is necessary, and that is sunlight. All three components, carbon dioxide, water, and the suns energy are necessary for photosynthesis to occur. These three components must meet in the chloroplast of the leaf cell for photosynthesis to occur. How do these three components get to the cells in the leaf? Chlorophyll is the green pigment in leaves that captures energy from the sun. Chlorophyll molecules are located in the thylakoid membranes inside chloroplasts. The veins in a plant carry water from the roots to the leaves. Carbon dioxide enters the leaf from the air through special openings called stomata ( Figure 1.2). "
light reactions of photosynthesis,T_3138,"What is produced by the plant cell during photosynthesis? The products of photosynthesis are glucose and oxygen. This means they are produced at the end of photosynthesis. Glucose, the food of plants, can be used to store energy in the form of large carbohydrate molecules. Glucose is a simple sugar molecule which can be combined with other glucose molecules to form large carbohydrates, such as starch. Oxygen is a waste product of photosynthesis. It is released into the atmosphere through the stomata. As you know, animals need oxygen to live. Without photosynthetic organisms like plants, there would not be enough oxygen in the atmosphere for animals to survive. "
light reactions of photosynthesis,T_3139,"The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO2 ) and 6 molecules of water (H2 O), with the addition of solar energy. This produces 1 molecule of glucose (C6 H12 O6 ) and 6 molecules of oxygen Stomata are special pores that allow gasses to enter and exit the leaf. (O2 ). Using chemical symbols, the equation is represented as follows: 6CO2 + 6H2 O  C6 H12 O6 + 6O2 . Though this equation may not seem that complicated, photosynthesis is a series of chemical reactions divided into two stages, the light reactions and the Calvin cycle ( Figure 1.3). "
light reactions of photosynthesis,T_3140,"Photosynthesis begins with the light reactions. It is during these reactions that the energy from sunlight is absorbed by the pigment chlorophyll in the thylakoid membranes of the chloroplast. The energy is then temporarily transferred to two molecules, ATP and NADPH, which are used in the second stage of photosynthesis. ATP and NADPH are generated by two electron transport chains. During the light reactions, water is used and oxygen is produced. These reactions can only occur during daylight as the process needs sunlight to begin. "
light reactions of photosynthesis,T_3141,"The second stage of photosynthesis is the production of glucose from carbon dioxide. This process occurs in a continuous cycle, named after its discover, Melvin Calvin. The Calvin cycle uses CO2 and the energy temporarily stored in ATP and NADPH to make the sugar glucose. "
limiting factors to population growth,T_3142,"For a population to be healthy, factors such as food, nutrients, water and space, must be available. What happens when there are not resources to support the population? Limiting factors are resources or other factors in the environment that can lower the population growth rate. Limiting factors include a low food supply and lack of space. Limiting factors can lower birth rates, increase death rates, or lead to emigration. When organisms face limiting factors, they show logistic growth (S-shaped curve, curve B: Figure 1.1). Compe- tition for resources like food and space cause the growth rate to stop increasing, so the population levels off. This flat upper line on a growth curve is the carrying capacity. The carrying capacity (K) is the maximum population size that can be supported in a particular area without destroying the habitat. Limiting factors determine the carrying capacity of a population. Recall that when there are no limiting factors, the population grows exponentially. In exponential growth (J-shaped curve, curve A: Figure 1.1), as the population size increases, the growth rate also increases. Exponential and Logistic Growth. Curve A shows exponential growth. shows logistic growth. Curve B Notice that the carrying capacity (K) is also shown. "
limiting factors to population growth,T_3143,"If there are 12 hamburgers at a lunch table and 24 people sit down at a lunch table, will everyone be able to eat? At first, maybe you will split hamburgers in half, but if more and more people keep coming to sit at the lunch table, you will not be able to feed everyone. This is what happens in nature. But in nature, organisms that cannot get food will die or find a new place to live. It is possible for any resource to be a limiting factor, however, a resource such as food can have dramatic consequences on a population. In nature, when the population size is small, there is usually plenty of food and other resources for each individual. When there is plenty of food and other resources, organisms can easily reproduce, so the birth rate is high. As the population increases, the food supply, or the supply of another necessary resource, may decrease. When necessary resources, such as food, decrease, some individuals will die. Overall, the population cannot reproduce at the same rate, so the birth rates drop. This will cause the population growth rate to decrease. When the population decreases to a certain level where every individual can get enough food and other resources, and the birth and death rates become stable, the population has leveled off at its carrying capacity. "
limiting factors to population growth,T_3144,"Other limiting factors include light, water, nutrients or minerals, oxygen, the ability of an ecosystem to recycle nutrients and/or waste, disease and/or parasites, temperature, space, and predation. Can you think of some other factors that limit populations? Weather can also be a limiting factor. Whereas most plants like rain, an individual cactus-like Agave americana plant actually likes to grow when it is dry. Rainfall limits reproduction of this plant which, in turn, limits growth rate. Can you think of some other factors like this? Human activities can also limit the growth of populations. Such activities include use of pesticides, such as DDT, use of herbicides, and habitat destruction. "
male reproductive structures,T_3156,"The male reproductive organs include the penis, testes, and epididymis ( Figure 1.1). The figure also shows other parts of the male reproductive system. The penis is a cylinder-shaped organ. It contains the urethra. The urethra is a tube that carries urine out of the body. The urethra also carries sperm out of the body. This drawing shows the organs of the male reproductive system. It shows the organs from the side. Find each organ in the drawing as you read about it in the text. The two testes (singular, testis) are egg-shaped organs. They produce sperm and secrete testosterone. The testes are found inside of the scrotum. The scrotum is a sac that hangs down outside the body. The scrotum also contains the epididymis. The testes, being in the scrotum outside the body, allow the temperature of the sperm to be maintained at a few degrees lower than body temperature. This is necessary for the stability of these reproductive cells. The epididymis is a tube that is about six meters (20 feet) long in adults. It is tightly coiled, so it fits inside the scrotum. It rests on top of the testes. The epididymis is where sperm grow larger and mature. The epididymis also stores sperm until they leave the body. Other parts of the male reproductive system include the vas deferens and the prostate gland. Both of these structures are pictured below ( Figure 1.1). The vas deferens is a tube that carries sperm from the epididymis to the urethra. The prostate gland secretes a fluid that mixes with sperm to help form semen. The prostate gland is located beneath the bladder. Semen is a ""milky"" liquid that carries sperm through the urethra and out of the body. In addition to sperm cells, semen contains sugars (fructose) which provide energy to the sperm cells, and enzymes and other substances which help the sperm survive. "
male reproductive system,T_3157,"Dogs have puppies. Cats have kittens. All organisms reproduce, obviously including humans. Like other mammals, humans have a body system that controls reproduction. It is called the reproductive system. It is the only human body system that is very different in males and females. The male and female reproductive systems have different organs and different functions. The male reproductive system has two main functions: 1. Producing sperm. 2. Releasing testosterone into the body. Sperm are male gametes, or reproductive cells. When a male gamete meets a female gamete, they can form a new organism. Sperm form when certain cells in the male reproductive system divide by meiosis, resulting in cells with half the amount of DNA as a regular ""body"" cell. More precisely, sperm cells are haploid sex cells, having one set of chromosomes. Regular body cells are diploid, having two set of chromosomes. As there are 46 chromosomes in a diploid human cell, how many are in a human sperm cell? When males grow older, they produce millions of sperm each day. The male reproductive system also maintains and transports and delivers sperm and a protective fluid, known as semen. Testosterone is the main sex hormone in males. Hormones are chemicals that control many body processes. Testosterone has two major roles: During the teen years, testosterone causes the reproductive organs to mature. It also causes other male traits to develop. For example, it causes hair to grow on the face and allows for muscle growth. During adulthood, testosterone helps a man to produce sperm. When a hormone is released into the body, we say it is ""secreted."" Testosterone is secreted by males, but it is not the only sex hormone that males secrete. Males also secrete small amounts of estrogen. Even though estrogen is the main female sex hormone, scientists think that estrogen is needed for normal sperm production in males. "
menstrual cycle,T_3172,"The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month. While the egg and follicle are developing in the ovary, tissues are building up inside the uterus, the reproductive organ where the baby would develop. The uterus develops a thick lining covered in tiny blood vessels. This prepares the uterus to receive an egg that could develop into a child (a fertilized egg). The occurs during the first part of the cycle. Ovulation, the release of an egg from the ovary, occurs at about the midpoint of the cycle. This would be around day 14 of a 28 day cycle. The egg is swept into the fallopian tube. If sperm is present, fertilization may occur. As sperm can only survive in the fallopian tube for up to a few days, fertilization can only occur within those few days post-ovulation. If the egg is fertilized, the egg makes its way through the fallopian tube into the uterus, where it imbeds into the thick lining. When this occurs, the monthly cycle stops. The monthly cycle does not resume until the pregnancy is over. If a sperm does not enter an egg, the lining of the uterus breaks down. Blood and other tissues from the lining break off from the uterus. They pass through the vagina and out of the body. This is called menstruation. Menstruation is also called a menstrual period. It lasts about 4 days, on average. When the menstrual period ends, the cycle repeats. Some women feel discomfort during this process. Some people think that the average length of a menstrual period is the same as the normal length. They assume that shorter or longer menstrual periods are not normal. In fact, menstrual periods can vary from 1 to 8 days in length. This is usually normal. The average length of the cycle (time between menstrual periods) is about 28 days, but there is no normal cycle length. Some women experience cramping and pain before and during menstruation. "
microscopes,T_3176,"Microscopes, tools that you may get to use in your class, are some of the most important tools in biology ( Figure Microscopy is the study of small objects using microscopes. Look at your fingertips. Before microscopes were invented in 1595, the smallest things you could see on yourself were the tiny lines in your skin. But what else is hidden in your skin? "
microscopes,T_3177,"Over four hundred years ago, two Dutch spectacle makers, Zaccharias Janssen and his son Hans, were experimenting with several lenses in a tube. They discovered that nearby objects appeared greatly enlarged, or magnified. This was the forerunner of the compound microscope and of the telescope. In 1665, Robert Hooke, an English natural scientist, used a microscope to zoom in on a piece of cork - the stuff that makes up the stoppers in wine bottles, which is made from tree bark. Inside of cork, he discovered tiny structures, which he called cells. It turns out that cells are the smallest structural unit of living organisms. This finding eventually led to the development of the theory that all living things are made of cells. Without microscopes, this discovery would not have been possible, and the cell theory would not have been developed. Hookes discovery of the cell set the stage for other scientists to discover other types of organisms. After Hooke, the ""father of microscopy,"" Dutch scientist Antoine van Leeuwenhoek ( Figure 1.2) taught himself to make one of the first microscopes. In one of his early experiments, van Leeuwenhoek took a sample of scum from his own teeth and used his microscope to discover bacteria, the smallest living organism on the planet. Using microscopes, van Leeuwenhoek also discovered one-celled protists and sperm cells. Today, microscopes are used by all types of scientists, including cell biologists, microbiologists, virologists, forensic scientists, entomologists, taxonomists, and many other types. Antoine van Leeuwenhoek, a Dutch cloth merchant with a passion for microscopy. "
microscopes,T_3178,"Some modern microscopes use light, as Hookes and van Leeuwenhoeks did. Others may use electron beams or sound waves. Researchers now use these four types of microscopes: 1. Light microscopes allow biologists to see small details of a specimen. Most of the microscopes used in schools and laboratories are light microscopes. Light microscopes use lenses, typically made of glass or plastic, to focus light either into the eye, a camera, or some other light detector. The most powerful light microscopes can make images up to 2,000 times larger. 2. Transmission electron microscopes (TEM) focus a beam of electrons through an object and can make an image up to two million times bigger, with a very clear image. 3. Scanning electron microscopes (SEM) allow scientists to find the shape and surface texture of extremely small objects, including a paperclip, a bedbug, or even an atom. These microscopes slide a beam of electrons across the surface of a specimen, producing detailed maps of the surface of objects. Magnification in a SEM can be controlled over a range from about 10 to 500,000 times. 4. Scanning acoustic microscopes use sound waves to scan a specimen. These microscopes are useful in biology and medical research. "
microscopes,T_3179,Scanning Electron Microscope at  (5:04) Click image to the left or use the URL below. URL:  1. How is the electron beam focused? 2. What part of a specimen does a scanning electron microscope look at? 3. Why is it important that a specimen for an electron microscope be placed in a vacuum? Why is this step unnecessary for a light microscope? 
mollusks,T_3188,"When you take a walk along a beach, what do you find there? Sand, the ocean, lots of sunlight. You may also find shells. The shells you find are most likely left by organisms in the phylum Mollusca. On the beach, you can find the shells of many different mollusks ( Figure 1.1), including clams, mussels, scallops, oysters, and snails. Mollusks are invertebrates that usually have a hard shell, a mantle, and a radula. Their glossy pearls, mother of pearl, and abalone shells are like pieces of jewelry. Some mollusks, such as squid and octopus, do not have shells. "
mollusks,T_3189,"The Mollusks body is often divided into different parts ( Figure 1.2): On the beach, you can find a wide variety of mollusk shells. 1. A head with eyes or tentacles. 2. In most species, a muscular foot, which helps the mollusk move. Some mollusks use the foot for burrowing into the sand, and others use it for jet-propulsion. 3. A mantle, or fold of the outer skin lining the shell. The mantle often releases calcium carbonate, which creates an external shell, just like the ones you find on the beach. The shell is made of chitin, a tough, semitransparent substance. 4. A mass housing the organs. 5. A complete digestive tract that begins at the mouth and runs to the anus. 6. Most ocean mollusks have a gill or gills to absorb oxygen from the water. 7. Many species have a feeding structure, the radula, found only in mollusks. The radula can be thought of as a ""tongue-like"" structure. The radula is made mostly of chitin. Types of radulae range from structures used to scrape algae off of rocks to the beaks of squid and octopuses. This is the basic body plan of a mollusk. Note the mantle, gills, and radula. Keep in mind the basic body plan can differ slightly among the mollusks. "
mollusks,T_3190,"Mollusks are probably most closely related to organisms in the phylum Annelida, also known as segmented worms. This phylum includes the earthworm and leech. Scientists believe these two groups are related because, when they are in the early stage of development, they look very similar. Mollusks also share features of their organ systems with segmented worms. Unlike segmented worms, however, mollusks do not have body segmentation. The basic mollusk body shape is usually quite different as well. "
muscles and exercise,T_3191,"Regular physical exercise is important in preventing lifestyle diseases such as cardiovascular disease, some types of cancer, type 2 diabetes, and obesity. Regular exercise also improves the health of the muscular system. Muscles that are exercised are bigger and stronger than muscles that are not exercised. Exercise improves both muscular strength and muscular endurance. Muscular strength is the ability of a muscle to use force during a contraction. Muscular endurance is the ability of a muscle to continue to contract over a long time without getting tired. Exercises are grouped into three types depending on the effect they have on the body: Aerobic exercises, such as cycling, walking, and running, increase muscular endurance and cardiovascular health. Anaerobic exercises, such as weight training or sprinting, increase muscle strength. Flexibility exercises, such as stretching, improve the range of motion of muscles and joints. Regular stretching helps people avoid activity-related injuries. "
muscles and exercise,T_3192,"Anaerobic exercises comprise brief periods of physical exertion and high-intensity, strength-training activities. Anaerobic exercises cause muscles to get bigger and stronger. Anaerobic exercises use a resistance against which the muscle has to work to lift or push away. The resistance can be a weight or a persons own body weight (Figure "
muscles and exercise,T_3193,"Aerobic exercises are exercises in which a low to moderate level of exertion can be sustained over long periods. These are exercises that cause your heart to beat faster and allow your muscles to use oxygen to contract. If you exercise aerobically, overtime, your muscles will not get easily tired, and you will use oxygen more efficiently. Aerobic exercise (Figure 1.2) also helps improve cardiac muscle. "
muscles and exercise,T_3194,"Sometimes muscles and tendons get injured when a person starts doing an activity before they have warmed up properly. A warm up is a slow increase in the intensity of a physical activity that prepares muscles for an activity. Warming up increases the blood flow to the muscles and increases the heart rate. Warmed-up muscles and tendons are less likely to get injured. For example, before running or playing soccer, a person might jog slowly to warm muscles and increase their heart rate. Even elite athletes need to warm up (Figure 1.3). When you dont do a proper warm-up, several types of injuries can occur. A strain happens when muscle or tendons tear. Strains are also known as ""pulled muscles."" Another common injury is tendinitis, the irritation of the tendons. Strains and tendinitis are usually treated with rest, cold compresses, and stretching exercises that a physical therapist designs for each patient. Injuries can also be prevented by proper rest and recovery. If you do not get enough rest, your body will become injured and will not react well to exercise, or improve. You can also rest by doing a different activity. For example, if you run, you can rest your running muscles and joints by swimming. Warming up before the game helps the players avoid injuries. Some warm-ups may include stretching exercises. "
muscles bones and movement,T_3195,"When skeletal muscles contract, bones move. But how do muscles make your bones move? A voluntary muscles usually works across a joint. It is attached to both the bones on either side of the joint by strong cords called tendons. A tendon is a tough band of connective tissue that connects a muscle to a bone. Tendons are similar to ligaments, except that ligaments join bones to each other. Muscles move the body by contracting against the skeleton. When muscles contract, they get shorter. By contracting, muscles pull on bones and allow the body to move. Muscles can only contract. They cannot actively extend, though they can move or relax back into the non-contracted neutral position. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. Each muscle in the pair works against the other to move bones at the joints of the body. The muscle that contracts to cause a joint to bend is called the flexor. The muscle that contracts to cause the joint to straighten is called the extensor. When one muscle is contracted, the other muscle from the pair is always elongated. For example, the biceps and triceps muscles work together to allow you to bend and straighten your elbow. When you want to bend your elbow, your biceps muscle contracts (Figure 1.1), and, at the same time, the triceps muscle relaxes. The biceps is the flexor, and the triceps is the extensor of your elbow joint. Other muscles that work together are the quadriceps and hamstrings used to bend and straighten the knee, and the pectorals and trapezius used to move the arms and shoulders forward and backward. During daily routines we do not use muscles equally. For example, we use our biceps more than our triceps due to lifting against gravity. "
muscles bones and movement,T_3196,"Smooth muscles and cardiac muscles are not attached to bone. Recall that these types of muscles are under involuntary control. Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder, or the uterus. Like skeletal muscles, smooth muscle fibers do contract together, causing the muscle to shorten. Smooth muscles have numerous functions, including the following. The smooth muscle in the uterus helps a woman to push out her baby. In the bladder, smooth muscle helps to push out urine. Smooth muscles move food through the digestive tract. In arteries, smooth muscle movements maintain the arteries diameter. Smooth muscle regulates air flow in lungs. Smooth muscle in the lungs helps the airways to expand and contract as necessary. Smooth muscles in arteries and veins are largely responsible for regulation of blood pressure. Cardiac muscle also contracts and gets shorter. This muscle is found only in the heart. The sudden burst of contraction forces blood throughout your body. When the cardiac muscle relaxes, the heart fills with blood. This rhythmic contraction must continue for your whole life, luckily the heart muscle never gets tired. If your heart beats 75 times a minute, how many times does it beat in an hour? A day? A year? 85 years? "
mutations,T_3197,"The process of DNA replication is not always 100% accurate. Sometimes the wrong base is inserted in the new strand of DNA. This wrong base could become permanent. A permanent change in the sequence of DNA is known as a mutation. Small changes in the DNA sequence are usually point mutations, which is a change in a single nucleotide. Once DNA has a mutation, that mutation will be copied each time the DNA replicates. After cell division, each resulting cell will carry the mutation. A mutation may have no effect. However, sometimes a mutation can cause a protein to be made incorrectly. A defect in the protein can affect how well the protein works, or whether it works at all. Usually the loss of a protein function is detrimental to the organism. In rare circumstances, though, the mutation can be beneficial. Mutations are a mechanism for how species evolve. For example, suppose a mutation in an animals DNA causes the loss of an enzyme that makes a dark pigment in the animals skin. If the population of animals has moved to a light colored environment, the animals with the mutant gene would have a lighter skin color and be better camouflaged. So in this case, the mutation is beneficial. "
mutations,T_3198,"If a single base is deleted (called a deletion, which is also a point mutation), there can be huge effects on the organism, because this may cause a frameshift mutation. Remember that the bases in the mRNA are read in groups of three by the tRNA. If the reading frame is off by even one base, the resulting sequence will consist of an entirely different set of codons. The reading of an mRNA is like reading three-letter words of a sentence. Imagine the sentence: The big dog ate the red cat. If you take out the second letter from ""big,"" the frame will be shifted so now it will read: The bgd oga tet her edc at. One single deletion makes the whole sentence impossible to read. A point mutation that adds a base (known as an insertion) would also result in a frameshift. "
mutations,T_3199,"Mutations may also occur in chromosomes ( Figure 1.1). These mutations are going to be fairly large mutations, possible affecting many genes. Possible types of mutations in chromosomes include: 1. Deletion: When a segment of DNA is lost, so there is a missing segment in the chromosome. These usually result in many genes missing from the chromosome. 2. Duplication: When a segment of DNA is repeated, creating a longer chromosome. These usually result in multiple copies of genes in the chromosome. 3. Inversion: When a segment of DNA is flipped and then reattached to the same chromosome. 4. Insertion: When a segment of DNA from one chromosome is added to another, unrelated chromosome. 5. Translocation: When two segments from different chromosomes change positions. "
mutations,T_3200,"Many mutations are not caused by errors in replication. Mutations can happen spontaneously, and they can be caused by mutagens in the environment. Some chemicals, such as those found in tobacco smoke, can be mutagens. Sometimes mutagens can also cause cancer. Tobacco smoke, for example, is often linked to lung cancer. "
nails and hair,T_3201,"Along with the skin, the integumentary system includes the nails and hair. Both the nails and hair contain the tough protein, keratin. The keratin forms fibers, which makes your nails and hair tough and strong. Keratin is similar in toughness to chitin, the carbohydrate found in the exoskeleton of arthropods. "
nails and hair,T_3202,"Nails are similar to claws in other animals. They cover the tips of fingers and toes. Fingernails and toenails both grow from nail beds. As the nail grows, more cells are added at the nail bed. Older cells get pushed away from the nail bed and the nail grows longer. There are no nerve endings in the nail. Otherwise cutting your nails would hurt a lot! Nails act as protective plates over the fingertips and toes. Fingernails also help in sensing the environment. The area under your nail has many nerve endings. These nerve endings allow you to receive more information about objects you touch. The Guinness Book of World Records began tracking record fingernail lengths in 1955. At that time the record was 1 foot 10.75 inches long. The current record-holder for men is from India, with a record of 20 feet 2.25 inches for all nails on his left hand, the longest being his thumbnail at 4 feet 9.6 inches. The record for women is held by an American woman. The record is 28 feet (850 cm) for all nails of both hands, with the longest nail on her right thumb at 2 feet 11 inches. Since adult nails grow at about 3 mm a month (1/10 of an inch), how long would it take to grow such long nails? "
nails and hair,T_3203,"Hair is one of the defining characteristics of mammals. In fact, mammals are the only animals to have hair. Hair sticks out from the epidermis, but it grows from the dermis ( Figure 1.1). Hair grows from inside the hair follicle. New cells grow in the bottom part of the hair, called the bulb. Older cells get pushed up, and the hair grows longer. The cells that make up the hair strand are dead and filled with the rope-like protein keratin. Hair, hair follicle, and oil glands. The oil, called sebum, helps to prevent water loss from the skin. The sebaceous gland secretes sebum, which waterproofs the skin and hair. In humans, hair grows everywhere on the body except the soles of the feet and the palms of the hands, the lips, and the eyelids (except for eyelashes). Hair grows at a rate of about half an inch (1.25 cm) each month, or about 6 inches (15 cm) a year. Hair, especially on the head, helps to keep the body warm. The air traps a layer of warm air near the skin and acts like a warm blanket. Hair can also act as a filter. Nose hair helps to trap particles in the air that may otherwise travel to the lungs. Eyelashes shield eyes from dust and sunlight. Eyebrows stop salty sweat and rain from flowing into the eye. The worlds longest documented hair, according to Guinness World Records, belongs to Xie Qiuping of China at just under 18 feet 6 inches (5.627 m) when measured on May 8, 2004. She had been growing her hair since 1973 when she was 13 years old. "
nervous system,T_3210,"Michelle was riding her scooter when she hit a hole in the street and started to lose control. She thought she would fall, but, in the blink of an eye, she shifted her weight and kept her balance. Her heart was pounding, but at least she didnt get hurt. How was she able to react so quickly? Michelle can thank her nervous system for that ( Figure 1.1). The nervous system, together with the endocrine system, controls all the other organ systems. The nervous system sends one type of signal around the body, and the endocrine system sends another type of signal around the body. The endocrine system makes and releases chemical messenger molecules, or hormones, which tell other body parts that a change or a reaction is necessary. So what type of signal does the nervous system send? Controlling muscles and maintaining balance are just two of the roles of the nervous system. The nervous system also lets you: Sense your surroundings with your eyes and other sense organs. Sense the environment inside of your body, including temperature. Control your internal body systems and keep them in balance. Staying balanced when riding a scooter requires control over the bodys muscles. The nervous system controls the muscles and maintains balance. Prepare your body to fight or flee in an emergency. Use language, think, learn, and remember. The nervous system works by sending and receiving electrical signals. The main organs of the nervous system are the brain and the spinal cord. The signals are carried by nerves in the body, similar to the wires that carry electricity all over a house. The signals travel from all over the body to the spinal cord and up to the brain, as well as moving in the other direction. For example, when Michelle started to fall off her scooter, her nervous system sensed that she was losing her balance. It responded by sending messages from her brain to muscles in her body. Some muscles tightened while others relaxed. Maybe these actions moved her hips or her arms. The nervous system, working together with the muscular and skeletal systems, allowed Michelle to react to the situation. As a result, Michelles body became balanced again. The messages released by the nervous system traveled through nerves. Just like the electricity that travels through wires, nerve quickly carry the electrical messages around the body. Think about how quickly all this happens. It has to be really fast, otherwise Michelle would not have been able to react. What would happen if a car pulled out unexpectedly in front of Michelle? A signal would have to go from her eyes to her brain and then to her muscles. What allows the nervous system to react so fast. It starts with the special cell of the nervous system, the neuron. "
non infectious reproductive system disorders,T_3211,"Many disorders of the reproductive system are not sexually transmitted infections. They are not caused by pathogens, so they dont spread from person to person. They develop for other reasons. The disorders are different between males and females. In both genders, the disorders could cause a little discomfort, or they could cause death. "
non infectious reproductive system disorders,T_3212,"Most common disorders of the male reproductive system involve the testes. For example, injuries to the testes are very common. In teenagers, injuries to the testes most often occur while playing sports. An injury such as a strike or kick to the testes can be very painful. It may also cause bruising and swelling. Such injuries do not usually last very long. Another disorder of the testes is cancer. Cancer of the testes is most common in males aged 15 to 35. It occurs when cells in the testes grow out of control. The cells form a lump called a tumor. If found early, cancer of the testes usually can be easily cured with surgery. "
non infectious reproductive system disorders,T_3213,"Disorders of the female reproductive system may affect the vagina, uterus, or ovaries. They may also affect the breasts. One of the most common disorders is vaginitis. This is redness and itching of the vagina. It may be due to irritation by soap or bubble bath. Another possible cause of vaginitis is a yeast infection. Yeast normally grow in the vagina. A yeast infection happens when the yeast multiply too fast and cause symptoms. A yeast infection can be treated with medication. Bubble baths may be fun, but for women and girls they can cause irritation to the vagina. A common disorder of the ovaries is an ovarian cyst. A cyst is a sac filled with fluid or other material. An ovarian cyst is usually harmless, but it may cause pain. Most cysts slowly disappear and do not need treatment. Very large or painful cysts can be removed with surgery. Many teen girls have painful menstrual periods. They typically have cramping in the lower abdomen. Generally, this is nothing to worry about. Taking a warm bath or using a heating pad often helps. Exercise can help as well. A pain reliever like ibuprofen may also work. If the pain is severe, a doctor can prescribe stronger medicine to relieve the pain. The most common type of cancer in females is breast cancer. The cancer causes the cells of the breast to grow out of control and form a tumor. Breast cancer is rare in teens. It becomes more common as women get older. If breast cancer is found early, it usually can be cured with surgery. "
nonrenewable resources,T_3217,"A nonrenewable resource is a natural resource that is consumed or used up faster than it can be made by nature. Two main types of nonrenewable resources are fossil fuels and nuclear power. Fossil fuels, such as petroleum, coal, and natural gas, formed from plant and animal remains over periods from 50 to 350 million years ago. They took millions of years to form. Humans have been consuming fossil fuels for less than 200 years, yet remaining reserves of oil can supply our needs only until around the year 2055. Natural gas can only supply us until around 2085. Coal will last longer, until around the year 2250. That is why it is so important to develop alternate forms of energy, especially for our cars. Today, electric cars are becoming more and more common. Considering the year 2055 is not that far away, what would happen if we ran out of gasoline? Alternative use of energy, especially in transportation, must become a standard feature of all cars and trucks and planes by the middle of the century. Nuclear power is the use of nuclear energy ( nuclear fission) to create energy inside of a nuclear reactor ( Figure uranium fuel supplies, which will last to about the year 2100 (or longer) at current rates of use. However, new technologies could make some uranium fuel reserves more useful. Population growth, especially in developing countries, should make people think about how fast they are consuming resources. Governments around the world should seriously consider these issues. Developing nations will also increase demands on natural resources as they build more factories ( Figure 1.2). Improvements in technology, conservation of resources, and controls in population growth could all help to decrease the demand on natural resources. Aerial photo of the Bruce Nuclear Gener- ating Station near Kincardine, Ontario. Per capita energy consumption (2003) shows the unequal distribution of wealth, technology, and energy use. "
organic compounds,T_3223,"The main chemical components of living organisms are known as organic compounds. Organic compounds are molecules built around the element carbon (C). Living things are made up of very large molecules. These large molecules are called macromolecules because macro means large; they are made by smaller molecules bonding together. Our body gets these smaller molecules, the ""building blocks"" or monomers, of organic molecules from the food we eat. Which organic molecules do you recognize from the list below? The four main types of macromolecules found in living organisms, shown in Table 1.1, are: 1. 2. 3. 4. Proteins. Carbohydrates. Lipids. Nucleic Acids. Proteins C, H, O, N, S Enzymes, muscle fibers, antibodies Elements Examples Monomer building molecule) (small block Amino acids Carbohydrates C, H, O Sugar, glucose, starch, glycogen, cellulose Monosaccharides (simple sugars) Lipids C, H, O, P Fats, oils, waxes, steroids, phospho- lipids in membranes Often include fatty acids Nucleic Acids C, H, O, P, N DNA, RNA, ATP Nucleotides "
organic compounds,T_3224,"Carbohydrates are sugars, or long chains of sugars. An important role of carbohydrates is to store energy. Glucose ( Figure 1.1) is an important simple sugar molecule with the chemical formula C6 H12 O6 . Simple sugars are known as monosaccharides. Carbohydrates also include long chains of connected sugar molecules. These long chains often consist of hundreds or thousands of monosaccharides bonded together to form polysaccharides. Plants store sugar in polysaccharides called starch. Animals store sugar in polysaccharides called glycogen. You get the carbohydrates you need for energy from eating carbohydrate-rich foods, including fruits and vegetables, as well as grains, such as bread, rice, or corn. A molecule of glucose, a type of carbohy- drate. "
organic compounds,T_3225,"Proteins are molecules that have many different functions in living things. All proteins are made of monomers called amino acids ( Figure 1.2) that connect together like beads on a necklace ( Figure 1.3). There are only 20 common amino acids needed to build proteins. These amino acids form in thousands of different combinations, making about 100,000 or more unique proteins in humans. Proteins can differ in both the number and order of amino acids. It is the number and order of amino acids that determines the shape of the protein, and it is the shape (structure) of the protein that determines the unique function of the protein. Small proteins have just a few hundred amino acids. The largest proteins have more than 25,000 amino acids. This model shows the general structure of all amino acids. Only the side chain, R, varies from one amino acid to another. KEY: H = hydrogen, N = nitrogen, C = carbon, O = oxygen, R = variable side chain. Many important molecules in your body are proteins. Examples include enzymes, antibodies, and muscle fiber. Enzymes are a type of protein that speed up chemical reactions. They are known as ""biological catalysts."" For example, your stomach would not be able to break down food if it did not have special enzymes to speed up the rate of digestion. Antibodies that protect you against disease are proteins. Muscle fiber is mostly protein ( Figure 1.4). Muscle fibers are made mostly of protein. Its important for you and other animals to eat food with protein, because we cannot make certain amino acids on our own. You can get proteins from plant sources, such as beans, and from animal sources, like milk or meat. When you eat food with protein, your body breaks the proteins down into individual amino acids and uses them to build new proteins. You really are what you eat! "
organic compounds,T_3226,"Have you ever tried to put oil in water? They dont mix. Oil is a type of lipid. Lipids are molecules such as fats, oils, and waxes. The most common lipids in your diet are probably fats and oils. Fats are solid at room temperature, whereas oils are fluid. Animals use fats for long-term energy storage and to keep warm. Plants use oils for long- term energy storage. When preparing food, we often use animal fats, such as butter, or plant oils, such as olive oil or canola oil. There are many more type of lipids that are important to life. One of the most important are the phospholipids that make up the protective outer membrane of all cells ( Figure 1.5). These lipid membranes are impermeable to most water soluble compounds. "
organic compounds,T_3227,"Nucleic acids are long chains of nucleotides. Nucleotides are made of a sugar, a nitrogen-containing base, and a phosphate group. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two main nucleic acids. DNA is a double-stranded nucleic acid. DNA is the molecule that stores our genetic information ( Figure 1.6). The single- stranded RNA is involved in making proteins. ATP (adenosine triphosphate), known as the ""energy currency"" of the cell, is also a nucleic acid. "
organization of the human body,T_3232,"Cells are grouped together to carry out specific functions. A group of cells that work together form a tissue. Your body has four main types of tissues, as do the bodies of other animals. These tissues make up all structures and contents of your body. An example of each tissue type is pictured in the Figure 1.1. Your body has four main types of tissue: nervous tissue, epithelial tissue, connective tissue, and muscle tissue. They are found throughout your body. 1. Epithelial tissue is made up of layers of tightly packed cells that line the surfaces of the body. Examples of epithelial tissue include the skin, the lining of the mouth and nose, and the lining of the digestive system. 2. Connective tissue is made up of many different types of cells that are all involved in supporting and binding other tissues of the body. Examples include tendon, cartilage, and bone. Blood is also classified as a specialized connective tissue. 3. Muscle tissue is made up of bands of cells that contract and allow movement. 4. Nervous tissue is made up of nerve cells that sense stimuli and transmit signals. Nervous tissue is found in nerves, the spinal cord, and the brain. "
organization of the human body,T_3233,A single tissue alone cannot do all the jobs that are needed to keep you alive and healthy. Two or more tissues working together can do a lot more. An organ is a structure made of two or more tissues that work together. The heart ( Figure 1.2) is made up of the four types of tissues. The four different tissue types work to- gether in the heart as they do in the other organs. 
organization of the human body,T_3234,"Your heart pumps blood around your body. But how does your heart get blood to and from every cell in your body? Your heart is connected to blood vessels such as veins and arteries. Organs that work together form an organ system. Together, your heart, blood, and blood vessels form your cardiovascular system. What other organ systems can you think of? "
organization of the human body,T_3235,"Your bodys 12 organ systems are shown below ( Table 1.1). Your organ systems do not work alone in your body. They must all be able to work together. For example, one of the most important functions of organ systems is to provide cells with oxygen and nutrients and to remove toxic waste products such as carbon dioxide. A number of organ systems, including the cardiovascular and respiratory systems, all work together to do this. Organ System Cardiovascular Major Tissues and Organs Heart; blood vessels; blood Lymphatic Lymph nodes; lymph vessels Digestive Esophagus; stomach; small intes- tine; large intestine Pituitary gland, hypothalamus; adrenal glands; ovaries; testes Endocrine Function Transports oxygen, hormones, and nutrients to the body cells. Moves wastes and carbon dioxide away from cells. Defend against infection and dis- ease, moves lymph between tissues and the blood stream. Digests foods and absorbs nutrients, minerals, vitamins, and water. Produces hormones that communi- cate between cells. Organ System Integumentary Major Tissues and Organs Skin, hair, nails Muscular Cardiac (heart) muscle; skeletal muscle; smooth muscle; tendons Brain, spinal cord; nerves Nervous Reproductive Respiratory Female: uterus; vagina; fallopian tubes; ovaries Male: penis; testes; seminal vesi- cles Trachea, larynx, pharynx, lungs Skeletal Bones, cartilage; ligaments Urinary Kidneys; urinary bladder Immune Bone marrow; spleen; white blood cells Function Provides protection from injury and water loss, physical defense against infection by microorganisms, and temperature control. Involved in movement and heat pro- duction. Collects, transfers, and processes information. Produces gametes (sex cells) and sex hormones. Brings air to sites where gas ex- change can occur between the blood and cells (around body) or blood and air (lungs). Supports and protects soft tissues of body; produces blood cells; stores minerals. Removes extra water, salts, and waste products from blood and body; controls pH; controls water and salt balance. Defends against diseases. "
origins of life,T_3241,There is good evidence that life has probably existed on Earth for most of Earths history. Fossils of blue-green algae found in Australia are the oldest fossils of life forms on Earth. They are at least 3.5 billion years old ( Figure 1.1). 
origins of life,T_3242,"How did life begin? In order to answer this question, scientists need to know what kinds of materials were available at that time. We know that the ingredients for life were present at the beginning of Earths history. Scientists believe early Earth did not contain oxygen gas (photosynthesis had yet to evolve), but did contain other gases, including: nitrogen gas, carbon dioxide, carbon monoxide, water vapor, hydrogen sulfide. Some of the oldest fossils on Earth were found along the coast of Australia, similar to the area shown here. Where did these ingredients come from? Some chemicals were in water and volcanic gases ( Figure 1.2). Other chemicals would have come from meteorites in space. Energy to drive chemical reactions was provided by volcanic eruptions and lightning. Today, we have evidence that life on Earth came from random reactions between chem- ical compounds, which formed molecules, or groups of atoms bonded together. Small molecules, such as those present in the early atmosphere, can provide the components (including the elements C, H, N, O and S) to make larger molecules. These early molecules further reacted and eventually formed even larger molecules and organic compounds, such as amino acids (which combine to form proteins), and nucleotides (which form nucleic acids - RNA or DNA). These organic molecules eventually came together in the right combinations to form basic cells. The components that were necessary for the formation of the first cells are still being studied. How long did it take to develop the first life forms? As much as 1 billion years. Many scientists still study the origin of the first life forms because there are many questions left unanswered, such as, ""Did proteins or nucleic acids develop first?"" or ""What exactly were early Earths atmospheric conditions like?"" There is a lot of work still left to answer these and similar questions. Some clues to the origins of life on Earth come from studying the early life forms that developed in hot springs, such as the Grand Prismatic Spring at Yellowstone National Park. This spring is approxi- mately 250 feet deep and 300 feet wide. "
outdoor air pollution,T_3243,"Air is all around us. Air is essential for life. Sometimes, humans can pollute the air. For example, releasing smoke and dust from factories and cars can cause air pollution. Air pollution is due to chemical substances and particles released into the air mainly by human actions. This pollution affects entire ecosystems around the world. Pollution can also cause many human health problems, and it can also cause death. Air pollution can be found both outdoors and indoors. Outdoor air pollution is made of chemical particles. When smoke or other pollutants enter the air, the particles found in the pollution mix with the air. Air is polluted when it contains many large toxic particles. Outdoor air pollution changes the natural characteristics of the atmosphere. Primary pollutants are added directly to the atmosphere. Fires add primary pollutants to the air. Particles released from the fire directly enter the air and cause pollution ( Figure 1.1). Burning of fossil fuels such as oil and coal is a major source of primary pollutants ( Figure Secondary pollutants are formed when primary pollutants interact with sunlight, air, or each other. They do not directly cause pollution. However, when they interact with other parts of the air, they do cause pollution. For example, ozone is created when some pollutants interact with sunlight. High levels of ozone in the atmosphere can cause problems for humans. Wildfires, either natural or human-caused, release particles into the air, one of the many causes of air pollution. A major source of air pollution is the burn- ing of fossil fuels from factories, power plants, and motor vehicles. "
outdoor air pollution,T_3244,"Most air pollutants can be traced to the burning of fossil fuels. Fossil fuels are burned during many processes, including in power plants to create electricity, in factories to make machinery run, in power stoves and furnaces for heating, and in waste facilities. Perhaps one of the biggest uses of fossil fuels is in transportation. Fossil fuels are used in cars, trains, and planes. Air pollution can also be caused by agriculture, such as cattle ranching and the use of fertilizers and pesticides. Other sources of air pollution include the production of plastics, refrigerants, and aerosols, in nuclear power and defense, from landfills and mining, and from biological warfare. "
outdoor air pollution,T_3245,"One result of air pollution is acid rain. Acid rain is precipitation with a low (acidic) pH. This rain can be very destructive to wildlife. When acid rain falls in forests, freshwater habitats, or soils, it can kill insects and aquatic life. It causes this damage because of its very low pH. Sulfur oxides and nitrogen oxides in the air both cause acid rain to form ( Figure 1.3). Sulfur oxides are chemicals that are released from coal-fired power plants. Nitrogen oxides are released from motor vehicle exhaust. A forest in the Jizera Mountains of the Czech Republic shows effects caused by acid rain. What do you observe? "
outdoor air pollution,T_3246,"Pollutants also affect the atmosphere through their contribution to global warming. Global warming is an increase in the Earths temperature. It is thought to be caused mostly by the increase of greenhouse gases like carbon dioxide. Greenhouse gases can be released by factories that burn fossil fuels. Over the past 20 years, burning fossil fuels has produced about three-quarters of the carbon dioxide from human activity. The rest of the carbon dioxide in the atmosphere is there because of deforestation, or cutting down trees ( Figure 1.4). Trees absorb carbon dioxide during cellular respiration, so when trees are cut down, they cannot remove carbon dioxide from the air. This increase in global temperature will cause the sea level to rise. It is also expected to produce an increase in extreme weather events and change the amount of precipitation. Global warming may also cause food shortages and species extinction. "
pathogens,T_3250,"Has this ever happened to you? A student sitting next to you in class has a cold. The other student is coughing and sneezing, but you feel fine. Two days later, you come down with a cold, too. Diseases like colds are contagious. Contagious diseases are also called infectious diseases. An infectious disease is a disease that spreads from person to person. Infectious diseases are caused by pathogens. A pathogen is a living thing or virus that causes disease. Pathogens are commonly called germs. They can travel from one person to another. "
pathogens,T_3251,"Living things that cause human diseases include bacteria, fungi, and protozoa. Most infectious diseases caused by these organisms can be cured with medicines. For example, medicines called antibiotics can cure most diseases caused by bacteria. Bacteria are one-celled organisms without a nucleus. Although most bacteria are harmless, some cause diseases. Worldwide, the most common disease caused by bacteria is tuberculosis (TB). TB is a serious disease of the lungs. Another common disease caused by bacteria is strep throat. You may have had strep throat yourself. Bacteria that cause strep throat are shown below ( Figure 1.1). Some types of pneumonia and many cases of illnesses from food are also caused by bacteria. The structures that look like strings of beads are bacteria. They belong to the genus Streptococcus. Bacteria of this genus cause diseases such as strep throat and pneumonia. They are shown here 900 times bigger than their actual size. Fungi are simple eukaryotic organisms that consist of one or more cells. They include mushrooms and yeasts. Human diseases caused by fungi include ringworm and athletes foot. Both are skin diseases that are not usually serious. A ringworm infection is pictured below ( Figure 1.2). A more serious fungus disease is histoplasmosis. It is a lung infection. Though fungal infections can be annoying, they are rarely as serious or deadly as bacterial or viral infections. Ringworm isnt a worm at all. Its a disease caused by a fungus. The fungus causes a ring-shaped rash on the skin, like the one shown here. Protozoa are one-celled organisms with a nucleus, making them eukaryotic organisms. They cause diseases such as malaria. Malaria is a serious disease that is common in warm climates. The protozoa infect people when they are bit by a mosquito. More than a million people die of malaria each year. Other protozoa cause diarrhea. An example is Giardia lamblia ( Figure 1.3). Viruses are nonliving collections of protein and DNA that must reproduce inside of living cells. Viruses cause many common diseases. For example, viruses cause colds and the flu. Cold sores are caused by the virus Herpes simplex This picture shows a one-celled organism called Giardia lamblia. It is a protozoan that causes diarrhea. ( Figure 1.4). Antibiotics do not affect viruses, because antibiotics only kill bacteria. But medicines called antiviral drugs can treat many diseases caused by viruses. Keep in mind that viruses are nonliving, so can they be killed? "
pathogens,T_3252,"Different pathogens spread in different ways. Some pathogens spread through food. They cause food borne illnesses, which are discussed in a previous concept. Some pathogens spread through water. Giardia lamblia is one example. Water can be boiled to kill Giardia and most other pathogens. Several pathogens spread through sexual contact. HIV is one example, which is discussed in the next concept. Other pathogens that spread through sexual contact are discussed in a separate concept. Many pathogens that cause respiratory diseases spread by droplets in the air. Droplets are released when a person sneezes or coughs. Thousands of tiny droplets are released when a person sneezes ( Figure 1.5). Each droplet can contain thousands of pathogens. Viruses that cause colds and the flu can spread in this way. You may get sick if you breathe in the pathogens. As this picture shows, thousands of tiny droplets are released into the air when a person sneezes. Each droplet may carry thousands of pathogens. You cant normally see the droplets from a sneeze because they are so small. However, you can breathe them in, along with any pathogens they carry. This is how many diseases of the respiratory system are spread. "
pathogens,T_3253,"Other pathogens spread when they get on objects or surfaces. A fungus may spread in this way. For example, you can pick up the fungus that causes athletes foot by wearing shoes that an infected person has worn. You can also pick up this fungus from the floor of a public shower or other damp areas. After acne, athletes foot is the most common skin disease in the United States. Therefore, the chance of coming in contact with the fungus in one of these ways is fairly high. Bacteria that cause the skin disease impetigo, which causes blisters, can spread when people share towels or clothes. The bacteria can also spread through direct skin contact in sports like wrestling. "
pathogens,T_3254,"Still other pathogens are spread by vectors. A vector is an organism that carries pathogens from one person or animal to another. Most vectors are insects, such as ticks and mosquitoes. These insects tend to transfer protozoan or viral parasites. When an insect bites an infected person or animal, it picks up the pathogen. Then the pathogen travels to the next person or animal it bites. Ticks carry the bacteria that cause Lyme disease. Mosquitoes ( Figure serious symptoms may develop. Other diseases spread by mosquitoes include Dengue Fever and Yellow Fever. The first case of West Nile virus in North America occurred in 1999. Within just a few years, the virus had spread throughout most of the United States. Birds as well as humans can be infected with the virus. Birds often fly long distances. This is one reason why West Nile virus spread so quickly. "
pedigree analysis,T_3255,"A pedigree is a chart that shows the inheritance of a trait over several generations. A pedigree is commonly created for families, and it outlines the inheritance patterns of genetic disorders and traits. A pedigree can help predict the probability that offspring will inherit a genetic disorder. Pictured below is a pedigree displaying recessive inheritance of a disorder through three generations ( Figure 1.1). From studying a pedigree, scientists can determine the following: If the trait is sex-linked (on the X or Y chromosome) or autosomal (on a chromosome that does not determine sex). If the trait is inherited in a dominant or recessive fashion. Sometimes pedigrees can also help determine whether individuals with the trait are heterozygous (two different alleles) or homozygous (two of the same allele). Some points to keep in mind when analyzing a pedigree are: 1. With autosomal recessive inheritance, all affected individuals will be homozygous recessive. 2. With dominant inheritance, all affected individuals will have at least one dominant allele. They will be either homozygous dominant or heterozygous. 3. With sex-linked inheritance, more males (XY) than females (XX) usually have the trait. Sex-linked inheritance is usually recessive. "
peripheral nervous system,T_3256,"There are other nerves in your body that are not found in the brain or spinal cord. The peripheral nervous system (PNS) ( Figure 1.1) contains all the nerves in the body that are found outside of the central nervous system. They include nerves of the hands, arms, feet, legs, and trunk. They also include nerves of the scalp, neck, and face. Nerves that send and receive messages to the internal organs are also part of the peripheral nervous system. The peripheral nervous system is divided into two parts, the sensory division and the motor division. How these divisions of the peripheral nervous system are related to the rest of the nervous system is shown below ( Figure 1.2). Refer to the figure as you read more about the peripheral nervous system in the text that follows. "
peripheral nervous system,T_3257,"The sensory division carries messages from sense organs and internal organs to the central nervous system. Human beings have several senses. They include sight, hearing, balance, touch, taste, and smell. We have special sense organs for each of these senses. What is the sense organ for sight? For hearing? Sensory neurons in each sense organ receive stimuli, or messages from the environment that cause a response in the body. For example, sensory neurons in the eyes send messages to the brain about light. Sensory neurons in the skin send messages to the brain about touch. Our sense organs recognize sensations, but they dont tell us what we are sensing. For example, when you breathe in chemicals given off by baking cookies, your nose does not tell you that you are smelling cookies. Thats your brains job. The sense organs send messages about sights, smells, and other stimuli to the brain ( Figure 1.3). The brain then reads the messages and tells you what they mean. A certain area of the brain receives and interprets information from each sense organ. For example, information from the nose is received and interpreted by the temporal lobe of the cerebrum. Which senses would be stimulated by these raspberries? "
peripheral nervous system,T_3258,"The motor division of the peripheral system carries messages from the central nervous system to internal organs and muscles. The motor division is also divided into two parts ( Figure 1.2), the somatic nervous system and the autonomic nervous system. The somatic nervous system carries messages that control body movements. It is responsible for activities that are under your control, such as waving your hand or kicking a ball. The girl pictured below ( Figure 1.4) is using her somatic nervous system to control the muscles needed to play the violin. Her brain sends messages to motor neurons that move her hands so she can play. Without the messages from her brain, she would not be able to move her hands and play the violin. The autonomic nervous system carries nerve impulses to internal organs. It controls activities that are not under your control, such as sweating and digesting food. The autonomic nervous system has two parts: 1. The sympathetic division controls internal organs and glands during emergencies. It prepares the body for fight or flight ( Figure 1.5). For example, it increases the heart rate and the flow of blood to the legs, so you can run away from danger. 2. The parasympathetic division controls internal organs and glands during the rest of the time. It controls processes like digestion, heartbeat, and breathing when there is not an emergency. Have you ever become frightened and felt your heart start pounding? How does this happen? The answer is your autonomic nervous system. The sympathetic division prepared you to deal with a possible emergency by increasing "
photosynthesis,T_3259,"If a plant gets hungry, it cannot walk to a local restaurant and buy a slice of pizza. So, how does a plant get the food it needs to survive? Plants are producers, which means they are able to make, or produce, their own food. They also produce the ""food"" for other organisms. Plants are also autotrophs. Autotrophs are the organisms that collect the energy from the sun and turn it into organic compounds. Using the energy from the sun, they produce complex organic compounds from simple inorganic molecules. So once again, how does a plant get the food it needs to survive? Through photosynthesis. Photosynthesis is the process plants use to make their own food from the suns energy, carbon dioxide, and water. During photosynthesis, carbon dioxide and water combine with solar energy to create glucose, a carbohydrate (C6 H12 O6 ), and oxygen. The process can be summarized as: in the presence of sunlight, carbon dioxide + water  glucose + oxygen. Glucose, the main product of photosynthesis, is a sugar that acts as the ""food"" source for plants. The glucose is then converted into usable chemical energy, ATP, during cellular respiration. The oxygen formed during photosynthesis, which is necessary for animal life, is essentially a waste product of the photosynthesis process. Actually, almost all organisms obtain their energy from photosynthetic organisms. For example, if a bird eats a caterpillar, then the bird gets the energy that the caterpillar gets from the plants it eats. So the bird indirectly gets energy that began with the glucose formed through photosynthesis. Therefore, the process of photosynthesis is central to sustaining life on Earth. In eukaryotic organisms, photosynthesis occurs in chloroplasts. Only cells with chloroplastsplant cells and algal (protist) cellscan perform photosynthesis. Animal cells and fungal cells do not have chloroplasts and, therefore, cannot photosynthesize. That is why these organisms, as well as the non- photosynthetic protists, rely on other organisms to obtain their energy. These organisms are heterotrophs. The Photosynthesis Song explaining photosynthesis, can be heard at  Click image to the left or use the URL below. URL: "
photosynthesis,T_3260,Why do leaves change color each fall? This MIT video demonstrates an experiment about the different pigments in leaves. See the video at  . Click image to the left or use the URL below. URL: 
polygenic traits,T_3277,"Another exception to Mendels rules is polygenic inheritance, which occurs when a trait is controlled by more than one gene. This means that each dominant allele ""adds"" to the expression of the next dominant allele. Usually, traits are polygenic when there is wide variation in the trait. For example, humans can be many different sizes. Height is a polygenic trait, controlled by at least three genes with six alleles. If you are dominant for all of the alleles for height, then you will be very tall. There is also a wide range of skin color across people. Skin color is also a polygenic trait, as are hair and eye color. Polygenic inheritance often results in a bell shaped curve when you analyze the population ( Figure 1.1). That means that most people fall in the middle of the phenotypic range, such as average height, while very few people are at the extremes, such as very tall or very short. At one end of the curve will be individuals who are recessive for all the alleles (for example, aabbcc); at the other end will be individuals who are dominant for all the alleles (for example, AABBCC). Through the middle of the curve will be individuals who have a combination of dominant and recessive alleles (for example, AaBbCc or AaBBcc). "
population growth patterns,T_3278,What does population growth mean? You can probably guess that it means the number of individuals in a population is increasing. The population growth rate tells you how quickly a population is increasing or decreasing. What determines the population growth rate for a particular population? 
population growth patterns,T_3279,"Population growth rate depends on birth rates and death rates, as well as migration. First, we will consider the effects of birth and death rates. You can predict the growth rate by using this simple equation: growth rate = birth rate death rate. If the birth rate is larger than the death rate, then the population grows. If the death rate is larger than the birth rate, what will happen to the population? The population size will decrease. If the birth and death rates are equal, then the population size will not change. Factors that affect population growth are: 1. 2. 3. 4. 5. 6. Age of organisms at first reproduction. How often an organism reproduces. The number of offspring of an organism. The presence or absence of parental care. How long an organism is able to reproduce. The death rate of offspring. For an ecosystem to be stable, populations in that system must be healthy, and that usually means reproducing as much as their environment allows. Do organisms reproduce yearly or every few years? Do organisms reproduce for much of their life, or just part of their life? Do organisms produce many offspring at once, or just a few, or even just one? Do many newborn organisms die, or do the majority survive? All these factors play a role in the growth of a population. Organisms can use different strategies to increase their reproduction rate. Altricial organisms are helpless at birth, and their parents give them a lot of care. This care is often seen in bird species. ( Figure 1.1). Altricial birds are usually born blind and without feathers. Compared to precocial organisms, altricial organisms have a longer period of development before they reach maturity. Precocial organisms, such as the geese shown below, can take care of themselves at birth and do not require help from their parents ( Figure 1.1). In order to reproduce as much as possible, altricial and precocial organisms must use very different strategies. (left) A hummingbird nest with young il- lustrates an altricial reproductive strategy, with a few small eggs, helpless young, and intensive parental care. (right) The Canada goose shows a precocial repro- ductive strategy. It lays a large number of large eggs, producing well-developed young. "
population growth patterns,T_3280,"Migration is the movement of individual organisms into, or out of, a population. Migration affects population growth rate. There are two types of migration: 1. Immigration is the movement of individuals into a population from other areas. This increases the population size and growth rate. 2. Emigration is the movement of individuals out of a population. This decreases the population size and growth rate. The earlier growth rate equation can be modified to account for migration: growth rate = (birth rate + immigration rate) (death rate + emigration rate). One type of migration that you are probably familiar with is the migration of birds. Maybe you have heard that birds fly south for the winter. In the fall, birds fly thousands of miles to the south where it is warmer. In the spring, they return to their homes. ( Figure 1.2). Monarch butterflies also migrate from Mexico to the northern U.S. in the summer and back to Mexico in the winter. These types of migrations move entire populations from one location to another. A flock of barnacle geese fly in formation during the autumn migration. "
population growth patterns,T_3281,"Population growth can be described with two models, based on the size of the population and necessary resources. These two types of growth are known as exponential growth and logistic growth. If a population is given unlimited amounts of resources, such as food and water, land if needed, moisture, oxygen, and other environmental factors, it will grow exponentially. Exponential growth occurs as a population grows larger, dramatically increasing the growth rate. This is shown as a ""J-shaped"" curve below ( Figure 1.3). You can see that the population grows slowly at first, but as time passes, growth occurs more and more rapidly. Growth of populations according to ex- ponential (or J-curve) growth model (left) and logistic (or S-curve) growth model (right). Time is plotted on the x-axis, and population size is plotted on the y-axis. In nature, organisms do not usually have ideal environments with unlimited food. In nature, there are limits. Sometimes, there will be plenty of food. Sometimes, a fire will wipe out all of the available nutrients. Sometimes a predator will kill many individuals in a population. How do you think these limits affect the way organisms grow? "
pregnancy and childbirth,T_3283,"While a woman is pregnant, the developing baby may be called an embryo or a fetus. Do these mean the same thing? No, in the very early stages the developing baby is called an embryo, while in the later stages it is called a fetus. When the ball of cells first implants into the uterus, it is called an embryo. The embryo stage lasts until the end of the 8th week after fertilization. After that point until birth, the developing baby is called a fetus. "
pregnancy and childbirth,T_3284,"During the embryo stage, the baby grows in size. 3rd week after fertilization: Cells of different types start to develop. Cells that will form muscles and skin, for example, start to develop at this time. 4th week after fertilization: Body organs begin to form. 8th week after fertilization: All the major organs have started to develop. Pictured below are some of the changes that take place during the 4th and 8th weeks ( Figure 1.1). Look closely at the two embryos in the figure. Do you think that the older embryo looks more human? Notice that it has arms and legs and lacks a tail. The face has also started to form, and it is much bigger. Embryonic Development (Weeks 48). Most organs develop in the embryo during weeks four through eight. (Note: the drawings of the embryos are not to scale.) "
pregnancy and childbirth,T_3285,"There are also many changes that take place after the embryo becomes a fetus. Some of the differences between them are obvious. For example, the fetus has ears and eyelids. Its fingers and toes are also fully formed. The fetus even has fingernails and toenails. In addition, the reproductive organs have developed to make the baby a male or female. The brain and lungs are also developing quickly. The fetus has started to move around inside the uterus. This is usually when the mother first feels the fetus moving. By the 28th week, the fetus is starting to look much more like a baby. Eyelashes and eyebrows are present. Hair has started to grow on the head. The body of the fetus is also starting to fill out as muscles and bones develop. Babies born after the 28th week are usually able to survive. However, they need help breathing because their lungs are not yet fully mature. A baby should not be delivered prior to this time, unless absolutely necessary. A baby born prior to week 28 will need considerable medical intervention to survive. During the last several weeks of the fetal period, all of the organs become mature. The most obvious change, however, is an increase in body size. The fetus rapidly puts on body fat and gains weight during the last couple of months. By the end of the 38th week, all of the organs are working, and the fetus is ready to be born. This is when birth normally occurs. A baby born before this time is considered premature. "
pregnancy and childbirth,T_3286,"During pregnancy, other structures also develop inside the mothers uterus. They are the amniotic sac, placenta, and umbilical cord ( Figure 1.2). Surrounding the fetus is the fluid-filled amniotic sac. The placenta and umbilical cord are also shown here. They provide a connection between the mothers and fetuss blood for the transfer of nutrients and gases. The amniotic sac is a membrane that surrounds the fetus. It is filled with water and dissolved substances, known as amniotic fluid. Imagine placing a small plastic toy inside a balloon and then filling the balloon with water. The toy would be cushioned and protected by the water. It would also be able to move freely inside the balloon. The amniotic sac and its fluid are like a water-filled balloon. They cushion and protect the fetus. They also let the fetus move freely inside the uterus. The placenta is a spongy mass of blood vessels. Some of the vessels come from the mother. Some come from the fetus. The placenta is attached to the inside of the mothers uterus. The fetus is connected to the placenta by a tube called the umbilical cord. The cord contains two arteries and a vein. Substances pass back and forth between the mothers and fetuss blood through the placenta and cord. Oxygen and nutrients pass from the mother to the fetus. Carbon dioxide passes from the fetus to the mother. It is important for the mother to eat plenty of nutritious foods during pregnancy. She must take in enough nutrients for the fetus as well as for herself. She needs extra calories, proteins, and lipids. She also needs more vitamins and minerals. In addition to eating well, the mother must avoid substances that could harm the embryo or fetus. These include alcohol, illegal drugs, and some medicines. It is especially important for her to avoid these substances during the first eight weeks after fertilization. This is when all the major organs are forming. Exposure to harmful substances during this time could damage the developing body systems. "
pregnancy and childbirth,T_3287,"During childbirth, a baby passes from the uterus, through the vagina, and out of the mothers body. Childbirth usually starts when the amniotic sac breaks. Then, the muscles of the uterus start contracting. The contractions get stronger and closer together. They may go on for hours. Eventually, the contractions squeeze the baby out of the uterus. Once the baby enters the vagina, the mother starts pushing. She soon pushes the baby through the vagina and out of her body. As soon as the baby is born, the umbilical cord is cut. After the cord is cut, the baby can no longer get rid of carbon dioxide through the cord and placenta. As a result, carbon dioxide builds up in the babys blood. This triggers the baby to start breathing. The amniotic sac and placenta pass through the vagina and out of the body shortly after the birth of the baby. "
preserving water sources,T_3288,"It might seem like there is plenty of water on Earth, but thats not really the case. Water is a limited resource. That means that it is used faster than it is replaced. Theoretically, at some point in time, the supply of fresh water could run out. Though this is unlikely, it is possible. But it is a significant issue in parts of the world with large populations. As these populations continue to grow, the supply of water becomes an increasingly important issue. Even though we have lots of water in our oceans, we cannot use that water whenever we want. It takes special equipment, such as a desalination plant, and a lot of energy (and money) to convert salt water into fresh water. Of all the water on Earth, only about 1% can be used for drinking water. Almost all of the rest of the water is either salt water in the ocean or ice in glaciers and ice caps. As a result, there are water shortages many places in the world. Since we have such a limited supply of water, it is important to preserve our water supplies. Therefore, steps have been taken to prevent water pollution. Technologies have also been developed to conserve water and prevent water pollution. Sub-Saharan African countries have the most vulnerable water supplies. Some scientists believe of a potential future crisis in both Asia and Africa from pollution and depletion of natural water resources. Many countries in the Middle East are at an extreme risk of water shortages. Diminished water supplies could increase the risk of both internal conflicts or wars between countries. "
preserving water sources,T_3289,"In the U.S., concern over water pollution has resulted in many federal laws. Some of these laws go all the way back to the 1800s! The laws prohibit the disposal of any waste into the nations rivers, lakes, streams, and other bodies of water, unless a person first has a permit. Growing concern for controlling water pollutants led to the enactment of the Clean Water Act in 1972. The Clean Water Act set water quality standards. It also limits the pollution that can enter the waterways. Other countries are also actively preventing water pollution and purifying water ( Figure 1.1). A water purification station in France. Contaminants are removed to make clean water. "
preserving water sources,T_3290,"Fresh water is also preserved by purifying wastewater. Wastewater is water that has been used for cleaning, washing, flushing, or manufacturing. It includes the water that goes down your shower drain and that is flushed down your toilet. Instead of dumping wastewater directly into rivers, wastewater can be purified at a water treatment plant ( Figure 1.2). When wastewater is recycled, waterborne diseases caused by pathogens in sewage can be prevented. What are some ways you can save water in your own house? "
preventing infectious diseases,T_3291,"Infectious diseases are diseases that spread from person to person. They are caused by pathogens such as bacteria, viruses or fungi. What can you do to avoid infectious diseases? Eating right and getting plenty of sleep are a good start. These habits will help keep your immune system healthy. With a healthy immune system, you will be able to fight off many pathogens. The next best way is to avoid pathogens. Though this is difficult, there are steps you can take to limit your exposure to pathogens. Here are the ten best ways to prevent the spread of infectious diseases. 1. Wash your hands frequently. 2. Dont share personal items. 3. Cover your mouth when you cough or sneeze. 4. 5. 6. 7. 8. 9. 10. Get vaccinated. Use safe cooking practices. Be a smart traveler. Practice safe sex. Dont pick your nose (or your mouth or eyes either). Exercise caution with animals. Watch the news, and be aware of disease outbreaks. "
preventing infectious diseases,T_3292,"You can also take steps to avoid pathogens in the first place. The best way to avoid pathogens is to wash your hands often. You should wash your hands after using the bathroom or handling raw meat or fish. You should also wash your hands before eating or preparing food. In addition, you should also wash the food that your eat, and the utensils and countertop where food is prepared. In addition, you should wash your hands after being around sick people. The correct way to wash your hands is demonstrated below ( Figure 1.1). If soap and water arent available, use some hand sanitizer. The best way to prevent diseases spread by vectors is to avoid contact with the vectors. Recall that a vector is an organism that carries pathogens from one person or animal to another. For example, ticks and mosquitoes are vectors, so you should wear long sleeves and long pants when appropriate to avoid tick and mosquito bites. Using insect repellent can also reduce your risk of insect bites. Many infectious diseases can be prevented with vaccinations. Immunization can drastically reduce your chances of contracting many diseases. You will read more about vaccinations in another concept. Vaccinations can help prevent measles, mumps, chicken pox, and several other diseases. If you do develop an infectious disease, try to avoid infecting others. Stay home from school until you are well. Also, take steps to keep your germs to yourself. Cover your mouth and nose with a tissue when you sneeze or cough, Watching the news will allow you to make informed decisions. If an outbreak of bad beef due to a bacterial infection is in the news, dont buy beef for a while. If tomatoes are making people sick, dont eat tomatoes until the outbreak is over. If a place has an unhealthy water supply, boil the water or drink bottled water. Local news can tell you of restaurants to avoid due to unhealthy conditions. And so on. "
preventing noninfectious diseases,T_3293,"Noninfectious diseases cant be passed from one person to another. Instead, these types of diseases are caused by factors such as the environment, genetics, and lifestyle. Examples of inherited noninfectious conditions include cystic fibrosis and Down syndrome. If youre born with these conditions, you must learn how to manage the symptoms. Examples of conditions caused by environmental or lifestyle factors include heart disease and skin cancer. We cant change our genetic codes, but there are plenty of ways to prevent other noninfectious diseases. For example, cutting down on exposure to cigarette smoke and the suns rays will prevent certain types of cancer. It is a fact that most chronic noninfectious diseases can be prevented. The chronic noninfectious diseases that cause the most deaths in many developed countries are largely preventable. These diseases are heart disease, stroke, diabetes and cancer, and though they do have some genetic components, they also have many lifestyle components. For example, some cancers have genetic risks, but people at high risk for cancers can have screening examinations to catch them early or sometimes can take other steps to prevent the cancers. Heart disease, stroke and diabetes are mostly linked to lifestyle choices, even when family history puts a person at higher risk for the diseases. Most allergies can be prevented by avoiding the substances that cause them. For example, you can avoid pollens by staying indoors as much as possible. You can learn to recognize plants like poison ivy and not touch them. A good way to remember how to avoid poison ivy is ""leaves of three, let it be."" Some people receive allergy shots to help prevent allergic reactions. The shots contain tiny amounts of allergens, which are the substances that cause an allergic reaction. After many months or years of shots, the immune system gets used to the allergens and no longer responds to them. Type 1 diabetes and other autoimmune diseases cannot be prevented. But choosing a healthy lifestyle can help prevent type 2 diabetes. Getting plenty of exercise, avoiding high-fat foods, and staying at a healthy weight can reduce the risk of developing this type of diabetes. This is especially important for people who have family members with the disease. Making these healthy lifestyle choices can also help prevent some types of cancer. In addition, you can lower the risk of cancer by avoiding carcinogens, which are substances that cause cancer. For example, you can reduce your risk of lung cancer by not smoking. You can reduce your risk of skin cancer by using sunscreen. How to choose a sunscreen that offers the most protection is explained below ( Figure 1.1). Some people think that tanning beds are a safe way to get a tan. This is a myth. Tanning beds expose the skin to UV radiation. Any exposure to UV radiation increases the risk of skin cancer. It doesnt matter whether the radiation comes from tanning lamps or the sun. Overall, people in many developed countries are contributing to higher rates of noninfectious diseases (heart disease, stroke, diabetes and cancer) by taking advantage of technology and social environments that encourage a less active lifestyle, and also encourages faster and cheaper meals. For example, many children now spend more time on their computer or watching TV then playing outdoors. The ""faster and cheaper"" meals are usually less healthy than other meals. Even though many people are living longer, they can choose to live more healthily by adopting regular exercise routines and healthy eating habits. When you choose a sunscreen, select one with an SPF (sun protection factor) of 30 or higher. Also, choose a sunscreen that protects against both UVB and UVA radiation. "
process of cellular respiration,T_3298,"Cellular respiration is the process of extracting energy in the form of ATP from the glucose in the food you eat. How does cellular respiration happen inside of the cell? Cellular respiration is a three step process. Briefly: 1. In stage one, glucose is broken down in the cytoplasm of the cell in a process called glycolysis. 2. In stage two, the pyruvate molecules are transported into the mitochondria. The mitochondria are the organelles known as the energy ""powerhouses"" of the cells (Figure 1.1). In the mitochondria, the pyruvate, which have been converted into a 2-carbon molecule, enter the Krebs cycle. Notice that mitochondria have an inner membrane with many folds, called cristae. These cristae greatly increase the membrane surface area where many of the cellular respiration reactions take place. 3. In stage three, the energy in the energy carriers enters an electron transport chain. During this step, this energy is used to produce ATP. Oxygen is needed to help the process of turning glucose into ATP. The initial step releases just two molecules of ATP for each glucose. The later steps release much more ATP. Most of the reactions of cellular respira- tion are carried out in the mitochondria. "
process of cellular respiration,T_3299,What goes into the cell? Oxygen and glucose are both reactants of cellular respiration. Oxygen enters the body when an organism breathes. Glucose enters the body when an organism eats. 
process of cellular respiration,T_3300,"What does the cell produce? The products of cellular respiration are carbon dioxide and water. Carbon dioxide is transported from your mitochondria out of your cell, to your red blood cells, and back to your lungs to be exhaled. ATP is generated in the process. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. This only occurs in the presence of oxygen. "
process of cellular respiration,T_3301,"The overall chemical reaction for cellular respiration is one molecule of glucose (C6 H12 O6 ) and six molecules of oxygen (O2 ) yields six molecules of carbon dioxide (CO2 ) and six molecules of water (H2 O). Using chemical symbols the equation is represented as follows: C6 H12 O6 + 6O2  6CO2 + 6H2 O ATP is generated during the process. Though this equation may not seem that complicated, cellular respiration is a series of chemical reactions divided into three stages: glycolysis, the Krebs cycle, and the electron transport chain. "
process of cellular respiration,T_3302,"Stage one of cellular respiration is glycolysis. Glycolysis is the splitting, or lysis of glucose. Glycolysis converts the 6-carbon glucose into two 3-carbon pyruvate molecules. This process occurs in the cytoplasm of the cell, and it occurs in the presence or absence of oxygen. During glycolysis a small amount of NADH is made as are four ATP. Two ATP are used during this process, leaving a net gain of two ATP from glycolysis. The NADH temporarily holds energy, which will be used in stage three. "
process of cellular respiration,T_3303,"In the presence of oxygen, under aerobic conditions, pyruvate enters the mitochondria to proceed into the Krebs cycle. The second stage of cellular respiration is the transfer of the energy in pyruvate, which is the energy initially in glucose, into two energy carriers, NADH and FADH2 . A small amount of ATP is also made during this process. This process occurs in a continuous cycle, named after its discover, Hans Krebs. The Krebs cycle uses a 2-carbon molecule (acetyl-CoA) derived from pyruvate and produces carbon dioxide. "
process of cellular respiration,T_3304,"Stage three of cellular respiration is the use of NADH and FADH2 to generate ATP. This occurs in two parts. First, the NADH and FADH2 enter an electron transport chain, where their energy is used to pump, by active transport, protons (H+ ) into the intermembrane space of mitochondria. This establishes a proton gradient across the inner membrane. These protons then flow down their concentration gradient, moving back into the matrix by facilitated diffusion. During this process, ATP is made by adding inorganic phosphate to ADP. Most of the ATP produced during cellular respiration is made during this stage. For each glucose that starts cellular respiration, in the presence of oxygen (aerobic conditions), 36-38 ATP are generated. Without oxygen, under anaerobic conditions, much less (only two!) ATP are produced. "
processes of breathing,T_3305,"Breathing is only part of the process of bringing oxygen to where it is needed in the body. After oxygen enters the lungs, what happens? 1. The oxygen enters the bloodstream from the alveoli, tiny sacs in the lungs where gas exchange takes place ( Figure 1.1). The transfer of oxygen into the blood is through simple diffusion. 2. The oxygen-rich blood returns to the heart. 3. Oxygen-rich blood is then pumped through the aorta, the large artery that receives blood directly from the heart. 4. From the aorta, oxygen-rich blood travels to the smaller arteries and, finally, to the capillaries, the smallest type of blood vessel. 5. The oxygen molecules move, by diffusion, out of the capillaries and into the body cells. 6. While oxygen moves from the capillaries and into body cells, carbon dioxide moves from the cells into the capillaries. Gas exchange is the movement of oxygen into the blood and carbon dioxide out of the blood. 7. Carbon dioxide is brought, through the blood, back to the heart and then to the lungs. Then it is released into the air during exhalation. Why is oxygen needed by each cell in your body? To make ATP, the usable form of cellular energy. Oxygen is needed in the final stage of cellular respiration, which is the process of converting glucose into ATP. This process is much more efficient in the presence of oxygen. Without oxygen, much less ATP is produced. As ATP is needed for the cells to function properly, every cell in your body needs oxygen. Getting that oxygen begins with inhaling. The oxygen moves into your blood, where it travels to every cell in your body. "
producers,T_3306,"Energy is the ability to do work. In organisms, this work can be physical work, like walking or jumping, or it can be the work used to carry out the chemical processes in their cells. Every biochemical reaction that occurs in an organisms cells needs energy. All organisms need a constant supply of energy to stay alive. Some organisms can get the energy directly from the sun. Other organisms get their energy from other organisms. Through predator-prey relationships, the energy of one organism is passed on to another. Energy is constantly flowing through a community. With just a few exceptions, all life on Earth depends on the suns energy for survival. The energy of the sun is first captured by producers ( Figure 1.1), organisms that can make their own food. Many producers make their own food through the process of photosynthesis. The ""food"" the producers make is the sugar, glucose. Producers make food for the rest of the ecosystem. As energy is not recycled, energy must consistently be captured by producers. This energy is then passed on to the organisms that eat the producers, and then to the organisms that eat those organisms, and so on. Recall that the only required ingredients needed for photosynthesis are sunlight, carbon dioxide (CO2 ), and wa- ter (H2 O). From these simple inorganic ingredients, photosynthetic organisms produce the carbohydrate glucose (C6 H12 O6 ), and other complex organic compounds. Essentially, these producers are changing the energy from the sunlight into a usable form of energy. They are also making the oxygen that we breathe. Oxygen is a waste product of photosynthesis. The survival of every ecosystem is dependent on the producers. Without producers capturing the energy from the sun and turning it into glucose, an ecosystem could not exist. On land, plants are the dominant producers. Phytoplankton, tiny photosynthetic organisms, are the most common producers in the oceans and lakes. Algae, which is the green layer you might see floating on a pond, are an example of phytoplankton. There are also bacteria that use chemical processes to produce food. They get their energy from sources other than the sun, but they are still called producers. This process is known as chemosynthesis, and is common in ecosystems without sunlight, such as certain marine ecosystems. Producers include (a) plants, (b) algae, and (c) diatoms. "
puberty and adolescence,T_3317,"Puberty is the stage of life when a child becomes sexually mature. Puberty lasts from about 12 to 18 years of age in boys and from about 10 to 16 years of age in girls. The age when puberty begins is different from one child to another. Children that begin puberty much earlier or later than their peers may feel self-conscious. They may also worry that something is wrong with them. Usually, an early or late puberty is perfectly normal. In boys, puberty begins when the pituitary gland tells the testes to secrete testosterone. Testosterone causes the following to happen: 1. 2. 3. 4. The penis and testes grow. The testes start making sperm. Pubic and facial hair grow. The shoulders broaden, and the voice becomes deeper. In girls, puberty begins when the pituitary gland tells the ovaries to secrete estrogen. Estrogen causes the following to happen: 1. 2. 3. 4. 5. The uterus and ovaries grow. The ovaries start releasing eggs. The menstrual cycle begins. Pubic hair grows. The hips widen, and the breasts develop. Boys and girls are close to the same height during childhood. In both boys and girls, growth in height and weight is very fast during puberty. But boys grow faster than girls during puberty. Their period of fast growth also lasts longer. By the end of puberty, boys are an average of 10 centimeters (4 inches) taller than girls. "
puberty and adolescence,T_3318,"Adolescence is the period of life between the start of puberty and the beginning of adulthood. Adolescence includes the physical changes of puberty. It also includes many other changes, including significant mental, emotional, and social changes. During adolescence: Teenagers develop new thinking abilities. For example, they can think about abstract ideas, such as freedom. They are also better at thinking logically. They are usually better at solving problems as well. Teenagers try to establish a sense of who they are as individuals. They may try to become more independent from their parents. Most teens also have emotional ups and downs. This is partly due to changing hormone levels. Teenagers usually spend much more time with peers than with family members. "
recombinant dna,T_3320,"Recombinant DNA is the combination of DNA from two different sources. For example, it is possible to place a human gene into bacterial DNA. Recombinant DNA technology is useful in gene cloning and in identifying the function of a gene. Recombinant DNA technology can also be used to produce useful proteins, such as insulin. To treat diabetes, many people need insulin. Previously, insulin had been taken from animals. Through recombinant DNA technology, bacteria were created that carry the human gene which codes for the production of insulin. These bacteria become tiny factories that produce this protein. Recombinant DNA technology helps create insulin so it can be used by humans. Recombinant DNA technology is used in gene cloning. A clone is an exact genetic copy. Genes are cloned for many reasons, including use in medicine and in agriculture. Below are the steps used to copy, or clone, a gene: 1. A gene or piece of DNA is put in a vector, or carrier molecule, producing a recombinant DNA molecule. 2. The vector is placed into a host cell, such as a bacterium. 3. The gene is copied (or cloned) inside of the bacterium. As the bacterial DNA is copied, so is the vector DNA. As the bacteria divide, the recombinant DNA molecules are divided between the new cells. Over a 12- to 24-hour period, millions of copies of the cloned DNA are made. 4. The cloned DNA can produce a protein (like insulin) that can be used in medicine or in research. "
recombinant dna,T_3321,"Bacteria have small rings of DNA in the cytoplasm, called plasmids ( Figure 1.1). A plasmid is not part of the bacterial chromosome, but an additional pieced of DNA. When putting foreign DNA into a bacterium, the plasmids are often used as a vector. Viruses can also be used as vectors. The manipulation of the plasmid DNA, and then the insertion of the recombinant plasmid into a bacterium, is an invaluable tool in scientific research. This image shows a drawing of a plasmid. The plasmid has two large segments and one small segment depicted. The two large segments (green and blue) indicate antibiotic resistances usually used in a screening procedure. The antibiotic resis- tance segments ensure only bacteria with the plasmid will grow. The small segment (red) indicates an origin of replication. The origin of replication is where DNA replication starts, copying the plasmid. "
reduce reuse and recycle,T_3322,"Why conserve resources? During your lifetime, it is possible that the world may run out of some nonrenewable resources, especially as the population passes 8 then 9 billion people. So it is necessary to try to make these resources last as long as possible. You may have heard people say, ""Reduce, Reuse, Recycle."" You may know that this is the slogan of the campaign to conserve resources. But what do each one of those words truly mean? "
reduce reuse and recycle,T_3323,"What exactly does it mean to reduce? Reducing means decreasing the amount of waste we create. That could also mean cutting down on use of natural resources. In addition, many ways to reduce also result in saving money. Minimizing of waste may be difficult to achieve for individuals and households, but here are some starting points that you can include in your daily routine to reduce the use of resources: Turn lights off when not using them. Turn the television off when no one is watching. Replace burned out bulbs with ones that are more energy-efficient ( Figure 1.1). Reduce water use by turning off faucets when not using water. Use low-flow shower heads, which save on water and use less energy. Use low-flush and composting toilets. Put kitchen and garden waste into a compost pile. In the summer, change filters on your air conditioner and use as little air conditioning as possible. The use of air conditioning uses a lot of energy. In winter, make sure your furnace is working properly and make sure there is enough insulation on windows and doors. Mend broken or worn items instead of buying new ones. When you go shopping for items, buy quantities you know you will use without waste. Walk or bicycle instead of using an automobile, in order to save on fuel usage and costs, and to cut down on pollution. When buying a new vehicle, check into hybrid, semi-hybrid, or electric models to cut down on gas usage and air pollution. These fluorescent light bulbs are much more energy efficient than standard light bulbs. "
reduce reuse and recycle,T_3324,"Lets now look at what we can reuse. Reusing includes using the same item again for the same function and also using an item again for a new function. Reuse can have both economic and environmental benefits. New packaging regulations are helping society to move towards these goals. Water is a resource that can be reused for numerous purposes. You may not drink used water, but it is quite useful for other purposes. Some ways of reusing resources include: Use reusable bags when shopping. Use gray water. Water that has been used for laundry, for example, can be used to water the garden or flush toilets. At the town level, purified sewage water can be used for fountains, watering public parks or golf courses, fire fighting, and irrigating crops. Rain can be caught in rain barrels and used to water your garden. What are some other ways to reuse resources? "
reduce reuse and recycle,T_3325,"Now we move on to recycle. Sometimes it may be difficult to understand the differences between reusing and recycling. Recycling involves processing used materials in order to make them suitable for other uses. That usually means taking a used item, breaking it down, and reusing the pieces. Even though recycling requires extra energy, it does often make use of items which are broken, worn out, or cannot be reused. The things that are commonly recycled include: Batteries. Biodegradable waste. Electronics. Iron and steel. Aluminum ( Figure 1.2). Glass. Paper. Plastic. Textiles, such as clothing. Timber. Tires. Each type of recyclable requires a different recycling technique. Here are some things you can do to recycle in your home, school, or community: Laws can also be created to make sure people and companies reduce, reuse, and recycle. Individuals can vote for leaders who stand for sustainable ecological practices. They can also tell their leaders to make wise use of natural resources. You can also influence companies. If you and your family only buy from companies and restaurants that support recycling or eco-friendly packaging, then other companies will also change to be more environmentally friendly. "
renewable resources and alternative energy sources,T_3326,"A resource is renewable if it is remade by natural processes at the same rate that humans use it up. Sunlight and wind are renewable resources because they will not be used up ( Figure 1.1). The rising and falling of ocean tides is another example of a resource in unlimited supply. A sustainable resource is a resource that is used in a way that meets the needs of the present without keeping future generations from meeting their needs. People can sustainably harvest wood, cork, and bamboo. Farmers can also grow crops sustainably by not planting the same crop in their soil year after year. Planting the same crop each year can remove nutrients from the soil. This means that wood, cork, bamboo, and crops can be sustainable resources. "
renewable resources and alternative energy sources,T_3327,"A nonrenewable resource is one that cannot be replaced as easily as it is consumed. Fossil fuels are an example of nonrenewable resources. They take millions of years to form naturally, and so they cannot be replaced as fast as they are consumed. To take the place of fossil fuel use, alternative energy resources are being developed. These alternative energy sources often utilize renewable resources. The following are examples of sustainable alternative energy resources: Solar power, which uses solar cells to turn sunlight into electricity ( Figure 1.2). The electricity can be used to power anything that uses normal coal-generated electricity. Wind power, which uses windmills to transform wind energy into electricity. It is used for less than 1% of the worlds energy needs. But wind energy is growing fast. Every year, 30% more wind energy is used to create electricity. Hydropower ( Figure 1.3), which uses the energy of moving water to turn turbines (similar to windmills) or water wheels, that create electricity. This form of energy produces no waste or pollution. It is a renewable resource. Geothermal power, which uses the natural flow of heat from the Earths core to produce steam. This steam is used to turn turbines which create electricity. Biomass is the mass of biological organisms. It is usually used to describe the amount of organic matter in a trophic level of an ecosystem. Biomass production involves using organic matter (""biomass"") from plants to create electricity. Using corn to make ethanol fuel is an example of biomass generated energy. Biomass is generally renewable. Tides in the ocean can also turn a turbine to create electricity. This energy can then be stored until needed ( Figure 1.4). Dam of the tidal power plant in the Rance River, Bretagne, France "
renewable resources and alternative energy sources,T_3328,"Scientists at the Massachusetts of Technology are turning trash into coal, which can readily be used to heat homes and cook food in developing countries. This coal burns cleaner than that from fossil fuels. It also save a tremendous amount of energy. See http://youtu.be/GzhFgEYiVyY?list=PLzMhsCgGKd1hoofiKuifwy6qRXZs7NG6a for more information. Click image to the left or use the URL below. URL: "
reproductive system health,T_3336,"As was discussed in previous concepts, both infectious and noninfectious diseases of the reproductive system can be very serious. But there are ways to keep your reproductive system healthy. What can you do to keep your reproductive system healthy? You can start by making the right choices for overall good health. To be as healthy as you can be, you should: Eat a balanced diet that is high in fiber and low in fat. Drink plenty of water. Get regular exercise. Maintain a healthy weight. Get enough sleep. Avoid using tobacco, alcohol, or other drugs. Manage stress in healthy ways. Keeping your genitals clean is also very important. A daily shower or bath is all that it takes. Females do not need to use special feminine hygiene products. In fact, using them may do more harm than good because they can irritate the vagina or other reproductive structures. You should also avoid other behaviors that can put you at risk. Do not get into contact with another persons blood or other body fluids. For example, never get a tattoo or piercing unless you are sure that the needles have not been used before. This is one of the most important ways to prevent an STI. Of course, the only way to be fully protected against STIs is to refrain from sexual activity. If you are a boy, you should always wear a protective cup when you play contact sports. Contact sports include football, boxing, and hockey. Wearing a cup will help protect the testes from injury. You should also do a monthly self-exam to check for cancer of the testes. If you are a girl and use tampons, be sure to change them every four to six hours. Leaving tampons in for too long can put you at risk of toxic shock syndrome. This is a serious condition. Signs and symptoms of toxic shock syndrome develop suddenly, and the disease can be fatal. The disease involves fever, shock, and problems with the function of several body organs. Girls should also get in the habit of doing a monthly self-exam to check for breast cancer. Although breast cancer is rare in teens, its a good idea to start doing the exam when you are young. It will help you get to know what is normal for you. "
respiration,T_3340,"Most of the time, you breathe without thinking about it. Breathing is mostly an involuntary action that is controlled by a part of your brain that also controls your heart beat. If you swim, do yoga, or sing, you know you can control your breathing, however. Taking air into the body through the nose and mouth is called inhalation. Pushing air out of the body through the nose or mouth is called exhalation. The woman pictured below is exhaling before she surfaces from the pool water (Figure 1.1). How do lungs allow air in? Air moves into and out of the lungs by the movement of muscles. The most important muscle in the process of breathing is the diaphragm, a sheet of muscle that spreads across the bottom of the rib cage. The diaphragm and rib muscles contract and relax to move air into and out of the lungs. During inhalation, the diaphragm contracts and moves downward. The rib muscles contract and cause the ribs to move outward. This causes the chest volume to increase. Because the chest volume is larger, the air pressure inside the lungs is lower than the air pressure outside. This difference in air pressures causes air to be sucked into the lungs. When the diaphragm and rib muscles relax, air is pushed out of the lungs. Exhalation is similar to letting the air out of a balloon. How does the inhaled oxygen get into the bloodstream? The exchange of gasses between the lungs and the blood happens in tiny sacs called alveoli. The walls of the alveoli are very thin and allow gases to pass though them. The alveoli are lined with capillaries (Figure 1.2). Oxygen moves from the alveoli to the blood in the capillaries that surround the alveoli. At the same time, carbon dioxide moves in the opposite direction, from capillary blood to the alveoli. The gases move by simple diffusion, passing from an area of high concentration to an area of low concentration. For example, initially there is more oxygen in the alveoli than in the blood, so oxygen moves by diffusion from the alveoli into the blood. "
respiration,T_3341,"The process of getting oxygen into the body and releasing carbon dioxide is called respiration. Sometimes breathing is called respiration, but there is much more to respiration than just breathing. Breathing is only the movement of oxygen into the body and carbon dioxide out of the body. The process of respiration also includes the exchange of oxygen and carbon dioxide between the blood and the cells of the body. "
respiratory system diseases,T_3342,"Respiratory diseases are diseases of the lungs, bronchial tubes, trachea, nose, and throat ( Figure 1.1). These diseases can range from a mild cold to a severe case of pneumonia. Respiratory diseases are common. Many are easily treated, while others may cause severe illness or death. Some respiratory diseases are caused by bacteria or viruses, while others are caused by environmental pollutants, such as tobacco smoke. Some diseases are genetic and, therefore, are inherited. This boy is suffering from whooping cough (also known as pertussis), which gets its name from the loud whooping sound that is made when the person inhales during a coughing fit. "
respiratory system diseases,T_3343,"Bronchitis is an inflammation of the bronchi, the air passages that conduct air into the lungs. The bronchi become red and swollen with infection. Acute bronchitis is usually caused by viruses or bacteria, and may last several days or weeks. It is characterized by a cough that produces phlegm, or mucus. Symptoms include shortness of breath and wheezing. Acute bronchitis is usually treated with antibiotics. "
respiratory system diseases,T_3344,"Asthma is a chronic illness in which the bronchioles, the tiny branches into which the bronchi are divided, become inflamed and narrow ( Figure 1.2). The muscles around the bronchioles contract, which narrows the airways. Large amounts of mucus are also made by the cells in the lungs. People with asthma have difficulty breathing. Their chests feel tight, and they wheeze. Asthma can be caused by different things, such as allergies. Asthma can also be caused by cold air, warm air, moist air, exercise, or stress. The most common asthma triggers are illnesses, like the common cold. Asthma is not contagious and cannot be passed on to other people. Children and adolescents who have asthma can still lead active lives if they control their asthma. Asthma can be controlled by taking medication and by avoiding contact with environmental triggers for asthma, like smoking. "
respiratory system diseases,T_3345,"Pneumonia is an illness that occurs when the alveoli, the tiny sacs in the lungs where gas exchange takes place, become inflamed and filled with fluid. When a person has pneumonia, gas exchange cannot occur properly across the alveoli. Pneumonia can be caused by many things. Infection by bacteria, viruses, fungi, or parasites can cause pneumonia. An injury caused by chemicals or a physical injury to the lungs can also cause pneumonia. Symptoms of pneumonia include cough, chest pain, fever, and difficulty breathing. Treatment depends on the cause of pneumonia. Bacterial pneumonia is treated with antibiotics. Pneumonia is a common illness that affects people in all age groups. It is a leading cause of death among the elderly and people who are chronically and terminally ill. "
respiratory system diseases,T_3346,"Tuberculosis (TB) is a common and often deadly disease caused by a genus of bacterium called Mycobacterium. Tuberculosis most commonly attacks the lungs but can also affect other parts of the body. TB is a chronic disease, but most people who become infected do not develop the full disease. Symptoms include a cough, which usually contains mucus and coughing up blood. The TB bacteria are spread in the air when people who have the disease cough, sneeze, or spit, so it is very contagious. To help prevent the spread of the disease, public health notices, such as the one pictured below ( Figure 1.3), remind people how to stop the spread of the disease. A public health notice from the early 20th century reminded people that TB could be spread very easily. "
respiratory system diseases,T_3347,"Lung cancer is a disease in which the cells found in the lungs grow out of control. The growing mass of cells can form a tumor that pushes into nearby tissues. The tumor will affect how these tissues work. Lung cancer is the most common cause of cancer-related death in men, and the second most common in women. It is responsible for 1.3 million deaths worldwide every year ( Figure 1.4). The most common symptoms are shortness of breath, coughing (including coughing up blood), and weight loss. The most common cause of lung cancer is exposure to tobacco smoke. The inside of a lung showing cancerous tissue. "
respiratory system diseases,T_3348,"Emphysema is a chronic lung disease caused by the breakdown of the lung tissue. Symptoms of emphysema include shortness of breath, especially during exercise, and chronic cough, usually due to cigarette smoking, and wheezing, especially during expiration. Damage to the alveoli ( Figure 1.5), is not curable. Smoking is the leading cause of emphysema. "
respiratory system diseases,T_3349,"Many respiratory diseases are caused by pathogens. A pathogen is an organism that causes disease in another organism. Certain bacteria, viruses, and fungi are pathogens of the respiratory system. The common cold and flu are caused by viruses. The influenza virus that causes the flu is pictured below ( Figure 1.6). Tuberculosis, whooping cough, and acute bronchitis are caused by bacteria. The pathogens that cause colds, flu, and TB can be passed from person to person by coughing, sneezing, and spitting. Illnesses caused by bacteria can be treated with antibiotics. Those caused by viruses cannot. Pollution is another common cause of respiratory disease. The quality of the air you breathe can affect the health of your lungs. Asthma, heart and lung diseases, allergies, and several types of cancers are all linked to air quality. Air pollution is not just found outdoors; indoor air pollution can also be responsible for health problems. Smoking is the major cause of chronic respiratory disease as well as cardiovascular disease and cancer. Exposure to tobacco smoke by smoking or by breathing air that contains tobacco smoke is the leading cause of preventable death in the United States. Regular smokers die about 10 years earlier than nonsmokers do. The Centers for Disease Control and Prevention (CDC) describes tobacco use as ""the single most important preventable risk to human health The lung of a smoker who had emphysema (left). Tar, a sticky, black substance found in tobacco smoke, is evident. Chronic obstructive pulmonary disease (right), is a tobacco-related disease that is characterized by emphysema. This represents the influenza virus that causes the swine flu, or H1N1. The Center for Disease Control and Prevention recommends that children between the ages of 6 months and 19 years get a flu vaccination each year. in developed countries and an important cause of [early] death worldwide."" Simply stated: Stopping smoking can prevent many respiratory diseases. "
respiratory system health,T_3350,"We know that many respiratory illnesses are caused by bacteria or viruses. There are steps you can take to help the spread of these pathogens, and also to prevent you from catching one. Furthermore, many respiratory illnesses are caused by poor habits, such as smoking. Many of the diseases related to smoking are called lifestyle diseases. Lifestyle diseases are diseases that are caused by choices that people make in their daily lives. For example, the choice to smoke can lead to emphysema, cancer and heart disease in later life. But you can make healthy choices instead. There are many things you can do to keep yourself healthy. "
respiratory system health,T_3351,"Cigarette smoking can cause serious diseases, so not smoking or quitting now are the most effective ways to reduce your risk of developing chronic respiratory diseases, such as lung cancer. Avoiding (or stopping) smoking is the single best way to prevent many respiratory and cardiovascular diseases. Also, do your best to avoid secondhand smoke. "
respiratory system health,T_3352,"Eating healthy foods, getting enough sleep, and being active every day can help keep your respiratory system, cardiovascular system and immune system strong. Getting enough exercise makes your lungs stronger and better at giving your body the oxygen it needs. It also helps to boost your body fight germs that could make you sick. These can also, of course, keep your skeletal and muscular systems strong. "
respiratory system health,T_3353,"Washing your hands often, especially after sneezing, coughing, or blowing your nose, helps to protect you and others from diseases. Washing your hands for 20 seconds with soap and warm water can help prevent colds and flu. In one respect, you can think of hand washing as a survival skill. Some viruses and bacteria can live from 20 minutes to two hours or more on surfaces like cafeteria tables, doorknobs, and desks. Washing your hands often can remove many of these pathogens. Never touch your mouth, nose, or eyes without washing your hands. "
respiratory system health,T_3354,"Do not go to school or to other public places when you are sick. You risk spreading your illness to other people. You may also get even sicker if you catch something else. Do not share food and other things that go in the mouth, as in guzzling milk from the carton or double dipping chips. You never know what pathogens can be lurking around. Cover your mouth with a tissue when you cough or sneeze and to dispose of the tissue yourself. No time to grab a tissue. Cough or sneeze into the inside of your elbow instead of your hands. "
respiratory system health,T_3355,"Getting the recommended vaccinations can help prevent diseases, such as whooping cough and flu. In fact, a yearly flu vaccine is recommended for everyone who is at least 6 months of age. The flu vaccine is especially important for people who are at high risk of developing serious complications (like pneumonia) if they get sick with the flu. People who have certain medical conditions including asthma, diabetes, and chronic lung disease, pregnant women, and people younger than 5 years (and especially those younger than 2), and people 65 years and older should also make sure they get the yearly flu vaccine. Seeking medical help for diseases like asthma can help stop the disease from getting worse. If you are unsure if you should go to the doctor, call the doctors office and ask. "
respiratory system organs,T_3356,"Your respiratory system is made up of the tissues and organs that allow oxygen to enter your body and carbon dioxide to leave your body. Organs in your respiratory system include your: Nose. Mouth. Larynx. Pharynx. Lungs. Diaphragm. The organs of the respiratory system move air into and out of the body. These structures are shown below (Figure 1.1). What do you think is the purpose of each of these organs? The nose and the nasal cavity filter, warm, and moisten the air you breathe. The nose hairs and the mucus produced by the cells in the nose catch particles in the air and keep them from entering the lungs. Behind the nasal cavity, air passes through the pharynx, a long tube. Both food and air pass through the pharynx. The larynx, also called the ""voice box,"" is found just below the pharynx. Your voice comes from your larynx. Air from the lungs passes across thin tissues in the larynx and produces sound. The trachea, or windpipe, is a long tube that leads down to the lungs, where it divides into the right and left bronchi. The bronchi branch out into smaller bronchioles in each lung. There is small flap called the epiglottis that covers your trachea when you eat or drink. The muscle controlling the epiglottis is involuntary and prevents food from entering your lungs or wind pipe. The bronchioles lead to the alveoli. Alveoli are the little sacs at the end of the bronchioles (Figure 1.2). They look like little bunches of grapes. Oxygen is exchanged for carbon dioxide in the alveoli. That means oxygen enters the blood, and carbon dioxide moves out of the blood. The gases are exchanged between the blood and alveoli by simple diffusion. The diaphragm is a sheet of muscle that spreads across the bottom of the rib cage. When the diaphragm contracts, the chest volume gets larger, and the lungs take in air. When the diaphragm relaxes, the chest volume gets smaller, and air is pushed out of the lungs. ""Grape-like"" alveoli in the lungs. "
rna,T_3357,"DNA contains the instructions to create proteins, but it does not make proteins itself. DNA is located in the nucleus, which it never leaves, while proteins are made on ribosomes in the cytoplasm. So DNA needs a messenger to bring its instructions to a ribosome located outside of the nucleus. DNA sends out a message, in the form of RNA (ribonucleic acid), describing how to make the protein. There are three types of RNA directly involved in protein synthesis: Messenger RNA ( mRNA) carries the instructions from the nucleus to the cytoplasm. mRNA is produced in the nucleus, as are all RNAs. The other two forms of RNA, ribosomal RNA ( rRNA) and transfer RNA ( tRNA), are involved in the process of ordering the amino acids to make the protein. rRNA becomes part of the ribosome, which is the site of protein synthesis, and tRNA brings an amino acid to the ribosome so it can be added to a growing chain during protein synthesis. There are numerous tRNAs, as each tRNA is specific for an amino acid. The amino acid actually attaches to the tRNA during this process. More about RNAs will be discussed during the Transcription and Translation Concepts. All three RNAs are nucleic acids, made of nucleotides, similar to DNA ( Figure 1.1). The RNA nucleotide is different from the DNA nucleotide in the following ways: RNA contains a different kind of sugar, called ribose. In RNA, the base uracil (U) replaces the thymine (T) found in DNA. RNA is a single strand molecule. A comparison of DNA and RNA, with the bases of each shown. Notice that in RNA, uracil replaces thymine. "
roundworms,T_3362,"The word ""worm"" is not very scientific. This informal term describes animals (usually invertebrates) that have long bodies with no arms or legs. Worms with round, non-segmented bodies are known as nematodes or roundworms ( Figure 1.1). They are classified in the phylum Nematoda, which has over 28,000 known species. Some scientists believe there could be over a million species of Nematodes. Nematodes are slender bilaterally symmetrical worms, typically less than 2.5 mm long. The smallest nematodes are microscopic, while free-living species can reach as much as 5 cm, and some parasitic species are larger still, reaching over a meter in length. The worm body is often covered with ridges, rings, bristles, or other distinctive structures. The radially symmetrical head of a nematode also has distinct features. The head is covered with sensory bristles and, in many cases, solid ""head-shields"" around the mouth region. The mouth has either three or six lips arranged around the mouth opening, which often have a series of teeth on their inner edges. Nematodes can be parasites of plants and animals. "
roundworms,T_3363,"1. Unlike the flatworms, the roundworms have a body cavity with internal organs. 2. A roundworm has a complete digestive system, which includes both a mouth and an anus. This is a significant difference from the incomplete digestive system of flatworms. The roundworm digestive system also include a large digestive organ known as the gut. Digestive enzymes that start to break down food are produced here. There is no stomach, but there is an intestine which produces enzymes that help absorb nutrients. The last portion of the intestine forms a rectum, which expels waste through the anus. 3. Roundworms also have a simple nervous system with a primitive brain. There are four nerves that run the length of the body and are connected from the top to the bottom of the body. At the anterior end of the animal (the head region), the nerves branch from a circular ring which serves as the brain. The head of a nematode has a few tiny sense organs, including chemoreceptors, which sense chemicals. Though still a relatively simple structure, the nervous system of roundworms is very different from that of the cnidarian nerve net. "
roundworms,T_3364,"Roundworms can be free-living organisms, but they are probably best known for their role as significant plant and animal parasites. Most Nematodes are parasitic, with over 16,000 parasitic species described. Heartworms, which cause serious disease in dogs while living in the heart and blood vessels, are a type of roundworm. Roundworms can also cause disease in humans. Elephantiasis, a disease characterized by the extreme swelling of the limbs ( Figure Most parasitic roundworm eggs or larvae are found in the soil and enter the human body when a person picks them up on the hands and then transfers them to the mouth. The eggs or larvae also can enter the human body directly through the skin. The best solution to these diseases is to try to prevent these diseases rather than treat or cure them. Diseases caused by roundworms are more common in developing countries. Many parasitic diseases caused by roundworms result from poor personal hygiene. Contributing factors may include lack of a clean water supply, inadequate sanitation measures, crowded living conditions, combined with a lack of access to health care and low levels of education. "
segmented worms,T_3388,"When you think of worms, you probably picture earthworms. There are actually many types of worms, including flatworms, roundworms, and segmented worms. Earthworms are segmented worms. Segmented worms are in the phylum Annelida, which has over 22,000 known species. These worms are known as the segmented worms because their bodies are segmented, or separated into repeating units. Besides the earthworm, the segmented worms also include leeches and some marine worms. Most segmented worms like the earthworm, feed on dead organic matter. Leeches (Figure 1.1), however, can live in fresh water and suck blood from their animal host. You may have noticed many earthworms in soil. Earthworms support terrestrial ecosystems both as prey and by aerating and enriching soil. "
segmented worms,T_3389,"Segmented worms have a number of characteristic features. 1. The basic form consists of multiple segments, each of which has the same sets of organs and, in most, a pair of parapodia that many species use for locomotion. 2. Segmented worms have a well-developed body cavity filled with fluid. This fluid-filled cavity serves as a hydroskeleton, a supportive structure that helps move the worms muscles. Only the most primitive worms (the flatworms) lack a body cavity. 3. Segmented worms also tend to have organ systems that are more developed than the roundworms or flat- worms. Earthworms, for example, have a complete digestive tract with two openings, as well as an esophagus and intestines. The circulatory system consists of paired hearts and blood vessels. Actually there are five pairs of hearts that pump blood along the two main vessels. And the nervous system consists of the brain and a ventral nerve cord. "
segmented worms,T_3390,The following table compares the three worm phyla (Table 1.1). Phylum Platyhelminthes Nematoda Annelida Common Name Flatworm Roundworm Segmented worm Body Cavity Segmented No Yes Yes No No Yes Digestive System Incomplete Complete Complete 
sex linked inheritance,T_3391,"What determines if a baby is a male or female? Recall that you have 23 pairs of chromosomesand one of those pairs is the sex chromosomes. Everyone has two sex chromosomes. Your sex chromosomes can be X or Y. Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). If a baby inherits an X chromosome from the father and an X chromosome from the mother, what will be the childs sex? The baby will have two X chromosomes, so it will be female. If the fathers sperm carries the Y chromosome, the child will be male. Notice that a mother can only pass on an X chromosome, so the sex of the baby is determined by the father. The father has a 50 percent chance of passing on the Y or X chromosome, so there is a 50 percent chance that a child will be male, and there is a 50 percent chance a child will be female. This 50:50 chance occurs for each baby. A couples first five children could all be boys. The sixth child still has a 50:50 chance of being a girl. One special pattern of inheritance that doesnt fit Mendels rules is sex-linked inheritance, referring to the inher- itance of traits that are located on genes on the sex chromosomes. Since males and females do not have the same sex chromosomes, there will be differences between the sexes in how these sex-linked traitstraits linked to genes located on the sex chromosomesare expressed. Sex-linked traits usually refer to traits due to genes on the X chromosome. One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot tell the difference between red and green. They often see these colors as shades of brown ( Figure 1.1). Boys are much more likely to be colorblind than girls ( Table 1.1). This is because colorblindness is a sex-linked, recessive trait. Boys only have one X chromosome, so if that chromosome carries the gene for colorblindness, they will be colorblind. As girls have two X chromosomes, a girl can have one X chromosome with the colorblind gene and one X chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal gene will mask the recessive colorblind gene. Females with one colorblindness allele and one normal allele are referred to as carriers. They carry the allele but do not express it. How would a female become colorblind? She would have to inherit two genes for colorblindness, which is very unlikely. Many sex-linked traits are inherited in a recessive manner. Xc Xc X (carrier female) Xc Y (colorblind male) X Y X XX (normal female) XY (normal male) According to this Punnett square ( Table 1.1), the son of a woman who carries the colorblindness trait and a male with normal vision has a 50% chance of being colorblind. "
sexually transmitted infections,T_3392,"A sexually transmitted infection (STI) is an infection that spreads through sexual contact. STIs are caused by pathogens, a living thing or virus that causes infection. The pathogens enter the body through the reproductive organs. Many STIs also spread through body fluids, such as blood. For example, a shared tattoo needle is one way an STI could spread. Some STIs can also spread from a mother to her baby during childbirth. STIs are more common in teens and young adults than in older people. One reason is that young people are more likely to take risks. They also may not know how STIs spread. They are likely to believe myths about STIs ( Table Myth If you are sexually active with just one person, you cant get STIs. If you dont have any symptoms, then you dont have an STI. Getting STIs is no big deal, because STIs can be cured with medicine. Fact The only way to avoid the risk of STIs is to practice abstinence from sexual activity. Many STIs do not cause symptoms, especially in fe- males. Only some STIs can be cured with medicine; other STIs cannot be cured. Most STIs are caused by bacteria or viruses. STIs caused by bacteria usually can be cured with drugs called antibiotics. But antibiotics are not effective against viruses. Therefore, STIs caused by viruses are not treated with antibiotics. Other drugs may be used to help control the symptoms of viral STIs, but they cannot be cured. Once you have a viral STI, you are usually infected for life. "
sexually transmitted infections,T_3393,"In the U.S., chlamydia is the most common STI caused by bacteria. Females are more likely than males to develop the infection. Rates of chlamydia among U.S. females in 2006 are shown below ( Figure 1.1). Rates were much higher in teens and young women than in other age groups. Chlamydia may cause a burning feeling during urination. It may also cause a discharge (leaking of fluids) from the vagina or penis. But in many cases it causes no symptoms. As a result, people do not know they are infected, so they dont go to the doctor for help. If chlamydia goes untreated, it may cause more serious problems in females. It may cause infections of the uterus, fallopian tubes, or ovaries. These infections may leave a woman unable to have children. Gonorrhea is another common STI. Gonorrhea may cause pain during urination. It may also cause a discharge from the vagina or penis. On the other hand, some people with gonorrhea have no symptoms. As a result, they dont seek treatment. Without treatment, gonorrhea may lead to infection of other reproductive organs. This can happen in males as well as females. Syphilis is a very serious STI. Luckily, it is less common than chlamydia or gonorrhea. Syphilis usually begins with a small sore on the genitals. This is followed a few months later by a rash and flu-like symptoms. If syphilis is not treated, it may damage the heart, brain, and other organs. It can even cause death. "
sexually transmitted infections,T_3394,"Genital warts are an STI caused by human papilloma virus, or HPV. They are one of the most common STIs in teenagers. HPV infections cannot be cured. But a new vaccine called Gardasil can prevent most HPV infections in females. Many doctors recommend that females between the ages of 9 and 26 years receive the vaccine. Preventing HPV infections in females is important because HPV can also cause cancer of the cervix. A related herpes virus causes cold sores on the lips ( Figure 1.2). Both viruses cause painful blisters. In the case of genital herpes, the blisters are on the penis or around the vaginal opening. The blisters go away on their own, but the virus remains in the body. The blisters may come back repeatedly, especially when a person is under stress. There is no cure for genital herpes. But drugs can help prevent or shorten outbreaks. Researchers are trying to find a vaccine to prevent genital herpes. Hepatitis B is a disease of the liver. It is caused by a virus called hepatitis B, which can be passed through sexual activity. Hepatitis B causes vomiting. It also causes yellowing of the skin and eyes. The disease goes away on its own in some people. Other people are sick for the rest of their lives. In these people, the virus usually damages the liver. It may also lead to liver cancer. Medicines can help prevent liver damage in these people. There is also a vaccine to protect against hepatitis B. HIV stands for ""human immunodeficiency virus."" It is the virus that causes AIDS. HIV and AIDS are described in a previous concept. HIV can spread through sexual contact. It can also spread through body fluids such as blood. There is no cure for HIV infection, and AIDS can cause death, although AIDS can be delayed for several years with medication. Researchers are trying to find a vaccine to prevent HIV infection. "
skeletal system joints,T_3395,"A joint is a point at which two or more bones meet. There are three main types of joints in the body: 1. Fixed joints do not allow any bone movement. Many of the joints in your skull are fixed ( Figure 1.1). There are eight bones that fuse together to form the cranium. The joints between these bones do not allow movement, which helps protect the brain. 2. Partly movable joints allow only a little movement. Your backbone has partly movable joints between the vertebrae ( Figure 1.2). The skull has fixed joints. Fixed joints do not allow any movement of the bones, which protects the brain from injury. 3. Movable joints allow the most movement. Movable joints are also the most common type of joint in your body. Your fingers, toes, hips, elbows, and knees all provide examples of movable joints. The surfaces of bones at movable joints are covered with a smooth layer of cartilage. The cartilage reduces friction between the bones. Ligaments often cross a joint, holding two nones together. For example, there are numerous ligaments connecting the leg bones across the knee joint. "
skeletal system joints,T_3396,"Four types of movable joints are discussed here. 1. In a ball-and-socket joint, the ball-shaped surface of one bone fits into the cup-like shape of another. Exam- ples of a ball-and-socket joint include the hip ( Figure 1.3) and the shoulder. 2. In a hinge joint, the ends of the bones are shaped in a way that allows motion in two directions, forward and backward. Examples of hinge joints are the knees ( Figure 1.4) and elbows. 3. The pivot joint ( Figure 1.5) only allows rotating movement. An example of a pivot joint is the joint between the radius and ulna that allows you to turn the palm of your hand up and down. 4. A gliding joint is a joint which allows only gliding movement. The gliding joint allows one bone to slide over the other. The gliding joint in your wrist allows you to flex your wrist. It also allows you to make very small side-to-side motions. There are also gliding joints in your ankles. "
skin,T_3397,"Did you know that you see the largest organ in your body every day? You wash it, dry it, cover it up to stay warm, and uncover it to cool off. Yes, your skin is your bodys largest organ. Your skin is part of your integumentary system ( Figure 1.1), which is the outer covering of your body. The integumentary system is made up of your skin, hair, and nails. Skin acts as a barrier that stops water and other things, like soap and dirt, from getting into your body. "
skin,T_3398,"The skin has many important functions. The skin: Provides a barrier. It keeps organisms that could harm the body out. It stops water from entering or leaving the body. Controls body temperature. It does this by making sweat (or perspiration), a watery substance that cools the body when it evaporates. Gathers information about your environment. Special nerve endings in your skin sense heat, pressure, cold, and pain. Helps the body get rid of some types of waste, which are removed in sweat. Acts as a sun block. A pigment called melanin blocks sunlight from getting to deeper layers of skin cells, which are easily damaged by sunlight. "
skin,T_3399,"Your skin is always exposed to your external environment, so it gets cut, scratched, and worn down. You also naturally shed many skin cells every day. Your body replaces damaged or missing skin cells by growing more of them. Did you know that the layer of skin you can see is actually dead? As the dead cells are shed or removed from the upper layer, they are replaced by the skin cells below them. Two different layers make up the skin: the epidermis and the dermis ( Figure 1.2). A fatty layer lies under the dermis, but it is not part of your skin. "
skin,T_3400,"The epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the bodys surface. Although the top layer of epidermis is only about as thick as a sheet of paper, it is made up of 25 to 30 layers of cells. The epidermis also contains cells that produce melanin. Melanin is the brownish pigment that gives skin and hair their color. Melanin-producing cells are found in the bottom layer of the epidermis. The epidermis does not have any blood vessels. The lower part of the epidermis receives blood by diffusion from blood vessels of the dermis. Skin is made up of two layers, the epider- mis on top and the dermis below. The tissue below the dermis is called the hy- podermis, but it is not part of the skin. "
skin,T_3401,"The dermis is the layer of skin directly under the epidermis. It is made of a tough connective tissue. The dermis contains hair follicles, sweat glands, oil glands, and blood vessels ( Figure 1.2). It also holds many nerve endings that give you your sense of touch, pressure, heat, and pain. Do you ever notice how your hair stands up when you are cold or afraid? Tiny muscles in the dermis pull on hair follicles which cause hair to stand up. The resulting little bumps in the skin are commonly called ""goosebumps"" ( Figure 1.3). "
skin,T_3402,"Glands and hair follicles open out into the epidermis, but they start in the dermis. Oil glands ( Figure 1.2) release, or secrete an oily substance, called sebum, into the hair follicle. Sebum waterproofs hair and the skin surface to prevent them from drying out. It can also stop the growth of bacteria on the skin. It is odorless, but the breakdown of sebum by bacteria can cause odors. If an oil gland becomes plugged and infected, it develops into a pimple. Up to 85% of teenagers get pimples, which usually go away by adulthood. Frequent washing can help decrease the amount of sebum on the skin. Sweat glands ( Figure 1.2) open to the skin surface through skin pores. They are found all over the body. Evaporation of sweat from the skin surface helps to lower skin temperature. The skin also releases excess water, salts, sugars, and other wastes, such as ammonia and urea, in sweat. The Integumentary System Song can be heard at  . Goosebumps are caused by tiny mus- cles in the dermis that pull on hair folli- cles, which causes the hairs to stand up straight. "
smooth skeletal and cardiac muscles,T_3403,"The muscular system consists of all the muscles in the body. This is the body system that allows us to move. You also depend on many muscles to keep you alive. Your heart, which is mostly muscle, pumps blood around your body. Each muscle in the body is made up of cells called muscle fibers. Muscle fibers are long, thin cells that can do something that other cells cannot dothey are able to get shorter. Shortening of muscle fibers is called contraction. Muscle fibers can contract because they are made of proteins, called actin and myosin, that form long filaments (or fibers). When muscles contract, these protein filaments slide or glide past one another, shortening the length of the cell. When your muscles relax, the length extends back to the previous position. Nearly all movement in the body is the result of muscle contraction. You can control some muscle movements. However, certain muscle movements happen without you thinking about them. Muscles that are under your conscious control are called voluntary muscles. Muscles that are not under your conscious control are called involuntary muscles. Muscle tissue is one of the four types of tissue found in animals. There are three different types of muscle in the body ( Figure 1.1): 1. Skeletal muscle is made up of voluntary muscles, usually attached to the skeleton. Skeletal muscles move the body. They can also contract involuntarily by reflexes. For example, you can choose to move your arm, but your arm would move automatically if you were to burn your finger on a stove top. This voluntary contraction begins with a thought process. A signal from your brain tells your muscles to contract or relax. Quickly contract and relax the muscles in your fingers a few times. Think about how quickly these signals must travel throughout your body to make this happen. 2. Smooth muscle is composed of involuntary muscles found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. These muscles push materials like food or blood through organs. Unlike skeletal muscle, smooth muscle can never be under your control. 3. Cardiac muscle is also an involuntary muscle, found only in the heart. The cardiac muscle fibers all contract together, generating enough force to push blood throughout the body. What would happen if this muscle was under conscious or voluntary control? There are three types of muscles in the body: cardiac, skeletal, and smooth. "
sources of water pollution,T_3407,"While to many people clean water may seem limitless and everywhere, to many others this is not so. Water pollution is a serious issue facing hundreds of millions of people world-wide, having harmful effects on the lives of those people. Water is not in unlimited supply and cannot just be made fresh when it is wanted. Water is actually a limited resource, and for many people, fresh, unpolluted water is hard to find. A limited resource is one that we use faster than we can remake it. It is a resource that can be used up. Water pollution happens when contaminants enter water bodies. Contaminants are any substances that harm the health of the environment or humans. Most contaminants enter the water because of humans. Surface water (river or lake) can be exposed to and contaminated by acid rain, storm water runoff, pesticide runoff, and industrial waste. This water is cleaned somewhat by exposure to sunlight, aeration, and microorganisms in the water. Groundwater (private wells and some public water supplies) generally takes longer to become contaminated, but the natural clean- ing process also may take much longer. Groundwater can be contaminated by disease-producing pathogens, careless disposal of hazardous household chemical-containing products, agricultural chemicals, and leaking underground storage tanks. Water pollution can cause harmful effects to ecology and human health. Shown is the pollution in Jakarta, Indonesia. Natural events, like storms, volcanic eruptions and earthquakes can cause major changes in water quality. But human-caused contaminants have a much greater impact on the quality of the water supply. Water is considered polluted either when it does not support a human use, like clean drinking water, or a use for other animals and plants. The overgrowth of algae, known as an algal bloom, can result from the runoff of fertilizer into bodies of water. This excess of nutrients allows the algae to grow beyond control, bring harm to the rest of the ecosystem. The main sources of water pollution can be grouped into two categories: Point source pollution results from the contaminants that enter a waterway or water body through a single site. Examples of this include untreated sewage, wastewater from a sewage treatment plant, and leaking underground tanks. Nonpoint source pollution is contamination that does not come from a single point source. Instead, it happens when there is a buildup of small amounts of contaminants that collect from a large area. Examples of this include fertilizer runoff from many farms flowing into groundwater or streams. "
taste and smell,T_3422,"The senses of taste and smell are more complicated than many people might think and have a surprisingly large impact on behavior, perception and overall health. Imagine your sense of smell disappearing as you age. Though this doesnt usually happen, it could provide clues about diseases of the nervous system. What about differences in taste? Do all foods taste the same to all people? Are there some foods you would never eat because you dont like the taste? Does this food taste good to other people? Genetic differences in taste could help predict what we eat, how well our metabolism works, and even whether or not were overweight. These two senses actually work together to provide some of the basic sensations of everyday life. "
taste and smell,T_3423,"Your sense of taste is controlled by sensory neurons, or nerve cells, on your tongue that sense the chemicals in food. The neurons are grouped in bundles within taste buds. Each taste bud actually has a pore that opens out to the surface of the tongue enabling molecules and ions taken into the mouth to reach the receptor cells inside. There are five different types of taste neurons on the tongue. Each type detects a different taste. The tastes are: 1. Sweet, which is produced by the presence of sugars, such as the common table sugar sucrose, and a few other substances. 2. Salty, which is produced primarily by the presence of sodium ions. Common salt is sodium chloride, NaCl. The use of salt can donate the sodium ion producing this taste. 3. Sour, which is the taste that detects acidity. The most common food group that contains naturally sour foods is fruit, such as lemon, grape, orange, and sometimes melon. Children show a greater enjoyment of sour flavors than adults, and sour candy such as Lemon Drops, Shock Tarts and sour versions of Skittles and Starburst, is popular. Many of these candies contain citric acid. 4. Bitter is an unpleasant, sharp, or disagreeable taste. Common bitter foods and beverages include coffee, unsweetened cocoa, beer (due to hops), olives, and citrus peel. 5. Umami, which is a meaty or savory taste. This taste can be found in fish, shellfish, cured meats, mushrooms, cheese, tomatoes, grains, and beans. A single taste bud contains 50100 taste cells representing all 5 taste sensations. A stimulated taste receptor cell triggers action potentials in a nearby sensory neuron, which send messages to the brain about the taste. The brain then decides what tastes you are sensing. "
taste and smell,T_3424,"Your sense of smell also involves sensory neurons that sense chemicals. The neurons are found in the nose, and they detect chemicals in the air. Unlike taste neurons, which can detect only five different tastes, the sensory neurons in the nose can detect thousands of different odors. Have you ever noticed that you lose your sense of taste when your nose is stuffed up? Thats because your sense of smell greatly affects your ability to taste food. As you eat, molecules of food chemicals enter your nose (actually your nasal cavity). You experience the taste and smell at the same time. Being able to smell as well as taste food greatly increases the number of different flavors you are able to sense. For example, you can use your sense of taste alone to learn that a food is sweet, but you have to also use your sense of smell to learn that the food tastes like strawberry cheesecake. Specific scents are often associated with our memories of places and events. Thats because scents are more novel or specific than shapes or other things you might see. So an odor similar to that of your grandmothers kitchen or pantry might be more quickly associated with your memories of that place than a similar sight, which might be more generalized. "
the carbon cycle,T_3428,"Carbon is one of the most common elements found in living organisms. Chains of carbon molecules form the backbones of many organic molecules, such as carbohydrates, proteins, and lipids. Carbon is constantly cycling between living organisms and the atmosphere ( Figure 1.1). The cycling of carbon occurs through the carbon cycle. Living organisms cannot make their own carbon, so how is carbon incorporated into living organisms? In the atmosphere, carbon is in the form of carbon dioxide gas (CO2 ). Recall that plants and other producers capture the carbon dioxide and convert it to glucose (C6 H12 O6 ) through the process of photosynthesis. Then as animals eat plants or other animals, they gain the carbon from those organisms. The chemical equation of photosynthesis is 6CO2 + 6H2 O  C6 H12 O6 + 6O2 . How does this carbon in living things end up back in the atmosphere? Remember that we breathe out carbon dioxide. This carbon dioxide is generated through the process of cellular respiration, which has the reverse chemical reaction as photosynthesis. That means when our cells burn food (glucose) for energy, carbon dioxide is released. We, like all animals, exhale this carbon dioxide and return it back to the atmosphere. Also, carbon is released to the atmosphere as an organism dies and decomposes. Cellular respiration and photosynthesis can be described as a cycle, as one uses carbon dioxide (and water) and makes oxygen (and glucose), and the other uses oxygen (and glucose) and makes carbon dioxide (and water). The carbon cycle. The cycling of car- bon dioxide in photosynthesis and cellular respiration are main components of the carbon cycle. Carbon is also returned to the atmosphere by the burning of organic matter (combustion) and fossil fuels and decomposition of organic matter. "
the carbon cycle,T_3429,"Millions of years ago, there were so many dead plants and animals that they could not completely decompose before they were buried. They were covered over by soil or sand, tar or ice. These dead plants and animals are organic matter made out of cells full of carbon-containing organic compounds (carbohydrates, lipids, proteins and nucleic acids). What happened to all this carbon? When organic matter is under pressure for millions of years, it forms fossil fuels. Fossil fuels are coal, oil, and natural gas. When humans dig up and use fossil fuels, we have an impact on the carbon cycle ( Figure 1.2). This carbon is not recycled until it is used by humans. The burning of fossil fuels releases more carbon dioxide into the atmosphere than is used by photosynthesis. So, there is more carbon dioxide entering the atmosphere than is coming out of it. Carbon dioxide is known as a greenhouse gas, since it lets in light energy but does not let heat escape, much like the panes of a greenhouse. The increase of greenhouse gasses in the atmosphere is contributing to a global rise in Earths temperature, known as global warming or global climate change. "
the nitrogen cycle,T_3430,"Like water and carbon, nitrogen is also repeatedly recycled through the biosphere. This process is called the nitrogen cycle. Nitrogen is one of the most common elements in living organisms. It is important for creating both proteins and nucleic acids, like DNA. The air that we breathe is mostly nitrogen gas (N2 ), but, unfortunately, animals and plants cannot use the nitrogen when it is a gas. In fact, plants often die from a lack of nitrogen even through they are surrounded by plenty of nitrogen gas. Nitrogen gas (N2 ) has two nitrogen atoms connected by a very strong triple bond. Most plants and animals cannot use the nitrogen in nitrogen gas because they cannot break that triple bond. In order for plants to make use of nitrogen, it must be transformed into molecules they can use. This can be accomplished several different ways ( Figure 1.1). Lightning: When lightening strikes, nitrogen gas is transformed into nitrate (NO3  ) that plants can use. Nitrogen fixation: Special nitrogen-fixing bacteria can also transform nitrogen gas into useful forms. These bacteria live in the roots of plants in the pea family. They turn the nitrogen gas into ammonium (NH4 + ) (a process called ammonification). In water environments, bacteria in the water can also fix nitrogen gas into ammonium. Ammonium can be used by aquatic plants as a source of nitrogen. Nitrogen also is released to the environment by decaying organisms or decaying wastes. These wastes release nitrogen in the form of ammonium. Ammonium in the soil can be turned into nitrate by a two-step process completed by two different types of bacteria. In the form of nitrate, nitrogen can be used by plants through the process of assimilation. It is then passed along to animals when they eat the plants. "
the nitrogen cycle,T_3431,"Turning nitrate back into nitrogen gas, the process of denitrification, happens through the work of denitrifying bacteria. These bacteria often live in swamps and lakes. They take in the nitrate and release it back to the atmosphere as nitrogen gas. Just like the carbon cycle, human activities impact the nitrogen cycle. These human activities include the burning of fossil fuels, which release nitrogen oxide gasses into the atmosphere. Releasing nitrogen oxide back into the atmosphere leads to problems like acid rain. "
the water cycle,T_3432,"Whereas energy flows through an ecosystem, water and elements like carbon and nitrogen are recycled. Water and nutrients are constantly being recycled through the environment. This process through which water or a chemical element is continuously recycled in an ecosystem is called a biogeochemical cycle. This recycling process involves both the living organisms (biotic components) and nonliving things (abiotic factors) in the ecosystem. Through biogeochemical cycles, water and other chemical elements are constantly being passed through living organisms to non-living matter and back again, over and over. Three important biogeochemical cycles are the water cycle, carbon cycle, and nitrogen cycle. The biogeochemical cycle that recycles water is the water cycle. The water cycle involves a series of interconnected pathways involving both the biotic and abiotic components of the biosphere. Water is obviously an extremely important aspect of every ecosystem. Life cannot exist without water. Many organisms contain a large amount of water in their bodies, and many live in water, so the water cycle is essential to life on Earth. Water continuously moves between living organisms, such as plants, and non-living things, such as clouds, rivers, and oceans ( Figure The water cycle does not have a real starting or ending point. It is an endless recycling process that involves the oceans, lakes and other bodies of water, as well as the land surfaces and the atmosphere. The steps in the water cycle are as follows, starting with the water in the oceans: 1. Water evaporates from the surface of the oceans, leaving behind salts. As the water vapor rises, it collects and is stored in clouds. 2. As water cools in the clouds, condensation occurs. Condensation is when gases turn back into liquids. 3. Condensation creates precipitation. Precipitation includes rain, snow, hail, and sleet. The precipitation allows the water to return again to the Earths surface. 4. When precipitation lands on land, the water can sink into the ground to become part of our underground water reserves, also known as groundwater. Much of this underground water is stored in aquifers, which are porous layers of rock that can hold water. "
the water cycle,T_3433,"Most precipitation that occurs over land, however, is not absorbed by the soil and is called runoff. This runoff collects in streams and rivers and eventually flows back into the ocean. "
the water cycle,T_3434,"Water also moves through the living organisms in an ecosystem. Plants soak up large amounts of water through their roots. The water then moves up the plant and evaporates from the leaves in a process called transpiration. The process of transpiration, like evaporation, returns water back into the atmosphere. "
timeline of evolution,T_3435,"For life to evolve from simple single-celled organisms to many millions of species of prokaryotic species to simple eukaryotic species to all the protists, fungi, plants, and animals, took some time. Well over 3 billion years. "
timeline of evolution,T_3436,"How old is Earth? How was it formed? How did life begin on Earth? These questions have fascinated scientists for centuries. During the 1800s, geologists, paleontologists, and naturalists found several forms of physical evidence that confirmed that Earth is very old. The evidence includes: Fossils of ancient sea life on dry land far from oceans. This supported the idea that the Earth changed over time and that some dry land today was once covered by oceans. The many layers of rock. When people realized that rock layers represent the order in which rocks and fossils appeared, they were able to trace the history of Earth and life on Earth. Indications that volcanic eruptions, earthquakes, and erosion that happened long ago shaped much of the Earths surface. This supported the idea of an older Earth. The Earth is at least as old as its oldest rocks. The oldest rock minerals found on Earth so far are crystals that are at least 4.404 billion years old. These tiny crystals were found in Australia. Likewise, Earth cannot be older than the solar system. The oldest possible age of Earth is 4.57 billion years old, the age of the solar system. Therefore, the age of Earth is between 4.4 and 4.57 billion years. "
timeline of evolution,T_3437,Geologists and other Earth scientists use geologic time scales to describe when events happened in the history of Earth. The time scales can be used to show when both geologic events and events affecting plant and animal life occurred. The geologic time scale pictured below ( Figure 1.1) illustrates the timing of events like: Earthquakes. Volcanic eruptions. Major erosion. Meteorites hitting Earth. The first signs of life forms. Mass extinctions. 
timeline of evolution,T_3438,"Life on Earth began about 3.5 to 4 billion years ago. The first life forms were single-celled organisms similar to bacteria. These first life forms were, of course, very basic, and this then allowed for the evolution of more complex life forms. The first multicellular organisms did not appear until about 610 million years ago. Many different types of organisms evolved during the next ten million years, in an event called the Cambrian Explosion. This sudden burst of evolution may have been caused by some environmental changes that made the Earths environment more suitable for a wider variety of life forms. Plants and fungi did not appear until roughly 500 million years ago. They were soon followed by arthropods (insects and spiders). Next came the amphibians about 300 million years ago, followed by mammals around 200 million years ago and birds around 100 million years ago. Even though large life forms have been very successful on Earth, most of the life forms on Earth today are still prokaryotessmall, relatively simple single-celled organisms. As it is difficult to identify, observe and study such small forms of life, most of these organisms remain unknown to scientists. Advancing technologies, however, do allow for the identification and study of such organisms. Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; in fact, it is estimated that 99% of the species that have ever lived on Earth no longer exist. The basic timeline of a 4.6 billion-year-old Earth includes the following: About 3.5 - 3.8 billion years of simple cells (prokaryotes). 3 billion years of photosynthesis. 2 billion years of complex cells (eukaryotes). 1 billion years of multicellular life. 600 million years of simple animals. 570 million years of arthropods (ancestors of insects, arachnids and crustaceans). 550 million years of complex animals. 500 million years of fish and proto-amphibians. 475 million years of land plants. 400 million years of insects and seeds. 360 million years of amphibians. 300 million years of reptiles. 200 million years of mammals. 150 million years of birds. 130 million years of flowers. 65 million years since the non-avian dinosaurs died out. 2.5 million years since the appearance of Homo. 200,000 years since the appearance of modern humans. 25,000 years since Neanderthals died out. "
touch,T_3439,"When you look at the prickly cactus pictured below ( Figure 1.1), does the word ""ouch"" come to mind? Touching the cactus would be painful. Touch is the sense of pain, pressure, or temperature. Touch depends on sensory neurons, or nerve cells, in the skin. The skin on the palms of the hands, soles of the feet, and face has the most sensory neurons and is especially sensitive to touch. The tongue and lips are very sensitive to touch as well. Neurons that sense pain are also found inside the body in muscles, joints, and organs. If you have a stomach ache or pain from a sprained ankle, its because of these sensory neurons found inside of your body. The following example shows how messages about touch travel from sensory neurons to the brain, as well as how the brain responds to the messages. Suppose you wanted to test the temperature of the water in a lake before jumping in. You might stick one bare foot in the water. Neurons in the skin on your foot would sense the temperature of the water and send a message about it to your central nervous system. The frontal lobe of the cerebrum would process the information. It might decide that the water is really cold and send a message to your muscles to pull your foot out of the water. In some cases, messages about pain or temperature dont travel all the way to and from the brain. Instead, they travel only as far as the spinal cord, and the spinal cord responds to the messages by giving orders to the muscles. This allows you to respond to pain more quickly. When messages avoid the brain in this way, it forms a reflex arc, like the one shown below ( Figure 1.2). "
touch,T_3440,"Our sense of touch is controlled by a huge network of nerve endings and touch receptors. This system is responsible for all the sensations we feel, including cold, hot, smooth, rough, pressure, tickle, itch, pain, vibrations, and more. There are four main types of receptors: mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors. Mechanoreceptors perceive sensations such as pressure, vibrations, and texture. Your brain gets an enormous amount of information about the texture of objects through your fingertips because the ridges that make up your fingerprints are full of these sensitive receptors. Thermoreceptors perceive sensations related to the temperature of objects. There are two basic categories of thermoreceptors: hot receptors and cold receptors. The highest concentration of thermoreceptors can be found in the face and ears. Pain receptors, or nociceptor detect pain or stimuli that can or does cause damage to the skin and other tissues of the body. There are over three million pain receptors throughout the body, found in skin, muscles, bones, blood vessels, and some organs. Proprioceptors detect the position of different parts of the body in relation to each other and the surrounding environment. These receptors are found in joints, tendons and muscles, and allow us to do fundamental things such as feeding or clothing ourselves. "
transcription of dna to rna,T_3444,"DNA is located in the nucleus. Proteins are made on ribosomes in the cytoplasm. Remember that information in a gene is converted into mRNA, which carries the information to the ribosome. In the nucleus, mRNA is created by using the DNA in a gene as a template. A template is a model provided for others to copy. The process of constructing an mRNA molecule from DNA is known as transcription ( Figure 1.1 and Figure of double stranded DNA. In transcription, only one strand of DNA is used as a template. First, the double helix of DNA unwinds and an enzyme, RNA Polymerase, builds the mRNA using the DNA as a template. The nucleotides follow basically the same base pairing rules as in DNA to form the correct sequence in the mRNA. This time, however, uracil (U) pairs with each adenine (A) in the DNA. For example, a DNA sequence ACGGGTAAGG will be transcribed into the mRNA sequence UGCCCAUUCC. In this manner, the information of the DNA is passed on to the mRNA. The mRNA will carry this code to the ribosomes to tell them how to make a protein. As not all genes are used in every cell, a gene must be ""turned on"" or expressed when the gene product is needed by the cell. Only the information in a gene that is being expressed is transcribed into an mRNA. Transcription is when RNA is created from a DNA template. Each gene (a) contains triplets of bases (b) that are transcribed into RNA (c). Every triplet in the DNA, or codon in the mRNA, encodes for a unique amino acid. Base-pairing ensures the accuracy of transcription. Notice how the helix must unwind for transcription to take place. The new mRNA is shown in green. "
translation of rna to protein,T_3445,"The mRNA, which is transcribed from the DNA in the nucleus, carries the directions for the protein-making process. mRNA tells the ribosome ( Figure 1.1) how to create a specific protein. Ribosomes translate RNA into a protein with a specific amino acid sequence. The tRNA binds and brings to the ribosome the amino acid encoded by the mRNA. The process of reading the mRNA code in the ribosome to make a protein is called translation ( Figure 1.2): the mRNA is translated from the language of nucleic acids (nucleotides) to the language of proteins (amino acids). Sets of three bases, called codons, are read in the ribosome, the organelle responsible for making proteins. This summary of how genes are ex- pressed shows that DNA is transcribed into RNA, which is translated, in turn, to protein. The one letter code represents amino acids. The following are the steps involved in translation: mRNA travels to the ribosome from the nucleus. The following steps occur in the ribosome: The base code in the mRNA determines the order of the amino acids in the protein. The genetic code in mRNA is read in words of three letters (triplets), called codons. Each codon codes for an amino acid. There are 20 amino acids used to make proteins, and different codons code for different amino acids. For example, GGU codes for the amino acid glycine, while GUC codes for valine. tRNA reads the mRNA code and brings a specific amino acid to attach to the growing chain of amino acids. The anticodon on the tRNA binds to the codon on the mRNA. Each tRNA carries only one type of amino acid and only recognizes one specific codon. For example, a GGC anticodon will bind to a CCG codon, and a CGA anticodon will bind to a GCU codon. tRNA is released from the amino acid. Three codons, UGA, UAA, and UAG, indicate that the protein should stop adding amino acids. They are called stop codons and do not code for an amino acid. Once tRNA comes to a stop codon, the protein is set free from the ribosome. The following chart ( Figure 1.3) is used to determine which amino acids correspond to which codons. "
types of echinoderms,T_3459,"The echinoderms can be divided into two major groups: 1. Eleutherozoa are the echinoderms that can move. This group includes the starfish and most other echinoderms. 2. Pelmatozoa are the immobile echinoderms. This group includes crinoids, such as the feather stars. Listed below are the four main classes of echinoderms present in the Eleutherozoa Group ( Table 1.1). Class Asteroidea Ophiuroidea Representative Organisms Starfish and asteroids Brittle stars ( Figure 1.1) Echinoidea Sea urchins and sand dollars Holothuroidea Sea cucumbers Characteristics Capture prey for their own food. Bottom feeders with long, narrow, flexible arms that allow relatively fast movement. Have movable spines which are used for movement, defense, and sensing the environment. Armless, elongated, generally soft- "
types of echinoderms,T_3460,"Echinoderms are spread all over the world at almost all depths, latitudes, and environments in the ocean. Most feather stars (crinoids) live in shallow water. In the deep ocean, sea cucumbers are common, sometimes making up 90% of the organisms. Most echinoderms, however, are found in reefs just lying beneath the surface of the water. No echinoderms are found in freshwater habitats or on land. This makes Echinodermata the largest animal phylum to only have ocean-based species. "
types of echinoderms,T_3461,"While almost all echinoderms live on the sea floor, some sea-lilies can swim at great speeds for brief periods of time, and a few sea cucumbers are fully floating. Some echinoderms find other ways of moving. For example, crinoids attach themselves to floating logs, and some sea cucumbers move by attaching to the sides of fish. On the underside side of a sea star, there are hundreds of tiny feet usually arranged into several rows on each ray of the star. These are called tube feet, or podia, and are filled with seawater in most echinoderms. The water vascular system within the body of the animal is also filled with seawater. By expanding and contracting chambers within the water vascular system, the echinoderm can force water into certain tube feet to extend them. The animal has muscles in the tube feet, which are used to retract them. By expanding and retracting the right tube feet in the proper order, the animal can walk. "
types of echinoderms,T_3462,Sea cucumbers at National Geographic http://animals.nationalgeographic.com/animals/invertebrates/sea-cucu 1. Where do sea cucumbers live? 2. How do sea cucumbers eat? 
types of nutrients,T_3467,"Carbohydrates, proteins, and lipids contain energy. When your body digests food, it breaks down the molecules of these nutrients. This releases the energy so your body can use it. "
types of nutrients,T_3468,"Carbohydrates are nutrients that include sugars, starches, and fiber. There are two types of carbohydrates: simple and complex. Pictured below are some foods that are good sources of carbohydrates ( Figure 1.1). "
types of nutrients,T_3469,"Sugars are small, simple carbohydrates that are found in foods such as fruits and milk. The sugar found in fruits is called fructose. The sugar found in milk is called lactose. These sugars are broken down by the body to form glucose (C6 H12 O6 ), the simplest sugar of all. Up to the age of 13 years, you need about 130 grams of carbohydrates a day. Most of the carbohydrates should be complex. They are broken down by the body more slowly than simple carbohydrates. There- fore, they provide energy longer and more steadily. Where does glucose come from? Recall that glucose is the product of photosynthesis, so some organisms such as plants are able to make their own glucose. As animals cannot photosynthesize, they must eat to obtain carbohydrates. Through the process of cellular respiration, glucose is converted by cells into energy that is usable by the cell (ATP). "
types of nutrients,T_3470,"Starch is a large, complex carbohydrate made of thousands of glucose units (monomers) joined together. Starches are found in foods such as vegetables and grains. Starches are broken down by the body into sugars that provide energy. Breads and pasta are good sources of complex carbohydrates. Fiber is another type of large, complex carbohydrate that is partly indigestible. Unlike sugars and starches, fiber does not provide energy. However, it has other important roles in the body. For example, fiber is important for maintaining the health of your gastrointestinal tract. Eating foods high in fiber also helps fill you up without providing too many calories. Most fruits and vegetables are high in fiber. Some examples are pictured below ( Figure 1.2). "
types of nutrients,T_3471,"Proteins are nutrients made up of smaller molecules called amino acids. Recall that there are 20 different amino acids arranged like ""beads on a string"" to form proteins. These amino acid chains then fold up into a three- dimensional molecule, giving the protein a specific function. Proteins have several important roles in the body. For example, proteins make up antibodies, muscle fibers and enzymes that help control cell and body processes. You need to make sure you have enough protein in your diet to obtain the necessary amino acids to make your proteins. Between the ages of 9 and 13 years, girls need about 26 grams of fiber per day, and boys need about 31 grams of fiber per day. If you eat more than you need for these purposes, the extra protein is used for energy. The image below shows how many grams of protein you need each day ( Figure 1.3). It also shows some foods that are good sources of protein. "
types of nutrients,T_3472,"Lipids are nutrients, such as fats that store energy. Lipids also have several other roles in the body. For example, lipids protect nerves and make up the membranes that surround cells. Fats are one type of lipid. Stored fat gives your body energy to use for later. Its like having money in a savings account: its there in case you need it. Stored fat also cushions and protects internal organs. In addition, it insulates the body. It helps keep you warm in cold weather. Between the ages of 9 and 13 years, you need about 34 grams of proteins a day. Seafood and eggs are other good sources of protein. There are two main types of fats, saturated and unsaturated. 1. Saturated fats can be unhealthy, even in very small amounts. They are found mainly in animal foods, such as meats, whole milk, and eggs. So even though these foods are good sources of proteins, they should be eaten in limited amounts. Saturated lipids increase cholesterol levels in the blood. Too much cholesterol in the blood Another type of lipid is called trans fat. Trans fats are manufactured and added to certain foods to keep them fresher for longer. Foods that contain trans fats include cakes, cookies, fried foods, and margarine. Eating foods that contain trans fats increases the risk of heart disease. Beginning with Denmark in 2003, many nations now limit the amount of trans fat that can be in food products or ban these products all together. On January 1, 2008, Calgary became the first city in Canada to ban trans fats from restaurants and fast food chains. Beginning in 2010, California banned trans fats from restaurant products, and in 2011, from all retail baked goods. "
urinary system,T_3473,"Sometimes, the urinary system ( Figure 1.1) is called the excretory system. But the urinary system is only one part of the excretory system. Recall that the excretory system is also made up of the skin, lungs, and large intestine, as well as the kidneys. The urinary system is the organ system that makes, stores, and gets rid of urine. "
urinary system,T_3474,"1. As you can see above ( Figure 1.1), the kidneys are two bean-shaped organs. Kidneys filter and clean the blood and form urine. They are about the size of your fists and are found near the middle of the back, just below your ribcage. 2. Ureters are tube-shaped and bring urine from the kidneys to the urinary bladder. 3. The urinary bladder is a hollow and muscular organ. It is shaped a little like a balloon. It is the organ that collects urine. 4. Urine leaves the body through the urethra. The kidneys filter the blood that passes through them, and the urinary bladder stores the urine until it is released from the body. "
urinary system,T_3475,"Urine is a liquid that is formed by the kidneys when they filter wastes from the blood. Urine contains mostly water, but it also contains salts and nitrogen-containing molecules. The amount of urine released from the body depends on many things. Some of these include the amount of fluid and food a person consumes and how much fluid they have lost from sweating and breathing. Urine ranges from colorless to dark yellow but is usually a pale yellow color. Light yellow urine contains mostly water. The darker the urine, the less water it contains. The urinary system also removes a type of waste called urea from your blood. Urea is a nitrogen-containing molecule that is made when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea and other wastes are carried in the bloodstream to the kidneys, where they are removed and form urine. "
vision correction,T_3488,You probably know people who need eyeglasses or contact lenses to see clearly. Maybe you need them yourself. Lenses are used to correct vision problems. Two of the most common vision problems are myopia and hyperopia. 
vision correction,T_3489,"Myopia is also called nearsightedness. It affects about one third of people. People with myopia can see nearby objects clearly, but distant objects appear blurry. The picture below shows how a person with myopia might see two boys that are a few meters away ( Figure 1.1). In myopia, the eye is too long. Below, you can see how images are focused on the retina of someone with myopia ( Figure 1.2). Myopia is corrected with a concave lens, which curves inward like the inside of a bowl. The lens changes the focus, so images fall on the retina as they should. Generally, nearsightedness first occurs in school-age children. There is some evidence that myopia is inherited. If one or both of your parents need glasses, there is an increased chance that you will too. Individuals who spend a lot of time reading, working or playing at a computer, or doing other close visual work may also be more likely to develop nearsightedness. Because the eye continues to grow during childhood, myopia typically progresses until On the left, you can see how a person with normal vision sees two boys. The right image shows how a person with myopia sees the boys. The eye of a person with myopia is longer than normal. As a result, images are focused in front of the retina (top left). A concave lens is used to correct myopia to help focus images on the retina (top right). Farsightedness, or hyperopia, oc- curs when objects are focused in back of the retina (bottom left). It is corrected with a convex lens (bottom right). about age 20. However, nearsightedness may also develop in adults due to visual stress or health conditions such as diabetes. A common sign of nearsightedness is difficulty seeing distant objects like a movie screen or the TV, or the whiteboard or chalkboard in school. Eyeglasses or contact lenses can easily help with myopia. Depending on the amount of myopia, you may only need to wear glasses or contact lenses for certain activities, like watching a movie or driving a car. Or, if you are very nearsighted, they may need to be worn all the time. "
vision correction,T_3490,"Farsightedness is also known as hyperopia. It affects about one fourth of people. People with hyperopia can see distant objects clearly, but nearby objects appear blurry. In hyperopia, the eye is too short. This results in images being focused in back of the retina ( Figure 1.2). Hyperopia is corrected with a convex lens, which curves outward like the outside of a bowl. The lens changes the focus so that images fall on the retina as they should. Common signs of farsightedness include difficulty in concentrating and maintaining a clear focus on close objects, eye strain, fatigue and headaches after close work, and aching or burning eyes, especially after intense concentration on close work. In addition to lenses, many cases of myopia and hyperopia can be corrected with surgery. For example, a procedure called LASIK (Laser-Assisted in situ Keratomileusis) uses a laser to permanently change the shape of the cornea so light is correctly focused on the retina. "
vitamins and minerals,T_3491,"Vitamins and minerals are also nutrients. They do not provide energy, but they are needed for good health. "
vitamins and minerals,T_3492,"Vitamins are organic compounds that the body needs in small amounts to function properly. Humans need 13 different vitamins. Some of them are listed below ( Table 1.1). The table also shows how much of each vitamin you need every day. Vitamins have many roles in the body. For example, Vitamin A helps maintain good vision. Vitamin B9 helps form red blood cells. Vitamin K is needed for blood to clot when you have a cut or other wound. Vitamin Necessary for Available from Daily Amount Required (at ages 913 years) Vitamin Necessary for Available from A Good vision B1 Healthy nerves B3 Healthy skin and nerves B9 Red blood cells B12 Healthy nerves C Growth and repair of tis- sues Healthy bones and teeth Blood to clot Carrots, spinach, milk, eggs Whole wheat, peas, meat, beans, fish, peanuts Beets, liver, pork, turkey, fish, peanuts Liver, peas, dried beans, leafy green vegetables Meat, liver, milk, shell- fish, eggs Oranges, grapefruits, red peppers, broccoli Milk, salmon, tuna, eggs Spinach, brussels sprouts, milk, eggs D K Daily Amount Required (at ages 913 years) 600 g (1 g = 1  106 g) 0.9 mg (1 mg = 1  103 g) 12 mg 300 g 1.8 g 45 mg 5 g 60 g Some vitamins are produced in the body. For example, vitamin D is made in the skin when it is exposed to sunlight. Vitamins B12 and K are produced by bacteria that normally live inside the body. Most other vitamins must come from foods. Foods that are good sources of vitamins include whole grains, vegetables, fruits, and milk ( Table 1.1). Not getting enough vitamins can cause health problems. For example, too little vitamin C causes a disease called scurvy. People with scurvy have bleeding gums, nosebleeds, and other symptoms. "
vitamins and minerals,T_3493,"Minerals are chemical elements that are needed for body processes. Minerals that you need in relatively large amounts are listed below ( Table 1.2). Minerals that you need in smaller amounts include iodine, iron, and zinc. Minerals have many important roles in the body. For example, calcium and phosphorus are needed for strong bones and teeth. Potassium and sodium are needed for muscles and nerves to work normally. Mineral Necessary for Available from Calcium Strong bones and teeth Chloride Magnesium Proper balance of water and salts in body Strong bones Phosphorus Strong bones and teeth Potassium Muscles and nerves to work normally Muscles and nerves to work normally Milk, soy milk, leafy green vegetables Table salt, most packaged foods Whole grains, leafy green vegetables, nuts Meat, poultry, whole grains Meats, grains, bananas, orange juice Table salt, most packaged foods Sodium Daily Amount Required (at ages 913 years) 1,300 mg 2.3 g 240 mg 1,250 mg 4.5 g 1.5 g Your body cannot produce any of the minerals that it needs. Instead, you must get minerals from the foods you eat. Good sources of minerals include milk, leafy green vegetables, and whole grains ( Table 1.2). Not getting enough minerals can cause health problems. For example, too little calcium may cause osteoporosis. This is a disease in which bones become soft and break easily. Getting too much of some minerals can also cause health problems. Many people get too much sodium. Sodium is added to most packaged foods. People often add more sodium to their food by using table salt. Too much sodium causes high blood pressure in some people. "
acids and bases,T_3517,"An acid is an ionic compound that produces positive hydrogen ions (H+ ) when dissolved in water. An example is hydrogen chloride (HCl). When it dissolves in water, its hydrogen ions and negative chloride ions (Cl ) separate, forming hydrochloric acid. This can be represented by the equation: HCl H2 O + ! H + Cl "
acids and bases,T_3518,"You already know that a sour taste is one property of acids. (Never taste an unknown substance to see whether it is an acid!) Acids have certain other properties as well. For example, acids can conduct electricity because they consist of charged particles in solution. Acids also react with metals to produce hydrogen gas. For example, when hydrochloric acid (HCl) reacts with the metal magnesium (Mg), it produces magnesium chloride (MgCl2 ) and hydrogen (H2 ). This is a single replacement reaction, represented by the chemical equation: Mg + 2HCl ! H2 + MgCl2 You can see an online demonstration of a similar reaction at this URL: "
acids and bases,T_3519,"Certain compounds, called indicators, change color when acids come into contact with them. They can be used to detect acids. An example of an indicator is a compound called litmus. It is placed on small strips of paper that may be red or blue. If you place a few drops of acid on a strip of blue litmus paper, the paper will turn red. You can see this in Figure 10.6. Litmus isnt the only indicator for detecting acids. Red cabbage juice also works well, as you can see in this entertaining video:  . "
acids and bases,T_3520,"Acids have many important uses, especially in industry. For example, sulfuric acid is used to manufacture a variety of different products, including paper, paint, and detergent. Some other uses of acids are illustrated in Figure 10.7. "
acids and bases,T_3521,"A base is an ionic compound that produces negative hydroxide ions (OH ) when dissolved in water. For example, when the compound sodium hydroxide (NaOH) dissolves in water, it produces hydroxide ions and positive sodium ions (Na+ ). This can be represented by the equation: NaOH H2 O ! OH + Na+ "
acids and bases,T_3522,"All bases share certain properties, including a bitter taste. (Never taste an unknown substance to see whether it is a base!) Did you ever taste unsweetened cocoa powder? It tastes bitter because it is a base. Bases also feel slippery. Think about how slippery soap feels. Soap is also a base. Like acids, bases conduct electricity because they consist of charged particles in solution. "
acids and bases,T_3523,"Bases change the color of certain compounds, and this property can be used to detect them. A common indicator of bases is red litmus paper. Bases turn red litmus paper blue. You can see an example in Figure 10.8. Red cabbage juice can detect bases as well as acids, as youll see by reviewing this video:  MEDIA Click image to the left or use the URL below. URL: "
acids and bases,T_3524,"Bases are used for a variety of purposes. For example, soaps contain bases such as potassium hydroxide. Other uses of bases are pictured in Figure 10.9. "
acids and bases,T_3525,The acid in vinegar is weak enough to safely eat on a salad. The acid in a car battery is strong enough to eat through skin. The base in antacid tablets is weak enough to take for an upset stomach. The base in drain cleaner is strong enough to cause serious burns. What causes these differences in strength of acids and bases? 
acids and bases,T_3526,"The strength of an acid depends on the concentration of hydrogen ions it produces when dissolved in water. A stronger acid produces a greater concentration of ions than a weaker acid. For example, when hydrogen chloride is added to water, all of it breaks down into H+ and Cl ions. Therefore, it is a strong acid. On the other hand, only about 1 percent of acetic acid breaks down into ions, so it is a weak acid. The strength of a base depends on the concentration of hydroxide ions it produces when dissolved in water. For example, sodium hydroxide completely breaks down into ions in water, so it is a strong base. However, only a fraction of ammonia breaks down into ions, so it is a weak base. "
acids and bases,T_3527,"The strength of acids and bases is measured on a scale called the pH scale (see Figure 10.10). The symbol pH represents acidity, or the concentration of hydrogen ions (H+ ) in a solution. Pure water, which is neutral, has a pH of 7. With a higher concentration of hydrogen ions, a solution is more acidic but has a lower pH. Therefore, acids have a pH less than 7, and the strongest acids have a pH close to zero. Bases have a pH greater than 7, and the strongest bases have a pH close to 14. You can watch a video about the pH scale at this URL:  MEDIA Click image to the left or use the URL below. URL: "
acids and bases,T_3528,"Acidity is an important factor for living things. For example, many plants grow best in soil that has a pH between 6 and 7. Fish also need a pH close to 7. Some air pollutants form acids when dissolved in water droplets in the air. This results in acid fog and acid rain, which may have a pH of 4 or even lower (see Figure 10.10). Figure 10.11 shows the effects of acid fog and acid rain on a forest. Acid rain also lowers the pH of surface waters such as streams and lakes. As a result, the water became too acidic for fish and many other water organisms to survive. Even normal (not acid) rain is slightly acidic. Thats because carbon dioxide in the air dissolves in raindrops, producing a weak acid called carbonic acid. When acidic rainwater soaks into the ground, it can slowly dissolve rocks, particularly those containing calcium carbonate. This is how water forms caves, like the one that opened this chapter. "
acids and bases,T_3529,"As you read above, an acid produces positive hydrogen ions and a base produces negative hydroxide ions. If an acid and base react together, the hydrogen and hydroxide ions combine to form water. This is represented by the equation: H+ + OH ! H2 O An acid also produces negative ions, and a base also produces positive ions. For example, the acid hydrogen chloride (HCl), when dissolved in water, produces negative chloride ions (Cl ) as well as hydrogen ions. The base sodium hydroxide (NaOH) produces positive sodium ions (Na+ ) in addition to hydroxide ions. These other ions also combine when the acid and base react. They form sodium chloride (NaCl). This is represented by the equation: Na+ + Cl ! NaCl Sodium chloride is called table salt, but salt is a more general term. A salt is any ionic compound that forms when an acid and base react. It consists of a positive ion from the base and a negative ion from the acid. Like pure water, a salt is neutral in pH. Thats why reactions of acids and bases are called neutralization reactions. Another example of a neutralization reaction is described in Figure 10.12. You can learn more about salts and how they form at this URL:  (13:21). MEDIA Click image to the left or use the URL below. URL: "
radioactivity,T_3530,"Radioactivity is the ability of an atom to emit, or give off, charged particles and energy from the nucleus. The charged particles and energy are called by the general term radiation. Only unstable nuclei emit radiation. When they do, they gain or lose protons. Then the atoms become different elements. (Be careful not to confuse this radiation with electromagnetic radiation, which has to do with the light given off by atoms as they absorb and then emit energy.) "
radioactivity,T_3531,"Radioactivity was discovered in 1896 by a French physicist named Antoine Henri Becquerel. Becquerel was experimenting with uranium, which glows after being exposed to sunlight. Becquerel wanted to see if the glow was caused by rays of energy, like rays of light and X-rays. He placed a bit of uranium on a photographic plate. The plate was similar to film thats used today to take X-rays. You can see an example of an X-ray in Figure 11.1. As Becquerel predicted, the uranium left an image on the photographic plate. This meant that uranium gives off rays after being exposed to sunlight. Becquerel was a good scientist, so he wanted to repeat his experiment to confirm his results. He placed more uranium on another photographic plate. However, the day had turned cloudy, so he tucked the plate and uranium in a drawer to try again another day. He wasnt expecting the uranium to leave an image on the plate without being exposed to sunlight. To his surprise, there was an image on the plate in the drawer the next day. Becquerel had discovered that uranium gives off rays without getting energy from light. He had discovered radioactivity, for which he received a Nobel prize. To learn more about the importance of Becquerels research, go to this URL: http://nobelprize.org/no Another scientist, who worked with Becquerel, actually came up with the term ""radioactivity."" The other scientist was the French chemist Marie Curie. She went on to discover the radioactive elements polonium and radium. She won two Nobel Prizes for her discoveries. You can learn more about Marie Curie at this URL: http://nobelprize.or "
radioactivity,T_3532,"Isotopes are atoms of the same element that differ from each other because they have different numbers of neutrons. Many elements have one or more isotopes that are radioactive. Radioactive isotopes are called radioisotopes. An example of a radioisotope is carbon-14. All carbon atoms have 6 protons, and most have 6 neutrons. These carbon atoms are called carbon-12, where 12 is the mass number (6 protons + 6 neutrons). A tiny percentage of carbon atoms have 8 neutrons instead of the usual 6. These atoms are called carbon-14 (6 protons + 8 neutrons). The nuclei of carbon-14 are unstable because they have too many neutrons. To be stable, a small nucleus like carbon, with just 6 protons, must have a 1:1 ratio of protons to neutrons. In other words, it must have the same number of neutrons as protons. In a large nucleus, with many protons, the ratio must be 2:1 or even 3:1 protons to neutrons. In elements with more than 83 protons, all the isotopes are radioactive (see Figure 11.2). The force of repulsion among all those protons overcomes the strong force holding them together. This makes the nuclei unstable and radioactive. Elements with more than 92 protons have such unstable nuclei that these elements do not even exist in nature. They exist only if they are created in a lab. "
radioactivity,T_3533,"A low level of radiation occurs naturally in the environment. This is called background radiation. It comes from various sources. One source is rocks, which may contain small amounts of radioactive elements such as uranium. Another source is cosmic rays. These are charged particles that arrive on Earth from outer space. Background radiation is generally considered to be safe for living things. A source of radiation that may be more dangerous is radon. Radon is a radioactive gas that forms in rocks underground. It can seep into basements and get trapped inside buildings. Then it may build up and become harmful to people who breathe it. Other sources of radiation are described in the interactive animation at this URL: http://w "
radioactivity,T_3534,"You may have seen a sign like the one in Figure 11.3. It warns people that there is radiation in the area. Exposure to radiation can be very dangerous. Radiation damages living things by knocking electrons out of atoms and changing them to ions. Radiation also breaks bonds in DNA and other biochemical compounds. A single large exposure to radiation can burn the skin and cause radiation sickness. Symptoms of this illness include extreme fatigue, destruction of blood cells, and loss of hair. Long-term exposure to lower levels of radiation can cause cancer. For example, radon in buildings can cause lung cancer. Marie Curie died of cancer, most likely because of exposure to radiation in her research. To learn more about the harmful health effects of radiation, go to this URL:  . Nonliving things can also be damaged by radiation. For example, high levels of radiation can remove electrons from metals. This may weaken metals in nuclear power plants and space vehicles, both of which are exposed to very high levels of radiation. "
radioactivity,T_3535,"One reason radiation is dangerous is that it cant be detected with the senses. You normally cant see it, smell it, hear it, or feel it. Fortunately, there are devices such as Geiger counters that can detect radiation. A Geiger counter, like the one in Figure 11.4, has a tube that contains atoms of a gas. If radiation enters the tube, it turns gas atoms to ions that carry electric current. The current causes the Geiger counter to click. The faster the clicks occur, the higher the level of radiation. You can see a video about the Geiger counter and how it was invented at the URL below. "
radioactivity,T_3536,"Despite its dangers, radioactivity has several uses. It can be used to determine the ages of ancient rocks and fossils. This use of radioactivity is explained in this chapters ""Radioactive Decay"" lesson. Radioactivity can also be used as a source of power to generate electricity. This use of radioactivity is covered later on in this chapter in the lesson ""Nuclear Energy."" Radioactivity can even be used to diagnose and treat diseases, including cancer. Cancer cells grow rapidly and take up a lot of glucose for energy. Glucose containing radioactive elements can be given to patients. Cancer cells will take up more of the glucose than normal cells do and give off radiation. The radiation can be detected with special machines (see Figure 11.5). Radioactive elements taken up by cancer cells may also be used to kill the cells and treat the disease. You can learn more about medical uses of radiation at the URL below. MEDIA Click image to the left or use the URL below. URL: "
radioactive decay,T_3537,"There are three types of radioactive decay: alpha, beta, and gamma decay. In all three types, nuclei emit radiation, but the nature of that radiation differs from one type of decay to another. You can watch a video about the three types at this URL:  (17:02). MEDIA Click image to the left or use the URL below. URL: "
radioactive decay,T_3538,"Alpha decay occurs when an unstable nucleus emits an alpha particle and energy. The diagram in Figure 11.6 represents alpha decay. An alpha particle contains two protons and two neutrons, giving it a charge of +2. A helium nucleus has two protons and two neutrons, so an alpha particle is represented in nuclear equations by the symbol 4 He. 2 The superscript 4 is the mass number (2 protons + 2 neutrons). The subscript 2 is the charge of the particle as well as the number of protons. An example of alpha decay is the decay of uranium-238 to thorium-234. In this reaction, uranium loses two protons and two neutrons to become the element thorium. The reaction can be represented by this equation: 238 92 U 4 !234 90 Th +2 He + Energy If you count the number of protons and neutrons on each side of this equation, youll see that the numbers are the same on both sides of the arrow. This means that the equation is balanced. The thorium-234 produced in this reaction is unstable, so it will undergo radioactive decay as well. The alpha particle (42 He) produced in the reaction can pick up two electrons to form the element helium. This is how most of Earths helium formed. Problem Solving ? 4 Problem: Fill in the missing subscript and superscript to balance this nuclear equation: 208 84 Po !? Pb +2 He + Energy Solution: The subscript is 82, and the superscript is 204. You Try It! ? 4 Problem: Fill in the missing subscript and superscript to balance this nuclear equation: 222 ? Ra !86 Rn+2 He+Energy "
radioactive decay,T_3539,"Beta decay occurs when an unstable nucleus emits a beta particle and energy. A beta particle is an electron. It has a charge of -1. In nuclear equations, a beta particle is represented by the symbol 01 e. The subscript -1 represents the particles charge, and the superscript 0 shows that the particle has virtually no mass. Nuclei contain only protons and neutrons, so how can a nucleus emit an electron? A neutron first breaks down into a proton and an electron (see Figure 11.7). Then the electron is emitted from the nucleus, while the proton stays inside the nucleus. The proton increases the atomic number by one, thus changing one element into another. An example of beta decay is the decay of thorium-234 to protactinium-234. In this reaction, thorium loses a neutron and gains a proton to become protactinium. The reaction can be represented by this equation: 234 90 Th !234 91 Pa + 0 1 e + Energy The protactinium-234 produced in this reaction is radioactive and decays to another element. The electron produced in the reaction (plus another electron) can combine with an alpha particle to form helium. Problem Solving Problem: Fill in the missing subscript and superscript in this nuclear equation: 131 I 53 !?? Xe + 14 C ? !?7 N + Solution: The subscript is 54, and the superscript is 131. 0 e + Energy 1 You Try It! Problem: Fill in the missing subscript and superscript in this nuclear equation: 0 e + Energy 1 "
radioactive decay,T_3540,"In alpha and beta decay, both particles and energy are emitted. In gamma decay, only energy is emitted. Gamma decay occurs when an unstable nucleus gives off gamma rays. Gamma rays, like rays of visible light and X-rays, are waves of energy that travel through space at the speed of light. Gamma rays have the greatest amount of energy of all such waves. By itself, gamma decay doesnt cause one element to change into another, but it is released in nuclear reactions that do. Some of the energy released in alpha and beta decay is in the form of gamma rays. You can learn more about gamma radiation at this URL:  (2:45). MEDIA Click image to the left or use the URL below. URL: "
radioactive decay,T_3541,The different types of radiation vary in how far they are able to travel and what they can penetrate (see Figure 11.8 and the URL below). MEDIA Click image to the left or use the URL below. URL:  Alpha particles can travel only a few centimeters through air. They cannot pass through a sheet of paper or thin layer of clothing. They may burn the skin but cannot penetrate tissues beneath the skin. Beta particles can travel up to a meter through air. They can pass through paper and cloth but not through a sheet of aluminum. They can penetrate and damage tissues beneath the skin. Gamma rays can travel thousands of meters through air. They can pass through a sheet of aluminum as well as paper and cloth. They can be stopped only by several centimeters of lead or several meters of concrete. They can penetrate and damage organs deep inside the body. 
radioactive decay,T_3542,"A radioactive isotope decays at a certain constant rate. The rate is measured in a unit called the half-life. This is the length of time it takes for half of a given amount of the isotope to decay. The concept of half-life is illustrated in Figure 11.9 for the beta decay of phosphorus-32 to sulfur-32. The half-life of this radioisotope is 14 days. After 14 days, half of the original amount of phosphorus-32 has decayed. After another 14 days, half of the remaining amount (or one-quarter of the original amount) has decayed, and so on. Different radioactive isotopes vary greatly in their rate of decay. As you can see from the examples in Table 11.1, the half-life of a radioisotope can be as short as a split second or as long as several billion years. You can simulate radioactive decay of radioisotopes with different half-lives at the URL below. Some radioisotopes decay much more quickly than others. Isotope Uranium-238 Potassium-40 Carbon-14 Hydrogen-3 Radon-222 Polonium-214 Half-life 4.47 billion years 1.28 billion years 5,730 years 12.3 years 3.82 days 0.00016 seconds Problem Solving Problem: If you had a gram of carbon-14, how many years would it take for radioactive decay to reduce it to one-quarter of a gram? Solution: One gram would decay to one-quarter of a gram in 2 half-lives years. 1 2 12 = 1 4 , or 2  5,730 years = 11,460 You Try It! Problem: What fraction of a given amount of hydrogen-3 would be left after 36.9 years of decay? "
radioactive decay,T_3543,Radioactive isotopes can be used to estimate the ages of fossils and rocks. The method is called radioactive dating. Carbon-14 dating is an example of radioactive dating. It is illustrated in the video at this URL:  MEDIA Click image to the left or use the URL below. URL: 
radioactive decay,T_3544,"Carbon-14 forms naturally in Earths atmosphere when cosmic rays strike atoms of nitrogen-14. Living things take in and use carbon-14, just as they do carbon-12. The carbon-14 in living things gradually decays to nitrogen-14. However, it is constantly replaced because living things keep taking in carbon-14. As a result, there is a fixed ratio of carbon-14 to carbon-12 in organisms as long as they are alive. This is illustrated in the top part of Figure 11.10. After organisms die, the carbon-14 they already contain continues to decay, but it is no longer replaced (see bottom part of Figure 11.10). Therefore, the carbon-14 in a dead organism constantly declines at a fixed rate equal to the half-life of carbon-14. Half of the remaining carbon-14 decays every 5,730 years. If you measure how much carbon- 14 is left in a fossil, you can determine how many half-lives (and how many years) have passed since the organism died. "
radioactive decay,T_3545,"Carbon-14 has a relatively short half-life (see Table 11.1). After about 50,000 years, too little carbon-14 is left in a fossil to be measured. Therefore, carbon-14 dating can only be used to date fossils that are less than 50,000 years old. Radioisotopes with a longer half-life, such as potassium-40, must be used to date older fossils and rocks. "
nuclear energy,T_3546,"Nuclear fission is the splitting of the nucleus of an atom into two smaller nuclei. This type of reaction releases a great deal of energy from a very small amount of matter. For example, nuclear fission of a tiny pellet of uranium-235, like the one pictured in Figure 11.11, can release as much energy as burning 1,000 kilograms of coal! Nuclear fission of uranium-235 can be represented by this equation: 235 92 U + 1 141 Neutron !92 36 Kr + 56 Ba + 3 Neutrons + Energy As shown in Figure 11.12, the reaction begins when a nucleus of uranium-235 absorbs a neutron. This can happen naturally or when a neutron is deliberately crashed into a uranium nucleus in a nuclear power plant. In either case, the nucleus of uranium becomes very unstable and splits in two. In this example, it forms krypton-92 and barium-141. The reaction also releases three neutrons and a great deal of energy. "
nuclear energy,T_3547,"The neutrons released in this nuclear fission reaction may be captured by other uranium nuclei and cause them to fission as well. This can start a nuclear chain reaction (see Figure 11.13). In a chain reaction, one fission reaction leads to others, which lead to others, and so on. A nuclear chain reaction is similar to a pile of wood burning. If you start one piece of wood burning, enough heat is produced by the burning wood to start the rest of the pile burning without any further help from you. You can see another example of a chain reaction at this URL: "
nuclear energy,T_3548,"If a nuclear chain reaction is uncontrolled, it produces a lot of energy all at once. This is what happens in an atomic bomb. If a nuclear chain reaction is controlled, it produces energy more slowly. This is what occurs in a nuclear power plant. The reaction may be controlled by inserting rods of material that do not undergo fission into the core of fissioning material (see Figure 11.14). The radiation from the controlled fission is used to heat water and turn it to steam. The steam is under pressure and causes a turbine to spin. The spinning turbine runs a generator, which produces electricity. "
nuclear energy,T_3549,"In the U.S., the majority of electricity is produced by burning coal or other fossil fuels. This causes air pollution, acid rain, and global warming. Fossil fuels are also limited and may eventually run out. Like fossil fuels, radioactive elements are limited. In fact, they are relatively rare, so they could run out sooner rather than later. On the other hand, nuclear fission does not release air pollution or cause the other environmental problems associated with burning fossil fuels. This is the major advantage of using nuclear fission as a source of energy. The main concern over the use of nuclear fission is the risk of radiation. Accidents at nuclear power plants can release harmful radiation that endangers people and other living things. Even without accidents, the used fuel that is left after nuclear fission reactions is still radioactive and very dangerous. It takes thousands of years for it to decay until it no longer releases harmful radiation. Therefore, used fuel must be stored securely to people and other living things. You can learn more about the problem of radioactive waste at this URL: "
nuclear energy,T_3550,"Nuclear fusion is the opposite of nuclear fission. In fusion, two or more small nuclei combine to form a single, larger nucleus. An example is shown in Figure 11.15. In this example, two hydrogen nuclei fuse to form a helium nucleus. A neutron and a great deal of energy are also released. In fact, fusion releases even more energy than fission does. "
nuclear energy,T_3551,"Nuclear fusion of hydrogen to form helium occurs naturally in the sun and other stars. It takes place only at extremely high temperatures. Thats because a great deal of energy is needed to overcome the force of repulsion between positively charged nuclei. The suns energy comes from fusion in its core, where temperatures reach millions of Kelvin (see Figure 11.16). "
nuclear energy,T_3552,"Scientists are searching for ways to create controlled nuclear fusion reactions on Earth. Their goal is develop nuclear fusion power plants, where the energy from fusion of hydrogen nuclei can be converted to electricity. How this might work is shown in Figure 11.17. The use of nuclear fusion for energy has several pros. Unlike nuclear fission, which involves dangerous radioiso- topes, nuclear fusion involves hydrogen and helium. These elements are harmless. Hydrogen is also very plentiful. There is a huge amount of hydrogen in ocean water. The hydrogen in just a gallon of water could produce as much energy by nuclear fusion as burning 1,140 liters (300 gallons) of gasoline! The hydrogen in the oceans would generate enough energy to supply all the worlds people for a very long time. Unfortunately, using energy from nuclear fusion is far from a reality. Scientists are a long way from developing the necessary technology. One problem is raising temperatures high enough for fusion to take place. Another problem is that matter this hot exists only in the plasma state. There are no known materials that can contain plasma, although a magnet might be able to do it. Thats because plasma consists of ions and responds to magnetism. You can learn more about research on nuclear fusion at the URL below. "
nuclear energy,T_3553,"Probably the most famous equation in the world is E = mc2 . You may have heard of it. You may have even seen it on a tee shirt or coffee mug. Its a simple equation that was derived in 1905 by the physicist Albert Einstein (see Figure 11.18). Although the equation is simple, it is incredibly important. It changed how scientists view two of the most basic concepts in science: matter and energy. The equation shows that matter and energy are two forms of the same thing. It also shows how matter and energy are related. In addition, Einsteins equation explains why nuclear fission and nuclear fusion produce so much energy. You can listen to a recording of Einstein explaining his famous equation at this URL: "
nuclear energy,T_3554,"In Einsteins equation, the variable E stands for energy and the variable m stands for mass. The c in the equation is a constant. It stands for the speed of light. The speed of light is 300,000 kilometers (186,000 miles) per second, so c2 is a very big number, no matter what units are used to measure it. Einsteins equation means that the energy in a given amount of matter is equal to its mass times the square of the speed of light. Thats a huge amount of energy from even a tiny amount of mass. Suppose, for example, that you have 1 gram of matter. Thats about the mass of a paperclip. Multiplying that mass by the square of the speed of light yields enough energy to power 3,600 homes for a year! "
nuclear energy,T_3555,"When the nucleus of a radioisotope undergoes fission or fusion, it loses a tiny amount of mass. What happens to the lost mass? It isnt really lost at all. It is converted to energy. How much energy? E = mc2 . The change in mass is tiny, but it results in a great deal of energy. What about the laws of conservation of mass and conservation of energy? Do they not apply to nuclear reactions? We dont need to throw out these laws. Instead, we just need to combine them. It is more correct to say that the sum of mass and energy is always conserved in a nuclear reaction. Mass may change to energy, but the amount of mass and energy combined remains the same. "
distance and direction,T_3556,"Assume that a school bus, like the one in Figure 12.2, passes by as you stand on the sidewalk. Its obvious to you that the bus is moving. It is moving relative to you and the trees across the street. But what about to the children inside the bus? They arent moving relative to each other. If they look only at the other children sitting near them, they will not appear to be moving. They may only be able to tell that the bus is moving by looking out the window and seeing you and the trees whizzing by. This example shows that how we perceive motion depends on our frame of reference. Frame of reference refers to something that is not moving with respect to an observer that can be used to detect motion. For the children on the bus, if they use other children riding the bus as their frame of reference, they do not appear to be moving. But if they use objects outside the bus as their frame of reference, they can tell they are moving. What is your frame of reference if you are standing on the sidewalk and see the bus go by? How can you tell the bus is moving? The video at the URL below illustrates other examples of how frame of reference is related to motion. MEDIA Click image to the left or use the URL below. URL: "
distance and direction,T_3557,"Did you ever go to a track meet like the one pictured in Figure 12.3? Running events in track include 100-meter sprints and 2000-meter races. Races are named for their distance. Distance is the length of the route between two points. The length of the route in a race is the distance between the starting and finishing lines. In a 100-meter sprint, for example, the distance is 100 meters. "
distance and direction,T_3558,"The SI unit for distance is the meter (1 m = 3.28 ft). Short distances may be measured in centimeters (1 cm = 0.01 m). Long distances may be measured in kilometers (1 km = 1000 m). For example, you might measure the distance a frogs tongue moves in centimeters and the distance a cheetah moves in kilometers. "
distance and direction,T_3559,Maps can often be used to measure distance. Look at the map in Figure 12.4. Find Mias house and the school. You can use the map key to directly measure the distance between these two points. The distance is 2 kilometers. Measure it yourself to see if you agree. 
distance and direction,T_3560,"Things dont always move in straight lines like the route from Mias house to the school. Sometimes they change direction as they move. For example, the route from Mias house to the post office changes from west to north at the school (see Figure 12.4). To find the total distance of a route that changes direction, you must add up the distances traveled in each direction. From Mias house to the school, for example, the distance is 2 kilometers. From the school to the post office, the distance is 1 kilometer. Therefore, the total distance from Mias house to the post office is 3 kilometers. You Try It! Problem: What is the distance from the post office to the park in Figure 12.4? Direction is just as important as distance in describing motion. For example, if Mia told a friend how to reach the post office from her house, she couldnt just say, ""go 3 kilometers."" The friend might end up at the park instead of the post office. Mia would have to be more specific. She could say, ""go west for 2 kilometers and then go north for 1 kilometer."" When both distance and direction are considered, motion is a vector. A vector is a quantity that includes both size and direction. A vector is represented by an arrow. The length of the arrow represents distance. The way the arrow points shows direction. The red arrows in Figure 12.4 are vectors for Mias route to the school and post office. If you want to learn more about vectors, watch the videos at these URLs:  (5:27) MEDIA Click image to the left or use the URL below. URL:  You Try It! Problem: Draw vectors to represent the route from the post office to the park in Figure 12.4. "
speed and velocity,T_3561,"Speed is an important aspect of motion. It is a measure of how fast or slow something moves. It depends on how far something travels and how long it takes to travel that far. Speed can be calculated using this general formula: speed = distance time A familiar example is the speed of a car. In the U.S., this is usually expressed in miles per hour (see Figure 12.6). If your family makes a car trip that covers 120 miles and takes 3 hours, then the cars speed is: speed = 120 mi = 40 mi/h 3h The speed of a car may also be expressed in kilometers per hour (km/h). The SI unit for speed is meters per second (m/s). "
speed and velocity,T_3562,"When you travel by car, you usually dont move at a constant speed. Instead you go faster or slower depending on speed limits, traffic, traffic lights, and many other factors. For example, you might travel 65 miles per hour on a highway but only 20 miles per hour on a city street (see Figure 12.7). You might come to a complete stop at traffic lights, slow down as you turn corners, and speed up to pass other cars. The speed of a moving car or other object at a given instant is called its instantaneous speed. It may vary from moment to moment, so it is hard to calculate. Its easier to calculate the average speed of a moving object than the instantaneous speed. The average speed is the total distance traveled divided by the total time it took to travel that distance. To calculate the average speed, you can use the general formula for speed that was given above. Suppose, for example, that you took a 75-mile car trip with your family. Your instantaneous speed would vary throughout the trip. If the trip took a total of 1.5 hours, your average speed for the trip would be: average speed = 75 mi = 50 mi/h 1.5 h You can see a video about instantaneous and average speed and how to calculate them at this URL:  MEDIA Click image to the left or use the URL below. URL:  You Try It! Problem: Terri rode her bike very slowly to the top of a big hill. Then she coasted back down the hill at a much faster speed. The distance from the bottom to the top of the hill is 3 kilometers. It took Terri 15 minutes to make the round trip. What was her average speed for the entire trip? "
speed and velocity,T_3563,The motion of an object can be represented by a distance-time graph like the one in Figure 12.8. A distance-time graph shows how the distance from the starting point changes over time. The graph in Figure 12.8 represents a bike trip. The trip began at 7:30 AM (A) and ended at 12:30 PM (F). The rider traveled from the starting point to a destination and then returned to the starting point again. 
speed and velocity,T_3564,"In a distance-time graph, the speed of the object is represented by the slope, or steepness, of the graph line. If the line is straight, like the line between A and B in Figure 12.8, then the speed is constant. The average speed can be calculated from the graph. The change in distance (represented by Dd) divided by the change in time (represented by Dt): speed = Dd Dt For example, the speed between A and B in Figure 12.8 is: speed = Dd 20 km 0 km 20 km = = = 20 km/h Dt 8:30 7:30 h 1h If the graph line is horizontal, as it is between B and C, then the slope and the speed are zero: speed = Dd 20 km 20 km 0 km = = = 0 km/h Dt 9:00 8:30 h 0.5 h You Try It! Problem: In Figure 12.8, calculate the speed of the rider between C and D. "
speed and velocity,T_3565,"If you know the speed of a moving object, you can also calculate the distance it will travel in a given amount of time. To do so, you would use this version of the general speed formula: distance = speed  time For example, if a car travels at a speed of 60 km/h for 2 hours, then the distance traveled is: distance = 60 km/h  2 h = 120 km You Try It! Problem: If Maria runs at a speed of 2 m/s, how far will she run in 60 seconds? "
speed and velocity,T_3566,"Speed tells you only how fast an object is moving. It doesnt tell you the direction the object is moving. The measure of both speed and direction is called velocity. Velocity is a vector that can be represented by an arrow. The length of the arrow represents speed, and the way the arrow points represents direction. The three arrows in Figure directions. They represent objects moving at the same speed but in different directions. Vector C is shorter than vector A or B but points in the same direction as vector A. It represents an object moving at a slower speed than A or B but in the same direction as A. If youre still not sure of the difference between speed and velocity, watch the cartoon at this URL:  (2:10). MEDIA Click image to the left or use the URL below. URL:  In general, if two objects are moving at the same speed and in the same direction, they have the same velocity. If two objects are moving at the same speed but in different directions (like A and B in Figure 12.9), they have different velocities. If two objects are moving in the same direction but at a different speed (like A and C in Figure 12.9), they have different velocities. A moving object that changes direction also has a different velocity, even if its speed does not change. "
acceleration,T_3567,"Acceleration is a measure of the change in velocity of a moving object. It shows how quickly velocity changes. Acceleration may reflect a change in speed, a change in direction, or both. Because acceleration includes both a size (speed) and direction, it is a vector. People commonly think of acceleration as an increase in speed, but a decrease in speed is also acceleration. In this case, acceleration is negative. Negative acceleration may be called deceleration. A change in direction without a change in speed is acceleration as well. You can see several examples of acceleration in Figure 12.11. If you are accelerating, you may be able to feel the change in velocity. This is true whether you change your speed or your direction. Think about what it feels like to ride in a car. As the car speeds up, you feel as though you are being pressed against the seat. The opposite occurs when the car slows down, especially if the change in speed is "
acceleration,T_3568,"Calculating acceleration is complicated if both speed and direction are changing. Its easier to calculate acceleration when only speed is changing. To calculate acceleration without a change in direction, you just divide the change in velocity (represented by Dv) by the change in time (represented by Dt). The formula for acceleration in this case is: Acceleration = Dv Dt Consider this example. The cyclist in Figure 12.12 speeds up as he goes downhill on this straight trail. His velocity changes from 1 meter per second at the top of the hill to 6 meters per second at the bottom. If it takes 5 seconds for him to reach the bottom, what is his acceleration, on average, as he flies down the hill? Acceleration = Dv 6 m/s 1 m/s 5 m/s 1 m/s = = = = 1 m/s2 Dt 5s 5s 1m In words, this means that for each second the cyclist travels downhill, his velocity increases by 1 meter per second (on average). The answer to this problem is expressed in the SI unit for acceleration: m/s2 (""meters per second squared""). You Try It! Problem: Tranh slowed his skateboard as he approached the street. He went from 8 m/s to 2 m/s in a period of 3 seconds. What was his acceleration? "
acceleration,T_3569,"The acceleration of an object can be represented by a velocity-time graph like the one in Figure 12.13. A velocity- time graph shows how velocity changes over time. It is similar to a distance-time graph except the y axis represents velocity instead of distance. The graph in Figure 12.13 represents the velocity of a sprinter on a straight track. The runner speeds up for the first 4 seconds of the race, then runs at a constant velocity for the next 3 seconds, and finally slows to a stop during the last 3 seconds of the race. In a velocity-time graph, acceleration is represented by the slope of the graph line. If the line slopes upward, like the line between A and B in Figure 12.13, velocity is increasing, so acceleration is positive. If the line is horizontal, as it is between B and C, velocity is not changing, so acceleration is zero. If the line slopes downward, like the line between C and D, velocity is decreasing, so acceleration is negative. You can review the concept of acceleration as well as other chapter concepts by watching the musical video at this URL: "
what is force,T_3570,"Force is defined as a push or a pull acting on an object. Examples of forces include friction and gravity. Both are covered in detail later in this chapter. Another example of force is applied force. It occurs when a person or thing applies force to an object, like the girl pushing the swing in Figure 13.1. The force of the push causes the swing to move. "
what is force,T_3571,"Force is a vector because it has both size and direction. For example, the girl in Figure 13.1 is pushing the swing away from herself. Thats the direction of the force. She can give the swing a strong push or a weak push. Thats the size, or strength, of the force. Like other vectors, forces can be represented with arrows. Figure 13.2 shows some examples. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force. How could you use an arrow to represent the girls push on the swing in Figure 13.1? "
what is force,T_3572,"The SI unit of force is the newton (N). One newton is the amount of force that causes a mass of 1 kilogram to accelerate at 1 m/s2 . Thus, the newton can also be expressed as kgm/s2 . The newton was named for the scientist Sir Isaac Newton, who is famous for his law of gravity. Youll learn more about Sir Isaac Newton later in the chapter. "
what is force,T_3573,"More than one force may act on an object at the same time. In fact, just about all objects on Earth have at least two forces acting on them at all times. One force is gravity, which pulls objects down toward the center of Earth. The other force is an upward force that may be provided by the ground or other surface. Consider the example in Figure 13.3. A book is resting on a table. Gravity pulls the book downward with a force of 20 newtons. At the same time, the table pushes the book upward with a force of 20 newtons. The combined forces acting on the book  or any other object  are called the net force. This is the overall force acting on an object that takes into account all of the individual forces acting on the object. You can learn more about the concept of net force at this URL:  . "
what is force,T_3574,"When two forces act on an object in opposite directions, like the book on the table, the net force is equal to the difference between the two forces. In other words, one force is subtracted from the other to calculate the net force. If the opposing forces are equal in strength, the net force is zero. Thats what happens with the book on the table. The upward force minus the downward force equals zero (20 N up - 20 N down = 0 N). Because the forces on the book are balanced, the book remains on the table and doesnt move. In addition to these downward and upward forces, which generally cancel each other out, forces may push or pull an object in other directions. Look at the dogs playing tug-of-war in Figure 13.4. One dog is pulling on the rope with a force of 10 newtons to the left. The other dog is pulling on the rope with a force of 12 newtons to the right. These opposing forces are not equal in strength, so they are unbalanced. When opposing forces are unbalanced, the net force is greater than zero. The net force on the rope is 2 newtons to the right, so the rope will move to the right. "
what is force,T_3575,"Two forces may act on an object in the same direction. You can see an example of this in Figure 13.5. After the man on the left lifts up the couch, he will push the couch to the right with a force of 25 newtons. At the same time, the man to the right is pulling the couch to the right with a force of 20 newtons. When two forces act in the same direction, the net force is equal to the sum of the forces. This always results in a stronger force than either of the individual forces alone. In this case, the net force on the couch is 45 newtons to the right, so the couch will move to the right. You Try It! Problem: The boys in the drawing above are about to kick the soccer ball in opposite directions. What will be the net force on the ball? In which direction will the ball move? "
friction,T_3576,"Friction is a force that opposes motion between two surfaces that are touching. Friction can work for or against us. For example, putting sand on an icy sidewalk increases friction so you are less likely to slip. On the other hand, too much friction between moving parts in a car engine can cause the parts to wear out. Other examples of friction are illustrated in Figure 13.7. You can see an animation showing how friction opposes motion at this URL: http://w "
friction,T_3577,"Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye appear rough or bumpy when viewed under a microscope. Look at the metal surfaces in Figure 13.8. The metal foil is so smooth that it is shiny. However, when highly magnified, the surface of metal appears to be very bumpy. All those mountains and valleys catch and grab the mountains and valleys of any other surface that contacts the metal. This creates friction. "
friction,T_3578,Rougher surfaces have more friction between them than smoother surfaces. Thats why we put sand on icy sidewalks and roads. The blades of skates are much smoother than the soles of shoes. Thats why you cant slide as far across ice with shoes as you can with skates (see Figure 13.9). The rougher surface of shoes causes more friction and slows you down. Heavier objects also have more friction because they press together with greater force. Did you ever try to push boxes or furniture across the floor? Its harder to overcome friction between heavier objects and the floor than it is between lighter objects and the floor. 
friction,T_3579,"You know that friction produces heat. Thats why rubbing your hands together makes them warmer. But do you know why the rubbing produces heat? Friction causes the molecules on rubbing surfaces to move faster, so they have more heat energy. Heat from friction can be useful. It not only warms your hands. It also lets you light a match (see Figure 13.10). On the other hand, heat from friction can be a problem inside a car engine. It can cause the car to overheat. To reduce friction, oil is added to the engine. Oil coats the surfaces of moving parts and makes them slippery so there is less friction. "
friction,T_3580,"There are different ways you could move heavy boxes. You could pick them up and carry them. You could slide them across the floor. Or you could put them on a dolly like the one in Figure 13.11 and roll them across the floor. This example illustrates three types of friction: static friction, sliding friction, and rolling friction. Another type of friction is fluid friction. All four types of friction are described below. In each type, friction works opposite the direction of the force applied to a move an object. You can see a video demonstration of the different types of friction at this URL:  (1:07). "
friction,T_3581,"Static friction acts on objects when they are resting on a surface. For example, if you are walking on a sidewalk, there is static friction between your shoes and the concrete each time you put down your foot (see Figure 13.12). Without this static friction, your feet would slip out from under you, making it difficult to walk. Static friction also allows you to sit in a chair without sliding to the floor. Can you think of other examples of static friction? "
friction,T_3582,"Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. Thats why its easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil and when you put on your bikes brakes. "
friction,T_3583,"Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker than sliding friction or static friction. This explains why it is much easier to move boxes on a wheeled dolly than by carrying or sliding them. It also explains why most forms of ground transportation use wheels, including cars, 4-wheelers, bicycles, roller skates, and skateboards. Ball bearings are another use of rolling friction (see Figure "
friction,T_3584,"Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If youve ever tried to push your open hand through the water in a tub or pool, then youve experienced fluid friction between your hand and the water. When a skydiver is falling toward Earth with a parachute, fluid friction between the parachute and the air slows the descent (see Figure 13.14). Fluid pressure with the air is called air resistance. The faster or larger a moving object is, the greater is the fluid friction resisting its motion. The very large surface area of a parachute, for example, has greater air resistance than a skydivers body. "
gravity,T_3585,"Gravity has traditionally been defined as a force of attraction between two masses. According to this conception of gravity, anything that has mass, no matter how small, exerts gravity on other matter. The effect of gravity is that objects exert a pull on other objects. Unlike friction, which acts only between objects that are touching, gravity also acts between objects that are not touching. In fact, gravity can act over very long distances. "
gravity,T_3586,"You are already very familiar with Earths gravity. It constantly pulls you toward the center of the planet. It prevents you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects above the surface, from meteors to skydivers, down to the ground. Gravity between Earth and the moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth moving around the sun. "
gravity,T_3587,"Weight measures the force of gravity pulling on an object. Because weight measures force, the SI unit for weight is the newton (N). On Earth, a mass of 1 kilogram has a weight of about 10 newtons because of the pull of Earths gravity On the moon, which has less gravity, the same mass would weigh less. Weight is measured with a scale, like the spring scale in Figure 13.16. The scale measures the force with which gravity pulls an object downward. "
gravity,T_3588,"People have known about gravity for thousands of years. After all, they constantly experienced gravity in their daily lives. They knew that things always fall toward the ground. However, it wasnt until Sir Isaac Newton developed his law of gravity in the late 1600s that people really began to understand gravity. Newton is pictured in Figure 13.17. "
gravity,T_3589,"Newton was the first one to suggest that gravity is universal and affects all objects in the universe. Thats why his law of gravity is called the law of universal gravitation. Universal gravitation means that the force that causes an apple to fall from a tree to the ground is the same force that causes the moon to keep moving around Earth. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. In fact, there is gravity between you and every mass around you  your desk, your book, your pen. Even tiny molecules of gas are attracted to one another by the force of gravity. Newtons law had a huge impact on how people thought about the universe. It explains the motion of objects not only on Earth but in outer space as well. You can learn more about Newtons law of gravity in the video at this URL: "
gravity,T_3590,"Newtons law also states that the strength of gravity between any two objects depends on two factors: the masses of the objects and the distance between them. Objects with greater mass have a stronger force of gravity. For example, because Earth is so massive, it attracts you and your desk more strongly than you and your desk attract each other. Thats why you and the desk remain in place on the floor rather than moving toward one another. Objects that are closer together have a stronger force of gravity. For example, the moon is closer to Earth than it is to the more massive sun, so the force of gravity is greater between the moon and Earth than between the moon and the sun. Thats why the moon circles around Earth rather than the sun. This is illustrated in Figure You can apply these relationships among mass, distance, and gravity by designing your own roller coaster at this URL:  . "
gravity,T_3591,"Newtons idea of gravity can predict the motion of most but not all objects. In the early 1900s, Albert Einstein came up with a theory of gravity that is better at predicting how all objects move. Einstein showed mathematically that gravity is not really a force in the sense that Newton thought. Instead, gravity is a result of the warping, or curving, of space and time. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Einstein theorized that Earth and other very massive bodies affect space and time around them in a similar way. This idea is represented in Figure 13.19. According to Einstein, objects curve toward one another because of the curves in space and time, not because they are pulling on each other with a force of attraction as Newton thought. You can see an animation of Einsteins theory of gravity at this URL: http://einstein. theory of gravity, go to this URL: "
gravity,T_3592,Regardless of what gravity is  a force between masses or the result of curves in space and time  the effects of gravity on motion are well known. You already know that gravity causes objects to fall down to the ground. Gravity affects the motion of objects in other ways as well. 
gravity,T_3593,"When gravity pulls objects toward the ground, it causes them to accelerate. Acceleration due to gravity equals 9.8 m/s2 . In other words, the velocity at which an object falls toward Earth increases each second by 9.8 m/s. Therefore, after 1 second, an object is falling at a velocity of 9.8 m/s. After 2 seconds, it is falling at a velocity of 19.6 m/s (9.8 m/s  2), and so on. This is illustrated in Figure 13.20. You can compare the acceleration due to gravity on Earth, the moon, and Mars with the interactive animation called ""Freefall"" at this URL: http://jersey.uoregon.edu/vlab/ . You might think that an object with greater mass would accelerate faster than an object with less mass. After all, its greater mass means that it is pulled by a stronger force of gravity. However, a more massive object accelerates at the same rate as a less massive object. The reason? The more massive object is harder to move because of its greater mass. As a result, it ends up moving at the same acceleration as the less massive object. Consider a bowling ball and a basketball. The bowling ball has greater mass than the basketball. However, if you were to drop both balls at the same time from the same distance above the ground, they would reach the ground together. This is true of all falling objects, unless air resistance affects one object more than another. For example, a falling leaf is slowed down by air resistance more than a falling acorn because of the leafs greater surface area. However, if the leaf and acorn were to fall in the absence of air (that is, in a vacuum), they would reach the ground at the same time. "
gravity,T_3594,"Earths gravity also affects the acceleration of objects that start out moving horizontally, or parallel to the ground. Look at Figure 13.21. A cannon shoots a cannon ball straight ahead, giving the ball horizontal motion. At the same time, gravity pulls the ball down toward the ground. Both forces acting together cause the ball to move in a curved path. This is called projectile motion. Projectile motion also applies to other moving objects, such as arrows shot from a bow. To hit the bulls eye of a target with an arrow, you actually have to aim for a spot above the bulls eye. Thats because by the time the arrow reaches the target, it has started to curve downward toward the ground. Figure 13.22 shows what happens if you aim at the bulls eye instead of above it. You can access interactive animations of projectile motion at these URLs: http://phet.colorado.edu/en/simulation/projectile-motion http://jersey.uoregon.edu/vlab/ (Select the applet entitled Cannon.) "
gravity,T_3595,"The moon moves around Earth in a circular path called an orbit. Why doesnt Earths gravity pull the moon down to the ground instead? The moon has enough forward velocity to partly counter the force of Earths gravity. It constantly falls toward Earth, but it stays far enough away from Earth so that it actually falls around the planet. As a result, the moon keeps orbiting Earth and never crashes into it. The diagram in Figure 13.23 shows how this happens. You can explore gravity and orbital motion in depth with the animation at this URL: http://phet.colorado You can see an animated version of this diagram at: http://en.wikipedia.org/wiki/File:Orbital_motion.gif . "
elastic force,T_3596,"Something that is elastic can return to its original shape after being stretched or compressed. This property is called elasticity. As you stretch or compress an elastic material, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force. Elastic force causes the material to spring back to its original shape as soon as the stretching or compressing force is released. You can watch a demonstration of elastic force at this URL:  (3:57). MEDIA Click image to the left or use the URL below. URL: "
elastic force,T_3597,"Elastic force can be very useful. You probably use it yourself every day. A few common uses of elastic force are pictured in Figure 13.25. Did you ever use a resistance band like the one in the figure? When you pull on the band, it stretches but doesnt break. The resistance you feel when you pull on it is elastic force. The resistance of the band to stretching is what gives your muscles a workout. After you stop pulling on the band, it returns to its original shape, ready for the next workout. Springs like the ones in Figure 13.26 also have elastic force when they are stretched or compressed. And like stretchy materials, they return to their original shape when the stretching or compressing force is released. Because of these properties, springs are used in scales to measure weight. They also cushion the ride in a car and provide springy support beneath a mattress. Can you think of other uses of springs? "
newtons first law,T_3598,"Newtons first law of motion states that an objects motion will not change unless an unbalanced force acts on the object. If the object is at rest, it will stay at rest. If the object is in motion, it will stay in motion and its velocity will remain the same. In other words, neither the direction nor the speed of the object will change as long as the net force acting on it is zero. You can watch a video about Newtons first law at this URL: http://videos.howstuffworks.com/ Look at the pool balls in Figure 14.2. When a pool player pushes the pool stick against the white ball, the white ball is set into motion. Once the white ball is rolling, it rolls all the way across the table and stops moving only after it crashes into the cluster of colored balls. Then, the force of the collision starts the colored balls moving. Some may roll until they bounce off the raised sides of the table. Some may fall down into the holes at the edges of the table. None of these motions will occur, however, unless that initial push of the pool stick is applied. As long as the net force on the balls is zero, they will remain at rest. "
newtons first law,T_3599,"Newtons first law of motion is also called the law of inertia. Inertia is the tendency of an object to resist a change in its motion. If an object is already at rest, inertia will keep it at rest. If the object is already moving, inertia will keep it moving. Think about what happens when you are riding in a car that stops suddenly. Your body moves forward on the seat. Why? The brakes stop the car but not your body, so your body keeps moving forward because of inertia. Thats why its important to always wear a seat belt. Inertia also explains the amusement park ride in Figure 14.1. The car keeps changing direction, but the riders keep moving in the same direction as before. They slide to the opposite side of the car as a result. You can see an animation of inertia at this URL: "
newtons first law,T_3600,"The inertia of an object depends on its mass. Objects with greater mass also have greater inertia. Think how hard it would be to push a big box full of books, like the one in Figure 14.3. Then think how easy it would be to push the box if it was empty. The full box is harder to move because it has greater mass and therefore greater inertia. "
newtons first law,T_3601,"To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object. Until the soccer player kicks the ball in Figure 14.4, the ball remains motionless on the ground. However, when the ball is kicked, the force on it is suddenly unbalanced. The ball starts moving across the field because its inertia has been overcome. "
newtons second law,T_3602,"Newton determined that two factors affect the acceleration of an object: the net force acting on the object and the objects mass. The relationships between these two factors and motion make up Newtons second law of motion. This law states that the acceleration of an object equals the net force acting on the object divided by the objects mass. This can be represented by the equation: Net force , or Mass F a= m Acceleration = You can watch a video about how Newtons second law of motion applies to football at this URL: http://science36 "
newtons second law,T_3603,"Newtons second law shows that there is a direct relationship between force and acceleration. The greater the force that is applied to an object of a given mass, the more the object will accelerate. For example, doubling the force on the object doubles its acceleration. The relationship between mass and acceleration, on the other hand, is an inverse relationship. The greater the mass of an object, the less it will accelerate when a given force is applied. For example, doubling the mass of an object results in only half as much acceleration for the same amount of force. Consider the example of a batter, like the boy in Figure 14.6. The harder he hits the ball, the greater will be its acceleration. It will travel faster and farther if he hits it with more force. What if the batter hits a baseball and a softball with the same amount of force? The softball will accelerate less than the baseball because the softball has greater mass. As a result, it wont travel as fast or as far as the baseball. "
newtons second law,T_3604,"The equation for acceleration given above can be used to calculate the acceleration of an object that is acted on by an unbalanced force. For example, assume you are pushing a large wooden trunk, like the one shown in Figure acceleration of the trunk, substitute these values in the equation for acceleration: a= F 20 N 2N = = m 10 kg kg Recall that one newton (1 N) is the force needed to cause a 1-kilogram mass to accelerate at 1 m/s2 . Therefore, force can also be expressed in the unit kgm/s2 . This way of expressing force can be substituted for newtons in the solution to the problem: a= 2 N 2 kg  m/s2 = = 2 m/s2 kg kg Why are there no kilograms in the final answer to this problem? The kilogram units in the numerator and denominator of the fraction cancel out. As a result, the answer is expressed in the correct units for acceleration: m/s2 . You Try It! Problem: Assume that you add the weights to the trunk in Figure 14.7. If you push the trunk and weights with a force of 20 N, what will be the trunks acceleration? Need more practice? You can find additional problems at this URL: "
newtons second law,T_3605,"Newtons second law of motion explains the weight of objects. Weight is a measure of the force of gravity pulling on an object of a given mass. Its the force (F) in the acceleration equation that was introduced above: a= F m This equation can also be written as: F = ma The acceleration due to gravity of an object equals 9.8 m/s2 , so if you know the mass of an object, you can calculate its weight as: F = m  9.8 m/s2 As this equation shows, weight is directly related to mass. As an objects mass increases, so does its weight. For example, if mass doubles, weight doubles as well. You can learn more about weight and acceleration at this URL: Problem Solving Problem: Daisy has a mass of 35 kilograms. How much does she weigh? Solution: Use the formula: F = m  9.8 m/s2 . F = 35 kg  9.8 m/s2 = 343.0 kg  m/s2 = 343.0 N You Try It! Problem: Daisys dad has a mass is 70 kg, which is twice Daisys mass. Predict how much Daisys dad weighs. Then calculate his weight to see if your prediction is correct. Helpful Hints The equation for calculating weight (F = m  a) works only when the correct units of measurement are used. Mass must be in kilograms (kg). Acceleration must be in m/s2 . Weight (F) is expressed in kgm/s2 or in newtons (N). "
newtons third law,T_3606,"Newtons third law of motion states that every action has an equal and opposite reaction. This means that forces always act in pairs. First an action occurs, such as the skateboarders pushing together. Then a reaction occurs that is equal in strength to the action but in the opposite direction. In the case of the skateboarders, they move apart, and the distance they move depends on how hard they first pushed together. You can see other examples of actions and reactions in Figure 14.9. You can watch a video about actions and reactions at this URL:  You might think that actions and reactions would cancel each other out like balanced forces do. Balanced forces, which are also equal and opposite, cancel each other out because they act on the same object. Action and reaction forces, in contrast, act on different objects, so they dont cancel each other out and, in fact, often result in motion. For example, in Figure 14.9, the kangaroos action acts on the ground, but the grounds reaction acts on the kangaroo. As a result, the kangaroo jumps away from the ground. One of the action-reaction examples in the Figure 14.9 does not result in motion. Do you know which one it is? "
newtons third law,T_3607,"What if a friend asked you to play catch with a bowling ball, like the one pictured in Figure 14.10? Hopefully, you would refuse to play! A bowling ball would be too heavy to catch without risk of injury assuming you could even throw it. Thats because a bowling ball has a lot of mass. This gives it a great deal of momentum. Momentum is a property of a moving object that makes the object hard to stop. It equals the objects mass times its velocity. It can be represented by the equation: Momentum = Mass  Velocity This equation shows that momentum is directly related to both mass and velocity. An object has greater momentum if it has greater mass, greater velocity, or both. For example, a bowling ball has greater momentum than a softball when both are moving at the same velocity because the bowling ball has greater mass. However, a softball moving at a very high velocity say, 100 miles an hour would have greater momentum than a slow-rolling bowling ball. If an object isnt moving at all, it has no momentum. Thats because its velocity is zero, and zero times anything is zero. "
newtons third law,T_3608,"Momentum can be calculated by multiplying an objects mass in kilograms (kg) by its velocity in meters per second (m/s). For example, assume that a golf ball has a mass of 0.05 kg. If the ball is traveling at a velocity of 50 m/s, its momentum is: Momentum = 0.05 kg  50 m/s = 2.5 kg  m/s Note that the SI unit for momentum is kgm/s. Problem Solving Problem: What is the momentum of a 40-kg child who is running straight ahead with a velocity of 2 m/s? Solution: The child has momentum of: 40 kg  2 m/s = 80 kgm/s. You Try It! Problem: Which football player has greater momentum? Player A: mass = 60 kg; velocity = 2.5 m/s Player B: mass = 65 kg; velocity = 2.0 m/s "
newtons third law,T_3609,"When an action and reaction occur, momentum is transferred from one object to the other. However, the com- bined momentum of the objects remains the same. In other words, momentum is conserved. This is the law of conservation of momentum. Consider the example of a truck colliding with a car, which is illustrated in Figure 14.11. Both vehicles are moving in the same direction before and after the collision, but the truck is moving faster than the car before the collision occurs. During the collision, the truck transfers some of its momentum to the car. After the collision, the truck is moving slower and the car is moving faster than before the collision occurred. Nonetheless, their combined momentum is the same both before and after the collision. You can see an animation showing how momentum is conserved in a head-on collision at this URL:  . "
newtons third law,T_3610,"Paul Doherty of the Exploratorium performs a ""sit-down"" lecture on one of Sir Issac Newtons most famous laws. For more information on Newtons laws of motion, see http://science.kqed.org/quest/video/quest-lab-newtons-laws- MEDIA Click image to the left or use the URL below. URL: "
newtons third law,T_3611,"At UC Berkeley, a team of undergrads is experimenting with velocity, force, and aerodynamics. But you wont find them in a lab  they work on a baseball diamond, throwing fast balls, sliders and curve balls. QUEST discovers how the principles of physics can make the difference between a strike and a home run. For more information on the physics of baseball, see http://science.kqed.org/quest/video/out-of-the-park-the-physics-of-baseball/ . MEDIA Click image to the left or use the URL below. URL: "
buoyancy of fluids,T_3623,Buoyancy is the ability of a fluid to exert an upward force on any object placed in the fluid. This upward force is called buoyant force. 
buoyancy of fluids,T_3624,"What explains buoyant force? Recall from the earlier lesson ""Pressure of Fluids"" that a fluid exerts pressure in all directions but the pressure is greater at greater depth. Therefore, the fluid below an object exerts greater force on the object than the fluid above the object. This is illustrated in Figure 15.12. Buoyant force explains why objects may float in water. No doubt youve noticed, however, that some objects do not float in water. If buoyant force applies to all objects in fluids, why do some objects sink instead of float? The answer has to do with their weight. "
buoyancy of fluids,T_3625,"Weight is a measure of the force of gravity pulling down on an object. Buoyant force pushes up on an object. Weight and buoyant force together determine whether an object sinks or floats. This is illustrated in Figure 15.13. If an objects weight is the same as the buoyant force acting on the object, then the object floats. This is the example on the left in Figure 15.13. If an objects weight is greater than the buoyant force acting on the object, then the object sinks. This is the example on the right in Figure 15.13. Because of buoyant force, objects seem lighter in water. You may have noticed this when you went swimming and could easily pick up a friend or sibling under the water. Some of the persons weight was countered by the buoyant force of the water. "
buoyancy of fluids,T_3626,"Density, or the amount of mass in a given volume, is also related to buoyancy. Thats because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why ice cubes float in a glass of water. This and other examples of density and buoyant force are illustrated in Figure 15.14 and in the video at this URL:  MEDIA Click image to the left or use the URL below. URL: "
buoyancy of fluids,T_3627,"Did you ever notice that when you get into a bathtub of water the level of the water rises? More than 2200 years ago, a Greek mathematician named Archimedes noticed the same thing. He observed that both a body and the water in a tub cant occupy the same space at the same time. As a result, some of the water is displaced, or moved out of the way. How much water is displaced? Archimedes determined that the volume of displaced water equals the volume of the submerged object. So more water is displaced by a bigger body than a smaller one. What does displacement have to do with buoyant force? Everything! Archimedes discovered that the buoyant force acting on an object in a fluid equals the weight of the fluid displaced by the object. This is known as Archimedes law (or Archimedes Principle). Archimedes law explains why some objects float in fluids even though they are very heavy. Remember the oil tanker that opened this chapter? It is extremely heavy, yet it stays afloat. If a steel ball with the same weight as the ship were put into water, it would sink to the bottom (see Figure 15.15). Thats because the volume of water displaced by the steel ball weighs less than the ball. As a result, the buoyant force is not as great as the force of gravity acting on the ball. The design of the ships hull, on the other hand, causes it to displace much more water than the ball. In fact, the weight of the displaced water is greater than the weight of the ship, so the buoyant force is greater than the force of gravity acting on the ship. As a result, the ship floats. You can check your understanding of Archimedes law by doing the brainteaser at this URL:  . For an entertaining video presentation of Archimedes law, go to this URL: http://videos.howstuffworks.com/disc "
work,T_3628,"Work is defined differently in physics than in everyday language. In physics, work means the use of force to move an object. The teen who is playing tennis in Figure 16.1 is using force to move her tennis racket, so she is doing work. The teen who is studying isnt moving anything, so she is not doing work. Not all force that is used to move an object does work. For work to be done, the force must be applied in the same direction that the object moves. If a force is applied in a different direction than the object moves, no work is done. Figure 16.2 illustrates this point. The stick person applies an upward force on the box when raising it from the ground to chest height. Work is done because the force is applied in the same direction as the box is moving. However, as the stick person walks from left to right while holding the box at chest height, no more work is done by the persons arms holding the box up. Thats because the force supporting the box acts in a different direction than the box is moving. A small amount of work in the horizontal direction is performed when the person is accelerating during the first step of the walk across the room. But other than that, there is no work, because there is no net force acting on the box horizontally. "
work,T_3629,"Work is directly related to both the force applied to an object and the distance the object moves. It can be represented by the equation: Work = Force  Distance This equation shows that the greater the force that is used to move an object or the farther the object is moved, the more work that is done. You can see a short video introduction to work as the product of force and distance at this link:  . To see the effects of force and distance on work, compare the weight lifters in Figure 16.3. The two weight lifters on the left are lifting the same amount of weight, but the bottom weight lifter is lifting the weight a longer distance. Therefore, this weight lifter is doing more work. The two weight lifters on the bottom right are both lifting the weight the same distance, but the weight lifter on the left is lifting a heavier weight. Therefore, this weight lifter is doing more work. "
work,T_3630,"The equation for work given above can be used to calculate the amount of work that is done if force and distance are known. For example, assume that one of the weight lifters in Figure 16.2 lifts a weight of 400 newtons over his head to a height of 2.2 meters off the ground. The amount of work he does is: Work = 400 N  2.2 m = 880 N  m Notice that the unit for work is the newton  meter. This is the SI unit for work, also called the joule (J). One joule equals the amount of work that is done when 1 newton of force moves an object over a distance of 1 meter. Problem Solving Problem: Todd pushed a 500 N box 4 meters across the floor. How much work did he do? Solution: Use the equation Work = Force  Distance. Work = 500 N  4 m = 2000 N  m, or 2000 J You Try It! Problem: Lara lifted a 100 N box 1.5 meters above the floor. How much work did she do? "
work,T_3631,"Did you ever rake leaves, like the woman in Figure 16.4? It can take a long time to do all that work. But if you use an electric leaf blower, like the man in the figure, the job gets done much sooner. Both the leaf blower and the rake do the work of removing leaves from the yard, but the leaf blower has more power. Thats why it can do the same amount of work in less time. "
work,T_3632,"Power is a measure of the amount of work that can be done in a given amount of time. Power can be represented by the equation: Power = Work Time In this equation, work is measured in joules and time is measured in seconds, so power is expressed in joules per second (J/s). This is the SI unit for work, also known as the watt (W). A watt equals 1 joule of work per second. The watt is named for James Watt, a Scottish inventor you will read about below. You may already be familiar with watts. Thats because light bulbs and small appliances such as hair dryers are labeled with the watts of power they provide. For example, the hair dryer in Figure 16.5 is labeled ""2000 watts."" This amount of power could also be expressed kilowatts. A kilowatt equals 1000 watts, so the 2000-watt hair dryer produces 2 kilowatts of power. Compared with a less powerful device, a more powerful device can either do more work in the same time or do the same work in less time. For example, compared with a low-power microwave, a high-power microwave can cook more food in the same time or the same amount of food in less time. "
work,T_3633,"Power can be calculated using the formula above, if the amount of work and time are known. For example, assume that a small engine does 3000 joules of work in 2 seconds. Then the power of the motor is: Power = 3000 J = 1500 J/s, or 1500 W 2s You can also calculate work if you know power and time by rewriting the power equation above as: Work = Power  Time For example, if you use a 2000-watt hair dryer for 30 seconds, how much work is done? First express 2000 watts in J/s and then substitute this value for power in the work equation: Work = 2000 J/s  30 s = 60, 000 J For a video presentation on calculating power and work, go to this link:  Problem Solving Problem: An electric mixer does 2500 joules of work in 5 seconds. What is its power? Solution: Use the equation: Power = Work Time . Power = 2500 J = 500 J/s, or 500 W 5s You Try It! Problem: How much work can be done in 30 seconds by a 1000-watt microwave? "
work,T_3634,"Sometimes power is measured in a unit called the horsepower. One horsepower is the amount of work a horse can do in 1 minute. It equals 745 watts of power. The horsepower was introduced by James Watt, who invented the first powerful steam engine in the 1770s. Watts steam engine led to a revolution in industry and agriculture because of its power. Watt wanted to impress people with the power of his steam engine, so he compared it with something familiar to people of his time: the power of workhorses, like those pictured in Figure 16.6. Watt said his steam engine could produce the power of 20 horses, or 20 horsepower. The most powerful engines today may produce more than 100,000 horsepower! How many watts of power is that? "
machines,T_3635,"A machine is any device that makes work easier by changing a force. When you use a machine, you apply force to the machine. This force is called the input force. The machine, in turn, applies force to an object. This force is called the output force. Recall that work equals force multiplied by distance: Work = Force  Distance The force you apply to a machine is applied over a given distance, called the input distance. The force applied by the machine to the object is also applied over a distance, called the output distance. The output distance may or may not be the same as the input distance. Machines make work easier by increasing the amount of force that is applied, increasing the distance over which the force is applied, or changing the direction in which the force is applied. Contrary to popular belief, machines do not increase the amount of work that is done. They just change how the work is done. So if a machine increases the force applied, it must apply the force over a shorter distance. Similarly, if a machine increases the distance over which the force is applied, it must apply less force. "
machines,T_3636,"Examples of machines that increase force are doorknobs and nutcrackers. Figure 16.8 explains how these machines work. In each case, the force applied by the user is less than the force applied by the machine, but the machine applies the force over a shorter distance. "
machines,T_3637,"Examples of machines that increase the distance over which force is applied are paddles and hammers. Figure 16.9 explains how these machines work. In each case, the machine increases the distance over which the force is applied, but it reduces the strength of the applied force. "
machines,T_3638,"Some machines change the direction of the force applied by the user. They may or may not also change the strength of the force or the distance over which it is applied. Two examples of machines that work in this way are claw hammers and the rope systems (pulleys) that raise or lower flags on flagpoles. Figure 16.10 explains how these machines work. In each case, the direction of the force applied by the user is reversed by the machine. How does this make it easier to do the job? "
machines,T_3639,"An exoskeleton suit may seem like science fiction, turning ordinary humans into super heroes. But wearable robots are moving forward into reality. And for paraplegics, the ability to stand and walk that these machines provide is a super power. QUEST meets Austin Whitney and Tamara Mena, two ""Exoskeleton Test Pilots"" who are now putting this new technology through its paces. For more information on exoskeleton suits, see http://science.kqed.org/ques MEDIA Click image to the left or use the URL below. URL: "
machines,T_3640,"You read above that machines do not increase the work done on an object. In other words, you cant get more work out of a machine than you put into it. In fact, machines always do less work on the object than the user does on the machine. Thats because all machines must use some of the work put into them to overcome friction. How much work? It depends on the efficiency of the machine. Efficiency is the percent of input work that becomes output work. It is a measure of how well a machine reduces friction. "
machines,T_3641,"Consider the ramp in Figure 16.11. Its easier to push the heavy piece of furniture up the ramp to the truck than to lift it straight up off the ground. However, pushing the furniture over the surface of the ramp creates a lot of friction. Some of the force applied to moving the furniture must be used to overcome the friction. It would be more efficient to use a dolly on wheels to roll the furniture up the ramp. Thats because rolling friction is much less than sliding friction. As a result, the efficiency of the ramp would be greater with a dolly. "
machines,T_3642,"Efficiency can be calculated with the equation: Efficiency = Output work 100% Input work Consider a machine that puts out 6000 joules of work. To produce that much work from the machine requires the user to put in 8000 joules of work. To find the efficiency of the machine, substitute these values into the equation for efficiency: Efficiency = 6000 J 100% = 75% 8000 J You Try It! Problem: Rani puts 10,000 joules of work into a car jack. The car jack, in turn, puts out 7000 joules of work to raise up the car. What is the efficiency of the jack? "
machines,T_3643,Another measure of the effectiveness of a machine is its mechanical advantage. Mechanical advantage is the number of times a machine multiplies the input force. It can be calculated with the equation: Mechanical Advantage = Output force Input force This equation computes the actual mechanical advantage of a machine. It takes into account the reduction in output force that is due to friction. It shows how much a machine actually multiplies force when it used in the real world. 
machines,T_3644,"It can be difficult to measure the input and output forces needed to calculate actual mechanical advantage. Its usually much easier to measure the input and output distances. These measurements can then be used to calculate the ideal mechanical advantage. The ideal mechanical advantage represents the multiplication of input force that would be achieved in the absence of friction. Therefore, it is greater than the actual mechanical advantage because all machines use up some work in overcoming friction. Ideal mechanical advantage is calculated with the equation: Ideal Mechanical Advantage = Input distance Output distance Compare this equation with the equation above for actual mechanical advantage. Notice how the input and output values are switched. This makes sense when you recall that when a machine increases force, it decreases distance and vice versa. You can watch a video about actual and ideal mechanical advantage at this link: http://video.goo Consider the simple ramp in Figure 16.12. A ramp can be used to raise an object up off the ground. The input distance is the length of the sloped surface of the ramp. The output distance is the height of the ramp, or the vertical distance the object is raised. Therefore, the ideal mechanical advantage of the ramp is: Ideal Mechanical Advantage = 6m =3 2m An ideal mechanical advantage of 3 means that the ramp ideally (in the absence of friction) multiplies the output force by a factor of 3. "
machines,T_3645,"As you read above, some machines increase the force put into the machine, while other machines increase the distance over which the force is applied. Still other machines change only the direction of the force. Which way a machine works affects its mechanical advantage. For machines that increase force including ramps, doorknobs, and nutcrackers the output force is greater than the input force. Therefore, the mechanical advantage is greater than 1. For machines that increase the distance over which force is applied, such as paddles and hammers, the output force is less than the input force. Therefore, the mechanical advantage is less than 1. For machines that change only the direction of the force, such as the rope systems on flagpoles, the output force is the same as the input force. Therefore, the mechanical advantage is equal to 1. "
simple machines,T_3646,"The man in Figure 16.14 is using a ramp to move a heavy dryer up to the back of a truck. The highway in the figure switches back and forth so it climbs up the steep hillside. Both the ramp and the highway are examples of inclined planes. An inclined plane is a simple machine consisting of a sloping surface that connects lower and higher elevations. The sloping surface of the inclined plane supports part of the weight of the object as it moves up the slope. As a result, it takes less force to move the object uphill. The trade-off is that the object must be moved over a greater distance than if it were moved straight up to the higher elevation. On the other hand, the output force is greater than the input force because it is applied over a shorter distance. Like other simple machines, the ideal mechanical advantage of an inclined plane is given by: Ideal Mechanical Advantage = Input distance Output distance For an inclined plane, the input distance is the length of the sloping surface, and the output distance is the maximum height of the inclined plane. This was illustrated in Figure 16.12. Because the sloping surface is always greater than the height of the inclined plane, the ideal mechanical advantage of an inclined plane is always greater than 1. An inclined plane with a longer sloping surface relative to its height has a gentler slope. An inclined plane with a gentler slope has a greater mechanical advantage and requires less input force to move an object to a higher elevation. "
simple machines,T_3647,Two simple machines that are based on the inclined plane are the wedge and the screw. Both increase the force used to move an object because the input force is applied over a greater distance than the output force. 
simple machines,T_3648,"Imagine trying to slice a tomato with a fork or spoon instead of a knife, like the one in Figure 16.15. The knife makes the job a lot easier because of the wedge shape of the blade. A wedge is a simple machine that consists of two inclined planes. But unlike one inclined plane, a wedge works only when it moves. It has a thin end and thick end, and the thin end is forced into an object to cut or split it. The chisel in Figure 16.15 is another example of a wedge. The input force is applied to the thick end of a wedge, and it acts over the length of the wedge. The output force pushes against the object on both sides of the wedge, so the output distance is the thickness of the wedge. Therefore, the ideal mechanical advantage of a wedge can be calculated as: Ideal Mechanical Advantage = Length of wedge Maximum thickness of wedge The length of a wedge is always greater than its maximum thickness. As a result, the ideal mechanical advantage of a wedge is always greater than 1. "
simple machines,T_3649,"The spiral staircase in Figure 16.16 also contains an inclined plane. Do you see it? The stairs that wrap around the inside of the walls make up the inclined plane. The spiral staircase is an example of a screw. A screw is a simple machine that consists of an inclined plane wrapped around a cylinder or cone. No doubt you are familiar with screws like the wood screw in Figure 16.16. The screw top of the container in the figure is another example. Screws move objects to a higher elevation (or greater depth) by increasing the force applied. When you use a wood screw, you apply force to turn the inclined plane. The output force pushes the screw into the wood. It acts along the length of the cylinder around which the inclined plane is wrapped. Therefore, the ideal mechanical advantage of a screw is calculated as: Ideal Mechanical Advantage = Length of inclined plane Length of screw The length of the inclined plane is always greater than the length of the screw. As a result, the mechanical advantage of a screw is always greater than 1. Look at the collection of screws and bolts in Figure 16.17. In some of them, the turns (or threads) of the inclined plane are closer together. The closer together the threads are, the longer the inclined plane is relative to the length of the screw or bolt, so the greater its mechanical advantage is. Therefore, if the threads are closer together, you need to apply less force to penetrate the wood or other object. The trade-off is that more turns of the screw or bolt are needed to do the job because the distance over which the input force must be applied is greater. "
simple machines,T_3650,"Did you ever use a hammer to pull a nail out of a board? If not, you can see how its done in Figure 16.18. When you pull down on the handle of the hammer, the claw end pulls up on the nail. A hammer is an example of a lever. A lever is a simple machine consisting of a bar that rotates around a fixed point called the fulcrum. For a video introduction to levers using skateboards as examples, go to this link:  MEDIA Click image to the left or use the URL below. URL:  A lever may or may not increase the force applied, and it may or may not change the direction of the force. It all depends on the location of the input and output forces relative to the fulcrum. In this regard, there are three basic types of levers, called first-class, second-class, and third-class levers. Figure 16.19 describes the three classes. "
simple machines,T_3651,"All three classes of levers make work easier, but they do so in different ways. When the input and output forces are on opposite sides of the fulcrum, the lever changes the direction of the applied force. This occurs only with a first-class lever. When both the input and output forces are on the same side of the fulcrum, the direction of the applied force does not change. This occurs with both second- and third-class levers. When the input force is applied farther from the fulcrum, the input distance is greater than the output distance, so the ideal mechanical advantage is greater than 1. This always occurs with second-class levers and may occur with first-class levers. When the input force is applied closer to the fulcrum, the input distance is less than the output distance, so the ideal mechanical advantage is less than 1. This always occurs with third-class levers and may occur with first-class levers. When both forces are the same distance from the fulcrum, the input distance equals the output distance, so the ideal mechanical advantage equals 1. This occurs only with first class-levers. "
simple machines,T_3652,"You may be wondering why you would use a third-class lever when it doesnt change the direction or strength of the applied force. The advantage of a third-class lever is that the output force is applied over a greater distance than the input force. This means that the output end of the lever must move faster than the input end. Why would this be useful when you are moving a hockey stick or baseball bat, both of which are third-class levers? "
simple machines,T_3653,"Did you ever ride on a Ferris wheel, like the one pictured in Figure 16.20? If you did, then you know how thrilling the ride can be. A Ferris wheel is an example of a wheel and axle. A wheel and axle is a simple machine that consists of two connected rings or cylinders, one inside the other, which both turn in the same direction around a single center point. The smaller, inner ring or cylinder is called the axle. The bigger, outer ring or cylinder is called the wheel. The car steering wheel in Figure 16.20 is another example of a wheel and axle. In a wheel and axle, force may be applied either to the wheel or to the axle. In both cases, the direction of the force does not change, but the force is either increased or applied over a greater distance. When the input force is applied to the axle, as it is with a Ferris wheel, the wheel turns with less force, so the ideal mechanical advantage is less than 1. However, the wheel turns over a greater distance, so it turns faster than the axle. The speed of the wheel is one reason that the Ferris wheel ride is so exciting. When the input force is applied to the wheel, as it is with a steering wheel, the axle turns over a shorter distance but with greater force, so the ideal mechanical advantage is greater than 1. This allows you to turn the steering wheel with relatively little effort, while the axle of the steering wheel applies enough force to turn the car. "
simple machines,T_3654,"Another simple machine that uses a wheel is the pulley. A pulley is a simple machine that consists of a rope and grooved wheel. The rope fits into the groove in the wheel, and pulling on the rope turns the wheel. Figure 16.21 shows two common uses of pulleys. Some pulleys are attached to a beam or other secure surface and remain fixed in place. They are called fixed pulleys. Other pulleys are attached to the object being moved and are moveable themselves. They are called moveable pulleys. Sometimes, fixed and moveable pulleys are used together. They make up a compound pulley. The three types of pulleys are compared in Figure 16.22. In all three types, the ideal mechanical advantage is equal to the number of rope segments pulling up on the object. The more rope segments that are helping to do the lifting work, the less force that is needed for the job. You can experiment with an interactive animation of compound pulleys with various numbers of pulleys at this link:  . In a single fixed pulley, only one rope segment lifts the object, so the ideal mechanical advantage is 1. This type of pulley doesnt increase the force, but it does change the direction of the force. This allows you to use your weight to pull on one end of the rope and more easily raise the object attached to the other end. In a single moveable pulley, two rope segments lift the object, so the ideal mechanical advantage is 2. This type of pulley doesnt change the direction of the force, but it does increase the force. In a compound pulley, two or more rope segments lift the object, so the ideal mechanical advantage is equal to or greater than 2. This type of pulley may or may not change the direction of the force, depending on the number and arrangement of pulleys. When several pulleys are combined, the increase in force may be very great. To learn more about the mechanical advantage of different types of pulleys, watch the video at this link: http://video "
compound machines,T_3655,"A compound machine is a machine that consists of more than one simple machine. Some compound machines consist of just two simple machines. For example, a wheelbarrow consists of a lever, as you read earlier in the lesson ""Simple Machines,"" and also a wheel and axle. Other compound machines, such as cars, consist of hundreds or even thousands of simple machines. Two common examples of compound machines are scissors and fishing rods with reels. To view a young students compound machine invention that includes several simple machines, watch the video at this link:  . To see if you can identify the simple machines in a lawn mower, go to the URL below and click on Find the Simple Machines. "
compound machines,T_3656,"Look at the scissors in Figure 16.24. As you can see from the figure, scissors consist of two levers and two wedges. You apply force to the handle ends of the levers, and the output force is exerted by the blade ends of the levers. The fulcrum of both levers is where they are joined together. Notice that the fulcrum lies between the input and output points, so the levers are first-class levers. They change the direction of force. They may or may not also increase force, depending on the relative lengths of the handles and blades. The blades themselves are wedges, with a sharp cutting edge and a thicker dull edge. "
compound machines,T_3657,"The fishing rod with reel shown in Figure 16.25 is another compound machine. The rod is a third-class lever, with the fulcrum on one end of the rod, the input force close to the fulcrum, and the output force at the other end of the rod. The output distance is greater than the input distance, so the angler can fling the fishing line far out into the water with just a flick of the wrist. The reel is a wheel and axle that works as a pulley. The fishing line is wrapped around the wheel. Using the handle to turn the axle of the wheel winds or unwinds the line. "
compound machines,T_3658,"Riding a bicycle might be easy. But the forces that allow humans to balance atop a bicycle are complex. QUEST visits Davis  a city that loves its bicycles  to take a ride on a research bicycle and explore a collection of antique bicycles. Scientists say studying the complicated physics of bicycling can lead to the design of safer, and more efficient bikes. For more information on the science of riding a bicycle, see  MEDIA Click image to the left or use the URL below. URL: "
compound machines,T_3659,"Because compound machines have more moving parts than simple machines, they generally have more friction to overcome. As a result, compound machines tend to have lower efficiency than simple machines. When a compound machine consists of a large number of simple machines, friction may become a serious problem, and it may produce a lot of heat. Lubricants such as oil or grease may be used to coat the moving parts so they slide over each other more easily. This is how a cars friction is reduced. Compound machines have a greater mechanical advantage than simple machines. Thats because the mechanical advantage of a compound machine equals the product of the mechanical advantages of all its component simple machines. The greater the number of simple machines it contains, the greater is its mechanical advantage. "
types of energy,T_3660,"The concept of energy was first introduced in the chapter ""States of Matter,"" where it is defined as the ability to cause change in matter. Energy can also be defined as the ability to do work. Work is done whenever a force is used to move matter. When work is done, energy is transferred from one object to another. For example, when the batter in Figure 17.2 uses energy to swing the bat, she transfers energy to the bat. The moving bat, in turn, transfers energy to the ball. Like work, energy is measured in the joule (J), or newtonmeter (Nm). Energy exists in different forms, which you can read about in the lesson ""Forms of Energy"" later in the chapter. Some forms of energy are mechanical, electrical, and chemical energy. Most forms of energy can also be classified as kinetic or potential energy. Kinetic and potential forms of mechanical energy are the focus of this lesson. Mechanical energy is the energy of objects that are moving or have the potential to move. "
types of energy,T_3661,"What do all the photos in Figure 17.3 have in common? All of them show things that are moving. Kinetic energy is the energy of moving matter. Anything that is moving has kinetic energy from the atoms in matter to the planets in solar systems. Things with kinetic energy can do work. For example, the hammer in the photo is doing the work of pounding the nail into the board. You can see a cartoon introduction to kinetic energy and its relation to work at this URL:  . The amount of kinetic energy in a moving object depends on its mass and velocity. An object with greater mass or greater velocity has more kinetic energy. The kinetic energy of a moving object can be calculated with the equation: 1 Kinetic Energy (KE) = mass  velocity2 2 This equation for kinetic energy shows that velocity affects kinetic energy more than mass does. For example, if mass doubles, kinetic energy also doubles. But if velocity doubles, kinetic energy increases by a factor of four. Thats because velocity is squared in the equation. You can see for yourself how mass and velocity affect kinetic energy by working through the problems below. Problem Solving Problem: Juan has a mass of 50 kg. If he is running at a velocity of 2 m/s, how much kinetic energy does he have? Solution: Use the formula: KE = 12 mass  velocity2 1 50 kg  (2 m/s2 ) 2 = 100 kg  m2 /s2 = 100 N  m, or 100 J KE = You Try It! Problem: What is Juans kinetic energy if he runs at a velocity of 4 m/s? Problem: Juans dad has a mass of 100 kg. How much kinetic energy does he have if he runs at a velocity of 2 m/s? "
types of energy,T_3662,"Did you ever see a scene like the one in Figure 17.4? In many parts of the world, trees lose their leaves in autumn. The leaves turn color and then fall from the trees to the ground. As the leaves are falling, they have kinetic energy. While they are still attached to the trees they also have energy, but its not because of motion. Instead, they have stored energy, called potential energy. An object has potential energy because of its position or shape. For example leaves on trees have potential energy because they could fall due to the pull of gravity. "
types of energy,T_3663,"Potential energy due to the position of an object above Earth is called gravitational potential energy. Like the leaves on trees, anything that is raised up above Earths surface has the potential to fall because of gravity. You can see examples of people with gravitational potential energy in Figure 17.5. Gravitational potential energy depends on an objects weight and its height above the ground. It can be calculated with the equation: Gravitational potential energy (GPE) = weight  height Consider the diver in Figure 17.5. If he weighs 70 newtons and the diving board is 5 meters above Earths surface, then his potential energy is: GPE = 70 N  5 m = 350 N  m, or 350 J "
types of energy,T_3664,"Potential energy due to an objects shape is called elastic potential energy. This energy results when elastic objects are stretched or compressed. Their elasticity gives them the potential to return to their original shape. For example, the rubber band in Figure 17.6 has been stretched, but it will spring back to its original shape when released. Springs like the handspring in the figure have elastic potential energy when they are compressed. What will happen when the handspring is released? "
types of energy,T_3665,"Remember the diver in Figure 17.5? What happens when he jumps off the diving board? His gravitational potential energy changes to kinetic energy as he falls toward the water. However, he can regain his potential energy by getting out of the water and climbing back up to the diving board. This requires an input of kinetic energy. These changes in energy are examples of energy conversion, the process in which energy changes from one type or form to another. "
types of energy,T_3666,"The law of conservation of energy applies to energy conversions. Energy is not used up when it changes form, although some energy may be used to overcome friction, and this energy is usually given off as heat. For example, the divers kinetic energy at the bottom of his fall is the same as his potential energy when he was on the diving board, except for a small amount of heat resulting from friction with the air as he falls. "
types of energy,T_3667,There are many other examples of energy conversions between potential and kinetic energy. Figure 17.7 describes how potential energy changes to kinetic energy and back again on swings and trampolines. You can see an animation of changes between potential and kinetic energy on a ramp at the URL below. Can you think of other examples? 
types of energy,T_3668,"QUEST teams up with Make Magazine to construct the latest must have, do-it-yourself device hacks and science projects. This week well show you how to make a tabletop linear accelerator that demonstrates the finer points of kinetic energy by shooting a steel ball. For more information on the tabletop linear accelerator, see http://science.k MEDIA Click image to the left or use the URL below. URL: "
forms of energy,T_3669,"Energy, or the ability to do work, can exist in many different forms. The photo in Figure 17.8 represents six of the eight different forms of energy that are described in this lesson. The guitarist gets the energy he needs to perform from chemical energy in food. He uses mechanical energy to pluck the strings of the guitar. The stage lights use electrical energy and give off both light energy and thermal energy, commonly called heat. The guitar also uses electrical energy, and it produces sound energy when the guitarist plucks the strings. For an introduction to all these forms of energy, go to this URL:  . For an interactive animation about the different forms of energy, visit this URL:  After you read below about different forms of energy, you can check your knowledge by doing the drag and drop quiz at this URL:  . "
forms of energy,T_3670,"Mechanical energy is the energy of an object that is moving or has the potential to move. It is the sum of an objects kinetic and potential energy. In Figure 17.9, the basketball has mechanical energy because it is moving. The arrow in the same figure has mechanical energy because it has the potential to move due to the elasticity of the bow. What are some other examples of mechanical energy? "
forms of energy,T_3671,"Energy is stored in the bonds between atoms that make up compounds. This energy is called chemical energy, and it is a form of potential energy. If the bonds between atoms are broken, the energy is released and can do work. The wood in the fireplace in Figure 17.10 has chemical energy. The energy is released as thermal energy when the wood burns. People and many other living things meet their energy needs with chemical energy stored in food. When food molecules are broken down, the energy is released and may be used to do work. "
forms of energy,T_3672,"Electrons are negatively charged particles in atoms. Moving electrons have a form of kinetic energy called electrical energy. If youve ever experienced an electric outage, then you know how hard it is to get by without electrical energy. Most of the electrical energy we use is produced by power plants and arrives in our homes through wires. Two other sources of electrical energy are pictured in Figure 17.11. "
forms of energy,T_3673,"The nuclei of atoms are held together by powerful forces. This gives them a tremendous amount of stored energy, called nuclear energy. The energy can be released and used to do work. This happens in nuclear power plants when nuclei fission, or split apart. It also happens in the sun and other stars when nuclei fuse, or join together. Some of the suns energy travels to Earth, where it warms the planet and provides the energy for photosynthesis (see Figure "
forms of energy,T_3674,"The atoms that make up matter are in constant motion, so they have kinetic energy. All that motion gives matter thermal energy. Thermal energy is defined as the total kinetic energy of all the atoms that make up an object. It depends on how fast the atoms are moving and how many atoms the object has. Therefore, an object with more mass has greater thermal energy than an object with less mass, even if their individual atoms are moving at the same speed. You can see an example of this in Figure 17.13. "
forms of energy,T_3675,"Energy that the sun and other stars release into space is called electromagnetic energy. This form of energy travels through space as electrical and magnetic waves. Electromagnetic energy is commonly called light. It includes visible light, as well as radio waves, microwaves, and X rays (Figure 17.14). "
forms of energy,T_3676,"The drummer in Figure 17.15 is hitting the drumheads with drumsticks. This causes the drumheads to vibrate. The vibrations pass to surrounding air particles and then from one air particle to another in a wave of energy called sound energy. We hear sound when the sound waves reach our ears. Sound energy can travel through air, water, and other substances, but not through empty space. Thats because the energy needs particles of matter to pass it on. "
forms of energy,T_3677,"Energy often changes from one form to another. For example, the mechanical energy of a moving drumstick changes to sound energy when it strikes the drumhead and causes it to vibrate. Any form of energy can change into any other form. Frequently, one form of energy changes into two or more different forms. For example, when wood burns, the woods chemical energy changes to both thermal energy and light energy. Other examples of energy conversions are described in Figure 17.16. You can see still others at this URL: http://fi.edu/guide/hughes/energychangeex.html . You can check your understanding of how energy changes form by doing the quizzes at these URLs:  Energy is conserved in energy conversions. No energy is lost when energy changes form, although some may be released as thermal energy due to friction. For example, not all of the energy put into a steam turbine in Figure 17.16 changes to electrical energy. Some changes to thermal energy because of friction of the turning blades and other moving parts. The more efficient a device is, the greater the percentage of usable energy it produces. Appliances with an ""Energy Star"" label like the one in Figure 17.17 use energy efficiently and thereby reduce energy use. "
energy resources,T_3678,Nonrenewable resources are natural resources that are limited in supply and cannot be replaced except over millions of years. Nonrenewable energy resources include fossil fuels and radioactive elements such as uranium. 
energy resources,T_3679,"Fossil fuels are mixtures of hydrocarbons that formed over millions of years from the remains of dead organisms. They include petroleum (commonly called oil), natural gas, and coal. Fossil fuels provide most of the energy used in the world today. They are burned in power plants to produce electrical energy, and they also fuel cars, heat homes, and supply energy for many other purposes. You can see examples of their use in Figure 17.19. Fossil fuels contain stored chemical energy that came originally from the sun. Ancient plants changed energy in "
energy resources,T_3680,"Like fossil fuels, the radioactive element uranium can be used to generate electrical energy in power plants. In a nuclear power plant, the nuclei of uranium atoms are split in the process of nuclear fission. This process releases a tremendous amount of energy from just a small amount of uranium. The total supply of uranium in the world is quite limited, however, and cannot be replaced once it is used up. This makes nuclear energy a nonrenewable resource. Although using nuclear energy does not release carbon dioxide or cause air pollution, it does produce dangerous radioactive wastes. Accidents at nuclear power plants also have the potential to release large amounts of radioactive material into the environment. Figure 17.21 describes the nuclear disaster caused by a Japanese tsunami in 2011. You can learn more about the disaster and its aftermath at the URLs below. "
energy resources,T_3681,"President Obama says the United States needs new nuclear reactors, to meet the countrys energy demands and counter climate change. But can nuclear power be produced more safely and affordably? A scientist at the University of California, Berkeley, is working to do just that. For more information about nuclear energy, see http://science.k MEDIA Click image to the left or use the URL below. URL: "
energy resources,T_3682,"Renewable resources are natural resources that can be replaced in a relatively short period of time or are virtually limitless in supply. Renewable energy resources include sunlight, moving water, wind, biomass, and geothermal energy. Each of these energy resources is described in Table 17.1. Resources such as sunlight and wind are limitless in supply, so they will never run out. Besides their availability, renewable energy resources also have the advantage of producing little if any pollution and not contributing to global warming. The technology needed to gather energy from renewable resources is currently expensive to install, but most of the resources themselves are free for the taking. here? Renewable Energy Resource Sunlight The energy in sunlight, or solar energy, can be used to heat homes. It can also be used to produce electricity in solar cells. However, solar energy may not be practical in areas that are often cloudy. Example Solar panels on the roof of this house generate enough electricity to supply a familys needs. Moving Water When water falls downhill, its potential energy is con- verted to kinetic energy that can turn a turbine and generate electricity. The water may fall naturally over a waterfall or flow through a dam. A drawback of dams is that they flood land upstream and reduce water flow downstream. Either effect may harm ecosystems. Wind Wind is moving air, so it has kinetic energy that can do work. Remember the wind turbines that opened this chapter? Wind turbines change the kinetic energy of the wind to electrical energy. Only certain areas of the world get enough steady wind to produce much electricity. Many people also think that wind turbines are noisy and unattractive in the landscape. Water flowing through Hoover dam between Arizona and Nevada generates electricity for both of these states and also by southern California. The dam spans the Colorado River. This old-fashioned windmill captures wind energy that is used for pumping water out of a well. Windmills like this one have been used for centuries. Renewable Energy Resource Biomass The stored chemical energy of trees and other plants is called biomass energy. When plant materials are burned, they produce thermal energy that can be used for heating, cooking, or generating electricity. Biomassespecially woodis an important energy source in countries where most people cant afford fossil fuels. Some plants can also be used to make ethanol, a fuel that is added to gasoline. Ethanol produces less pollution than gasoline, but large areas of land are needed to grow the plants needed to make it. Geothermal Heat below Earths surfacecalled geothermal en- ergycan be used to produce electricity. A power plant pumps water underground where it is heated. Then it pumps the water back to the plant and uses its thermal energy to generate electricity. On a small scale, geothermal energy can be used to heat homes. Installing a geothermal system can be very costly, how- ever, because of the need to drill through underground rocks. Example This large machine is harvesting and grinding plants to be used for biomass energy. This geothermal power plant is located in Italy where hot magma is close to the surface. "
energy resources,T_3683,"The largest solar thermal plant in the world opens in Californias Mojave Desert, after a debate that pitted renewable energy against a threatened tortoise. The Ivanpah solar plant is one of seven big solar farms scheduled to open in California in the coming months, as a result of the states push to produce one third of its electricity from renewable energy. Some 30 states have similar mandates. For more information on this solar plant, see http://science.kqed.org/ MEDIA Click image to the left or use the URL below. URL: "
energy resources,T_3684,"On the windswept tarmac of the former Alameda Naval Air Station, an inventive group of scientists and engineers are test-flying a kite-like tethered wing that may someday help revolutionize clean energy. QUEST explores the potential of wind energy and new airborne wind turbines designed to harness the stronger and more consistent winds found at higher altitudes. For more information on wind energy, see http://science.kqed.org/quest/video/airborne MEDIA Click image to the left or use the URL below. URL: "
energy resources,T_3685,"Solar and wind power may get the headlines when it comes to renewable energy. But another type of clean power is heating up in the hills just north of Sonoma wine country. Geothermal power uses heat from deep inside the Earth to generate electricity. The Geysers, the worlds largest power-producing geothermal field, has been providing electricity for roughly 850,000 Northern California households, and is set to expand even further. For more information on geothermal energy, see http://science.kqed.org/quest/video/geothermal-heats-up/ . MEDIA Click image to the left or use the URL below. URL: "
energy resources,T_3686,"Figure 17.22 shows the mix of energy resources used worldwide in 2006. Fossil fuels still provide most of the worlds energy, with oil being the single most commonly used energy resource. Natural gas is used less than the other two fossil fuels, but even natural gas is used more than all renewable energy resources combined. Wind, solar, and geothermal energy contribute the least to global energy use, despite the fact that they are virtually limitless in supply and nonpolluting. "
energy resources,T_3687,"People in the richer nations of the world use far more energy, especially energy from fossil fuels, than people in the poorer nations do. Figure 17.23 compares the amounts of oil used by the top ten oil-consuming nations. The U.S. uses more oil than several other top-ten countries combined. If you also consider the population size in these countries, the differences are even more stunning. The average person in the U.S. uses a whopping 23 barrels of oil a year! In comparison, the average person in India or China uses just 1 or 2 barrels a year. Because richer nations use more fossil fuels, they also cause more air pollution and global warming than poorer nations do. "
energy resources,T_3688,We can reduce our use of energy resources and the pollution they cause by conserving energy. Conservation means saving resources by using them more efficiently or not using them at all. Figure 17.24 shows several ways that people can conserve energy in their daily lives. You can find more energy-saving tips at the URL below. What do you do to save energy? What else could you do? 
energy resources,T_3689,"QUEST teams up with Climate Watch to give you an inside look at home energy efficiency. Tag along with Sustainable Spaces on a home efficiency ""green-up"" and learn tips on how to make your home more energy efficient. For more information on home energy audits, see http://science.kqed.org/quest/video/web-extra-home-energy-audit/ . MEDIA Click image to the left or use the URL below. URL: "
energy resources,T_3690,"With the race on to reduce global warming and fossil fuel dependency, experts in alternative energy see a bright future for renewable resources like wind, solar, hydro-power and geothermal energy. QUEST and Climate Watch team up to look at the ""Smart Grid"" of the future and how it might be improved to more cleanly and efficiently keep the lights on in California. For more information on the ""Smart Grid"", see http://science.kqed.org/quest/video/clim MEDIA Click image to the left or use the URL below. URL: "
temperature and heat,T_3691,"No doubt you already have a good idea of what temperature is. You might define it as how hot or cold something feels. In physics, temperature is defined as the average kinetic energy of the particles in an object. When particles move more quickly, temperature is higher and an object feels warmer. When particles move more slowly, temperature is lower and an object feels cooler. "
temperature and heat,T_3692,"If two objects have the same mass, the object with the higher temperature has greater thermal energy. Temperature affects thermal energy, but temperature isnt the same thing as thermal energy. Thats because an objects mass also affects its thermal energy. The examples in Figure 18.1 make this clear. In the figure, the particles of cocoa are moving faster than the particles of bathwater. Therefore, the cocoa has a higher temperature. However, the bath water has more thermal energy because there is so much more of it. It has many more moving particles. Bill Nye the Science Guy cleverly discusses these concepts at this URL:  MEDIA Click image to the left or use the URL below. URL:  If youre still not clear about the relationship between temperature and thermal energy, watch the animation ""Tem- perature"" at this URL:  . "
temperature and heat,T_3693,"Temperature is measured with a thermometer. A thermometer shows how hot or cold something is relative to two reference temperatures, usually the freezing and boiling points of water. Scientists often use the Celsius scale for temperature. On this scale, the freezing point of water is 0C and the boiling point is 100C. To learn more about measuring temperature, watch the animation Measuring Temperature at this URL:  Did you ever wonder how a thermometer works? Look at the thermometer in Figure 18.2. Particles of the red liquid have greater energy when they are warmer, so they move more and spread apart. This causes the liquid to expand and rise higher in the glass tube. Like the liquid in a thermometer, most types of matter expand to some degree when they get warmer. Gases usually expand the most when heated, followed by liquids. Solids generally expand the least. (Water is an exception; it takes up more space as a solid than as a liquid.) "
temperature and heat,T_3694,"Something that has a high temperature is said to be hot. Does temperature measure heat? Is heat just another word for thermal energy? The answer to both questions is no. Heat is the transfer of thermal energy between objects that have different temperatures. Thermal energy always moves from an object with a higher temperature to an object with a lower temperature. When thermal energy is transferred in this way, the warm object becomes cooler and the cool object becomes warmer. Sooner or later, both objects will have the same temperature. Only then does the transfer of thermal energy end. For a visual explanation of these concepts, watch the animation ""Temperature vs. Heat"" at this URL:  . "
temperature and heat,T_3695,"Figure 18.3 illustrates an example of thermal energy transfer. Before the spoon was put into the steaming hot coffee, it was cool to the touch. Once in the coffee, the spoon heated up quickly. The fast-moving particles of the coffee transferred some of their energy to the slower-moving particles of the spoon. The spoon particles started moving faster and became warmer, causing the temperature of the spoon to rise. Because the coffee particles lost some of their kinetic energy to the spoon particles, the coffee particles started to move more slowly. This caused the temperature of the coffee to fall. Before long, the coffee and spoon had the same temperature. "
temperature and heat,T_3696,"The girls in Figure 18.4 are having fun at the beach. Its a warm, sunny day, and the sand feels hot under their bare hands and feet. The water, in contrast, feels much cooler. Why does the sand get so hot while the water does not? The answer has to do with specific heat. Specific heat is the amount of energy (in joules) needed to raise the temperature of 1 gram of a substance by 1C. Specific heat is a property that is specific to a given type of matter. Table 18.1 lists the specific heat of four different substances. Metals such as iron have relatively low specific heat. It doesnt take much energy to raise their temperature. Thats why a metal spoon heats up quickly when placed in hot coffee. Sand also has a relatively low specific heat, whereas water has a very high specific heat. It takes a lot more energy to increase the temperature of water than sand. This explains why the sand on a beach gets hot while the water stays cool. Differences in the specific heat of water and land also affect climate. To learn how, watch the video at this URL:  MEDIA Click image to the left or use the URL below. URL:  In Table 18.1, how much greater is the specific heat of water than sand? Substances iron sand wood water Specific Heat (joules) 0.45 0.67 1.76 4.18 "
temperature and heat,T_3697,"The roadway across the Golden Gate Bridge rises and falls as much as 16 feet depending on the temperature. When the sun hits the bridge, the metal expands and the bridge cables stretch. As the fog rolls in, the cables contract and the bridge goes up. Curators from the Outdoor Exploratorium in San Francisco have set up a scope two miles away so you can see how the bridge is moving up or down depending on the weather. For more information on how the bridge moves due to temperature, see http://science.kqed.org/quest/video/quest-lab-bridge-thermometer/ . Heat is the transfer of thermal energy between objects that have different temperatures. Thermal energy always moves from an object with a higher temperature to an object with a lower temperature. Specific heat is the amount of energy (in joules) needed to raise the temperature of 1 gram of a substance by 1C. Substances differ in their specific heat. "
transfer of thermal energy,T_3698,"Conduction is the transfer of thermal energy between particles of matter that are touching. When energetic particles collide with nearby particles, they transfer some of their thermal energy. From particle to particle, like dominoes falling, thermal energy moves throughout a substance. In Figure 18.5, conduction occurs between particles of the metal in the pot and between particles of the pot and the water. Figure 18.6 shows additional examples of conduction. For a deeper understanding of this method of heat transfer, watch the animation ""Conduction"" at this URL: http://w "
transfer of thermal energy,T_3699,"Conduction is usually faster in liquids and certain solids than in gases. Materials that are good conductors of thermal energy are called thermal conductors. Metals are excellent thermal conductors. They have freely moving electrons that can transfer energy quickly and easily. Thats why the metal pot in Figure 18.5 soon gets hot all over, even though it gains thermal energy from the fire only at the bottom of the pot. In Figure 18.6, the metal heating element of the curling iron heats up almost instantly and quickly transfers energy to the strands of hair that it touches. "
transfer of thermal energy,T_3700,"Particles of gases are farther apart and have fewer collisions, so they are not good at transferring thermal energy. Materials that are poor thermal conductors are called thermal insulators. Figure 18.7 shows several examples. Fluffy yellow insulation inside the roof of a home is full of air. The air prevents the transfer of thermal energy into the house on hot days and out of the house on cold days. A puffy down jacket keeps you warm in the winter for the same reason. Its feather filling holds trapped air that prevents energy transfer from your warm body to the cold air outside. Solids like plastic and wood are also good thermal insulators. Thats why pot handles and cooking utensils are often made of these materials. "
transfer of thermal energy,T_3701,"Everyday, women living in the refugee camps of Darfur, Sudan must walk for up to seven hours outside the safety of the camps to collect firewood for cooking, putting them at risk for violent attacks. Now, researchers at Lawrence Berkeley National Laboratory have engineered a more efficient wood-burning stove, which is greatly reducing both the womens need for firewood and the threats against them. For more information on these stoves, see http://scien MEDIA Click image to the left or use the URL below. URL: "
transfer of thermal energy,T_3702,"Convection is the transfer of thermal energy by particles moving through a fluid. Particles transfer energy by moving from warmer to cooler areas. Thats how energy is transferred in the soup in Figure 18.7. Particles of soup near the bottom of the pot get hot first. They have more energy so they spread out and become less dense. With lower density, these particles rise to the top of the pot (see Figure 18.8). By the time they reach the top of the pot they have cooled off. They have less energy to move apart, so they become denser. With greater density, the particles sink to the bottom of the pot, and the cycle repeats. This loop of moving particles is called a convection current. Convection currents move thermal energy through many fluids, including molten rock inside Earth, water in the oceans, and air in the atmosphere. In the atmosphere, convection currents create wind. You can see one way this happens in Figure 18.9. Land heats up and cools off faster than water because it has lower specific heat. Therefore, land is warmer during the day and cooler at night than water. Air close to the surface gains or loses heat as well. Warm air rises because it is less dense, and when it does, cool air moves in to take its place. This creates a convection current that carries air from the warmer to the cooler area. You can learn more about convection currents by watching ""Convection"" at this URL:  . "
transfer of thermal energy,T_3703,"Both conduction and convection transfer energy through matter. Radiation is the only way of transferring energy that doesnt require matter. Radiation is the transfer of energy by waves that can travel through empty space. When the waves reach objects, they transfer energy to the objects, causing them to warm up. This is how the suns energy reaches Earth and heats its surface (see Figure 18.10). Radiation is also how thermal energy from a campfire warms people nearby. You might be surprised to learn that all objects radiate thermal energy, including people. In fact, when a room is full of people, it may feel noticeably warmer because of all the thermal energy the people radiate! To learn more about thermal radiation, watch ""Radiation"" at the URL below. "
using thermal energy,T_3704,Warming homes and other buildings is an obvious way that thermal energy can be used. Two common types of home heating systems are hot-water and warm-air heating systems. Both types are described below. You can watch an animation showing how a solar heating system works at this URL: 
using thermal energy,T_3705,"A hot-water heating system uses thermal energy to heat water and then pumps the hot water throughout the building in a system of pipes and radiators. You can see a diagram of this type of heating system in Figure 18.12. Typically, the water is heated in a boiler that burns natural gas or heating oil. There is usually a radiator in each room that gets warm when the hot water flows through it. The radiator transfers thermal energy to the air around it by conduction and radiation. The warm air then circulates throughout the room in convection currents. The hot water cools as it flows through the system and transfers its thermal energy. When it finally returns to the boiler, it is heated again and the cycle repeats. "
using thermal energy,T_3706,"A warm-air heating system uses thermal energy to heat air. It then forces the warm air through a system of ducts. You can see a diagram of this type of heating system in Figure 18.13. Typically, the air is heated in a furnace that burns natural gas or heating oil. When the air is warm, a fan blows it through the ducts and out through vents that are located in each room. Warm air blowing out of a vent moves across the room, pushing cold air out of the way. The cold air enters an intake vent on the opposite side of the room and returns to the furnace with the help of another fan. In the furnace, the cold air is heated, and the cycle repeats. "
using thermal energy,T_3707,"Its easy to see how thermal energy can be used to keep things warm. But did you know that thermal energy can also be used to keep things cool? Cooling systems such as air conditioners and refrigerators transfer thermal energy in order to keep homes and cars cool or to keep food cold. In a refrigerator, for example, thermal energy is transferred from the cool air inside the refrigerator to the warmer air in the kitchen. You read in this chapters ""Transfer of Thermal Energy"" lesson that thermal energy always moves from a warmer area to a cooler area, so how can it move from the cooler refrigerator to the warmer room? The answer is work. The refrigerator does work to transfer thermal energy in this way. Doing this work takes energy, which is usually provided by electricity. Figure 18.14 explains how a refrigerator does its work. For an animated demonstration of how a refrigerator works, go to this URL:  The key to how a refrigerator or other cooling system works is the refrigerant. A refrigerant is a substance, such as FreonTM, that has a low boiling point and changes between liquid and gaseous states as it passes through the cooling system. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator and changes to a gas. As a gas, it releases thermal energy to the warm air outside the refrigerator and changes back to a liquid. "
using thermal energy,T_3708,A combustion engine is a complex machine that burns fuel to produce thermal energy and then uses the energy to do work. Two basic types of combustion engines are external and internal combustion engines. 
using thermal energy,T_3709,"An external combustion engine burns fuel externally, or outside the engine. The burning fuel releases thermal energy that is used to turn water to steam. The pressure of the steam is then used to move a piston back and forth in a cylinder. The kinetic energy of the moving piston can be used to turn a turbine or other device. Figure 18.15 explains in greater detail how this type of engine works. You can see an animated version of an external combustion engine at this URL: http://science.howstuffworks.com/transport/engines-equipment/steam1.htm . "
using thermal energy,T_3710,"An internal combustion engine (see Figure 18.16) burns fuel internally, or inside the engine. This type of engine is found in most cars and other motor vehicles. It works in these steps, which keep repeating: 1. A mixture of fuel and air is pulled into a cylinder through a valve, which then closes. 2. The piston is pushed upward, compressing the fuel-air mixture in the closed cylinder. The mixture is now under a lot of pressure and very warm. 3. A spark from a spark plug is used to ignite the fuel-air mixture, causing it to burn explosively within the confined space of the closed cylinder. 4. The pressure of the hot gases from combustion forces the piston downward. 5. When the piston moves up again, it forces the exhaust gases of combustion out of the cylinder though another valve. Then the process repeats. In a car, the piston is connected by the piston rod to the crankshaft. The crankshaft rotates when the piston moves up and down. The kinetic energy of the moving crankshaft is used to turn the driveshaft, which causes the wheels of the car to turn. Most cars have at least four cylinders connected to the crankshaft. Their pistons move up and down in sequence, one after the other. You can watch animations of internal combustion engines in action at these URLs: http://auto.howstuffworks.com/engine1.htm "
characteristics of waves,T_3711,"A mechanical wave is a disturbance in matter that transfers energy from place to place. A mechanical wave starts when matter is disturbed. An example of a mechanical wave is pictured in Figure 19.1. A drop of water falls into a pond. This disturbs the water in the pond. What happens next? The disturbance travels outward from the drop in all directions. This is the wave. A source of energy is needed to start a mechanical wave. In this case, the energy comes from the falling drop of water. "
characteristics of waves,T_3712,"The energy of a mechanical wave can travel only through matter. This matter is called the medium (plural, media). The medium in Figure 19.1 is a liquid the water in the pond. But the medium of a mechanical wave can be any state of matter, including a solid or a gas. Its important to note that particles of matter in the medium dont actually travel along with the wave. Only the energy travels. The particles of the medium just vibrate, or move back-and- forth or up-and-down in one spot, always returning to their original positions. As the particles vibrate, they pass the energy of the disturbance to the particles next to them, which pass the energy to the particles next to them, and so on. "
characteristics of waves,T_3713,"There are three types of mechanical waves. They differ in how they travel through a medium. The three types are transverse, longitudinal, and surface waves. All three types are described in detail below. "
characteristics of waves,T_3714,"A transverse wave is a wave in which the medium vibrates at right angles to the direction that the wave travels. An example of a transverse wave is a wave in a rope, like the one pictured in Figure 19.2. In this wave, energy is provided by a persons hand moving one end of the rope up and down. The direction of the wave is down the length of the rope away from the persons hand. The rope itself moves up and down as the wave passes through it. You can see a brief video of a transverse wave in a rope at this URL:  . To see a transverse wave in slow motion, go to this URL:  (0:22). MEDIA Click image to the left or use the URL below. URL: "
characteristics of waves,T_3715,"A transverse wave can be characterized by the high and low points reached by particles of the medium as the wave passes through. This is illustrated in Figure 19.3. The high points are called crests, and the low points are called troughs. "
characteristics of waves,T_3716,"Another example of transverse waves occurs with earthquakes. The disturbance that causes an earthquake sends transverse waves through underground rocks in all directions from the disturbance. Earthquake waves that travel this way are called secondary, or S, waves. An S wave is illustrated in Figure 19.4. "
characteristics of waves,T_3717,"A longitudinal wave is a wave in which the medium vibrates in the same direction that the wave travels. An example of a longitudinal wave is a wave in a spring, like the one in Figure 19.5. In this wave, the energy is provided by a persons hand pushing and pulling the spring. The coils of the spring first crowd closer together and then spread farther apart as the disturbance passes through them. The direction of the wave is down the length of the spring, or the same direction in which the coils move. You can see a video of a longitudinal wave in a spring at this URL: http "
characteristics of waves,T_3718,"A longitudinal wave can be characterized by the compressions and rarefactions of the medium. This is illustrated in Figure 19.6. Compressions are the places where the coils are crowded together, and rarefactions are the places where the coils are spread apart. "
characteristics of waves,T_3719,"Earthquakes cause longitudinal waves as well as transverse waves. The disturbance that causes an earthquake sends longitudinal waves through underground rocks in all directions from the disturbance. Earthquake waves that travel this way are called primary, or P, waves. They are illustrated in Figure 19.7. "
characteristics of waves,T_3720,"A surface wave is a wave that travels along the surface of a medium. It combines a transverse wave and a longitudinal wave. Ocean waves are surface waves. They travel on the surface of the water between the ocean and the air. In a surface wave, particles of the medium move up and down as well as back and forth. This gives them an overall circular motion. This is illustrated in Figure 19.8 and at the URL below. MEDIA Click image to the left or use the URL below. URL:  In deep water, particles of water just move in circles. They dont actually move closer to shore with the energy of the waves. However, near the shore where the water is shallow, the waves behave differently. They start to drag on the bottom, creating friction (see Figure 19.9). The friction slows down the bottoms of the waves, while the tops of the waves keep moving at the same speed. This causes the waves to get steeper until they topple over and crash on the shore. The crashing waves carry water onto the shore as surf. "
measuring waves,T_3721,The height of a wave is its amplitude. Another measure of wave size is wavelength. Both wave amplitude and wave- length are described in detail below. Figure 19.11 shows these wave measures for both transverse and longitudinal waves. You can also simulate waves with different amplitudes and wavelengths by doing the interactive animation at this URL: http://sci-culture.com/advancedpoll/GCSE/sine%20wave%20simulator.html . 
measuring waves,T_3722,"Wave amplitude is the maximum distance the particles of a medium move from their resting position when a wave passes through. The resting position is where the particles would be in the absence of a wave. In a transverse wave, wave amplitude is the height of each crest above the resting position. The higher the crests are, the greater the amplitude. In a longitudinal wave, amplitude is a measure of how compressed particles of the medium become when the wave passes through. The closer together the particles are, the greater the amplitude. What determines a waves amplitude? It depends on the energy of the disturbance that causes the wave. A wave caused by a disturbance with more energy has greater amplitude. Imagine dropping a small pebble into a pond of still water. Tiny ripples will move out from the disturbance in concentric circles, like those in Figure 19.1. The ripples are low-amplitude waves. Now imagine throwing a big boulder into the pond. Very large waves will be generated by the disturbance. These waves are high-amplitude waves. "
measuring waves,T_3723,Another important measure of wave size is wavelength. Wavelength is the distance between two corresponding points on adjacent waves (see Figure 19.11). Wavelength can be measured as the distance between two adjacent crests of a transverse wave or two adjacent compressions of a longitudinal wave. It is usually measured in meters. Wavelength is related to the energy of a wave. Short-wavelength waves have more energy than long-wavelength waves of the same amplitude. You can see examples of waves with shorter and longer wavelengths in Figure 19.12. 
measuring waves,T_3724,"Imagine making transverse waves in a rope, like the waves in Figure 19.2. You tie one end of the rope to a doorknob or other fixed point and move the other end up and down with your hand. You can move the rope up and down slowly or quickly. How quickly you move the rope determines the frequency of the waves. "
measuring waves,T_3725,"The number of waves that pass a fixed point in a given amount of time is wave frequency. Wave frequency can be measured by counting the number of crests or compressions that pass the point in 1 second or other time period. The higher the number is, the greater is the frequency of the wave. The SI unit for wave frequency is the hertz (Hz), where 1 hertz equals 1 wave passing a fixed point in 1 second. Figure 19.13 shows high-frequency and low- frequency transverse waves. You can simulate transverse waves with different frequencies at this URL: http://zonal The frequency of a wave is the same as the frequency of the vibrations that caused the wave. For example, to generate a higher-frequency wave in a rope, you must move the rope up and down more quickly. This takes more energy, so a higher-frequency wave has more energy than a lower-frequency wave with the same amplitude. "
measuring waves,T_3726,"Assume that you move one end of a rope up and down just once. How long will take the wave to travel down the rope to the other end? This depends on the speed of the wave. Wave speed is how far the wave travels in a given amount of time, such as how many meters it travels per second. Wave speed is not the same thing as wave frequency, but it is related to frequency and also to wavelength. This equation shows how the three factors are related: Speed = Wavelength  Frequency In this equation, wavelength is measured in meters and frequency is measured in hertz, or number of waves per second. Therefore, wave speed is given in meters per second. The equation for wave speed can be used to calculate the speed of a wave when both wavelength and wave frequency are known. Consider an ocean wave with a wavelength of 3 meters and a frequency of 1 hertz. The speed of the wave is: Speed = 3 m  1 wave/s = 3 m/s You Try It! Problem: Jera made a wave in a spring by pushing and pulling on one end. The wavelength is 0.1 m, and the wave frequency is 0.2 m/s. What is the speed of the wave? If you want more practice calculating wave speed from wavelength and frequency, try the problems at this URL: http The equation for wave speed (above) can be rewritten as: Frequency = Speed Speed or Wavelength = Wavelength Frequency Therefore, if you know the speed of a wave and either the wavelength or wave frequency, you can calculate the missing value. For example, suppose that a wave is traveling at a speed of 2 meters per second and has a wavelength of 1 meter. Then the frequency of the wave is: Frequency = 2 m/s = 2 waves/s, or 2 Hz 1m You Try It! Problem: A wave is traveling at a speed of 2 m/s and has a frequency of 2 Hz. What is its wavelength? "
measuring waves,T_3727,"The speed of most waves depends on the medium through which they are traveling. Generally, waves travel fastest through solids and slowest through gases. Thats because particles are closest together in solids and farthest apart in gases. When particles are farther apart, it takes longer for the energy of the disturbance to pass from particle to particle. "
measuring waves,T_3728,"The organizers of the famous Maverick surf contest have voted that the conditions are right for hanging ten this weekend. The monster waves at Mavericks attract big wave surfers from around the world. But what exactly makes these Half Moon Bay waves so big? For more information on waves, see http://science.kqed.org/quest/video/scie MEDIA Click image to the left or use the URL below. URL: "
wave interactions and interference,T_3729,"Waves interact with matter in several ways. The interactions occur when waves pass from one medium to another. Besides bouncing back like an echo, waves may bend or spread out when they strike a new medium. These three ways that waves may interact with matter are called reflection, refraction, and diffraction. Each type of interaction is described in detail below. For animations of the three types of wave interactions, go to this URL: "
wave interactions and interference,T_3730,"An echo is an example of wave reflection. Reflection occurs when waves bounce back from a barrier they cannot pass through. Reflection can happen with any type of waves, not just sound waves. For example, Figure 19.15 shows the reflection of ocean waves off a rocky coast. Light waves can also be reflected. In fact, thats how we see most objects. Light from a light source, such as the sun or a light bulb, shines on the object and some of the light is reflected. When the reflected light enters our eyes, we can see the object. Reflected waves have the same speed and frequency as the original waves before they were reflected. However, the direction of the reflected waves is different. When waves strike an obstacle head on, the reflected waves bounce straight back in the direction they came from. When waves strike an obstacle at any other angle, they bounce back at the same angle but in a different direction. This is illustrated in Figure 19.16. "
wave interactions and interference,T_3731,"Refraction is another way that waves interact with matter. Refraction occurs when waves bend as they enter a new medium at an angle. You can see an example of refraction in Figure 19.17. Light bends when it passes from air to water. The bending of the light causes the pencil to appear broken. Why do waves bend as they enter a new medium? Waves usually travel at different speeds in different media. For example, light travels more slowly in water than air. This causes it to refract when it passes from air to water. "
wave interactions and interference,T_3732,"Did you ever notice that when youre walking down a street, you can hear sounds around the corners of buildings? Figure 19.18 shows why this happens. As you can see from the figure, sound waves spread out and travel around obstacles. This is called diffraction. It also occurs when waves pass through an opening in an obstacle. All waves may be diffracted, but it is more pronounced in some types of waves than others. For example, sound waves bend around corners much more than light does. Thats why you can hear but not see around corners. For a given type of waves, such as sound waves, how much the waves diffract depends on two factors: the size of the obstacle or opening in the obstacle and the wavelength. This is illustrated in Figure 19.19. Diffraction is minor if the length of the obstacle or opening is greater than the wavelength. Diffraction is major if the length of the obstacle or opening is less than the wavelength. "
wave interactions and interference,T_3733,"Waves interact not only with matter in the ways described above. Waves also interact with other waves. This is called wave interference. Wave interference may occur when two waves that are traveling in opposite directions meet. The two waves pass through each other, and this affects their amplitude. How amplitude is affected depends on the type of interference. Interference can be constructive or destructive. "
wave interactions and interference,T_3734,"Constructive interference occurs when the crests of one wave overlap the crests of the other wave. This is illustrated in Figure 19.20. As the waves pass through each other, the crests combine to produce a wave with greater amplitude. You can see an animation of constructive interference at this URL: http://phys23p.sl.psu.edu/phys_anim/waves/em "
wave interactions and interference,T_3735,"Destructive interference occurs when the crests of one wave overlap the troughs of another wave. This is illustrated in Figure 19.21. As the waves pass through each other, the crests and troughs cancel each other out to produce a wave with less amplitude. You can see an animation of destructive interference at this URL: http://phys23p.sl.psu.ed "
wave interactions and interference,T_3736,"When a wave is reflected straight back from an obstacle, the reflected wave interferes with the original wave and creates a standing wave. This is a wave that appears to be standing still. A standing wave occurs because of a combination of constructive and destructive interference between a wave and its reflected wave. You can see animations of standing waves at the URLs below. http://skullsinthestars.com/2008/05/04/classic-science-paper-otto-wieners-experiment-1890/  Its easy to generate a standing wave in a rope by tying one end to a fixed object and moving the other end up and down. When waves reach the fixed object, they are reflected back. The original wave and the reflected wave interfere to produce a standing wave. Try it yourself and see if the wave appears to stand still. "
characteristics of sound,T_3770,"Why does a tree make sound when it crashes to the ground? How does the sound reach peoples ears if they happen to be in the forest? And in general, how do sounds get started, and how do they travel? Keep reading to find out. "
characteristics of sound,T_3771,"All sounds begin with vibrating matter. It could be the ground vibrating when a tree comes crashing down. Or it could be guitar strings vibrating when they are plucked. You can see a guitar string vibrating in Figure 20.2. The vibrating string repeatedly pushes against the air particles next to it. The pressure of the vibrating string causes these air particles to vibrate. The air particles alternately push together and spread apart. This starts waves of vibrations that travel through the air in all directions away from the strings. The vibrations pass through the air as longitudinal waves, with individual air particles vibrating back and forth in the same direction that the waves travel. You can see an animation of sound waves moving through air at this URL: "
characteristics of sound,T_3772,"Sound waves are mechanical waves, so they can travel only though matter and not through empty space. This was demonstrated in the 1600s by a scientist named Robert Boyle. Boyle placed a ticking clock in a sealed glass jar. The clock could be heard ticking through the air and glass of the jar. Then Boyle pumped the air out of the jar. The clock was still running, but the ticking could no longer be heard. Thats because the sound couldnt travel away from the clock without air particles to pass the sound energy along. You can see an online demonstration of the same experimentwith a modern twistat this URL:  (4:06). MEDIA Click image to the left or use the URL below. URL:  Sound waves can travel through many different kinds of matter. Most of the sounds we hear travel through air, but sounds can also travel through liquids such as water and solids such as glass and metal. If you swim underwater or even submerge your ears in bathwater any sounds you hear have traveled to your ears through water. You can tell that sounds travel through glass and other solids because you can hear loud outdoor sounds such as sirens through closed windows and doors. "
characteristics of sound,T_3773,"Sound has certain characteristic properties because of the way sound energy travels in waves. Properties of sound include speed, loudness, and pitch. "
characteristics of sound,T_3774,"The speed of sound is the distance that sound waves travel in a given amount of time. You probably already know that sound travels more slowly than light. Thats why you usually see the flash of lightning before you hear the boom of thunder. However, the speed of sound isnt constant. It varies depending on the medium of the sound waves. Table 20.1 lists the speed of sound in several different media. Generally, sound waves travel fastest through solids and slowest through gases. Thats because the particles of solids are close together and can quickly pass the energy of vibrations to nearby particles. You can explore the speed of sound in different media at this URL:  Medium (20C) Air Water Wood Glass Aluminum Speed of Sound Waves (m/s) 343 1437 3850 4540 6320 The speed of sound also depends on the temperature of the medium. For a given medium such as air, sound has a slower speed at lower temperatures. You can compare the speed of sound in air at different temperatures in Table transfer the energy of the sound waves. The amount of water vapor in the air affects the speed of sound as well. Do you think sound travels faster or slower when the air contains more water vapor? (Hint: Compare the speed of sound in water and air in Table 20.1.) Temperature of Air 0C 20C 100C Speed of Sound (m/s) 331 343 386 KQED: Speed of Sound Along with cable cars and seagulls, the Golden Gate Bridge foghorn is one of San Franciscos most iconic sounds. But did you know that if you hear that foghorn off in the distance, you can calculate how many miles you are from the bridge? Using the Speed of Sound exhibit at the Outdoor Exploratorium at Fort Mason, Shawn Lani shows us how sound perception is affected by distance. For more information on the speed of sound, see http://science.kqed. MEDIA Click image to the left or use the URL below. URL: "
characteristics of sound,T_3775,"A friend whispers to you in class in a voice so soft that you have to lean very close to hear what hes saying. Later that day, your friend shouts to you across the football field. Now his voice is loud enough for you to hear him clearly even though hes many meters away. Obviously, sounds can vary in loudness. Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of sound. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (dB). You can see typical decibel levels of several different sounds in Figure 20.3. As decibel levels get higher, sound waves have greater intensity and sounds are louder. For every 10-decibel increase in the intensity of sound, loudness is 10 times greater. Therefore, a 30-decibel ""quiet"" room is 10 times louder than a 20-decibel whisper, and a 40- decibel light rainfall is 100 times louder than a 20-decibel whisper. How much louder than a 20-decibel whisper is the 60-decibel sound of a vacuum cleaner? The intensity of sound waves determines the loudness of sounds, but what determines intensity? Intensity is a function of two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound. Remember that sound waves start at a source of vibrations and spread out from the source in all directions. The farther the sound waves travel away from the source, the more spread out their energy becomes. This is illustrated in Figure 20.4. The decrease in intensity with distance from a sound source explains why even loud sounds fade away as you move farther from the source. It also explains why low-amplitude sounds can be heard only over short distances. For a video demonstration of the amplitude and loudness of sounds, go to this URL: interactive animation at this URL: "
characteristics of sound,T_3776,"A marching band is parading down the street. You can hear it coming from several blocks away. When the different instruments finally pass by you, their distinctive sounds can be heard. The tiny piccolos trill their bird-like high notes, and the big tubas rumble out their booming bass notes (see Figure 20.5). Clearly, some sounds are higher or lower than others. But do you know why? How high or low a sound seems to a listener is its pitch. Pitch, in turn, depends on the frequency of sound waves. Recall that the frequency of waves is the number of waves that pass a fixed point in a given amount of time. High-pitched sounds, like the sounds of a piccolo, have high-frequency waves. Low-pitched sounds, like the sounds of a tuba, have low-frequency waves. For a video demonstration of frequency and pitch, go to this URL:  (3:20). MEDIA Click image to the left or use the URL below. URL:  To explore an interactive animation of sound wave frequency, go to this URL:  The frequency of sound waves is measured in hertz (Hz), or the number of waves that pass a fixed point in a second. Human beings can normally hear sounds with a frequency between about 20 Hz and 20,000 Hz. Sounds with frequencies below 20 hertz are called infrasound. Sounds with frequencies above 20,000 hertz are called ultrasound. Some other animals can hear sounds in the ultrasound range. For example, dogs can hear sounds with frequencies as high as 50,000 Hz. You may have seen special whistles that dogs but not people can hear. The whistles produce a sound with a frequency too high for the human ear to detect. Other animals can hear even higher-frequency sounds. Bats, for example, can hear sounds with frequencies higher than 100,000 Hz. "
characteristics of sound,T_3777,"Look at the police car in Figure 20.6. The sound waves from its siren travel outward in all directions. Because the car is racing forward (toward the right), the sound waves get bunched up in front of the car and spread out behind it. As the car approaches the person on the right (position B), the sound waves get closer and closer together. In other words, they have a higher frequency. This makes the siren sound higher in pitch. After the car speeds by the person on the left (position A), the sound waves get more and more spread out, so they have a lower frequency. This makes the siren sound lower in pitch. A change in the frequency of sound waves, relative to a stationary listener, when the source of the sound waves is moving is called the Doppler effect. Youve probably experienced the Doppler effect yourself. The next time a vehicle with a siren races by, listen for the change in pitch. For an online animation of the Doppler effect, go to the URL below. "
hearing sound,T_3778,"Figure 20.7 shows the three main parts of the ear: the outer, middle, and inner ear. It also shows the specific structures in each part. The roles of these structures in hearing are described below and in the animations at these URLS:  (1:43) MEDIA Click image to the left or use the URL below. URL: "
hearing sound,T_3779,"The outer ear includes the pinna, ear canal, and eardrum. The pinna is the only part of the ear that extends outward from the head. Its position and shape make it good at catching sound waves and funneling them into the ear canal. The ear canal is a tube that carries sound waves into the ear. The sound waves travel through the air inside the ear canal to the eardrum. The eardrum is like the head of a drum. Its a thin membrane stretched tight across the end of the ear canal. The eardrum vibrates when sound waves strike it, and it sends the vibrations on to the middle ear. "
hearing sound,T_3780,"The middle ear contains three tiny bones (ossicles) called the hammer, anvil, and stirrup. If you look at these bones in Figure 20.7, you might notice that they resemble the objects for which they are named. The three bones transmit vibrations from the eardrum to the inner ear. They also amplify the vibrations. The arrangement of the three bones allows them to work together as a lever that increases the amplitude of the waves as they pass to the inner ear. "
hearing sound,T_3781,"The stirrup passes the amplified sound waves to the inner ear through the oval window (see Figure 20.7). When the oval window vibrates, it causes the cochlea to vibrate as well. The cochlea is a shell-like structure that is full of fluid and lined with nerve cells called hair cells. Each hair cell has tiny hair-like projections, as you can see in Figure and this triggers electrical impulses. The electrical impulses travel to the brain through nerves. Only after the nerve impulses reach the brain do we hear the sound. "
hearing sound,T_3782,"All these structures of the ear must work well for normal hearing. Damage to any of them, through illness or injury, may cause hearing loss. Total hearing loss is called deafness. To learn more about hearing loss, watch the animation at this URL:  (1:39). MEDIA Click image to the left or use the URL below. URL:  Most adults experience at least some hearing loss as they get older. The most common cause is exposure to loud sounds, which damage hair cells. The louder a sound is, the less exposure is needed for damage to occur. Even a single brief exposure to a sound louder than 115 decibels can cause hearing loss. Figure 20.9 shows the relationship between loudness, exposure time, and hearing loss. "
hearing sound,T_3783,"Hearing loss caused by loud sounds is permanent. However, this type of hearing loss can be prevented by protecting the ears from loud sounds. "
hearing sound,T_3784,People who work in jobs that expose them to loud sounds must wear hearing protectors. Examples include construc- tion workers who work around loud machinery for many hours each day (see Figure 20.10). But anyone exposed to loud sounds for longer than the permissible exposure time should wear hearing protectors. Many home and yard chores and even recreational activities are loud enough to cause hearing loss if people are exposed to them for very long. 
hearing sound,T_3785,"You can see two different types of hearing protectors in Figure 20.11. Earplugs are simple hearing protectors that just muffle sounds by partially blocking all sound waves from entering the ears. This type of hearing protector is suitable for lower noise levels, such as the noise of a lawnmower or snowmobile engine. Electronic ear protectors work differently. They identify high-amplitude sound waves and send sound waves through them in the opposite direction. This causes destructive interference with the waves, which reduces their amplitude to zero or nearly zero. This changes even the loudest sounds to just a soft hiss. Sounds that people need to hear, such as the voices of co-workers, are not interfered with in this way and may be amplified instead so they can be heard more clearly. This type of hearing protector is recommended for higher noise levels and situations where its important to be able to hear lower-decibel sounds. "
using sound,T_3786,"People have been using sound to make music for thousands of years. They have invented many different kinds of musical instruments for this purpose. Despite their diversity, however, musical instruments share certain similarities. All musical instruments create sound by causing matter to vibrate. The vibrations start sound waves moving through the air. Most musical instruments use resonance to amplify the sound waves and make the sounds louder. Resonance occurs when an object vibrates in response to sound waves of a certain frequency. In a musical instrument such as a guitar, the whole instrument and the air inside it may vibrate when a single string is plucked. This causes constructive interference with the sound waves, which increases their amplitude. Most musical instruments have a way of changing the frequency of the sound waves they produce. This changes the pitch of the sounds. There are three basic categories of musical instruments: percussion, wind, and stringed instruments. In Figure "
using sound,T_3787,"Researchers at Lawrence Berkeley National Laboratory are pioneering a new way to recover 100-year-old record- ings. Found on fragile wax cylinders and early lacquer records, the sounds reveal a rich acoustic heritage, including languages long lost. For more information on how to recover recordings, see http://science.kqed.org/quest/video/ MEDIA Click image to the left or use the URL below. URL: "
using sound,T_3788,"Ultrasound has frequencies higher than the human ear can detect (higher than 20,000 hertz). Although we cant hear ultrasound, it is very useful. Uses include echolocation, sonar, and ultrasonography. "
using sound,T_3789,"Animals such as bats, whales, and dolphins send out ultrasound waves and use their echoes, or reflected waves, to identify the locations of objects they cannot see. This is called echolocation. Animals use echolocation to find prey and avoid running into objects in the dark. Figure 20.13 and the animation at the URL below show how a bat uses echolocation to locate insect prey. "
using sound,T_3790,"Sonar uses ultrasound in a way that is similar to echolocation. Sonar stands for sound navigation and ranging. It is used to locate underwater objects such as sunken ships or to determine how deep the water is. A sonar device is usually located on a boat at the surface of the water. The device is both a sender and a receiver (see Figure 20.14). It sends out ultrasound waves and detects reflected waves that bounce off underwater objects or the bottom of the water. If you watch the video at the URL below, you can see how sonar is used on a submarine. The distance to underwater objects or the bottom of the water can be calculated from the known speed of sound in water and the time it takes for the waves to travel to the object. The equation for the calculation is: Distance = Speed  Time Assume, for example, that a sonar device on a ship sends an ultrasound wave to the bottom of the ocean. The speed of the sound through ocean water is 1437 m/s, and the wave travels to the bottom and back in 2 seconds. What is the distance from the surface to the bottom of the water? The sound wave travels to the bottom and back in 2 seconds, so it travels from the surface to the bottom in 1 second. Therefore, the distance from the surface to the bottom is: Distance = 1437 m/s  1 s = 1437 m You Try It! Problem: The sonar device on a ship sends an ultrasound wave to the bottom of the water at speed of 1437 m/s. The wave is reflected back to the device in 4 seconds. How deep is the water? "
using sound,T_3791,"Ultrasound can be used to ""see"" inside the human body. This use of ultrasound is called ultrasonography. Harmless ultrasound waves are sent inside the body, and the reflected waves are used to create an image on a screen. This technology is used to examine internal organs and unborn babies without risk to the patient. You can see an ultrasound image in Figure 20.15. You can see an animation showing how ultrasonography works at this URL: "
using sound,T_3792,"In this QUEST web extra, Stanford University astrophysicist Todd Hoeksema explains how solar sound waves are a vital ingredient to the science of helioseismology, in which the interior properties of the sun are probed by analyzing and tracking the surface sound waves that bounce into and out of the Sun. For more information on solar sound waves, see http://science.kqed.org/quest/video/web-extra-music-of-the-sun/ . MEDIA Click image to the left or use the URL below. URL: "
electromagnetic waves,T_3793,"An electromagnetic wave is a wave that consists of vibrating electric and magnetic fields. A familiar example will help you understand the fields that make up an electromagnetic wave. Think about a common bar magnet. It exerts magnetic force in an area surrounding it, called the magnetic field. You can see the magnetic field of a bar magnet in Figure 21.1. Because of this force field, a magnet can exert force on objects without touching them. They just have to be in its magnetic field. An electric field is similar to a magnetic field (see Figure 21.1). An electric field is an area of electrical force surrounding a charged particle. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them. "
electromagnetic waves,T_3794,"An electromagnetic wave begins when an electrically charged particle vibrates. This is illustrated in Figure 21.2. When a charged particle vibrates, it causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field (you can learn how this happens in the chapter ""Electromagnetism""). The two types of vibrating fields combine to create an electromagnetic wave. You can see an animation of an electromagnetic wave at this URL:  (1:31). MEDIA Click image to the left or use the URL below. URL: "
electromagnetic waves,T_3795,"As you can see in Figure 21.2, the electric and magnetic fields that make up an electromagnetic wave occur are at right angles to each other. Both fields are also at right angles to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. "
electromagnetic waves,T_3796,"Unlike a mechanical transverse wave, which requires a medium, an electromagnetic transverse wave can travel through space without a medium. Waves traveling through a medium lose some energy to the medium. However, when an electromagnetic wave travels through space, no energy is lost, so the wave doesnt get weaker as it travels. However, the energy is ""diluted"" as it spreads out over an ever-larger area as it travels away from the source. This is similar to the way a sound wave spreads out and becomes less intense farther from the sound source. "
electromagnetic waves,T_3797,"Electromagnetic waves can travel through matter as well as across space. When they strike matter, they interact with it in the same ways that mechanical waves interact with matter. They may reflect (bounce back), refract (bend when traveling through different materials), or diffract (bend around objects). They may also be converted to other forms of energy. Microwaves are a familiar example. They are a type of electromagnetic wave that you can read about later on in this chapter, in the lesson ""The Electromagnetic Spectrum."" When microwaves strike food in a microwave oven, they are converted to thermal energy, which heats the food. "
electromagnetic waves,T_3798,"Electromagnetic radiation behaves like waves of energy most of the time, but sometimes it behaves like particles. As evidence accumulated for this dual nature of electromagnetic radiation, the famous physicist Albert Einstein developed a new theory about electromagnetic radiation, called the wave-particle theory. This theory explains how electromagnetic radiation can behave as both a wave and a particle. In brief, when an electron returns to a lower energy level, it is thought to give off a tiny ""packet"" of energy called a photon (see Figure 21.3). The amount of energy in a photon may vary. It depends on the frequency of electromagnetic radiation. The higher the frequency is, the more energy a photon has. "
electromagnetic waves,T_3799,"The most important source of electromagnetic radiation on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves that people use depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes. Youll learn about all these types of electromagnetic waves in this chapters lesson on ""The Electromagnetic Spectrum."" "
properties of electromagnetic waves,T_3800,"All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light, is 300 million meters per second (3.0  108 m/s). Nothing else in the universe is known to travel this fast. If you could move that fast, you would be able to travel around Earth 7.5 times in just 1 second! The sun is about 150 million kilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth from the sun. Electromagnetic waves travel more slowly through a medium, and their speed may vary from one medium to another. For example, light travels more slowly through water than it does through air (see Figure 21.4). You can learn more about the speed of light at this URL: http://videos.howstuffworks.com/discovery/29407-assignme "
properties of electromagnetic waves,T_3801,"Although all electromagnetic waves travel at the same speed, they may differ in their wavelength and frequency. "
properties of electromagnetic waves,T_3802,Wavelength and frequency are defined in the same way for electromagnetic waves as they are for mechanical waves. Both properties are illustrated in Figure 21.5. Wavelength is the distance between corresponding points of adjacent waves. Wavelengths of electromagnetic waves range from many kilometers to a tiny fraction of a millimeter. Frequency is the number of waves that pass a fixed point in a given amount of time. Frequencies of electro- magnetic waves range from thousands to trillions of waves per second. Higher frequency waves have greater energy. 
properties of electromagnetic waves,T_3803,"The speed of a wave is a product of its wavelength and frequency. Because all electromagnetic waves travel at the same speed through space, a wave with a shorter wavelength must have a higher frequency, and vice versa. This relationship is represented by the equation: Speed = Wavelength  Frequency The equation for wave speed can be rewritten as: Frequency = Speed Speed or Wavelength = Wavelength Frequency Therefore, if either wavelength or frequency is known, the missing value can be calculated. Consider an electromag- netic wave that has a wavelength of 3 meters. Its speed, like the speed of all electromagnetic waves, is 3.0  108 meters per second. Its frequency can be found by substituting these values into the frequency equation: Frequency = 3.0  108 m/s = 1.0  108 waves/s, or 1.0  108 hertz (Hz) 3.0 m You Try It! Problem: What is the wavelength of an electromagnetic wave that has a frequency of 3.0  108 hertz? For more practice calculating the frequency and wavelength of electromagnetic waves, go to these URLs: "
the electromagnetic spectrum,T_3804,"Electromagnetic radiation occurs in waves of different wavelengths and frequencies. Infrared light and visible light make up just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum. The electromagnetic spectrum is summarized in the diagram in Figure 21.7. On the far left of the diagram are radio waves, which include microwaves. They have the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount of energy. On the far right are X rays and gamma rays. The have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the greatest amount of energy. Between these two extremes, wavelength, frequency, and energy change continuously from one side of the spectrum to the other. Waves in this middle section of the electromagnetic spectrum are commonly called light. As you will read below, the properties of electromagnetic waves influence how the different waves behave and how they can be used. "
the electromagnetic spectrum,T_3805,"Radio waves are the broad range of electromagnetic waves with the longest wavelengths and lowest frequencies. In Figure 21.7, you can see that the wavelength of radio waves may be longer than a soccer field. With their low frequencies, radio waves have the least energy of electromagnetic waves, but they still are extremely useful. They are used for radio and television broadcasts, microwave ovens, cell phone transmissions, and radar. You can learn more about radio waves, including how they were discovered, at this URL:  MEDIA Click image to the left or use the URL below. URL: "
the electromagnetic spectrum,T_3806,"In radio broadcasts, sounds are encoded in radio waves that are sent out through the atmosphere from a radio tower. A receiver detects the radio waves and changes them back to sounds. Youve probably listened to both AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts. AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the amplitude of radio waves. AM broadcasts use longerwavelength radio waves than FM broadcasts. Because of their longer wavelengths, AM radio waves reflect off a layer of the upper atmosphere called the ionosphere. You can see how this happens in Figure 21.8. This allows AM radio waves to reach radio receivers that are very far away from the radio tower. FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequency of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude modulation, so FM broadcasts usually sound clearer than AM broadcasts. However, because of their shorter wavelength, FM waves do not reflect off the ionosphere. Instead, they pass right through it and out into space (see Figure 21.8). As a result, FM waves cannot reach very distant receivers. "
the electromagnetic spectrum,T_3807,"Television broadcasts also use radio waves. Sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded radio waves are broadcast from a TV tower like the one in Figure 21.9. When the waves are received by television sets, they are decoded and changed back to sounds and pictures. "
the electromagnetic spectrum,T_3808,"The shortest wavelength, highest frequency radio waves are called microwaves (see Figure 21.7). Microwaves have more energy than other radio waves. Thats why they are useful for heating food in microwave ovens. Microwaves have other important uses as well, including cell phone transmissions and radar, which is a device for determining the presence and location of an object by measuring the time for the echo of a radio wave to return from it and the direction from which it returns. These uses are described in Figure 21.10. You can learn more about microwaves and their uses in the video at this URL:  (3:23). MEDIA Click image to the left or use the URL below. URL: "
the electromagnetic spectrum,T_3809,"Mid-wavelength electromagnetic waves are commonly called light. This range of electromagnetic waves has shorter wavelengths and higher frequencies than radio waves, but not as short and high as X rays and gamma rays. Light includes visible light, infrared light, and ultraviolet light. If you look back at Figure 21.7, you can see where these different types of light waves fall in the electromagnetic spectrum. "
the electromagnetic spectrum,T_3810,"The only light that people can see is called visible light. It refers to a very narrow range of wavelengths in the electromagnetic spectrum that falls between infrared light and ultraviolet light. Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength. You can see the spectrum of colors of visible light in Figure 21.11. When all of the wavelengths are combined, as they are in sunlight, visible light appears white. You can learn more about visible light in the chapter ""Visible Light"" and at the URL below. "
the electromagnetic spectrum,T_3811,"Light with the longest wavelengths is called infrared light. The term infrared means ""below red."" Infrared light is the range of light waves that have longer wavelengths than red light in the visible spectrum. You cant see infrared light waves, but you can feel them as heat on your skin. The sun gives off infrared light as do fires and living things. The picture of a cat that opened this chapter was made with a camera that detects infrared light waves and changes their energy to colored light in the visible range. Night vision goggles, which are used by law enforcement and the military, also detect infrared light waves. The goggles convert the invisible waves to visible images. For a deeper understanding of infrared light, watch the video at this URL:  MEDIA Click image to the left or use the URL below. URL: "
the electromagnetic spectrum,T_3812,"Light with wavelengths shorter than visible light is called ultraviolet light. The term ultraviolet means ""above violet."" Ultraviolet light is the range of light waves that have shorter wavelengths than violet light in the visible spectrum. Humans cant see ultraviolet light, but it is very useful nonetheless. It has higher-frequency waves than visible light, so it has more energy. It can be used to kill bacteria in food and to sterilize laboratory equipment (see Figure 21.12). The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D is needed for strong bones and teeth. You can learn more about ultraviolet light and its discovery at this URL:  MEDIA Click image to the left or use the URL below. URL:  Too much exposure to ultraviolet light can cause sunburn and skin cancer. You can protect your skin from ultraviolet light by wearing clothing that covers your skin and by applying sunscreen to any exposed areas. The SPF, or sun- protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (see Figure 21.13). A sunscreen with a higher SPF protects the skin longer. You should use sunscreen with an SPF of at least 15 even on cloudy days, because ultraviolet light can travel through clouds. Sunscreen should be applied liberally and often. You can learn more about the effects of ultraviolet light on the skin at this URL:  MEDIA Click image to the left or use the URL below. URL: "
the electromagnetic spectrum,T_3813,"The shortest-wavelength, highest-frequency electromagnetic waves are X rays and gamma rays. These rays have so much energy that they can pass through many materials. This makes them potentially very harmful, but it also makes them useful for certain purposes. "
the electromagnetic spectrum,T_3814,"X rays are high-energy electromagnetic waves. They have enough energy to pass through soft tissues such as skin but not enough to pass through bones and teeth, which are very dense. The bright areas on the X ray film in Figure also to screen luggage at airports (see Figure 21.14). Too much X ray exposure may cause cancer. If youve had dental X rays, you may have noticed that a heavy apron was placed over your body to protect it from stray X rays. The apron is made of lead, which X rays cannot pass through. You can learn about the discovery of X rays as well as other uses of X rays at this URL: "
the electromagnetic spectrum,T_3815,"Gamma rays are the most energetic of all electromagnetic waves. They can pass through most materials, including bones and teeth. Nonetheless, even these waves are useful. For example, they can be used to treat cancer. A medical device sends gamma rays the site of the cancer, and the rays destroy the cancerous cells. If you want to learn more about gamma rays, watch the video at the URL below. MEDIA Click image to the left or use the URL below. URL: "
the electromagnetic spectrum,T_3816,"Scientists in Berkeley have developed a powerful new microscope which uses X rays to scan a whole cell and in a manner of minutes, generate a 3D view of the cell and its genetic material. This groundbreaking tool is helping to advance research into the development of biofuels, the treatment of malaria and it may even help to more rapidly diagnose cancer. For more information on X ray microscopes, see http://science.kqed.org/quest/video/x-ray-micros MEDIA Click image to the left or use the URL below. URL: "
the light we see,T_3817,Look at the classroom in Figure 22.1. It has several sources of visible light. One source of visible light is the sun. Sunlight enters the classroom through the windows. The sun provides virtually all of the visible light that living things need. Visible light travels across space from the sun to Earth in electromagnetic waves. But how does the sun produce light? Read on to find out. 
the light we see,T_3818,"The sun and other stars produce light because they are so hot. They glow with light due to their extremely high temperatures. This way of producing light is called incandescence. Some objects produce light without becoming very hot. They generate light through chemical reactions or other processes. Producing light without heat is called luminescence. Objects that produce light by luminescence are said to be luminous. Luminescence, in turn, can occur in different ways: One type of luminescence is called fluorescence. In this process, a substance absorbs shorter-wavelength light, such as ultraviolet light, and then gives off light in the visible range of wavelengths. Certain minerals produce light in this way. Another type of luminescence is called electroluminescence. In this process, a substance gives off light when an electric current runs through it. Some gases produce light in this way. A third type of luminescence is called bioluminescence. This is the production of light by living things as a result of chemical reactions. Examples of bioluminescent organisms are pictured in Figure 22.2. You can learn more about bioluminescence in the video at this URL:  Many other objects appear to produce their own light, but they actually just reflect light from another source. The moon is a good example. It appears to glow in the sky from its own light, but in reality it is just reflecting light from the sun. Objects like the moon that are lit up by another source of light are said to be illuminated. Everything you can see that doesnt produce its own light is illuminated. "
the light we see,T_3819,"The classroom in Figure 22.1 has artificial light sources in addition to natural sunlight. There are fluorescent lights on the ceiling of the room. There are also projectors on the ceiling that are shining light on screens. In these and most other artificial light sources, electricity provides the energy and some type of light bulb converts the electrical energy to visible light. How a light bulb produces visible light varies by type of bulb, as you can see in Table 22.1. Incandescent light bulbs, which produce light by incandescence, give off a lot of heat as well as light, so they waste energy. Other light bulbs produce light by luminescence, so they produce little if any heat. These light bulbs use energy more efficiently. Which types of light bulbs do you use? Type of Light Bulb Incandescent Light Description An incandescent light bulb produces visible light by incandescence. The bulb contains a thin wire filament made of tungsten. When electric current passes through the filament, it gets extremely hot and glows. You can learn more about incandescent light bulbs at the URL below. Fluorescent Light A fluorescent light bulb produces visible light by flu- orescence. The bulb contains mercury gas that gives off ultraviolet light when electricity passes through it. The inside of the bulb is coated with a substance called phosphor. The phosphor absorbs the ultraviolet light and then gives off most of the energy as visible light. You can learn more about fluorescent light bulbs at this URL: http://science.discovery.com/videos/deco Type of Light Bulb Neon Light Vapor Light LED Light Description A neon light produces visible light by electrolumines- cence. The bulb is a glass tube that contains the noble gas neon. When electricity passes through the gas, it excites electrons of neon atoms, causing them to give off visible light. Neon produces red light. Other noble gases are also used in lights, and they produce light of different colors. For example, krypton produces violet light, and argon produces blue light. A vapor light produces visible light by electrolumi- nescence. The bulb contains a small amount of solid sodium or mercury as well as a mixture of neon and argon gases. When an electric current passes through the gases, it causes the solid sodium or mercury to change to a gas and emit visible light. Sodium vapor lights, like these streetlights, produce yellowish light. Mercury vapor lights produce bluish light. Vapor lights are very bright and energy efficient. The bulbs are also long lasting. LED stands for light-emitting diode. This type of light contains a material, called a semi-conductor, which gives off visible light when a current runs through it. LED lights are used for traffic lights and indicator lights on computers, cars, and many other devices. This type of light is very reliable and durable. "
the light we see,T_3820,"When visible light strikes matter, it interacts with it. How light interacts with matter depends on the type of matter. "
the light we see,T_3821,"Light may interact with matter in several ways. Light may be reflected by matter. Reflected light bounces back when it strikes matter. Reflection of light is similar to reflection of sound waves. You can read more about reflection of light later on in this chapter in the lesson Optics. Light may be refracted by matter. The light is bent when it passes from one type of matter to another. Refraction of light is similar to refraction of sound waves. You can also read more about refraction of light in the lesson Optics. Light may pass through matter. This is called transmission of light. As light is transmitted, it may be scattered by particles of matter and spread out in all directions. This is called scattering of light. Light may be absorbed by matter. This is called absorption of light. When light is absorbed, it doesnt reflect from or pass through matter. Instead, its energy is transferred to particles of matter, which may increase the temperature of matter. "
the light we see,T_3822,"Matter can be classified on the basis of how light interacts with it. Matter may be transparent, translucent, or opaque. Each type of matter is illustrated in Figure 22.3. Transparent matter is matter that transmits light without scattering it. Examples of transparent matter include air, pure water, and clear glass. You can see clearly through a transparent object, such as the revolving glass doors in the figure, because all the light passes straight through it. Translucent matter is matter that transmits but scatters light. Light passes through a translucent object but you cannot see clearly through the object because the light is scattered in all directions. The frosted glass doors in the figure are translucent. Opaque matter is matter that does not let any light pass through it. Matter may be opaque because it absorbs light, reflects light, or does both. Examples of opaque objects are solid wooden doors and glass mirrors. A wooden door absorbs most of the light that strikes it and reflects just a few wavelengths of visible light. A mirror, which is a sheet of glass with a shiny metal coating on the back, reflects all the light that strikes it. "
the light we see,T_3823,"Visible light consists of a range of wavelengths. The wavelength of visible light determines the color that the light appears. As you can see in Figure 22.4, light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between is a continuum of all the other colors of light. Only a few colors of light are represented in the figure. "
the light we see,T_3824,"A prism, like the one in Figure 22.5, can be used to separate visible light into its different colors. A prism is a pyramid-shaped object made of transparent matter, usually clear glass. It transmits light but slows it down. When light passes from the air to the glass of the prism, the change in speed causes the light to bend. Different wavelengths of light bend at different angles. This causes the beam of light to separate into light of different wavelengths. What we see is a rainbow of colors. Look back at the rainbow that opened this chapter. Do you see all the different colors of light, from red at the top to violet at the bottom? Individual raindrops act as tiny prisms. They separate sunlight into its different wavelengths and create a rainbow. For an animated version of Figure 22.5, go to the URL: http://en.wikipedia.org/wiki/File:Light_dispersion_conce "
the light we see,T_3825,"We see an opaque object, such as the apple in Figure 22.6, because it reflects some wavelengths of visible light. The wavelengths that are reflected determine the color that the object appears. For example, the apple in the figure appears red because it reflects red light and absorbs light of other wavelengths. We see a transparent or translucent object, such as the bottle in Figure 22.6, because it transmits light. The wavelength of the transmitted light determines the color that the object appears. For example, the bottle in the figure appears blue because it transmits blue light. The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes a red apple, the blue light is absorbed and no light is reflected. When no light reflects from an object, it looks black. Black isnt a color. It is the absence of light. "
the light we see,T_3826,"The human eye can distinguish only red, green, and blue light. These three colors of light are called primary colors. All other colors of light can be created by combining the primary colors. As you can see in Figure 22.7, when red and green light combine, they form yellow. When red and blue light combine, they form magenta, a dark pinkish color, and when blue and green light combine, they form cyan, a bluish green color. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram in Figure 22.7. When all three primary colors combine, they form white light. White is the color of the full spectrum of visible light when all of its wavelengths are combined. You can explore the colors of visible light and how they combine with the interactive animations at this URL:  . "
the light we see,T_3827,"Many objects have color because they contain pigments. A pigment is a substance that colors materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. A very common pigment is chlorophyll, which is found in plants. This dark green pigment absorbs all but green wavelengths of visible light. It is responsible for capturing the light energy needed for photosynthesis. Pigments are also found in paints, inks, and dyes. Just three pigments, called primary pigments, can be combined to produce all other colors. The primary pigment colors are the same as the secondary colors of light: cyan, magenta, and yellow. The printer ink cartridges in Figure 22.8 come in just these three colors. They are the only colors needed for full-color printing. "
the light we see,T_3828,"Artist Kate Nichols longed to paint with the iridescent colors of butterfly wings, but no such pigments existed. So she became the first artist-in-residence at Lawrence Berkeley National Laboratory to synthesize nanoparticles and incorporate them into her artwork. From the laboratory to the studio, see how Kate uses the phenomenon known as ""structural color"" to transform nanotechnology into creativity. For more information on using nanoparticles to create colors, see http://science.kqed.org/quest/video/science-on-the-spot-color-by-nano-the-art-of-kate-nichols/ . MEDIA Click image to the left or use the URL below. URL: "
optics,T_3829,Almost all surfaces reflect some of the light that strikes them. The still water of the lake in Figure 22.9 reflects almost all of the light that strikes it. The reflected light forms an image of nearby objects. An image is a copy of an object that is formed by reflected or refracted light. 
optics,T_3830,"If a surface is extremely smooth, like very still water, then an image formed by reflection is sharp and clear. This is called regular reflection. If the surface is even slightly rough, an image may not form, or if there is an image, it is blurry or fuzzy. This is called diffuse reflection. Both types of reflection are represented in Figure 22.10. You can also see animations of both types of reflection at this URL: http://toolboxes.flexiblelearning.net.au/demosites/serie In Figure 22.10, the waves of light are represented by arrows called rays. Rays that strike the surface are referred to as incident rays, and rays that reflect off the surface are known as reflected rays. In regular reflection, all the rays are reflected in the same direction. This explains why regular reflection forms a clear image. In diffuse reflection, in contrast, the rays are reflected in many different directions. This is why diffuse reflection forms, at best, a blurry image. "
optics,T_3831,"One thing is true of both regular and diffuse reflection. The angle at which the reflected rays bounce off the surface is equal to the angle at which the incident rays strike the surface. This is the law of reflection, and it applies to the reflection of all light. The law is illustrated in Figure 22.11 and in the animation at this URL: "
optics,T_3832,"Mirrors are usually made of glass with a shiny metal backing that reflects all the light that strikes it. Mirrors may have flat or curved surfaces. The shape of a mirrors surface determines the type of image the mirror forms. For example, the image may be real or virtual. A real image forms in front of a mirror where reflected light rays actually meet. It is a true image that could be projected on a screen. A virtual image appears to be on the other side of the mirror. Of course, reflected rays dont actually go behind a mirror, so a virtual image doesnt really exist. It just appears to exist to the human eye and brain. "
optics,T_3833,"Most mirrors are plane mirrors. A plane mirror has a flat reflective surface and forms only virtual images. The image formed by a plane mirror is also life sized. But something is different about the image compared with the real object in front of the mirror. Left and right are reversed. Look at the man shaving in Figure 22.12. He is using his right hand to hold the razor, but his image appears to be holding the razor in the left hand. Almost all plane mirrors reverse left and right in this way. "
optics,T_3834,"Some mirrors have a curved rather than flat surface. Curved mirrors can be concave or convex. A concave mirror is shaped like the inside of a bowl. This type of mirror forms either real or virtual images, depending on where the object is placed relative to the focal point. The focal point is the point in front of the mirror where the reflected rays intersect. You can see how concave mirrors form images in Figure 22.13 and in the interactive animation at the URL below. The animation allows you to move an object to see how its position affects the image. Concave mirrors are used behind car headlights. They focus the light and make it brighter. They are also used in some telescopes. "
optics,T_3835,"The other type of curved mirror, a convex mirror, is shaped like the outside of a bowl. This type of mirror forms only virtual images. The image is always right-side up and smaller than the actual object, which makes the object appear farther away than it really is. You can see how a convex mirror forms an image in Figure 22.14 and in the animation at the URL below. Because of their shape, convex mirrors can gather and reflect light from a wide area. This is why they are used as side mirrors on cars. They give the driver a wider view of the area around the vehicle than a plane mirror would. "
optics,T_3836,"Although the speed of light is constant in a vacuum, light travels at different speeds in different kinds of matter. For example, light travels more slowly in glass than in air. Therefore, when light passes from air to glass, it slows down. If light strikes a sheet of glass straight on, or perpendicular to the glass, it slows down but passes straight through. However, if light enters the glass at an angle other than 90 , the wave refracts, or bends. This is illustrated in Figure change in speed, the more light bends. "
optics,T_3837,"Lenses make use of the refraction of light to create images. A lens is a transparent object, typically made of glass, with one or two curved surfaces. The more curved the surface of a lens is, the more it refracts light. Like mirrors, lenses may be concave or convex. "
optics,T_3838,"Concave lenses are thicker at the edges than in the middle. They cause rays of light to diverge, or spread apart. Figure 22.16 shows how a concave lens forms an image. The image is always virtual and on the same side of the lens as the object. The image is also right-side up and smaller than the object. Concave lenses are used in cameras. They focus reduced images inside the camera, where they are captured and stored. You can explore the formation of images by a concave lens with the interactive animation at this URL: http://phet.colorado.edu/sims/geometric-opti "
optics,T_3839,"Convex lenses are thicker in the middle than at the edges. They cause rays of light to converge, or meet, at a point called the focus (F). Convex lenses form either real or virtual images. It depends on how close an object is to the lens relative to the focus. Figure 22.17 shows how a convex lens works. You can also interact with an animated convex lens at the URL below. An example of a convex lens is a hand lens. "
optics,T_3840,"Mirrors and lenses are used in optical instruments to reflect and refract light. Optical instruments include micro- scopes, telescopes, cameras, and lasers. "
optics,T_3841,"A light microscope is an instrument that uses lenses to make enlarged images of objects that are too small for the unaided eye to see. A common type of light microscope is a compound microscope, like the one in Figure 22.18. A compound microscope has at least two convex lenses: one or more objective lenses and one or more eyepiece lenses. The objective lenses are close to the object being viewed. They form an enlarged image of the object inside the microscope. The eyepiece lenses are close to the viewers eyes. They form an enlarged image of the first image. The magnifications of all the lenses are multiplied together to yield the overall magnification of the microscope. Some light microscopes can magnify objects more than 1000 times! For more on light microscopes and the images they create, watch the video at this URL:  (7:29). MEDIA Click image to the left or use the URL below. URL: "
optics,T_3842,"Like microscopes, telescopes use convex lenses to make enlarged images. However, telescopes make enlarged images of objectssuch as distant starsthat only appear tiny because they are very far away. There are two basic types of telescopes: reflecting telescopes and refracting telescopes. The two types are compared in Figure 22.19. You can learn more about telescopes and how they evolved in the video at this URL: "
optics,T_3843,"A camera is an optical instrument that records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way, as demonstrated in Figure 22.20 and at the URL below. Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture. As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus. The shutter controls the amount of light that strikes the film (or sensor). It stays open longer in dim light to let more light in. For a series of animations showing how a camera works, go to this URL:  . "
optics,T_3844,"Did you ever see a cat chase after a laser light, like the one in Figure 22.21? A laser is a device that produces a very focused beam of light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up (see Figure 22.21). Laser light is created in a tube like the one shown in Figure 22.22. Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength. At each end of the tube is a concave mirror. The photons of light bounce back and forth in the tube off the mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. You can see an animation showing how a laser works at this URL:  (1:12). MEDIA Click image to the left or use the URL below. URL:  Besides entertaining a cat, laser light has many other uses. It is used to scan bar codes, for example, and to carry communication signals in optical fibers. Optical fibers are extremely thin glass tubes that are used to guide laser light (see Figure 22.23). Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber. All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances. More than one signal can travel through an optic fiber at the same time, as you can see in Figure 22.23. Optical fibers are used to carry telephone, cable TV, and Internet signals. "
vision,T_3845,"The structure of the human eye is shown in Figure 22.24. Find each structure in the diagram as you read about it below. The cornea is the transparent outer covering of the eye. It protects the eye and also acts as a convex lens, helping to focus light that enters the eye. The pupil is an opening in the front of the eye. It looks black because it doesnt reflect any light. It allows light to enter the eye. The pupil automatically gets bigger or smaller to let more or less light in as needed. The iris is the colored part of the eye. It controls the size of the pupil. The lens is a convex lens that fine-tunes the focus so an image forms on the back of the eye. Tiny muscles control the shape of the lens to focus images of close or distant objects. The retina is a membrane lining the back of the eye. The retina has nerve cells called rods and cones that change images to electrical signals. Rods are good at sensing dim light but cant distinguish different colors of light. Cones can sense colors but not in dim light. There are three different types of cones. Each type senses one of the three primary colors of light. The optic nerve carries electrical signals from the rods and cones to the brain. "
vision,T_3846,"As just described, the eyes collect and focus visible light. The lens and other structures of the eye work together to focus a real image on the retina. The image is upside-down and reduced in size, as you can see in Figure 22.25. The image reaches the brain as electrical signals that travel through the optic nerve. The brain interprets the signals as shape, color, and brightness. It also interprets the image as though it were right-side up. The brain does this automatically, so what we see is always right-side up. The brain also tells us what we are seeing. "
vision,T_3847,"Many people have vision problems. The problems often can be corrected with contact lenses or lenses in eyeglasses. Some vision problems can also be corrected with laser surgery, which reshapes the cornea. Two of the most common vision problems are nearsightedness and farsightedness. You may even have one of these conditions yourself. Both are illustrated in Figure 22.26 and in the video at this URL:  (1:08). MEDIA Click image to the left or use the URL below. URL:  Nearsightedness, or myopia, is the condition in which nearby objects are seen clearly, but distant objects are blurry. It occurs when the eyeball is longer than normal. This causes images to be focused in front of the retina. Myopia can be corrected with concave lenses. The lenses focus images farther back in the eye, so they are on the retina instead of in front of it. Farsightedness, or hyperopia, is the condition in which distant objects are seen clearly, but nearby objects are blurry. It occurs when the eyeball is shorter than normal. This causes images to be focused in back of the retina. Hyperopia can be corrected with convex lenses. The lenses focus images farther forward in the eye, so they are on the retina instead of behind it. "
magnets and magnetism,T_3883,"A magnet is an object that attracts certain materials such as iron. Youre probably familiar with common bar magnets, like the one in Figure 24.2. Like all magnets, this bar magnet has north and south poles and attracts objects such as paper clips that contain iron. "
magnets and magnetism,T_3884,"All magnets have two magnetic poles. The poles are regions where the magnet is strongest. The poles are called north and south because they always line up with Earths north-south axis if the magnet is allowed to move freely. (Earths axis is the imaginary line around which the planet rotates.) What do you suppose would happen if you cut the bar magnet in Figure 24.2 in half along the line between the north and south poles? Both halves would also have north and south poles. If you cut each of the halves in half, all those pieces would have north and south poles as well. Pieces of a magnet always have both north and south poles no matter how many times you cut the magnet. "
magnets and magnetism,T_3885,"The force that a magnet exerts on certain materials is called magnetic force. Like electric force, magnetic force is exerted over a distance and includes forces of attraction and repulsion. North and south poles of two magnets attract each other, while two north poles or two south poles repel each other. "
magnets and magnetism,T_3886,"Like the electric field that surrounds a charged particle, a magnetic field surrounds a magnet. This is the area around the magnet where it exerts magnetic force. Figure 24.3 shows the magnetic field surrounding a bar magnet. Tiny bits of iron, called iron filings, were placed under a sheet of glass. When the magnet was placed on the glass, it attracted the iron filings. The pattern of the iron filings shows the lines of force that make up the magnetic field of the magnet. The concentration of iron filings near the poles indicates that these areas exert the strongest force. To see an animated magnetic field of a bar magnet, go to this URL: http://elgg.norfolk.e2bn.org/jsmith112/files/68/149/ When two magnets are brought close together, their magnetic fields interact. You can see how in Figure 24.4. The drawings show how lines of force of north and south poles attract each other whereas those of two north poles repel each other. The animations at the URL below show how magnetic field lines change as two or more magnets move in relation to each other. You can take an animated quiz to check your understanding of magnetic field interactions at this URL: http://elgg. "
magnets and magnetism,T_3887,"Magnetism is the ability of a material to be attracted by a magnet and to act as a magnet. No doubt youve handled refrigerator magnets like the ones in Figure 24.5. You probably know first-hand that they stick to a metal refrigerator but not to surfaces such as wooden doors and glass windows. Wood and glass arent attracted to a magnet, whereas the steel refrigerator is. Obviously, only certain materials respond to magnetic force. "
magnets and magnetism,T_3888,"Magnetism is due to the movement of electrons within atoms of matter. When electrons spin around the nucleus of an atom, it causes the atom to become a tiny magnet, with north and south poles and a magnetic field. In most materials, the electrons orbiting the nuclei of the atoms are arranged in such a way that the materials have no magnetic properties. Also, in most types of matter, the north and south poles of atoms point in all different directions, so overall the matter is not magnetic. Examples of nonmagnetic materials include wood, glass, plastic, paper, copper, and aluminum. These materials are not attracted to magnets and cannot become magnets. In other materials, electrons fill the orbitals of the atoms that make up the material in a way to allow for each atom to have a tiny magnetic field, giving each atom a tiny north and south pole. There are large areas where the north and south poles of atoms are all lined up in the same direction. These areas are called magnetic domains. Generally, the magnetic domains point in different directions, so the material is still not magnetic. However, the material can be magnetized by placing it in a magnetic field. When this happens, all the magnetic domains become aligned, and the material becomes a magnet. This is illustrated in Figure 24.6. Materials that can be magnetized are called ferromagnetic materials. They include iron, cobalt, and nickel. "
magnets and magnetism,T_3889,"Materials that have been magnetized may become temporary or permanent magnets. An example of each type of magnet is described below. Both are demonstrated in Figure 24.7. If you bring a bar magnet close to pile of paper clips, the paper clips will become temporarily magnetized, as all their magnetic domains align. As a result, the paper clips will stick to the magnet and also to each other. However, if you remove the paper clips from the bar magnets magnetic field, their magnetic domains will no longer align. As a result, the paper clips will no longer be magnetized or stick together. If you stroke an iron nail with a bar magnet, the nail will become a permanent (or at least long-lasting) magnet. Its magnetic domains will remain aligned even after you remove it from the magnetic field of the bar magnet. Permanent magnets can be demagnetized, however, if they are dropped or heated to high temperatures. These actions move the magnetic domains out of alignment. "
earth as a magnet,T_3890,"Imagine a huge bar magnet passing through Earths axis, as illustrated in Figure 24.10. This is a good representation of Earth as a magnet. Like a bar magnet, Earth has north and south magnetic poles and a magnetic field. "
earth as a magnet,T_3891,"Although a compass always points north, it doesnt point to Earths geographic north pole, which is located at 90 north latitude (see Figure 24.11). Instead, it points to Earths magnetic north pole, which is located at about 80 north latitude. Earths magnetic south pole is also located several degrees of latitude away from the geographic south pole. A compass pointer has north and south poles, and its north pole points to Earths magnetic north pole. Why does this happen if opposite poles attract? Why doesnt the compass needle point south instead? The answer may surprise you. Earths magnetic north pole is actually the south pole of magnet Earth! Its called the magnetic north pole to avoid confusion. Because its close to the geographic north pole, it would be confusing to call it the magnetic south pole. "
earth as a magnet,T_3892,"Like all magnets, Earth has a magnetic field. Earths magnetic field is called the magnetosphere. It is a huge region that extends outward from Earth for several thousand kilometers but is strongest at the poles. You can see the extent of the magnetosphere in Figure 24.12. For an animated version of the magnetosphere, watch the video at this URL: MEDIA Click image to the left or use the URL below. URL: "
earth as a magnet,T_3893,"Do you like to read science fiction? Science fiction writers are really creative. For example, an author might write about a time in the distant past when compasses pointed south instead of north. Actually, this idea isnt fictionits a fact! Earths magnetic poles have switched places repeatedly over the past hundreds of millions of years, each time reversing Earths magnetic field. This is illustrated in Figure 24.13. Scientists dont know for certain why magnetic reversals occur, but there is hard evidence showing that they have occurred. The evidence comes from rocks on the ocean floor. Look at Figure 24.14, which shows a ridge on the ocean floor. At the center of the ridge, hot magma pushes up through the crust and hardens into rock. Once the magma hardens, the alignment of magnetic domains in the rock is frozen in place forever. The newly hardened rock is then gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. Rock samples from many places on the ocean floor reveal that magnetic domains of rocks from different time periods are aligned in opposite directions. The evidence shows that Earths magnetic field reversed hundreds of times over the last 330 million years. The last reversal was less than a million years ago. What might happen if a magnetic reversal occurred in your lifetime? How might it affect you? You can learn more about Earths magnetic reversals at this URL:  . "
earth as a magnet,T_3894,"The idea that Earth is a magnet is far from new. It was first proposed in 1600 by a British physician named William Gilbert. However, explaining why Earth acts like a magnet is a relatively recent discovery. It had to wait until the development of technologies such as seismographs, which detect and measure earthquake waves. Then scientists could learn about Earths inner structure (see Figure 24.15). They discovered that Earth has an inner and outer core and that the outer core consists of liquid metals, mainly iron and nickel. Scientists think that Earths magnetic field is generated by the movement of charged particles through the molten metals in the outer core. The particles move as Earth spins on its axis. The video at the URL below takes a closer look at how this occurs. MEDIA Click image to the left or use the URL below. URL: "
earth as a magnet,T_3895,"Earths magnetic field helps protect Earth and its organisms from harmful particles given off by the sun. Most of the particles are attracted to the north and south magnetic poles, where Earths magnetic field is strongest. This is also where relatively few organisms live. Another benefit of Earths magnetic field is its use for navigation. People use compasses to detect Earths magnetic north pole and tell direction. Many animals have natural ""compasses"" that work just as well. Birds like the garden warbler in Figure 24.16 use Earths magnetic field to guide their annual migrations. Recent research suggests that warblers and other migrating birds have structures in their eyes that let them see Earths magnetic field as a visual pattern. You can learn more about animals and Earths magnetic field, including the potential effects of magnetic field reversals, at this URL:  . "
earth as a magnet,T_3896,"Northern California residents may not be able to see the northern lights like people in Alaska can, but Bay Area scientists are playing a key role in understanding them. Find out more about the spectacular light shows up north and what scientists at UC Berkeley are discovering about the Earths magnetic field. For more information on the northern lights, see http://science.kqed.org/quest/video/illuminating-the-northern-lights/ . MEDIA Click image to the left or use the URL below. URL: "
types of matter,T_3921,An element is a pure substance. It cannot be separated into any other substances. There are more than 90 different elements that occur in nature. Some are much more common than others. Hydrogen is the most common element in the universe. Oxygen is the most common element in Earths crust. Figure 3.7 shows other examples of elements. Still others are described in the video below. MEDIA Click image to the left or use the URL below. URL: 
types of matter,T_3922,"Each element has a unique set of properties that make it different from all other elements. As a result, elements can be identified by their properties. For example, the elements iron and nickel are both metals that are good conductors of heat and electricity. However, iron is attracted by a magnet, whereas nickel is not. How could you use this property to separate iron objects from nickel objects? "
types of matter,T_3923,"The idea of elements is not new. It dates back about 2500 years to ancient Greece. The ancient Greek philosopher Aristotle thought that all matter consists of just four elements. He identified the elements as earth, air, water, and fire. He thought that different kinds of matter contain only these four elements but in different combinations. Aristotles ideas about elements were accepted for the next 2000 years. Then, scientists started discovering the many unique substances we call elements today. You can read when and how each of the elements was discovered at the link below. Scientists soon realized that there are far more than just four elements. Eventually, they discovered a total of 92 naturally occurring elements. "
types of matter,T_3924,"The smallest particle of an element that still has the elements properties is an atom. All the atoms of an element are alike, and they are different from the atoms of all other elements. For example, atoms of gold are the same whether they are found in a gold nugget or a gold ring (see Figure 3.8). All gold atoms have the same structure and properties. "
types of matter,T_3925,"There are millions of different substances in the world. Thats because elements can combine in many different ways to form new substances. In fact, most elements are found in compounds. A compound is a unique substance that forms when two or more elements combine chemically. An example is water, which forms when hydrogen and oxygen combine chemically. A compound always has the same components in the same proportions. It also has the same composition throughout. You can learn more about compounds and how they form by watching this video: MEDIA Click image to the left or use the URL below. URL: "
types of matter,T_3926,"A compound has different properties than the substances it contains. For example, hydrogen and oxygen are gases at room temperature. But when they combine chemically, they form liquid water. Another example is table salt, or sodium chloride. It contains sodium and chlorine. Sodium is a silvery solid that reacts explosively with water, and chlorine is a poisonous gas (see Figure 3.9). But together, sodium and chlorine form a harmless, unreactive compound that you can safely sprinkle on food. "
types of matter,T_3927,"The smallest particle of a compound that still has the compounds properties is a molecule. A molecule consists of two or more atoms that are joined together. For example, a molecule of water consists of two hydrogen atoms joined to one oxygen atom (see Figure 3.10). You can learn more about molecules at this link:  Some compounds form crystals instead of molecules. A crystal is a rigid, lattice-like framework of many atoms bonded together. Table salt is an example of a compound that forms crystals (see Figure 3.11). Its crystals are made up of many sodium and chloride ions. Ions are electrically charged forms of atoms. You can actually watch crystals forming in this video:  . "
types of matter,T_3928,"Not all combined substances are compounds. Some are mixtures. A mixture is a combination of two or more substances in any proportion. The substances in a mixture may be elements or compounds. The substances dont combine chemically to form a new substance, as they do in a compound. Instead, they keep their original properties and just intermix. Examples of mixtures include salt and water in the ocean and gases in the atmosphere. Other examples are pictured in Figure 3.12. "
types of matter,T_3929,"Some mixtures are homogeneous. This means they have the same composition throughout. An example is salt water in the ocean. Ocean water everywhere is about 3.5 percent salt. Some mixtures are heterogeneous. This means they vary in their composition. An example is trail mix. No two samples of trail mix, even from the same package, are likely to be exactly the same. One sample might have more raisins, another might have more nuts. "
types of matter,T_3930,"Mixtures have different properties depending on the size of their particles. Three types of mixtures based on particle size are described below. Figure 3.13 shows examples of each type. You can watch videos about the three types of mixtures at these links: MEDIA Click image to the left or use the URL below. URL:  MEDIA Click image to the left or use the URL below. URL:  A solution is a homogeneous mixture with tiny particles. An example is salt water. The particles of a solution are too small to reflect light. As a result, you cannot see them. Thats why salt water looks the same as pure water. The particles of solutions are also too small to settle or be filtered out of the mixture. A suspension is a heterogeneous mixture with large particles. An example is muddy water. The particles of a suspension are big enough to reflect light, so you can see them. They are also big enough to settle or be filtered out. Anything that you have to shake before using, such as salad dressing, is usually a suspension. A colloid is a homogeneous mixture with medium-sized particles. Examples include homogenized milk and gelatin. The particles of a colloid are large enough to reflect light, so you can see them. But they are too small to settle or filter out of the mixture. "
types of matter,T_3931,"The components of a mixture keep their own identity when they combine. Therefore, they usually can be easily separated again. Their different physical properties are used to separate them. For example, oil is less dense than water, so a mixture of oil and water can be separated by letting it stand until the oil floats to the top. Other ways of separating mixtures are shown in Figure 3.14 and in the videos below.  (2:30) MEDIA Click image to the left or use the URL below. URL:  (2:41) MEDIA Click image to the left or use the URL below. URL: "
inside the atom,T_3963,"Figure 5.1 represents a simple model of an atom. You will learn about more complex models in later lessons, but this model is a good place to start. You can see similar, animated models of atoms at this URL: http://web.jjay.cuny "
inside the atom,T_3964,"At the center of an atom is the nucleus (plural, nuclei). The nucleus contains most of the atoms mass. However, in size, its just a tiny part of the atom. The model in Figure 5.1 is not to scale. If an atom were the size of a football stadium, the nucleus would be only about the size of a pea. The nucleus, in turn, consists of two types of particles, called protons and neutrons. These particles are tightly packed inside the nucleus. Constantly moving about the nucleus are other particles called electrons. You can see a video about all three types of atomic particles at this URL:  (1:57). "
inside the atom,T_3965,"A proton is a particle in the nucleus of an atom that has a positive electric charge. All protons are identical. It is the number of protons that gives atoms of different elements their unique properties. Atoms of each type of element have a characteristic number of protons. For example, each atom of carbon has six protons, as you can see in Figure "
inside the atom,T_3966,"A neutron is a particle in the nucleus of an atom that has no electric charge. Atoms of an element often have the same number of neutrons as protons. For example, most carbon atoms have six neutrons as well as six protons. This is also shown in Figure 5.2. "
inside the atom,T_3967,"An electron is a particle outside the nucleus of an atom that has a negative electric charge. The charge of an electron is opposite but equal to the charge of a proton. Atoms have the same number of electrons as protons. As a result, the negative and positive charges ""cancel out."" This makes atoms electrically neutral. For example, a carbon atom has six electrons that ""cancel out"" its six protons. "
inside the atom,T_3968,"When it comes to atomic particles, opposites attract. Negative electrons are attracted to positive protons. This force of attraction keeps the electrons moving about the nucleus. An analogy is the way planets orbit the sun. What about particles with the same charge, such as protons in the nucleus? They push apart, or repel, each other. So why doesnt the nucleus fly apart? The reason is a force of attraction between protons and neutrons called the strong force. The name of the strong force suits it. It is stronger than the electric force pushing protons apart. However, the strong force affects only nearby particles (see Figure 5.3). It is not effective if the nucleus gets too big. This puts an upper limit on the number of protons an atom can have and remain stable. You can learn more about atomic forces in the colorful tutorial at this URL:  . "
inside the atom,T_3969,"Electrons have almost no mass. Instead, almost all the mass of an atom is in its protons and neutrons in the nucleus. The nucleus is very small, but it is densely packed with matter. The SI unit for the mass of an atom is the atomic mass unit (amu). One atomic mass unit equals the mass of a proton, which is about 1.7  10 24 g. Each neutron also has a mass of 1 amu. Therefore, the sum of the protons and neutrons in an atom is about equal to the atoms total mass in atomic mass units. Two numbers are commonly used to distinguish atoms: atomic number and mass number. Figure 5.4 shows how these numbers are usually written. The atomic number is the number of protons in an atom. This number is unique for atoms of each kind of element. For example, the atomic number of all helium atoms is 2. The mass number is the number of protons plus the number of neutrons in an atom. For example, most atoms of helium have 2 neutrons, so their mass number is 2 + 2 = 4. This mass number means that an atom of helium has a mass of about 4 amu. Problem Solving Problem: An atom has an atomic number of 12 and a mass number of 24. How many protons and neutrons does the atom have? Solution: The number of protons is the same as the atomic number, or 12. The number of neutrons is equal to the mass number minus the atomic number, or 24 12 = 12. You Try It! Problem: An atom has an atomic number of 8 and a mass number of 16. How many neutrons does it have? What is the atoms mass in atomic mass units? "
inside the atom,T_3970,"The number of protons per atom is always the same for a given element. However, the number of neutrons may vary, and the number of electrons can change. "
inside the atom,T_3971,"Sometimes atoms lose or gain electrons. Then they become ions. Ions have a positive or negative charge. Thats because they do not have the same number of electrons as protons. If atoms lose electrons, they become positive ions, or cations. If atoms gain electrons, they become negative ions, or anions. Consider the example of fluorine in Figure 5.5. A fluorine atom has nine protons and nine electrons, so it is electrically neutral. If a fluorine atom gains an electron, it becomes a fluoride ion with a negative charge of minus one. "
inside the atom,T_3972,"Some atoms of the same element may have different numbers of neutrons. For example, some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in number of neutrons are called isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes of an element generally have the same chemical properties. Thats because they have the same numbers of protons and electrons. For a video explanation of isotopes, go to this URL:  MEDIA Click image to the left or use the URL below. URL: "
inside the atom,T_3973,Hydrogen is a good example of isotopes because it has the simplest atoms. Three isotopes of hydrogen are modeled in Figure 5.6. Most hydrogen atoms have just one proton and one electron and lack a neutron. They are just called hydrogen. Some hydrogen atoms have one neutron. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium. 
inside the atom,T_3974,"For most other elements, isotopes are named for their mass number. For example, carbon atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13). These atoms are the isotope called carbon-13. Some carbon atoms have 8 neutrons. What is the name of this isotope of carbon? You can learn more about this isotope at the URL below. It is used by scientists to estimate the ages of rocks and fossils. "
inside the atom,T_3975,Remember the quarks from the first page of this chapter? Quarks are even tinier particles of matter that make up protons and neutrons. There are three quarks in each proton and three quarks in each neutron. The charges of quarks are balanced exactly right to give a positive charge to a proton and a neutral charge to a neutron. It might seem strange that quarks are never found alone but only as components of other particles. This is because the quarks are held together by very strange particles called gluons. 
inside the atom,T_3976,"Gluons make quarks attract each other more strongly the farther apart the quarks get. To understand how gluons work, imagine holding a rubber band between your fingers. If you try to move your hands apart, they will be pulled back together by the rubber band. The farther apart you move your hands, the stronger the force of the rubber band pulling your hands together. Gluons work the same way on quarks inside protons and neutrons (and other, really rare particles too). If you were to move your hands apart with enough force, the rubber band holding them together would break. The same is true of quarks. If they are given enough energy, they pull apart with enough force to ""break"" the binding from the gluons. However, all the energy that is put into a particle to make this possible is then used to create a new set of quarks and gluons. And so a new proton or neutron appears. "
inside the atom,T_3977,"The existence of quarks was first proposed in the 1960s. Since then, scientists have done experiments to show that quarks really do exist. In fact, they have identified six different types of quarks. However, much remains to be learned about these tiny, fundamental particles of matter. They are very difficult and expensive to study. If you want to learn more about them, including how they are studied, the URL below is a good place to start. "
inside the atom,T_3978,"QUEST journeys back to find out how physicists on the UC Berkeley campus in the 1930s, and at the Stanford Linear Accelerator Center in the 1970s, created ""atom smashers"" that led to key discoveries about the tiny constituents of the atom and paved the way for the Large Hadron Collider in Switzerland. For more information on particle accelerators, see http://science.kqed.org/quest/video/homegrown-particle-accelerators/ . MEDIA Click image to the left or use the URL below. URL: "
history of the atom,T_3979,"The history of the atom begins around 450 B.C. with a Greek philosopher named Democritus (see Figure 5.7). Democritus wondered what would happen if you cut a piece of matter, such as an apple, into smaller and smaller pieces. He thought that a point would be reached where matter could not be cut into still smaller pieces. He called these ""uncuttable"" pieces atomos. This is where the modern term atom comes from. Democritus was an important philosopher. However, he was less influential than the Greek philosopher Aristotle, who lived about 100 years after Democritus. Aristotle rejected Democrituss idea of atoms. In fact, Aristotle thought "
history of the atom,T_3980,"Around 1800, a British chemist named John Dalton revived Democrituss early ideas about the atom. Dalton is pictured in Figure 5.8. He made a living by teaching and just did research in his spare time. Nonetheless, from his research results, he developed one of the most important theories in science. "
history of the atom,T_3981,"Dalton did many experiments that provided evidence for atoms. For example, he studied the pressure of gases. He concluded that gases must consist of tiny particles in constant motion. Dalton also researched the properties of compounds. He showed that a compound always consists of the same elements in the same ratio. On the other hand, different compounds always consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of tiny particles that can combine in an endless variety of ways. From his research, Dalton developed a theory of the atom. You can learn more about Dalton and his research by watching the video at this URL:  (9:03). MEDIA Click image to the left or use the URL below. URL: "
history of the atom,T_3982,The atomic theory Dalton developed consists of three ideas: All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles. They also cannot be created or destroyed. All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses. Atoms join together to form compounds. A given compound always consists of the same kinds of atoms in the same ratio. Daltons theory was soon widely accepted. Most of it is still accepted today. The only part that is no longer accepted is his idea that atoms are the smallest particles. Scientists now know that atoms consist of even smaller particles. 
history of the atom,T_3983,"Dalton incorrectly thought that atoms are tiny solid particles of matter. He used solid wooden balls to model them. The sketch in the Figure 5.9 shows how Daltons model atoms looked. He made holes in the balls so they could be joined together with hooks. In this way, the balls could be used to model compounds. When later scientists discovered subatomic particles (particles smaller than the atom itself), they realized that Daltons models were too simple. They didnt show that atoms consist of even smaller particles. Models including these smaller particles were later developed. "
history of the atom,T_3984,The next major advance in the history of the atom was the discovery of electrons. These were the first subatomic particles to be identified. They were discovered in 1897 by a British physicist named J. J. Thomson. You can learn more about Thomson and his discovery at this online exhibit:  . 
history of the atom,T_3985,"Thomson was interested in electricity. He did experiments in which he passed an electric current through a vacuum tube. The experiments are described in Figure 5.10. Thomsons experiments showed that an electric current consists of flowing, negatively charged particles. Why was this discovery important? Many scientists of Thomsons time thought that electric current consists of rays, like rays of light, and that it is positive rather than negative. Thomsons experiments also showed that the negative particles are all alike and smaller than atoms. Thomson concluded that the negative particles couldnt be fundamental units of matter because they are all alike. Instead, they must be parts of atoms. The negative particles were later named electrons. "
history of the atom,T_3986,"Thomson knew that atoms are neutral in electric charge. So how could atoms contain negative particles? Thomson thought that the rest of the atom must be positive to cancel out the negative charge. He said that an atom is like a plum pudding, which has plums scattered through it. Thats why Thomsons model of the atom is called the plum pudding model. You can see it in Figure 5.11. It shows the atom as a sphere of positive charge (the pudding) with negative electrons (the plums) scattered through it. "
history of the atom,T_3987,A physicist from New Zealand named Ernest Rutherford made the next major discovery about atoms. He discovered the nucleus. You can watch a video about Rutherford and his discovery at this URL:  MEDIA Click image to the left or use the URL below. URL: 
history of the atom,T_3988,"In 1899, Rutherford discovered that some elements give off positively charged particles. He named them alpha particles (a). In 1911, he used alpha particles to study atoms. He aimed a beam of alpha particles at a very thin sheet of gold foil. Outside the foil, he placed a screen of material that glowed when alpha particles struck it. If Thomsons plum pudding model were correct, the alpha particles should be deflected a little as they passed through the foil. Why? The positive ""pudding"" part of gold atoms would slightly repel the positive alpha particles. This would cause the alpha particles to change course. But Rutherford got a surprise. Most of the alpha particles passed straight through the foil as though they were moving through empty space. Even more surprising, a few of the alpha particles bounced back from the foil as though they had struck a wall. This is called back scattering. It happened only in very small areas at the centers of the gold atoms. "
history of the atom,T_3989,"Based on his results, Rutherford concluded that all the positive charge of an atom is concentrated in a small central area. He called this area the nucleus. Rutherford later discovered that the nucleus contains positively charged particles. He named the positive particles protons. Rutherford also predicted the existence of neutrons in the nucleus. However, he failed to find them. One of his students, a physicist named James Chadwick, went on to discover neutrons in 1932. You learn how at this URL:  . "
history of the atom,T_3990,"Rutherfords discoveries meant that Thomsons plum pudding model was incorrect. Positive charge is not spread out everywhere in an atom. It is all concentrated in the tiny nucleus. The rest of the atom is empty space, except for the electrons moving randomly through it. In Rutherfords model, electrons move around the nucleus in random orbits. He compared them to planets orbiting a star. Thats why Rutherfords model is called the planetary model. You can see it in Figure 5.13. "
modern atomic theory,T_3991,"Bohrs research focused on electrons. In 1913, he discovered evidence that the orbits of electrons are located at fixed distances from the nucleus. Remember, Rutherford thought that electrons orbit the nucleus at random. Figure 5.14 shows Bohrs model of the atom. "
modern atomic theory,T_3992,"Basic to Bohrs model is the idea of energy levels. Energy levels are areas located at fixed distances from the nucleus of the atom. They are the only places where electrons can be found. Energy levels are a little like rungs on a ladder. You can stand on one rung or another but not between the rungs. The same goes for electrons. They can occupy one energy level or another but not the space between energy levels. The model of an atom in Figure 5.15 has six energy levels. The level with the least energy is the one closest to the nucleus. As you go farther from the nucleus, the levels have more and more energy. Electrons can jump from one energy level to another. If an atom absorbs energy, some of its electrons can jump to a higher energy level. If electrons jump to a lower energy level, the atom emits, or gives off, energy. You can see an animation at this happening at the URL below. "
modern atomic theory,T_3993,"Bohrs idea of energy levels is still useful today. It helps explain how matter behaves. For example, when chemicals in fireworks explode, their atoms absorb energy. Some of their electrons jump to a higher energy level. When the electrons move back to their original energy level, they give off the energy as light. Different chemicals have different arrangements of electrons, so they give off light of different colors. This explains the blue- and purple- colored fireworks in Figure 5.16. "
modern atomic theory,T_3994,"In the 1920s, physicists discovered that electrons do not travel in fixed paths. In fact, they found that electrons only have a certain chance of being in any particular place. They could only describe where electrons are with mathematical formulas. Thats because electrons have wave-like properties as well as properties of particles of matter. It is the ""wave nature"" of electrons that lets them exist only at certain distances from the nucleus. The negative electrons are attracted to the positive nucleus. However, because the electrons behave like waves, they bend around the nucleus instead of falling toward it. Electrons exist only where the wave is stable. These are the orbitals. They do not exist where the wave is not stable. These are the places between orbitals. "
modern atomic theory,T_3995,"Today, these ideas about electrons are represented by the electron cloud model. The electron cloud is an area around the nucleus where electrons are likely to be. Figure 5.17 shows an electron cloud model for a helium atom. "
modern atomic theory,T_3996,"Some regions of the electron cloud are denser than others. The denser regions are areas where electrons are most likely to be. These regions are called orbitals. Each orbital has a maximum of just two electrons. Different energy levels in the cloud have different numbers of orbitals. Therefore, different energy levels have different maximum numbers of electrons. Table 5.1 lists the number of orbitals and electrons for the first four energy levels. Energy levels farther from the nucleus have more orbitals. Therefore, these levels can hold more electrons. Energy Level Number of Orbitals 1 2 3 4 1 4 9 16 Max. No. of Electrons (@ 2 per orbital) 2 8 18 32 Figure 5.18 shows the arrangement of electrons in an atom of magnesium as an example. The most stable arrange- ment of electrons occurs when electrons fill the orbitals at the lowest energy levels first before more are added at higher levels. You can learn more about orbitals and their electrons at the URL below: "
how elements are organized,T_3997,"Mendeleev was a teacher as well as a chemist. He was writing a chemistry textbook and needed a way to organize the elements so it would be easier for students to learn about them. He made a set of cards of the elements, similar to a deck of playing cards, with one element per card. On the card, he wrote the elements name, atomic mass, and known properties. He arranged and rearranged the cards in many different ways, looking for a pattern. He finally found it when he placed the elements in order by atomic mass. "
how elements are organized,T_3998,"You can see how Mendeleev organized the elements in Figure 6.2. From left to right across each row, elements are arranged by increasing atomic mass. Mendeleev discovered that if he placed eight elements in each row and then continued on to the next row, the columns of the table would contain elements with similar properties. He called the columns groups. They are sometimes called families, because elements within a group are similar but not identical to one another, like people in a family. Mendeleevs table of the elements is called a periodic table because of its repeating pattern. Anything that keeps repeating is referred to as periodic. Other examples of things that are periodic include the monthly phases of the moon and the daily cycle of night and day. The term period refers to the interval between repetitions. In a periodic table, the periods are the rows of the table. In Mendeleevs table, each period contains eight elements, and then the pattern repeats in the next row. "
how elements are organized,T_3999,"Did you notice the blanks in Mendeleevs table (Figure 6.2)? They are spaces that Mendeleev left for elements that had not yet been discovered when he created his table. He predicted that these missing elements would eventually be discovered. Based on their position in the table, he could even predict their properties. For example, he predicted a missing element in row 5 of his group 3. He said it would have an atomic mass of about 68 and be a soft metal like other group 3 elements. Scientists searched for the missing element. They found it a few years later and named it gallium. Scientists searched for the other missing elements. Eventually, all of them were found. An important measure of a good model is its ability to make accurate predictions. This makes it a useful model. Clearly, Mendeleevs periodic table was a useful model. It helped scientists discover new elements and make sense of those that were already known. "
how elements are organized,T_4000,A periodic table is still used today to classify the elements. Figure 6.3 shows the modern periodic table. You can see an interactive version at this URL:  . 
how elements are organized,T_4001,"In the modern periodic table, elements are organized by atomic number. The atomic number is the number of protons in an atom of an element. This number is unique for each element, so it seems like an obvious way to organize the elements. (Mendeleev used atomic mass instead of atomic number because protons had not yet been discovered when he made his table.) In the modern table, atomic number increases from left to right across each period. It also increases from top to bottom within each group. How is this like Mendeleevs table? "
how elements are organized,T_4002,"Besides atomic number, the periodic table includes each elements chemical symbol and class. Some tables include other information as well. The chemical symbol consists of one or two letters that come from the chemicals name in English or another language. The first letter is always written in upper case. The second letter, if there is one, is always written in lower case. For example, the symbol for lead is Pb. It comes from the Latin word plumbum, which means ""lead."" Find lead in Figure 6.3. What is its atomic number? You can access videos about lead and other elements in the modern periodic table at this URL:  . The classes of elements are metals, metalloids, and nonmetals. They are color-coded in the table. Blue stands for metals, orange for metalloids, and green for nonmetals. You can read about each of these three classes of elements later in the chapter, in the lesson ""Classes of Elements."" "
how elements are organized,T_4003,"Rows of the modern table are called periods, as they are in Mendeleevs table. From left to right across a period, each element has one more proton than the element before it. In each period, elements change from metals on the left side of the table, to metalloids, and then to nonmetals on the right. Figure 6.4 shows this for period 4. Some periods in the modern periodic table are longer than others. For example, period 1 contains only two elements. Periods 6 and 7, in contrast, are so long that some of their elements are placed below the main part of the table. They are the elements starting with lanthanum (La) in period 6 and actinium (Ac) in period 7. Some elements in period 7 have not yet been named. They are represented by temporary symbols, such as Uub. Many of these elements have only recently been shown to exist. Elements 114 and 116 were added to the table in 2011. Four more elements (113, 115, 117, and 118) were approved for addition in December 2015 and will be named at some later date. "
how elements are organized,T_4004,"Columns of the modern table are called groups, as they are in Mendeleevs table. However, the modern table has many more groups  18 to be exact. Elements in the same group have similar properties. For example, all elements in group 18 are colorless, odorless gases. You can read about the different groups of elements in this chapters lesson on ""Groups of Elements."" "
classes of elements,T_4005,"Metals are elements that are good conductors of electricity. They are the largest of the three classes of elements. In fact, most elements are metals. Look back at the modern periodic table (Figure 6.3) in this chapters lesson ""How Elements Are Organized."" Find the metals in the table. They are all the elements that are color-coded blue. Examples include sodium (Na), silver (Ag), and zinc (Zn). Metals have relatively high melting points, so almost all are solids at room temperature. The only exception is mercury (Hg), which is a liquid. Most metals are also good conductors of heat. Thats why they are used for cooking pots and stovetops. Metals have other characteristic properties as well. Most are shiny, ductile, and malleable. These properties are illustrated in Figure 6.5. You can dig deeper into the properties of metals at this URL: "
classes of elements,T_4006,"Nonmetals are elements that do not conduct electricity. They are the second largest class of elements. Find the nonmetals in Figure 6.3. They are all the elements on the right side of the table that are color-coded green. Examples of nonmetals include helium (He), carbon (C), and oxygen (O). Nonmetals generally have properties that are the opposite of those of metals. They also tend to vary more in their properties than metals do. For example, nonmetals have relatively low boiling points, so many of them are gases at room temperature. But several nonmetals are solids, including carbon and phosphorus (P). One nonmetal, bromine (Br), is a liquid at room temperature. Generally, nonmetals are also poor conductors of heat. In fact, they may be used for insulation. For example, the down filling in a down jacket is mostly air, which consists mainly of nitrogen (N) and oxygen (O). These nonmetal gases are poor conductors of heat, so they keep body heat in and cold air out. Solid nonmetals are dull rather than shiny. They are also brittle rather than ductile or malleable. You can see examples of solid nonmetals in Figure 6.6. You can learn more about specific nonmetals with the interactive table at this URL: http://library.thinkquest.org/36 "
classes of elements,T_4007,"Metalloids are elements that fall between metals and nonmetals in the periodic table. Just seven elements are metalloids, so they are the smallest class of elements. In Figure 6.3, they are color-coded orange. Examples of metalloids include boron (B), silicon (Si), and germanium (Ge). Metalloids have some properties of metals and some properties of nonmetals. For example, many metalloids can conduct electricity but only at certain temperatures. These metalloids are called semiconductors. Silicon is an example. It is used in computer chips. It is also the most common metalloid on Earth. It is shiny like a metal but brittle like a nonmetal. You see a sample of silicon in Figure 6.7. The figure also shows other examples of metalloids. You can learn more about the properties of metalloids at this URL: http://library.thinkquest.org/3659/p "
classes of elements,T_4008,"From left to right across the periodic table, each element has one more proton than the element to its left. Because atoms are always electrically neutral, for each added proton, one electron is also added. Electrons are added first to the lowest energy level possible until that level is full. Only then are electrons added to the next higher energy level. "
classes of elements,T_4009,"The increase in electrons across the periodic table explains why elements go from metals to metalloids and then to nonmetals from left to right across the table. Look at period 2 in Figure 6.8 as an example. Lithium (Li) is a metal, boron (B) a metalloid, and fluorine (F) and neon (Ne) are nonmetals. The inner energy level is full for all four elements. This level has just one orbital and can hold a maximum of two electrons. The outer energy level is a different story. This level has four orbitals and can hold a maximum of eight electrons. Lithium has just one electron in this level, boron has three, fluorine has seven, and neon has eight. "
classes of elements,T_4010,"The electrons in the outer energy level of an atom are called valence electrons. It is valence electrons that are potentially involved in chemical reactions. The number of valence electrons determines an elements reactivity, or how likely the element is to react with other elements. The number of valence electrons also determines whether the element can conduct electric current. Thats because electric current is the flow of electrons. Table 6.1 shows how these properties vary in elements from each class. Metals such as lithium have an outer energy level that is almost empty. They ""want"" to give up their few valence electrons so they will have a full outer energy level. As a result, metals are very reactive and good conductors of electricity. Metalloids such as boron have an outer energy level that is about half full. These elements need to gain or lose too many electrons for a full outer energy level to come about easily. As a result, these elements are not very reactive. They may be able to conduct electricity but not very well. Some nonmetals, such as bromine, have an outer energy level that is almost full. They ""want"" to gain electrons so they will have a full outer energy level. As a result, these nonmetals are very reactive. Because they only accept electrons and do not give them up, they do not conduct electricity. Other nonmetals, such as neon, have a completely full outer energy level. Their electrons are already in the most stable arrangement possible. They are unreactive and do not conduct electricity. Element Description Element Lithium Description Lithium (Li) is a highly reactive metal. It has just one electron in its outer energy level. Lithium reacts explosively with water (see picture). It can react with moisture on skin and cause serious burns. Boron Boron (B) is a metalloid. It has three valence electrons and is less reactive than lithium. Boron compounds dissolved in water form boric acid. Dilute boric acid is weak enough to use as eye wash. Bromine Bromine (Br) is an extremely reactive nonmetal. In fact, reactions with fluorine are often explosive, as you can see in the URL below. Neon (Ne) is a nonmetal gas with a completely filled outer energy level. This makes it unreactive, so it doesnt combine with other elements. Neon is used for lighted signs like this one. You can learn why neon gives off light at this link:  Neon "
groups of elements,T_4011,"All the elements in group 1 have just one valence electron, so they are highly reactive. Group 1 is shown in Figure element in the universe. All the other elements in group 1 are alkali metals. They are the most reactive of all metals, and along with the elements in group 17, the most reactive elements. Because alkali metals are so reactive, they are only found in nature combined with other elements. The alkali metals are soft. Most are soft enough to cut with a knife. They are also low in density. Some of them even float on water. All are solids at room temperature. You can see a video demonstrating the reactivity of alkali metals with water at this URL:  (2:22). MEDIA Click image to the left or use the URL below. URL: "
groups of elements,T_4012,"The alkaline Earth metals include all the elements in group 2 (see Figure 6.10). These metals have just two valence electrons, so they are very reactive, although not quite as reactive as the alkali metals. In nature, they are always found combined with other elements. Alkaline Earth metals are silvery grey in color. They are harder and denser than the alkali metals. All are solids at room temperature. "
groups of elements,T_4013,"Groups 3-12 of the periodic table contain transition metals (see Figure 6.11). Transition metals have more valence electrons and are less reactive than metals in the first two metal groups. The transition metals are shiny. Many are silver colored. They tend to be very hard, with high melting and boiling points. All except mercury (Hg) are solids at room temperature. Transition metals include the elements that are placed below the periodic table. Those that follow lanthanum (La) are called lanthanides. They are all shiny, relatively reactive metals. Those that follow Actinium (Ac) are called actinides. They are all radioactive metals. This means they are unstable. They break down into different, more stable elements. You can read more about radioactive elements in the chapter Nuclear Chemistry. Many of the actinides do not occur in nature but are made in laboratories. "
groups of elements,T_4014,"Groups 13-16 each contain one or more metalloids. These groups are shown in Figure 6.12. Group 13 is called the boron group. The only metalloid in this group is boron (B). The other four elements are metals. All group 13 elements have three valence electrons and are fairly reactive. All are solids at room temperature. Group 14 is called the carbon group. Carbon (C) is a nonmetal. The next two elements are metalloids, and the final two are metals. All the elements in the carbon group have four valence electrons. They are not very reactive. All are solids at room temperature. Group 15 is called the nitrogen group. The first two elements in this group are nonmetals. These are followed by two metalloids and one metal. All the elements in the nitrogen group have five valence electrons, but they vary in their reactivity. Nitrogen (N) in not reactive at all. Phosphorus (P), in contrast, is quite reactive. In fact, it is found naturally only in combination with other substances. Nitrogen is a gas at room temperature. The other group 15 elements are solids. Group 16 is called the oxygen group. The first three elements in this group are nonmetals. They are followed by one metalloid and one metal. All the elements in the oxygen group have six valence electrons, and all are "
groups of elements,T_4015,"Elements in group 17 are called halogens (see Figure 6.13). They are highly reactive nonmetals with seven valence electrons. The halogens react violently with alkali metals, which have one valence electron. The two elements combine to form a salt. For example, the halogen chlorine (Cl) and the alkali metal sodium (Na) react to form table salt, or sodium chloride (NaCl). The halogen group includes gases, liquids, and solids. For example, chlorine is a gas at room temperature, bromine (Br) is a liquid, and iodine (I) is a solid. You can watch a video demonstrating the reactivity of halogens at this URL:  . "
groups of elements,T_4016,"Group 18 elements are nonmetals called noble gases (see Figure 6.14). They are all colorless, odorless gases. Their outer energy level is also full, so they are the least reactive elements. In nature, they seldom combine with other substances. For a short video about the noble gases and their properties, go to this URL: "
introduction to chemical bonds,T_4017,"Elements form compounds when they combine chemically. Their atoms join together to form molecules, crystals, or other structures. The atoms are held together by chemical bonds. A chemical bond is a force of attraction between atoms or ions. It occurs when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom. You can learn more about chemical bonds in this video:  MEDIA Click image to the left or use the URL below. URL:  Look at the example of water in Figure 7.1. A water molecule consists of two atoms of hydrogen and one atom of oxygen. Each hydrogen atom has just one electron. The oxygen atom has six valence electrons. In a water molecule, two hydrogen atoms share their two electrons with the six valence electrons of one oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. This gives it a more stable arrangement of electrons that takes less energy to maintain. "
introduction to chemical bonds,T_4018,"Water (H2 O) is an example of a chemical compound. Water molecules always consist of two atoms of hydrogen and one atom of oxygen. Like water, all other chemical compounds consist of a fixed ratio of elements. It doesnt matter how much or how little of a compound there is. It always has the same composition. "
introduction to chemical bonds,T_4019,"Elements are represented by chemical symbols. Examples are H for hydrogen and O for oxygen. Compounds are represented by chemical formulas. Youve already seen the chemical formula for water. Its H2 O. The subscript 2 after the H shows that there are two atoms of hydrogen in a molecule of water. The O for oxygen has no subscript. When there is just one atom of an element in a molecule, no subscript is used. Table 7.1 shows some other examples of compounds and their chemical formulas. Name of Compound Electron Dot Diagram Numbers of Atoms Chemical Formula Name of Compound Hydrogen chloride Electron Dot Diagram Numbers of Atoms H=1 Cl = 1 Chemical Formula HCl Methane C=1 H=4 CH4 Hydrogen peroxide H=2 O=2 H2 O2 Carbon dioxide C=1 O=2 CO2 Problem Solving Problem: A molecule of ammonia consists of one atom of nitrogen (N) and three atoms of hydrogen (H). What is its chemical formula? Solution: The chemical formula is NH3 . You Try It! Problem: A molecule of nitrogen dioxide consists of one atom of nitrogen (N) and two atoms of oxygen (O). What is its chemical formula? "
introduction to chemical bonds,T_4020,"The same elements may combine in different ratios. If they do, they form different compounds. Figure 7.2 shows some examples. Both water (H2 O) and hydrogen peroxide (H2 O2 ) consist of hydrogen and oxygen. However, they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If youve ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water! Both carbon dioxide (CO2 ) and carbon monoxide (CO) consist of carbon and oxygen, but in different ratios. How do their properties differ? "
introduction to chemical bonds,T_4021,"There are different types of compounds. They differ in the nature of the bonds that hold their atoms together. The type of bonds in a compound determines many of its properties. Three types of bonds are ionic, covalent, and metallic bonds. You will read about these three types in later lessons. You can also learn more about them by watching this video:  (7:18). MEDIA Click image to the left or use the URL below. URL: "
introduction to chemical bonds,T_4022,"Chocolate: Its been revered for millennia by cultures throughout the world. But while its easy to appreciate all of its delicious forms, creating this confection is a complex culinary feat. Local chocolate makers explain the elaborate engineering and chemistry behind this tasty treat. And learn why its actually good for your health! For more information on the science of chocolate, see http://science.kqed.org/quest/video/the-sweet-science-of-chocolate/ . MEDIA Click image to the left or use the URL below. URL: "
ionic bonds,T_4023,"An ionic bond is the force of attraction that holds together positive and negative ions. It forms when atoms of a metallic element give up electrons to atoms of a nonmetallic element. Figure 7.3 shows how this happens. In row 1 of Figure 7.3, an atom of sodium donates an electron to an atom of chlorine (Cl). By losing an electron, the sodium atom becomes a sodium ion. It now has one less electron than protons, giving it a charge of +1. Positive ions such as sodium are given the same name as the element. The chemical symbol has a plus sign to distinguish the ion from an atom of the element. The symbol for a sodium ion is Na+ . By gaining an electron, the chlorine atom becomes a chloride ion. It now has one more electron than protons, giving it a charge of -1. Negative ions are named by adding the suffix ide to the first part of the element name. The symbol for chloride is Cl . Sodium and chloride ions have equal but opposite charges. Opposites attract, so sodium and chloride ions attract each other. They cling together in a strong ionic bond. You can see this in row 2 of Figure 7.3. Brackets separate the ions in the diagram to show that the ions in the compound do not share electrons. You can see animations of sodium chloride forming at these URLs: http://web.jjay.cuny.edu/~acarpi/NSC/salt.htm "
ionic bonds,T_4024,"Ionic bonds form only between metals and nonmetals. Metals ""want"" to give up electrons, and nonmetals ""want"" to gain electrons. Find sodium (Na) in Figure 7.4. Sodium is an alkali metal in group 1. Like other group 1 elements, it has just one valence electron. If sodium loses that one electron, it will have a full outer energy level. Now find fluorine (F) in Figure 7.4. Fluorine is a halogen in group 17. It has seven valence electrons. If fluorine gains one electron, it will have a full outer energy level. After sodium gives up its valence electron to fluorine, both atoms have a more stable arrangement of electrons. "
ionic bonds,T_4025,"It takes energy to remove valence electrons from an atom. The force of attraction between the negative electrons and positive nucleus must be overcome. The amount of energy needed depends on the element. Less energy is needed to remove just one or a few electrons than many. This explains why sodium and other alkali metals form positive ions so easily. Less energy is also needed to remove electrons from larger atoms in the same group. For example, in group 1, it takes less energy to remove an electron from francium (Fr) at the bottom of the group than from lithium (Li) at the top of the group (see Figure 7.4). In bigger atoms, valence electrons are farther from the nucleus. As a result, the force of attraction between the electrons and nucleus is weaker. What happens when an atom gains an electron and becomes a negative ion? Energy is released. Halogens release the most energy when they form ions. As a result, they are very reactive. "
ionic bonds,T_4026,"Ionic compounds contain ions of metals and nonmetals held together by ionic bonds. Ionic compounds do not form molecules. Instead, many positive and negative ions bond together to form a structure called a crystal. You can see an example of a crystal in Figure 7.5. It shows the ionic compound sodium chloride. Positive sodium ions (Na+ ) alternate with negative chloride ions (Cl ). The oppositely charged ions are strongly attracted to each other. Helpful Hints Naming Ionic Compounds Ionic compounds are named for their positive and negative ions. The name of the positive "
ionic bonds,T_4027,"The crystal structure of ionic compounds is strong and rigid. It takes a lot of energy to break all those strong ionic bonds. As a result, ionic compounds are solids with high melting and boiling points (see Table 7.2). The rigid crystals are brittle and more likely to break than bend when struck. As a result, ionic crystals tend to shatter. You can learn more about the properties of ionic compounds by watching the video at this URL:  MEDIA Click image to the left or use the URL below. URL:  Compare the melting and boiling points of these ionic compounds with those of water (0C and 100C), which is not an ionic compound. Ionic Compound Sodium chloride (NaCl) Calcium chloride (CaCl2 ) Barium oxide (BaO) Iron bromide (FeBr3 ) Melting Point (C) 801 772 1923 684 Boiling Point (C) 1413 1935 2000 934 Solid ionic compounds are poor conductors of electricity. The strong bonds between ions lock them into place in the crystal. However, in the liquid state, ionic compounds are good conductors of electricity. Most ionic compounds dissolve easily in water. When they dissolve, they separate into individual ions. The ions can move freely, so they are good conductors of electricity. Dissolved ionic compounds are called electrolytes. "
ionic bonds,T_4028,Ionic compounds have many uses. Some are shown in Figure 7.6. Many ionic compounds are used in industry. The human body also needs several ions for good health. Having low levels of the ions can endanger important functions such as heartbeat. Solutions of ionic compounds can be used to restore the ions. 
covalent bonds,T_4029,"A covalent bond is the force of attraction that holds together two atoms that share a pair of electrons. The shared electrons are attracted to the nuclei of both atoms. Covalent bonds form only between atoms of nonmetals. The two atoms may be the same or different elements. If the bonds form between atoms of different elements, a covalent compound forms. Covalent compounds are described in detail later in the lesson. To see a video about covalent bonding, go to this URL:  (6:20). MEDIA Click image to the left or use the URL below. URL:  Figure 7.7 shows an example of a covalent bond forming between two atoms of the same element, in this case two atoms of hydrogen. The two atoms share a pair of electrons. Hydrogen normally occurs in two-atom, or diatomic, molecules like this (di- means ""two""). Several other elements also normally occur as diatomic molecules: nitrogen, oxygen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). "
covalent bonds,T_4030,"Covalent bonds form because they give atoms a more stable arrangement of electrons. Look at the hydrogen atoms in Figure 7.7. Alone, each hydrogen atom has just one electron. By sharing electrons with another hydrogen atom, it has two electrons: its own and the one in the other hydrogen atom. The shared electrons are attracted to both hydrogen nuclei. This force of attraction holds the two atoms together as a molecule of hydrogen. Some atoms need to share more than one pair of electrons to have a full outer energy level. For example, an oxygen atom has six valence electrons. It needs two more electrons to fill its outer energy level. Therefore, it must form two covalent bonds. This can happen in many different ways. One way is shown in Figure 7.8. The oxygen atom in the figure has covalent bonds with two hydrogen atoms. This forms the covalent compound water. "
covalent bonds,T_4031,"In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar bonds. Figure 7.9 shows this for water. The oxygen atom attracts the shared electrons more strongly because its nucleus has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge. The hydrogen atoms attract the electrons less strongly. They become slightly positive in charge. For another example of polar bonds, see the video at this URL:  (0:52). MEDIA Click image to the left or use the URL below. URL:  In other covalent bonds, electrons are shared equally. These bonds are called nonpolar bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral. Figure 7.10 shows an example of nonpolar bonds. "
covalent bonds,T_4032,"Covalent bonds between atoms of different elements form covalent compounds. The smallest, simplest covalent compounds have molecules with just two atoms. An example is hydrogen chloride (HCl). It consists of one hydrogen atom and one chlorine atom. The largest, most complex covalent molecules have thousands of atoms. Examples include proteins and carbohydrates. These are compounds in living things. Helpful Hints Naming Covalent Compounds Follow these rules in naming simple covalent compounds: The element closer to the left of the periodic table is named first. The second element gets the suffix ide. Prefixes such as di- (2) and tri- (3) show the number of each atom in the compound. These are written with subscripts in the chemical formula. Example: The gas that consists of one carbon atom and two oxygen atoms is named carbon dioxide. Its chemical formula is CO2 . You Try It! Problem: What is the name of the compound that contains three oxygen atoms and two nitrogen atoms? What is its chemical formula? "
covalent bonds,T_4033,"Covalent compounds have different properties than ionic compounds because of their bonds. Covalent compounds exist as individual molecules rather than crystals. It takes less energy for individual molecules than ions in a crystal to pull apart. As a result, covalent compounds have lower melting and boiling points than ionic compounds. Many are gases or liquids at room temperature. Covalent compounds have shared electrons. These are not free to move like the transferred electrons of ionic compounds. This makes covalent compounds poor conductors of electricity. Many covalent compounds also do not dissolve in water as all ionic compounds do. "
covalent bonds,T_4034,"Having polar bonds may make a covalent compound polar. A polar compound is one in which there is a slight difference in charge between opposite ends of the molecule. All polar compounds contain polar bonds. But having polar bonds does not necessarily result in a polar compound. It depends on how the atoms are arranged. This is illustrated in Figure 7.11. Both molecules in the figure contain polar bonds, but only formaldehyde is a polar compound. Why is carbon dioxide nonpolar? The molecules of polar compounds are attracted to each other. You can see this in Figure 7.12 for water. A bond forms between the positive hydrogen end of one water molecule and the negative oxygen end of another water molecule. This type of bond is called a hydrogen bond. Hydrogen bonds are weak, but they still must be overcome when a polar substance changes from a solid to a liquid or from a liquid to a gas. As a result, polar covalent compounds may have higher melting and boiling points than nonpolar covalent compounds. To learn more about hydrogen bonding and when it occurs, see the video at this URL:  (0:58). MEDIA Click image to the left or use the URL below. URL: "
metallic bonds,T_4035,"A metallic bond is the force of attraction between a positive metal ion and the valence electrons it shares with other ions of the metal. The positive ions form a lattice-like structure. You can see an example in Figure 7.13. (For an animated version, go to the URL below.) The ions are held together in the lattice by bonds with the valence electrons around them. These valence electrons include their own and those of other ions. Why do metallic bonds form? Recall that metals ""want"" to give up their valence electrons. This means that their valence electrons move freely. The electrons form a ""sea"" of negative charge surrounding the positive ions. MEDIA Click image to the left or use the URL below. URL: "
metallic bonds,T_4036,"Because of their freely moving electrons, metals are good conductors of electricity. Metals also can be shaped without breaking. They are ductile (can be shaped into wires) and malleable (can be shaped into thin sheets). Metals have these properties because of the nature of their metallic bonds. A metallic lattice, like the one in Figure 7.13, may resemble a rigid ionic crystal. However, it is much more flexible. Look at Figure 7.14. It shows a blacksmith hammering a piece of red-hot iron in order to shape it. Why doesnt the iron shatter, as an ionic crystal would? The ions of the metal can move within the ""sea"" of electrons without breaking the metallic bonds that hold them together. The ions can shift closer together or farther apart. In this way, the metal can change shape without breaking. You can learn more about metallic bonds and the properties of metals at this URL:  (6:12). MEDIA Click image to the left or use the URL below. URL: "
metallic bonds,T_4037,"Metals are useful for many purposes because of their unique properties. However, pure metals may be less useful than mixtures of metals. For example, iron is not as strong as steel, which is a mixture of iron and small amounts of carbon. Steel is so strong that it can hold up huge bridges, like the one Figure 7.15. Steel is also used to make skyscrapers, cargo ships, cars, and trains. Steel is an example of an alloy. An alloy is a mixture of a metal with one or more other elements. The other elements may be metals, nonmetals, or both. An alloy is a solid solution. It is formed by melting a metal and dissolving the other elements in it. The molten solution is then allowed to cool and harden. Several other examples of alloys and their uses are shown in Figure 7.16. You can learn about an amazing alloy called memory wire at the URL below. If you have braces on your teeth, you may even have this alloy in your mouth! "
introduction to chemical reactions,T_4038,"A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. A chemical reaction can be represented by this general equation: Reactants ! Products The arrow (!) shows the direction in which the reaction occurs. The reaction may occur quickly or slowly. For example, foam shoots out of a fire extinguisher as soon as the lever is pressed. But it might take years for metal to rust. "
introduction to chemical reactions,T_4039,"In chemical reactions, bonds break in the reactants and new bonds form in the products. The reactants and prod- ucts contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. Look at the example in Figure 8.2. It shows how water forms. Bonds break in molecules of hydrogen and oxygen. Then new bonds form in molecules of water. In both reactants and products, there are four hydrogen atoms and two oxygen atoms. But the atoms are combined differently in water. You can see another example at this URL: http://w "
introduction to chemical reactions,T_4040,"The arrow in Figure 8.2 shows that the reaction goes from left to right, from hydrogen and oxygen to water. The reaction can also go in the reverse direction. If an electric current passes through water, water molecules break down into molecules of hydrogen and oxygen. This reaction would be represented by a right-to-left arrow ( ) in Figure Many other reactions can also go in both forward and reverse directions. Often, a point is reached at which the forward and reverse reactions occur at the same rate. When this happens, there is no overall change in the amount of reactants and products. This point is called equilibrium, which refers to a balance between any opposing changes. You can see an animation of a chemical reaction reaching equilibrium at this URL: "
introduction to chemical reactions,T_4041,"Not all changes in matter involve chemical reactions. For example, there are no chemical reactions involved in changes of state. When liquid water freezes or evaporates, it is still water. No bonds are broken and no new products are formed. How can you tell whether a change in matter involves a chemical reaction? Often, there is evidence. Four common signs that a chemical reaction has occurred are: Change in color: the products are a different color than the reactants. Change in temperature: heat is released or absorbed during the reaction. Production of a gas: gas bubbles are released during the reaction. Production of a solid: a solid settles out of a liquid solution. The solid is called a precipitate. You can see examples of each type of evidence in Figure 8.3 and at this URL:  MEDIA Click image to the left or use the URL below. URL: "
chemical equations,T_4042,"A chemical equation is a symbolic representation of a chemical reaction. It is a shorthand way of showing how atoms are rearranged in the reaction. The general form of a chemical equation was introduced in this chapters lesson ""Introduction to Chemical Reactions."" It is: Reactants ! Products Consider the simple example in Figure 8.4. When carbon (C) reacts with oxygen (O2 ), it produces carbon dioxide (CO2 ). The chemical equation for this reaction is: C + O2 ! CO2 The reactants are one atom of carbon and one molecule of oxygen. When there is more than one reactant, they are separated by plus signs (+). The product is one molecule of carbon dioxide. If more than one product were produced, plus signs would be used between them as well. "
chemical equations,T_4043,"Some chemical equations are more challenging to write. Consider the reaction in which hydrogen (H2 ) and oxygen (O2 ) combine to form water (H2 O). Hydrogen and oxygen are the reactants, and water is the product. To write a chemical equation for this reaction, you would start by writing symbols for the reactants and products: Equation 1: H2 + O2 ! H2 O Like equations in math, equations in chemistry must balance. There must be the same number of each type of atom in the products as there is in the reactants. In equation 1, count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced. "
chemical equations,T_4044,"Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2H2 . A coefficient of 1 usually isnt written. Coefficients can be used to balance equation 1 (above) as follows: Equation 2: 2H2 + O2 ! 2H2 O Equation 2 shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms. There are now four hydrogen atoms in both reactants and products. Is equation 2 balanced? Count the oxygen atoms to find out. "
chemical equations,T_4045,"Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps: 1. Count the number of each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step 2. 2. Add coefficients to increase the number of atoms or molecules of reactants or products. Use the smallest coefficients possible. 3. Repeat steps 1 and 2 until the equation is balanced. Helpful Hint When you balance chemical equations, never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients. Work through the Problem Solving examples below. Then do the You Try It! problems to check your understand- ing. If you need more help, go to this URL:  (14:28). MEDIA Click image to the left or use the URL below. URL:  Problem Solving Problem: Balance this chemical equation: N2 + H2 ! NH3 Hints for balancing 1. Two N are needed in the products to match the two N (N2 ) in the reactants. Add the coefficient 2 in front of NH3 . Now N is balanced. 2. Six H are now needed in the reactants to match the six H in the products. Add the coefficient 3 in front of H2 . Now H is balanced. Solution: N2 + 3H2 ! 2NH3 Problem: Balance this chemical equation: CH4 + O2 ! CO2 + H2 O Solution: CH4 + 2O2 ! CO2 + 2H2 O You Try It! Problem: Balance these chemical equations: Zn + HCl ! ZnCl2 + H2 Cu + O2 ! CuO "
chemical equations,T_4046,"Why must chemical equations be balanced? Its the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants. Balanced chemical equations show that mass is conserved in chemical reactions. How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. For this and other contributions, Lavoisier has been called the father of modern chemistry. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar, like the one in Figure 8.5. As a result, any gases involved in the reactions were captured and could be measured. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions. Another outcome of Lavoisiers research was his discovery of oxygen. You can learn more about Lavoisier and his important research at: "
types of chemical reactions,T_4047,"A synthesis reaction occurs when two or more reactants combine to form a single product. A synthesis reaction can be represented by the general equation: A+B !C In this general equation (and others like it in this lesson), the letters A, B,C, and so on represent atoms or ions of elements. The arrow shows the direction of the reaction. The letters on the left side of the arrow are the reactants that begin the chemical reaction. The letters on the right side of the arrow are the product of the reaction. Two examples of synthesis reactions are described below. You can see more examples at this URL: "
types of chemical reactions,T_4048,"An example of a synthesis reaction is the combination of sodium (Na) and chlorine (Cl) to produce sodium chloride (NaCl). This reaction is represented by the chemical equation: 2Na + Cl2 ! 2NaCl Sodium is a highly reactive metal, and chlorine is a poisonous gas (see Figure 8.6). The compound they synthesize has very different properties. It is table salt, which is neither reactive nor poisonous. In fact, salt is a necessary component of the human diet. "
types of chemical reactions,T_4049,"Another example of a synthesis reaction is illustrated in Figure 8.7. The brown haze in the air over the city of Los Angeles is smog. A major component of smog is nitrogen dioxide (NO2 ). It forms when nitric oxide (NO), from sources such as car exhaust, combines with oxygen (O2 ) in the air. The equation for this reaction is: 2NO + O2 ! 2NO2 Nitrogen dioxide is a toxic gas with a sharp odor. It can irritate the eyes and throat and trigger asthma attacks. It is a major air pollutant. "
types of chemical reactions,T_4050,"A decomposition reaction is the reverse of a synthesis reaction. In a decomposition reaction, one reactant breaks down into two or more products. This can be represented by the general equation: AB ! A + B Two examples of decomposition reactions are described below. You can see other examples at this URL: http://w "
types of chemical reactions,T_4051,"An example of a decomposition reaction is the breakdown of carbonic acid (H2 CO3 ) to produce water (H2 O) and carbon dioxide (CO2 ). The equation for this reaction is: H2 CO3 ! H2 O + CO2 Carbonic acid is synthesized in the reverse reaction. It forms when carbon dioxide dissolves in water. For example, some of the carbon dioxide in the atmosphere dissolves in the ocean and forms carbonic acid. The amount of carbon dioxide in the atmosphere has increased over recent decades (see Figure 8.8). As a result, the acidity of ocean water is also increasing. How do you think this might affect ocean life? "
types of chemical reactions,T_4052,"Another example of a decomposition reaction is illustrated in Figure 8.9. Water (H2 O) decomposes to hydrogen (H2 ) and oxygen (O2 ) when an electric current passes through it. This reaction is represented by the equation: 2H2 O ! 2H2 + O2 What is the reverse of this decomposition reaction? (Hint: How is water synthesized? You can look at this chapters ""Introduction to Chemical Reactions"" lesson to find out.) "
types of chemical reactions,T_4053,Replacement reactions involve ions. They occur when ions switch places in compounds. There are two types of replacement reactions: single and double. Both types are described below. 
types of chemical reactions,T_4054,"A single replacement reaction occurs when one ion takes the place of another in a single compound. This type of reaction has the general equation: A + BC ! B + AC Do you see how A has replaced B in the compound? The compound BC has become the compound AC. An example of a single replacement reaction occurs when potassium (K) reacts with water (H2 O). A colorless solid called potassium hydroxide (KOH) forms, and hydrogen gas (H2 ) is released. The equation for the reaction is: 2K + 2H2 O ! 2KOH + H2 Potassium is a highly reactive group 1 alkali metal, so its reaction with water is explosive. You can actually watch this reaction occurring at: http://commons.wikimedia.org/wiki/File:Potassium_water_20.theora.ogv . "
types of chemical reactions,T_4055,A double replacement reaction occurs when two compounds exchange ions. This produces two new compounds. A double replacement reaction can be represented by the general equation: AB +CD ! AD +CB Do you see how B and D have changed places? Both reactant compounds have changed. An example of a double replacement reaction is sodium chloride (NaCl) reacting with silver fluoride (AgF). This reaction is represented by the equation: NaCl + AgF ! NaF + AgCl Cl and F have changed places. Can you name the products of this reaction? 
types of chemical reactions,T_4056,A combustion reaction occurs when a substance reacts quickly with oxygen (O2 ). You can see an example of a combustion reaction in Figure 8.10. Combustion is commonly called burning. The substance that burns is usually referred to as fuel. The products of a combustion reaction include carbon dioxide (CO2 ) and water (H2 O). The reaction typically gives off heat and light as well. The general equation for a combustion reaction can be represented by: Fuel + O2 ! CO2 + H2 O 
types of chemical reactions,T_4057,"The fuel that burns in a combustion reaction is often a substance called a hydrocarbon. A hydrocarbon is a compound that contains only carbon (C) and hydrogen (H). Fossil fuels, such as natural gas, consist of hydrocarbons. Natural gas is a fuel that is commonly used in home furnaces and gas stoves (see Figure 8.11). The main component of natural gas is the hydrocarbon called methane (CH4 ). The combustion of methane is represented by the equation: CH4 + 2O2 ! CO2 + 2H2 O "
types of chemical reactions,T_4058,"Your own body cells burn fuel in combustion reactions. The fuel is glucose (C6 H12 O6 ), a simple sugar. The process in which combustion of glucose occurs in body cells is called cellular respiration. This combustion reaction provides energy for life processes. Cellular respiration can be summed up by the equation: C6 H12 O6 + 6O2 ! 6CO2 + 6H2 O Where does glucose come from? It is produced by plants during photosynthesis. In this process, carbon dioxide and water combine to form glucose. Which type of chemical reaction is photosynthesis? "
chemical reactions and energy,T_4059,"In an endothermic reaction, it takes more energy to break bonds in the reactants than is released when new bonds form in the products. The word ""endothermic"" literally means ""taking in heat."" A constant input of energy, often in the form of heat, is needed in an endothermic reaction. Not enough energy is released when products form to break more bonds in the reactants. Additional energy is needed to keep the reaction going. The general equation for an endothermic reaction is: Reactants + Energy ! Products In many endothermic reactions, heat is absorbed from the surroundings. As a result, the temperature drops. The drop in temperature may be great enough to cause liquid products to freeze. Thats what happens in the endothermic reaction at this URL:  One of the most important endothermic reactions is photosynthesis. In this reaction, plants synthesize glucose (C6 H12 O6 ) from carbon dioxide (CO2 ) and water (H2 O). They also release oxygen (O2 ). The energy for photo- synthesis comes from light (see Figure 8.12). Without light energy, photosynthesis cannot occur. The chemical equation for photosynthesis is: 6CO2 + 6H2 O ! C6 H12 O6 + 6O2 "
chemical reactions and energy,T_4060,"In an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products. The word ""exothermic"" literally means ""turning out heat."" Energy, often in the form of heat, is released as an exothermic reaction occurs. The general equation for an exothermic reaction is: Reactants ! Products + Energy If the energy is released as heat, an exothermic reaction results in a rise in temperature. Thats what happens in the exothermic reaction at the URL below. Combustion reactions are examples of exothermic reactions. When substances burn, they usually give off energy as heat and light. Look at the big bonfire in Figure 8.13. You can see the light energy it is giving off. If you were standing near the fire, you would also feel its heat. "
chemical reactions and energy,T_4061,"Whether a reaction absorbs energy or releases energy, there is no overall change in the amount of energy. Energy cannot be created or destroyed. This is the law of conservation of energy. Energy can change form for example, from electricity to light but the same amount of energy always remains. If energy cannot be destroyed, what happens to the energy that is absorbed in an endothermic reaction? The energy is stored in the chemical bonds of the products. This form of energy is called chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. The excess energy in the reactants is released to the surroundings when the reaction occurs. The graphs in Figure 8.14 show the chemical energy of reactants and products in each type of reaction. "
chemical reactions and energy,T_4062,"All chemical reactions, even exothermic reactions, need a certain amount of energy to get started. This energy is called activation energy. For example, activation energy is needed to start a car. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas wont occur without the spark of energy to begin the reaction. Why is activation energy needed? A reaction wont occur unless atoms or molecules of reactants come together. This happens only if the particles are moving, and movement takes energy. Often, reactants have to overcome forces that push them apart. This takes energy as well. Still more energy is needed to start breaking bonds in reactants. The graphs in Figure 8.15 show the changes in energy in endothermic and exothermic reactions. Both reactions need the same amount of activation energy in order to begin. You have probably used activation energy to start a chemical reaction. For example, if youve ever used a match to light a campfire, then you provided the activation energy needed to start a combustion reaction. Combustion is exothermic. Once a fire starts to burn, it releases enough energy to activate the next reaction, and the next, and so on. However, wood will not burst into flames on its own. "
chemical reactions and energy,T_4063,"Any factor that helps reactants come together so they can react lowers the amount of activation energy needed to start the reaction. If the activation energy is lowered, more reactant particles can react, and the reaction occurs more quickly. How fast a reaction occurs is called the reaction rate. Factors that affect the reaction rate include: temperature of reactants concentration of reactants surface area of reactants presence of catalysts "
chemical reactions and energy,T_4064,"When the temperature of reactants is higher, the rate of the reaction is faster. At higher temperatures, particles of reactants have more energy, so they move faster. They are more likely to bump into one another and to collide with greater force. For example, when you fry an egg, turning up the heat causes the egg to cook faster. The same principle explains why storing food in a cold refrigerator reduces the rate at which food spoils (see Figure 8.16). Both food frying and food spoiling are chemical reactions that happen faster at higher temperatures. "
chemical reactions and energy,T_4065,"Concentration is the number of particles of a substance in a given volume. When the concentration of reactants is higher, the reaction rate is faster. At higher concentrations, particles of reactants are crowded closer together, so they are more likely to collide and react. Did you ever see a sign like the one in Figure 8.17? You might see it where someone is using a tank of pure oxygen for a breathing problem. The greater concentration of oxygen in the air makes combustion rapid if a fire starts burning. "
chemical reactions and energy,T_4066,"When a solid substance is involved in a chemical reaction, only the matter at the surface of the solid is exposed to other reactants. If a solid has more surface area, more of it is exposed and able to react. Therefore, increasing the surface area of solid reactants increases the reaction rate. For example, crushing a solid into a powder exposes more of the substance to other reactants. This may greatly speed up the reaction. You can see another example in Figure 8.18. Iron rusts when it combines with oxygen in the air. The iron hammer head and iron nails will both rust eventually. Which will rust faster? "
chemical reactions and energy,T_4067,"Some reactions need extra help to occur quickly. They need another substance, called a catalyst. A catalyst is a substance that increases the rate of a chemical reaction but is not changed or used up in the reaction. The catalyst can go on to catalyze many more reactions. Catalysts are not reactants, but they help reactants come together so they can react. You can see one way this happens in the animation at the URL below. By helping reactants come together, a catalyst decreases the activation energy needed to start a chemical reaction. This speeds up the reaction. Living things depend on catalysts to speed up many chemical reactions inside their cells. Catalysts in living things are called enzymes. Enzymes may be extremely effective. A reaction that takes a split second to occur with an enzyme might take billions of years without it! "
properties of carbon,T_4068,"Carbon is a nonmetal in group 14 of the periodic table. Like other group 14 compounds, carbon has four valence electrons. Valence electrons are the electrons in the outer energy level of an atom that are involved in chemical bonds. The valence electrons of carbon are shown in Figure 9.1. "
properties of carbon,T_4069,"Because it has four valence electrons, carbon needs four more electrons to fill its outer energy level. It can achieve this by forming four covalent bonds. Covalent bonds are chemical bonds that form between nonmetals. In a covalent bond, two atoms share a pair of electrons. By forming four covalent bonds, carbon shares four pairs of electrons, thus filling its outer energy level. A carbon atom can form bonds with other carbon atoms or with the atoms of other elements. Carbon often forms bonds with hydrogen. You can see an example in Figure 9.2. The compound represented in the figure is methane (CH4 ). The carbon atom in a methane molecule forms bonds with four hydrogen atoms. The diagram on the left shows all the shared electrons. The diagram on the right represents each pair of shared electrons with a dash (). This type of diagram is called a structural formula. "
properties of carbon,T_4070,"Carbon can form single, double, or even triple bonds with other carbon atoms. In a single bond, two carbon atoms share one pair of electrons. In a double bond, they share two pairs of electrons, and in a triple bond they share three pairs of electrons. Examples of compounds with these types of bonds are shown in Figure 9.3. "
properties of carbon,T_4071,"Because of carbons ability to form so many covalent bonds, it often forms polymers. A polymer is a large molecule that consists of many smaller molecules joined together by covalent bonds. The smaller molecules are called monomers. (The prefix mono means ""one,"" and the prefix poly means ""many."") Polymers may consist of just one type of monomer or of more than one type. Polymers are a little like the strings of beads in Figure 9.4. What do the individual beads represent? Many polymers occur naturally. You will read about natural polymers in this chapters ""Hydrocarbons"" and ""Carbon and Living Things"" lessons. Other polymers are synthetic. This means that they are produced in labs or factories. Synthetic polymers are created in synthesis reactions in which monomers bond together to form much larger compounds. Plastics are examples of synthetic polymers. The plastic items in Figure 9.5 are all made of polythene (also called polyethylene). It consists of repeating monomers of ethene (C2 H4 ). To learn more about polymers and how they form, go to this URL:  (2:13). "
properties of carbon,T_4072,"Exploratorium Staff Scientist Julie Yu changes and manipulates the physical and chemical properties of plastic bottles by exposing them to heat. This is how plastic bags and bottles can be recycled and used over and over again. For more information on properties of plastic, see http://science.kqed.org/quest/video/quest-lab-properties-of-plas MEDIA Click image to the left or use the URL below. URL: "
properties of carbon,T_4073,"Pure carbon can exist in different forms, depending on how its atoms are arranged. The forms include diamond, graphite, and fullerenes. All three forms exist as crystals, but they have different structures. Their different structures, in turn, give them different properties. You can learn more about them in Table 9.1. atoms affect the properties of the substances formed? Structure Diamond crystal Description Diamond Diamond is a form of carbon in which each carbon atom is bonded to four other carbon atoms. This forms a strong, rigid, three- dimensional structure. Diamond is the hardest natural substance. It is used for cutting and grinding tools as well as for rings and other pieces of jewelry. Graphite Graphite is a form of carbon in which carbon atoms are arranged in layers. Bonds are strong between carbon atoms within each layer but relatively weak between atoms in different layers. The weak bonds between layers allow the layers to slide over one another. This makes graphite relatively soft and slippery. It is used as a lubricant. It also makes up the ""lead"" in pencils. Fullerene A fullerene (also called a bucky- ball) is a form of carbon in which carbon atoms are arranged in hol- low spheres. Each carbon atom is bonded to three others by sin- gle covalent bonds. The pattern of atoms resembles the pattern on the surface of a soccer ball. Fullerenes were first discovered in 1985. They have been found in soot and me- teorites. Possible commercial uses of fullerenes are under investiga- tion. To learn how this form of carbon got its funny names, go to this URL:  This metal cutter has a diamond blade. "
hydrocarbons,T_4074,"Hydrocarbons are compounds that contain only carbon and hydrogen. Hydrocarbons are the simplest type of carbon-based compounds. Nonetheless, they can vary greatly in size. The smallest hydrocarbons have just one or two carbon atoms, but large hydrocarbons may have hundreds. The size of hydrocarbon molecules influences their properties. For example, it influences their boiling and melting points. As a result, some hydrocarbons are gases at room temperature, while others are liquids or solids. Hydrocarbons are generally nonpolar and do not dissolve in water. In fact, they tend to repel water. Thats why they are used in floor wax and similar products. Hydrocarbons can be classified in two basic classes. The classes are saturated hydrocarbons and unsaturated hydrocarbons. This classification is based on the number of bonds between carbon atoms. You can learn more about both types of hydrocarbons at this URL:  (6:41). MEDIA Click image to the left or use the URL below. URL: "
hydrocarbons,T_4075,"Saturated hydrocarbons contain only single bonds between carbon atoms. They are the simplest hydrocarbons. They are called saturated because each carbon atom is bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogen. You can see an example of a saturated hydrocarbon in Figure Saturated hydrocarbons are given the general name of alkanes. The name of specific alkanes always ends in -ane. The first part of the name indicates how many carbon atoms each molecule of the alkane has. The smallest alkane is methane. It has just one carbon atom. The next largest is ethane, with two carbon atoms. The chemical formulas and properties of methane, ethane, and several other alkanes are listed in Table 9.2. The boiling and melting points of alkanes are determined mainly by the number of carbon atoms they have. Alkanes with more carbon atoms generally have higher boiling and melting points. This table shows only alkanes with relatively few carbon atoms. Some alkanes have many more carbon atoms. What properties might larger alkanes have? For example, do you think that any of them might be solids? Alkane Methane Ethane Propane Butane Pentane Hexane Heptane Octane Chemical Formula CH4 C2 H6 C3 H8 C4 H10 C5 H12 C6 H14 C7 H16 C8 H18 Boiling Point (C) -162 -89 -42 0 36 69 98 126 Melting Point (C) -183 -172 -188 -138 -130 -95 -91 -57 State (at 20C) gas gas gas gas liquid liquid liquid liquid "
hydrocarbons,T_4076,"Structural formulas are often used to represent hydrocarbon compounds because the molecules can have different shapes, or structures. Hydrocarbons may form straight chains, branched chains, or rings. Figure 9.8 shows an example of an alkane with each shape. In straight-chain molecules, all the carbon atoms are lined up in a row like cars of a train. They form what is called the backbone of the molecule. In branched-chain molecules, at least one of the carbon atoms branches off to the side from the backbone. In cyclic molecules, the chain of carbon atoms is joined at the two ends to form a ring. "
hydrocarbons,T_4077,"Even compounds with the same number of carbon and hydrogen atoms can have different shapes. These compounds are called isomers. Look at the examples in Figure 9.9. The figure shows the structural formulas of butane and its isomer iso-butane. Both molecules have four carbon atoms and ten hydrogen atoms (C4 H10 ), but the atoms are arranged differently. Butane is a straight-chain molecule. Iso-butane is branched. You can see three-dimensional models of these two isomers at the URLs below. You can rotate the molecule models to get a better idea of their shapes. "
hydrocarbons,T_4078,"Ring-shaped alkanes are called cycloalkanes. They usually contain just five or six carbon atoms because larger rings are not very stable. However, rings can join together to create larger molecules consisting of two or more rings. Compared with the straight- and branched-chain alkanes, cycloalkanes have higher boiling and melting points. "
hydrocarbons,T_4079,"Unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms. As a result, the carbon atoms are unable to bond with as many hydrogen atoms as they would if they were joined only by single bonds. This makes them unsaturated with hydrogen. Unsaturated hydrocarbons are classified on the basis of their bonds as alkenes, alkynes, or aromatic hydrocarbons. "
hydrocarbons,T_4080,"Unsaturated hydrocarbons that contain at least one double bond are called alkenes. The name of a specific alkene always ends in ene, with a prefix indicating the number of carbon atoms. Figure 9.10 shows the structural formula for the smallest alkene. It has just two carbon atoms and is named ethene. Ethene is produced by most fruits and vegetables. It speeds up ripening and also rotting. Figure 9.11 shows the effects of ethene on bananas. Like alkanes, alkenes can have different shapes. They can form straight chains, branched chains, or rings. Alkenes can also form isomers, or compounds with the same atoms but different shapes. Generally, the physical properties of alkenes are similar to those of alkanes. Smaller alkenes, such as ethene, have relatively high boiling and melting points. They are gases at room temperature. Larger alkenes have lower boiling and melting points. They are liquids or waxy solids at room temperature. "
hydrocarbons,T_4081,"Unsaturated hydrocarbons that contain at least one triple bond are called alkynes. The name of specific alkynes always end in yne, with a prefix for the number of carbon atoms. Figure 9.12 shows the smallest alkyne, called ethyne, which has just two carbon atoms. Ethyne is also called acetylene. It is burned in acetylene torches, like the one in Figure 9.13. Acetylene produces so much heat when it burns that it can melt metal. Breaking all those bonds between carbon atoms releases a lot of energy. Alkynes may form straight or branched chains. They rarely occur as cycloalkynes. In fact, alkynes of all shapes are relatively rare, at least in nature. "
hydrocarbons,T_4082,"Unsaturated cyclic hydrocarbons are called aromatic hydrocarbons. Thats because they have a strong aroma, or scent. Their molecules consist of six carbon atoms in a ring shape, connected by alternating single and double bonds. Aromatic hydrocarbons may have a single ring or multiple rings joined together by bonds between their carbon atoms. Benzene is the smallest aromatic hydrocarbon. It has just one ring. You can see its structural formula in Figure 9.14. Benzene has many uses. For example, it is used in air fresheners and mothballs because of its strong scent. You can learn more about benzene and other aromatic hydrocarbons at this URL:  MEDIA Click image to the left or use the URL below. URL: "
hydrocarbons,T_4083,"It is hard to overstate the importance of hydrocarbons to modern life. Hydrocarbons have even been called the driving force of western civilization. You saw some ways they are used in Figure 9.6. Several other ways are illustrated in Figure 9.15. Their most important use is as fuels. Gasoline, natural gas, fuel oil, diesel fuel, jet fuel, coal, kerosene, and propane are just some of the hydrocarbon compounds that are burned for fuel. Hydrocarbons are also used to manufacture many products, including plastics and synthetic fabrics such as polyester. The main source of hydrocarbons is fossil fuels coal, petroleum, and natural gas. Fossil fuels form over hundreds of millions of years when dead organisms are covered with sediments and put under great pressure. Giant ferns in ancient swamps turned into coal deposits. Dead organisms in ancient seas gradually formed deposits of petroleum and natural gas. You can read more about these sources of hydrocarbons in the chapter Introduction to Energy and at the URL below. "
carbon and living things,T_4084,"A biochemical compound is any carbon-based compound found in living things. Like hydrocarbons, all biochemi- cal compounds contain hydrogen as well as carbon. However, biochemical compounds also contain other elements, such as oxygen and nitrogen. Almost all biochemical compounds are polymers. They consist of many, smaller monomer molecules. Biochemical polymers are referred to as macromolecules. The prefix macro means ""large,"" and many biochemical molecules are very large indeed. They may contain thousands of monomer molecules. Biochemical compounds make up the cells and tissues of organisms. They are also involved in life processes, such as making and using food for energy. Given their diversity of functions, its not surprising that there are millions of different biochemical compounds. However, they can be grouped into just four main classes: carbohydrates, proteins, lipids, and nucleic acids. The classes are summarized in Table 9.3 and described in the rest of this lesson. Class Carbohydrates Elements carbon hydrogen oxygen Examples sugars starches cellulose Proteins carbon hydrogen oxygen nitrogen sulfur carbon hydrogen oxygen carbon hydrogen oxygen nitrogen phosphorus enzymes hormones Lipids Nucleic acids Functions provide energy to cells store energy in plants makes up the cell walls of plants speed up biochemical re- actions regulate life processes fats oils store energy in animals store energy in plants DNA RNA stores genetic information in cells helps cells make proteins "
carbon and living things,T_4085,"Carbohydrates are biochemical compounds that include sugars, starches, and cellulose. They contain oxygen in addition to carbon and hydrogen. Organisms use carbohydrates mainly for energy. "
carbon and living things,T_4086,"Sugars are simple carbohydrates. Molecules of sugar have just a few carbon atoms. The simplest sugar is glucose (C6 H12 O6 ). Glucose is the sugar that the cells of living things use for energy. Plants and some other organisms make glucose in the process of photosynthesis. Living things that cannot make glucose obtain it by consuming plants or these other organisms. You can see the structural formula of glucose and two other sugars in Figure 9.16. The other sugars in the figure are fructose and sucrose. Fructose is an isomer of glucose. It is found in fruits. It has the same atoms as glucose, but they are arranged differently. Sucrose is table sugar. It consists of one molecule of glucose and one molecule of fructose. "
carbon and living things,T_4087,"Starches are complex carbohydrates. They are polymers of glucose. They consist of hundreds of glucose monomers bonded together. Plants make starch to store extra sugars. Consumers get starch from plants. Common sources of starch in the human diet are pictured in Figure 9.17. Our digestive system breaks down starch to simple sugars, which our cells use for energy. "
carbon and living things,T_4088,"Cellulose is another complex carbohydrate that is a polymer of glucose. However, the glucose molecules are bonded together differently in cellulose than they are in starches. Cellulose molecules bundle together to form long, tough fibers (see Figure 9.18). Have you ever eaten raw celery? If you have, then you probably noticed that the stalks contain long, stringy fibers. The fibers are mostly cellulose. Cellulose is the most abundant biochemical compound. It makes up the cell walls of plants and gives support to trunks and stems. Cellulose also provides needed fiber in the human diet. We cant digest cellulose, but it helps keep food wastes moving through the digestive tract. "
carbon and living things,T_4089,"Proteins are biochemical compounds that contain oxygen, nitrogen, and sulfur in addition to carbon and hydrogen. Protein molecules consist of one or more chains of small molecules called amino acids. "
carbon and living things,T_4090,"Amino acids are the ""building blocks"" of proteins. There are 20 different common amino acids. The structural formula of the simplest amino acid, called glycine, is shown in Figure 9.19. Other amino acids have a similar structure. The sequence of amino acids and the number of amino acid chains in a protein determine the proteins shape. The shape of a protein, in turn, determines its function. Shapes may be very complex. You can learn more about the structure of proteins at the URL below. MEDIA Click image to the left or use the URL below. URL: "
carbon and living things,T_4091,"Proteins are the most common biochemicals. They have many different functions, including: making up tissues as components of muscle. speeding up biochemical reactions as enzymes. regulating life processes as hormones. helping defend against infections as antibodies. transporting materials as components of the blood (see the example in Figure 9.20). "
carbon and living things,T_4092,"Lipids are biochemical compounds such as fats and oils. Organisms use lipids to store energy. In addition to carbon and hydrogen, lipids contain oxygen. "
carbon and living things,T_4093,"Lipids are made up of long carbon chains called fatty acids. Like hydrocarbons, fatty acids may be saturated or unsaturated. Figure 9.21 shows structural formulas for two small fatty acids. One is saturated and one is unsaturated. In saturated fatty acids, there are only single bonds between carbon atoms. As a result, the carbons are saturated with hydrogen atoms. Saturated fatty acids are found in fats. Fats are solid lipids that animals use to store energy. In unsaturated fatty acids, there is at least one double bond between carbon atoms. As a result, some carbons are not bonded to as many hydrogen atoms as possible. Unsaturated fatty acids are found in oils. Oils are liquid lipids that plants use to store energy. "
carbon and living things,T_4094,"Some lipids contain the element phosphorus as well as oxygen, carbon, and hydrogen. These lipids are called phospholipids. Two layers of phospholipid molecules make up most of the cell membrane in the cells of living things. Figure 9.22 shows how phospholipid molecules are arranged in a cell membrane. One end (the head) of each phospholipid molecule is polar and attracts water. This end is called hydrophilic (""water loving""). The other end (the tail) is nonpolar and repels water. This end is called hydrophobic (""water hating""). The nonpolar tails are on the inside of the membrane. The polar heads are on the outside of the membrane. These differences in polarity allow some molecules to pass through the membrane while keeping others out. You can see how this works in the video at the URL below. "
carbon and living things,T_4095,"Nucleic acids are biochemical molecules that contain oxygen, nitrogen, and phosphorus in addition to carbon and hydrogen. There are two main types of nucleic acids. They are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). "
carbon and living things,T_4096,"Nucleic acids consist of chains of small molecules called nucleotides. The structure of a nucleotide is shown in Figure 9.23. Each nucleotide contains a phosphate group (PO4 ), a sugar (C5 H8 O4 ) in DNA, and a nitrogen- containing base. (A base is a compound that is not neither acidic nor neutral.) There are four different nitrogenous bases in DNA. They are adenine, thymine, guanine, and cytosine. In RNA, the only difference is that thymine is replaced with a different base, uracil. DNA consists of two long chains of nucleotides. Nitrogen bases on the two chains form hydrogen bonds with each other. Adenine always bonds with thymine, and guanine always bonds with cytosine. These bonds hold the two chains together and give DNA is characteristic double helix, or spiral, shape. You can see the shape of the DNA molecule in Figure 9.24. Sugars and phosphate groups form the ""backbone"" of each chain of DNA. The bonded bases are called base pairs. RNA, in contrast to DNA, consists of just one chain of nucleotides. Determining the structure of DNA was a big scientific breakthrough. You can read the interesting story of its discovery at the URL below. "
carbon and living things,T_4097,"DNA stores genetic information in the cells of all living things. It contains the genetic code. This is the code that instructs cells how to make proteins. The instructions are encoded in the sequence of nitrogen bases in the nucleotide chains of DNA. RNA ""reads"" the genetic code in DNA and is involved in the synthesis of proteins based on the code. This video shows how:  (2:51). MEDIA Click image to the left or use the URL below. URL: "
biochemical reactions,T_4098,Most of the energy used by living things comes either directly or indirectly from the sun. Sunlight provides the energy for photosynthesis. This is the process in which plants and certain other organisms (see Figure 9.26) synthesize glucose (C6 H12 O6 ). The process uses carbon dioxide and water and also produces oxygen. The overall chemical equation for photosynthesis is: 6CO2 + 6H2 O + Light Energy ! C6 H12 O6 + 6O2 Photosynthesis changes light energy to chemical energy. The chemical energy is stored in the bonds of glucose molecules. Glucose is used for energy by the cells of almost all living things. Plants make their own glucose. Other organisms get glucose by consuming plants (or organisms that consume plants). How do living things get energy from glucose? The answer is cellular respiration. 
biochemical reactions,T_4099,"Cellular respiration is the process in which the cells of living things break down glucose with oxygen to produce carbon dioxide, water, and energy. The overall chemical equation for cellular respiration is: C6 H12 O6 + 6O2 ! 6CO2 + 6H2 O + Heat and Chemical Energy Cellular respiration releases some of the energy in glucose as heat. It uses the rest of the energy to form many, even smaller molecules. The smaller molecules contain just the right amount of energy to power chemical reactions inside cells. You can look at cellular respiration in more detail at this URL:  MEDIA Click image to the left or use the URL below. URL: "
biochemical reactions,T_4100,"Human body temperature must remain within a narrow range around 37C (98.6F). At this temperature, most biochemical reactions would occur too slowly to keep us alive. Thats where enzymes come in. Enzymes are biochemical catalysts. They speed up biochemical reactions, not only in humans but in virtually all living things. Most enzymes are proteins. Two are described in Figure 9.27. "
acceleration,T_4101,"Acceleration is a measure of the change in velocity of a moving object. It measures the rate at which velocity changes. Velocity, in turn, is a measure of the speed and direction of motion, so a change in velocity may reflect a change in speed, a change in direction, or both. Both velocity and acceleration are vectors. A vector is any measurement that has both size and direction. People commonly think of acceleration as in increase in speed, but a decrease in speed is also acceleration. In this case, acceleration is negative and called deceleration. A change in direction without a change in speed is acceleration as well. Q: Can you think of an example of acceleration that doesnt involve a change in speed? A: Driving at a constant speed around a bend in a road is one example. Use your imagination to think of others. "
acceleration,T_4102,"You can see several examples of acceleration in the pictures from the Figure 1.1. In each example, velocity is changing but in different ways. For example, direction may be changing but not speed, or vice versa. Figure out what is moving and how its moving in each of the photos. Q: Describe how velocity is changing in each of the motions you identified from the Figure 1.1. A: You should describe how both direction and speed are changing. For example, the boy on the carousel is moving up and down and around in a circle, so his direction is constantly changing, but his speed changes only at the beginning and end of the ride. The skydiver is falling straight down toward the ground so her direction isnt changing, but her speed keeps increasing as she falls until she opens her parachute. "
acceleration,T_4103,"If you are accelerating, you may be able to feel the change in velocity. This is true whether the change is in speed, direction, or both. You often feel acceleration when you ride in a car. As the car speeds up, you feel as though you are being pressed against the seat. When the car slows down, you feel like you are being pushed forward, especially if the change in speed is sudden. If the car changes direction and turns right, you feel as though you are being pushed to the left. With a left turn, you feel a push to the right. The next time you ride in a car, notice how it feels as the car accelerates in each of these ways. "
acceleration due to gravity,T_4104,"Gravity is a force that pulls objects down toward the ground. When objects fall to the ground, gravity causes them to accelerate. Acceleration is a change in velocity, and velocity, in turn, is a measure of the speed and direction of motion. Gravity causes an object to fall toward the ground at a faster and faster velocity the longer the object falls. In fact, its velocity increases by 9.8 m/s2, so by 1 second after an object starts falling, its velocity is 9.8 m/s. By 2 seconds after it starts falling, its velocity is 19.6 m/s (9.8 m/s + 9.8 m/s), and so on. The acceleration of a falling object due to gravity is illustrated in the Figure 1.1. Q: In this diagram, the boy drops the object at time t= 0 s. By t = 1 s, the object is falling at a velocity of 9.8 m/s. What is its velocity by t = 5 s? What will its velocity be at t = 6 s if it keeps falling? A: Its velocity at t = 5 s is 49.0 m/s, and at t = 6 s, it will be 58.8 m/s (49.0 m/s + 9.8 m/s). "
acceleration due to gravity,T_4105,"What if you were to drop a bowling ball and a soccer ball at the same time from the same distance above the ground? The bowling ball has greater mass than the basketball, so the pull of gravity on it is greater. Would it fall to the ground faster? No, the bowling ball and basketball would reach the ground at the same time. The reason? The more massive bowling ball is also harder to move because of its greater mass, so it ends up moving at the same acceleration as the soccer ball. This is true of all falling objects. They all accelerate at the same rate due to gravity, unless air resistance affects one object more than another. For example, a falling leaf is slowed down by air resistance more than a falling acorn because of the leafs greater surface area. Q: If a leaf and an acorn were to fall to the ground in the absence of air (that is, in a vacuum), how would this affect their acceleration due to gravity? A: They would both accelerate at the same rate and reach the ground at the same time. "
accuracy and precision,T_4106,"The accuracy of a measurement is how close the measurement is to the true value. If you were to hit four different golf balls toward an over-sized hole, all of them might land in the hole. These shots would all be accurate because they all landed in the hole. This is illustrated in the sketch below. "
accuracy and precision,T_4107,"As you can see from the sketch above, the four golf balls did not land as close to one another as they could have. Each one landed in a different part of the hole. Therefore, these shots are not very precise. The precision of measurements is how close they are to each other. If you make the same measurement twice, the answers are precise if they are the same or at least very close to one another. The golf balls in the sketch below landed quite close together in a cluster, so they would be considered precise. However, they are all far from the hole, so they are not accurate. Q: If you were to hit four golf balls toward a hole and your shots were both accurate and precise, where would the balls land? A: All four golf balls would land in the hole (accurate) and also very close to one another (precise). "
acid base neutralization,T_4108,"An acid is a compound that produces positive hydrogen ions (H+ ) and negative nonmetal ions when it dissolves in water. (Ions are atoms that have become charged by losing or gaining electrons.) Hydrochloric acid (HCl) is an example of an acid. When it dissolves in water, it produces positive hydrogen ions and negative chloride ions (Cl ). This can be represented by the chemical equation: H O 2 HCl H+ + Cl A base is a compound that produces negative hydroxide ions (OH ) and positive metal ions when it dissolves in water. For example, when the base sodium hydroxide (NaOH) dissolves in water, it produces negative hydroxide ions and positive sodium ions (Na+ ). This can be represented by the chemical equation: H O 2 NaOH OH + Na+ Q: If you were to combine acid and base solutions, what products do you think would be produced? A: Combining acid and base solutions produces water and a neutral ionic compound. "
acid base neutralization,T_4109,"When an acid and a base react, the reaction is called a neutralization reaction. Thats because the reaction produces neutral products. Water is always one product, and a salt is also produced. A salt is a neutral ionic compound. Lets see how a neutralization reaction produces both water and a salt, using as an example the reaction between solutions of hydrochloric acid and sodium hydroxide. The overall equation for this reaction is: NaOH + HCl  H2 O and NaCl Now lets break this reaction down into two parts to see how each product forms. Positive hydrogen ions from HCl and negative hydroxide ions from NaOH combine to form water. This part of the reaction can be represented by the equation: H+ + OH  H2 O Positive sodium ions from NaOH and negative chloride ions from HCL combine to form the salt sodium chloride (NaCl), commonly called table salt. This part of the reaction can be represented by the equation: Na+ + Cl  NaCl Another example of a neutralization reaction can be seen in the Figure 1.1. Q: What products are produced when antacid tablets react with hydrochloric acid in the stomach? A: The products are water and the salt calcium chloride (CaCl2 ). Carbon dioxide (CO2 ) is also produced. The reaction is represented by the chemical equation: CaCO3 + 2HCl  H2 O + CaCl2 + CO2 "
activation energy,T_4110,"Chemical reactions also need energy to be activated. They require a certain amount of energy just to get started. This energy is called activation energy. For example, activation energy is needed to start a car engine. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas wont occur without the spark of energy to begin the reaction. Q: Why is activation energy needed? Why wont a reaction occur without it? A: A reaction wont occur unless atoms or molecules of reactants come together. This happens only if the particles are moving, and movement takes energy. Often, reactants have to overcome forces that push them apart. This takes energy as well. Still more energy is needed to start breaking bonds in reactants. "
activation energy,T_4111,"Some chemical reactions need a constant input of energy to take place. They are called endothermic reactions. Other chemical reactions release energy when they occur, so they can keep going without any added energy. They are called exothermic reactions. Q: It makes sense that endothermic reactions need activation energy. But do exothermic reactions also need activation energy? A: All chemical reactions need energy to get started, even exothermic reactions. Look at the Figure 1.1. They compare energy changes that occur during endothermic and exothermic reactions. From the graphs, you can see that both types of reactions need the same amount of activation energy in order to get started. Only after it starts does the exothermic reaction produce more energy than it uses. "
activation energy,T_4112,"You have probably used activation energy to start a chemical reaction. For example, if youve ever struck a match to light it, then you provided the activation energy needed to start a combustion reaction. When you struck the match on the box, the friction started the match head burning. Combustion is exothermic. Once a match starts to burn, it releases enough energy to activate the next reaction, and the next, and so on. However, the match wont burst into flames on its own. "
alkaline earth metals,T_4116,"Barium (Ba) is one of six elements in group 2 of the periodic table, which is shown in Figure 1.1. Elements in this group are called alkaline Earth metals. These metals are silver or gray in color. They are relatively soft and low in density, although not as soft and lightweight as alkali metals. "
alkaline earth metals,T_4117,"All alkaline Earth metals have similar properties because they all have two valence electrons. They readily give up their two valence electrons to achieve a full outer energy level, which is the most stable arrangement of electrons. As a result, they are very reactive, although not quite as reactive as the alkali metals in group 1. For example, alkaline Earth metals will react with cold water, but not explosively as alkali metals do. Because of their reactivity, alkaline Earth metals never exist as pure substances in nature. Instead, they are always found combined with other elements. The reactivity of alkaline Earth metals increases from the top to the bottom of the group. Thats because the atoms get bigger from the top to the bottom, so the valence electrons are farther from the nucleus. When valence electrons are farther from the nucleus, they are attracted less strongly by the nucleus and more easily removed from the atom. This makes the atom more reactive. Q: Alkali metals have just one valence electron. Why are alkaline Earth metals less reactive than alkali metals? A: It takes more energy to remove two valence electrons from an atom than one valence electron. This makes alkaline Earth metals with their two valence electrons less reactive than alkali metals with their one valence electron. "
alkaline earth metals,T_4118,"For a better understanding of alkaline Earth metals, lets take a closer look at two of them: calcium (Ca) and strontium (Sr). Calcium is a soft, gray, nontoxic alkaline Earth metal. Although pure calcium doesnt exist in nature, calcium compounds are very common in Earths crust and in sea water. Calcium is also the most abundant metal in the human body, occurring as calcium compounds such as calcium phosphate and calcium carbonate. These calcium compounds are found in bones and make them hard and strong. The skeleton of the average adult contains about a kilogram of calcium. Because calciumlike bariumabsorbs x-rays, bones show up white in x-ray images. Calcium is an important component of a healthy human diet. Good food sources of calcium are pictured in Figure Q: What health problems might result from a diet low in calcium? A: Children who dont get enough calcium while their bones are forming may develop a deficiency disease called rickets, in which their bones are softer than normal and become bent and stunted. Adults who dont get enough calcium may develop a condition called osteoporosis, in which the bones lose calcium and become weak and brittle. People with osteoporosis are at high risk of bone fractures. Strontium is a silver-colored alkaline Earth metal that is even softer than calcium. Strontium compounds are quite common and have a variety of usesfrom fireworks to cement to toothpaste. In fireworks, strontium compounds produce deep red explosions. In toothpaste, the compound strontium chloride reduces tooth sensitivity. "
alloys,T_4119,"An alloy is a mixture of a metal with one or more other elements. The other elements may be metals, nonmetals, or both. An alloy is formed by melting a metal and dissolving the other elements in it. The molten solution is then allowed to cool and harden. Alloys generally have more useful properties than pure metals. Several examples of alloys are described and pictured below. If you have braces on your teeth, you might even have this alloy in your mouth! Click image to the left or use the URL below. URL: "
alloys,T_4120,"Most metal objects are made of alloys rather than pure metals. Objects made of four different alloys are shown in the Figure 1.1. Brass saxophone: Brass is an alloy of copper and zinc. It is softer than bronze and easier to shape. Its also very shiny. Notice the curved pieces in this shiny brass saxophone. Brass is used for shap- ing many other curved objects, such as doorknobs and plumbing fixtures. Stain- less steel sink: Stainless steel is a type of steel that contains nickel and chromium in addition to carbon and iron. It is shiny, strong, and resistant to rusting. This makes it useful for sinks, eating utensils, and other objects that are exposed to wa- ter. ""Gold"" bracelet: Pure gold is relatively soft, so it is rarely used for jewelry. Most ""gold"" jewelry is actually made of an alloy of gold, copper and silver. Bronze statue: Bronze was the first alloy ever made. The earliest bronze dates back many thou- sands of years. Bronze is a mixture of copper and tin. Both copper and tin are relatively soft metals, but mixed together in bronze they are much harder. Bronze has been used for statues, coins, and other objects. Q: Sterling silver is an alloy that is used to make fine jewelry. What elements do you think sterling silver contains? What properties might sterling silver have that make it more useful than pure silver? A: Most sterling silver is about 93 percent silver and about 7 percent copper. Sterling silver is harder and stronger than pure silver, while retaining the malleability and luster of pure silver. "
alpha decay,T_4121,"Radioactive elements and isotopes have unstable nuclei. To become more stable, the nuclei undergo radioactive decay. In radioactive decay, the nuclei give off, or emit, radiation in the form of energy and often particles as well. There are several types of radioactive decay, including alpha, beta, and gamma decay. Energy is emitted in all three types of decay, but only alpha and beta decay also emit particles. "
alpha decay,T_4122,"Alpha decay occurs when a nucleus is unstable because it has too many protons. The Figure 1.1 shows what happens during alpha decay. The nucleus emits an alpha particle and energy. An alpha particle consists of two protons and two neutrons, which is actually a helium nucleus. Losing the protons and neutrons makes the nucleus more stable. "
alpha decay,T_4123,"Radioactive nuclei and particles are represented by nuclear symbols that indicate their numbers of protons and neutrons. For example, an alpha particle (helium nucleus) is represented by the symbol 42 He, where He is the chemical symbol for helium, the subscript 2 is the number of protons, and the superscript 4 is the mass number (2 protons + 2 neutrons). Nuclear symbols are used to write nuclear equations for radioactive decay. Lets consider an example. Uranium-238 undergoes alpha decay to become thorium-234. (The numbers following the chemical names refer to the number of protons plus neutrons.) In this reaction, uranium-238 loses two protons and two neutrons to become the element thorium-234. The reaction can be represented by this nuclear equation: 238 U 92 4 234 90 Th + 2 He + Energy If you count the number of protons (subscripts) as well as the number of protons plus neutrons (superscripts), youll see that the total numbers are the same on both sides of the arrow. This means that the equation is balanced. The thorium-234 produced in this reaction is also unstable, so it will undergo radioactive decay as well. The alpha particle (42 He) produced in the reaction can join with two free electrons to form the element helium. This is how most of Earths helium formed. Q: Fill in the missing subscript and superscript to balance the following nuclear equation for alpha decay of Polonium-210. 210 Po 84 ?? Pb + 42 He + Energy A: The subscript of Pb is 82, and the superscript is 206. This means that the new element produced in the reaction has 82 protons. You can find the element with this number of protons in the periodic table. It is the element lead (Pb). The new element also has 124 neutrons (206 - 82 protons = 124 neutrons). "
alpha decay,T_4124,"All types of radioactive decay pose risks to living things, but alpha decay is the least dangerous. Thats because alpha particles are relatively heavy, so they can travel only a few centimeters through the air. They also are not very penetrating. For example, they cant pass through a sheet of paper or thin layer of clothing. They may burn the skin, but they cant penetrate to the tissues underneath the skin. However, if alpha particles are emitted inside the body, they can do more damage. One way this can happen is by inhaling cigarette smoke. People who smoke actually inhale the radioactive element polonium-210. It undergoes alpha decay in the lungs. Over time, exposure to alpha particles may cause lung cancer. "
archimedes law,T_4128,"Did you ever notice when you get into a bathtub of water that the level of the water rises? More than 2000 years ago, a Greek mathematician named Archimedes noticed the same thing. He observed that both a body and the water in a tub cant occupy the same space at the same time. As a result, some of the water is displaced, or moved out of the way. How much water is displaced? Archimedes determined that the volume of displaced water equals the volume of the submerged object. So more water is displaced by a bigger body than a smaller one. Q: If you jump into swimming pool, how much water does your body displace? A: The water displaced by your body is equal to your bodys volume. Depending on your size, this volume might be about 0.07 m3 . "
archimedes law,T_4129,Objects such as ships may float in a fluid like water because of buoyant force. This is an upward force that a fluid exerts on any object that is placed in it. Archimedes discovered that the buoyant force acting on an object equals the weight of the fluid displaced by the object. This is known as Archimedes law (or Archimedes principle). 
archimedes law,T_4130,"Archimedes law explains why some objects float in fluids even though they are very heavy. It all depends on how much fluid they displace. The cruise ship pictured in the opening image is extremely heavy, yet it stays afloat. If a steel ball with the same weight as the ship were placed in water, it would sink to the bottom. This is modeled in the Figure 1.1. The reason the ball sinks is that its shape is very compact, so it displaces relatively little water. The volume of water displaced by the steel ball weighs less than the ball itself, so the buoyant force is not as great as the force of gravity pulling down on the ball. Thus, the ball sinks. Now look at the ships hull in the Figure 1.1. Its shape causes the ship to displace much more water than the ball. In fact, the weight of the displaced water is greater than the weight of the ship. As a result, the buoyant force is greater than the force of gravity acting on the ship, so the ship floats. Q: Why might you be more likely to float in water if you stretch out your body rather than curl up into a ball? A: You would displace more water by stretching out your body, so there would be more buoyant force acting on it. Therefore, you would be more likely to float in this position. "
artificial light,T_4131,"If youre like most people, you dont give it a thought when you flick a switch to turn on a lightat least not until the power goes out and youre left in the dark! When you flick on a light switch, electricity normally flows through the light, and some type of light bulb converts the electrical energy to visible light. This can happen in various ways, depending on the type of light bulb. Several different types of light bulbs are described below. All of them are examples of artificial light, as opposed to natural light from the sun or other sources in nature. "
artificial light,T_4132,"An incandescent light bulb like the one pictured in the Figure 1.1 produces visible light by incandescence. Incan- descence occurs when something gets so hot that it glows. An incandescent light bulb contains a thin wire filament made of tungsten. When electric current passes through the filament, it gets extremely hot and emits light. "
artificial light,T_4133,A fluorescent light bulb produces visible light by fluorescence. Fluorescence occurs when a substance absorbs shorter-wavelength ultraviolet light and then gives off the energy as visible light. The compact fluorescent light bulb (CFL) in the Figure 1.2 contains mercury gas that gives off ultraviolet light when electricity passes through it. The inside of the bulb is coated with a substance called phosphor. Phosphor absorbs the ultraviolet light and then gives off most of the energy as visible light. 
artificial light,T_4134,"A neon light produces visible light by electroluminescence. In this process, neon or some other gas gives off light when an electric current passes through it. Other halogen gases besides neonincluding krypton and argonalso produce light in this way. The word OPEN in the sign 1.3 is a neon light. It is a long glass tube that contains neon gas. When electricity passes through the gas, it excites electrons of neon atoms, and the electrons jump to a higher energy level. As the excited electrons return to their original energy level, they give off visible light. Neon produces red light. Other gases produce light of different colors. For example, krypton produces violet light, and argon produces blue light. "
artificial light,T_4135,"A vapor light also produces visible light by electroluminescence The bulb contains a small amount of solid sodium or mercury as well as a mixture of neon and argon gases. When an electric current passes through the gases, it causes the solid sodium or mercury to change to a gas and emit visible light. Sodium vapor lights, like the streetlight pictured in the Figure 1.4, produce yellowish light. Mercury vapor lights produce bluish light. In addition to lighting city streets, vapor lights are used to light highways and stadiums. The bulbs are very bright and long lasting so they are a good choice for these places. "
artificial light,T_4136,"LED stands for light-emitting diode. An LED light contains a material called a semi-conductor, which gives off visible light when an electric current flows through it. LED lights are used for traffic lights (see Figure 1.5) and also indicator lights on computers, cars, and many other devices. This type of light is very reliable and durable. Q: Some light bulbs produce a lot of heat in addition to visible light, so they waste energy. Other bulbs produce much less heat, so they use energy more efficiently. Which light bulbs described above would you place in each category? A: Incandescent light bulbs, which produce light by incandescence, give off a lot of heat as well as light, so they waste energy. The other light bulbs produce light by some type of luminescence, in which light is produced without heat. These light bulbs use energy more efficiently. Which types of light bulbs do you use? "
atomic forces,T_4137,"Electromagnetic force is a force of attraction or repulsion between all electrically charged particles. This force is transferred between charged particles of matter by fundamental force-carrying particles called photons. Because of electromagnetic force, particles with opposite charges attract each other and particles with the same charge repel each other. Inside the atom, two types of subatomic particles have electric charge: electrons, which have an electric charge of -1, and protons, which have an opposite but equal electric charge of +1. The model of an atom in the Figure 1.1 shows both types of charged particles. Protons are found inside the nucleus at the center of the atom, and they give the nucleus a positive charge. (There are also neutrons in the nucleus, but they have no electric charge.) Negative electrons stay in the area surrounding the positive nucleus because of the electromagnetic force of attraction between them. Q: Why do you think protons cluster together in the nucleus of the atom instead of repelling each other because of their like charges? A: The electromagnetic force of repulsion between positively charged protons is overcome by a stronger force, called the strong nuclear force. "
atomic forces,T_4138,"The strong nuclear force is a force of attraction between fundamental particles called quarks, which have a type of charge called color charge. The strong nuclear force is transferred between quarks by fundamental force-carrying particles called gluons. Both protons and neutrons consist of quarks. The exchange of gluons holds quarks together within a proton or neutron. Excess, or residual, strong force holds together protons and neutrons in the nucleus. The strong nuclear force is strong enough to overcome the electromagnetic force of repulsion pushing protons apart. Both forces are represented in the Figure 1.2. The strong nuclear force works only over very short distances. As a result, it isnt effective if the nucleus gets too big. As more protons are added to the nucleus, the electromagnetic force of repulsion between them gets stronger, while the strong nuclear force of attraction between them gets weaker. This puts an upper limit on the number of protons an atom can have and remain stable. If atoms have more than 83 protons, the electromagnetic repulsion between them is greater than the strong nuclear force of attraction between them. This makes the nucleus unstable, or radioactive, so it breaks down. The following video discusses the strong nuclear force and its role in the atom. The types of quarks found in protons and neutrons are called up quarks (u) and down quarks (d). Each proton consists of two up quarks and one down quark (uud), and each neutron consists of one up quark and two down quarks (udd). This diagram represents two protons. Click image to the left or use the URL below. URL: "
atomic forces,T_4139,"The weak nuclear force is transferred by the exchange of force-carrying fundamental particles called W and Z bosons. This force is also a very short-range force that works only within the nucleus of the atom. It is much weaker than the strong force or electromagnetic force that are also at work inside the atom. Unlike these other two forces, the weak nuclear force does not bind subatomic particles together in an atom. Instead, it changes subatomic particles from one type to another. The Figure 1.3 shows one way this can happen. In this figure, an up quark in a proton is changed by the weak force to a down quark. This changes the proton (uud) to a neutron (udd). Q: If the weak force causes a proton to change to a neutron, how does this change the atom? A: The resulting atom represents a different element. Thats because each element has a unique number of protons. For example, all atoms of helium have two protons. If one of the protons in a helium atom changes to a neutron, the resulting atom would have just one proton, so the atom would no longer be a helium atom. Instead it would be a hydrogen atom, because all hydrogen atoms have a single proton. "
atomic nucleus,T_4140,"The nucleus (plural, nuclei) is a positively charged region at the center of the atom. It consists of two types of subatomic particles packed tightly together. The particles are protons, which have a positive electric charge, and neutrons, which are neutral in electric charge. Outside of the nucleus, an atom is mostly empty space, with orbiting negative particles called electrons whizzing through it. The Figure 1.1 shows these parts of the atom. "
atomic nucleus,T_4141,"The nucleus of the atom is extremely small. Its radius is only about 1/100,000 of the total radius of the atom. If an atom were the size of a football stadium, the nucleus would be about the size of a pea! Click image to the left or use the URL below. URL:  Electrons have virtually no mass, but protons and neutrons have a lot of mass for their size. As a result, the nucleus has virtually all the mass of an atom. Given its great mass and tiny size, the nucleus is very dense. If an object the size of a penny had the same density as the nucleus of an atom, its mass would be greater than 30 million tons! Click image to the left or use the URL below. URL: "
atomic nucleus,T_4142,"Particles with opposite electric charges attract each other. This explains why negative electrons orbit the positive nucleus. Particles with the same electric charge repel each other. This means that the positive protons in the nucleus push apart from one another. So why doesnt the nucleus fly apart? An even stronger forcecalled the strong nuclear forceholds protons and neutrons together in the nucleus. Click image to the left or use the URL below. URL:  Q: Can you guess why an atomic bomb releases so much energy when it explodes? A: When an atomic bomb explodes, the nuclei of atoms undergo a process called fission, in which they split apart. This releases the huge amount of energy that was holding together subatomic particles in the nucleus. "
atomic number,T_4143,"Its often useful to have ways to signify different people or objects like athletes on teams. The same is true of atoms. Its important to be able to distinguish atoms of one element from atoms of other elements. Elements are pure substances that make up all other matter, so each one is given a unique name. The names of elements are also represented by unique one- or two-letter symbols, such as H for hydrogen, C for carbon, and He for helium. You can see other examples in the Figure 1.1. Q: The table shown above is called the periodic table of the elements. Each symbol stands for a different element. What do you think the symbol K stands for? A: The symbol K stands for the element potassium. The symbol comes from the Latin name for potassium, which is kalium. The symbols in the table above would be more useful if they revealed more information about the atoms they represent. For example, it would be useful to know the numbers of protons and neutrons in the atoms. Thats where atomic number and mass number come in. "
atomic number,T_4144,"The number of protons in an atom is called its atomic number. This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms. For example, all helium atoms have two protons, and no other elements have atoms with two protons. In the case of helium, the atomic number is 2. The atomic number of an element is usually written in front of and slightly below the elements symbol, like in the Figure 1.2 for helium. Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons. Therefore, the atomic number of an atom also tells you how many electrons the atom has. This, in turn, determines many of the atoms properties. "
atomic number,T_4145,"There is another number in the box above for helium. That number is the mass number, which is the mass of the atom in a unit called the atomic mass unit (amu). One atomic mass unit is the mass of a proton, or about 1.67 1027 kilograms, which is an extremely small mass. A neutron has just a tiny bit more mass than a proton, so its mass is often assumed to be one atomic mass unit as well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units. Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units (2 amu for the protons + 2 amu for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they have a different mass number than the one given in the box above. Q: What is the mass number of a helium atom that has three neutrons? A: The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons, the mass number is 2 (protons) + 3 (neutrons) = 5. Q: How would you represent this isotope of helium to show its atomic number and mass number? A: You would represent it by the elements symbol and both numbers, with the mass number on top and the atomic number on the bottom: 5 2 He "
balancing chemical equations,T_4153,"A chemical equation represents the changes that occur during a chemical reaction. A chemical equation has the general form: Reactants  Products An example of a simple chemical reaction is the reaction in which hydrogen (H2 ) and oxygen (O2 ) combine to produce water (H2 O). In this reaction, the reactants are hydrogen and oxygen and the product is water. To write the chemical equation for this reaction, you would start by writing the reactants on the left and the product on the right, with an arrow between them to show the direction in which the reaction occurs: Equation 1: H2 + O2  H2 O Q: Look closely at equation 1. Theres something wrong with it. Do you see what it is? A: All chemical equations must be balanced. This means that there must be the same number of each type of atom on both sides of the arrow. Thats because mass is always conserved in chemical reactions. Count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced. "
balancing chemical equations,T_4154,"Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2 H2 , and two molecules of water would be written 2 H2 O. A coefficient of 1 usually isnt written. Coefficients can be used to balance equation 1 (above) as follows: Equation 2: 2 H2 + O2  2 H2 O Equation 2 shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms and so do the two molecules of water. Therefore, there are now four hydrogen atoms in both reactants and products. Q: Is equation 2 balanced? A: Count the oxygen atoms to find out. There are two oxygen atoms in the one molecule of oxygen in the reactants. There are also two oxygen atoms in the products, one in each of the two water molecules. Therefore, equation 2 is balanced. "
balancing chemical equations,T_4155,"Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps: 1. Count each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step 2. 2. Place coefficients, as needed, in front of the symbols or formulas to increase the number of atoms or molecules of the substances. Use the smallest coefficients possible. Warning! Never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients. 3. Repeat steps 1 and 2 until the equation is balanced. Q: Balance this chemical equation for the reaction in which nitrogen (N2 ) and hydrogen (H2 ) combine to form ammonia (NH3 ): N2 + H2  NH3 A: First count the nitrogen atoms on both sides of the arrow. There are two nitrogen atoms in the reactants so there must be two in the products as well. Place the coefficient 2 in front of NH3 to balance nitrogen: N2 + H2  2 NH3 Now count the hydrogen atoms on both sides of the arrow. There are six hydrogen atoms in the products so there must also be six in the reactants. Place the coefficient 3 in front of H2 to balance hydrogen: N2 + 3 H2  2 NH3 "
beta decay,T_4158,"Atoms with unstable nuclei are radioactive. To become more stable, the nuclei undergo radioactive decay. In radioactive decay, the nuclei emit energy and usually particles of matter as well. There are several types of radioactive decay, including alpha, beta, and gamma decay. Energy is emitted in all three types of decay, but only alpha and beta decay also emit particles. "
beta decay,T_4159,"Beta decay occurs when an unstable nucleus emits a beta particle and energy. A beta particle is either an electron or a positron. An electron is a negatively charged particle, and a positron is a positively charged electron (or anti- electron). When the beta particle is an electron, the decay is called beta-minus decay. When the beta particle is a positron, the decay is called beta-plus decay. Beta-minus decay occurs when a nucleus has too many neutrons relative to protons, and beta-plus decay occurs when a nucleus has too few neutrons relative to protons. Q: Nuclei contain only protons and neutrons, so how can a nucleus emit an electron in beta-minus decay or a positron in beta-plus decay? A: Beta decay begins with a proton or neutron. You can see how in the Figure 1.1. Q: How does beta decay change an atom to a different element? A: In beta-minus decay an atom gains a proton, and it beta-plus decay it loses a proton. In each case, the atom becomes a different element because it has a different number of protons. "
beta decay,T_4160,"Radioactive nuclei and particles are represented by nuclear symbols.. For example, a beta-minus particle (electron) is represented by the symbol 01 e. The subscript -1 represents the particles charge, and the superscript 0 shows that the particle has virtually no mass (no protons or neutrons). Another example is the radioactive nucleus of thorium-234. It is represented by the symbol 234 90 Th, where the subscript 90 stands for the number of protons and the superscript 234 for the number of protons plus neutrons. Nuclear symbols are used to write nuclear equations for radioactive decay. Lets consider the example of the beta- minus decay of thorium-234 to protactinium-234. This reaction is represented by the equation: 234 Th 90 0 234 91 Pa + 1 e + energy The equation shows that thorium-234 becomes protactinium-234 and loses a beta particle and energy. The protactinium- 234 produced in the reaction is also radioactive, so it will decay as well. A nuclear equation is balanced if the total numbers of protons and neutrons are the same on both sides of the arrow. If you compare the subscripts and superscripts on both sides of the equation above, youll see that they are the same. Q: What happens to the electron produced in the reaction above? A: Along with another electron, it can combine with an alpha particle to form a helium atom. An alpha particle, which is emitted during alpha decay, consists of two protons and two neutrons. Q: Try to balance the following nuclear equation for beta-minus decay by filling in the missing subscript and superscript. 131 I 53 ?? Xe + 01 e + energy A: The subscript of Xe is 54, and the superscript is 131. "
beta decay,T_4161,Beta particles can travel about a meter through air. They can pass through a sheet of paper or a layer of cloth but not through a sheet of aluminum or a few centimeters of wood. They can also penetrate the skin and damage underlying tissues. They are even more harmful if they are ingested or inhaled. 
biochemical compound classification,T_4162,"Glucose is an example of a biochemical compound. The prefix bio- comes from the Greek word that means life. A biochemical compound is any carbon-based compound that is found in living things. Biochemical compounds make up the cells and tissues of living things. They are also involved in all life processes, including making and using food for energy. Given their diversity of functions, its not surprising that there are millions of different biochemical compounds. Q: Plants make food in the process of photosynthesis. What biochemical compound is synthesized in photosynthe- sis? A: Glucose is synthesized in photosynthesis. Virtually all living things use glucose for energy, but glucose is just one of many examples of biochemical compounds that are found in most or all living things. In fact the similarity in biochemical compounds between living things provides some of the best evidence for the evolution of species from common ancestors. A classic example is the biochemical compound called cytochrome c. It is found in all living organisms because it performs essential life functions. Only slight variations in the molecule exist between closely related species, as you can see in the Figure and the single-celled tetrahymena (pictured in the Figure 1.1), the cytochrome c molecule is nearly 50 percent the same. "
biochemical compound classification,T_4163,"All biochemical molecules contain hydrogen and oxygen as well as carbon. They may also contain nitrogen, phosphorus, and/or sulfur. Almost all biochemical compounds are polymers. Polymers are large molecules that consist of many smaller, repeating molecules, called monomers. Glucose is a monomer of biochemical compounds called starches. In starches and all other biochemical polymers, monomers are joined together by covalent bonds, in which atoms share pairs of valence electrons. Click image to the left or use the URL below. URL: "
biochemical compound classification,T_4164,"Most biochemical molecules are macromolecules. The prefix macro- means large, and many biochemical molecules are very large indeed. They may contain thousands of monomer molecules. The largest known biochemical molecule is called titin. It plays an important role in muscle contraction. The human form of the molecule contains more than 34,000 monomers. Its chemical formula is C169723 H270464 N45688 O52243 S912 . Its chemical name contains almost 190,000 letters, and it has been called the longest word in any language. "
biochemical compound classification,T_4165,"Although there are millions of biochemical compounds, all of them can be grouped into just four main classes: carbohydrates, proteins, lipids, and nucleic acids. The classes are summarized in the Table 1.1. Class Carbohydrates Elements carbon hydrogen oxygen Examples sugars starches cellulose Proteins carbon hydrogen oxygen nitrogen sulfur carbon hydrogen oxygen carbon hydrogen oxygen nitrogen phosphorus enzymes hormones Lipids Nucleic acids Functions provide energy to cells store energy in plants makes up the cell walls of plants speed up biochemical re- actions regulate life processes fats oils store energy in animals store energy in plants DNA RNA stores genetic information in cells helps cells make proteins Q: In which class of biochemical compounds would you place glucose? A: Glucose is a sugar in the class carbohydrates. Like other carbohydrates, it contains only carbon, hydrogen, and oxygen. It provides energy to the cells of living things. Q: Look back at the chemical formula for titin. In which class of biochemical compounds should it be placed? A: Titin is a protein. You can tell because it contains sulfur, and proteins are the only biochemical compounds that contain this element. "
biochemical reaction chemistry,T_4166,Chemical reactions that take place inside living things are called biochemical reactions (bio- means life). Its not just for energy that living things depend on biochemical reactions. Every function and structure of a living organism depends on thousands of biochemical reactions taking place in each cell. The sum of all these biochemical reactions is called metabolism. 
biochemical reaction chemistry,T_4167,"Biochemical reactions of metabolism can be divided into two general categories: catabolic reactions and anabolic reactions. Catabolic reactions involve breaking bonds. Larger molecules are broken down to smaller ones. For example, complex carbohydrates are broken down to simple sugars. Catabolic reactions release energy, so they are exothermic. Anabolic reactions involve forming bonds. Smaller molecules are combined to form larger ones. For example, simple sugars are combined to form complex carbohydrates. Anabolic reactions require energy, so they are endothermic. Q: Imagine! Each of the trillions of cells in your body is continuously performing thousands of catabolic and anabolic reactions. Thats an amazing number of biochemical reactionsfar more than the number of reactions that might take place in a lab or factory. How can so many biochemical reactions take place simultaneously in our cells? A: So many reactions can occur because biochemical reactions are amazingly fast. Q: In a lab or factory, reactants can be heated to very high temperatures or placed under great pressure so they will react very quickly. These ways of speeding up chemical reactions cant occur inside the delicate cells of living things. So how do cells speed up biochemical reactions? A: The answer is enzymes. "
biochemical reaction chemistry,T_4168,"Enzymes are proteins that increase the rate of chemical reactions by reducing the amount of activation energy needed for reactants to start reacting. Enzymes are synthesized in the cells that need them, based on instructions encoded in the cells DNA. Enzymes arent changed or used up in the reactions they catalyze, so they can be used to speed up the same reaction over and over again. Enzymes are highly specific for certain chemical reactions, so they are very effective. A reaction that would take years to occur without its enzyme might occur in a split second with the enzyme. Enzymes are also very efficient, so waste products rarely form. "
biochemical reaction chemistry,T_4169,"Some of the most important biochemical reactions are the reactions involved in photosynthesis and cellular respira- tion. Together, these two processes provide energy to almost all of Earths organisms. The two processes are closely related, as you can see in the Figure 1.1. In photosynthesis, light energy from the sun is converted to stored chemical energy in glucose. In cellular respiration, stored energy is released from glucose and stored in smaller amounts that cells can use. A: In photosynthesis, carbon dioxide (CO2 ) and water (H2 O) are the reactants. They combine using energy from light to produce oxygen (O2 ) and glucose (C6 H12 O6 ). Oxygen and glucose, in turn, are the reactants in cellular respiration. They combine to produce carbon dioxide, water, and energy. "
bohrs atomic model,T_4170,"The existence of the atom was first demonstrated around 1800 by John Dalton. Then, close to a century went by before J.J. Thomson discovered the first subatomic particle, the negatively charged electron. Because atoms are neutral in charge, Thomson thought that they must consist of a sphere of positive charge with electrons scattered through it. In 1910, Ernest Rutherford showed that this idea was incorrect. He demonstrated that all of the positive charge of an atom is actually concentrated in a tiny central region called the nucleus. Rutherford surmised that electrons move around the nucleus like planets around the sun. Rutherfords idea of atomic structure was an improvement on Thomsons model, but it wasnt the last word. Rutherford focused on the nucleus and didnt really clarify where the electrons were in the empty space surrounding the nucleus. The next major advance in atomic history occurred in 1913, when the Danish scientist Niels Bohr published a description of a more detailed model of the atom. His model identified more clearly where electrons could be found. Although later scientists would develop more refined atomic models, Bohrs model was basically correct and much of it is still accepted today. It is also a very useful model because it explains the properties of different elements. Bohr received the 1922 Nobel prize in physics for his contribution to our understanding of the structure of the atom. You can see a picture of Bohr 1.1. "
bohrs atomic model,T_4171,"As a young man, Bohr worked in Rutherfords lab in England. Because Rutherfords model was weak on the position of the electrons, Bohr focused on them. He hypothesized that electrons can move around the nucleus only at fixed distances from the nucleus based on the amount of energy they have. He called these fixed distances energy levels, or electron shells. He thought of them as concentric spheres, with the nucleus at the center of each sphere. In other words, the shells consisted of sphere within sphere within sphere. Furthermore, electrons with less energy would be found at lower energy levels, closer to the nucleus. Those with more energy would be found at higher energy levels, farther from the nucleus. Bohr also hypothesized that if an electron absorbed just the right amount of energy, it would jump to the next higher energy level. Conversely, if it lost the same amount of energy, it would jump back to its original energy level. However, an electron could never exist in between two energy levels. These ideas are illustrated in the Figure 1.2. Q: How is an atom like a ladder? A: Energy levels in an atom are like the rungs of a ladder. Just as you can stand only on the rungs and not in between them, electrons can orbit the nucleus only at fixed distances from the nucleus and not in between them. "
bohrs atomic model,T_4172,"Bohrs model of the atom is actually a combination of two different ideas: Rutherfords atomic model of electrons orbiting the nucleus and German scientist Max Plancks idea of a quantum, which Planck published in 1901. A quantum (plural, quanta) is the minimum amount of energy that can be absorbed or released by matter. It is a discrete, or distinct, amount of energy. If energy were water and you wanted to add it to matter in the form of a drinking glass, you couldnt simply pour the water continuously into the glass. Instead, you could add it only in small fixed quantities, for example, by the teaspoonful. Bohr reasoned that if electrons can absorb or lose only fixed quantities of energy, then they must vary in their energy by these fixed amounts. Thus, they can occupy only fixed energy levels around the nucleus that correspond to quantum increases in energy. This is a two-dimensional model of a three-dimensional atom. The concen- tric circles actually represent concentric spheres. Q: The idea that energy is transferred only in discrete units, or quanta, was revolutionary when Max Planck first proposed it in 1901. However, what scientists already knew about matter may have made it easier for them to accept the idea of energy quanta. Can you explain? A: Scientists already knew that matter exists in discrete units called atoms. This idea had been demonstrated by John Dalton around 1800. Knowing this may have made it easier for scientists to accept the idea that energy exists in discrete units as well. "
bond polarity,T_4176,"Covalent bonds are chemical bonds between atoms of nonmetals that share valence electrons. In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar covalent bonds. The Figure than the hydrogen atoms do because the nucleus of the oxygen atom has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge, and the hydrogen atoms become slightly positive in charge. Click image to the left or use the URL below. URL:  In other covalent bonds, electrons are shared equally. These bonds are called nonpolar covalent bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral in charge. The oxygen (O2 ) molecule in the Figure 1.2 has two nonpolar bonds. The two oxygen nuclei have an equal force of attraction for their four shared electrons. "
bond polarity,T_4177,"A covalent compound is a compound in which atoms are held together by covalent bonds. If the covalent bonds are polar, then the covalent compound as a whole may be polar. A polar covalent compound is one in which there is a slight difference in electric charge between opposite sides of the molecule. All polar compounds contain polar bonds. But having polar bonds does not necessarily result in a polar compound. It depends on how the atoms are arranged. This is illustrated in the Figure 1.3. In both molecules, the oxygen atoms attract electrons more strongly than the carbon or hydrogen atoms do, so both molecules have polar bonds. However, only formaldehyde is a polar compound. Carbon dioxide is nonpolar. Q: Why is carbon dioxide nonpolar? A: The symmetrical arrangement of atoms in carbon dioxide results in opposites sides of the molecule having the same charge. "
buoyancy,T_4182,"Buoyant force is an upward force that fluids exert on any object that is placed in them. The ability of fluids to exert this force is called buoyancy. What explains buoyant force? A fluid exerts pressure in all directions, but the pressure is greater at greater depth. Therefore, the fluid below an object, where the fluid is deeper, exerts greater pressure on the object than the fluid above it. You can see in the Figure 1.1 how this works. Buoyant force explains why the girl pictured above can float in water. Q: Youve probably noticed that some things dont float in water. For example, if you drop a stone in water, it will sink to the bottom rather than floating. If buoyant force applies to all objects in fluids, why do some objects sink instead of float? A: The answer has to do with their weight. "
buoyancy,T_4183,"Weight is a measure of the force of gravity pulling down on an object, whereas buoyant force pushes up on an object. Which force is greater determines whether an object sinks or floats. Look at the Figure 1.2. On the left, the objects weight is the same as the buoyant force acting on it, so the object floats. On the right, the objects weight is greater than the buoyant force acting on it, so the object sinks. "
buoyancy,T_4184,"Density, or the amount of mass in a given volume, is also related to the ability of an object to float. Thats because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why the giant ice berg in the Figure 1.3 is floating in the ocean. Q: Can you think of more examples of substances that float in a fluid because they are low in density? A: Oil is less dense than water, so oil from a spill floats on ocean water. Helium is less dense than air, so balloons filled with helium float in air. "
calculating acceleration from force and mass,T_4185,"A change in an objects motionsuch as Xander speeding up on his scooteris called acceleration. Acceleration occurs whenever an object is acted upon by an unbalanced force. The greater the net force acting on the object, the greater its acceleration will be, but the mass of the object also affects its acceleration. The smaller its mass is, the greater its acceleration for a given amount of force. Newtons second law of motion summarizes these relationships. According to this law, the acceleration of an object equals the net force acting on it divided by its mass. This can be represented by the equation: Acceleration = Net force Mass or a = F m "
calculating acceleration from force and mass,T_4186,"This equation for acceleration can be used to calculate the acceleration of an object that is acted on by a net force. For example, Xander and his scooter have a total mass of 50 kilograms. Assume that the net force acting on Xander and the scooter is 25 Newtons. What is his acceleration? Substitute the relevant values into the equation for acceleration: F = 25 N = 0.5 N a= m 50 kg kg The Newton is the SI unit for force. It is defined as the force needed to cause a 1-kilogram mass to accelerate at 1 m/s2 . Therefore, force can also be expressed in the unit kg  m/s2 . This way of expressing force can be substituted for Newtons in Xanders acceleration so the answer is expressed in the SI unit for acceleration, which is m/s2 : 2 0.5 kgm/s a = 0.5kgN = = 0.5 m/s2 kg Q: Why are there no kilograms in the final answer to this problem? A: The kilogram units in the numerator and denominator of the fraction cancel out. As a result, the answer is expressed in the correct SI units for acceleration. "
calculating acceleration from force and mass,T_4187,"Its often easier to measure the mass and acceleration of an object than the net force acting on it. Mass can be measured with a balance, and average acceleration can be calculated from velocity and time. However, net force may be a combination of many unseen forces, such as gravity, friction with surfaces, and air resistance. Therefore, it may be more useful to know how to calculate the net force acting on an object from its mass and acceleration. The equation for acceleration above can be rewritten to solve for net force as: Net Force = Mass  Acceleration, or F=ma Look at Xander in the Figure 1.1. Hes riding his scooter down a ramp. Assume that his acceleration is 0.8 m/s2 . How much force does it take for him to accelerate at this rate? Substitute the relevant values into the equation for force to find the answer: F = m  a = 50 kg  0.8 m/s2 = 40 kg  m/s2 , or 40 N Q: If Xander and his scooter actually had a mass of 40 kg instead of 50 kg, how much force would it take for him to accelerate at 0.8 m/s2 ? "
calculating acceleration from velocity and time,T_4188,"Calculating acceleration is complicated if both speed and direction are changing or if you want to know acceleration at any given instant in time. However, its relatively easy to calculate average acceleration over a period of time when only speed is changing. Then acceleration is the change in velocity (represented by v) divided by the change in time (represented by t): acceleration = v t "
calculating acceleration from velocity and time,T_4189,"Look at the cyclist in the Figure 1.1. With the help of gravity, he speeds up as he goes downhill on a straight part of the trail. His velocity changes from 1 meter per second at the top of the hill to 6 meters per second by the time he reaches the bottom. If it takes him 5 seconds to reach the bottom, what is his average acceleration as he races down the hill? v t 6 m/s  1 m/s = 5s 5 m/s = 5s 1 m/s = 1s = 1 m/s2 acceleration = In words, this means that for each second the cyclist travels downhill, his velocity (in this case, his speed) increases by 1 meter per second on average. Note that the answer to this problem is expressed in m/s2 , which is the SI unit for acceleration. Q: The cyclist slows down at the end of the race. His velocity changes from 6 m/s to 2 m/s during a period of 4 seconds without any change in direction. What was his average acceleration during these 4 seconds? A: Use the equation given above for acceleration: v t 6 m/s  2 m/s = 4s 4 m/s = 4s 1 m/s = 1s = 1 m/s2 acceleration = "
calculating work,T_4195,Work is the use of force to move an object. It is directly related to both the force applied to the object and the distance the object moves. Work can be calculated with this equation: Work = Force x Distance. 
calculating work,T_4196,"The equation for work can be used to calculate work if force and distance are known. To use the equation, force is expressed in Newtons (N), and distance is expressed in meters (m). For example, assume that Clarissa uses 100 Newtons of force to push the mower and that she pushes it for a total of 200 meters as she cuts the grass in her grandmothers yard. Then, the amount of work Clarissa does is: Work = 100 N  200 m = 20,000 N  m Notice that the unit for work in the answer is the Newton  meter (N  m). This is the SI unit for work, also called the joule (J). One joule equals the amount of work that is done when 1 N of force moves an object over a distance of 1 m. Q: After Clarissa mows her grandmothers lawn, she volunteers to mow a neighbors lawn as well. If she pushes the mower with the same force as before and moves it over a total of 234 meters, how much work does she do mowing the neighbors lawn? A: The work Clarissa does can be calculated as: Work = 100 N  234 m = 23,400 N  m, or 23,400 J "
calculating work,T_4197,"The work equation given above can be rearranged to find force or distance if the other variables are known: Force = Work Distance Distance = Work Force After Clarissa finishes mowing both lawns, she pushes the lawn mower down the sidewalk to her own house. If she pushes the mower over a distance of 30 meters and does 2700 joules of work, how much force does she use? Substitute the known values into the equation for force: J Force = 2700 30 m = 90 N Q: When Clarissa gets back to her house, she hangs the 200-Newton lawn mower on some hooks in the garage (see the Figure 1.1). To lift the mower, she does 400 joules of work. How far does she lift the mower to hang it? A: Substitute the known values into the equation for distance: "
carbohydrate classification,T_4198,"Carbohydrates are one of four classes of biochemical compounds. The other three classes are proteins, lipids, and nucleic acids. In addition to cellulose, carbohydrates include sugars and starches. Carbohydrate molecules contain atoms of carbon, hydrogen, and oxygen. Living things use carbohydrates mainly for energy. Q: Which carbohydrates do you use for energy? A: You may eat a wide variety of carbohydratesfrom sugars in fruits to starches in potatoes. However, body cells use only sugars for energy. "
carbohydrate classification,T_4199,"Sugars are simple carbohydrates. Molecules of sugars have relatively few carbon atoms. Glucose (C6 H12 O6 ) is one of the smallest sugar molecules. Plants and some other organisms make glucose in the process of photosynthesis. Living things that cannot make glucose obtain it by consuming plants or these other organisms. In the Figure 1.1, you can see structural formulas for glucose and two other sugars, named fructose and sucrose. Fructose is a sugar that is found in fruits. It is an isomer of glucose. Isomers are compounds that have the same atoms but different arrangements of atoms. Do you see how the atoms are arranged differently in fructose than in glucose? Youre probably most familiar with the sugar sucrose, because sucrose is table sugar. Its the sugar that you spoon onto your cereal or into your iced tea. Q: Compare the structure of sucrose with the structures of glucose and fructose. How is sucrose related to the other two sugars? A: Sucrose consists of one molecule of glucose and one molecule of fructose bonded together. "
carbohydrate classification,T_4200,"Starches are complex carbohydrates. They are polymers of glucose. A polymer is a large molecule that consists of many smaller, repeating molecules, called monomers. The monomers are joined together by covalent bonds. Starches contain hundreds of glucose monomers. Plants make starches to store extra glucose. Consumers get starches by eating plants. Common sources of starches in the human diet are pictured in the Figure 1.2. Our digestive system breaks down starches to sugar, which our cells use for energy. "
carbohydrate classification,T_4201,"Cellulose is another complex carbohydrate that is a polymer of glucose. However, glucose molecules are bonded together differently in cellulose than they are in starches. Cellulose molecules bundle together to form long, tough fibers, as you can see in the Figure 1.3. Have you ever eaten raw celery? If you have, then you probably noticed that Foods that are good sources of starches. the stalks contain long, stringy fibers. The fibers are mostly cellulose. Cellulose is the most abundant biochemical compound. It makes up the cell walls of plants and gives support to stems and tree trunks. Cellulose also provides needed fiber in the human diet. We cant digest cellulose, but it helps keep food wastes moving through the digestive tract. "
carbon bonding,T_4202,"Carbon is a very common ingredient of matter because it can combine with itself and with many other elements. It can form a great diversity of compounds, ranging in size from just a few atoms to thousands of atoms. There are millions of known carbon compounds, and carbon is the only element that can form so many different compounds. "
carbon bonding,T_4203,"Carbon is a nonmetal in group 14 of the periodic table. Like other group 14 elements, carbon has four valence electrons. Valence electrons are the electrons in the outer energy level of an atom that are involved in chemical bonds. The valence electrons of carbon are shown in the electron dot diagram in the Figure 1.1. Q: How many more electrons does carbon need to have a full outer energy level? A: Carbon needs four more valence electrons, or a total of eight valence electrons, to fill its outer energy level. A full outer energy level is the most stable arrangement of electrons. Q: How can carbon achieve a full outer energy level? A: Carbon can form four covalent bonds. Covalent bonds are chemical bonds that form between nonmetals. In a covalent bond, two atoms share a pair of electrons. By forming four covalent bonds, carbon shares four pairs of electrons, thus filling its outer energy level and achieving stability. "
carbon bonding,T_4204,"A carbon atom can form covalent bonds with other carbon atoms or with the atoms of other elements. Carbon often forms bonds with hydrogen. Compounds that contain only carbon and hydrogen are called hydrocarbons. Methane (CH4 ), which is modeled in the Figure 1.2, is an example of a hydrocarbon. In methane, a single carbon atom forms covalent bonds with four hydrogen atoms. The diagram on the left in the Figure 1.2 shows all the shared valence electrons. The diagram on the right in the Figure 1.2, called a structural formula, represents each pair of shared electrons with a dash (-). Methane (CH4 ) "
carbon bonding,T_4205,"Carbon can form single, double, or even triple bonds with other carbon atoms. In a single bond, two carbon atoms share one pair of electrons. In a double bond, they share two pairs of electrons, and in a triple bond they share three pairs of electrons. Examples of compounds with these types of bonds are represented by the structural formulas in the Figure 1.3. Q: How many bonds do the carbon atoms share in each of these compounds? A: In ethane, the two carbon atoms share a single bond. In ethene they share a double bond, and in ethyne they share a triple bond. "
carbon monomers and polymers,T_4206,"Carbon has a unique ability to form covalent bonds with many other atoms. It can bond with other carbon atoms as well as with atoms of other elements. Because of this ability, carbon often forms polymers. A polymer is a large molecule that is made out of many smaller molecules that are joined together by covalent bonds. The smaller, repeating molecules are called monomers. (The prefix mono- means one and the prefix poly- means many.) Polymers may consist of just one type of monomer or of more than one type. Polymers are similar to the strings of beads pictured in the Figure 1.1. Like beads on a string, monomers in a polymer may be all the same or different from one another. "
carbon monomers and polymers,T_4207,"Many polymers of carbon occur naturally. Two examples are rubber and cellulose. Rubber is a natural polymer of the monomer named isoprene (C5 H8 ). This polymer comes from rubber trees, which grow in tropical areas. Structural formulas for rubber and isoprene are shown in the Figure 1.2. Note that just a small section of the rubber polymer is represented by the structural formula. Cellulose is a natural polymer of the monomer named glucose (C6 H12 O6 ). This polymer makes up the cell walls of plants and is the most common compound in living things. Structural formulas for cellulose and glucose are also shown in the Figure 1.2). As you can see from the structural formula for cellulose, when two glucose monomers bond together, a molecule of water (H2 O) is released. Q: How are the glucose molecules arranged in the cellulose polymer? A: The glucose molecules alternate between right-side up and upside down. "
carbon monomers and polymers,T_4208,Synthetic carbon polymers are produced in labs or factories. Plastics are common examples of synthetic carbon polymers. You are probably familiar with the plastic called polyethylene. All of the plastic items pictured in the Figure 1.3 are made of polyethylene. It consists of repeating monomers of ethylene (C2 H4 ). Structural formulas for ethylene and polyethylene are also shown in the Figure 1.4. Click image to the left or use the URL below. URL: 
catalysts,T_4209,"A catalyst is a substance that increases the rate of a chemical reaction. The presence of a catalyst is one of several factors that influence the rate of chemical reactions. (Other factors include the temperature, concentration, and surface area of reactants.) A catalyst isnt a reactant in the chemical reaction it speeds up. As a result, it isnt changed or used up in the reaction, so it can go on to catalyze many more reactions. Q: How is a catalyst like a tunnel through a mountain? A: Like a tunnel through a mountain, a catalyst provides a faster pathway for a chemical reaction to occur. "
catalysts,T_4210,"Catalysts interact with reactants so the reaction can occur by an alternate pathway that has a lower activation energy. Activation energy is the energy needed to start a reaction. When activation energy is lower, more reactant particles have enough energy to react so the reaction goes faster. Many catalysts work like the one in the Figure 1.1. The catalyst brings the reactants together by temporarily bonding with them. This makes it easier and quicker for the reactants to react together. Q: In the Figure 1.1, look at the energy needed in the catalytic and non-catalytic pathways of the reaction. How does the amount of energy compare? How does this affect the reaction rate along each pathway? A: The catalytic pathway of the reaction requires far less energy. Therefore, the reaction will occur faster by this pathway because more reactants will have enough energy to react. "
catalysts,T_4211,"Chemical reactions constantly occur inside living things. Many of these reactions require catalysts so they will occur quickly enough to support life. Catalysts in living things are called enzymes. Enzymes may be extremely effective. A reaction that takes a split second to occur with an enzyme might take many years without it! More than 1000 different enzymes are necessary for human life. Many enzymes are needed for the digestion of food. An example is amylase, which is found in the mouth and small intestine. Amylase catalyzes the breakdown of starch to sugar. You can see how it affects the rate of starch digestion in the Figure 1.2. A: The starches in the cracker start to break down to sugars with the help of the enzyme amylase. Try this yourself and see if you can taste the reaction. "
cellular respiration reactions,T_4212,"Cellular Respiration is the process in which the cells of living things break down the organic compound glucose with oxygen to produce carbon dioxide and water. The overall chemical equation for cellular respiration is: C6 H12 O6 + 6O2  6CO2 + 6H2 O As the Figure 1.1 shows, cellular respiration occurs in the cells of all kinds of organisms, including those that make their own food (autotrophs) as well as those that get their food by consuming other organisms (heterotrophs). Q: How is cellular respiration related to breathing? A: Breathing consists of inhaling and exhaling, and its purpose is to move gases into and out of the body. Oxygen needed for cellular respiration is brought into the body with each inhalation. Carbon dioxide and water vapor produced by cellular respiration are released from the body with each exhalation. "
cellular respiration reactions,T_4213,"The reactions of cellular respiration are catabolic reactions. In catabolic reactions, bonds are broken in larger molecules and energy is released. In cellular respiration, bonds are broken in glucose, and this releases the chemical energy that was stored in the glucose bonds. Some of this energy is converted to heat. The rest of the energy is used to form many small molecules of a compound called adenosine triphosphate, or ATP. ATP molecules contain just the right amount of stored chemical energy to power biochemical reactions inside cells. Click image to the left or use the URL below. URL: "
chemical bond,T_4220,A chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom that may be involved in chemical interactions. Valence electrons are the basis of all chemical bonds. Q: Why do you think that chemical bonds form? A: Chemical bonds form because they give atoms a more stable arrangement of electrons. 
chemical bond,T_4221,"To understand why chemical bonds form, consider the common compound known as water, or H2 O. It consists of two hydrogen (H) atoms and one oxygen (O) atom. As you can see in the on the left side of the Figure 1.1, each hydrogen atom has just one electron, which is also its sole valence electron. The oxygen atom has six valence electrons. These are the electrons in the outer energy level of the oxygen atom. In the water molecule on the right in the Figure 1.1, each hydrogen atom shares a pair of electrons with the oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. The hydrogen atoms each have a pair of shared electrons, so their first and only energy level is full. The oxygen atom has a total of eight valence electrons, so its outer energy level is full. A full outer energy level is the most stable possible arrangement of electrons. It explains why elements form chemical bonds with each other. "
chemical bond,T_4222,"Not all chemical bonds form in the same way as the bonds in water. There are actually three different types of chemical bonds, called covalent, ionic, and metallic bonds. Each type of bond is described below. Click image to the left or use the URL below. URL:  A covalent bond is the force of attraction that holds together two nonmetal atoms that share a pair of electrons. One electron is provided by each atom, and the pair of electrons is attracted to the positive nuclei of both atoms. The water molecule represented in the Figure 1.1 contains covalent bonds. An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals instead of molecules. Table salt contains ionic bonds. A metallic bond is the force of attraction between a positive metal ion and the valence electrons that surround itboth its own valence electrons and those of other ions of the same metal. The ions and electrons form a lattice-like structure. Only metals, such as the copper pictured in the Figure 1.2, form metallic bonds. "
chemical equations,T_4226,"A chemical equation is a shorthand way to sum up what occurs in a chemical reaction. The general form of a chemical equation is: Reactants  Products The reactants in a chemical equation are the substances that begin the reaction, and the products are the substances that are produced in the reaction. The reactants are always written on the left side of the equation and the products on the right. The arrow pointing from left to right shows that the reactants change into the products during the reaction. This happens when chemical bonds break in the reactants and new bonds form in the products. As a result, the products are different chemical substances than the reactants that started the reaction. Q: What is the general equation for the reaction in which iron rusts? A: Iron combines with oxygen to produce rust, which is the compound named iron oxide. This reaction could be represented by the general chemical equation below. Note that when there is more than one reactant, they are separated by plus signs (+). If more than one product were produced, plus signs would be used between them as well. Iron + Oxygen  Iron Oxide "
chemical equations,T_4227,"When scientists write chemical equations, they use chemical symbols and chemical formulas instead of names to represent reactants and products. Look at the chemical reaction illustrated in the Figure 1.1. In this reaction, carbon reacts with oxygen to produce carbon dioxide. Carbon is represented by the chemical symbol C. The chemical symbol for oxygen is O, but pure oxygen exists as diatomic (two-atom) molecules, represented by the chemical formula O2 . A molecule of the compound carbon dioxide consists of one atom of carbon and two atoms of oxygen, so carbon dioxide is represented by the chemical formula CO2 . Q: What is the chemical equation for this reaction? A: The chemical equation is: C + O2  CO2 Q: How have the atoms of the reactants been rearranged in the products of the reaction? What bonds have been broken, and what new bonds have formed? A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms. "
chemical equations,T_4228,"All chemical equations, like equations in math, must balance. This means that there must be the same number of each type of atom on both sides of the arrow. Thats because matter is always conserved in a chemical reaction. This is the law of conservation of mass. Look at the equation above for the reaction between carbon and oxygen in the formation of carbon dioxide. Count the number of atoms of each type. Are the numbers the same on both sides of the arrow? The answer is yes, so the equation is balanced. "
chemical equations,T_4229,"Lets return to the chemical reaction in which iron (Fe) combines with oxygen (O2 ) to form rust, or iron oxide (Fe2 O3 ). The equation for this reaction is: 4Fe+ 3O2  2Fe2 O3 This equation illustrates the use of coefficients to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula that shows how many atoms or molecules of the substance are involved in the reaction. From the equation for rusting, you can see that four atoms of iron combine with three molecules of oxygen to form two molecules of iron oxide. Q: Is the equation for the rusting reaction balanced? How can you tell? A: Yes, the equation is balanced. You can tell because there is the same number of each type of atom on both sides of the arrow. First count the iron atoms. There are four iron atoms in the reactants. There are also four iron atoms in the products (two in each of the two iron oxide molecules). Now count the oxygen atoms. There are six on each side of the arrow, confirming that the equation is balanced in terms of oxygen as well as iron. "
chemical formula,T_4230,"In a chemical formula, the elements in a compound are represented by their chemical symbols, and the ratio of different elements is represented by subscripts. Consider the compound water as an example. Each water molecule contains two hydrogen atoms and one oxygen atom. Therefore, the chemical formula for water is: H2 O The subscript 2 after the H shows that there are two atoms of hydrogen in the molecule. The O for oxygen has no subscript. When there is just one atom of an element in a molecule, no subscript is used in the chemical formula. "
chemical formula,T_4231,"The Table 1.1 shows four examples of compounds and their chemical formulas. The first two compounds are ionic compounds, and the second two are covalent compounds. Each formula shows the ratio of ions or atoms that make up the compound. Name of Compound Type of Compound Sodium chloride ionic Calcium iodide ionic Hydrogen peroxide covalent Carbon dioxide covalent Ratio of Ions or Atoms of Each Element 1 sodium ion (Na+ ) 1 chloride ion (Cl ) 1 calcium ion (Ca2+ ) 2 io- dide ions (I ) 2 hydrogen atoms (H) 2 oxygen atoms (O) 1 carbon atom (C) 2 oxy- gen atoms (O) Chemical Formulas NaCl CaI2 H2 O2 CO2 There is a different rule for writing the chemical formula for each type of compound. Ionic compounds are compounds in which positive metal ions and negative nonmetal ions are joined by ionic bonds. In these compounds, the chemical symbol for the positive metal ion is written first, followed by the symbol for the negative nonmetal ion. Click image to the left or use the URL below. URL:  Q: The ionic compound lithium fluoride consists of a ratio of one lithium ion (Li+ ) to one fluoride ion (F ). What is the chemical formula for this compound? A: The chemical formula is LiF. Covalent compounds are compounds in which nonmetals are joined by covalent bonds. In these compounds, the element that is farther to the left in the periodic table is written first, followed by the element that is farther to the right. If both elements are in the same group of the periodic table, the one with the higher period number is written first. Click image to the left or use the URL below. URL:  Q: A molecule of the covalent compound nitrogen dioxide consists of one nitrogen atom (N) and two oxygen atoms (O). What is the chemical formula for this compound? A: The chemical formula is NO2 . "
chemical reaction overview,T_4235,"A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right). Reactants  Products Chemical reactions may occur quickly or slowly. Look at the two pictures in the Figure 1.1. Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust. Q: What happens during a chemical reaction? Where do the reactants go, and where do the products come from? A: During a chemical reaction, chemical changes take place. Some chemical bonds break and new chemical bonds form. "
chemical reaction overview,T_4236,"The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products. Consider the chemical reaction in which water forms from oxygen and hydrogen gases. The Figure 1.2 represents this reaction. Bonds break in molecules of hydrogen and oxygen, and then new bonds form in molecules of water. In both reactants and products there are four hydrogen atoms and two oxygen atoms, but the atoms are combined differently in water. "
chemical reaction overview,T_4237,"The chemical reaction in the Figure 1.2, in which water forms from hydrogen and oxygen, is an example of a synthesis reaction. In this type of reaction, two or more reactants combine to synthesize a single product. There are several other types of chemical reactions, including decomposition, replacement, and combustion reactions. The Table 1.1 compares these four types of chemical reactions. Type of Reaction Synthesis Decomposition General Equation A+B  C AB  A + B Example 2Na + Cl2  2NaCl 2H2 O 2H2 + O2 Type of Reaction Single Replacement Double Replacement Combustion General Equation A+BC  B+ AC AB+ CD  AD + CB fuel + oxygen  carbon dioxide + water Example 2K + 2H2 O  2KOH + H2 NaCl+ AgF  NaF + AgCl CH4 + 2O2  CO2 + 2H2 O Q: The burning of wood is a chemical reaction. Which type of reaction is it? A: The burning of woodor of anything elseis a combustion reaction. In the combustion example in the table, the fuel is methane gas (CH4 ). Click image to the left or use the URL below. URL: "
chemical reaction overview,T_4238,"All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions. In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light. In endothermic reactions, more energy is used to break bonds in reactants than is released when bonds form in products. These reactions absorb energy from the environment. Q: When it comes to energy, which type of reaction is the burning of wood? Is it an endothermic reaction or an exothermic reaction? How can you tell? A: The burning of wood is an exothermic reaction. You can tell by the heat and light energy given off by a wood fire. "
chemical reaction rate,T_4239,How fast a chemical reaction occurs is called the reaction rate. Several factors affect the rate of a given chemical reaction. They include the: temperature of reactants. concentration of reactants. surface area of reactants. presence of a catalyst. 
chemical reaction rate,T_4240,"When the temperature of reactants is higher, the rate of the reaction is faster. At higher temperatures, particles of reactants have more energy, so they move faster. As a result, they are more likely to bump into one another and to collide with greater force. For example, food spoils because of chemical reactions, and these reactions occur faster at higher temperatures (see the bread on the left in the Figure 1.1). This is why we store foods in the refrigerator or freezer (like the bread on the right in the Figure 1.1). The lower temperature slows the rate of spoilage. Left image: Bread after 1 month on a warm countertop. Right image: Bread after 1 month in a cold refrigerator. "
chemical reaction rate,T_4241,"Concentration is the number of particles of a substance in a given volume. When the concentration of reactants is higher, the reaction rate is faster. At higher concentrations, particles of reactants are crowded closer together, so they are more likely to collide and react. Did you ever see a sign like the one in the Figure 1.2? You might see it where someone is using a tank of pure oxygen for a breathing problem. Combustion, or burning, is a chemical reaction in which oxygen is a reactant. A greater concentration of oxygen in the air makes combustion more rapid if a fire starts burning. Q: It is dangerous to smoke or use open flames when oxygen is in use. Can you explain why? A: Because of the higher-than-normal concentration of oxygen, the flame of a match, lighter, or cigarette could spread quickly to other materials or even cause an explosion. "
chemical reaction rate,T_4242,"When a solid substance is involved in a chemical reaction, only the matter at the surface of the solid is exposed to other reactants. If a solid has more surface area, more of it is exposed and able to react. Therefore, increasing the surface area of solid reactants increases the reaction rate. Look at the hammer and nails pictured in the Figure 1.3. Both are made of iron and will rust when the iron combines with oxygen in the air. However, the nails have a greater surface area, so they will rust faster. "
chemical reaction rate,T_4243,"Some reactions need extra help to occur quickly. They need another substance called a catalyst. A catalyst is a substance that increases the rate of a chemical reaction. A catalyst isnt a reactant, so it isnt changed or used up in the reaction. Therefore, it can catalyze many other reactions. "
chemistry of compounds,T_4244,"A compound is a unique substance that forms when two or more elements combine chemically. Compounds form as a result of chemical reactions. The elements in compounds are held together by chemical bonds. A chemical bond is a force of attraction between atoms or ions that share or transfer valence electrons. Click image to the left or use the URL below. URL:  Water is an example of a common chemical compound. As you can see in the Figure 1.1, each water molecule consists of two atoms of hydrogen and one atom of oxygen. Water always has this 2:1 ratio of hydrogen to oxygen. Like water, all compounds consist of a fixed ratio of elements. It doesnt matter how much or how little of a compound there is. It always has the same composition. Q: Sometimes the same elements combine in different ratios. How can this happen if a compound always consists of the same elements in the same ratio? A: If the same elements combine in different ratios, they form different compounds. "
chemistry of compounds,T_4245,"Look at the Figure 1.2 of water (H2 O) and hydrogen peroxide (H2 O2 ), and read about these two compounds. Both compounds consist of hydrogen and oxygen, but they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If youve ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water! Q: Read the Figure 1.3 about carbon dioxide (CO2 ) and carbon monoxide (CO). Both compounds consist of carbon and oxygen, but in different ratios. How can you tell that carbon dioxide and carbon monoxide are different compounds? Carbon Dioxide: Every time you exhale, you release carbon dioxide into the air. Its an odorless, colorless gas. Car- bon dioxide contributes to global climate change, but it isnt directly harmful to hu- man health. Carbon Monoxide: Carbon monoxide is produced when matter burns. Its a colorless, odorless gas that is very harmful to human health. In fact, it can kill people in minutes. Because you cant see or smell carbon monoxide, it must be detected with an alarm. "
chemistry of compounds,T_4246,"There are two basic types of compounds that differ in the nature of the bonds that hold their atoms or ions together. They are covalent and ionic compounds. Both types are described below. Click image to the left or use the URL below. URL:  Covalent compounds consist of atoms that are held together by covalent bonds. These bonds form between nonmetals that share valence electrons. Covalent compounds exist as individual molecules. Water is an example of a covalent compound. Ionic compounds consist of ions that are held together by ionic bonds. These bonds form when metals transfer electrons to nonmetals. Ionic compounds exist as a matrix of many ions, called a crystal. Sodium chloride (table salt) is an example of an ionic compound. "
color,T_4247,"Visible light is light that has wavelengths that can be detected by the human eye. The wavelength of visible light determines the color that the light appears. As you can see in the Figure 1.1, light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between are all the other colors of light that we can see. Only seven main colors of light are actually represented in the diagram. "
color,T_4248,"A prism, like the one in the Figure 1.2, can be used to separate visible light into its different colors. A prism is a pyramid-shaped object made of transparent matter, usually clear glass or plastic. Matter that is transparent allows light to pass through it. A prism transmits light but slows it down. When light passes from air to the glass of the prism, the change in speed causes the light to change direction and bend. Different wavelengths of light bend at different angles. This makes the beam of light separate into light of different wavelengths. What we see is a rainbow of colors. Q: Look back at the rainbow that opened this article. Do you see all the different colors of light, from red at the top to violet at the bottom? What causes a rainbow to form? A: Individual raindrops act as tiny prisms. They separate sunlight into its different wavelengths and create a rainbow of colors. "
color,T_4249,"An opaque object is one that doesnt let light pass through it. Instead, it reflects or absorbs the light that strikes it. Many objects, such as the leaves pictured in the Figure 1.3, reflect just one or a few wavelengths of visible light and absorb the rest. The wavelengths that are reflected determine the color that an object appears to the human eye. For example, the leaves appear green because they reflect green light and absorb light of other wavelengths. A transparent or translucent material, such as window glass, transmits some or all of the light that strikes it. This means that the light passes through the material rather than being reflected by it. In this case, we see the material because of the transmitted light. Therefore, the wavelength of the transmitted light determines the color that the object appears. Look at the beautiful stained glass windows in the Figure 1.4. The different colors of glass transmit The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes green leaves, the blue light is absorbed and no light is reflected. Q: What color do you see if an object absorbs all of the light that strikes it? A: When all of the light is absorbed, none is reflected, so the object looks black. But black isnt a color of light. Black is the absence of light. "
color,T_4250,"The human eye can distinguish only red, green, and blue light. These three colors are called the primary colors of light. All other colors of light can be created by combining the primary colors. Look at the Venn diagram 1.5. Red and green light combine to form yellow light. Red and blue light combine to form magenta light, and blue and green light combine to form cyan light. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram, where all three primary colors of light combine. The result is white light. "
color,T_4251,"Many objects have color because they contain pigments. A pigment is a substance that colors materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. A very common pigment is the dark green pigment called chlorophyll, which is found in plants. Chlorophyll absorbs all but green wavelengths of visible light. Pigments are also found in many manufactured products. They are used to color paints, inks, and dyes. Just three pigments, called primary pigments, can be combined to produce all other colors. The primary colors of pigments are the same as the secondary colors of light: cyan, magenta, and yellow. Q: A color printer needs just three colors of ink to print all of the colors that we can see. Which colors are they? A: The three colors of ink in a color printer are the three primary pigment colors: cyan, magenta, and yellow. These three colors can be combined in different ratios to produce all other colors, so they are the only colors needed for full-color printing. "
combining forces,T_4252,"You have probably heard of the famous equation E = mc2 . The ""E"" represent the amount of energy. The ""m"" represents mass. The ""c"" represent the speed of light. Writing a ""c"" is much easier than writing the actual speed of light. The speed of light is a really large number. The speed of light is about 300 million meters per second. Thats really, really fast. Light always travels at the same speed through space. In outer space, there is not any matter to get in its way. Think about riding your bicycle. When you ride on a hard surface, it is easy to pedal. You can go really fast. Imagine how your speed would change if you were riding through deep sand. You would find it hard to pedal. You would not be able to go as fast. The same is true for light. When there is no matter around, like in outer space, it can go fast. When matter gets in its way, it slows down. Light travels through some matter faster than through others. Table 1.1 gives the speed of light in six common materials. Material Air Water Glass Vegetable oil Alcohol Diamond Speed of Light (m/s) 299 million meters per second 231 million meters per second 200 million meters per second 150 million meters per second 140 million meters per second 125 million meters per second No matter how slow light travels, it still goes really, really fast. The important thing to remember is that it does travel. It is hard for us to imagine light taking time to cover a distance. Think about when you enter your science classroom. You step through the door. You tell your teacher, ""Hello."" You walk to your desk and sit down. It may take around 10 to 20 seconds to walk this distance. Imagine now your teacher turns the light off. She carries a small lamp over to the door you just entered. She asks you to watch carefully as she switches on the light. She flips the switch and you immediately see the light. The light just covered the same distance you just walked. Thats how fast light is. For us, it is hard to imagine that it moves. Now lets think about light traveling between the Sun and Earth. The Sun is 93 million miles away. What if we were able to turn off the Sun for just a second? How long would it take us to notice? Would we notice instantly like in the classroom? Remember, the Sun is a long way away. We wouldnt notice the change for a little over 8 minutes. That is because the Sun is a long way away. Even when moving as fast as light, it takes time to travel from the Sun to Earth. What do you think happens when it hits the air in our atmosphere? Air is made up of matter. When light travels through matter it slows down. How do scientists know it slows down? What evidence do scientists have? When sunlight hits Earths atmosphere it bends just a little. If sunlight goes through water droplets it bends even more. The bending of light through droplets of water is why we can see rainbows. It also explains why the straw in a glass of water appears to be broken. "
combining forces,T_4253,"When light passes from one medium (or type of matter) to another, it changes speed. You can actually see this happen. If light strikes a new substance at an angle, the light appears to bend. This is what explains the straw looking broken in the picture above. So, does light always bend as it travels into a new medium? If light travels straight into a new substance it is not bent. You may know this angle as perpendicular. The light still slows down, just does not appear to bend. Any angle other than perpendicular the light will bend as it slows down. The bending of light is called refraction. Figure 1.1 shows how refraction occurs. Notice that the angle of light changes again as it passes from the glass back to the air. In this case, the speed increases, and the ray of light resumes its initial direction. For a more detailed explanation of refraction, watch this video: Click image to the left or use the URL below. URL: "
combustion reactions,T_4254,"A combustion reaction occurs when a substance reacts quickly with oxygen (O2 ). For example, in the Figure usually referred to as fuel. The products of a complete combustion reaction include carbon dioxide (CO2 ) and water vapor (H2 O). The reaction typically gives off heat and light as well. The general equation for a complete combustion reaction is: Fuel + O2  CO2 + H2 O The burning of charcoal is a combustion reaction. "
combustion reactions,T_4255,"The fuel that burns in a combustion reaction contains compounds called hydrocarbons. Hydrocarbons are compounds that contain only carbon (C) and hydrogen (H). The charcoal pictured in the Figure 1.1 consists of hydrocarbons. So do fossil fuels such as natural gas. Natural gas is a fuel that is commonly used in home furnaces and gas stoves. The main component of natural gas is the hydrocarbon called methane (CH4 ). You can see a methane flame in the Figure 1.2. The combustion of methane is represented by the equation: CH4 + 2O2  CO2 + 2H2 O The combustion of methane gas heats a pot on a stove. Q: Sometimes the flame on a gas stove isnt just blue but has some yellow or orange in it. Why might this occur? A: If the flame isnt just blue, the methane isnt getting enough oxygen to burn completely, leaving some of the carbon unburned. The flame will also not be as hot as a completely blue flame for the same reason. "
compound machine,T_4258,"A compound machine is a machine that consists of more than one simple machine. Some compound machines consist of just two simple machines. You can read below about two examplesthe wheelbarrow and corkscrew. Other compound machines, such as bicycles, consist of many simple machines. Big compound machines such as cars may consist of hundreds or even thousands of simple machines. "
compound machine,T_4259,"Look at the wheelbarrow in the Figure 1.1. It is used to carry heavy objects. It consists of two simple machines: a lever and a wheel and axle. Effort is applied to the lever by picking up the handles of the wheelbarrow. The lever, in turn, applies upward force to the load. The force is increased by the lever, making the load easier to lift. Effort is applied to the wheel of the wheelbarrow by pushing it over the ground. The rolling wheel turns the axle and increases the force, making it easier to push the load. "
compound machine,T_4260,"The corkscrew in the Figure 1.2 is also a compound machine. It is used to pierce a cork and pull it out of the neck of a bottle. It consists of a screw and two levers. Turning the handle on top twists the screw down into the center of the cork. Then, pushing down on the two levers causes the screw to pull upward, bringing the cork with it. The levers increase the force and change its direction. "
compound machine,T_4261,"Friction is a force that opposes motion between any surfaces that are touching. All machines have moving parts and friction, so they have to use some of the work that is applied to them to overcome friction. This makes all machines less than 100 percent efficient. Because compound machines have more moving parts than simple machines, they generally have more friction to overcome. As a result, compound machines tend to have lower efficiency than simple machines. When a compound machine consists of many simple machines, friction can become a serious problem, and it may produce a lot of heat. Lubricants such as oil or grease may be used to coat the moving parts of a machine so they slide over each other more easily. This is how friction is reduced in a car engine. "
compound machine,T_4262,"The mechanical advantage of a machine is the factor by which it changes the force applied to the machine. Many machines increase the force applied to them, and this is how they make work easier. Compound machines tend to have a greater mechanical advantage than simple machines. Thats because the mechanical advantage of a compound machine equals the product of the mechanical advantages of all its component simple machines. The greater the number of simple machines it contains, the greater its mechanical advantage tends to be. Q: Assume that the lever and the wheel and axle of a wheelbarrow each have a mechanical advantage of 2. What is the mechanical advantage of the wheelbarrow? A: The mechanical advantage of the wheelbarrow is the product of the mechanical advantage of the lever (2) and the mechanical advantage of the wheel and axle (2). Therefore, the wheelbarrow has a mechanical advantage of 4. "
compounds,T_4263,"A compound is a unique substance that forms when two or more elements combine chemically. For example, the compound carbon dioxide forms when one atom of carbon (grey in the model above) combines with two atoms of oxygen (red in the model). Another example of a compound is water. It forms when two hydrogen atoms combine with one oxygen atom. Click image to the left or use the URL below. URL:  Q: How could a water molecule be represented? A: It could be represented by a model like the one for carbon dioxide in the opening image. You can see a sample Figure 1.1. A model of water. Two things are true of all compounds: A compound always has the same elements in the same proportions. For example, carbon dioxide always has two atoms of oxygen for each atom of carbon, and water always has two atoms of hydrogen for each atom of oxygen. A compound always has the same composition throughout. For example, all the carbon dioxide in the atmosphere and all the water in the ocean have these same proportions of elements. Q: How do you think the properties of compounds compare with the properties of the elements that form them? A: You might expect the properties of a compound to be similar to the properties of the elements that make up the compound. But you would be wrong. "
compounds,T_4264,"The properties of compounds are different from the properties of the elements that form themsometimes very different. Thats because elements in a compound combine and become an entirely different substance with its own unique properties. Do you put salt on your food? Table salt is the compound sodium chloride. It contains sodium and chlorine. As shown in the Figure 1.2, sodium is a solid that reacts explosively with water, and chlorine is a poisonous gas. But together in table salt, sodium and chlorine form a harmless unreactive compound that you can safely eat. Q: The compound sodium chloride is very different from the elements sodium and chlorine that combine to form it. What are some properties of sodium chloride? A: Sodium chloride is an odorless white solid that is harmless unless consumed in large quantities. In fact, it is a necessary component of the human diet. "
compounds,T_4265,"Compounds like sodium chloride form structures called crystals. A crystal is a rigid framework of many ions locked together in a repeating pattern. Ions are electrically charged forms of atoms. You can see a crystal of sodium chloride in the Figure 1.3. It is made up of many sodium and chloride ions. Sodium and chlorine combine to form sodium chloride, or table salt. A sodium chloride crystal consists of many sodium ions (blue) and chloride ions (green) arranged in a rigid framework. Click image to the left or use the URL below. URL:  Compounds such as carbon dioxide and water form molecules instead of crystals. A molecule is the smallest particle of a compound that still has the compounds properties. It consists of two or more atoms bonded together. You saw models of carbon dioxide and water molecules above. "
conservation of energy in chemical reactions,T_4269,"All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. Like the combustion reaction in a furnace, some chemical reactions require less energy to break bonds in reactants than is released when bonds form in products. These reactions, called exothermic reactions, release energy. In other chemical reactions, it takes more energy to break bonds in reactants than is released when bonds form in products. These reactions, called endothermic reactions, absorb energy. "
conservation of energy in chemical reactions,T_4270,"Whether a chemical reaction absorbs or releases energy, there is no overall change in the amount of energy during the reaction. Thats because energy cannot be created or destroyed. This is the law of conservation of energy. Energy may change form during a chemical reactionfor example, from chemical energy to heat energy when gas burns in a furnacebut the same amount of energy remains after the reaction as before. This is true of all chemical reactions. Q: If energy cant be destroyed during a chemical reaction, what happens to the energy that is absorbed in an endothermic reaction? A: The energy is stored in the bonds of the products as chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. This is represented by the graph on the left in the Figure 1.1. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. You can see this in the graph on the right in the Figure 1.1. Note: H represents the change in en- ergy. Q: What happens to the excess energy in the reactants of an exothermic reaction? A: The excess energy is generally released to the surroundings when the reaction occurs. In a home heating system, for example, the energy that is released during combustion in the furnace is used to heat the home. "
conservation of mass and energy in nuclear reactions,T_4273,"Einsteins equation is possibly the best-known equation of all time. Theres reason for that. The equation is incredibly important. It changed how scientists view energy and matter, which are two of the most basic concepts in all of science. The equation shows that energy and matter are two forms of the same thing. This new idea turned science upside down when Einstein introduced it in the early 1900s. Amazingly, the idea has withstood the test of time as more and more evidence has been gathered to support it. You can listen to an explanation of Einsteins equation at URL: https://youtu.be/hW7DW9NIO9M Q: What do the letters in Einsteins equation stand for? A: E stands for energy, m stands for mass, and c stands for the speed of light. The speed of light is 300,000 kilometers (186,000 miles) per second, so c2 is a very big number. Therefore, the amount of energy in even a small mass of matter is tremendous. Suppose, for example, that you have 1 gram of matter. Thats about the mass of a paperclip. Multiplying this mass by c2 would yield enough energy to power 3,600 homes for a year! "
conservation of mass and energy in nuclear reactions,T_4274,"Einsteins equation helps scientists understand what happens in nuclear reactions and why they produce so much energy. When the nucleus of a radioisotope undergoes fission or fusion in a nuclear reaction, it loses a tiny amount of mass. What happens to the lost mass? It isnt really lost at all. It is converted to energy. How much energy? E = mc2 . The change in mass is tiny, but it results in a great deal of energy. Q: In a nuclear reaction, mass decreases and energy increases. What about the laws of conservation of mass and conservation of energy? Are mass and energy not conserved in nuclear reactions? Do we need to throw out these laws when it comes to nuclear reactions? A: No, the laws still apply. However, its more correct to say that the sum of mass and energy is always conserved in a nuclear reaction. Mass changes to energy, but the total amount of mass and energy combined remains the same. "
conservation of mass in chemical reactions,T_4275,"A chemical reaction occurs when some substances change chemically to other substances. Chemical reactions are represented by chemical equations. Consider a simple chemical reaction, the burning of methane. In this reaction, methane (CH4 ) combines with oxygen (O2 ) in the air and produces carbon dioxide (CO2 ) and water vapor (H2 O). The reaction is represented by the following chemical equation: CH4 + 2O2  CO2 + 2H2 O This equation shows that one molecule of methane combines with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water vapor. All chemical equations must be balanced. This means that the same number of each type of atom must appear on both sides of the arrow. Q: Is the chemical equation for the burning of methane balanced? Count the atoms of each type on both sides of the arrow to find out. A: Yes, the equation is balanced. There is one carbon atom on both sides of the arrow. There are also four hydrogen atoms and four oxygen atoms on both sides of the arrow. "
conservation of mass in chemical reactions,T_4276,"Why must chemical equations be balanced? Its the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants. Balanced chemical equations show that mass is conserved in chemical reactions. "
conservation of mass in chemical reactions,T_4277,"How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar, like the one in the Figure 1.1. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions. Another outcome of Lavoisiers research was the discovery of oxygen. Click image to the left or use the URL below. URL:  Q: Lavoisier carried out his experiments inside a sealed glass jar. Why was sealing the jar important for his results? What might his results have been if he hadnt sealed the jar? A: Sealing the jar was important so that any gases produced in the reactions were captured and could be measured. If he hadnt sealed the jar, gases might have escaped detection. Then his results would have shown that there was less mass after the reactions than before. In other words, he would not have been able to conclude that mass is conserved in chemical reactions. "
convection,T_4278,"Convection is the transfer of thermal energy by particles moving through a fluid (either a gas or a liquid). Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Convection is one of three ways that thermal energy can be transferred (the other ways are conduction and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
convection,T_4279,"The Figure 1.1 shows how convection occurs, using hot water in a pot as an example. When particles in one area of a fluid (in this case, the water at the bottom of the pot) gain thermal energy, they move more quickly, have more collisions, and spread farther apart. This decreases the density of the particles, so they rise up through the fluid. As they rise, they transfer their thermal energy to other particles of the fluid and cool off in the process. With less energy, the particles move more slowly, have fewer collisions, and move closer together. This increases their density, so they sink back down through the fluid. When they reach the bottom of the fluid, the cycle repeats. The result is a loop of moving particles called a convection current. "
convection,T_4280,"Convection currents transfer thermal energy through many fluids, not just hot water in a pot. For example, convection currents transfer thermal energy through molten rock below Earths surface, through water in the oceans, and through air in the atmosphere. Convection currents in the atmosphere create winds. You can see one way this happens in the Figure 1.2. The land heats up and cools off faster than the water because it has lower specific heat. Therefore, the land gets warmer during the day and cooler at night than the water does. During the day, warm air rises above the land and cool air from the water moves in to take its place. During the night, the opposite happens. Warm air rises above the water and cool air from the land moves out to take its place. Q: During the day, in which direction is thermal energy of the air transferred? In which direction is it transferred during the night? A: During the day, thermal energy is transferred from the air over the land to the air over the water. During the night, thermal energy is transferred in the opposite direction. "
cooling systems,T_4281,"A refrigerator is an example of a cooling system. Another example is an air conditioner. The purpose of any cooling system is to transfer thermal energy in order to keep things cool. A refrigerator, for example, transfers thermal energy from the cool air inside the refrigerator to the warm air in the kitchen. If youve ever noticed how warm the back of a running refrigerator gets, then you know that it releases a lot of thermal energy into the room. Q: Thermal energy always moves from a warmer area to a cooler area. How can thermal energy move from the cooler air inside a refrigerator to the warmer air in a room? A: The answer is work. "
cooling systems,T_4282,"A refrigerator must do work to reverse the normal direction of thermal energy flow. Work involves the use of force to move something, and doing work takes energy. In a refrigerator, the energy is usually provided by electricity. You can read in detail in the Figure 1.1 how a refrigerator does its work. "
cooling systems,T_4283,"The key to how a refrigerator or other cooling system works is the refrigerant. A refrigerant is a substance such as FreonTM that has a low boiling point and changes between liquid and gaseous states as it passes through the refrigerator. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator and changes to a gas. As a gas, it transfers thermal energy to the warm air outside the refrigerator and changes back to a liquid. Work is done by a refrigerator to move the refrigerant through the different components of the refrigerator. "
covalent bonding,T_4284,A covalent bond is the force of attraction that holds together two atoms that share a pair of valence electrons. The shared electrons are attracted to the nuclei of both atoms. This forms a molecule consisting of two or more atoms. Covalent bonds form only between atoms of nonmetals. 
covalent bonding,T_4285,"The two atoms that are held together by a covalent bond may be atoms of the same element or different elements. When atoms of different elements form covalent bonds, a new substance, called a covalent compound, results. Water is an example of a covalent compound. A water molecule is modeled in the Figure 1.1. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. Q: How many valence electrons does the oxygen atom (O) share with each hydrogen atom (H)? How many covalent bonds hold the water molecule together? A: The oxygen atom shares one pair of valence electrons with each hydrogen atom. Each pair of shared electrons represents one covalent bond, so two covalent bonds hold the water molecule together. The diagram in the Figure 1.2 shows an example of covalent bonds between two atoms of the same element, in this case two atoms of oxygen. The diagram represents an oxygen molecule, so its not a new compound. Oxygen normally occurs in diatomic (two-atom) molecules. Several other elements also occur as diatomic molecules: hydrogen, nitrogen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). Q: How many electrons do these two oxygen atoms share? How many covalent bonds hold the oxygen molecule together? A: The two oxygen atoms share two pairs of electrons, so two covalent bonds hold the oxygen molecule together. "
covalent bonding,T_4286,"Covalent bonds form because they give atoms a more stable arrangement of electrons. Look at the oxygen atoms in the Figure 1.2. Alone, each oxygen atom has six valence electrons. By sharing two pairs of valence electrons, each oxygen atom has a total of eight valence electrons. This fills its outer energy level, giving it the most stable arrangement of electrons. The shared electrons are attracted to both oxygen nuclei, and this force of attraction holds the two atoms together in the oxygen molecule. "
crystalline carbon,T_4287,"Graphite is one of three forms of crystalline, or crystal-forming, carbon. Carbon also exists in an amorphous, or shapeless, form in substances such as coal and charcoal. Different forms of the same element are called allotropes. Besides graphite, the other allotropes of crystalline carbon are diamond and fullerenes. All three forms exist as crystals rather than molecules. In a crystal, many atoms are bonded together in a repeating pattern that may contains thousands of atoms. The arrangement of atoms in the crystal differs for each form of carbon and explains why the different forms have different properties. Click image to the left or use the URL below. URL:  Q: How do you think the properties of diamond might differ from the properties of graphite? A: Diamond is clear whereas graphite is black. Diamond is also very hard, so it doesnt break easily. Graphite, in contrast, is soft and breaks very easily. "
crystalline carbon,T_4288,"Diamond is a form of carbon in which each carbon atom is covalently bonded to four other carbon atoms. This forms a strong, rigid, three-dimensional structure (see Figure 1.1). Diamond is the hardest natural substance, and no other natural substance can scratch it. This property makes diamonds useful for cutting and grinding tools as well as for rings and other jewelry (see Figure 1.2). "
crystalline carbon,T_4289,"Graphite is a form of crystalline carbon in which each carbon atom is covalently bonded to three other carbon atoms. The carbon atoms are arranged in layers, with strong bonds within each layer but only weak bonds between layers (see Figure 1.3). The weak bonds between layers allow the layers to slide over one another, so graphite is relatively soft and slippery. This makes it useful as a lubricant. Q: Why do graphites properties make it useful for pencil leads? A: Being slippery, graphite slides easily over paper when you write. Being soft, it rubs off on the paper, allowing you to leave marks. Graphites softness also allows you to sharpen it easily. (Imagine trying to sharpen a diamond!) "
crystalline carbon,T_4290,"A fullerene (also called a Bucky ball) is a form of carbon in which carbon atoms are arranged in a hollow sphere resembling a soccer ball (see Figure 1.4). Each sphere contains 60 carbon atoms, and each carbon atom is bonded to three others by single covalent bonds. The bonds are relatively weak, so fullerenes can dissolve and form solutions. Fullerenes were first discovered in 1985 and have been found in soot and meteorites. Possible commercial uses of fullerenes are under investigation. Fullerene Crystal "
daltons atomic theory,T_4291,"Around 1800, the English chemist John Dalton brought back Democritus ancient idea of the atom. You can see a picture of Dalton 1.1. Dalton grew up in a working-class family. As an adult, he made a living by teaching and just did research in his spare time. Nonetheless, from his research he developed one of the most important theories in all of science. Based on his research results, he was able to demonstrate that atoms actually do exist, something that Democritus had only guessed. "
daltons atomic theory,T_4292,"Dalton did many experiments that provided evidence for the existence of atoms. For example: He investigated pressure and other properties of gases, from which he inferred that gases must consist of tiny, individual particles that are in constant, random motion. He researched the properties of compounds, which are substances that consist of more than one element. He showed that a given compound is always comprised of the same elements in the same whole-number ratio and that different compounds consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of separate, discrete particles that cannot be subdivided. "
daltons atomic theory,T_4293,"From his research, Dalton developed a theory about atoms. Daltons atomic theory consists of three basic ideas: All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles, created, or destroyed. All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses. Atoms join together to form compounds, and a given compound always consists of the same kinds of atoms in the same proportions. Daltons atomic theory was accepted by many scientists almost immediately. Most of it is still accepted today. However, scientists now know that atoms are not the smallest particles of matter. Atoms consist of several types of smaller particles, including protons, neutrons, and electrons. "
daltons atomic theory,T_4294,"Because Dalton thought atoms were the smallest particles of matter, he envisioned them as solid, hard spheres, like billiard (pool) balls, so he used wooden balls to model them. Three of his model atoms are pictured in the Figure and used to model compounds. Q: When scientists discovered smaller particles inside the atom, they realized that Daltons atomic models were too simple. How do modern atomic models differ from Daltons models? A: Modern atomic models, like the one pictured at the top of this article, usually represent subatomic particles, including electrons, protons, and neutrons. "
dangers and uses of radiation,T_4295,"A low level of radiation occurs naturally in the environment. This is called background radiation. One source of background radiation is rocks, which may contain small amounts of radioactive elements such as uranium. Another source is cosmic rays. These are charged particles that arrive on Earth from outer space. Background radiation is generally considered to be safe for living things. "
dangers and uses of radiation,T_4296,"Long-term or high-dose exposure to radiation can harm both living and nonliving things. Radiation knocks electrons out of atoms and changes them to ions. It also breaks bonds in DNA and other compounds in living things. One source of radiation that is especially dangerous to people is radon. Radon is a radioactive gas that forms in rocks underground. It can seep into basements and get trapped inside buildings. Then it may build up and become harmful to people who breathe it. Long-term exposure to radon can cause lung cancer. Exposure to higher levels of radiation can be very dangerous, even if the exposure is short-term. A single large dose of radiation can burn the skin and cause radiation sickness. Symptoms of this illness include extreme fatigue, destruction of blood cells, and loss of hair. Nonliving things can also be damaged by radiation. For example, high levels of radiation can weaken metals by removing electrons. This is a problem in nuclear power plants and space vehicles because they are exposed to very high levels of radiation. Q: Can you tell when you are being exposed to radiation? For example, can you sense radon in the air? A: Radiation cant be detected with the senses. This adds to its danger. However, there are other ways to detect it. "
dangers and uses of radiation,T_4297,"You generally cant see, smell, taste, hear, or feel radiation. Fortunately, there are devices such as Geiger counters that can detect radiation. A Geiger counter, like the one pictured in the Figure 1.1, contains atoms of a gas that is ionized if it encounters radiation. When this happens, the gas atoms change to ions that can carry an electric current. The current causes the Geiger counter to click. The faster the clicks occur, the higher the level of radiation. "
dangers and uses of radiation,T_4298,"Despite its dangers, radioactivity has several uses. For example, it can be used to determine the ages of ancient rocks and fossils. It can also be used as a source of power to generate electricity. Radioactivity can even be used to diagnose and treat diseases, including cancer. Cancer cells grow rapidly and take up a lot of glucose for energy. Glucose containing radioactive elements can be given to patients. Cancer cells take up more of the glucose than normal cells do and give off radiation. The radiation can be detected with special machines like the one in the Figure 1.2. The radiation may also kill cancer cells. "
decomposition reactions,T_4299,"A decomposition reaction occurs when one reactant breaks down into two or more products. It can be represented by the general equation: AB  A + B In this equation, AB represents the reactant that begins the reaction, and A and B represent the products of the reaction. The arrow shows the direction in which the reaction occurs. Q: What is the chemical equation for the decomposition of hydrogen peroxide (H2 O2 ) to water (H2 O) and oxygen (O2 )? A: The equation for this decomposition reaction is: 2 H2 O2  2 H2 O + O2 "
decomposition reactions,T_4300,"Two more examples of decomposition reactions are described below. Carbonic acid (H2 CO3 ) is an ingredient in soft drinks. A decomposition reaction takes place when carbonic acid breaks down to produce water (H2 O) and carbon dioxide (CO2 ). This occurs when you open a can of soft drink and some of the carbon dioxide fizzes out. The equation for this reaction is: H2 CO3  H2 O + CO2 Another decomposition reaction occurs when water (H2 O) breaks down to produce hydrogen (H2 ) and oxygen (O2 ) gases (see Figure 1.1). This happens when an electric current passes through the water, as illustrated below. The equation for this reaction is: 2 H2 O  2 H2 + O2 Decomposition of water. Q: What ratio of hydrogen molecules (H2 ) to oxygen molecules (O2 ) is produced in the decomposition of water? A: Two hydrogen molecules per oxygen molecule are produced because water (H2 O) has a ratio of two hydrogen atoms to one oxygen atom. "
democrituss idea of the atom,T_4301,"Democritus lived in Greece from about 460 to 370 B.C.E. Like many other ancient Greek philosophers, he spent a lot of time wondering about the natural world. Democritus wondered, for example, what would happen if you cut a chunk of mattersuch as a piece of cheese into smaller and smaller pieces. He thought that a point would be reached at which the cheese could not be cut into still smaller pieces. He called these pieces atomos, which means uncuttable in Greek. This is where the modern term atom comes from. "
democrituss idea of the atom,T_4302,"Democritus idea of the atom has been called the best guess in antiquity. Thats because it was correct in many ways, yet it was based on pure speculation. It really was just a guess. Heres what Democritus thought about the atom: How many times could you cut this piece of cheese in half? How small would the smallest pieces be? All matter consists of atoms, which cannot be further subdivided into smaller particles. Atoms are extremely smalltoo small to see. Atoms are solid particles that are indestructible. Atoms are separated from one another by emptiness, or void. Q: How are Democrituss ideas about atoms similar to modern ideas about atoms? A: Modern ideas agree that all matter is made up of extremely small building blocks called atoms. Q: How are Democrituss ideas different from modern ideas? A: Although atoms are extremely small, it is now possible to see them with very powerful microscopes. Atoms also arent the solid, uncuttable particles Democritus thought. Instead, they consist of several kinds of smaller, simpler particles as well as a lot of empty space. In addition, atoms arent really indestructible because they can be changed to other forms of matter or energy. "
democrituss idea of the atom,T_4303,"Did you ever notice dust motes moving in still air where a beam of sunlight passes through it? You can see an example in the forest scene in the Figure 1.2. This sort of observation gave Democritus the idea that atoms are in constant, random motion. If this were true, Democritus thought, then atoms must always be bumping into each other. When they do, he surmised, they either bounce apart or stick together to form clumps of atoms. Eventually, the clumps could grow big enough to be visible matter. Q: Which modern theory of matter is similar to Democritus ideas about the motion of atoms? A: The modern kinetic theory of matter is remarkably similar to Democritus ideas about the motion of atoms. According to this theory, atoms of matter are in constant random motion. This motion is greater in gases than in liquids, and it is greater in liquids than in solids. But even in solids, atoms are constantly vibrating in place. "
democrituss idea of the atom,T_4304,"Democritus thought that different kinds of matter vary because of the size, shape, and arrangement of their atoms. For example, he suggested that sweet substances are made of smooth atoms and bitter substances are made of sharp atoms. He speculated that atoms of liquids are slippery, which allows them to slide over each other and liquids to flow. Atoms of solids, in contrast, stick together, so they cannot move apart. Differences in the weight of matter, he argued, could be explained by the closeness of atoms. Atoms of lighter matter, he thought, were more spread out and separated by more empty space. Q: Democritus thought that different kinds of atoms make up different types of matter. How is this similar to modern ideas about atoms? A: The modern view is that atoms of different elements differ in their numbers of protons and electrons and this gives them different physical and chemical properties. Dust motes dance in a beam of sunlight. "
democrituss idea of the atom,T_4305,"Democritus was an important philosopher, but he was less influential than another Greek philosopher named Aristo- tle, who lived about 100 years after Democritus. Aristotle rejected Democritus idea of the atom. In fact, Aristotle thought the idea was ridiculous. Unfortunately, Aristotles opinion was accepted for more than 2000 years, and Democritus idea was more or less forgotten. However, the idea of the atom was revived around 1800 by the English scientist John Dalton. Dalton developed an entire theory about the atom, much of which is still accepted today. He based his theory on experimental evidence, not on lucky guesses. "
descriptive statistics,T_4310,"The girls in the picture above make up a small samplethere are only four of them. In scientific investigations, samples may include hundreds or even thousands of people or other objects of study. Especially when samples are very large, its important to be able to summarize their overall characteristics with a few numbers. Thats where descriptive statistics come in. Descriptive statistics are measures that show the central tendency, or center, of a sample or the variation in a sample. "
descriptive statistics,T_4311,"The central tendency of a sample can be represented by the mean, median, or mode. The mean is the average value. It is calculated by adding the individual measurements and dividing the sum by the total number of measurements. The median is the middle value. To find the median, rank all the measurements from smallest to largest and then find the measurement that is in the middle. The mode is the most common value. It is the value that occurs most often. Q: A sample of five children have the following heights: 60 cm, 58 cm, 54 cm, 62 cm, and 58 cm. What are the mean, median, and mode of this sample? A: The mean is (60 cm + 58 cm + 54 cm + 62 cm + 58 cm)  5 = 58 cm. The median and mode are both 58 cm as well. The mean, median, and mode are not always the same, as they are for this sample. In fact, sometimes these three statistics are very different from one another for the same sample. "
descriptive statistics,T_4312,Many samples have a lot of variation in measurements. Variation can be described with a statistic called the range. The range is the total spread of values in a sample. It is calculated by subtracting the smallest value from the largest value. Q: What is the range of heights in the sample of children in the previous question? A: The range is 62 cm - 54 cm = 8 cm. 
direction,T_4315,"Direction can be described in relative terms, such as up, down, in, out, left, right, forward, backward, or sideways. Direction can also be described with the cardinal directions: north, south, east, or west. On maps, cardinal directions are indicated with a compass rose. You can see one in the bottom left corner of the map in the Figure 1.1. You can use the compass rose to find directions on the map. For example, to go to the school from Jordans house, you would travel from east to west. If you wanted to go on to the post office, you would change direction at the school and then travel from south to north. "
direction,T_4316,"Look again at the map in the Figure 1.1. The distance from Jordans house to the post office is 3 km. But if Jordan told a friend how to reach the post office from his house, he couldnt just say go 3 kilometers. The friend might end up at the park instead of the post office. Jordan would have to include direction as well as distance. He could say, go west for 2 kilometers and then go north for 1 kilometer. "
direction,T_4317,"When both distance and direction are considered, motion can be represented by a vector. A vector is a measurement that has both size and direction. It may be represented by an arrow. If you are representing motion with an arrow, the length of the arrow represents distance, and the way the arrow points represents direction. The red arrows on the map in the Figure 1.1 are vectors for Jordans route from his house to the school and from the school to the post office. Q: How would you draw arrows to represent the distances and directions from the post office to the park on the map in the Figure 1.1? A: The vectors would look like this: "
distance,T_4322,"Distance is the length of the route between two points. The distance of a race, for example, is the length of the track between the starting and finishing lines. In a 100-meter sprint, that distance is 100 meters. "
distance,T_4323,"The SI unit for distance is the meter (m). Short distances may be measured in centimeters (cm), and long distances may be measured in kilometers (km). For example, you might measure the distance from the bottom to the top of a sheet of paper in centimeters and the distance from your house to your school in kilometers. "
distance,T_4324,Maps can often be used to measure distance. The map in the Figure 1.1 shows the route from Jordans house to his school. You can use the scale at the bottom of the map to measure the distance between these two points. Q: What is the distance from Jordans house to his school? A: The distance is 2 kilometers. 
doppler effect,T_4325,"The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. (It can also occur when the sound source is stationary and the listener is moving.) The Figure 1.1 shows how the Doppler effect occurs. The sound waves from the police car siren travel outward in all directions. Because the car is racing forward (to the left), the sound waves get bunched up in front of the car and spread out behind it. Sound waves that are closer together have a higher frequency, and sound waves that are farther apart have a lower frequency. The frequency of sound waves, in turn, determines the pitch of the sound. Sound waves with a higher frequency produce sound with a higher pitch, and sound waves with a lower frequency produce sound with a lower pitch. "
doppler effect,T_4326,"As the car approaches listener A, the sound waves get closer together, increasing their frequency. This listener hears the pitch of the siren get higher. As the car speeds away from listener B, the sound waves get farther apart, decreasing their frequency. This listener hears the pitch of the siren get lower. Q: What will the siren sound like to listener A after the police car passes him? A: The siren will suddenly get lower in pitch because the sound waves will be much more spread out and have a lower frequency. "
earth as a magnet,T_4327,"Imagine a huge bar magnet passing through Earths axis, as in the Figure 1.1. This is a good representation of Earth as a magnet. Like a bar magnet, Earth has north and south magnetic poles. A magnetic pole is the north or south end of a magnet, where the magnet exerts the most force. "
earth as a magnet,T_4328,"Although the needle of a compass always points north, it doesnt point to Earths north geographic pole. Find the north geographic pole in the Figure 1.2. As you can see, it is located at 90 north latitude. Where does a compass Q: The north end of a compass needle points toward Earths north magnetic pole. The like poles of two magnets repel each other, and the opposite poles attract. So why doesnt the north end of a compass needle point to Earths south magnetic pole instead? A: The answer may surprise you. The compass needle actually does point to the south pole of magnet Earth. However, it is called the north magnetic pole because it is close to the north geographic pole. This naming convention was adopted a long time ago to avoid confusion. "
earth as a magnet,T_4329,"Like all magnets, Earth has a magnetic field. Earths magnetic field is called the magnetosphere. You can see a model of the magnetosphere in the Figure 1.3. It is a huge region that extends outward from Earth in all directions. Earth exerts magnetic force over the entire field, but the force is strongest at the poles, where lines of force converge. Click image to the left or use the URL below. URL: "
efficiency,T_4330,"A dolly is a machine because it changes a force to make work easier. What is work? In physics, work is defined as the use of force to move an object over a distance. It is represented by the equation: Work = Force x Distance All machines make work easier, but they dont increase the amount of work that is done. You can never get more work out of a machine than you put into it. In fact, a machine always does less work on an object than the user does on the machine. Thats because a machine must use some of the work put into it to overcome friction. Friction is the force that opposes motion between any surfaces that are touching. All machines involve motion, so they all have friction. How much work is needed to overcome friction in a machine depends on the machines efficiency. "
efficiency,T_4331,"Efficiency is the percent of work put into a machine by the user (input work) that becomes work done by the machine (output work). The output work is always less than the input work because some of the input work is used to overcome friction. Therefore, efficiency is always less than 100 percent. The closer to 100 percent a machines efficiency is, the better it is at reducing friction. Look at the ramp in the Figure 1.1. A ramp is a type of simple machine called an inclined plane. It is easier to push the heavy piece of furniture up the ramp to the truck than to lift it straight up off the ground, but pushing the furniture over the surface of the ramp creates a lot of friction. Some of the force applied to moving the furniture must be used to overcome the friction with the ramp. Q: Why would it be more efficient to use a dolly to roll the furniture up the ramp? A: There would be less friction to overcome if you used a dolly because of the wheels. So the efficiency of the ramp would be greater with the dolly. "
efficiency,T_4332,"Efficiency can be calculated with the equation: Output work Efficiency = Input work  100% Consider a machine that puts out 6000 joules of work. To produce that much work from the machine requires the user to put in 8000 joules of work. To find the efficiency of the machine, substitute these values into the equation for efficiency: 6000 J 100% = 75% 8000 J Q: Rani puts 7500 joules of work into pushing a box up a ramp, but only 6700 joules of work actually go into moving the box. The rest of the work overcomes friction between the box and the ramp. What is the efficiency of the ramp? A: The efficiency of the ramp is: 6700 J 100% = 90% 7500 J "
einsteins concept of gravity,T_4333,"In the late 1600s, Isaac Newton introduced his law of gravity, which identifies gravity as a force of attraction between all objects with mass in the universe. The law also states that the strength of gravity between two objects depends on their mass and distance apart. Newtons law of gravity was accepted for more than two centuries. It can predict the motion of most objects and was even used by NASA to land astronauts on the moon. Its still used for most practical purposes. However, Newtons law doesnt explain why gravity occurs. It only describes how gravity seems to affect objects. There are also some cases in which Newtons law doesnt even describe what happens. Q: Newton expressed his ideas about gravity as a law. A law in science is a description of what always occurs in nature. For example, according to Newtons law, objects on Earth always fall down, not up. What is needed to explain gravity? A: A theory is needed to explain gravity. In science, a theory is a broad explanation that is supported by a great deal of evidence. "
einsteins concept of gravity,T_4334,"In the early 1900s, Albert Einstein came up with a theory of gravity that actually explains gravity rather than simply describing its effects. Einstein showed mathematically that gravity is not really a force that of attraction between all objects with mass, as Newton thought. Instead, Einstein showed that gravity is a result of the warping, or curving, of space and time, which made up the same space-time fabric. These ideas about space-time and gravity became known as Einsteins theory of general relativity. "
einsteins concept of gravity,T_4335,"Einstein derived his theory using mathematics. However, you can get a good grasp of it with the help of a simple visual analogy. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Now imagine placing a lighter ball at the edge of the trampoline. What will happen? It will roll down toward the bowling ball. This apparent attraction to the bowling ball occurs because the trampoline curves downward, not because the two balls are actually attracted to one another by an invisible force called gravity. Einstein theorized that the sun and other very massive bodies affect space and time around them in a way that is similar to the effect of the bowling ball on the trampoline. The more massive a body is, the more it causes space-time to curve. This idea is represented by the Figure 1.1. According to Einstein, objects move toward one another because of the curves in space-time, not because they are pulling on each other with a force of attraction. Einsteins theory is supported by evidence and widely accepted today, although Newtons law is still used for many calculations. "
elastic force,T_4336,"Something that is elastic can return to its original shape after being stretched or compressed. This property is called elasticity. As you stretch or compress an elastic material like a bungee cord, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force. The farther the material is stretched or compressed, the greater the elastic force becomes. As soon as the stretching or compressing force is released, elastic force causes the material to spring back to its original shape. Click image to the left or use the URL below. URL:  Q: What force stretches the bungee cord after the jumper jumps? When does the bungee cord snap back to its original shape? A: After the bungee jumper jumps, he accelerates toward the ground due to gravity. His weight stretches the bungee cord. As the bungee cord stretches, it exerts elastic force upward against the jumper, which slows his descent and brings him to a momentary stop. Then the bungee cord springs back to its original shape, and the jumper bounces upward. "
elastic force,T_4337,"Elastic force can be very useful and not just for bungee jumping. In fact, you probably use elastic force every day. A few common uses of elastic force are shown in the Figure 1.1. Do you use elastic force in any of these ways? Q: How does the resistance band work? How does it use elastic force? A: When you pull on the band, it stretches but doesnt break. The resistance you feel when you pull on it is elastic force. The farther you stretch the band, the greater the resistance is. The resistance of the band to stretching is what gives your muscles a workout. After you stop pulling on the band, it returns to its original shape, ready for the next stretch. Springs like the spring toy pictured in the Figure 1.2 also have elastic force when they are stretched or compressed. Q: Can you think of other uses of springs? A: Bedsprings provide springy support beneath a mattress. The spring in a door closer pulls the door shut. The spring in a retractable ballpoint pen retracts the point of the pen. The spring in a pogo stick bounces the rider up off the ground. "
electromagnetic spectrum,T_4379,"Electromagnetic radiation is energy that travels in waves across space as well as through matter. Most of the electromagnetic radiation on Earth comes from the sun. Like other waves, electromagnetic waves are characterized by certain wavelengths and wave frequencies. Wavelength is the distance between two corresponding points on adjacent waves. Wave frequency is the number of waves that pass a fixed point in a given amount of time. Electromagnetic waves with shorter wavelengths have higher frequencies and more energy. "
electromagnetic spectrum,T_4380,"Visible light and infrared light are just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum. You can see the waves of the electromagnetic spectrum in the Figure 1.1. At the top of the diagram, the wavelengths of the waves are given. Also included are objects that are about the same size as the corresponding wavelengths. The frequencies and energy levels of the waves are shown at the bottom of the diagram. Some sources of the waves are also given. On the left side of the electromagnetic spectrum diagram are radio waves and microwaves. Radio waves have the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount of energy. On the right side of the diagram are X rays and gamma rays. They have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the most energy. Between these two extremes are waves that are commonly called light. Light includes infrared light, visible light, and ultraviolet light. The wavelengths, frequencies, and energy levels of light fall in between those of radio waves on the left and X rays and gamma rays on the right. Q: Which type of light has the longest wavelengths? A: Infrared light has the longest wavelengths. Q: What sources of infrared light are shown in the diagram? A: The sources in the diagram are people and light bulbs, but all living things and most other objects give off infrared light. "
electromagnetic waves,T_4381,"Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. Like other waves, electro- magnetic waves transfer energy from one place to another. The transfer of energy by electromagnetic waves is called electromagnetic radiation. Electromagnetic waves can transfer energy through matter or across empty space. Click image to the left or use the URL below. URL:  Q: How do microwaves transfer energy inside a microwave oven? A: They transfer energy through the air inside the oven to the food. "
electromagnetic waves,T_4382,"A familiar example may help you understand the vibrating electric and magnetic fields that make up electromagnetic waves. Consider a bar magnet, like the one in the Figure 1.1. The magnet exerts magnetic force over an area all around it. This area is called a magnetic field. The field lines in the diagram represent the direction and location of the magnetic force. Because of the field surrounding a magnet, it can exert force on objects without touching them. They just have to be within its magnetic field. Q: How could you demonstrate that a magnet can exert force on objects without touching them? A: You could put small objects containing iron, such as paper clips, near a magnet and show that they move toward the magnet. An electric field is similar to a magnetic field. It is an area of electrical force surrounding a positively or negatively charged particle. You can see electric fields in the following Figure 1.2. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them. "
electromagnetic waves,T_4383,"An electromagnetic wave begins when an electrically charged particle vibrates. The Figure 1.3 shows how this happens. A vibrating charged particle causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field. The two types of vibrating fields combine to create an electromagnetic wave. "
electromagnetic waves,T_4384,"As you can see in the Figure 1.3, the electric and magnetic fields that make up an electromagnetic wave are perpendicular (at right angles) to each other. Both fields are also perpendicular to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. However, unlike a mechanical transverse wave, which can only travel through matter, an electromagnetic transverse wave can travel through empty space. When waves travel through matter, they lose some energy to the matter as they pass through it. But when waves travel through space, no energy is lost. Therefore, electromagnetic waves dont get weaker as they travel. However, the energy is diluted as it travels farther from its source because it spreads out over an ever-larger area. "
electromagnetic waves,T_4385,"When electromagnetic waves strike matter, they may interact with it in the same ways that mechanical waves interact with matter. Electromagnetic waves may: reflect, or bounce back from a surface; refract, or bend when entering a new medium; diffract, or spread out around obstacles. Electromagnetic waves may also be absorbed by matter and converted to other forms of energy. Microwaves are a familiar example. When microwaves strike food in a microwave oven, they are absorbed and converted to thermal energy, which heats the food. "
electromagnetic waves,T_4386,"The most important source of electromagnetic waves on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes. "
electron cloud atomic model,T_4390,"Up until about 1920, scientists accepted Niels Bohrs model of the atom. In this model, negative electrons circle the positive nucleus at fixed distances from the nucleus, called energy levels. You can see the model in Figure 1.1 for an atom of the element nitrogen. Bohrs model is useful for understanding properties of elements and their chemical interactions. However, it doesnt explain certain behaviors of electrons, except for those in the simplest atom, the hydrogen atom. "
electron cloud atomic model,T_4391,"In the mid-1920s, an Austrian scientist named Erwin Schrdinger thought that the problem with Bohrs model was restricting the electrons to specific orbits. He wondered if electrons might behave like light, which scientists already knew had properties of both particles and waves. Schrdinger speculated that electrons might also travel in waves. Q: How do you pin down the location of an electron in a wave? A: You cant specify the exact location of an electron. However, Schrdinger showed that you can at least determine where an electron is most likely to be. Schrdinger developed an equation that could be used to calculate the chances of an electron being in any given place around the nucleus. Based on his calculations, he identified regions around the nucleus where electrons are most likely to be. He called these regions orbitals. As you can see in the Figure 1.2, orbitals may be shaped like spheres, dumbbells, or rings. In each case, the nucleus of the atom is at the center of the orbital. "
electron cloud atomic model,T_4392,"Schrdingers work on orbitals is the basis of the modern model of the atom, which scientists call the quantum mechanical model. The modern model is also commonly called the electron cloud model. Thats because each orbital around the nucleus of the atom resembles a fuzzy cloud around the nucleus, like the ones shown in the Figure 1.3 for a helium atom. The densest area of the cloud is where the electrons have the greatest chances of being. Q: In the model pictured in the Figure 1.3, where are the two helium electrons most likely to be? A: The two electrons are most likely to be inside the sphere closest to the nucleus where the cloud is darkest. "
electrons,T_4403,"Electrons are one of three main types of particles that make up atoms. The other two types are protons and neutrons. Unlike protons and neutrons, which consist of smaller, simpler particles, electrons are fundamental particles that do not consist of smaller particles. They are a type of fundamental particles called leptons. All leptons have an electric charge of -1 or 0. Click image to the left or use the URL below. URL: "
electrons,T_4404,"Electrons are extremely small. The mass of an electron is only about 1/2000 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have an electric charge of -1, which is equal but opposite to the charge of proton, which is +1. All atoms have the same number of electrons as protons, so the positive and negative charges cancel out, making atoms electrically neutral. "
electrons,T_4405,"Unlike protons and neutrons, which are located inside the nucleus at the center of the atom, electrons are found outside the nucleus. Because opposite electric charges attract each other, negative electrons are attracted to the positive nucleus. This force of attraction keeps electrons constantly moving through the otherwise empty space around the nucleus. The Figure shown 1.1 is a common way to represent the structure of an atom. It shows the electron as a particle orbiting the nucleus, similar to the way that planets orbit the sun. "
electrons,T_4406,"The atomic model above is useful for some purposes, but its too simple when it comes to the location of electrons. In reality, its impossible to say what path an electron will follow. Instead, its only possible to describe the chances of finding an electron in a certain region around the nucleus. The region where an electron is most likely to be is called an orbital. Each orbital can have at most two electrons. Some orbitals, called S orbitals, are shaped like spheres, with the nucleus in the center. An S orbital is pictured in Figure 1.2. Where the dots are denser, the chance of finding an electron is greater. Also pictured in Figure 1.2 is a P orbital. P orbitals are shaped like dumbbells, with the nucleus in the pinched part of the dumbbell. Click image to the left or use the URL below. URL:  Q: How many electrons can there be in each type of orbital shown above? A: There can be a maximum of two electrons in any orbital, regardless of its shape. Q: Where is the nucleus in each orbital? A: The nucleus is at the center of each orbital. It is in the middle of the sphere in the S orbital and in the pinched part of the P orbital. "
electrons,T_4407,"Electrons are located at fixed distances from the nucleus, called energy levels. You can see the first three energy levels in the Figure 1.3. The diagram also shows the maximum possible number of electrons at each energy level. Electrons at lower energy levels, which are closer to the nucleus, have less energy. At the lowest energy level, which has the least energy, there is just one orbital, so this energy level has a maximum of two electrons. Only when a lower energy level is full are electrons added to the next higher energy level. Electrons at higher energy levels, which are farther from the nucleus, have more energy. They also have more orbitals and greater possible numbers of electrons. Electrons at the outermost energy level of an atom are called valence electrons. They determine many of the properties of an element. Thats because these electrons are involved in chemical reactions with other atoms. Atoms may share or transfer valence electrons. Shared electrons bind atoms together to form chemical compounds. Q: If an atom has 12 electrons, how will they be distributed in energy levels? A: The atom will have two electrons at the first energy level, eight at the second energy level, and the remaining two at the third energy level. Q: Sometimes, an electron jumps from one energy level to another. How do you think this happens? A: To change energy levels, an electron must either gain or lose energy. Thats because electrons at higher energy levels have more energy than electrons at lower energy levels. "
elements,T_4408,"A pure substance is called an element. An element is a pure substance because it cannot be separated into any other substances. Currently, 92 different elements are known to exist in nature, although additional elements have been formed in labs. All matter consists of one or more of these elements. Some elements are very common; others are relatively rare. The most common element in the universe is hydrogen, which is part of Earths atmosphere and a component of water. The most common element in Earths atmosphere is nitrogen, and the most common element in Earths crust is oxygen. Click image to the left or use the URL below. URL: "
elements,T_4409,"Each element has a unique set of properties that is different from the set of properties of any other element. For example, the element iron is a solid that is attracted by a magnet and can be made into a magnet, like the compass needle shown in the Figure 1.1. The element neon, on the other hand, is a gas that gives off a red glow when electricity flows through it. The lighted sign in the Figure 1.2 contains neon. The needle of this compass is made of the element iron. Q: Do you know properties of any other elements? For example, what do you know about helium? A: Helium is a gas that has a lower density than air. Thats why helium balloons have to be weighted down so they wont float away. Q: Living things, like all matter, are made of elements. Do you know which element is most common in living things? A: Carbon is the most common element in living things. It has the unique property of being able to combine with many other elements as well as with itself. This allows carbon to form a huge number of different substances. "
elements,T_4410,"For thousands of years, people have wondered about the substances that make up matter. About 2500 years ago, the Greek philosopher Aristotle argued that all matter is made up of just four elements, which he identified as earth, air, water, and fire. He thought that different substances vary in their properties because they contain different proportions of these four elements. Aristotle had the right idea, but he was wrong about which substances are elements. Nonetheless, his four elements were accepted until just a few hundred years ago. Then scientists started discovering many of the elements with which we are familiar today. Eventually they discovered dozens of different elements. "
elements,T_4411,"The smallest particle of an element that still has the properties of that element is the atom. Atoms actually consist of smaller particles, including protons and electrons, but these smaller particles are the same for all elements. All the atoms of an element are like one another, and are different from the atoms of all other elements. For example, the atoms of each element have a unique number of protons. Consider carbon as an example. Carbon atoms have six protons. They also have six electrons. All carbon atoms are the same whether they are found in a lump of coal or a teaspoon of table sugar (Figure 1.3). On the other hand, carbon atoms are different from the atoms of hydrogen, which are also found in coal and sugar. Each hydrogen atom has just one proton and one electron. Carbon is the main element in coal (left). Carbon is also a major component of sugar (right). Q: Why do you think coal and sugar are so different from one another when carbon is a major component of each A: Coal and sugar differ from one another because they contain different proportions of carbon and other elements. For example, coal is about 85 percent carbon, whereas table sugar is about 42 percent carbon. Both coal and sugar also contain the elements hydrogen and oxygen but in different proportions. In addition, coal contains the elements nitrogen and sulfur. "
endothermic reactions,T_4412,"All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In some chemical reactions, called exothermic reactions, more energy is released when new bonds form in the products than is needed to break bonds in the reactants. The opposite is true of endothermic reactions. In an endothermic reaction, it takes more energy to break bonds in the reactants than is released when new bonds form in the products. "
endothermic reactions,T_4413,"The word endothermic literally means taking in heat. A constant input of energy, often in the form of heat, is needed to keep an endothermic reaction going. This is illustrated in the Figure 1.1. Energy must be constantly added because not enough energy is released when the products form to break more bonds in the reactants. The general equation for an endothermic reaction is: Reactants + Energy  Products Note: H represents the change in en- ergy. In endothermic reactions, the temperature of the products is typically lower than the temperature of the reactants. The drop in temperature may be great enough to cause liquids to freeze. Q: Now can you guess how an instant cold pack works? A: Squeezing the cold pack breaks an inner bag of water, and the water mixes with a chemical inside the pack. The chemical and water combine in an endothermic reaction. The energy needed for the reaction to take place comes from the water, which gets colder as the reaction proceeds. "
endothermic reactions,T_4414,"One of the most important series of endothermic reactions is photosynthesis. In photosynthesis, plants make the simple sugar glucose (C6 H12 O6 ) from carbon dioxide (CO2 ) and water (H2 O). They also release oxygen (O2 ) in the process. The reactions of photosynthesis are summed up by this chemical equation: 6 CO2 + 6 H2 O  C6 H12 O6 + 6 O2 The energy for photosynthesis comes from light. Without light energy, photosynthesis cannot occur. As you can see in the Figure 1.2, plants can get the energy they need for photosynthesis from either sunlight or artificial light. "
energy,T_4415,"Energy is defined in science as the ability to move matter or change matter in some other way. Energy can also be defined as the ability to do work, which means using force to move an object over a distance. When work is done, energy is transferred from one object to another. For example, when the boy in the Figure 1.1 uses force to swing the racket, he transfers some of his energy to the racket. Q: It takes energy to play tennis. Where does this boy get his energy? A: He gets energy from the food he eats. "
energy,T_4416,"Because energy is the ability to do work, it is expressed in the same unit that is used for work. The SI unit for both work and energy is the joule (J), or Newton  meter (N  m). One joule is the amount of energy needed to apply a force of 1 Newton over a distance of 1 meter. For example, suppose the boy in the Figure 1.1 applies 20 Newtons of force to his tennis racket over a distance of 1 meter. The energy needed to do this work is 20 N m, or 20 J. "
energy,T_4417,"If you think about different sources of energysuch as batteries and the sunyou probably realize that energy can take different forms. For example, when the boy swings his tennis racket, the energy of the moving racket is an example of mechanical energy. To move his racket, the boy needs energy stored in food, which is an example of chemical energy. Other forms of energy include electrical, thermal, light, and sound energy. The different forms of energy can also be classified as either kinetic energy or potential energy. Kinetic energy is the energy of moving matter. Potential energy is energy that is stored in matter. Q: Is the chemical energy in food kinetic energy or potential energy? A: The chemical energy in food is potential energy. It is stored in the chemical bonds that make up food molecules. The stored energy is released when we digest food. Then we can use it for many purposes, such as moving (mechanical energy) or staying warm (thermal energy). Q: What is an example of kinetic energy? A: Anything that is moving has kinetic energy. An example is a moving tennis racket. "
energy conversion,T_4418,"Gravity is a force, but not like other forces you may know. Gravity is a bit special. You know that a force is a push or pull. If you push a ball, it starts to roll. If you lift a book, it moves upward. Now, imagine you drop a ball. It falls to the ground. Can you see the force pulling it down? That is what makes gravity really cool. It is invisible. Invisible means you cannot see it. But wait, it has even more surprises. Gravity holds planets in place around the Sun. Gravity keeps the Moon from flying off into space. Gravity exerts a force on objects that are not even touching. In fact, gravity can act over very large distances. However, the force does get weaker the farther apart the objects are. "
energy conversion,T_4419,"You are already very familiar with Earths gravity. It constantly pulls you toward Earths center. What might happen if there was no gravity? You know that the Earth is rotating on its axis. This motion causes our day and night cycle. The Earth also orbits the Sun. All this motion may cause you to fly off the Earth! You can thank gravity for keeping you in place. Gravity keeps us firmly down on the ground. Gravity also pulls on objects that are in the sky. It also pulls on objects that are in space. Meteors and skydivers are pulled down by gravity. Gravity also keeps the moon orbiting the Earth. Without gravity, the moon would float away. It also holds artificial satellites in their orbit. Many of these satellites help to connect the world. They allow you to pick up a phone a call in many parts of the world. You can also thank gravity for all your TV channels. Gravity keeps Earth and the other planets moving around the much more massive Sun. "
energy conversion,T_4420,"""What goes up must come down."" You have probably heard that statement before. At one time this statement was true, but no longer. Since the 1960s, we have sent many spacecraft into space. Some are still traveling away from Earth. So it is possible to overcome gravity. Do you need a giant rocket to overcome gravity? No, you actually overcome gravity every day. Think about when you climb a set of stairs. When you do, you are overcoming gravity. What if you jump on a trampoline? You are overcoming gravity for a few seconds. Everyone can overcome gravity. You just need to apply a force larger than gravity. Think about that the next time you jump into the air. You are overcoming gravity for a brief second. Enjoy it while it lasts. Eventually, gravity will win the battle. "
energy conversion,T_4421,1. What is the traditional definition of gravity? 2. Identify factors that influence the strength of gravity between two objects. 
energy conversion,T_4422,"By clicking a link below, you will leave the CK-12 site and open an external site in a new tab. This page will remain open in the original tab. "
energy level,T_4423,"Energy levels (also called electron shells) are fixed distances from the nucleus of an atom where electrons may be found. Electrons are tiny, negatively charged particles in an atom that move around the positive nucleus at the center. Energy levels are a little like the steps of a staircase. You can stand on one step or another but not in between the steps. The same goes for electrons. They can occupy one energy level or another but not the space between energy levels. The model in the Figure 1.1 shows the first four energy levels of an atom. Electrons in energy level I (also called energy level K) have the least amount of energy. As you go farther from the nucleus, electrons at higher levels have more energy, and their energy increases by a fixed, discrete amount. Electrons can jump from a lower to the next higher energy level if they absorb this amount of energy. Conversely, if electrons jump from a higher to a lower energy level, they give off energy, often in the form of light. This explains the fireworks pictured above. When the fireworks explode, electrons gain energy and jump to higher energy levels. When they jump back to their original energy levels, they release the energy as light. Different atoms have different arrangements of electrons, so they give off light of different colors. Q: In the atomic model Figure 1.1, where would you find electrons that have the most energy? A: Electrons with the most energy would be found in energy level IV. "
energy level,T_4424,"The smallest atoms are hydrogen atoms. They have just one electron orbiting the nucleus. That one electron is in the first energy level. Bigger atoms have more electrons. Electrons are always added to the lowest energy level first until it has the maximum number of electrons possible. Then electrons are added to the next higher energy level until that level is full, and so on. How many electrons can a given energy level hold? The maximum numbers of electrons possible for the first four energy levels are shown in the Figure 1.1. For example, energy level I can hold a maximum of two electrons, and energy level II can hold a maximum of eight electrons. The maximum number depends on the number of orbitals at a given energy level. An orbital is a volume of space within an atom where an electron is most likely to be found. As you can see by the images in the Figure 1.2, some orbitals are shaped like spheres (S orbitals) and some are shaped like dumbbells (P orbitals). There are other types of orbitals as well. Regardless of its shape, each orbital can hold a maximum of two electrons. Energy level I has just one orbital, so two electrons will fill this energy level. Energy level II has four orbitals, so it takes eight electrons to fill this energy level. Q: Energy level III can hold a maximum of 18 electrons. How many orbitals does this energy level have? A: At two electrons per orbital, this energy level must have nine orbitals. "
energy level,T_4425,"Electrons in the outermost energy level of an atom have a special significance. These electrons are called valence electrons, and they determine many of the properties of an atom. An atom is most stable if its outermost energy level contains as many electrons as it can hold. For example, helium has two electrons, both in the first energy level. This energy level can hold only two electrons, so heliums only energy level is full. This makes helium a very stable element. In other words, its atoms are unlikely to react with other atoms. Consider the elements fluorine and lithium, modeled in the Figure 1.3. Fluorine has seven of eight possible electrons in its outermost energy level, which is energy level II. It would be more stable if it had one more electron because this would fill its outermost energy level. Lithium, on the other hand, has just one of eight possible electrons in its outermost energy level (also energy level II). It would be more stable if it had one less electron because it would have a full outer energy level (now energy level I). Both fluorine and lithium are highly reactive elements because of their number of valence electrons. Fluorine will readily gain one electron and lithium will just as readily give up one electron to become more stable. In fact, lithium and fluorine will react together as shown in the Figure 1.4. When the two elements react, lithium transfers its one extra electron to fluorine. Q: A neon atom has ten electrons. How many electrons does it have in its outermost energy level? How stable do you think a neon atom is? A: A neon atom has two electrons in energy level I and its remaining eight electrons in energy level II, which can "
enzymes as catalysts,T_4426,"Chemical reactions constantly occur inside the cells of living things. However, under the conditions inside cells, most biochemical reactions would occur too slowly to maintain life. Thats where enzymes come in. Enzymes are catalysts in living things. Like other catalysts, they speed up chemical reactions. Enzymes are proteins that are synthesized in the cells that need them, based on instructions encoded in the cells DNA. "
enzymes as catalysts,T_4427,"Enzymes increase the rate of chemical reactions by reducing the amount of activation energy needed for reactants to start reacting. One way this can happen is modeled in the Figure 1.1. Enzymes arent changed or used up in the reactions they catalyze, so they can be used to speed up the same reaction over and over again. Each enzyme is highly specific for the particular reaction is catalyzes, so enzymes are very effective. A reaction that would take many years to occur without its enzyme might occur in a split second with the enzyme. Enzymes are also very efficient, so waste products rarely form. Q: This model of enzyme action is called the lock-and-key model. Explain why. A: The substrates (reactants) fit precisely into the active site of the enzyme like a key into a lock. Being brought together in the enzyme in this way helps the reactants react more easily. After the product is formed, it is released by the enzyme. The enzyme is now ready to pick up more reactants and catalyze another reaction. Click image to the left or use the URL below. URL: "
enzymes as catalysts,T_4428,More than 1000 different enzymes are necessary for human life. Many enzymes are needed for the digestion of food. Two examples are amylase and pepsin. Both are described in the Figure 1.2. 
exothermic reactions,T_4435,"All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In some chemical reactions, called endothermic reactions, less energy is released when new bonds form in the products than is needed to break bonds in the reactants. The opposite is true of exothermic reactions. In an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products. "
exothermic reactions,T_4436,"The word exothermic means releasing heat. Energy, often in the form of heat, is released as an exothermic reaction proceeds. This is illustrated in the Figure 1.1. The general equation for an exothermic reaction is: Reactants  Products + Energy If the energy produced in an exothermic reaction is released as heat, it results in a rise in temperature. As a result, the products are likely to be warmer than the reactants. Note: H represents the change in en- ergy. Q: You turn on the hot water faucet, and hot water pours out. How does the water get hot? Do you think that an exothermic reaction might be involved? A: A hot water heater increases the temperature of water in most homes. Many hot water heaters burn a fuel such as natural gas. The burning fuel causes the water to get hot because combustion is an exothermic reaction. "
exothermic reactions,T_4437,"All combustion reactions are exothermic reactions. During a combustion reaction, a substance burns as it combines with oxygen. When substances burn, they usually give off energy as heat and light. Look at the big bonfire in the Figure 1.2. The combustion of wood is an exothermic reaction that releases a lot of energy as heat and light. You can see the light energy the fire is giving off. If you were standing near the fire, you would also feel its heat. "
external combustion engines,T_4438,A combustion engine is a complex machine that burns fuel to produce thermal energy and then uses the thermal energy to do work. There are two types of combustion engines: external and internal. A steam engine is an external combustion engine. 
external combustion engines,T_4439,"An external combustion engine burns fuel externally, or outside the engine. The burning fuel releases thermal energy, which is used to heat water and change it to steam. The pressure of the steam moves a piston back and forth inside a cylinder. The kinetic energy of the moving piston can be used to turn a vehicles wheels, a turbine, or other mechanical device. The Figure 1.1 explains in greater detail how this type of engine works. Q: What type of energy does the piston have when it moves back and forth inside the cylinder? A: Like anything else that is moving, the moving piston has kinetic energy. "
ferromagnetic material,T_4440,"Magnetism is the ability of a material to be attracted by a magnet and to act as a magnet. Magnetism is due to the movement of electrons within atoms of matter. When electrons spin around the nucleus of an atom, it causes the atom to become a tiny magnet, with north and south poles and a magnetic field. In most materials, the north and south poles of atoms point in all different directions, so overall the material is not magnetic. Examples of nonmagnetic materials include wood, glass, plastic, paper, copper, and aluminum. These materials are not attracted to magnets and cannot become magnets. In other materials, there are regions where the north and south poles of atoms are all lined up in the same direction. These regions are called magnetic domains. Generally, the magnetic domains point in different directions, so the material is still not magnetic. However, the material can be magnetized (made into a magnet) by placing it in a magnetic field. When this happens, all the magnetic domains line up, and the material becomes a magnet. You can see this in the Figure 1.1. Materials that can be magnitized are called ferromagnetic materials. They include iron, cobalt, and nickel. "
ferromagnetic material,T_4441,"Materials that have been magnetized may become temporary or permanent magnets. If you bring a bar magnet close to pile of paper clips, the paper clips will become temporarily magnetized, as all their magnetic domains line up. As a result, the paper clips will stick to the magnet and also to each other (see the Figure 1.2). However, if you remove the paper clips from the bar magnets magnetic field, their magnetic domains will no longer align. As a result, the paper clips will no longer be magnetized or stick together. If you stroke an iron nail with a bar magnet, the nail will become a permanent (or at least long-lasting) magnet. You can see how its done in the Figure 1.3. The nails magnetic domains will remain aligned even after you remove the nail from the magnetic field of the bar magnet. Q: Even permanent magnets can be demagnetized if they are dropped or heated to high temperatures. Can you explain why? "
ferromagnetic material,T_4442,"Some materials are natural permanent magnets. The most magnetic material in nature is the mineral magnetite, also called lodestone (see Figure 1.4). The magnetic domains of magnetite naturally align with Earths axis. The picture on the left shows a chunk of magnetite attracting small bits of iron. The magnetite spoon compass shown on the right dates back about 2000 years and comes from China. The handle of the spoon always points north. Clearly, the magnetic properties of magnetite have been recognized for thousands of years. "
force,T_4445,"Force is defined as a push or pull acting on an object. There are several fundamental forces in the universe, including the force of gravity, electromagnetic force, and weak and strong nuclear forces. When it comes to the motion of everyday objects, however, the forces of interest include mainly gravity, friction, and applied force. Applied force is force that a person or thing applies to an object. Q: What forces act on Carsons scooter? A: Gravity, friction, and applied forces all act on Carsons scooter. Gravity keeps pulling both Carson and the scooter toward the ground. Friction between the wheels of the scooter and the ground prevent the scooter from sliding but also slow it down. In addition, Carson applies forces to his scooter to control its speed and direction. "
force,T_4446,"Forces cause all motions. Everytime the motion of an object changes, its because a force has been applied to it. Force can cause a stationary object to start moving or a moving object to change its speed or direction or both. A change in the speed or direction of an object is called acceleration. Look at Carsons brother Colton in the Figure starts the scooter moving in the opposite direction. The harder he pushes against the ground, the faster the scooter will go. How much an object accelerates when a force is applied to it depends not only on the strength of the force but also on the objects mass. For example, a heavier scooter would be harder to accelerate. Colton would have to push with more force to start it moving and move it faster. Q: What units do you think are used to measure force? A: The SI unit for force is the Newton (N). A Newton is the force needed to cause a mass of 1 kilogram to accelerate at 1 m/s2 , so a Newton equals 1 kg  m/s2 . The Newton was named for the scientist Sir Isaac Newton, who is famous for his laws of motion and gravity. "
force,T_4447,"Force is a vector, or a measure that has both size and direction. For example, Colton pushes on the ground in the opposite direction that the scooter moves, so thats the direction of the force he is applies. He can give the scooter a strong push or a weak push. Thats the size of the force. Like other vectors, a force can be represented with an arrow. You can see some examples in the Figure 1.2. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force. Q: How could you use arrows to represent the forces that start Coltons scooter moving? A: Colton pushes against the ground behind him (to the right in the Figure 1.1). The ground pushes back with equal force to the left, causing the scooter to move in that direction. Force arrows A and B in example 2 in the Figure 1.1) could represent these forces. "
forms of energy,T_4448,"Energy, or the ability to cause changes in matter, can exist in many different forms. Energy can also change from one form to another. The photo above of the guitar player represents six forms of energy: mechanical, chemical, electrical, light, thermal, and sound energy. Another form of energy is nuclear energy. Q: Can you find the six different forms of energy in the photo of the guitar player (See opening image)? A: The guitarist uses mechanical energy to pluck the strings of the guitar. He gets the energy he needs to perform from chemical energy in food he ate earlier in the day. The stage lights use electrical energy, which they change to light energy and thermal energy (commonly called heat). The guitar produces sound energy when the guitarist plucks the strings. "
forms of energy,T_4449,"The different forms of energy are defined and illustrated below. 1. Mechanical energy is the energy of movement. It is found in objects that are moving or have the potential to move. 2. Chemical energy is energy that is stored in the bonds between the atoms of compounds. If the bonds are broken, the energy is released and can be converted to other forms of energy. This portable guitar amplifier can run on batteries. Batteries store chemical energy and change it to electrical energy. 3. Electrical energy is the energy of moving electrons. Electrons flow through wires to create electric current. 4. Electromagnetic energy is energy that travels through space as electrical and magnetic waves. The light flooding the stage in the Figure 1.3 is one type of electromagnetic energy. Other types include radio waves, microwaves, X rays, and gamma rays. 5. Thermal energy is the energy of moving atoms of matter. All matter has thermal energy because atoms of all matter are constantly moving. An object with more mass has greater thermal energy than an object with less mass because it has more atoms. Why is this jogger sweating so much? His sweat is soaking up his shirt because he has so much thermal energy. Jogging is hot work because of the heat from the sun and the hard work he puts into his run. 6. Sound energy is a form of mechanical energy that starts with a vibration in matter. For example, the singers voice 7. Nuclear energy is energy that is stored in the nuclei of atoms because of the strong forces that hold the nucleus together. The energy can be released in nuclear power plants by splitting nuclei apart. It is also released when unstable (radioactive) nuclei break down, or decay. Q: The fans at a rock concert also produce or use several forms of energy. What are they? A: The fans see the concert because of electromagnetic energy (light) that enters their eyes from the well-lit musicians on stage. They hear the music because of the sound energy that reaches their ears from the amplifiers. They use mechanical energy when they clap their hands and jump from their seats in excitement. Their bodies generate thermal energy, using the chemical energy stored in food they have eaten. "
frequency and pitch of sound,T_4452,"How high or low a sound seems to a listener is its pitch. Pitch, in turn, depends on the frequency of sound waves. Wave frequency is the number of waves that pass a fixed point in a given amount of time. High-pitched sounds, like the sounds of the piccolo in the Figure 1.1, have high-frequency waves. Low-pitched sounds, like the sounds of the tuba Figure 1.1, have low-frequency waves. "
frequency and pitch of sound,T_4453,"The frequency of sound waves is measured in hertz (Hz), or the number of waves that pass a fixed point in a second. Human beings can normally hear sounds with a frequency between about 20 Hz and 20,000 Hz. Sounds with frequencies below 20 hertz are called infrasound. Infrasound is too low-pitched for humans to hear. Sounds with frequencies above 20,000 hertz are called ultrasound. Ultrasound is too high-pitched for humans to hear. Some other animals can hear sounds in the ultrasound range. For example, dogs can hear sounds with frequencies as high as 50,000 Hz. You may have seen special whistles that dogsbut not peoplecan hear. The whistles produce sounds with frequencies too high for the human ear to detect. Other animals can hear even higher-frequency sounds. Bats, like the one pictured in the Figure 1.2, can hear sounds with frequencies higher than 100,000 Hz! Q: Bats use ultrasound to navigate in the dark. Can you explain how? A: Bats send out ultrasound waves, which reflect back from objects ahead of them. They sense the reflected sound waves and use the information to detect objects they cant see in the dark. This is how they avoid flying into walls and trees and also how they find flying insects to eat. "
friction,T_4454,"Friction is a force that opposes motion between any surfaces that are touching. Friction can work for or against us. For example, putting sand on an icy sidewalk increases friction so you are less likely to slip. On the other hand, too much friction between moving parts in a car engine can cause the parts to wear out. Other examples of friction are illustrated in the two Figures 1.1 and 1.2. "
friction,T_4455,"Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye make look rough or bumpy when viewed under a microscope. Look at the metal surfaces in the Figure 1.3. The aluminum foil These photos show two ways that friction is useful These photos show two ways that friction can cause problems is so smooth that its shiny. However, when highly magnified, the surface of metal appears to be very bumpy. All those mountains and valleys catch and grab the mountains and valleys of any other surface that contacts the metal. This creates friction. "
friction,T_4456,"Rougher surfaces have more friction between them than smoother surfaces. Thats why we put sand on icy sidewalks and roads. You cant slide as far across ice with shoes as you can on the blades of skates (see Figure 1.4). The rougher surface of the soles of the shoes causes more friction and slows you down. Q: Heavier objects also have more friction. Can you explain why? A: Heavier objects press together with greater force, and this causes greater friction between them. Did you ever try to furniture across the floor? Its harder to overcome friction between a heavier piece of furniture and the floor than between lighter pieces and the floor. "
friction,T_4457,"You know that friction produces heat. Thats why rubbing your hands together makes them warmer. But do you know why? Friction causes the molecules on rubbing surfaces to move faster, so they have more energy. This gives them a higher temperature, and they feel warmer. Heat from friction can be useful. It not only warms your hands. It also lets you light a match as shown in the Figure 1.5. On the other hand, heat from friction between moving parts inside a car engine can be a big problem. It can cause the car to overheat. Q: How is friction reduced between the moving parts inside a car engine? A: To reduce friction, oil is added to the engine. The oil coats the surfaces of the moving parts and makes them slippery. They slide over each other more easily, so there is less friction. "
fundamental particles,T_4458,"Scientists have long wanted to find the most basic building blocks of the universe. They asked, what are the fundamental particles of matter that cannot be subdivided into smaller, simpler particles, and what holds these particles together? The quest for fundamental particles began thousands of years ago. Scientists thought they had finally found them when John Dalton discovered the atom in 1803 (see the timeline in Table 1.1). The word atom means indivisible, and Dalton thought that the atom could not be divided into smaller, simpler particles. Year Discovery Year 1803 Discovery John Dalton discovers the atom. 1897 J.J. Thomson discovers the electron, the first lepton to be discovered. 1905 Albert Einstein discovers the photon, the first boson to be discovered. 1911 Ernest Rutherford discovers the proton, the first particle to be discovered in the nucleus of the atom. Year 1932 Discovery James Chadwick discovers the neutron, another particle in the nucleus. 1964 Murray Gell-Mann proposes the existence of quarks, the fundamental particles that make up protons and neutrons. 1964-present Through the research of many scientists, many other fundamental particles (except gravitons) are shown to exist. For almost 100 years after Dalton discovered atoms, they were accepted as the fundamental particles of matter. But starting in the late 1890s with the discovery of electrons, particles smaller and simpler than atoms were identified. Within a few decades, protons and neutrons were also discovered. Ultimately, hundreds of subatomic particles were found. "
fundamental particles,T_4459,"Today, scientists think that electrons truly are fundamental particles that cannot be broken down into smaller, simpler particles. They are a type of fundamental particles called leptons. Protons and neutrons, on the other hand, are no longer thought to be fundamental particles. Instead, they are now thought to consist of smaller, simpler particles of matter called quarks. Scientists theorize that leptons and quarks are held together by yet another type of fundamental particles called bosons. All three types of fundamental particlesleptons, quarks, and bosonsare described below. The following Figure 1.1 shows the variety of particles of each type. There are six types of quarks. In ordinary matter, virtually all quarks are of the types called up and down quarks. All quarks have mass, and they have an electric charge of either +2/3 or -1/3. For example, up quarks have a charge of +2/3, and down quarks have a charge of -1/3. Quarks also have a different type of charge, called color charge, although it has nothing to do with the colors that we see. Quarks are never found alone but instead always occur in groups of two or three quarks. There are also six types of leptons, including electrons. Leptons have an electric charge of either -1 or 0. Electrons, for example, have a charge of -1. Leptons have mass, although the mass of electrons is extremely small. There are four known types of bosons, which are force-carrying particles. Each of these bosons carries a different fundamental force between interacting particles. In addition, there is a particle which may exist, called the ""Higgs Boson"", which gives objects the masses they have. Some types of bosons have mass; others are massless. Bosons have an electric charge of +1, -1, or 0. Q: Protons consist of three quarks: two up quarks and one down quark. Neutrons also consist of three quarks: two down quarks and one up quark. Based on this information, what is the total electric charge of a proton? Of a neutron? A: These combinations of quarks give protons a total electric charge of +1 (2/3 + 2/3 - 1/3 = 1) and neutrons a total electric charge of 0 (2/3 - 1/3 - 1/3 = 0). "
fundamental particles,T_4460,"The interactions of matter particles are subject to four fundamental forces: gravity, electromagnetic force, weak nuclear force, and strong nuclear force. All of these forces are thought to be transmitted by bosons, the force- carrying fundamental particles. The different types of bosons and the forces they carry are shown in Table 1.2. Consider the examples of gluons, the bosons that carry the strong nuclear force. A continuous exchange of gluons between quarks binds them together in both protons and neutrons. Note that force-carrying particles for gravity (gravitons) have not yet been found. Type of Bosons Gluons Fundamental Force They Carry strong nuclear force Particles They Affect quarks Distance over Which They Carry Force only within the nucleus Type of Bosons W bosons Z bosons Photons Gravitons (hypothetical) Fundamental Force They Carry weak nuclear force Particles They Affect leptons and quarks Distance over Which They Carry Force only within the nucleus electromagnetic force force of gravity leptons and quarks leptons and quarks all distances all distances Q: Which type of boson carries force between the negative electrons and positive protons of an atom? A: Photons carry electromagnetic force. They are responsible for the force of attraction or repulsion between all electrically charged matter, including the force of attraction between negative electrons and positive protons in an atom. Q: Gravitons have not yet been discovered so they have only been hypothesized to exist. What evidence do you think leads scientists to think that these hypothetical particles affect both leptons and quarks and that they carry force over all distances? A: Gravity is known to affect all matter that has mass, and both quarks and leptons have mass. Gravity is also known to work over long as well as short distances. For example, Earths gravity keeps you firmly planted on the ground and also keeps the moon orbiting around the planet. "
fundamental particles,T_4461,"Based on their knowledge of subatomic particles, scientists have developed a theory called the standard model to explain all the matter in the universe and how it is held together. The model includes only the fundamental particles in the Table 1.2. No other particles are needed to explain all kinds of matter. According to the model, all known matter consists of quarks and leptons that interact by exchanging bosons, which transmit fundamental forces. The standard model is a good theory because all of its predictions have been verified by experimental data. However, the model doesnt explain everything, including the force of gravity and why matter has mass. Scientists continue to search for evidence that will allow them to explain these aspects of force and matter as well. "
gamma decay,T_4462,"Gamma rays are electromagnetic waves. Electromagnetic waves are waves of electric and magnetic energy that travel through space at the speed of light. The energy travels in tiny packets of energy, called photons. Photons of gamma energy are called gamma particles. Other electromagnetic waves include microwaves, light rays, and X rays. Gamma rays have the greatest amount of energy of all electromagnetic waves. Click image to the left or use the URL below. URL: "
gamma decay,T_4463,"Gamma rays are produced when radioactive elements decay. Radioactive elements are elements with unstable nuclei. To become more stable, the nuclei undergo radioactive decay. In this process, the nuclei give off energy and may also emit charged particles of matter. Types of radioactive decay include alpha, beta, and gamma decay. In alpha and beta decay, both particles and energy are emitted. In gamma decay, only energy, in the form of gamma rays, is emitted. Alpha and beta decay occur when a nucleus has too many protons or an unstable ratio of protons to neutrons. When the nucleus emits a particle, it gains or loses one or two protons, so the atom becomes a different element. Gamma decay, in contrast, occurs when a nucleus is in an excited state and has too much energy to be stable. This often happens after alpha or beta decay has occurred. Because only energy is emitted during gamma decay, the number of protons remains the same. Therefore, an atom does not become a different element during this type of decay. Q: The Figure 1.1 shows how helium-3 (He-3) decays by emitting a gamma particle. How can you tell that the atom is still the same element after gamma decay occurs? A: The nucleus of the atom has two protons (red) before the reaction occurs. After the nucleus emits the gamma particle, it still has two protons, so the atom is still the same element. "
gamma decay,T_4464,Gamma rays are the most dangerous type of radiation. They can travel farther and penetrate materials more deeply than can the charged particles emitted during alpha and beta decay. Gamma rays can be stopped only by several centimeters of lead or several meters of concrete. Its no surprise that they can penetrate and damage cells deep inside the body. 
gamma rays,T_4465,"Electromagnetic waves transfer energy across space as well as through matter. They vary in their wavelengths and frequencies, and higher-frequency waves have more energy. The full range of wavelengths of electromagnetic waves, shown in the Figure 1.1, is called the electromagnetic spectrum. "
gamma rays,T_4466,"As you can see in the Figure 1.1, gamma rays have the shortest wavelengths and highest frequencies of all electromagnetic waves. Their wavelengths are shorter than the diameter of atomic nuclei, and their frequencies are greater than 1019 hertz (Hz). Thats 10 quadrillion waves per second! Because of their high frequencies, gamma rays are also the most energetic of all electromagnetic waves. "
gamma rays,T_4467,"Gamma rays are given off by radioactive atoms and nuclear explosions. They are also given off by the sun and other stars, as well as by collapsing stars in gamma ray bursts. Fortunately, gamma rays from space are absorbed by Earths atmosphere before they can reach the surface. Q: Predict how gamma rays might affect living things on Earth if they werent absorbed by the atmosphere. A: Gamma rays would destroy most living things on Earth because they have so much energy. "
gamma rays,T_4468,"The extremely high energy of gamma rays allows them to penetrate just about anything. They can even pass through bones and teeth. This makes gamma rays very dangerous. They can destroy living cells, produce gene mutations, and cause cancer. Ironically, the deadly effects of gamma rays can be used to treat cancer. In this type of treatment, a medical device sends out focused gamma rays that target cancerous cells. The gamma rays kill the cells and destroy the cancer. "
gravity,T_4472,"Gravity has traditionally been defined as a force of attraction between things that have mass. According to this conception of gravity, anything that has mass, no matter how small, exerts gravity on other matter. Gravity can act between objects that are not even touching. In fact, gravity can act over very long distances. However, the farther two objects are from each other, the weaker is the force of gravity between them. Less massive objects also have less gravity than more massive objects. "
gravity,T_4473,"You are already very familiar with Earths gravity. It constantly pulls you toward the center of the planet. It prevents you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects that are above the surfacefrom meteors to skydiversdown to the ground. Gravity between Earth and the moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth and the other planets moving around the much more massive sun. Q: There is a force of gravity between Earth and you and also between you and all the objects around you. When you drop a paper clip, why doesnt it fall toward you instead of toward Earth? A: Earth is so much more massive than you that its gravitational pull on the paper clip is immensely greater. "
gravity,T_4474,"Weight measures the force of gravity pulling downward on an object. The SI unit for weight, like other forces, is the Newton (N). On Earth, a mass of 1 kilogram has a weight of about 10 Newtons because of the pull of Earths gravity. On the moon, which has less gravity, the same mass would weigh less. Weight is measured with a scale, like the spring scale shown in the Figure 1.1. The scale measures the force with which gravity pulls an object downward. To delve a little deeper into weight and gravity, watch this video: Click image to the left or use the URL below. URL: "
gravity,T_4475,"At the following URL, read about gravity and tides. Watch the animation and look closely at the diagrams. Then answer the questions below. 1. 2. 3. 4. 5. What causes tides? Which has a greater influence on tides, the moon or the sun? Why? Why is there a tidal bulge of water on the opposite side of Earth from the moon? When are tides highest? What causes these tides to be highest? When are tides lowest? What causes these tides to be lowest? "
groups with metalloids,T_4476,"Groups 13-16 of the periodic table (orange in the Figure 1.1) are the only groups that contain elements classified as metalloids. Unlike other groups of the periodic table, which contain elements in just one class, groups 13-16 contain elements in at least two different classes. In addition to metalloids, they also contain metals, nonmetals, or both. Groups 13-16 fall between the transition metals (in groups 3-12) and the nonmetals called halogens (in group 17). "
groups with metalloids,T_4477,"Metalloids are the smallest class of elements, containing just six members: boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), and tellurium (Te). Metalloids have some properties of metals (elements that can conduct electricity) and some properties of nonmetals (elements that cannot conduct electricity). For example, most metalloids can conduct electricity, but not as well as metals. Metalloids also tend to be shiny like metals, but brittle like nonmetals. Chemically, metalloids may behave like metals or nonmetals, depending on their number of valence electrons. Q: Why does the chemical behavior of an element depend on its number of valence electrons? A: Valence electrons are the electrons in an atoms outer energy level that may be involved in chemical reactions with other atoms. "
groups with metalloids,T_4478,"Group 13 of the periodic table is also called the boron group because boron (B) is the first element at the top of the group (see Figure 1.2). Boron is also the only metalloid in this group. The other four elements in the groupaluminum (Al), gallium (Ga), indium (In), and thallium (Tl)are all metals. Group 13 elements have three valence electrons and are fairly reactive. All of them are solids at room temperature. "
groups with metalloids,T_4479,"Group 14 of the periodic table is headed by the nonmetal carbon (C), so this group is also called the carbon group. Carbon is followed by silicon (Si) and germanium (Ge) (Figure 1.3), which are metalloids, and then by tin (Sn) and lead (Pb), which are metals. Group 14 elements group have four valence electrons, so they generally arent very reactive. All of them are solids at room temperature. "
groups with metalloids,T_4480,"Group 15 of the periodic table is also called the nitrogen group. The first element in the group is the nonmetal nitrogen (N), followed by phosphorus (P), another nonmetal. Arsenic (As) (Figure 1.4) and antimony (Sb) are the metalloids in this group, and bismuth (Bi) is a metal. All group 15 elements have five valence electrons, but they Germanium is a brittle, shiny, silvery- white metalloid. Along with silicon, it is used to make the tiny electric cir- cuits on computer chips. It is also used to make fiber optic cableslike the one pictured herethat carry telephone and other communication signals. vary in their reactivity. Nitrogen, for example, is not very reactive at all, whereas phosphorus is very reactive and found naturally only in combination with other substances. All group 15 elements are solids, except for nitrogen, which is a gas. "
groups with metalloids,T_4481,"Group 16 of the periodic table is also called the oxygen group. The first three elementsoxygen (O), sulfur (S), and selenium (Se)are nonmetals. They are followed by tellurium (Te) (Figure 1.5), a metalloid, and polonium (Po), a metal. All group 16 elements have six valence electrons and are very reactive. Oxygen is a gas at room temperature, and the other elements in the group are solids. Q: With six valence electrons, group 16 elements need to attract two electrons from another element to have a stable electron arrangement of eight valence electrons. Which group of elements in the periodic table do you think might The most common form of the metalloid arsenic is gray and shiny. Arsenic is extremely toxic, so it is used as rat poison. Surprisingly, we need it (in tiny amounts) for normal growth and a healthy nervous system. form compounds with elements in group 16? A: Group 2 elements, called the alkaline Earth metals, form compounds with elements in the oxygen group. Thats because group 2 elements have two valence electrons that they are eager to give up. An example of a group 2 and group 6 compound is calcium oxide (CaO). "
halogens,T_4482,"Halogens are highly reactive nonmetallic elements in group 17 of the periodic table. As you can see in the periodic table 1.1, the halogens include the elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). All of them are relatively common on Earth except for astatine. Astatine is radioactive and rapidly decays to other, more stable elements. As a result, it is one of the least common elements on Earth. Q: Based on their position in the periodic table from the Figure 1.1, how many valence electrons do you think halogens have? A: The number of valence electrons starts at one for elements in group 1. It then increases by one from left to right across each period (row) of the periodic table for groups 1-2 and 13-18 (numbered 3-0 in the periodic table above.) Therefore, halogens have seven valence electrons. "
halogens,T_4483,"The halogens are among the most reactive of all elements, although reactivity declines from the top to the bottom of the halogen group. Because all halogens have seven valence electrons, they are eager to gain one more electron. Doing so gives them a full outer energy level, which is the most stable arrangement of electrons. Halogens often combine with alkali metals in group 1 of the periodic table. Alkali metals have just one valence electron, which they are equally eager to donate. Reactions involving halogens, especially halogens near the top of the group, may be explosive. You can see some examples in the video below. (Warning: Dont try any of these reactions at home!) Click image to the left or use the URL below. URL: "
halogens,T_4484,"The halogen group is quite diverse. It includes elements that occur in three different states of matter at room temperature. Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. Halogens also vary in color, as you can see in the Figure 1.2. Fluorine and chlorine are green, bromine is red, and iodine and astatine are nearly black. Like other nonmetals, halogens cannot conduct electricity or heat. Compared with most other elements, halogens have relatively low melting and boiling points. "
halogens,T_4485,Most halogens have a variety of important uses. A few are described in the Figure 1.3. Q: Can you relate some of these uses of halogens to the properties of these elements? A: The ability of halogens to kill germs and bleach clothes relates to their highly reactive nature. 
hearing and the ear,T_4486,"Sound is a form of energy that travels in waves through matter. The ability to sense sound energy and perceive sound is called hearing. The organ that we use to sense sound energy is the ear. Almost all the structures in the ear are needed for this purpose. Together, they gather sound waves, amplify the waves, and change their kinetic energy to electrical signals. The electrical signals travel to the brain, which interprets them as the sounds we hear. The Figure 1.1 shows the three main parts of the ear: the outer, middle, and inner ear. It also shows the specific structures in each part of the ear. "
hearing and the ear,T_4487,"The outer ear includes the pinna, ear canal, and eardrum. The pinna is the only part of the ear that extends outward from the head. Its position and shape make it good at catching sound waves and funneling them into the ear canal. The ear canal is a tube that carries sound waves into the ear. The sound waves travel through the air inside the ear canal to the eardrum. The eardrum is like the head of a drum. It is a thin membrane stretched tight across the end of the ear canal. The eardrum vibrates when sound waves strike it, and it sends the vibrations on to the middle ear. Q: How might cupping his hands behind his ears help the boy pictured in the opening image hear better? A: His hands might help the pinna of his ears catch sound waves and direct them into the ear canal. "
hearing and the ear,T_4488,"The middle ear contains three tiny bones (ossicles) called the hammer, anvil, and stirrup. If you look at these bones in the Figure 1.1, you might notice that they resemble the objects for which they are named. The three bones transmit vibrations from the eardrum to the inner ear. The arrangement of the three bones allows them to work together as a lever that increases the amplitude of the waves as they pass to the inner ear. Q: Wave amplitude is the maximum distance particles of matter move when a wave passes through them. Why would amplifying the sound waves as they pass through the middle ear improve hearing? A: Amplified sound waves have more energy. This increases the intensity and loudness of the sounds, so they are easier to hear. "
hearing and the ear,T_4489,"The stirrup in the middle ear passes the amplified sound waves to the inner ear through the oval window. When the oval window vibrates, it causes the cochlea to vibrate as well. The cochlea is a shell-like structure that is full of fluid and lined with nerve cells called hair cells. Each hair cell has many tiny hairs, as you can see in the magnified image 1.2. When the cochlea vibrates, it causes waves in the fluid inside. The waves bend the hairs on the hair cells, and this triggers electrical impulses. The electrical impulses travel to the brain through nerves. Only after the nerve impulses reach the brain do we hear the sound. "
hearing loss,T_4490,"The ear is a complex organ that senses sound energy so we can hear. Hearing is the ability to sense sound energy and perceive sound. All of the structures of the ear that are involved in hearing must work well for a person to have normal hearing. Damage to any of the structures, through illness or injury, may cause hearing loss. Total hearing loss is called deafness. "
hearing loss,T_4491,"The most common cause of hearing loss is exposure to loud sounds. Loud sounds can damage hair cells inside the ears. Hair cells change sound waves to electrical signals that the brain can interpret as sounds. Louder sounds, which have greater intensity than softer sounds, can damage hair cells more quickly than softer sounds. You can see the relationship between sound intensity, exposure time, and hearing loss in the following Figure 1.1. The intensity of sounds is measured in decibels (dB). Q: What is the maximum amount of time you should be exposed to a sound as intense as 100 dB? What might make a sound this intense? A: You should be exposed to a 100-dB sound for no longer than 15 minutes. An example of a sound this intense is the sound of a car horn. "
hearing loss,T_4492,"Hearing loss caused by loud sounds is permanent. However, this type of hearing loss can be prevented by protecting the ears from loud sounds. People who work in jobs that expose them to loud sounds must wear hearing protectors. Examples include construction workers who work around loud machinery for many hours each day. But anyone exposed to loud sounds for longer than the permissible exposure time should wear hearing protectors. Many home and yard chores and even recreational activities are loud enough to cause hearing loss if people are exposed to them for too much time. You can see examples in the Figure 1.2. "
hearing loss,T_4493,"You can see two different types of hearing protectors in the Figure 1.3. Earplugs are simple hearing protectors that just muffle sounds by partially blocking all sound waves from entering the ears. This type of hearing protector is suitable for lower noise levels, such as the noise of a lawnmower or snowmobile. Electronic ear protectors work differently. They identify high-amplitude sound waves and send sound waves through them in the opposite direction. This causes destructive interference with the waves, which reduces their amplitude to zero or nearly zero. This changes even the loudest sounds to just a soft hiss. Sounds that people need to hear, such as the voices of co-workers, are not interfered with in this way and may be amplified instead so they can be heard more clearly. This type of hearing protector is recommended for higher noise levels and situations where its important to be able to hear lower-decibel sounds. "
heat,T_4494,"Heat is the transfer of thermal energy between substances. Thermal energy is the kinetic energy of moving particles of matter, measured by their temperature. Thermal energy always moves from matter with greater thermal energy to matter with less thermal energy, so it moves from warmer to cooler substances. You can see this in the Figure particles of the cooler substance. Thermal energy is transferred in this way until both substances have the same thermal energy and temperature. Q: How is thermal energy transferred in an oven? A: Thermal energy of the hot oven is transferred to the cooler food, raising its temperature. "
heat,T_4495,"How do you cool down a glass of room-temperature cola? You probably add ice cubes to it, as in the Figure 1.2. You might think that the ice cools down the cola, but in fact, it works the other way around. The warm cola heats up the ice. Thermal energy from the warm cola is transferred to the much colder ice, causing it to melt. The cola loses thermal energy in the process, so its temperature falls. "
heat conduction,T_4496,"Conduction is the transfer of thermal energy between particles of matter that are touching. Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Conduction is one of three ways that thermal energy can be transferred (the other ways are convection and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature. "
heat conduction,T_4497,"To understand how conduction works, you need to think about the tiny particles that make up matter. The particles of all matter are in constant random motion, but the particles of warmer matter have more energy and move more quickly than the particles of cooler matter. When particles of warmer matter collide with particles of cooler matter, they transfer some of their thermal energy to the cooler particles. From particle to particle, like dominoes falling, thermal energy moves through matter. In the opening photo above, conduction occurs between particles of metal in the cookie sheet and anything cooler that comes into contact with ithopefully, not someones bare hands! "
heat conduction,T_4498,"The cookie sheet in the opening image transfers thermal energy to the cookies and helps them bake. There are many other common examples of conduction. The Figure 1.1 shows a few situations in which thermal energy is transferred in this way. Q: How is thermal energy transferred in each of the situations pictured in the Figure 1.1? A: Thermal energy is transferred by conduction from the hot iron to the shirt, from the hot cup to the hand holding it, from the flame of the camp stove to the bottom of the pot as well as from the bottom of the pot to the food inside, and from the feet to the snow. The shirt, hand, pot, food, and snow become warmer because of the transferred energy. Because the feet lose thermal energy, they feel colder. "
heating systems,T_4499,"Modern home heating systems keep us comfortable in cold weather. We may even depend on them for our survival. But we often take them for granted. Two common types of home heating systems are hot-water and warm-air heating systems. Both types are described below. Thermal energy is the total energy of moving particles of matter. The transfer of thermal energy is called heat. Therefore, a heating system is a system for the transfer of thermal energy. Regardless of the type of heating system in a home, the basic function is the same: to produce thermal energy and transfer it to air throughout the house. "
heating systems,T_4500,"A hot-water heating system produces thermal energy to heat water and then pumps the hot water throughout the building in a system of pipes and radiators. You can see a simple diagram of this type of heating system in the Figure 1.1. Water is heated in a boiler that burns a fuel such as natural gas or heating oil. The boiler converts the chemical energy stored in the fuel to thermal energy. The heated water is pumped from the boiler through pipes and radiators throughout the house. There is usually a radiator in each room. The radiators get warm when the hot water flows through them. The warm radiators radiate thermal energy to the air around them. The warm air then circulates throughout the rooms in convection currents. The hot water cools as it flows through the system and transfers its thermal energy. When it finally returns to the boiler, it is heated again and the cycle repeats. Q: Look closely at the hot-water heating system in the Figure 1.1. The radiator is a coiled pipe through which hot water flows. What happens to the water as it flows through the radiator? Why is each radiator connected to two pipes? Why cant water flow directly from one radiator to another through a single pipe? A: The radiator is where most of the energy transfer occurs. Water passes through such a great length of pipe in the radiator that it transfers a lot of thermal energy to the radiator. As the water transfers thermal energy, it gets cooler. The cool water flows into a return pipe rather than going directly to another radiator because the cool water no longer has enough thermal energy to heat a room. "
heating systems,T_4501,"A warm-air heating system uses thermal energy to heat air and then forces the warm air through a system of ducts and registers. You can see a this type of heating system in the Figure 1.2. The air is heated in a furnace that burns fuel such as natural gas, propane, or heating oil. After the air gets warm, a fan blows it through the ducts and out through the registers that are located in each room. Warm air blowing out of a register moves across the room, pushing cold air out of the way. The cold air enters a return register across the room and returns to the furnace with the help of another fan. In the furnace, the cold air is heated, and the cycle repeats. Q: How does a home heating system know when to run and when to stop running? A: A home heating system is turned on or off by a thermostat. "
heating systems,T_4502,"A thermostat, like the one seen in the Figure 1.3, is an important part of any home heating system. It is like the brain of the entire system. It constantly monitors the temperature in the home and tells the boiler or furnace when to turn on or off. The thermostat is set at a selected temperature, say 71  F. When the temperature in the home starts to fall below this point, the thermostat triggers the boiler or furnace to start running. When the temperature starts to rise above this point, the thermostat triggers the boiler or furnace to stop running. In this way, the thermostat maintains the homes temperature at the set point. "
hydrocarbons,T_4508,"Hydrocarbons are compounds that contain only carbon and hydrogen. Hydrocarbons are the simplest type of carbon-based compounds, but they can vary greatly in size. The smallest hydrocarbons have just one or two carbon atoms. The largest hydrocarbons may have thousands of carbon atoms. Q: How are hydrocarbons involved in each of the photos pictured above? A: The main ingredient of mothballs is the hydrocarbon naphthalene. The main ingredient in nail polish remover is the hydrocarbon acetone. The lawn mower runs on a mixture of hydrocarbons called gasoline, and the camp stove burns a hydrocarbon fuel named isobutane. "
hydrocarbons,T_4509,"The size of hydrocarbon molecules influences their properties, including their melting and boiling points. As a result, some hydrocarbons are gases at room temperature, while others are liquids or solids. Hydrocarbons are generally nonpolar, which means that their molecules do not have oppositely charged sides. Therefore, they do not dissolve in water, which is a polar compound. In fact, hydrocarbons tend to repel water. Thats why they are used in floor wax and similar products. "
hydrocarbons,T_4510,"Hydrocarbons are placed in two different classes: saturated hydrocarbons and unsaturated hydrocarbons. This classification is based on the number of bonds between carbon atoms. Saturated hydrocarbons have only single bonds between carbon atoms, so the carbon atoms are bonded to as many hydrogen atoms as possible. In other words, they are saturated with hydrogen atoms. Unsaturated hydrocarbons have at least one double or triple bond between carbon atoms, so the carbon atoms are not bonded to as many hydrogen atoms as possible. In other words, they are unsaturated with hydrogen atoms. "
hydrocarbons,T_4511,"It is hard to overstate the importance of hydrocarbons to modern life. Hydrocarbons have even been called the driving force of western civilization. You saw some ways they are used in the opening image. Several other ways are pictured in the Figure 1.1. The most important use of hydrocarbons is for fuel. Gasoline, natural gas, fuel oil, diesel fuel, jet fuel, coal, kerosene, and propane are just some of the commonly used hydrocarbon fuels. Hydrocarbons are also used to make things, including plastics and synthetic fabrics such as polyester. Motor oil: Motor oil consists of several hydrocarbons. It lubricates the moving parts of car engines. Asphalt: Asphalt pavement on highways is made of hy- drocarbons found in petroleum. Candle: Many candles are made of paraffin wax, a solid mixture of hydrocarbons. Lighter: This lighter burns the hydrocarbon named butane. Rain Boots: These rain boots are made of a mixture of several hydro- carbons. Transportation: These forms of transportation are fueled by different mixtures of hydrocarbons. "
hydrocarbons,T_4512,"The main source of hydrocarbons is fossil fuelscoal, petroleum, and natural gas. Fossil fuels formed over hundreds of millions of years, as dead organisms were covered with sediments and put under great pressure. Giant ferns in ancient swamps turned into coal deposits. The Figure 1.2 shows one way that coal deposits are mined. Dead organisms in ancient seas gradually formed deposits of petroleum and natural gas. Open-Pit Coal Mine "
hydrogen and alkali metals,T_4513,"Sodium (Na) is an element in group 1 of the periodic table of the elements. This group (column) of the table is shown in Figure below. It includes the nonmetal hydrogen (H) and six metals that are called alkali metals. Elements in the same group of the periodic table have the same number of valence electrons. These are the electrons in their outer energy level that can be involved in chemical reactions. Valence electrons determine many of the properties of an element, so elements in the same group have similar properties. All the elements in group 1 have just one valence electron. This makes them very reactive. Q: Why does having just one valence electron make group 1 elements very reactive? A: With just one valence electron, group 1 elements are eager to lose that electron. Doing so allows them to achieve a full outer energy level and maximum stability. "
hydrogen and alkali metals,T_4514,"Hydrogen is a very reactive gas, and the alkali metals are even more reactive. In fact, they are the most reactive metals and, along with the elements in group 17, are the most reactive of all elements. The reactivity of alkali metals increases from the top to the bottom of the group, so lithium (Li) is the least reactive alkali metal and francium (Fr) is the most reactive. Because alkali metals are so reactive, they are found in nature only in combination with other elements. They often combine with group 17 elements, which are very eager to gain an electron. Click image to the left or use the URL below. URL: "
hydrogen and alkali metals,T_4515,"Besides being very reactive, alkali metals share a number of other properties. Alkali metals are all solids at room temperature. Alkali metals are low in density, and some of them float on water. Alkali metals are relatively soft. Some are even soft enough to cut with a knife, like the sodium pictured in the Figure 1.1. "
hydrogen and alkali metals,T_4516,"Although all group 1 elements share certain properties, such as being very reactive, they are not alike in every way. Three different group 1 elements are described in more detail below. Notice the ways in which they differ from one another. Q: Why do you think hydrogen gas usually exists as diatomic molecules? A: Each hydrogen atom has just one electron. When two hydrogen atoms bond together, they share a pair of electrons. The shared electrons fill their only energy level, giving them the most stable arrangement of electrons. Potassium is a soft, silvery metal that ignites explosively in water. It easily loses its one valence electron to form positive potassium ions (K+ ), which are needed by all living cells. Potassium is so impor- tant for plants that it is found in almost all fertilizers, like the one shown here. Potassium is abundant in Earths crust in minerals such as feldspar. Francium has one of the largest, heaviest atoms of all elements. Its one valence electron is far removed from the nucleus, as you can see in the atomic model on the right, so it is easily removed from the atom. Francium is radioactive and quickly decays to form other elements such as radium. This is why francium is extremely rare in nature. Less than an ounce of francium is present on Earth at any given time. Q: Francium decays too quickly to form compounds with other elements. Which elements to you think it would bond with if it could? A: With one valence electron, francium would bond with a halogen element in group 17, which has seven valence electrons and needs one more to fill its outer energy level. Elements in group 17 include fluorine and chlorine. "
hydrogen bonding,T_4517,"Polar compounds, such as water, are compounds that have a partial negative charge on one side of each molecule and a partial positive charge on the other side. All polar compounds contain polar bonds (although not all compounds that contain polar bonds are polar.) In a polar bond, two atoms share electrons unequally. One atom attracts the shared electrons more strongly, so it has a partial negative charge. The other atom attracts the shared electrons less strongly, so it is has a partial positive charge. In a water molecule, the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do. This explains why the oxygen side of the water molecule has a partial negative charge and the hydrogen side of the molecule has a partial positive charge. Q: If a molecule is polar, how might this affect its interactions with nearby molecules of the same compound? A: Opposite charges on different molecules of the same compound might cause the molecules to be attracted to each other. "
hydrogen bonding,T_4518,"Because of waters polarity, individual water molecules are attracted to one another. You can see this in the Figure of a nearby water molecule. This force of attraction is called a hydrogen bond. Hydrogen bonds are intermolecular (between-molecule) bonds, rather than intramolecular (within-molecule) bonds. They occur not only in water but in other polar molecules in which positive hydrogen atoms are attracted to negative atoms in nearby molecules. Hydrogen bonds are relatively weak as chemical bonds go. For example, they are much weaker than the bonds holding atoms together within molecules of covalent compounds. Click image to the left or use the URL below. URL: "
hydrogen bonding,T_4519,"Changes of state from solid to liquid and from liquid to gas occur when matter gains energy. The energy allows individual molecules to separate and move apart from one another. It takes more energy to bring about these changes of state for polar molecules. Although hydrogen bonds are weak, they add to the energy needed for molecules to move apart from one another, so it takes higher temperatures for these changes of state to occur in polar compounds. This explains why polar compounds have relatively high melting and boiling points. The Table 1.1 compares melting and boiling points for some polar and nonpolar covalent compounds. Name of Compound (Chemical Formula) Methane (CH4 ) Ethylene (C2 H2 ) Ammonia (NH3 ) Water (H2 O) Polar or Nonpolar? Melting Point( C) Boiling Point ( C) nonpolar nonpolar polar polar -182 -169 -78 0 -162 -104 -33 100 Q: Which compound in the Table 1.1 do you think is more polar, ammonia or water? "
inclined plane,T_4525,"An inclined plane is a simple machine that consists of a sloping surface connecting a lower elevation to a higher elevation. An inclined plane is one of six types of simple machines, and it is one of the oldest and most basic. In fact, two other simple machines, the wedge and the screw, are variations of the inclined plane. A ramp like the one in the Figure 1.1 is another example of an inclined plane. Inclined planes make it easier to move objects to a higher elevation. The sloping surface of the inclined plane supports part of the weight of the object as it moves up the slope. As a result, it takes less force to move the object uphill. The trade-off is that the object must be moved over a greater distance than if it were moved straight up to the higher elevation. "
inclined plane,T_4526,"The mechanical advantage of a simple machine is the factor by which it multiplies the force applied to the machine. It is the ratio of output force (the force put out by the machined) to input force (the force put into the machine). For an inclined plane, less force is put into moving an object up the slope than if the object were lifted straight up, so the mechanical advantage is greater than 1. The more gradual the slope of the inclined plane, the less input force is needed and the greater the mechanical advantage. Q: Which inclined plane pictured in the Figure 1.2 has a greater mechanical advantage? A: The inclined plane on the right has a more gradual slope, so it has a greater mechanical advantage. Less force is needed to move objects up the gentler slope, yet the objects attain the same elevation as they would if more force were used to push them up the steeper slope. "
inertia,T_4527,"Inertia is the tendency of an object to resist a change in its motion. All objects have inertia, whether they are stationary or moving. Inertia explains Newtons first law of motion, which states that an object at rest will remain at rest and an object in motion will stay in motion unless it is acted on by an unbalanced force. Thats why Newtons first law of motion is sometimes called the law of inertia. Q: You probably dont realize it, but you experience inertia all the time, and you dont have to ride a skateboard. For example, think about what happens when you are riding in a car that stops suddenly. Your body moves forward on the seat and strains against the seat belt. Why does this happen? A: The brakes stop the car but not your body, so your body keeps moving forward because of inertia. "
inertia,T_4528,"The inertia of an object depends on its mass. Objects with greater mass also have greater inertia. It would be easier for Lauren to push just one of her cousins on her skateboard than both of them. With just one twin, there would be only about half as much mass on the skateboard, so there would be less inertia to overcome. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
inertia,T_4529,"To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object. The unbalanced force that starts Laurens cousins rolling along on the skateboard is applied by Lauren when she gives it a push. Once an object starts moving, inertia keeps it moving without any additional force being applied. In fact, it wont stop moving unless another unbalanced force opposes its motion. For example, Lauren can stop the rolling skateboard by moving to the other end and pushing in the opposite direction. Q: What if Lauren didnt stop the skateboard in this way? If it remained on a smooth, flat surface, would it just keep rolling forever? A: The inertia of the moving skateboard would keep it rolling forever if no other unbalanced force opposed its motion. However, another unbalanced force does act on the skateboard Q: What other force is acting on the skateboard? A: The other force is rolling friction between the skateboards wheels and the ground. The force of friction opposes the motion of the rolling skateboard and would eventually bring it to a stop without any help from Lauren. Friction opposes the motion of all moving objects, solike the skateboardall moving objects eventually stop moving even if no other forces oppose their motion. Later that day, Jonathan rode his skateboard and did some jumps. You can see him in the picture 1.2. When hes in the air, there is no rolling friction between his wheels and the ground, but another unbalanced force is acting on the skateboard and changing its motion. Q: What force is acting on the skateboard when it is in the air above the ground? And how will this force change the skateboards motion? A: The force of gravity is acting on the skateboard. It will pull the skateboard back down to the ground. Once its on the ground, friction will slow its motion. "
intensity and loudness of sound,T_4530,"Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of the sound waves. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (dB). "
intensity and loudness of sound,T_4531,"The Figure 1.1 shows decibel levels of several different sounds. As decibel levels get higher, sound waves have greater intensity and sounds are louder. For every 10-decibel increase in the intensity of sound, loudness is 10 times greater. Therefore, a 30-decibel quiet room is 10 times louder than a 20-decibel whisper, and a 40-decibel light rainfall is 100 times louder than the whisper. High-decibel sounds are dangerous. They can damage the ears and cause loss of hearing. Q: How much louder than a 20-decibel whisper is the 60-decibel sound of a vacuum cleaner? A: The vacuum cleaner is 10,000 times louder than the whisper! "
intensity and loudness of sound,T_4532,"The intensity of sound waves determines the loudness of sounds, but what determines intensity? Intensity results from two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound. Amplitude is a measure of the size of sound waves. It depends on the amount of energy that started the waves. Greater amplitude waves have more energy and greater intensity, so they sound louder. As sound waves travel farther from their source, the more spread out their energy becomes. You can see how this works in the Figure 1.2. As distance from the sound source increases, the area covered by the sound waves increases. The same amount of energy is spread over a greater area, so the intensity and loudness of the sound is less. This explains why even loud sounds fade away as you move farther from the source. Q: Why can low-amplitude sounds like whispers be heard only over short distances? A: The sound waves already have so little energy that spreading them out over a wider area quickly reduces their intensity below the level of hearing. "
internal combustion engines,T_4533,"A combustion engine is a complex machine that burns fuel to produce thermal energy and then uses the energy to do work. In a car, the engine does the work of providing kinetic energy that turns the wheels. The combustion engine in a car is a type of engine called an internal combustion engine. (Another type of combustion engine is an external combustion engine.) "
internal combustion engines,T_4534,"An internal combustion engine burns fuel internally, or inside the engine. This type of engine is found not only in cars but in most other motor vehicles as well. The engine works in a series of steps, which keep repeating. You can follow the steps in the Figure 1.1. 1. A mixture of fuel and air is pulled-into a cylinder through a valve, which then closes. 2. A piston inside the cylinder moves upward, compressing the fuel-air mixture in the closed cylinder. The mixture is now under a lot of pressure and very warm. 3. A spark from a spark plug ignites the fuel-air mixture, causing it to burn explosively within the confined space of the closed cylinder. 4. The pressure of the hot gases from combustion pushes the piston downward. 5. The piston moves up again, pushing exhaust gases out of the cylinder through another valve. 6. The piston moves downward again, and the cycle repeats. Q: The internal combustion engine converts thermal energy to another form of energy. Which form of energy is it? A: The engine converts thermal energy to kinetic energy, or the energy of a moving objectin this case, the moving piston. "
internal combustion engines,T_4535,"In a car, the piston in the engine is connected by the piston rod to the crankshaft. The crankshaft rotates when the piston moves up and down. The crankshaft, in turn, is connected to the driveshaft. When the crankshaft rotates, so does the driveshaft. The rotating driveshaft turns the wheels of the car. "
internal combustion engines,T_4536,"Most cars have at least four cylinders connected to the crankshaft. Their pistons move up and down in sequence, one after the other. A powerful car may have eight pistons, and some race cars may have even more. The more cylinders a car engine has, the more powerful its engine can be. "
international system of units,T_4537,"The example of the Mars Climate Orbiter shows the importance of using a standard system of measurement in science and technology. The measurement system used by most scientists and engineers is the International System of Units, or SI. There are a total of seven basic SI units, including units for length (meter) and mass (kilogram). SI units are easy to use because they are based on the number 10. Basic units are multiplied or divided by powers of ten to arrive at bigger or smaller units. Prefixes are added to the names of the units to indicate the powers of ten, as shown in the Table 1.1. Prefix kilo- (k) Multiply Basic Unit 1000 Basic Unit of Length = Meter (m) kilometer (km) = 1000 m Prefix deci- (d) centi- (c) milli- (m) micro- () nano- (n) Multiply Basic Unit 0.1 0.01 0.001 0.000001 0.000000001 Basic Unit of Length = Meter (m) decimeter (dm) = 0.1 m centimeter (cm) = 0.01 m millimeter (mm) = 0.001 m micrometer (m) = 0.000001 m nanometer (nm) = 0.000000001 m Q: What is the name of the unit that is one-hundredth (0.01) of a meter? A: The name of this unit is the centimeter. Q: What fraction of a meter is a decimeter? A: A decimeter is one-tenth (0.1) of a meter. "
international system of units,T_4538,"In the Table 1.2, two basic SI units are compared with their English system equivalents. You can use the information in the table to convert SI units to English units or vice versa. For example, from the table you know that 1 meter equals 39.37 inches. How many inches are there in 3 meters? 3 m = 3(39.37 in) = 118.11 in Measure Length Mass SI Unit meter (m) kilogram (kg) English Unit Equivalent 1 m = 39.37 in 1 kg = 2.20 lb Q: Rod needs to buy a meter of wire for a science experiment, but the wire is sold only by the yard. If he buys a yard of wire, will he have enough? (Hint: There are 36 inches in a yard.) A: Rod needs 39.37 inches (a meter) of wire, but a yard is only 36 inches, so if he buys a yard of wire he wont have enough. "
ionic bonding,T_4539,"An ionic bond is the force of attraction that holds together positive and negative ions. It forms when atoms of a metallic element give up electrons to atoms of a nonmetallic element. The Figure 1.1 shows how this happens. In row 1 of the Figure 1.1, an atom of sodium (Na) donates an electron to an atom of chlorine (Cl). By losing an electron, the sodium atom becomes a sodium ion. It now has more protons than electrons and a charge of +1. Positive ions such as sodium are given the same name as the element. The chemical symbol has a plus sign to distinguish the ion from an atom of the element. The symbol for a sodium ion is Na+ . By gaining an electron, the chlorine atom becomes a chloride ion. It now has more electrons than protons and a charge of -1. Negative ions are named by adding the suffix -ide to the first part of the element name. The symbol for chloride is Cl . Sodium and chloride ions have equal but opposite charges. Opposite electric charges attract each other, so sodium and chloride ions cling together in a strong ionic bond. You can see this in row 2 of the Figure 1.1. (Brackets separate the ions in the diagram to show that the ions in the compound do not actually share electrons.) When ionic bonds hold ions together, they form an ionic compound. The compound formed from sodium and chloride ions is named sodium chloride. It is commonly called table salt. "
ionic bonding,T_4540,"Ionic bonds form only between metals and nonmetals. Thats because metals want to give up electrons, and nonmetals want to gain electrons. Find sodium (Na) in the Figure 1.2. Sodium is an alkali metal in group 1. Like all group 1 elements, it has just one valence electron. If sodium loses that one electron, it will have a full outer energy level, which is the most stable arrangement of electrons. Now find fluorine (F) in the periodic table Figure gains one electron, it will also have a full outer energy level and the most stable arrangement of electrons. Q: Predict what other elements might form ionic bonds. A: Metals on the left and in the center of the periodic table form ionic bonds with nonmetals on the right of the periodic table. For example, alkali metals in group 1 form ionic bonds with halogen nonmetals in group 17. "
ionic bonding,T_4541,"It takes energy to remove valence electrons from an atom because the force of attraction between the negative electrons and the positive nucleus must be overcome. The amount of energy needed depends on the element. Less energy is needed to remove just one or a few valence electrons than many. This explains why sodium and other alkali metals form positive ions so easily. Less energy is also needed to remove electrons from larger atoms in the same group. For example, in group 1, it takes less energy to remove an electron from francium (Fr) at the bottom of the group than from lithium (Li) at the top of the group (see the Figure 1.2). In bigger atoms, valence electrons are farther from the nucleus. As a result, the force of attraction between the valence electrons and the nucleus is weaker. Q: What do you think happens when an atom gains an electron and becomes a negative ion? A: Energy is released when an atom gains an electron. Halogens release the most energy when they form ions. As a result, they are very reactive elements. "
ionic compounds,T_4542,"All compounds form when atoms of different elements share or transfer electrons. Compounds in which electrons are transferred from one atom to another are called ionic compounds. In this type of compound, electrons actually move between the atoms, rather than being shared between them. When atoms give up or accept electrons in this way, they become charged particles called ions. The ions are held together by ionic bonds, which form an ionic compound. Ionic compounds generally form between elements that are metals and elements that are nonmetals. For example, the metal calcium (Ca) and the nonmetal chlorine (Cl) form the ionic compound calcium chloride (CaCl2 ). In this compound, there are two negative chloride ions for each positive calcium ion. Because the positive and negative charges cancel out, an ionic compound is neutral in charge. Q: Now can you explain why calcium chloride prevents ice from forming on a snowy road? A: If calcium chloride dissolves in water, it breaks down into its ions (Ca2+ and Cl ). When water has ions dissolved in it, it has a lower freezing point. Pure water freezes at 0 C. With calcium and chloride ions dissolved in it, it wont freeze unless the temperature reaches -29 C or lower. "
ionic compounds,T_4543,"Many compounds form molecules, but ionic compounds form crystals instead. A crystal consists of many alternating positive and negative ions bonded together in a matrix. Look at the crystal of sodium chloride (NaCl) in the Figure bonds. Sodium chloride crystals are cubic in shape. Other ionic compounds may have crystals with different shapes. "
ionic compounds,T_4544,"Ionic compounds are named for their positive and negative ions. The name of the positive ion always comes first, followed by the name of the negative ion. For example, positive sodium ions and negative chloride ions form the compound named sodium chloride. Similarly, positive calcium ions and negative chloride ions form the compound named calcium chloride. Q: What is the name of the ionic compound that is composed of positive barium ions and negative iodide ions? A: The compound is named barium iodide. "
ionic compounds,T_4545,"The crystal structure of ionic compounds is strong and rigid. It takes a lot of energy to break all those ionic bonds. As a result, ionic compounds are solids with high melting and boiling points. You can see the melting and boiling points of several different ionic compounds in the Table 1.1. To appreciate how high they are, consider that the melting and boiling points of water, which is not an ionic compound, are 0 C and 100 C, respectively. Ionic Compound Sodium chloride (NaCl) Calcium chloride (CaCl2 ) Barium oxide (BaO) Iron bromide (FeBr3 ) Melting Point ( C) 801 772 1923 684 Boiling Point ( C) 1413 1935 2000 934 Solid ionic compounds are poor conductors of electricity. The strong bonds between their oppositely charged ions lock them into place in the crystal. Therefore, the charged particles cannot move freely and carry electric current, which is a flow of charge. But all that changes when ionic compounds dissolve in water. When they dissolve, they separate into individual ions. The ions can move freely, so they can carry current. Dissolved ionic compounds are called electrolytes. The rigid crystals of ionic compounds are brittle. They are more likely to break than bend when struck. As a result, ionic crystals tend to shatter easily. Try striking salt crystals with a hammer and youll find that they readily break into smaller pieces. Click image to the left or use the URL below. URL: "
ionic compounds,T_4546,Ionic compounds have many uses. Some are shown in the Figure 1.2. Many ionic compounds are used in industry. The human body needs several ions for good health. Having low levels of the ions can endanger important functions such as heartbeat. Solutions of ionic compounds can be used to restore the ions. 
ions,T_4547,"The northern lights arent caused by atoms, because atoms are not charged particles. An atom always has the same number of electrons as protons. Electrons have an electric charge of -1 and protons have an electric charge of +1. Therefore, the charges of an atoms electrons and protons cancel out. This explains why atoms are neutral in electric charge. Q: What would happen to an atoms charge if it were to gain extra electrons? A: If an atom were to gain extra electrons, it would have more electrons than protons. This would give it a negative charge, so it would no longer be neutral. "
ions,T_4548,"Atoms cannot only gain extra electrons. They can also lose electrons. In either case, they become ions. Ions are atoms that have a positive or negative charge because they have unequal numbers of protons and electrons. If atoms lose electrons, they become positive ions, or cations. If atoms gain electrons, they become negative ions, or anions. Consider the example of fluorine (see Figure 1.1). A fluorine atom has nine protons and nine electrons, so it is electrically neutral. If a fluorine atom gains an electron, it becomes a fluoride ion with an electric charge of -1. "
ions,T_4549,"Like fluoride, other negative ions usually have names ending in -ide. Positive ions, on the other hand, are just given the element name followed by the word ion. For example, when a sodium atom loses an electron, it becomes a positive sodium ion. The charge of an ion is indicated by a plus (+) or minus sign (-), which is written to the right of and just above the ions chemical symbol. For example, the fluoride ion is represented by the symbol F , and the sodium ion is represented by the symbol Na+ . If the charge is greater than one, a number is used to indicate it. For example, iron (Fe) may lose two electrons to form an ion with a charge of plus two. This ion would be represented by the symbol Fe2+ . This and some other common ions are listed with their symbols in the Table 1.1. Cations Name of Ion Calcium ion Hydrogen ion Iron(II) ion Iron(III) ion Chemical Symbol Ca2+ H+ Fe2+ Fe3+ Anions Name of Ion Chloride Fluoride Bromide Oxide Chemical Symbol Cl F Br O2 Q: How does the iron(III) ion differ from the iron(II) ion? A: The iron(III) ion has a charge of +3, so it has one less electron than the iron(II) ion, which has a charge of +2. Q: What is the charge of an oxide ion? How does its number of electrons compare to its number of protons? A: An oxide ion has a charge of -2. It has two more electrons than protons. "
ions,T_4550,"The process in which an atom becomes an ion is called ionization. It may occur when atoms are exposed to high levels of radiation. The radiation may give their outer electrons enough energy to escape from the attraction of the positive nucleus. However, most ions form when atoms transfer electrons to or from other atoms or molecules. For example, sodium atoms may transfer electrons to chlorine atoms. This forms positive sodium ions (Na+ ) and negative chloride ions (Cl ). Click image to the left or use the URL below. URL: "
ions,T_4551,"Ions are highly reactive, especially as gases. They usually react with ions of opposite charge to form neutral compounds. For example, positive sodium ions and negative chloride ions react to form the neutral compound sodium chloride, commonly known as table salt. This occurs because oppositely charged ions attract each other. Ions with the same charge, on the other hand, repel each other. Ions are also deflected by a magnetic field, as you saw in the opening image of the northern lights. "
isomers,T_4552,"Hydrocarbons are compounds that contain only carbon and hydrogen atoms. The smallest hydrocarbon, methane (CH4 ), contains just one carbon atom and four hydrogen atoms. Larger hydrocarbons contain many more. Hydro- carbons with four or more carbon atoms can have different shapes. Although they have the same chemical formula, with the same numbers of carbon and hydrogen atoms, they form different compounds, called isomers. Isomers are compounds whose properties are different because their atoms are bonded together in different arrangements. "
isomers,T_4553,"The smallest hydrocarbon that has isomers is butane, which has just four carbon atoms. In the Figure 1.1 you can see structural formulas for normal butane (or n-butane) and its only isomer, named iso-butane. Both molecules have four carbon atoms as well as ten hydrogen atoms (C4 H10 ), but the atoms are arranged differently in the two compounds. In n-butane, all four carbon atoms are lined up in a straight chain. In iso-butane, one of the carbon atoms branches off from the main chain. The next smallest hydrocarbon is pentane, which has five carbon atoms and twelve hydrogen atoms (C5 H12 ). Pentane has three isomers: n-pentane, iso-pentane, and neo-pentane. Their structural formulas are shown in the images below. Look at the carbon atoms in each isomer. In n-pentane (see Figure 1.2), the carbon atoms form a straight chain. In iso-pentane (see Figure 1.3), one carbon atom branches off from the main chain. In neo-pentane (see Figure 1.4), two carbon atoms branch off from the main chain. "
isomers,T_4554,"Butane has only two isomers and pentane has just three, but some hydrocarbons have many more isomers than these. As you increase the number of carbon atoms in a hydrocarbon, the number of isomers quickly increases. For example, heptane, with seven carbon atoms, has nine isomers; and dodecane, with twelve carbon atoms, has 355 isomers. Some hydrocarbons with many more carbon atoms have billions of isomers! Q: Why does the number of carbon atoms in a hydrocarbon determine how many isomers it has? A: The more carbon atoms there are, the greater the number of possible arrangements of carbon atoms. "
isomers,T_4555,"Because isomers are different compounds, they have different properties. Generally, branched-chain isomers have lower boiling and melting points than straight-chain isomers. For example, the boiling and melting points of iso- butane are -12  C and -160  C, respectively, compared with 0  C and -138  C for n-butane. The more branching there is, the lower the boiling and melting points are. Q: The boiling point of n-pentane is 36  C. Predict the boiling points of iso-pentane and neo-pentane. A: The boiling point of iso-pentane is 28  C, and the boiling point of neo-pentane is 10  C. "
isotopes,T_4556,"All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes of an element generally have the same physical and chemical properties. Thats because they have the same numbers of protons and electrons. Click image to the left or use the URL below. URL: "
isotopes,T_4557,"Hydrogen is an example of an element that has isotopes. Three isotopes of hydrogen are modeled in the Figure hydrogen. Some hydrogen atoms have one neutron as well. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium. Q: The mass number of an atom is the sum of its protons and neutrons. What is the mass number of each isotope of hydrogen shown above? A: The mass numbers are: hydrogen = 1, deuterium = 2, and tritium = 3. "
isotopes,T_4558,"For most elements other than hydrogen, isotopes are named for their mass number. For example, carbon atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13). These atoms are the isotope called carbon-13. Q: Some carbon atoms have 8 neutrons. What is the name of this isotope of carbon? A: Carbon atoms with 8 neutrons have an atomic mass of 14 (6 protons + 8 neutrons = 14), so this isotope of carbon is named carbon-14. "
isotopes,T_4559,"Atoms need a certain ratio of neutrons to protons to have a stable nucleus. Having too many or too few neutrons relative to protons results in an unstable, or radioactive, nucleus that will sooner or later break down to a more stable form. This process is called radioactive decay. Many isotopes have radioactive nuclei, and these isotopes are referred to as radioisotopes. When they decay, they release particles that may be harmful. This is why radioactive isotopes are dangerous and why working with them requires special suits for protection. The isotope of carbon known as carbon-14 is an example of a radioisotope. In contrast, the carbon isotopes called carbon-12 and carbon-13 are stable. "
kinetic energy,T_4560,"Kinetic energy is the energy of moving matter. Anything that is moving has kinetic energyfrom atoms in matter to stars in outer space. Things with kinetic energy can do work. For example, the spinning saw blade in the photo above is doing the work of cutting through a piece of metal. "
kinetic energy,T_4561,"The amount of kinetic energy in a moving object depends directly on its mass and velocity. An object with greater mass or greater velocity has more kinetic energy. You can calculate the kinetic energy of a moving object with this equation: Kinetic Energy (KE) = 12 mass  velocity2 This equation shows that an increase in velocity increases kinetic energy more than an increase in mass. If mass doubles, kinetic energy doubles as well, but if velocity doubles, kinetic energy increases by a factor of four. Thats because velocity is squared in the equation. Lets consider an example. The Figure 1.1 shows Juan running on the beach with his dad. Juan has a mass of 40 kg and is running at a velocity of 1 m/s. How much kinetic energy does he have? Substitute these values for mass and velocity into the equation for kinetic energy: m2 2 KE = 12  40 kg  (1 m s ) = 20 kg  s2 = 20 N  m, or 20 J Notice that the answer is given in joules (J), or N  m, which is the SI unit for energy. One joule is the amount of energy needed to apply a force of 1 Newton over a distance of 1 meter. What about Juans dad? His mass is 80 kg, and hes running at the same velocity as Juan (1 m/s). Because his mass is twice as great as Juans, his kinetic energy is twice as great: m2 2 KE = 12  80 kg  (1 m s ) = 40 kg  s2 = 40 N  m, or 40 J Q: What is Juans kinetic energy if he speeds up to 2 m/s from 1 m/s? A: By doubling his velocity, Juan increases his kinetic energy by a factor of four: m2 2 KE = 12  40 kg  (2 m s ) = 80 kg  s2 = 80 N  m, or 80 J "
kinetic theory of matter,T_4562,"Energy is the ability to cause changes in matter. For example, your body uses chemical energy when you lift your arm or take a step. In both cases, energy is used to move matteryou. Any matter that is moving has energy just because its moving. The energy of moving matter is called kinetic energy. Scientists think that the particles of all matter are in constant motion. In other words, the particles of matter have kinetic energy. The theory that all matter consists of constantly moving particles is called the kinetic theory of matter. "
kinetic theory of matter,T_4563,"Differences in kinetic energy explain why matter exists in different states. Particles of matter are attracted to each other, so they tend to pull together. The particles can move apart only if they have enough kinetic energy to overcome this force of attraction. Its like a tug of war between opposing sides, with the force of attraction between particles on one side and the kinetic energy of individual particles on the other side. The outcome of the war determines the state of matter. If particles do not have enough kinetic energy to overcome the force of attraction between them, matter exists as a solid. The particles are packed closely together and held rigidly in place. All they can do is vibrate. This explains why solids have a fixed volume and a fixed shape. If particles have enough kinetic energy to partly overcome the force of attraction between them, matter exists as a liquid. The particles can slide past one another but not pull apart completely. This explains why liquids can change shape but have a fixed volume. If particles have enough kinetic energy to completely overcome the force of attraction between them, matter exists as a gas. The particles can pull apart and spread out. This explains why gases have neither a fixed volume nor a fixed shape. Look at the Figure 1.1. It sums up visually the relationship between kinetic energy and state of matter. Q: How could you use a bottle of cola to demonstrate these relationships between kinetic energy and state of matter? A: You could shake a bottle of cola and then open it. Shaking causes carbon dioxide to come out of the cola solution and change to a gas. The gas fizzes out of the bottle and spreads into the surrounding air, showing that its particles have enough kinetic energy to spread apart. Then you could tilt the open bottle and pour out a small amount of the cola on a table, where it will form a puddle. This shows that particles of the liquid have enough kinetic energy to slide over each other but not enough to pull apart completely. If you do nothing to the solid glass of the cola bottle, it will remain the same size and shape. Its particles do not have enough energy to move apart or even to slide over each other. "
law of conservation of momentum,T_4564,"When skater 2 runs into skater 1, hes going faster than skater 1 so he has more momentum. Momentum is a property of a moving object that makes it hard to stop. Its a product of the objects mass and velocity. At the moment of the collision, skater 2 transfers some of his momentum to skater 1, who shoots forward when skater 2 runs into him. Whenever an action and reaction such as this occur, momentum is transferred from one object to the other. However, the combined momentum of the objects remains the same. In other words, momentum is conserved. This is the law of conservation of momentum. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
law of conservation of momentum,T_4565,"The Figure 1.1 shows how momentum is conserved in the two colliding skaters. The total momentum is the same after the collision as it was before. However, after the collision, skater 1 has more momentum and skater 2 has less momentum than before. Q: What if two skaters have a head-on collision? Do you think momentum is conserved then? A: As in all actions and reactions, momentum is also conserved in a head-on collision. "
law of reflection,T_4566,"Reflection is one of several ways that light can interact with matter. Light reflects off surfaces such as mirrors that do not transmit or absorb light. When light is reflected from a smooth surface, it may form an image. An image is a copy of an object that is formed by reflected (or refracted) light. Q: Is an image an actual object? If not, what is it? A: No, an image isnt an actual object. It is focused rays of light that make a copy of an object, like a picture projected on a screen. "
law of reflection,T_4567,"If a surface is extremely smooth, as it is in a mirror, then the image formed by reflection is sharp and clear. This is called regular reflection (also called specular reflection). However, if the surface is even slightly rough or bumpy, an image may not form, or if there is an image, it is blurry or fuzzy. This is called diffuse reflection. Q: Look at the boats and their images in the Figure 1.1. Which one represents regular reflection, and which one represents diffuse reflection? A: Reflection of the boat on the left is regular reflection. The water is smooth and the image is sharp and clear. Reflection of the boat on the right is diffuse reflection. The water has ripples and the image is blurry and wavy. In the Figure 1.2, you can see how both types of reflection occur. Waves of light are represented by arrows called rays. Rays that strike the surface are referred to as incident rays, and rays that reflect off the surface are known as reflected rays. In regular reflection, all the rays are reflected in the same direction. This explains why regular reflection forms a clear image. In diffuse reflection, the rays are reflected in many different directions. This is why diffuse reflection forms, at best, a blurry image. "
law of reflection,T_4568,One thing is true of both regular and diffuse reflection. The angle at which the reflected rays leave the surface is equal to the angle at which the incident rays strike the surface. This is known as the law of reflection. The law is illustrated in the Figure 1.3. 
lens,T_4569,"A lens is a transparent object with one or two curved surfaces. It is typically made of glass (or clear plastic in the case of a contact lens). A lens refracts, or bends, light and forms an image. An image is a copy of an objected formed by the refraction (or reflection) of visible light. The more curved the surface of a lens is, the more it refracts the light that passes through it. There are two basic types of lenses: concave and convex. The two types of lenses have different shapes, so they bend light and form images in different ways. "
lens,T_4570,"A concave lens is thicker at the edges than it is in the middle. You can see the shape of a concave lens in the Figure Note that the image formed by a concave lens is on the same side of the lens as the object. It is also smaller than the object and right-side up. However, it isnt a real image. It is a virtual image. Your brain tricks you into seeing an image there. The light rays actually pass through the glass to the other side and spread out in all directions. "
lens,T_4571,"A convex lens is thicker in the middle than at the edges. You can see the shape of a convex lens in the Figure 1.2. A convex lens causes rays of light to converge, or meet, at a point called the focus (F). A convex lens forms either a real or virtual image. It depends on how close the object is to the lens relative to the focus. Q: An example of a convex lens is a hand lens. Which of the three convex lens diagrams in the Figure 1.2 shows how a hand lens makes an image? A: Youve probably looked through a hand lens before. If you have, then you know that the image it produces is right-side up. Therefore, the first diagram must show how a hand lens makes an image. Its the only one that produces a right-side up image. "
lever,T_4572,"A lever is a simple machine consisting of a bar that rotates around a fixed point. The fixed point of a lever is called the fulcrum. Like other machines, a lever makes work easier by changing the force applied to the machine or the distance over which the force is applied. How does a hammer make it easier to pull a nail out of a board? First, it changes the direction of the force applied to the hammerthe hand pushes down on the handle while the claw end of the hammer head pulls up. Often, you can push down with more force than you can push up because you can put your own weight behind it. The hammer also increases the strength of the force that is applied to it. It easily pulls the nail out of the board, which you couldnt do with your hands alone. On the other hand, the hammer decreases the distance over which the force is applied. The hand pushing down on the handle moves the handle over a distance of several inches, whereas the hammer pulls up on the nail only an inch or two. Q: Where is the fulcrum of the hammer when it is used to pull a nail out of a board? In other words, around what point does the hammer rotate? A: The fulcrum is the point where the head of the hammer rests on the surface of the board. "
lever,T_4573,"Other levers change force or distance in different ways than a hammer removing a nail. How a lever changes force or distance depends on the location of the input and output forces relative to the fulcrum. The input force is the force applied by the user to the lever. The output force is the force applied by the lever to the object. Based on the location of input and output forces, there are three basic types of levers, called first-class, second-class, and third-class levers. The Table 1.1 describes the three classes. Class of Lever Example of Lever in This Class First class Location of Input & Output Forces & Fulcrum* Ideal Mechanical Advantage Change in Direction of Force? Seesaw 1 <1 >1 yes yes yes Second class Wheelbarrow >1 no Third class Hockey stick <1 no = fulcrum I = input force O = output force The Table 1.1 includes the ideal mechanical advantage of each class of lever. The mechanical advantage is the factor by which a machine changes the input force. The ideal mechanical advantage is the increase or decrease in force that would occur if there were no friction to overcome in the use of the machine. Because all machines must overcome some friction, the ideal mechanical advantage is always somewhat greater than the actual mechanical advantage of the machine as it is used in the real world. Q: Which class of lever is a hammer when it is used to pry a nail out of a board? What is its mechanical advantage? A: To pry a nail out of a board, the fulcrum is located between the input and output forces. Therefore, when a hammer is used in this way it is a first class lever. The fulcrum is closer to the output force than the input force, so the mechanical advantage is >1. In other words, the hammer increases the force applied to it, making it easier to pry the nail out of the board. "
lever,T_4574,"All three classes of levers make work easier, but they do so in different ways. When the input and output forces are on opposite sides of the fulcrum, the lever changes the direction of the applied force. This occurs only with first-class levers. When both the input and output forces are on the same side of the fulcrum, the direction of the applied force does not change. This occurs with both second-class and third-class levers. When the input force is applied farther from the fulcrum than the output force is, the output force is greater than the input force, and the ideal mechanical advantage is greater than 1. This always occurs with second-class levers and may occur with first-class levers. When the input force is applied closer to the fulcrum than the output force is, the output force is less than the input force, and the ideal mechanical advantage is less than 1. This always occurs with third-class levers and may occur with first-class levers. When the input and output forces are the same distance from the fulcrum, the output force equals the input force, and the ideal mechanical advantage is 1. This occurs only with first some first-class levers. "
lever,T_4575,"You may be wondering why you would use a third-class lever when it doesnt change the direction or strength of the applied force. The advantage of a third-class lever is that the output force is applied over a greater distance than the input force. The output end of the lever must move faster than the input end in order to cover the greater distance. Q: A broom is a third-class lever when it is used to sweep a floor (see the Figure 1.1), so the output end of the lever moves faster than the input end. Why is this useful? A: By moving more quickly over the floor, the broom does the work faster. "
light,T_4576,"Electromagnetic waves are waves that carry energy through matter or space as vibrating electric and magnetic fields. Electromagnetic waves have a wide range of wavelengths and frequencies. Sunlight contains the complete range of wavelengths of electromagnetic waves, which is called the electromagnetic spectrum. The Figure 1.1 shows all the waves in the spectrum. "
light,T_4577,"Light includes infrared light, visible light, and ultraviolet light. As you can see from the Figure 1.1, light falls roughly in the middle of the electromagnetic spectrum. It has shorter wavelengths and higher frequencies than microwaves, but not as short and high as X rays. Q: Which type of light do you think is harmful to the skin? A: Waves of light with the highest frequencies have the most energy and are harmful to the skin. Use the electro- magnetic spectrum in the Figure 1.1 to find out which of the three types of light have the highest frequencies. "
light,T_4578,"Light with the longest wavelengths is called infrared light. The term infrared means below red. Infrared light is the range of light waves that have longer wavelengths and lower frequencies than red light in the visible range of light waves. The sun gives off infrared light as do flames and living things. You cant see infrared light waves, but you can feel them as heat. But infrared cameras and night vision goggles can detect infrared light waves and convert them to visible images. "
light,T_4579,"The only light that people can see is called visible light. This light consists of a very narrow range of wavelengths that falls between infrared light and ultraviolet light. Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength (see Figure 1.2). When all of the wavelengths of visible light are combined, as they are in sunlight, visible light appears white. "
light,T_4580,"Light with wavelengths shorter than visible light is called ultraviolet light. The term ultraviolet means above violet. Ultraviolet light is the range of light waves that have shorter wavelengths and higher frequencies than violet light in the visible range of light. With higher frequencies than visible light, ultraviolet light has more energy. It can be used to kill bacteria in food and to sterilize surgical instruments. The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D, in turn, is needed for strong bones and teeth. Too much exposure to ultraviolet light can cause sunburn and skin cancer. As the slip, slop, slap slogan suggests, you can protect your skin from ultraviolet light by wearing clothing that covers your skin, applying sunscreen to any exposed areas, and wearing a hat to protect your head from exposure. The SPF, or sun-protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (see Figure 1.3). A sunscreen with a higher SPF value protects the skin longer. Sunscreen must be applied liberally and often to be effective, and no sunscreen is completely waterproof. Q: You should apply sunscreen even on cloudy days. Can you explain why? A: Ultraviolet light can travel through clouds, so it can harm unprotected skin even on cloudy days. "
lipid classification,T_4581,"Lipids are one of four classes of biochemical compounds, which are compounds that make up living things and carry out life processes. (The other three classes of biochemical compounds are carbohydrates, proteins, and nucleic acids.) Living things use lipids to store energy. Lipids are also the major components of cell membranes in living things. Types of lipids include fats and oils. Fats are solid lipids that animals use to store energy. Oils are liquid lipids that plants use to store energy. Q: Can you name some lipids that are fats? What are some lipids that are oils? A: Lipids that are fats include butter and the fats in meats. Lipids that are oils include olive oil and vegetable oil. Examples of both types of lipids are pictured in the Figure 1.1. "
lipid classification,T_4582,"Lipids consist only or mainly of carbon, hydrogen, and oxygen. Both fats and oils are made up of long chains of carbon atoms that are bonded together. These chains are called fatty acids. Fatty acids may be saturated or (A) The white bands on these lamb chops are fat. (B) The yellow liquid in this bottle is olive oil. unsaturated. In the Figure 1.2 you can see structural formulas for two small fatty acids, one saturated and one unsaturated. Saturated fatty acids have only single bonds between carbon atoms. As a result, the carbon atoms are bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogens. Saturated fatty acids are found in fats. Unsaturated fatty acids have at least one double bond between carbon atoms. As a result, some carbon atoms are not bonded to as many hydrogen atoms as possible. They are unsaturated with hydrogens. Unsaturated fatty acids are found in oils. Q: Both of these fatty acid molecules have six carbon atoms and two oxygen atoms. How many hydrogen atoms does each fatty acid molecule contain? What else is different about the two molecules? A: The saturated fatty acid molecule has 12 hydrogen atoms. This is as many hydrogen atoms as can possibly be bonded to carbon atoms in this molecule. The unsaturated fatty acid molecule has 10 hydrogen atoms, or two less than the maximum possible number. The saturated fatty acid has only single bonds between its carbon atoms. The unsaturated fatty acid has a double bond between two of its carbon atoms. "
lipid classification,T_4583,"Some lipids contain the element phosphorus as well as carbon, hydrogen, and oxygen. These lipids are called phospholipids. Two layers of phospholipid molecules make up the cell membranes of living things. In the Figure One end of each phospholipid molecule is polar, so it has a partial electric charge. Water is also polar and has electrically charged ends, so it is attracted to the oppositely charged end of a phospholipid molecule. This end of the phospholipid molecule is described as hydrophilic, which means water loving. The other end of each phospholipid molecule is nonpolar and has no electric charge. This end of the phospho- lipid molecule repels polar water and is described as hydrophobic, or water hating. In the Figure 1.3, the hydrophilic ends of the phospholipid molecules are on the outsides of the cell membrane, and the hydrophobic ends are on the inside of the cell membrane. This arrangement of phospholipids allows some substances to pass through the cell membrane while keeping other substances out. "
longitudinal wave,T_4586,"A longitudinal wave is a type of mechanical wave. A mechanical wave is a wave that travels through matter, called the medium. In a longitudinal wave, particles of the medium vibrate in a direction that is parallel to the direction that the wave travels. You can see this in the Figure 1.1. The persons hand pushes and pulls on one end of the spring. The energy of this disturbance passes through the coils of the spring to the other end. Click image to the left or use the URL below. URL: "
longitudinal wave,T_4587,"Notice in the Figure 1.1 that the coils of the spring first crowd closer together and then spread farther apart as the wave passes through them. Places where particles of a medium crowd closer together are called compressions, and places where the particles spread farther apart are called rarefactions. The more energy the wave has, the closer together the particles are in compressions and the farther apart they are in rarefactions. "
longitudinal wave,T_4588,Earthquakes cause longitudinal waves called P waves. The disturbance that causes an earthquake sends longitudinal waves through underground rocks in all directions away from the disturbance. P waves are modeled in the Figure Q: Where are the compressions and rarefactions of the medium in this model of P waves? A: The compressions are the places where the vertical lines are closest together. The rarefactions are the places where the vertical lines are farthest apart. 
magnetic field reversal,T_4589,"Earths magnetic poles have switched places repeatedly in the past. As you can see in the Figure 1.1, each time the switch occurred, Earths magnetic field was reversed. The magnetic field is the region around a magnet over which it exerts magnetic force. We think of todays magnetic field direction as normal, but thats only because its what were used to. "
magnetic field reversal,T_4590,"Scientists dont know for certain why magnetic reversals occur, but there is hard evidence that they have for hundreds of millions of years. The evidence comes from rocks on the ocean floor. Look at Figure 1.2. They show the same ridge on the ocean floor during different periods of time. A. At the center of the ridge, hot magma pushes up through the crust and hardens into rock. Once the magma hardens, the alignment of magnetic domains in the rock is frozen in place forever. Magnetic domains are regions in the rocks where all the atoms are lined up and pointing toward Earths north magnetic pole. B. The newly hardened rock is gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. The alignment of magnetic domains in this new rock is in the opposite direction, showing that a magnetic reversal has occurred. C. A magnetic reversal occurs again. It is frozen in rock to document the change. Rock samples from many places on the ocean floor show that the north and south magnetic poles reversed hundreds of times over the last 330 million years. The last reversal was less than a million years ago. "
magnets,T_4591,"A magnet is an object that attracts certain materials such as iron. Youre probably familiar with common bar magnets, like the one shown in the Figure 1.1. Like all magnets, this bar magnet has north and south magnetic poles. The red end of the magnet is the north pole and the blue end is the south pole. The poles are regions where the magnet is strongest. The poles are called north and south because they always line up with Earths north-south axis if the magnet is allowed to move freely. (Earths axis is the imaginary line around which the planet rotates.) Q: What do you suppose would happen if you cut the bar magnet pictured in the Figure 1.1 along the line between the north and south poles? A: Both halves of the magnet would also have north and south poles. If you cut each of the halves in half, all those pieces would have north and south poles as well. Pieces of a magnet always have both north and south poles no matter how many times you cut the magnet. "
magnets,T_4592,"The force that a magnet exerts on certain materials, including other magnets, is called magnetic force. The force is exerted over a distance and includes forces of attraction and repulsion. North and south poles of two magnets attract each other, while two north poles or two south poles repel each other. A magnet can exert force over a distance because the magnet is surrounded by a magnetic field. In the Figure 1.2, you can see the magnetic field surrounding a bar magnet. Tiny bits of iron, called iron filings, were placed under a sheet of glass. When the magnet was placed on the glass, it attracted the iron filings. The pattern of the iron filings shows the lines of force that make up the magnetic field of the magnet. The concentration of iron filings near the poles indicates that these areas exert the strongest force. You can also see how the magnetic field affects the compasses placed above the magnet. When two magnets are brought close together, their magnetic fields interact. You can see how they interact in the Figure 1.3. The lines of force of north and south poles attract each other whereas those of two north poles repel each other. "
mechanical advantage,T_4596,"How much a machine changes the input force is its mechanical advantage. Mechanical advantage is the ratio of the output force to the input force, so it can be represented by the equation: Actual Mechanical Advantage = Output force Input force Note that this equation represents the actual mechanical advantage of a machine. The actual mechanical advantage takes into account the amount of the input force that is used to overcome friction. The equation yields the factor by which the machine changes the input force when the machine is actually used in the real world. "
mechanical advantage,T_4597,"It can be difficult to measure the input and output forces needed to calculate the actual mechanical advantage of a machine. Generally, an unknown amount of the input force is used to overcome friction. Its usually easier to measure the input and output distances than the input and output forces. The distance measurements can then be used to calculate the ideal mechanical advantage. The ideal mechanical advantage represents the change in input force that would be achieved by the machine if there were no friction to overcome. The ideal mechanical advantage is always greater than the actual mechanical advantage because all machines have to overcome friction. Ideal mechanical advantage can be calculated with the equation: Ideal Mechanical Advantage = Input Distance Output Distance "
mechanical advantage,T_4598,"Look at the ramp in the Figure 1.1. A ramp is a type of simple machine called an inclined plane. It can be used to raise an object off the ground. The input distance is the length of the sloped surface of the ramp. This is the distance over which the input force is applied. The output distance is the height of the ramp, or the vertical distance the object is raised. For this ramp, the input distance is 6 m and the output distance is 2 meters. Therefore, the ideal mechanical advantage of this ramp is: Input distance Ideal Mechanical Advantage = Output distance = 62 m m =3 An ideal mechanical advantage of 3 means that the ramp ideally (in the absence of friction) multiplies the input force by a factor of 3. The trade-off is that the input force must be applied over a greater distance than the object is lifted. Q: Assume that another ramp has a sloping surface of 8 m and a vertical height of 4 m. What is the ideal mechanical advantage of this ramp? A: The ramp has an ideal mechanical advantage of: Ideal Mechanical Advantage = 84 m m =2 "
mechanical advantage,T_4599,"Many machinesincluding inclined planes such as rampsincrease the strength of the force put into the machine but decrease the distance over which the force is applied. Other machines increase the distance over which the force is applied but decrease the strength of the force. Still other machines change the direction of the force, with or without also increasing its strength or distance. Which way a machine works determines its mechanical advantage, as shown in the Table 1.1. Strength of Force increases decreases stays the same (changes direction only) Distance Over Force is Applied decreases increases stays the same which Mechanical Advantage Example >1 <1 =1 ramp hammer flagpole pulley "
mechanical wave,T_4600,"The waves in the picture above are examples of mechanical waves. A mechanical wave is a disturbance in matter that transfers energy through the matter. A mechanical wave starts when matter is disturbed. A source of energy is needed to disturb matter and start a mechanical wave. Q: Where does the energy come from in the water wave pictured above? A: The energy comes from the falling droplets of water, which have kinetic energy because of their motion. "
mechanical wave,T_4601,"The energy of a mechanical wave can travel only through matter. The matter through which the wave travels is called the medium (plural, media). The medium in the water wave pictured above is water, a liquid. But the medium of a mechanical wave can be any state of matter, even a solid. Q: How do the particles of the medium move when a wave passes through them? A: The particles of the medium just vibrate in place. As they vibrate, they pass the energy of the disturbance to the particles next to them, which pass the energy to the particles next to them, and so on. Particles of the medium dont actually travel along with the wave. Only the energy of the wave travels through the medium. "
mechanical wave,T_4602,"There are three types of mechanical waves: transverse, longitudinal, and surface waves. They differ in how particles of the medium move. You can see this in the Figure 1.1. In a transverse wave, particles of the medium vibrate up and down perpendicular to the direction of the wave. In a longitudinal wave, particles of the medium vibrate back and forth parallel to the direction of the wave. In a surface wave, particles of the medium vibrate both up and down and back and forth, so they end up moving in a circle. Q: How do you think surface waves are related to transverse and longitudinal waves? A: A surface wave is combination of a transverse wave and a longitudinal wave. "
mendeleevs periodic table,T_4606,"For many years, scientists looked for a good way to organize the elements. This became increasingly important as more and more elements were discovered. An ingenious method of organizing elements was developed in 1869 by a Russian scientist named Dmitri Mendeleev, who is pictured 1.1. Mendeleevs method of organizing elements was later revised, but it served as a basis for the method that is still used today. Mendeleev was a teacher as well as a chemist. He was writing a chemistry textbook and wanted to find a way to organize the 63 known elements so it would be easier for students to learn about them. He made a set of cards of the elements, similar to a deck of playing cards. On each card, he wrote the name of a different element, its atomic mass, and other known properties. Mendeleev arranged and rearranged the cards in many different ways, looking for a pattern. He finally found it when he placed the elements in order by increasing atomic mass. Q: What is atomic mass? Why might it be a good basis for organizing elements? A: Atomic mass is the mass of one atom of an element. It is about equal to the mass of the protons plus the neutrons in an atom. It is a good basis for organizing elements because each element has a unique number of protons and atomic mass is an indirect way of organizing elements by number of protons. "
mendeleevs periodic table,T_4607,"You can see how Mendeleev organized the elements in the Figure 1.2. From left to right across each row, elements are arranged by increasing atomic mass. Mendeleev discovered that if he placed eight elements in each row and then continued on to the next row, the columns of the table would contain elements with similar properties. He called the columns groups. They are sometimes called families, because elements within a group are similar but not identical to one another, like people in a family. Mendeleevs table of the elements is called a periodic table because of its repeating pattern. Anything that keeps repeating is referred to as periodic. Other examples of things that are periodic include the monthly phases of the moon and the daily cycle of night and day. The term period refers to the interval between repetitions. For example, the moons phases repeat every four weeks. In a periodic table of the elements, the periods are the rows of the table. In Mendeleevs table, each period contains eight elements, and then the pattern repeats in the next row. "
mendeleevs periodic table,T_4608,"Did you notice the blanks in Mendeleevs table? They are spaces that Mendeleev left blank for elements that had not yet been discovered when he created his table. He predicted that these missing elements would eventually be discovered. Based on their position in the table, he even predicted their properties. For example, he predicted a missing element in row 5 of group III. He also predicted that the missing element would have an atomic mass of 68 and be a relatively soft metal like other elements in this group. Scientists searched for the missing element, and they found it just a few years later. They named the new element gallium. Scientists searched for the other missing elements in Mendeleevs table and eventually found all of them. An important measure of a good model is its ability to make accurate predictions. This makes it a useful model. Clearly, Mendeleevs periodic table was a useful model. It helped scientists discover new elements and made sense of those that were already known. "
metallic bonding,T_4609,"Metallic bonds are forces of attraction between positive metal ions and the valence electrons that are constantly moving around them (see the Figure 1.1). The valence electrons include their own and those of other, nearby ions of the same metal. The valence electrons of metals move freely in this way because metals have relatively low electronegativity, or attraction to electrons. The positive metal ions form a lattice-like structure held together by all the metallic bonds. Click image to the left or use the URL below. URL:  Q: Why do metallic bonds form only in elements that are metals? Why dont similar bonds form in elements that are nonmetals? A: Metal atoms readily give up valence electrons and become positive ions whenever they form bonds. When nonmetals bond together, the atoms share valence electrons and do not become ions. For example, when oxygen atoms bond together they form oxygen molecules in which two oxygen atoms share two pairs of valence electrons equally, so neither atom becomes charged. "
metallic bonding,T_4610,"The valence electrons surrounding metal ions are constantly moving. This makes metals good conductors of electricity. The lattice-like structure of metal ions is strong but quite flexible. This allows metals to bend without breaking. Metals are both ductile (can be shaped into wires) and malleable (can be shaped into thin sheets). Q: Look at the metalworker in the Figure 1.2. Hes hammering a piece of hot iron in order to shape it. Why doesnt the iron crack when he hits it? A: The iron ions can move within the sea of electrons around them. They can shift a little closer together or farther apart without breaking the metallic bonds between them. Therefore, the metal can bend rather than crack when the hammer hits it. "
metalloids,T_4611,"Metalloids are the smallest class of elements. (The other two classes of elements are metals and nonmetals). There are just six metalloids. In addition to silicon, they include boron, germanium, arsenic, antimony, and tellurium. Metalloids fall between metals and nonmetals in the periodic table. They also fall between metals and nonmetals in terms of their properties. Q: How does the position of an element in the periodic table influence its properties? A: Elements are arranged in the periodic table by their atomic number, which is the number of protons in their atoms. Atoms are neutral in electric charge, so they always have the same number of electrons as protons. It is the number of electrons in the outer energy level of atoms that determines most of the properties of elements. "
metalloids,T_4612,"How metalloids behave in chemical interactions with other elements depends mainly on the number of electrons in the outer energy level of their atoms. Metalloids have from three to six electrons in their outer energy level. Boron, pictured in the Figure 1.1, is the only metalloid with just three electrons in its outer energy level. It tends to act like metals by giving up its electrons in chemical reactions. Metalloids with more than four electrons in their outer energy level (arsenic, antimony, and tellurium) tend to act like nonmetals by gaining electrons in chemical reactions. Those with exactly four electrons in their outer energy level (silicon and germanium) may act like either metals or nonmetals, depending on the other elements in the reaction. "
metalloids,T_4613,"Most metalloids have some physical properties of metals and some physical properties of nonmetals. For example, metals are good conductors of both heat and electricity, whereas nonmetals generally cannot conduct heat or electricity. And metalloids? They fall between metals and nonmetals in their ability to conduct heat, and if they can conduct electricity, they usually can do so only at higher temperatures. Metalloids that can conduct electricity at higher temperatures are called semiconductors. Silicon is an example of a semiconductor. It is used to make the tiny electric circuits in computer chips. You can see a sample of silicon and a silicon chip in the Figure 1.2. Metalloids tend to be shiny like metals but brittle like nonmetals. Because they are brittle, they may chip like glass or crumble to a powder if struck. Other physical properties of metalloids are more variable, including their boiling and melting points, although all metalloids exist as solids at room temperature. Click image to the left or use the URL below. URL: "
metals,T_4614,"Metals are elements that can conduct electricity. They are one of three classes of elements (the other two classes are nonmetals and metalloids). Metals are by far the largest of the three classes. In fact, most elements are metals. All of the elements on the left side and in the middle of the periodic table, except for hydrogen, are metals. There are several different types of metals, including alkali metals in group 1 of the periodic table, alkaline Earth metals in group 2, and transition metals in groups 3-12. The majority of metals are transition metals. "
metals,T_4615,"Elements in the same class share certain basic similarities. In addition to conducting electricity, many metals have several other shared properties, including those listed below. Metals have relatively high melting points. This explains why all metals except for mercury are solids at room temperature. Most metals are good conductors of heat. Thats why metals such as iron, copper, and aluminum are used for pots and pans. Metals are generally shiny. This is because they reflect much of the light that strikes them. The mercury pictured above is very shiny. The majority of metals are ductile. This means that they can be pulled into long, thin shapes, like the aluminum electric wires pictured in the Figure 1.1. Metals tend to be malleable. This means that they can be formed into thin sheets without breaking. An example is aluminum foil, also pictured in the Figure 1.1. Q: The defining characteristic of metals is their ability to conduct electricity. Why do you think metals have this property? A: The properties of metalsas well as of elements in the other classesdepend mainly on the number and arrangement of their electrons. "
metals,T_4616,"To understand why metals can conduct electricity, consider the metal lithium as an example. An atom of lithium is modeled below. Look at lithiums electrons. There are two electrons at the first energy level. This energy level can hold only two electrons, so it is full in lithium. The second energy level is another story. It can hold a maximum of eight electrons, but in lithium it has just one. A full outer energy level is the most stable arrangement of electrons. Lithium would need to gain seven electrons to fill its outer energy level and make it stable. Its far easier for lithium to give up its one electron in energy level 2, leaving it with a full outer energy level (now level 1). Electricity is a flow of electrons. Because lithium (like most other metals) easily gives up its extra electron, it is a good conductor of electricity. This tendency to give up electrons also explains other properties of metals such as lithium. "
microwaves,T_4617,Electromagnetic waves carry energy through matter or space as vibrating electric and magnetic fields. Electromag- netic waves have a wide range of wavelengths and frequencies. The complete range is called the electromagnetic spectrum. The Figure 1.1 shows all the waves of the spectrum. The waves used in radar guns are microwaves. 
microwaves,T_4618,"Find the microwave in the Figure 1.1. A microwave is an electromagnetic wave with a relatively long wavelength and low frequency. Microwaves are often classified as radio waves, but they have higher frequencies than other radio waves. With higher frequencies, they also have more energy. Thats why microwaves are useful for heating food in microwave ovens. Microwaves have other important uses as well, including cell phone transmissions and radar. These uses are described below. Click image to the left or use the URL below. URL: "
microwaves,T_4619,"Cell phone signals are carried through the air as microwaves. You can see how this works in the Figure 1.2. A cell phone encodes the sounds of the callers voice in microwaves by changing the frequency of the waves. This is called frequency modulation. The encoded microwaves are then sent from the phone through the air to a cell tower. From the cell tower, the waves travel to a switching center. From there they go to another cell tower and from the tower to the receiver of the person being called. The receiver changes the encoded microwaves back to sounds. Q: Cell towers reach high above the ground. Why do you think such tall towers are used? A: Microwaves can be interrupted by buildings and other obstructions, so cell towers must be placed high above the ground to prevent the interruption of cell phone signals. "
microwaves,T_4620,"Radar stands for radio detection and ranging. In police radar, a radar gun sends out short bursts of microwaves. The microwaves reflect back from oncoming vehicles and are detected by a receiver in the radar gun. The frequency of the reflected waves is used to compute the speed of the vehicles. Radar is also used for tracking storms, detecting air traffic, and other purposes. Q: How are reflected microwaves used to determine the speed of oncoming cars (see Figure 1.3)? A: As the car approaches the radar gun, the reflected microwaves get bunched up in front of the car. Therefore, the waves the receiver detects have a higher frequency than they would if they were being reflected from a stationary object. The faster the car is moving, the greater the increase in the frequency of the waves. This is an example of the Doppler effect, which can also occur with sound waves. "
mirrors,T_4621,"A mirror is typically made of glass with a shiny metal backing that reflects all the light that strikes it. When a mirror reflects light, it forms an image. An image is a copy of an object that is formed by reflection or refraction. Mirrors may have flat or curved surfaces. The shape of a mirrors surface determines the type of image it forms. For example, some mirrors form real images, and other mirrors form virtual images. Whats the difference between real and virtual images? A real image forms in front of a mirror where reflected light rays actually meet. It is a true image that could be projected on a screen. A virtual image appears to be on the other side of the mirror. Of course, reflected rays dont actually go through the mirror to the other side, so a virtual image doesnt really exist. It just appears to exist to the human brain. Q: Look back at the image of the girl pointing at her image in the mirror. Which type of image is it, real or virtual? A: The image of the girl is a virtual image. It appears to be on the other side of the mirror from the girl. "
mirrors,T_4622,"The mirror in the opening photo is a plane mirror. This is the most common type of mirror. It has a flat reflective surface and forms only virtual images. The image formed by a plane mirror is also right-side up and life sized. But something is different about the image compared with the real object in front of the mirror. Left and right are reversed. Look at the girl brushing her teeth in the Figure 1.1. She is using her left hand to brush her teeth, but her image (on the left) appears to be brushing her teeth with the right hand. All plane mirrors reverse left and right in this way. The term mirror image refers to how left and right are reversed in an image compared with the object. "
mirrors,T_4623,"Some mirrors have a curved rather than flat surface. Curved mirrors can be concave or convex. A concave mirror is shaped like the inside of a bowl. This type of mirror forms either real or virtual images, depending on where the object is placed relative to the focal point. The focal point is the point in front of the mirror where the reflected rays meet. You can see how concave mirrors form images in the Figure 1.2. Concave mirrors are used behind car headlights. They focus the light and make it brighter. Concave mirrors are also used in some telescopes. "
mirrors,T_4624,"The other type of curved mirror, a convex mirror, is shaped like the outside of a bowl. Because of its shape, it can gather and reflect light from a wide area. As you can see in the Figure 1.3, a convex mirror forms only virtual images that are right-side up and smaller than the actual object. Q: Convex mirrors are used as side mirrors on cars. You can see one in the Figure 1.4. Why is a convex mirror good for this purpose? A: Because it gathers light over a wide area, a convex mirror gives the driver a wider view of the area around the vehicle than a plane mirror would. "
modern periodic table,T_4629,"In the 1860s, a scientist named Dmitri Mendeleev also saw the need to organize the elements. He created a table in which he arranged all of the elements by increasing atomic mass from left to right across each row. When he placed eight elements in each row and then started again in the next row, each column of the table contained elements with similar properties. He called the columns of elements groups. Mendeleevs table is called a periodic table and the rows are called periods. Thats because the table keeps repeating from row to row, and periodic means repeating. "
modern periodic table,T_4630,"A periodic table is still used today to organize the elements. You can see a simple version of the modern periodic table in the Figure 1.1. The modern table is based on Mendeleevs table, except the modern table arranges the elements by increasing atomic number instead of atomic mass. Atomic number is the number of protons in an atom, and this number is unique for each element. The modern table has more elements than Mendeleevs table because many elements have been discovered since Mendeleevs time. "
modern periodic table,T_4631,"In the Figure 1.1, each element is represented by its chemical symbol, which consists of one or two letters. The first letter of the symbol is always written in upper case, and the second letterif there is oneis always written in lower case. For example, the symbol for copper is Cu. It stands for cuprum, which is the Latin word for copper. The number above each symbol in the table is its unique atomic number. Notice how the atomic numbers increase from left to right and from top to bottom in the table. Q: Find the symbol for copper in the Figure 1.1. What is its atomic number? What does this number represent? A: The atomic number of copper is 29. This number represents the number of protons in each atom of copper. (Copper is the element that makes up the coil of wire in photo A of the opening sequence of photos.) "
modern periodic table,T_4632,"Rows of the modern periodic table are called periods, as they are in Mendeleevs table. From left to right across a period, each element has one more proton than the element before it. Some periods in the modern periodic table are longer than others. For example, period 1 contains only two elements: hydrogen (H) and helium (He). In contrast, periods 6 and 7 are so long that many of their elements are placed below the main part of the table. They are the elements starting with lanthanum (La) in period 6 and actinium (Ac) in period 7. Some elements in period 7 have not yet been named. They are represented by temporary three-letter symbols, such as Uub. The number of each period represents the number of energy levels that have electrons in them for atoms of each element in that period. Q: Find calcium (Ca) in the Figure 1.1. How many energy levels have electrons in them for atoms of calcium? A: Calcium is in period 4, so its atoms have electrons in them for the first four energy levels. "
modern periodic table,T_4633,"Columns of the modern table are called groups, as they are in Mendeleevs table. However, the modern table has many more groups18 compared with just 8 in Mendeleevs table. Elements in the same group have similar properties. For example, all elements in group 18 are colorless, odorless gases, such as neon (Ne). (Neon is the element inside the light in opening photo C.) In contrast, all elements in group 1 are very reactive solids. They react explosively with water, as you can see in the video and Figure 1.2. Click image to the left or use the URL below. URL:  The alkali metal sodium (Na) reacting with water. "
modern periodic table,T_4634,"All elements can be classified in one of three classes: metals, metalloids, or nonmetals. Elements in each class share certain basic properties. For example, elements in the metals class can conduct electricity, whereas elements in the nonmetals class generally cannot. Elements in the metalloids class fall in between the metals and nonmetals in their properties. An example of a metalloid is arsenic (As). (Arsenic is the element in opening photo B.) In the periodic table above, elements are color coded to show their class. As you move from left to right across each period of the table, the elements change from metals to metalloids to nonmetals. Q: To which class of elements does copper (Cu) belong: metal, metalloid, or nonmetal? Identify three other elements in this class. A: In the Figure 1.1, the cell for copper is colored blue. This means that copper belongs to the metals class. Other elements in the metals class include iron (Fe), sodium (Na), and gold (Au). It is apparent from the table that the majority of elements are metals. "
molecular compounds,T_4635,"Compounds that form from two or more nonmetallic elements, such as carbon and hydrogen, are called covalent compounds. In a covalent compound, atoms of the different elements are held together in molecules by covalent bonds. These are chemical bonds in which atoms share valence electrons. The force of attraction between the shared electrons and the positive nuclei of both atoms holds the atoms together in the molecule. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. The largest, most complex covalent molecules have thousands of atoms. Examples include proteins and carbohy- drates, which are compounds in living things. The smallest, simplest covalent compounds have molecules with just two atoms. An example is hydrogen chloride (HCl). It consists of one hydrogen atom and one chlorine atom, as you can see in the Figure 1.1. "
molecular compounds,T_4636,"To name simple covalent compounds, follow these rules: Start with the name of the element closer to the left side of the periodic table. Follow this with the name of element closer to the right of the periodic table. Give this second name the suffix -ide. Use prefixes to represent the numbers of the different atoms in each molecule of the compound. The most commonly used prefixes are shown in the Table 1.1. Number 1 2 3 4 5 6 Prefix mono- (or none) di- tri- tetra- penta- hexa- Q: What is the name of the compound that contains three oxygen atoms and two nitrogen atoms? A: The compound is named dinitrogen trioxide. Nitrogen is named first because it is farther to the left in the periodic table than oxygen. Oxygen is given the -ide suffix because it is the second element named in the compound. The prefix di- is added to nitrogen to show that there are two atoms of nitrogen in each molecule of the compound. The prefix tri- is added to oxygen to show that there are three atoms of oxygen in each molecule. In the chemical formula for a covalent compound, the numbers of the different atoms in a molecule are represented by subscripts. For example, the formula for the compound named carbon dioxide is CO2 . Q: What is the chemical formula for dinitrogen trioxide? A: The chemical formula is N2 O3 . "
molecular compounds,T_4637,"The covalent bonds of covalent compounds are responsible for many of the properties of the compounds. Because valence electrons are shared in covalent compounds, rather than transferred between atoms as they are in ionic compounds, covalent compounds have very different properties than ionic compounds. Many covalent compounds, especially those containing carbon and hydrogen, burn easily. In contrast, many ionic compounds do not burn. Many covalent compounds do not dissolve in water, whereas most ionic compounds dissolve well in water. Unlike ionic compounds, covalent compounds do not have freely moving electrons, so they cannot conduct Name of Compound(Chemical For- mula) Sodium chloride (NaCl) Lithium fluoride (LiF) Type of Compound Boiling Point ( C) ionic ionic 1413 1676 Q: The two covalent compounds in the table are gases at room temperature, which is 20 C. For a compound to be a liquid at room temperature, what does its boiling point have to be? A: To be a liquid at room temperature, a covalent compound has to have a boiling point higher than 20 C. Water is an example of a covalent compound that is a liquid at room temperature. The boiling point of water is 100 C. "
momentum,T_4638,"Momentum is a property of a moving object that makes it hard to stop. The more mass it has or the faster its moving, the greater its momentum. Momentum equals mass times velocity and is represented by the equation: Momentum = Mass  Velocity Q: What is Codys momentum as he stands at the top of the ramp? A: Cody has no momentum as he stands there because he isnt moving. In other words, his velocity is zero. However, Cody will gain momentum as he starts moving down the ramp and picks up speed. Q: Codys older brother Jerod is pictured in the Figure 1.1. If Jerod were to travel down the ramp at the same velocity as Cody, who would have greater momentum? Who would be harder to stop? A: Jerod obviously has greater mass than Cody, so he would have greater momentum. He would also be harder to stop. "
momentum,T_4639,"To calculate momentum with the equation above, mass is measured in (kg), and velocity is measured in meters per second (m/s). For example, Cody and his skateboard have a combined mass of 40 kg. If Cody is traveling at a velocity of 1.1 m/s by the time he reaches the bottom of the ramp, then his momentum is: Momentum = 40 kg  1.1 m/s = 44 kg  m/s Note that the SI unit for momentum is kg  m/s. Q: The combined mass of Jerod and his skateboard is 68 kg. If Jerod goes down the ramp at the same velocity as Cody, what is his momentum at the bottom of the ramp? A: His momentum is: Momentum = 68 kg  1.1 m/s = 75 kg  m/s "
motion,T_4640,"In science, motion is defined as a change in position. An objects position is its location. Besides the wings of the hummingbird in the opening image, you can see other examples of motion in the Figure 1.1. In each case, the position of something is changing. Q: In each picture in the Figure 1.1, what is moving and how is its position changing? A: The train and all its passengers are speeding straight down a track to the next station. The man and his bike are racing along a curving highway. The geese are flying over their wetland environment. The meteor is shooting through the atmosphere toward Earth, burning up as it goes. "
motion,T_4641,"Theres more to motion than objects simply changing position. Youll see why when you consider the following example. Assume that the school bus pictured in the Figure 1.2 passes by you as you stand on the sidewalk. Its obvious to you that the bus is moving, but what about to the children inside the bus? The bus isnt moving relative to them, and if they look at the other children sitting on the bus, they wont appear to be moving either. If the ride is really smooth, the children may only be able to tell that the bus is moving by looking out the window and seeing you and the trees whizzing by. This example shows that how we perceive motion depends on our frame of reference. Frame of reference refers to something that is not moving with respect to an observer that can be used to detect motion. For the children on the bus, if they use other children riding the bus as their frame of reference, they do not appear to be moving. But if they use objects outside the bus as their frame of reference, they can tell they are moving. Q: What is your frame of reference if you are standing on the sidewalk and see the bus go by? How can you tell that the bus is moving? A: Your frame of reference might be the trees and other stationary objects across the street. As the bus goes by, it momentarily blocks your view of these objects, and this helps you detect the bus motion. "
musical instruments,T_4642,"People have been using sound to make music for thousands of years. They have invented many different kinds of musical instruments. Despite their diversity, however, musical instruments share certain similarities. All musical instruments create sound by causing matter to vibrate. The vibrations start sound waves moving through the air. Most musical instruments use resonance to amplify the sound waves and make the sounds louder. Resonance occurs when an object vibrates in response to sound waves of a certain frequency. In a musical instrument such as a drum, the whole instrument and the air inside it may vibrate when the head of the drum is struck. Most musical instruments have a way of changing the frequency of the sound waves they produce. This changes the pitch of the sounds, or how high or low the sounds seem to a listener. "
musical instruments,T_4643,"There are three basic categories of musical instruments: percussion, wind, and stringed instruments. You can read in the Figure 1.1 how instruments in each category make sound and change pitch. Q: Can you name other instruments in each of the three categories of musical instruments? A: Other percussion instruments include drums and cymbals. Other wind instruments include trumpets and flutes. Other stringed instruments include guitars and harps. "
nature of technology,T_4647,"Printers like the one that made the plastic bicycle are a new type of technology. Technology is the application of science to solve problems. Because technology finds solutions to practical problems, new technologies may have major impacts on society, science, and industry. For example, some people predict that 3-D printing will revolutionize manufacturing. Q: Making products with 3-D printers has several advantages over making them with machines in factories. What do you think some of the advantages might be? A: Making products with 3-D printers would allow anyone anywhere to make just about anything, provided they have the printer, powder, and computer program. Suppose, for example, that you live in a remote location and need a new part for your car. The solution? Just download the design on your computer and print the part on your 3-D printer. Manufacturing would no longer require specially designed machines in factories that produce pollution. Another advantage of using 3-D printers to make products is that no materials are wasted. This would lower manufacturing costs as well as save natural resources. "
nature of technology,T_4648,"New technologies such as 3-D printers often evolve slowly as new materials, designs, or processes are invented. Solar-powered cars are a good example. For several decades, researchers have been working on developing practical solar-powered cars. Why? Cars powered by sunlight have at least two important advantages over gas-powered cars. The energy they use is free and available almost everywhere, and they produce no pollution. The timeline in Table Milestone 1954: First modern solar cell 1955: First solar car 1983: First practical solar car 1987: First World Solar Challenge 2008: First Commercial solar car The first modern solar cell was invented in 1954 by a team of researchers at Bell Labs in the U.S. It could convert light energy to enough electricity to power devices. In 1955, William G. Cobb of General Motors demon- strated his 15-inch-long Sunmobile, the worlds first solar-powered automobile. Its tiny electric motor was powered by 12 solar cells on top of the car. In 1983, the first drivable solar car was created by Hans Tholstrup, a Danish inventor who was influenced by the earlier Sunmobile. Called the Quiet Achiever, Tholstrups car was driven 4000 km across Australia. However, its average speed was only 23 km/h, despite having more than 700 solar cells on its top panel. Inspired by his success with the Quiet Achiever, in 1987 Tholstrup launched the first World Solar Chal- lenge. This was the worlds first solar car race. The race is now held every other year. In that first race, the winner was General Motors Sunraycer, shown here. It had an average speed of 67 km/h. Its aerodynamic shape helped it achieve that speed. In 2008, the first commercial solar car was introduced. Called the Venturi Astrolab, it has a top speed of 120 km/h. To go this fast while using very little energy, it is built of ultra-light materials. Its oversized body protects the driver in case of collision and provides a lot of surface area for solar cells. Q: Why was the invention of the solar cell important to the evolution of solar car technology? A: The solar car could not exist without the solar cell. This invention provided a way to convert light energy to electricity that could be used to run a device such as a car. Q: The 1955 Sunmobile was just a model car. It was too small for people to drive. Why was it an important achievement in the evolution of solar car technology? A: The car wasnt practical, but it was a working solar car. It showed people that solar car technology is possible. It spurred others, including Hans Tholstrup, to work on solar cars that people could actually drive. Q: How have the World Solar Challenge races influenced the development of solar cars? A: The races have drawn a lot of attention to solar car development. The challenge of winning a race has also stimulated developers to keep improving the performance of solar cars so they can go faster and farther on solar power alone. "
neutrons,T_4649,"A neutron is one of three main particles that make up the atom. The other two particles are the proton and electron. Atoms of all elementsexcept for most atoms of hydrogenhave neutrons in their nucleus. The nucleus is the small, dense region at the center of an atom where protons are also found. Atoms generally have about the same number of neutrons as protons. For example, all carbon atoms have six protons and most also have six neutrons. A model of a carbon atom is shown in the Figure 1.1. Click image to the left or use the URL below. URL: "
neutrons,T_4650,"Unlike protons and electrons, which are electrically charged, neutrons have no charge. In other words, they are electrically neutral. Thats why the neutrons in the diagram above are labeled n0 . The zero stands for zero charge. The mass of a neutron is slightly greater than the mass of a proton, which is 1 atomic mass unit (amu). (An atomic mass unit equals about 1.67  1027 kilograms.) A neutron also has about the same diameter as a proton, or 1.7 1017 meters. "
neutrons,T_4651,"All the atoms of a given element have the same number of protons and electrons. The number of neutrons, however, may vary for atoms of the same element. For example, almost 99 percent of carbon atoms have six neutrons, but the rest have either seven or eight neutrons. Atoms of an element that differ in their numbers of neutrons are called isotopes. The nuclei of these isotopes of carbon are shown in the Figure 1.2. The isotope called carbon-14 is used to find the ages of fossils. Q: Notice the names of the carbon isotopes in the diagram. Based on this example, infer how isotopes of an element are named. A: Isotopes of an element are named for their total number of protons and neutrons. Q: The element oxygen has 8 protons. How many protons and neutrons are there in oxygen-17? A: Oxygen-17like all atoms of oxygenhas 8 protons. Its name provides the clue that it has a total of 17 protons and neutrons. Therefore, it must have 9 neutrons (8 + 9 = 17). "
neutrons,T_4652,"Neutrons consist of fundamental particles known as quarks and gluons. Each neutron contains three quarks, as shown in the diagram below. Two of the quarks are called down quarks (d) and the third quark is called an up quark (u). Gluons (represented by wavy black lines in the diagram) are fundamental particles that are given off or absorbed by quarks. They carry the strong nuclear force that holds together quarks in a neutron. "
newtons first law,T_4653,"Did you ever ride a skateboard? Even if you didnt, you probably know that to start a skateboard rolling over a level surface, you need to push off with one foot against the ground. Thats what Coreys friend Nina is doing in this picture 1.1. Do you know how to stop a skateboard once it starts rolling? Look how Ninas friend Laura does it in the Figure the skateboard. Even if Laura didnt try to stop the skateboard, it would stop sooner or later. Thats because theres also friction between the wheels and the pavement. Friction is a force that counters all kinds of motion. It occurs whenever two surfaces come into contact. "
newtons first law,T_4654,"If you understand how a skateboard starts and stops, then you already know something about Newtons first law of motion. This law was developed by English scientist Isaac Newton around 1700. Newton was one of the greatest scientists of all time. He developed three laws of motion and the law of gravity, among many other contributions. Newtons first law of motion states that an object at rest will remain at rest and an object in motion will stay in motion unless it is acted on by an unbalanced force. Without an unbalanced force, a moving object will not only keep moving, but its speed and direction will also remain the same. Newtons first law of motion is often called the law of inertia because inertia is the tendency of an object to resist a change in its motion. If an object is already at rest, inertia will keep it at rest. If an object is already in motion, inertia will keep it moving. "
newtons first law,T_4655,"Coreys friend Jerod likes to skate on the flat banks at Newtons Skate Park. Thats Jerod in the Figure 1.3. As he reaches the top of a bank, he turns his skateboard to go back down. To change direction, he presses down with his heels on one edge of the skateboard. This causes the skateboard to turn in the opposite direction. "
newtons first law,T_4656,"Q: How does Nina use Newtons first law to start her skateboard rolling? A: The skateboard wont move unless Nina pushes off from the pavement with one foot. The force she applies when she pushes off is stronger than the force of friction that opposes the skateboards motion. As a result, the force on the skateboard is unbalanced, and the skateboard moves forward. Q: How does Nina use Newtons first law to stop her skateboard? A: Once the skateboard starts moving, it would keep moving at the same speed and in the same direction if not for another unbalanced force. That force is friction between the skateboard and the pavement. The force of friction is unbalanced because Nina is no longer pushing with her foot to keep the skateboard moving. Thats why the skateboard stops. Q: How does Jerod use Newtons first law of motion to change the direction of his skateboard? A: Pressing down on just one side of a skateboard creates an unbalanced force. The unbalanced force causes the skateboard to turn toward the other side. In the picture, Jerod is pressing down with his heels, so the skateboard turns toward his toes. "
newtons law of gravity,T_4657,"Newton was the first one to suggest that gravity is universal and affects all objects in the universe. Thats why Newtons law of gravity is called the law of universal gravitation. Universal gravitation means that the force that causes an apple to fall from a tree to the ground is the same force that causes the moon to keep moving around Earth. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. In fact, there is gravity between you and every mass around youyour desk, your book, your pen. Even tiny molecules of gas are attracted to one another by the force of gravity. Q: Newtons law of universal gravitation had a huge impact on how people thought about the universe. Why do you think it was so important? A: Newtons law was the first scientific law that applied to the entire universe. It explains the motion of objects not only on Earth but in outer space as well. "
newtons law of gravity,T_4658,"Newtons law also states that the strength of gravity between any two objects depends on two factors: the masses of the objects and the distance between them. Objects with greater mass have a stronger force of gravity between them. For example, because Earth is so massive, it attracts you and your desk more strongly that you and your desk attract each other. Thats why you and the desk remain in place on the floor rather than moving toward one another. Objects that are closer together have a stronger force of gravity between them. For example, the moon is closer to Earth than it is to the more massive sun, so the force of gravity is greater between the moon and Earth than between the moon and the sun. Thats why the moon circles around Earth rather than the sun. You can see this in the Figure 1.1. "
newtons second law,T_4659,"Whenever an object speeds up, slows down, or changes direction, it accelerates. Acceleration occurs whenever an unbalanced force acts on an object. Two factors affect the acceleration of an object: the net force acting on the object and the objects mass. Newtons second law of motion describes how force and mass affect acceleration. The law states that the acceleration of an object equals the net force acting on the object divided by the objects mass. This can be represented by the equation: Acceleration = or a = Net force Mass F m Q: While Tony races along on his rollerblades, what net force is acting on the skates? A: Tony exerts a backward force against the ground, as you can see in the Figure 1.1, first with one skate and then with the other. This force pushes him forward. Although friction partly counters the forward motion of the skates, it is weaker than the force Tony exerts. Therefore, there is a net forward force on the skates. "
newtons second law,T_4660,"Newtons second law shows that there is a direct relationship between force and acceleration. The greater the force that is applied to an object of a given mass, the more the object will accelerate. For example, doubling the force on the object doubles its acceleration. The relationship between mass and acceleration is different. It is an inverse relationship. In an inverse relationship, when one variable increases, the other variable decreases. The greater the mass of an object, the less it will accelerate when a given force is applied. For example, doubling the mass of an object results in only half as much acceleration for the same amount of force. Q: Tony has greater mass than the other two boys he is racing (pictured in the opening image). How will this affect his acceleration around the track? A: Tonys greater mass will result in less acceleration for the same amount of force. "
newtons third law,T_4661,"Newtons third law of motion explains how Jerod starts his skateboard moving. This law states that every action has an equal and opposite reaction. This means that forces always act in pairs. First an action occursJerod pushes against the ground with his foot. Then a reaction occursJerod moves forward on his skateboard. The reaction is always equal in strength to the action but in the opposite direction. Q: If Jerod pushes against the ground with greater force, how will this affect his forward motion? A: His action force will be greater, so the reaction force will be greater as well. Jerod will be pushed forward with more force, and this will make him go faster and farther. "
newtons third law,T_4662,"The forces involved in actions and reactions can be represented with arrows. The way an arrow points shows the direction of the force, and the size of the arrow represents the strength of the force. Look at the skateboarders in the Figure 1.1. In the top row, the arrows represent the forces with which the skateboarders push against each other. This is the action. In the bottom row, the arrows represent the forces with which the skateboarders move apart. This is the reaction. Compare the top and bottom arrows. They point in different directions, but they are the same size. This shows that the reaction forces are equal and opposite to the action forces. "
newtons third law,T_4663,"Because action and reaction forces are equal and opposite, you might think they would cancel out, as balanced forces do. But you would be wrong. Balanced forces are equal and opposite forces that act on the same object. Thats why they cancel out. Action-reaction forces are equal and opposite forces that act on different objects, so they dont cancel out. In fact, they often result in motion. Think about Jerod again. He applies force with his foot to the ground, whereas the ground applies force to Jerod and the skateboard, causing them to move forward. Q: Actions and reactions occur all the time. Can you think of an example in your daily life? A: Heres one example. If you lean on something like a wall or your locker, you are applying force to it. The wall or locker applies an equal and opposite force to you. If it didnt, you would go right through it or else it would tip over. "
noble gases,T_4664,"Noble gases are nonreactive, nonmetallic elements in group 18 of the periodic table. As you can see in the periodic table below, noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). All noble gases are colorless and odorless. They also have low boiling points, explaining why they are gases at room temperature. Radon, at the bottom of the group, is radioactive, so it constantly decays to other elements. Click image to the left or use the URL below. URL:  Q: Based on their position in the periodic table (Figure 1.1), how many valence electrons do you think noble gases have? A: The number of valence electrons starts at one for elements in group 1. It then increases by one from left to right across each period (row) of the periodic table for groups 1-2 and 13-18 (numbered 3-0 in the table above). Therefore, noble gases have eight valence electrons. "
noble gases,T_4665,"Noble gases are the least reactive of all known elements. Thats because with eight valence electrons, their outer energy levels are full. The only exception is helium, which has just two electrons. But helium also has a full outer energy level, because its only energy level (energy level 1) can hold a maximum of two electrons. A full outer energy level is the most stable arrangement of electrons. As a result, noble gases cannot become more stable by reacting with other elements and gaining or losing valence electrons. Therefore, noble gases are rarely involved in chemical reactions and almost never form compounds with other elements. "
noble gases,T_4666,"Because the noble gases are the least reactive of all elements, their eight valence electrons are used as the standard for nonreactivity and to explain how other elements interact. This is stated as the octet (group of eight) rule. According to this rule, atoms react to form compounds that allow them to have a group of eight valence electrons like the noble gases. For example, sodium (with one valence electron) reacts with chlorine (with seven valence electrons) to form the stable compound sodium chloride (table salt). In this reaction, sodium donates an electron and chlorine accepts it, giving each element an octet of valence electrons. "
noble gases,T_4667,"Did you ever get a birthday balloon like the one pictured 1.2? The balloon is filled with the noble gas helium. The gas is pumped from a tank into a Mylar balloon. Unlike a balloon filled with air, a balloon filled with helium needs to be weighted down so it wont float away. Q: Why does a helium balloon float away if its not weighted down? A: Helium atoms have just two protons, two neutrons, and two electrons, so they have less mass than any other atoms except hydrogen. As a result, helium is lighter than air, explaining why a helium balloon floats up into the air unless weighted down. Early incandescent light bulbs, like the one pictured in the Figure 1.3, didnt last very long. The filaments quickly burned out. Although air was pumped out of the bulb, it wasnt a complete vacuum. Oxygen in the small amount of air remaining inside the light bulb reacted with the metal filament. This corroded the filament and caused dark deposits on the glass. Filling a light bulb with argon gas prevents these problems. Thats why modern light bulbs are filled with argon. A: As a noble gas with eight electrons, argon doesnt react with the metal in the filament. This protects the filament and keeps the glass blub free of deposits. Noble gases are also used to fill the glass tubes of lighted signs like the one in the Figure 1.4. Although noble gases are chemically nonreactive, their electrons can be energized by sending an electric current through them. When this happens, the electrons jump to a higher energy level. When the electrons return to their original energy level, they give off energy as light. Different noble gases give off light of different colors. Neon gives off reddish-orange light, like the word Open in the sign below. Krypton gives off violet light and xenon gives off blue light. "
nonmetals,T_4668,"Nonmetals are elements that generally do not conduct electricity. They are one of three classes of elements (the other two classes are metals and metalloids.) Nonmetals are the second largest of the three classes after metals. They are the elements located on the right side of the periodic table. Q: From left to right across each period (row) of the periodic table, each element has atoms with one more proton and one more electron than the element before it. How might this be related to the properties of nonmetals? A: Because nonmetals are on the right side of the periodic table, they have more electrons in their outer energy level than elements on the left side or in the middle of the periodic table. The number of electrons in the outer energy level of an atom determines many of its properties. "
nonmetals,T_4669,"As their name suggests, nonmetals generally have properties that are very different from the properties of metals. Properties of nonmetals include a relatively low boiling point, which explains why many of them are gases at room temperature. However, some nonmetals are solids at room temperature, including the three pictured above, and one nonmetalbromineis a liquid at room temperature. Other properties of nonmetals are illustrated and described in the Figure 1.1. "
nonmetals,T_4670,"Reactivity is how likely an element is to react chemically with other elements. Some nonmetals are extremely reactive, whereas others are completely nonreactive. What explains this variation in nonmetals? The answer is their number of valence electrons. These are the electrons in the outer energy level of an atom that are involved in interactions with other atoms. Lets look at two examples of nonmetals, fluorine and neon. Simple atomic models of these two elements are shown in the Figure 1.2. Q: Which element, fluorine or neon, do you predict is more reactive? A: Fluorine is more reactive than neon. Thats because it has seven of eight possible electrons in its outer energy level, whereas neon already has eight electrons in this energy level. Although neon has just one more electron than fluorine in its outer energy level, that one electron makes a huge difference. Fluorine needs one more electron to fill its outer energy level in order to have the most stable arrangement of electrons. Therefore, fluorine readily accepts an electron from any element that is equally eager to give one up, Click image to the left or use the URL below. URL: "
nonmetals,T_4671,"Like most other nonmetals, fluorine cannot conduct electricity, and its electrons explain this as well. An electric current is a flow of electrons. Elements that readily give up electrons (the metals) can carry electric current because their electrons can flow freely. Elements that gain electrons instead of giving them up cannot carry electric current. They hold onto their electrons so they cannot flow. "
nuclear fission,T_4672,"Nuclear fission is the splitting of the nucleus of a radioactive atom into two smaller nuclei. This type of reaction releases a great deal of energy from a very small amount of matter. Fission of a tiny pellet of radioactive uranium- 235, like the one pictured in the Figure 1.1, releases as much energy as burning 1,000 kilograms of coal! Q: What causes the nucleus of uranium-235 atom to fission? A: Another particle collides with it. "
nuclear fission,T_4673,"The Figure 1.2 shows how nuclear fission of uranium-235 occurs. It begins when a uranium nucleus gains a neutron. This can happen naturally when a free neutron strikes it, or it can occur deliberately when a neutron is crashed into it in a nuclear power plant. In either case, the nucleus of uranium-235 becomes extremely unstable with the extra neutron. As a result, it splits into two smaller nuclei, krypton-92 and barium-141. The reaction also releases three neutrons and a great deal of energy. It can be represented by this nuclear equation: 235 U 92 141 + 1 neutron  92 36 Kr + 56 Ba + 3 neutrons + energy Note that the subscripts of the element symbols represent numbers of protons and the superscripts represent numbers of protons plus neutrons. "
nuclear fission,T_4674,"The neutrons released when uranium-235 fissions may crash into other uranium nuclei and cause them to fission as well. This can start a nuclear chain reaction. You can see how this happens in the Figure 1.3. In a chain reaction, one fission reaction leads to others, which lead to others, and so on. A nuclear chain reaction is similar to a pile of wood burning. If you start one piece of wood burning, enough heat is produced by the burning wood to start the rest of the pile burning without any further help from you. Click image to the left or use the URL below. URL: "
nuclear fission,T_4675,"If a nuclear chain reaction is uncontrolled, it produces a lot of energy all at once. This is what happens in an atomic bomb. However, if a nuclear chain reaction is controlled, it produces energy much more slowly. This is what occurs in a nuclear power plant. The reaction is controlled by inserting rods of nonfissioning material into the fissioning material. You can see this in the Figure 1.4. The radiation from the controlled fission is used to heat water and turn it to steam. The steam is under pressure and causes a turbine to spin. The spinning turbine runs a generator, which produces electricity. "
nuclear fission,T_4676,"In the U.S., the majority of electricity is produced by burning coal or other fossil fuels. This causes air pollution that harms the health of living things. The air pollution also causes acid rain and contributes to global warming. In addition, fossil fuels are nonrenewable resources, so if we keep using them, they will eventually run out. The main advantage of nuclear energy is that it doesnt release air pollution or cause the other environmental problems associated with the burning of fossil fuels. On the other other hand, radioactive elements are nonrenewable like fossil fuels and could eventually be used up. The main concern over the use of nuclear energy is the risk of radiation. Accidents at nuclear power plants can release harmful radiation that endangers people and other living things. Even without accidents, the used fuel that is left after nuclear fission reactions is still radioactive and very dangerous. It takes thousands of years for it to decay until it no longer releases harmful radiation. Therefore, used fuel must be stored securely to protect people and other living things. Click image to the left or use the URL below. URL: "
nuclear fusion,T_4677,"In nuclear fusion, two or more small nuclei combine to form a single, larger nucleus. You can see an example in the Figure 1.1. In this example, nuclei of two hydrogen isotopes (tritium and deuterium) fuse to form a helium nucleus. A neutron and a tremendous amount of energy are also released. "
nuclear fusion,T_4678,"Nuclear fusion of hydrogen to form helium occurs naturally in the sun and other stars. It takes place only at extremely high temperatures. Thats because a great deal of energy is needed to overcome the force of repulsion between the positively charged nuclei. The suns energy comes from fusion in its core, shown in the Figure 1.2. In the core, temperatures reach millions of degrees Kelvin. Click image to the left or use the URL below. URL:  The Sun Q: Why doesnt nuclear fusion occur naturally on Earth? A: Nuclear fusion doesnt occur naturally on Earth because it requires temperatures far higher than Earth tempera- tures. "
nuclear fusion,T_4679,"Scientists are searching for ways to create controlled nuclear fusion reactions on Earth. Their goal is develop nuclear fusion power plants, where the energy from fusion of hydrogen nuclei can be converted to electricity. You can see how this might work in the Figure 1.3. In the thermonuclear reactor, radiation from fusion is used to heat water and produce steam. The steam can then be used to turn a turbine and generate electricity. The use of nuclear fusion for energy has several pros. Unlike nuclear fission, which involves dangerous radioactive elements, nuclear fusion involves just hydrogen and helium. These elements are harmless. Hydrogen is also very plentiful. There is a huge amount of hydrogen in ocean water. The hydrogen in just a gallon of water could produce as much energy by nuclear fusion as burning 1,140 liters (300 gallons) of gasoline! The hydrogen in the oceans would generate enough energy to supply all the worlds people for a very long time. Unfortunately, using energy from nuclear fusion is far from a reality. Scientists are a long way from developing the necessary technology. One problem is raising temperatures high enough for fusion to take place. Another problem is that matter this hot exists only in the plasma state. There are no known materials that can contain plasma, although a magnet might be able to do it. Thats because plasma consists of ions and responds to magnetism. Click image to the left or use the URL below. URL: "
nucleic acid classification,T_4680,"Nucleic acids are one of four classes of biochemical compounds. (The other three classes are carbohydrates, proteins, and lipids.) Nucleic acids include RNA (ribonucleic acid) as well as DNA (deoxyribonucleic acid). Both types of nucleic acids contain the elements carbon, hydrogen, oxygen, nitrogen, and phosphorus. Q: Which of the elements in DNA is not identified with any other class of biochemical compounds? A: All biochemical compounds contain carbon, hydrogen, and oxygen; and proteins as well as nucleic acids contain nitrogen. Phosphorus is the only element that is identified with nucleic acids. "
nucleic acid classification,T_4681,"Nucleic acids consist of chains of small molecules called nucleotides, which are held together by covalent bonds. The structure of a nucleotide is shown in the Figure 1.1. Each nucleotide consists of: 1. a phosphate group, which contains phosphorus and oxygen (PO4 ). 2. a sugar, which is deoxyribose (C5 H8 O4 ) in DNA and ribose (C5 H10 O5 ) in RNA. 3. one of four nitrogen-containing bases. (A base is a compound that is not neither acidic nor neutral.) In DNA, the bases are adenine, thymine, guanine, and cytosine. RNA has the base uracil instead of thymine, but the other three bases are the same. "
nucleic acid classification,T_4682,"RNA consists of just one chain of nucleotides. DNA consists of two chains. Nitrogen bases on the two chains of DNA form hydrogen bonds with each other. Hydrogen bonds are relatively weak bonds that form between a positively charged hydrogen atom in one molecule and a negatively charged atom in another molecule. Hydrogen bonds form only between adenine and thymine, and between guanine and cytosine. These bonds hold the two chains together and give DNA is characteristic double helix, or spiral, shape. You can see the shape of the DNA molecule in the Figure 1.2. Sugars and phosphate groups form the backbone of each chain of DNA. The bonded bases are called base pairs. Determining the structure of DNA was a huge scientific breakthrough. Q: Compare the structure of DNA to a spiral staircase. What part of the molecule do the stair steps represent? A: The steps represent the base pairs. "
nucleic acid classification,T_4683,"DNA stores genetic information in the cells of all living things. It contains the genetic code. This is the code that instructs cells how to make proteins. The instructions are encoded in the sequence of nitrogen bases in the nucleotide chains of DNA. RNA copies and interprets the genetic code in DNA and is also involved in the synthesis of proteins based on the code. Click image to the left or use the URL below. URL:  Q: DNA is found only in the nucleus of cells, but proteins are synthesized in the cytoplasm of cells, outside of the nucleus. How do you think the instructions encoded in DNA reach the cytoplasm so they can be used to make proteins? A: After RNA copies the instructions in DNA, it carries them from the nucleus to a site of protein synthesis in the cytoplasm, where the instructions are translated into a protein. "
optical instruments,T_4691,"Optics is the study of visible light and the ways it can be used to extend human vision and do other tasks. Knowledge of light was needed for the invention of optical instruments such as microscopes, telescopes, and cameras, in addition to optical fibers. These instruments use mirrors and lenses to reflect and refract light and form images. Q: What is an image? A: An image is a copy of an object created by the reflection or refraction of visible light. "
optical instruments,T_4692,"A light microscope is an instrument that uses lenses to make enlarged images of objects that are too small for the unaided eye to see. A common type of light microscope is a compound microscope, like the one shown in the Figure lenses. The objective lenses are close to the object being viewed. They form an enlarged image of the object inside the microscope. The eyepiece lenses are close to the viewers eyes. They form an enlarged image of the first image. The magnifications of all the lenses are multiplied together to yield the overall magnification of the microscope. Some light microscopes can magnify objects more than 1000 times! Q: How has the microscope advanced scientific knowledge? A: The microscope has revealed secrets of the natural world like no other single invention. The microscope let scientists see entire new worlds, leading to many discoveriesespecially in biology and medicinethat could not have been made without it. Some examples include the discovery of cells and the identification of bacteria and other single-celled organisms. With the development of more powerful microscopes, viruses were discovered and even atoms finally became visible. These discoveries changed our ideas about the human body and the nature of life itself. "
optical instruments,T_4693,"Like microscopes, telescopes use convex lenses to make enlarged images. However, telescopes make enlarged images of objectssuch as distant starsthat only appear tiny because they are very far away. There are two basic types of telescopes: reflecting telescopes and refracting telescopes. The two types are compared in the Figure 1.2. They differ in how they collect light, but both use convex lenses to form enlarged images. Click image to the left or use the URL below. URL: "
optical instruments,T_4694,"A camera is an optical instrument that forms and records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way, as shown in the Figure 1.3. Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture. As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus. The shutter controls the amount of light that actually strikes the film (or sensor). It stays open longer in dim light to let more light in. "
optical instruments,T_4695,"Did you ever see a cat chase after a laser light, like the one in Figure 1.4? A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up. The diagram in Figure 1.4 shows why a beam of laser light is so focused compared with ordinary light from a flashlight. The following Figure 1.5 provides a closer look at the tube where laser light is created. Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength. At each end of the tube is a concave mirror. The photons of light reflect back and forth in the tube off these mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. Click image to the left or use the URL below. URL: "
optical instruments,T_4696,"Besides entertaining a cat, laser light has many other uses. One use is carrying communication signals in optical fibers. Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber. All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances. More than one signal can travel through an optical fiber at the same time, as you can see in the Figure Q: When lasers were invented in 1960, they were called ""a solution looking for a problem. Since then, they have been put to thousands of different uses. Can you name other ways that lasers are used? A: The first widespread use of lasers was the supermarket barcode scanner, introduced in 1974. The compact disc (CD) player was the first laser-equipped device commonly used by consumers, starting in 1982. The CD player was quickly followed by the laser printer. Some other uses of lasers include bloodless surgery, cutting and welding of metals, guiding missiles, thermometers, laser light shows, and acne treatments. The optical fiber in the diagram is much larger than a real optical fiber, which is only about as wide as a human hair. "
orbital motion,T_4697,"Earth and many other bodiesincluding asteroids, comets, and the other planetsmove around the sun in curved paths called orbits. Generally, the orbits are elliptical, or oval, in shape. You can see the shape of Earths orbit in the Figure 1.1. Because of the suns relatively strong gravity, Earth and the other bodies constantly fall toward the sun, but they stay far enough away from the sun because of their forward velocity to fall around the sun instead of into it. As a result, they keep orbiting the sun and never crash to its surface. The motion of Earth and the other bodies around the sun is called orbital motion. Orbital motion occurs whenever an object is moving forward and at the same time is pulled by gravity toward another object. "
orbital motion,T_4698,"Just as Earth orbits the sun, the moon also orbits Earth. The moon is affected by Earths gravity more than it is by the gravity of the sun because the moon is much closer to Earth. The gravity between Earth and the moon pulls the moon toward Earth. At the same time, the moon has forward velocity that partly counters the force of Earths gravity. So the moon orbits Earth instead of falling down to the surface of the planet. The Figure 1.2 shows the forces involved in the moons orbital motion around Earth. In the diagram, v represents the forward velocity of the moon, and a represents the acceleration due to gravity between Earth and the moon. The line encircling Earth shows the moons actual orbit, which results from the combination of v and a. "
ph concept,T_4703,Acids are ionic compounds that produce positively charged hydrogen ions (H+ ) when dissolved in water. Acids taste sour and react with metals. Bases are ionic compounds that produce negatively charged hydroxide ions (OH ) when dissolved in water. Bases taste bitter and do not react with metals. Examples of acids are vinegar and battery acid. The acid in vinegar is weak enough to safely eat on a salad. The acid in a car battery is strong enough to eat through skin. Examples of bases include those in antacid tablets and drain cleaner. Bases in antacid tablets are weak enough to take for an upset stomach. Bases in drain cleaner are strong enough to cause serious burns. Q: What do you think causes these differences in the strength of acids and bases? A: The strength of an acid or a base depends on how much of it breaks down into ions when it dissolves in water. 
ph concept,T_4704,"The strength of an acid depends on how many hydrogen ions it produces when it dissolves in water. A stronger acid produces more hydrogen ions than a weaker acid. For example, sulfuric acid (H2 SO4 ), which is found in car batteries, is a strong acid because nearly all of it breaks down into ions when it dissolves in water. On the other hand, acetic acid (CH3 CO2 H), which is the acid in vinegar, is a weak acid because less than 1 percent of it breaks down into ions in water. The strength of a base depends on how many hydroxide ions it produces when it dissolves in water. A stronger base produces more hydroxide ions than a weaker base. For example, sodium hydroxide (NaOH), a base in drain cleaner, is a strong base because all of it breaks down into ions when it dissolves in water. Calcium carbonate (CaCO3 ), a base in antacids, is a weak base because only a small percentage of it breaks down into ions in water. "
ph concept,T_4705,"The strength of acids and bases is measured on a scale called the pH scale, which is shown in the Figure 1.1. By definition, pH represents the acidity, or hydrogen ion (H+ ) concentration, of a solution. Pure water, which is neutral, has a pH of 7. With a higher the concentration of hydrogen ions, a solution is more acidic and has a lower pH. Acids have a pH less than 7, and the strongest acids have a pH close to zero. Bases have a pH greater than 7, and the strongest bases have a pH close to 14. Its important to realize that the pH scale is based on powers of ten. For example, a solution with a pH of 8 is 10 times more basic than a solution with a pH of 7, and a solution with a pH of 9 is 100 times more basic than a solution with a pH of 7. Q: How much more acidic is a solution with a pH of 4 than a solution with a pH of 7? A: A solution with a pH of 4 is 1000 (10  10  10, or 103 ) times more acidic than a solution with a pH of 7. Q: Which solution on the pH scale in the Figure 1.1 is the weakest acid? Which solution is the strongest base? A: The weakest acid on the scale is milk, which has a pH value between 6.5 and 6.8. The strongest base on the scale is liquid drain cleaner, which has a pH of 14. "
ph concept,T_4706,"Acidity is an important factor for living things. For example, many plants grow best in soil that has a pH between 6 and 7. Fish may also need a pH between 6 and 7. Certain air pollutants form acids when dissolved in water droplets in the air. This results in acid fog and acid rain, which may have a pH of 4 or even lower. The pH chart in the Figure lowers the pH of surface waters such as ponds and lakes. As a result, the water may become too acidic for fish and other water organisms to survive. Acid fog and acid rain killed the trees in this forest. Even normal (clean) rain is somewhat acidic. Thats because carbon dioxide (CO2 ) in the air dissolves in raindrops, producing a weak acid called carbonic acid (H2 CO3 ), which has a pH of about 5.5. When rainwater soaks into the ground, it can slowly dissolve rocks, particularly those containing calcium carbonate. This is how water forms underground caves. Q: How do you think acid rain might affect buildings and statues made of stone? A: Acid rain dissolves and damages stone buildings and statues. The Figure 1.3 shows a statue that has been damaged by acid rain. "
photosynthesis reactions,T_4707,"Most of the energy used by living things comes either directly or indirectly from the sun. Thats because sunlight provides the energy for photosynthesis. This is the process in which plants and certain other organisms synthesize glucose (C6 H12 O6 ). The process uses carbon dioxide and water and also produces oxygen. The overall chemical equation for photosynthesis is: 6CO2 + 6H2 O + Light Energy  C6 H12 O6 + 6O2 Photosynthesis changes light energy to chemical energy. The chemical energy is stored in the bonds of glucose molecules. Glucose, in turn, is used for energy by the cells of almost all living things. Photosynthetic organisms such as plants make their own glucose. Other organisms get glucose by consuming plants (or organisms that consume plants). Q: How do living things get energy from glucose? A: They break bonds in glucose and release the stored energy in the process of cellular respiration. "
photosynthesis reactions,T_4708,"The organisms pictured in the Figures 1.1, 1.2, and 1.3 all use sunlight to make glucose in the process of photo- synthesis. In addition to plants, they include bacteria and algae. All of these organisms contain the green pigment chlorophyll, which is needed to capture light energy. A tremendous amount of photosynthesis takes place in the plants of this lush tropi- cal rainforest. "
position time graphs,T_4725,"The motion of an object can be represented by a position-time graph like Graph 1 in the Figure 1.1. In this type of graph, the y-axis represents position relative to the starting point, and the x-axis represents time. A position-time graph shows how far an object has traveled from its starting position at any given time since it started moving. Q: In the Figure 1.1, what distance has the object traveled from the starting point by the time 5 seconds have elapsed? A: The object has traveled a distance of 50 meters. "
position time graphs,T_4726,"In a position-time graph, the velocity of the moving object is represented by the slope, or steepness, of the graph line. If the graph line is horizontal, like the line after time = 5 seconds in Graph 2 in the Figure 1.2, then the slope is zero and so is the velocity. The position of the object is not changing. The steeper the line is, the greater the slope of the line is and the faster the objects motion is changing. "
position time graphs,T_4727,"Its easy to calculate the average velocity of a moving object from a position-time graph. Average velocity equals the change in position (represented by d) divided by the corresponding change in time (represented by t): velocity = d t For example, in Graph 2 in the Figure 1.2, the average velocity between 0 seconds and 5 seconds is: d t 25 m  0 m = 5 s0 s 25 m = 5s = 5 m/s velocity = "
potential energy,T_4728,"The diver has energy because of her position high above the pool. The type of energy she has is called potential energy. Potential energy is energy that is stored in a person or object. Often, the person or object has potential energy because of its position or shape. Q: What is it about the divers position that gives her potential energy? A: Because the diver is high above the water, she has the potential to fall toward Earth because of gravity. This gives her potential energy. "
potential energy,T_4729,"Potential energy due to the position of an object above Earths surface is called gravitational potential energy. Like the diver on the diving board, anything that is raised up above Earths surface has the potential to fall because of gravity. You can see another example of people with gravitational potential energy in the Figure 1.1. Gravitational potential energy depends on an objects weight and its height above the ground. It can be calculated with the equation: Gravitational potential energy (GPE) = weight  height Consider the little girl on the sled, pictured in the Figure 1.1. She weighs 140 Newtons, and the top of the hill is 4 meters higher than the bottom of the hill. As she sits at the top of the hill, the childs gravitational potential energy is: GPE = 140 N  4 m = 560 N  m Notice that the answer is given in Newton  meters (N  m), which is the SI unit for energy. A Newton  meter is the energy needed to move a weight of 1 Newton over a distance of 1 meter. A Newton  meter is also called a joule (J). Q: The gymnast on the balance beam pictured in the Figure 1.1 weighs 360 Newtons. If the balance beam is 1.2 meters above the ground, what is the gymnasts gravitational potential energy? A: Her gravitational potential energy is: GPE = 360 N  1.2 m = 432 N  m, or 432 J "
potential energy,T_4730,"Potential energy due to an objects shape is called elastic potential energy. This energy results when an elastic object is stretched or compressed. The farther the object is stretched or compressed, the greater its potential energy is. A point will be reached when the object cant be stretched or compressed any more. Then it will forcefully return to its original shape. Look at the pogo stick in the Figure 1.2. Its spring has elastic potential energy when it is pressed down by the boys weight. When it cant be compressed any more, it will spring back to its original shape. The energy it releases will push the pogo stickand the boyoff the ground. Q: The girl in the Figure 1.3 is giving the elastic band of her slingshot potential energy by stretching it. Shes holding a small stone against the stretched band. What will happen when she releases the band? A: The elastic band will spring back to its original shape. When that happens, watch out! Some of the bands elastic potential energy will be transferred to the stone, which will go flying through the air. "
potential energy,T_4731,"All of the examples of potential energy described above involve movement or the potential to move. The form of energy that involves movement is called mechanical energy. Other forms of energy also involve potential energy, including chemical energy and nuclear energy. Chemical energy is stored in the bonds between the atoms of compounds. For example, food and batteries both contain chemical energy. Nuclear energy is stored in the nuclei of atoms because of the strong forces that hold the nucleus together. Nuclei of radioactive elements such as uranium are unstable, so they break apart and release the stored energy. "
power,T_4732,"Power is a measure of the amount of work that can be done in a given amount of time. Power can be represented by the equation: Power = Work Time In this equation, work is measured in joules (J) and time is measured in seconds (s), so power is expressed in joules per second (J/s). This is the SI unit for power, also known as the watt (W). A watt equals 1 joule of work per second. Youre probably already familiar with watts. Light bulbs and small appliances such as microwave ovens are labeled with the watts of power they provide. For example, the package of light bulbs in the Figure 1.1 is labeled 14 watts. Q: Assume you have two light bulbs of the same type, such as two compact fluorescent light bulbs like the one pictured in the Figure 1.1. If one light bulb is a 25-watt bulb and the other is a 60-watt bulb, which bulb produces brighter light? A: The 60-watt bulb is more powerful, so it produces brighter light. Compared with a less powerful device, a more powerful device can either do more work in the same time or do the same work in less time. For example, compared with a low-power microwave oven, a high-power microwave oven can cook more food in the same time or the same amount of food in less time. "
power,T_4733,"Power can be calculated using the formula above if the amount of work and time are known. For example, assume that a microwave oven does 24,000 joules of work in 30 seconds. Then the power of the microwave is: 24000 J Power = Work Time = 30 s = 800 J/s, or 800 W Q: Another microwave oven does 5,000 joules of work in 5 seconds. What is its power? A: The power of the other microwave oven is: J Power = 5000 5 s = 1000 J/s, or 1000 W Q: Which microwave oven will heat the same amount of food in less time? A: The 1000-watt microwave oven has more power, so it will heat the same amount of food in less time. "
power,T_4734,"You can also calculate work if you know power and time by rewriting the power equation above as: Work = Power  Time For example, if you use a 1000-watt microwave oven for 20 seconds, how much work does it do? First express 1000 watts in J/s and then substitute this value for power the work equation: Work = 1000 J/s  20 s = 20,000 J "
power,T_4735,"Sometimes power is measured in a unit called the horsepower. For example, the power of car engines is usually expressed in horsepowers. One horsepower is the amount of work a horse can do in 1 minute. It equals 745 watts of power. Compare the horsepowers in the Figure 1.2 to the other Figure 1.3. This team of three horses provides 3 horsepowers of power. This big tractor provides 180 horsepowers of power. Q: If the team of horses and the tractor do the same amount of work plowing a field, which will get the job done faster? A: The tractor will get the job done faster because it has more power. In fact, because the tractor has 30 times the power of the six-horse team, ideally it can do the same work 30 times faster! "
projectile motion,T_4741,"When the archer releases the bowstring, the arrow will be flung forward toward the top of the target where shes aiming. But another force will also act on the arrow in a different direction. The other force is gravity, and it will pull the arrow down toward Earth. The two forces combined will cause the arrow to move in the curved path shown in the Figure 1.1. This type of motion is called projectile motion. It occurs whenever an object curves down toward the ground because it has both a horizontal force and the downward force of gravity acting on it. Because of projectile motion, to hit the bulls eye of a target with an arrow, you actually have to aim for a spot above the bulls eye. You can see in theFigure 1.2 what happens if you aim at the bulls eye instead of above it. "
projectile motion,T_4742,"You can probably think of other examples of projectile motion. One is shown in the Figure 1.3. The cannon shoots a ball straight ahead, giving it horizontal motion. At the same time, gravity pulls the ball down toward the ground. Q: How would you show the force of gravity on the cannon ball in the Figure 1.3? A: You would add a line pointing straight down from the cannon to the ground. "
properties of acids,T_4743,"Acids are ionic compounds that produce positive hydrogen ions (H+ ) when dissolved in water. Ionic compounds are compounds that contain positive metal ions and negative nonmetal ions held together by ionic bonds. (Ions are atoms that have become charged particles by gaining or losing electrons.) An example of an acid is hydrogen chloride (HCl). When it dissolves in water, it separates into positive hydrogen ions and negative chloride ions (Cl ). This is represented by the chemical equation: H O 2 HCl H+ + Cl "
properties of acids,T_4744,"You already know that a sour taste is one property of acids. (Warning: Never taste an unknown substance to see whether it is an acid!) Acids have certain other properties as well. For example, acids can conduct electricity when dissolved in water because they consist of charged particles in solution. (Electric current is a flow of charged particles.) Acids can also react with metals, and when they do they produce hydrogen gas. An example of this type of reaction is hydrochloric acid reacting with the metal zinc (Zn). The reaction is pictured in the Figure 1.1. It can be represented by the chemical equation: Zn + 2HCl  H2 + ZnCl2 Q: What sign indicates that a gas is being produced in this reaction? A: The bubbles are hydrogen gas rising through the acid. Q: Besides hydrogen gas, what else is produced in this reaction? A: This reaction also produces zinc chloride ZnCl2 , which is a neutral ionic compound called a salt. "
properties of acids,T_4745,"Certain compounds, called indicators, change color when acids come into contact with them, so indicators can be used to detect acids. An example of an indicator is the compound called litmus. It is placed on small strips of paper that may be red or blue. If you place a few drops of acid on a strip of blue litmus paper, the paper will turn red. You can see this in the Figure 1.2. Litmus isnt the only indicator for detecting acids. Red cabbage juice also works well, as you can see in this entertaining video. Click image to the left or use the URL below. URL:  Drawing of blue litmus paper turning red in acid. "
properties of acids,T_4746,"The strength of acids is measured on a scale called the pH scale. The pH value of a solution represents its concentration of hydrogen ions. A pH value of 7 indicates a neutral solution, and a pH value less than 7 indicates an acidic solution. The lower the pH value is, the greater is the concentration of hydrogen ions and the stronger the acid. The strongest acids, such as battery acid, have pH values close to zero. "
properties of acids,T_4747,"Acids have many important uses, especially in industry. For example, sulfuric acid is used to manufacture a variety of different products, including paper, paint, and detergent. Some other uses of acids are be seen in the Figure 1.3. "
properties of bases,T_4748,"Bases are ionic compounds that produce negative hydroxide ions (OH ) when dissolved in water. An ionic com- pound contains positive metal ions and negative nonmetal ions held together by ionic bonds. (Ions are atoms that have become charged particles because they have either lost or gained electrons.) An example of a base is sodium hydroxide (NaOH). When it dissolves in water, it produces negative hydroxide ions and positive sodium ions (Na+ ). This can be represented by the equation: H O 2 NaOH OH + Na+ "
properties of bases,T_4749,"All bases share certain properties, including a bitter taste. (Warning: Never taste an unknown substance to see whether it is a base!) Bases also feel slippery. Think about how slippery soap feels. Thats because its a base. In addition, bases conduct electricity when dissolved in water because they consist of charged particles in solution. (Electric current is a flow of charged particles.) Q: Bases are closely related to compounds called acids. How are their properties similar? How are they different? A: A property that is shared by bases and acids is the ability to conduct electricity when dissolved in water. Some ways bases and acids are different is that acids taste sour whereas bases taste bitter. Also, acids but not bases react with metals. "
properties of bases,T_4750,"Certain compounds, called indicators, change color when bases come into contact with them, so they can be used to detect bases. An example of an indicator is a compound called litmus. It is placed on small strips of paper that may be red or blue. If you place a few drops of a base on a strip of red litmus paper, the paper will turn blue. You can see this in the Figure 1.1. Litmus isnt the only detector of bases. Red cabbage juice can also detect bases, as you can see in this video. Click image to the left or use the URL below. URL:  Drawing of red litmus paper turning blue in a base. "
properties of bases,T_4751,"The strength of bases is measured on a scale called the pH scale, which ranges from 0 to 14. On this scale, a pH value of 7 indicates a neutral solution, and a pH value greater than 7 indicates a basic solution. The higher the pH value is, the stronger the base. The strongest bases, such as drain cleaner, have a pH value close to 14. "
properties of bases,T_4752,"Bases are used for a variety of purposes. For example, soaps contain bases such as potassium hydroxide (KOH). Other uses of bases can be seen in the Figure 1.2. "
properties of electromagnetic waves,T_4753,"All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light, is about 300 million meters per second (3.0 x 108 m/s). Nothing else in the universe is known to travel this fast. The sun is about 150 million kilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth from the sun. If you could move that fast, you would be able to travel around Earth 7.5 times in just 1 second! "
properties of electromagnetic waves,T_4754,"Although all electromagnetic waves travel at the same speed across space, they may differ in their wavelengths, frequencies, and energy levels. Wavelength is the distance between corresponding points of adjacent waves (see the Figure 1.1). Wavelengths of electromagnetic waves range from longer than a soccer field to shorter than the diameter of an atom. Wave frequency is the number of waves that pass a fixed point in a given amount of time. Frequencies of electromagnetic waves range from thousands of waves per second to trillions of waves per second. The energy of electromagnetic waves depends on their frequency. Low-frequency waves have little energy and are normally harmless. High-frequency waves have a lot of energy and are potentially very harmful. Q: Which electromagnetic waves do you think have higher frequencies: visible light or X rays? A: X rays are harmful but visible light is harmless, so you can infer that X rays have higher frequencies than visible light. "
properties of electromagnetic waves,T_4755,"The speed of a wave is a product of its wavelength and frequency. Because all electromagnetic waves travel at the same speed through space, a wave with a shorter wavelength must have a higher frequency, and vice versa. This relationship is represented by the equation: Speed = Wavelength  Frequency The equation for wave speed can be rewritten as: Speed Speed Frequency = Wavelength or Wavelength = Frequency Therefore, if either wavelength or frequency is known, the missing value can be calculated. Consider an electromag- netic wave that has a wavelength of 3 meters. Its speed, like the speed of all electromagnetic waves, is 3.0  108 meters per second. Its frequency can be found by substituting these values into the frequency equation: Frequency = 3.0108 m/s 3.0 m = 1.0  108 waves/s, or 1.0  108 Hz Q: What is the wavelength of an electromagnetic wave that has a frequency of 3.0  108 hertz? A: Use the wavelength equation: Wavelength = 3.0108 m/s 3.0108 waves/s = 1.0 m "
protein classification,T_4759,"Hemoglobin is a compound in the class of compounds called proteins. Proteins are one of four classes of biochemi- cal compounds, which are compounds in living things. (The other three classes are carbohydrates, lipids, and nucleic acids.) Proteins contain carbon, hydrogen, oxygen, nitrogen, and sulfur. Protein molecules consist of one or more chains of small molecules called amino acids. "
protein classification,T_4760,"Amino acids are the building blocks of proteins. There are 20 different amino acids. The structural formula of the simplest amino acid, called glycine, is shown in the Figure 1.1. Other amino acids have slightly different structures. A protein molecule is made from one or more long chains of amino acids, each linked to its neighbors by covalent bonds. If a protein has more than one chain, the chains are held together by weaker bonds, such as hydrogen bonds. The sequence of amino acids in chains and the number of chains in a protein determine the proteins shape. The shape of a protein, in turn, determines its function. Shapes may be very complex. Click image to the left or use the URL below. URL:  Q: What do you think the ribbons in the colorful hemoglobin molecule pictured in the opening image represent? A: The ribbons represent chains of amino acids. "
protein classification,T_4761,"Proteins are the most numerous and diverse biochemical compounds, and they have many different functions. Some of their functions include: making up tissues as components of muscle. speeding up biochemical reactions as enzymes. regulating life processes as hormones. helping to defend against infections as antibodies. carrying materials around the body as transport proteins (see the example of hemoglobin in the Figure 1.2). "
protons,T_4762,"A proton is one of three main particles that make up the atom. The other two particles are the neutron and electron. Protons are found in the nucleus of the atom. This is a tiny, dense region at the center of the atom. Protons have a positive electrical charge of one (+1) and a mass of 1 atomic mass unit (amu), which is about 1.67  1027 kilograms. Together with neutrons, they make up virtually all of the mass of an atom. Click image to the left or use the URL below. URL:  Q: How do you think the sun is related to protons? A: The suns tremendous energy is the result of proton interactions. In the sun, as well as in other stars, protons from hydrogen atoms combine, or fuse, to form nuclei of helium atoms. This fusion reaction releases a huge amount of energy and takes place in nature only at the extremely high temperatures of stars such as the sun. "
protons,T_4763,"All protons are identical. For example, hydrogen protons are exactly the same as protons of helium and all other elements, or pure substances. However, atoms of different elements have different numbers of protons. In fact, atoms of any given element have a unique number of protons that is different from the numbers of protons of all other elements. For example, a hydrogen atom has just one proton, whereas a helium atom has two protons. The number of protons in an atom determines the electrical charge of the nucleus. The nucleus also contains neutrons, but they are neutral in charge. The one proton in a hydrogen nucleus, for example, gives it a charge of +1, and the two protons in a helium nucleus give it a charge of +2. "
protons,T_4764,"Protons are made of fundamental particles called quarks and gluons. As you can see in the Figure 1.1, a proton contains three quarks (colored circles) and three streams of gluons (wavy white lines). Two of the quarks are called up quarks (u), and the third quark is called a down quark (d). The gluons carry the strong nuclear force between quarks, binding them together. This force is needed to overcome the electric force of repulsion between positive protons. Although protons were discovered almost 100 years ago, the quarks and gluons inside them were discovered much more recently. Scientists are still learning more about these fundamental particles. "
pulley,T_4765,"A pulley is a simple machine that consists of a rope and grooved wheel. The rope fits into the groove in the wheel, and pulling on the rope turns the wheel. Pulleys are generally used to lift objects, especially heavy objects. The object lifted by a pulley is called the load. The force applied to the pulley is called the effort. Q: Can you guess what the pulley pictured above is used for? A: The pulley is used to lift heavy buckets full of water out of the well. "
pulley,T_4766,"Some pulleys are attached to a beam or other secure surface and remain fixed in place. They are called fixed pulleys. Other pulleys are attached to the object being moved and are moveable themselves. They are called moveable pulleys. Sometimes, fixed and moveable pulleys are used together. They make up a compound pulley. The three types of pulleys are compared in the Table 1.1. Q: Which type of pulley is the old pulley in the opening image? A: The old pulley is a single fixed pulley. It is securely attached to the beam above it. Type of Pulley How It Works Example Single fixed pul- ley Flagpole pulley No. of Rope Segments Pulling Up 1 Ideal Mechani- cal Advantage 1 Change Direction Force? yes Single moveable pulley Zip-line pulley 2 2 no Compound pulley (fixed & moveable pulleys) Crane pulley 2 2 varies in of "
pulley,T_4767,"The mechanical advantage of a simple machine such as a pulley is the factor by which the machine changes the force applied to it. The ideal mechanical advantage of a machine is its mechanical advantage in the absence of friction. All machines must overcome friction, so the ideal mechanical advantage is always somewhat greater than the actual mechanical advantage of the machine as it is used in the real world. In a pulley, the ideal mechanical advantage is equal to the number of rope segments pulling up on the object. The more rope segments that are helping to do the lifting work, the less force that is needed for the job. Look at the table of types of pulleys. It gives the ideal mechanical advantage of each type. In the single fixed pulley, only one rope segment pulls up on the load, so the ideal mechanical advantage is 1. In other words, this type of pulley doesnt increase the force that is applied to it. However, it does change the direction of the force. This allows you to use your weight to pull on one end of the rope and more easily raise the load attached to the other end. In the single moveable pulley, two rope segments pull up on the load, so the ideal mechanical advantage is 2. This type of pulley doesnt change the direction of the force applied to it, but it increases the force by a factor of 2. In a compound pulley, two or more rope segments pull up on the load, so the ideal mechanical advantage is 2 or greater than 2. This type of pulley may or may not change the direction of the force applied to itit depends on the number and arrangement of pulleysbut the increase in force may be great. Q: If a compound pulley has four rope segments pulling up on the load, by what factor does it multiply the force applied to the pulley? A: With four rope segments, the ideal mechanical advantage is 4. This means that the compound pulley multiplies the force applied to it by a factor of 4. For example if 400 Newtons of force were applied to the pulley, the pulley would apply 1600 Newtons of force to the load. "
radio waves,T_4768,"Electromagnetic waves consist of vibrating electric and magnetic fields. They transfer energy across space as well as through matter. Electromagnetic waves vary in their wavelengths and frequencies, and higher-frequency waves have more energy. The full range of wavelengths of electromagnetic waves is called the electromagnetic spectrum. It is outlined in the following Figure 1.1. "
radio waves,T_4769,"Electromagnetic waves on the left side of the Figure 1.1 are called radio waves. Radio waves are electromagnetic waves with the longest wavelengths. They may have wavelengths longer than a soccer field. They are also the electromagnetic waves with the lowest frequencies. With their low frequencies, they have the least energy of all electromagnetic waves. Nonetheless, radio waves are very useful. They are used for radio and television broadcasts and many other purposes. Click image to the left or use the URL below. URL:  Q: Based on the electromagnetic spectrum Figure 1.1, what is the range of frequencies of radio waves? A: The range of frequencies of radio waves is between 105 and 1012 Hz, or waves per second. "
radio waves,T_4770,"In radio broadcasts, sounds are encoded in radio waves, and then the waves are sent out through the atmosphere from a radio tower. A radio receiver detects the waves and changes them back to sounds. You may have listened to both AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts. AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the am- plitude, or maximum height, of radio waves. AM broadcasts use longer wavelength radio waves than FM broadcasts. Because of their longer wavelengths, AM waves reflect off a layer of the upper atmosphere called the ionosphere. You can see how this happens in the Figure 1.2. Because the waves are reflected, they can reach radio receivers that are very far away from the radio tower. FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequency of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude modulation, so FM broadcasts usually produce clearer sounds than AM broadcasts. However, the relatively short wavelengths of FM waves means that they dont reflect off the ionosphere as AM waves do. Instead, FM waves pass through the ionosphere and out into space. This is also shown in the Figure 1.2. As a result, FM waves cannot reach very distant receivers. Q: The composition of the ionosphere changes somewhat from day to night. The changes make the nighttime ionosphere even better at reflecting AM radio waves. How do you think this might affect the distance AM radio waves travel at night? A: With greater reflection off the ionosphere, AM waves can travel even farther at night than they can during the day. Radio receivers can often pick up radio broadcasts at night from cities that are hundreds of miles away. "
radio waves,T_4771,"Television broadcasts also use radio waves (see Figure 1.2). For TV broadcasts, sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded waves are broadcast from a TV tower. When the waves are received by television sets, they are decoded and changed back to sounds and pictures. "
radioactive decay,T_4772,"Radioactive decay is the process in which the nuclei of radioactive atoms emit charged particles and energy, which are called by the general term radiation. Radioactive atoms have unstable nuclei, and when the nuclei emit radiation, they become more stable. Radioactive decay is a nuclearrather than chemicalreaction because it involves only the nuclei of atoms. In a nuclear reaction, one element may change into another. Click image to the left or use the URL below. URL: "
radioactive decay,T_4773,"There are several types of radioactive decay, including alpha, beta, and gamma decay. In all three types, nuclei emit radiation, but the nature of the radiation differs. The Table 1.1 shows the radiation emitted in each type of decay. Type Alpha decay Beta decay Gamma decay Radiation Emitted alpha particle (2 protons and 2 neutrons) + energy beta particle (1 electron or 1 positron) + energy energy (gamma ray) "
radioactive decay,T_4774,"Both alpha and beta decay change the number of protons in an atoms nucleus, thereby changing the atom to a different element. In alpha decay, the nucleus loses two protons. In beta decay, the nucleus either loses a proton or gains a proton. In gamma decay, no change in proton number occurs, so the atom does not become a different element. Q: If the radioactive element polonium (Po) undergoes alpha decay, what element does it become? A: From the periodic table, the atomic number of polonium is 84, so it has 84 protons. If it loses two protons through alpha decay, it will have 82 protons. Atoms with 82 protons are the element lead (Pb). "
radioactive decay,T_4775,"The charged particles and energy emitted during radioactive decay can harm living things, but the three types of radioactive decay arent equally dangerous. Thats because they differ in how far they can travel and what they can penetrate. You can see this in the Figure 1.1. "
radioactivity,T_4776,"For an atom of one element to change into a different element, the number of protons in its nucleus must change. Thats because each element has a unique number of protons. For example, lead atoms always have 82 protons, and gold atoms always have 79 protons. Q: So how can one element change into another? A: The starting element must be radioactive, and its nuclei must gain or lose protons. "
radioactivity,T_4777,"Radioactivity is the ability of an atom to emit, or give off, charged particles and energy from its nucleus. The charged particles and energy are called by the general term radiation. Only unstable nuclei emit radiation. They are unstable because they have too much energy, too many protons, or an unstable ratio of protons to neutrons. For example, all elements with more than 83 protonssuch as uranium, radium, and poloniumhave unstable nuclei. They are called radioactive elements. The nuclei of these elements must lose protons to become more stable. When they do, they become different elements. "
radioactivity,T_4778,"Radioactivity was discovered in 1896 by a French physicist named Antoine Henri Becquerel, who is pictured 1.1. Becquerel was experimenting with uranium, which was known to glow after being exposed to sunlight. Becquerel wanted to see if the glow was caused by rays of energy, like rays of light or X-rays. He placed a bit of uranium on a photographic plate after exposing the uranium to sunlight. The plate was similar to the film that is used today to take X-rays, and Becquerel expected the uranium to leave an image on the plate. The next day, there was an image on the plate, just as Becquerel expected. This meant that uranium gives off rays after being exposed to sunlight. Becquerel was a good scientist, so he wanted to repeat his experiment to confirm his results. He placed more uranium on another photographic plate. However, the day had turned cloudy, so he tucked the plate and uranium in a drawer to try again another day. He wasnt expecting the uranium to leave an image on the plate without first being exposed to sunlight. To his surprise, there was an image on the plate in the drawer the next day. Becquerel had discovered that uranium gives off rays of energy on its own. He had discovered radioactivity, for which he received a Nobel prize. Another scientist, who worked with Becquerel, actually came up with the term radioactivity. The other scientist was the French chemist Marie Curie. She went on to discover the radioactive elements polonium and radium. She won two Nobel Prizes for her discoveries. "
radioisotopes,T_4779,"All the atoms of a given element have the same number of protons in their nucleus, but they may have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Many elements have one or more isotopes that are radioactive. These isotopes are called radioisotopes. Their nuclei are unstable, so they break down, or decay, and emit radiation. Q: What makes the nucleus of a radioisotope unstable? A: The nucleus may be unstable because it has too many protons or an unstable ratio of protons to neutrons. For a nucleus with a small number of protons to be stable, the ratio of protons to neutrons should be 1:1. For a nucleus with a large number of protons to be stable, the ratio should be about 1:1.5. "
radioisotopes,T_4780,"Find carbon in the Figure 1.1, and youll see that its atomic number is 6. This means that all carbon atoms have 6 protons per nucleus. Almost all carbon atoms also have 6 neutrons per nucleus. These carbon atoms are called carbon-12, where 12 is the number of protons (6) plus neutrons (6). This gives carbon-12 nuclei a 1:1 ratio of protons to neutrons, so carbon-12 nuclei are stable. Some carbon atoms have more than 6 neutrons, either 7 or 8. Carbon atoms with 8 neutrons are called carbon-14 (6 protons + 8 neutrons). The nuclei of carbon-14 atoms are unstable because they have too many neutrons relative to protons, so they gradually decay. Q: What is the proton-to-neutron ratio of carbon-14 nuclei? A: With six protons and 8 neutrons, the ratio is 6:8, or 1:1.3. Q: How is carbon-14 used to estimate the ages of fossils? A: Living things take in carbon, including tiny amounts of carbon-14, throughout life. The carbon-14 constantly decays, but more carbon-14 is taken in all the time to replace it. After living things die, no new carbon-14 is taken in, and the carbon-14 they already have keeps decaying. The older a fossil is, the less carbon-14 it still has, so the remaining amount can be measured to estimate the fossils age. Click image to the left or use the URL below. URL:  Periodic Table of the Elements "
radioisotopes,T_4781,"In elements with more than 83 protons, all of the isotopes are radioactive. In the Figure 1.1, these are the elements with a yellow background. The force of repulsion among all those protons makes the nuclei unstable. Elements with more than 92 protons have such unstable nuclei that they dont even exist in nature. They have only been created in labs. "
reactants and products,T_4786,"All chemical reactionsincluding a candle burninginvolve reactants and products. Reactants are substances that start a chemical reaction. Products are substances that are produced in the reaction. When a candle burns, the reactants are fuel (the candlewick and wax) and oxygen (in the air). The products are carbon dioxide gas and water vapor. "
reactants and products,T_4787,"The relationship between reactants and products in a chemical reaction can be represented by a chemical equation that has this general form: Reactants  Products The arrow () shows the direction in which the reaction occurs. In many reactions, the reaction also occurs in the opposite direction. This is represented with another arrow pointing in the opposite direction (). Q: Write a general chemical equation for the reaction that occurs when a fuel such as candle wax burns. A: The burning of fuel is a combustion reaction. The general equation for this type of reaction is: Fuel + O2  CO2 + H2 O Q: How do the reactants in a chemical reaction turn into the products? A: Bonds break in the reactants, and new bonds form in the products. "
reactants and products,T_4788,"The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms end up in different combinations in the products. This makes the products new substances that are chemically different from the reactants. Consider the example of water forming from hydrogen and oxygen. Both hydrogen and oxygen gases exist as diatomic (two-atom) molecules. These molecules are the reactants in the reaction. The Figure 1.1 shows that bonds must break to separate the atoms in the hydrogen and oxygen molecules. Then new bonds must form between hydrogen and oxygen atoms to form water molecules. The water molecules are the products of the reaction. Q: Watch the animation of a similar chemical reaction at the following URL. Can you identify the reactants and the product in the reaction? Click image to the left or use the URL below. URL:  A: The reactants are hydrogen (H2 ) and fluorine (F2 ), and the product is hydrogen fluoride (HF). "
recognizing chemical reactions,T_4789,A change in color is just one of several potential signs that a chemical reaction has occurred. Other potential signs include: Change in temperature-Heat is released or absorbed during the reaction. Production of a gas-Gas bubbles are released during the reaction. Production of a solid-A solid settles out of a liquid solution. The solid is called a precipitate. Click image to the left or use the URL below. URL: 
recognizing chemical reactions,T_4790,"Look carefully at the Figures 1.1, 1.2, and 1.3. All of the photos demonstrate chemical reactions. For each photo, identify a sign that one or more chemical reactions have taken place. A burning campfire can warm you up on a cold day. Dissolving an antacid tablet in water produces a fizzy drink. Adding acid to milk produces solid curds of cottage cheese. Q: Did you ever make a volcano by pouring vinegar over a mountain of baking soda? If you did, you probably saw the mixture bubble up and foam over. Did a chemical reaction occur? How do you know? A: Yes, a chemical reaction occurred. You know because the bubbles are evidence that a gas has been produced and production of a gas is a sign of a chemical reaction. "
replacement reactions,T_4794,"A replacement reaction occurs when elements switch places in compounds. This type of reaction involves ions (electrically charged versions of atoms) and ionic compounds. These are compounds in which positive ions of a metal and negative ions of a nonmetal are held together by ionic bonds. Generally, a more reactive element replaces an element that is less reactive, and the less reactive element is set free from the compound. There are two types of replacement reactions: single and double. Both types are described below. Q: Can you predict how single and double replacement reactions differ? A: One way they differ is that a single replacement reaction involves one reactant compound, whereas a double replacement reaction involves two reactant compounds. Keep reading to learn more about these two types of reactions. "
replacement reactions,T_4795,"A single replacement reaction occurs when one element replaces another in a single compound. This type of reaction has the general equation: A + BC  B + AC In this equation, A represents a more reactive element and BC represents the original compound. During the reaction, A replaces B, forming the product compound AC and releasing the less reactive element B. An example of a single replacement reaction occurs when potassium (K) reacts with water (H2 O). A colorless solid compound named potassium hydroxide (KOH) forms, and hydrogen gas (H2 ) is set free. The equation for the reaction is: 2K + 2H2 O  2KOH + H2 In this reaction, a potassium ion replaces one of the hydrogen atoms in each molecule of water. Potassium is a highly reactive group 1 alkali metal, so its reaction with water is explosive. Q: Find potassium in the periodic table of the elements. What other element might replace hydrogen in water in a similar replacement reaction? A: Another group 1 element, such as lithium or sodium, might be involved in a similar replacement reaction with water. "
replacement reactions,T_4796,"A double replacement reaction occurs when two ionic compounds exchange ions. This produces two new ionic compounds. A double replacement reaction can be represented by the general equation: AB + CD  AD + CB AB and CD are the two reactant compounds, and AD and CB are the two product compounds that result from the reaction. During the reaction, the ions B and D change places. Q: Could the product compounds be DA and BC? A: No, they could not. In an ionic compound, the positive metal ion is always written first, followed by the negative nonmetal ion. Therefore, A and C must always come first, followed by D or B. An example of a double replacement reaction is sodium chloride (NaCl) reacting with silver fluoride (AgF). This reaction is represented by the equation: NaCl + AgF  NaF + AgCl During the reaction, chloride and fluoride ions change places, so two new compounds are formed in the products: sodium fluoride (NaF) and silver chloride (AgCl). Q: When iron sulfide (FeS) and hydrogen chloride (HCl) react together, a double replacement reaction occurs. What are the products of this reaction? What is the chemical equation for this reaction? A: The products of the reaction are iron chloride (FeCl2 ) and hydrogen sulfide (H2 S). The chemical equation for this reaction is: FeS + 2HCl  H2 S + FeCl2 "
rutherfords atomic model,T_4799,"In 1804, almost a century before the nucleus was discovered, the English scientist John Dalton provided evidence for the existence of the atom. Dalton thought that atoms were the smallest particles of matter, which couldnt be divided into smaller particles. He modeled atoms with solid wooden balls. In 1897, another English scientist, named J. J. Thomson, discovered the electron. It was first subatomic particle to be identified. Because atoms are neutral in electric charge, Thomson assumed that atoms must also contain areas of positive charge to cancel out the negatively charged electrons. He thought that an atom was like a plum pudding, consisting mostly of positively charged matter with negative electrons scattered through it. The nucleus of the atom was discovered next. It was discovered in 1911 by a scientist from New Zealand named Ernest Rutherford, who is pictured in Figure 1.1. Through his clever research, Rutherford showed that the positive charge of an atom is confined to a tiny massive region at the center of the atom, rather than being spread evenly throughout the pudding of the atom as Thomson had suggested. "
rutherfords atomic model,T_4800,"The way Rutherford discovered the atomic nucleus is a good example of the role of creativity in science. His quest actually began in 1899 when he discovered that some elements give off positively charged particles that can penetrate just about anything. He called these particles alpha () particles (we now know they were helium nuclei). Like all good scientists, Rutherford was curious. He wondered how he could use alpha particles to learn about the structure of the atom. He decided to aim a beam of alpha particles at a sheet of very thin gold foil. He chose gold because it can be pounded into sheets that are only 0.00004 cm thick. Surrounding the sheet of gold foil, he placed a screen that glowed when alpha particles struck it. It would be used to detect the alpha particles after they passed through the foil. A small slit in the screen allowed the beam of alpha particles to reach the foil from the particle emitter. You can see the setup for Rutherfords experiment in the Figure 1.2. Q: What would you expect to happen when the alpha particles strike the gold foil? A: The alpha particles would penetrate the gold foil. Alpha particles are positive, so they might be repelled by any areas of positive charge inside the gold atoms. Assuming a plum pudding model of the atom, Rutherford predicted that the areas of positive charge in the gold atoms would deflect, or bend, the path of all the alpha particles as they passed through. You can see what really happened in the Figure 1.2. Most of the alpha particles passed straight through the gold foil as though it wasnt there. The particles seemed to be passing through empty space. Only a few of the alpha particles were deflected from their straight path, as Rutherford had predicted. Surprisingly, a tiny percentage of the particles bounced back from the foil like a basketball bouncing off a backboard! Q: What can you infer from these observations? A: You can infer that most of the alpha particles were not repelled by any positive charge, whereas a few were repelled by a strong positive charge. "
rutherfords atomic model,T_4801,"Rutherford made the same inferences. He concluded that all of the positive charge and virtually all of the mass of an atom are concentrated in one tiny area and the rest of the atom is mostly empty space. Rutherford called the area of concentrated positive charge the nucleus. He predictedand soon discoveredthat the nucleus contains positively charged particles, which he named protons. Rutherford also predicted the existence of neutral nuclear particles called neutrons, but he failed to find them. However, his student James Chadwick discovered them several years later. "
rutherfords atomic model,T_4802,"Rutherfords discoveries meant that Thomsons plum pudding model was incorrect. Positive charge is not spread evenly throughout an atom. Instead, it is all concentrated in the tiny nucleus. The rest of the atom is empty space except for the electrons scattered through it. In Rutherfords model of the atom, which is shown in the Figure 1.3, the electrons move around the massive nucleus like planets orbiting the sun. Thats why his model is called the planetary model. Rutherford didnt know exactly where or how electrons orbit the nucleus. That research would be undertaken by later scientists, beginning with Niels Bohr in 1913. New and improved atomic models would also be developed. Nonetheless, Rutherfords model is still often used to represent the atom. "
saturated hydrocarbons,T_4806,"Saturated hydrocarbons are hydrocarbons that contain only single bonds between carbon atoms. They are the simplest class of hydrocarbons. They are called saturated because each carbon atom is bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogen. You can see an example of a saturated hydrocarbon in the Figure 1.1. In this compound, named ethane, each carbon atom is bonded to three hydrogen atoms. In the structural formula, each dash (-) represents a single covalent bond, in which two atoms share one pair of valence electrons. Q: What is the chemical formula for ethane? A: The chemical formula is C2 H6 . "
saturated hydrocarbons,T_4807,"Saturated hydrocarbons are given the general name of alkanes. The name of specific alkanes always ends in -ane. The first part of the name indicates how many carbon atoms each molecule of the alkane has. The smallest alkane is methane. It has just one carbon atom. The next largest is ethane with two carbon atoms. The chemical formulas and properties of methane, ethane, and other small alkanes are listed in the Table 1.1. The boiling and melting points of alkanes are determined mainly by the number of carbon atoms they have. Alkanes with more carbon atoms generally boil and melt at higher temperatures. Alkane Methane Ethane Propane Butane Pentane Hexane Heptane Octane Chemical Formula CH4 C2 H6 C3 H8 C4 H10 C5 H12 C6 H14 C7 H16 C8 H18 Boiling Point( C) -162 -89 -42 0 36 69 98 126 Melting Point( C) -183 -172 -188 -138 -130 -95 -91 -57 State (at 20  C) gas gas gas gas liquid liquid liquid liquid Q: The Table 1.1 shows only alkanes that have relatively few carbon atoms. Some alkanes have many more carbon atoms. What properties might larger alkanes have? A: Alkanes with more carbon atoms have higher boiling and melting points, so some of them are solids at room temperature. "
saturated hydrocarbons,T_4808,"Structural formulas are often used to represent hydrocarbon compounds because the molecules can have different shapes and a structural formula shows how the atoms are arranged. Hydrocarbons may form straight chains, A) In a straight-chain molecule, all the carbon atoms are lined up in a row like cars of a train. The carbon atoms form the backbone of the molecule. B) In a branched-chain molecule, at least one of the carbon atoms branches off from the backbone. C) In a cyclic molecule, the chain of carbon atoms is joined at the two ends to form a ring. Each ring usually contains just five or six carbon atoms, but rings can join together to form larger molecules. A cyclic molecule generally has higher boiling and melting points than straight-chain and branched- chain molecules. "
scientific graphing,T_4814,"Graphs are very useful tools in science. They can help you visualize a set of data. With a graph, you can actually see what all the numbers in a data table mean. Three commonly used types of graphs are bar graphs, circle graphs, and line graphs. Each type of graph is suitable for showing a different type of data. "
scientific graphing,T_4815,The data in Table 1.1 shows the average number of tornadoes per year for the ten U.S. cities that have the most tornadoes. The data were averaged over the time period 1950-2007. 
scientific graphing,T_4816,"Rank City 1 2 3 4 5 6 7 8 9 10 Clearwater, FL Oklahoma City, OK Tampa-St. Petersburg, FL Houston, TX Tulsa, OK New Orleans, LA Melbourne, FL Indianapolis, IN Fort Worth, TX Lubbock, TX Average Number of Tornadoes(per 1000 Square Miles) 7.4 2.2 2.1 2.1 2.1 2.0 1.9 1.7 1.7 1.6 Bar graphs are especially useful for comparing values for different things, such as the average numbers of tornadoes for different cities. Therefore, a bar graph is a good choice for displaying the data in theTable 1.1. The bar graph below shows one way that these data could be presented. Q: What do the two axes of this bar graph represent? A: The x-axis represents cities, and the y-axis represents average numbers of tornadoes. Q: Could you switch what the axes represent? If so, how would the bar graph look? A: Yes; the x-axis could represent average numbers of tornadoes, and the y-axis could represent cities. The bars of the graph would be horizontal instead of vertical. "
scientific graphing,T_4817,"The data in Table 1.2 shows the percent of all U.S. tornadoes by tornado strength for the years 1986 to 1995. In this table, tornadoes are rated on a scale called the F scale. On this scale, F0 tornadoes are the weakest and F5 tornadoes are the strongest. Tornado Scale(F-scale rating) F0 F1 F2 F3 F4 F5 Percent of all U.S. Tornadoes 55.0% 31.6% 10.0% 2.6% 0.7% 0.1% Circle graphs are used to show percents (or fractions) of a whole, such as the percents of F0 to F5 tornadoes out of all tornadoes. Therefore, a circle graph is a good choice for the data in the table. The circle graph below displays these data. Q: What if the Table 1.2 on tornado strength listed the numbers of tornadoes rather than the percents of tornadoes? Could a circle graph be used to display these data? A: No, a circle graph can only be used to show percents (or fractions) of a whole. However, the numbers could be used to calculate percents, which could then be displayed in a circle graph. "
scientific graphing,T_4818,"Consider the data in Table 1.3. It lists the number of tornadoes in the U.S. per month, averaged over the years 2009 to 2011. Month January February March April May June July August September October November December Average Number of Tornadoes 17 33 74 371 279 251 122 57 39 65 39 34 Line graphs are especially useful for showing changes over time, or time trends in data, such as how the average number of tornadoes varies throughout the year. Therefore, a line graph would be a good choice to display the data in the Table 1.3. The line graph below shows one way this could be done. Q: Based on the line graph above, describe the trend in tornado numbers by month throughout the course of a year. A: The number of tornadoes rises rapidly from a low in January to a peak in April. This is followed by a relatively slow decline throughout the rest of the year. "
scientific modeling,T_4828,"A model is a representation of an object, system, or process. For example, a road map is a representation of an actual system of roads on the ground. Models are very useful in science. They provide a way to investigate things that are too small, large, complex, or distant to investigate directly. To be useful, a model must closely represent the real thing in important ways, but it must be simpler and easier to understand than the real thing. Q: What might be examples of things that would be modeled in physical science because they are difficult to investigate directly? A: Examples include extremely small things such as atoms, very distant objects such as stars, and complex systems such as the electric grid that carries electricity throughout the country. Q: What are ways that these things might be modeled? A: Types of models include two-dimensional diagrams, three-dimensional structures, mathematical formulas, and computer simulations. Examples of simple two-dimensional models in physical science are described below. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
scientific modeling,T_4829,"The diagram below is a simple two-dimensional model of a water molecule. This is the smallest particle of water that still has the properties of water. The model shows that each molecule of water consists of one atom of oxygen and two atoms of hydrogen. Q: What else can you learn about water molecules from this model? A: The model shows the number of atomic particlesprotons, neutrons, and electronsin each type of atom. It also shows that each hydrogen atom in a water molecule shares its electron with the oxygen atom. Q: Do you think this water molecule model satisfies the criteria of a useful model? In other words, does it represent a real water molecule in important ways while being simpler and easier to understand than a real water molecule? A: The model shows the basic structure of a water molecule and how the atoms in the molecule share electrons. These features of the water molecule explain important properties of water. The model is also simpler and easier to understand than a real water molecule. In a real molecule, electrons spin around the nuclei at the center of the atoms in a cloud, rather than in neat, circular orbits, as shown in the model. The atoms of a real water molecule also contain even smaller particles than protons, neutrons, and electrons. For many purposes, however, its not necessary to represent these more complex features of a real water molecule. The diagram below shows another example of a simple model in physical science. This diagram is a model of an electric circuit. It represents the main parts of the circuit with simple symbols. Horizontal lines with + and - signs represent a battery. The parts labeled R1 , R2 , and R3 are devices that use electricity provided by the battery. For example, these parts might be a series of three light bulbs. Q: In the electric circuit diagram, what do the black lines connecting the battery and electric devices represent? A: The black lines represent electric wires. The wires are necessary to carry electric current from the battery to the electric devices and back to the battery again. Q: How is a circuit diagram simpler and easier to understand than an actual electric circuit? A: A circuit diagram shows only the parts of the circuit that carry electric current, and it uses simple symbols to represent them. "
scope of chemistry,T_4836,"Chemistry is the study of matter and energy and how they interact, mainly at the level of atoms and molecules. Basic concepts in chemistry include chemicals, which are specific types of matter, and chemical reactions. In a chemical reaction, atoms or molecules of certain types of matter combine chemically to form other types of matter. All chemical reactions involve energy. Q: How do you think chemistry explains why the copper on the Statue of Liberty is green instead of brownish red? A: The copper has become tarnished. The tarnishalso called patinais a compound called copper carbonate, which is green. Copper carbonate forms when copper undergoes a chemical reaction with carbon dioxide in moist air. The green patina that forms on copper actually preserves the underlying metal. Thats why its not removed from the statue. Some people also think that the patina looks attractive. "
scope of chemistry,T_4837,"Chemistry can help you understand the world around you. Everything you touch, taste, or smell is made of chemicals, and chemical reactions underlie many common changes. For example, chemistry explains how food cooks, why laundry detergent cleans your clothes, and why antacid tablets relieve an upset stomach. Other examples are illustrated in the Figure 1.1. Chemistry even explains you! Your body is made of chemicals, and chemical changes constantly take place within it. Each of these pictures represents a way that chemicals and chemical reactions af- fect our lives. "
scope of physics,T_4840,"Physics is the study of energy, matter, and their interactions. Its a very broad field because it is concerned with matter and energy at all levelsfrom the most fundamental particles of matter to the entire universe. Some people would even argue that physics is the study of everything! Important concepts in physics include motion, forces such as magnetism and gravity, and forms of energy such as light, sound, and electrical energy. Q: How do you think physics explains the distorted images formed by a funhouse mirror? A: Physics explains how energy interacts with matter. In this case, for example, physics explains how visible light reflects from mirrors to form images. Most mirrors, such as bathroom mirrors, have a flat surface. Light reflected from a flat mirror forms an image that looks the same as the object in front of it. Funhouse mirrors, like the one pictured above, are different. They have a curved surface that reflects light at different angles. This explains why the images they form are distorted. "
scope of physics,T_4841,"Physics can help you understand just about everything in the world around you. Thats because everything around you consists of matter and energy. Several examples of matter and energy interacting are pictured in the Figure 1.1. Read how physics explains each example. Examples of how matter and energy interact. Q: Based on the examples in Figure 1.1, what might be other examples of energy and matter interacting? A: Like the strings of cello, anything that vibrates produces waves of energy that travel through matter. For example, when you throw a pebble into a pond, waves of energy travel from the pebble through the water in all directions. Like an incandescent light bulb, anything that glows consists of matter that produces light energy. For example, fireflies use chemicals to produce light energy. Like a moving tennis racket, anything that moves has energy because it is moving, including your eyes as they read this sentence. "
screw,T_4842,"A screw is a simple machine that consists of an inclined plane wrapped around a central cylinder. No doubt you are familiar with screws like the wood screw in the left-hand side of the Figure 1.1. The cap of the bottle pictured on the right is another example of a screw. Screws move objects to a greater depth (or higher elevation) by increasing the force applied to the screw. Many screws are used to hold things together, such as two pieces of wood or a screw cap and bottle. When you use a screw, you apply force to turn the inclined plane. The screw, in turn, applies greater force to the object, such as the wood or bottle top. Q: Can you identify the inclined plane in each example of a screw pictured in the Figure 1.1? A: The inclined plane of the screw on the left consists of the ridges, or threads, that wrap around the central cylinder of the screw. The inclined plane of the cap on the right consists of the ridges that wrap around the inner sides of the cap. "
screw,T_4843,"The mechanical advantage of a simple machine is the factor by which it multiplies the force applied to the machine. It is the ratio of the output force to the input force. The force applied by the screw (output force) is always greater than the force applied to the screw (input force). Therefore, the mechanical advantage of a screw is always greater than 1. Look at the two screws in the Figure 1.2. In the screw on the right, the threads of the inclined plane are closer together. This screw has a greater mechanical advantage and is easier to turn than the screw on the left, so it takes less force to penetrate the wood with the right screw. The trade-off is that more turns of the screw are needed to do the job because the distance over which the input force must be applied is greater. Q: Why is it harder to turn a screw with more widely spaced threads? A: The screw moves farther with each turn when the threads are more widely space, so more force must be applied to turn the screw and cover the greater distance. "
significant figures,T_4847,"In any measurement, the number of significant figures is the number of digits thought to be correct by the person doing the measuring. It includes all digits that can be read directly from the measuring device plus one estimated digit. Look at the sketch of a beaker below. How much blue liquid does the beaker contain? The top of the liquid falls between the mark for 40 mL and 50 mL, but its closer to 50 mL. A reasonable estimate is 47 mL. In this measurement, the first digit (4) is known for certain and the second digit (7) is an estimate, so the measurement has two significant figures. Now look at the graduated cylinder sketched below. How much blue liquid does it contain? First, its important to note that you should read the amount of liquid at the bottom of its curved surface. This falls about half way between the mark for 36 mL and the mark for 37 mL, so a reasonable estimate would be 36.5 mL. Q: How many significant figures does this measurement have? A: There are three significant figures in this measurement. You know that the first two digits (3 and 6) are accurate. The third digit (5) is an estimate. "
significant figures,T_4848,"The examples above show that its easy to count the number of significant figures when you are making a measure- ment. But what if someone else has made the measurement? How do you know which digits are known for certain and which are estimated? How can you tell how many significant figures there are in the measurement? There are several rules for counting significant figures: Leading zeros are never significant. For example, in the number 006.1, only the 6 and 1 are significant. Zeros within a number between nonzero digits are always significant. For example, in the number 106.1, the zero is significant, so this number has four significant figures. Zeros that show only where the decimal point falls are not significant. For example, the number 470,000 has just two significant figures (4 and 7). The zeros just show that the 4 represents hundreds of thousands and the 7 represents tens of thousands. Therefore, these zeros are not significant. Trailing zeros that arent needed to show where the decimal point falls are significant. For example, 4.00 has three significant figures. Q: How many significant figures are there in each of these numbers: 20,080, 2.080, and 2000? A: Both 20,080 and 2.080 contain four significant figures, but 2000 has just one significant figure. "
significant figures,T_4849,"When measurements are used in a calculation, the answer cannot have more significant figures than the measurement with the fewest significant figures. This explains why the homework answer above is wrong. It has more significant figures than the measurement with the fewest significant figures. As another example, assume that you want to calculate the volume of the block of wood shown below. The volume of the block is represented by the formula: Volume = length  width  height Therefore, you would do the following calculation: Volume = 1.2 cm  1.0 cm  1 cm = 1.2 cm3 Q: Does this answer have the correct number of significant figures? A: No, it has too many significant figures. The correct answer is 1 cm3 . Thats because the height of the block has just one significant figure. Therefore, the answer can have only one significant figure. "
significant figures,T_4850,"To get the correct answer in the volume calculation above, rounding was necessary. Rounding is done when one or more ending digits are dropped to get the correct number of significant figures. In this example, the answer was rounded down to a lower number (from 1.2 to 1). Sometimes the answer is rounded up to a higher number. How do you know which way to round? Follow these simple rules: If the digit to be rounded (dropped) is less than 5, then round down. For example, when rounding 2.344 to three significant figures, round down to 2.34. If the digit to be rounded is greater than 5, then round up. For example, when rounding 2.346 to three significant figures, round up to 2.35. If the digit to be rounded is 5, round up if the digit before 5 is odd, and round down if digit before 5 is even. For example, when rounding 2.345 to three significant figures, round down to 2.34. This rule may seem arbitrary, but in a series of many calculations, any rounding errors should cancel each other out. "
simple machines,T_4851,"A machine is any device that makes work easier by changing a force. Work is done whenever a force moves an object over a distance. The amount of work done is represented by the equation: Work = Force x Distance When you use a machine, you apply force to the machine. This force is called the input force. The machine, in turn, applies force to an object. This force is called the output force. The output force may or may not be the same as the input force. The force you apply to the machine is applied over a given distance, called the input distance. The force applied by the machine to the object is also applied over a distance, called the output distance. The output distance may or may not be the same as the input distance. "
simple machines,T_4852,"Contrary to popular belief, machines do not increase the amount of work that is done. They just change how the work is done. Machines make work easier by increasing the amount of force that is applied, increasing the distance over which the force is applied, or changing the direction in which the force is applied. Q: If a machine increases the force applied, what does this tell you about the distance over which the force is applied by the machine: A: The machine must apply the force over a shorter distance. Thats because a machine doesnt change the amount of work and work equals force times distance. Therefore, if force increases, distance must decrease. For the same reason, if a machine increases the distance over which the force is applied, it must apply less force. "
simple machines,T_4853,"Examples of machines that increase force are steering wheels and pliers (see Figure 1.1). Read below to find out how both of these machines work. In each case, the machine applies more force than the user applies to the machine, but the machine applies the force over a shorter distance. "
simple machines,T_4854,"Examples of machines that increase the distance over which force is applied are leaf rakes and hammers (see Figure which the force is applied, but it reduces the strength of the force. "
simple machines,T_4855,"Some machines change the direction of the force applied by the user. They may or may not also change the strength of the force or the distance over which the force is applied. Two examples of machines that work this way are the claw ends of hammers and flagpole pulleys. You can see in the Figure 1.3 how each of these machines works. In both cases, the direction of the force applied by the user is reversed by the machine. Q: If the pulley only changes the direction of the force, how does it make the work of raising the flag easier? A: The pulley makes it easier to lift the flag because it allows a person to pull down on the rope and add his or her own weight to the effort, rather than simply lifting the load. "
simple machines,T_4856,"There are six types of simple machines that are the basis of all other machines. They are the inclined plane, lever, wedge, screw, pulley, and wheel and axle. The six types are pictured in the Figure 1.4. Youve probably used some of these simple machines yourself. Most machines are combinations of two or more simple machines. These machines are called compound machines. An example of a compound machine is a wheelbarrow (see bottom of Figure 1.4). It consists of two simple machines: a lever and a wheel and axle. Many compound machines are much more complex and consist of many simple machines. Examples include washing machines and cars. "
sound waves,T_4875,"In science, sound is defined as the transfer of energy from a vibrating object in waves that travel through matter. Most people commonly use the term sound to mean what they hear when sound waves enter their ears. The tree above generated sound waves when it fell to the ground, so it made sound according to the scientific definition. But the sound wasnt detected by a persons ears if there was nobody in the forest. So the answer to the riddle is both yes and no! "
sound waves,T_4876,"All sound waves begin with vibrating matter. Look at the first guitar string on the left in the Figure 1.1. Plucking the string makes it vibrate. The diagram below the figure shows the wave generated by the vibrating string. The moving string repeatedly pushes against the air particles next to it, which causes the air particles to vibrate. The vibrations spread through the air in all directions away from the guitar string as longitudinal waves. In longitudinal waves, particles of the medium vibrate back and forth parallel to the direction that the waves travel. Q: If there were no air particles to carry the vibrations away from the guitar string, how would sound reach the ear? A: It wouldnt unless the vibrations were carried by another medium. Sound waves are mechanical waves, so they can travel only though matter and not through empty space. "
sound waves,T_4877,"The fact that sound cannot travel through empty space was first demonstrated in the 1600s by a scientist named Robert Boyle. Boyle placed a ticking clock in a sealed glass jar. The clock could be heard ticking through the air and glass of the jar. Then Boyle pumped the air out of the jar. The clock was still ticking, but the ticking sound could no longer be heard. Thats because the sound couldnt travel away from the clock without air particles to pass the sound energy along. Click image to the left or use the URL below. URL: "
sound waves,T_4878,"Most of the sounds we hear reach our ears through the air, but sounds can also travel through liquids and solids. If you swim underwateror even submerge your ears in bathwaterany sounds you hear have traveled to your ears through the water. Some solids, including glass and metals, are very good at transmitting sounds. Foam rubber and heavy fabrics, on the other hand, tend to muffle sounds. They absorb rather than pass on the sound energy. Q: How can you tell that sounds travel through solids? A: One way is that you can hear loud outdoor sounds such as sirens through closed windows and doors. You can also hear sounds through the inside walls of a house. For example, if you put your ear against a wall, you may be able to eavesdrop on a conversation in the next roomnot that you would, of course. "
sources of visible light,T_4879,"Visible light includes all the wavelengths of light that the human eye can detect. It allows us to see objects in the world around us. Without visible light, we would only be able to sense most objects by sound, touch, or smell. Like humans, most other organisms also depend on visible light, either directly or indirectly. Many animalsincluding predators of jellyfishuse visible light to see. Plants and certain other organisms use visible light to make food in the process of photosynthesis. Without this food, most other organisms would not be able to survive. Q: Do you think that some animals might be able to see light that isnt visible to humans? A: Some animals can see light in the infrared or ultraviolet range of wavelengths. For example, mosquitoes can see infrared light, which is emitted by warm objects. By seeing infrared light, mosquitoes can tell where the warmest, blood-rich areas of the body are located. "
sources of visible light,T_4880,"Most of the visible light on Earth comes from the sun. The sun and other stars produce light because they are so hot. They glow with light due to their extremely high temperatures. This way of producing light is called incandescence. Incandescent light bulbs also produce light in this way. When electric current passes through a wire filament inside an incandescent bulb, the wire gets so hot that it glows. Do you see the glowing filament inside the incandescent light bulb in the Figure 1.1? Q: What are some other sources of incandescent light? A: Flames also produce incandescent light. For example, burning candles, oil lamps, and bonfires produce light in this way. "
sources of visible light,T_4881,"Some objects produce light without becoming very hot. They generate light through chemical reactions or other processes. Producing light without heat is called luminescence. Luminescence, in turn, can occur in several different ways: One type of luminescence is called fluorescence. In this process, a substance absorbs shorter-wavelength ultraviolet light and then gives off light in the visible range of wavelengths. Certain minerals produce light in this way, including gemstones such as amethyst, diamond, and emerald. Another type of luminescence is called electroluminescence. In this process, a substance gives off light when an electric current passes through it. Gases such as neon, argon, and krypton produce light by this means. The car dash lights in the Figure 1.2 are produced by electroluminescence. A third type of luminescence is called bioluminescence. This is the production of light by living things as a result of chemical reactions. The jellyfish in the opening photo above produces light by bioluminescence. So does the firefly in the Figure 1.3. Fireflies give off visible light to attract mates. "
sources of visible light,T_4882,"Many other objects appear to produce their own light, but they actually just reflect light from another source. Being lit by another source is called illumination. The moon in the Figure 1.4 is glowing so brightly that you can see shadows under the trees. It appears to glow from its own light, but its really just illuminated by light from the sun. Everything you can see that doesnt produce its own light is illuminated by light from some other source. "
speed,T_4885,"How fast or slow something moves is its speed. Speed determines how far something travels in a given amount of time. The SI unit for speed is meters per second (m/s). Speed may be constant, but often it varies from moment to moment. "
speed,T_4886,"Even if speed varies during the course of a trip, its easy to calculate the average speed by using this formula: speed = distance time For example, assume you go on a car trip with your family. The total distance you travel is 120 miles, and it takes 3 hours to travel that far. The average speed for the trip is: 120 mi 3h = 40 mi/h speed = Q: Terri rode her bike very slowly to the top of a big hill. Then she coasted back down the hill at a much faster speed. The distance from the bottom to the top of the hill is 3 kilometers. It took Terri 41 hour to make the round trip. What was her average speed for the entire trip? (Hint: The round-trip distance is 6 km.) A: Terris speed can be calculated as follows: 6 km 0.25 h = 24 km/h speed = "
speed,T_4887,"When you travel by car, you usually dont move at a constant speed. Instead you go faster or slower depending on speed limits, traffic lights, the number of vehicles on the road, and other factors. For example, you might travel 65 miles per hour on a highway but only 20 miles per hour on a city street (see the pictures in the Figure 1.1.) You might come to a complete stop at traffic lights, slow down as you turn corners, and speed up to pass other cars. Therefore, your speed at any given instant, or your instantaneous speed, may be very different than your speed at other times. Instantaneous speed is much more difficult to calculate than average speed. Cars race by in a blur of motion on an open highway but crawl at a snails pace when they hit city traffic. "
speed,T_4888,"If you know the average speed of a moving object, you can calculate the distance it will travel in a given period of time or the time it will take to travel a given distance. To calculate distance from speed and time, use this version of the average speed formula given above: distance = speed  time For example, if a car travels at an average speed of 60 km/h for 5 hours, then the distance it travels is: distance = 60 km/h  5 h = 300 km To calculate time from speed and distance, use this version of the formula: time = distance speed Q: If you walk 6 km at an average speed of 3 km/h, how much time does it take? A: Use the formula for time as follows: distance speed 6 km = 3 km/h =2h time = "
speed of sound,T_4889,"The speed of sound is the distance that sound waves travel in a given amount of time. Youll often see the speed of sound given as 343 meters per second. But thats just the speed of sound under a certain set of conditions, specifically, through dry air at 20  C. The speed of sound may be very different through other matter or at other temperatures. "
speed of sound,T_4890,"Sound waves are mechanical waves, and mechanical waves can only travel through matter. The matter through which the waves travel is called the medium (plural, media). The Table 1.1 gives the speed of sound in several different media. Generally, sound waves travel most quickly through solids, followed by liquids, and then by gases. Particles of matter are closest together in solids and farthest apart in gases. When particles are closer together, they can more quickly pass the energy of vibrations to nearby particles. Medium (20  C) Dry Air Speed of Sound Waves (m/s) 343 Medium (20  C) Water Wood Glass Aluminum Speed of Sound Waves (m/s) 1437 3850 4540 6320 Q: The table gives the speed of sound in dry air. Do you think that sound travels more or less quickly through air that contains water vapor? (Hint: Compare the speed of sound in water and air in the table.) A: Sound travels at a higher speed through water than air, so it travels more quickly through air that contains water vapor than it does through dry air. "
speed of sound,T_4891,"The speed of sound also depends on the temperature of the medium. For a given medium, sound has a slower speed at lower temperatures. You can compare the speed of sound in dry air at different temperatures in the following Table 1.2. At a lower temperature, particles of the medium are moving more slowly, so it takes them longer to transfer the energy of the sound waves. Temperature of Air 0 C 20  C 100  C Speed of Sound Waves (m/s) 331 343 386 Q: What do you think the speed of sound might be in dry air at a temperature of -20  C? A: For each 1 degree Celsius that temperature decreases, the speed of sound decreases by 0.6 m/s. So sound travels through dry, -20  C air at a speed of 319 m/s. "
static electricity and static discharge,T_4895,"Static electricity is a buildup of electric charges on objects. Charges build up when negative electrons are transferred from one object to another. The object that gives up electrons becomes positively charged, and the object that accepts the electrons becomes negatively charged. This can happen in several ways. One way electric charges can build up is through friction between materials that differ in their ability to give up or accept electrons. When you wipe your rubber-soled shoes on the wool mat, for example, electrons rub off the mat onto your shoes. As a result of this transfer of electrons, positive charges build up on the mat and negative charges build up on you. Once an object becomes electrically charged, it is likely to remain charged until it touches another object or at least comes very close to another object. Thats because electric charges cannot travel easily through air, especially if the air is dry. Q: Youre more likely to get a shock in the winter when the air is very dry. Can you explain why? A: When the air is very dry, electric charges are more likely to build up objects because they cannot travel easily through the dry air. This makes a shock more likely when you touch another object. "
static electricity and static discharge,T_4896,"What happens when you have become negatively charged and your hand approaches the metal doorknocker? Your negatively charged hand repels electrons in the metal, so the electrons move to the other side of the knocker. This makes the side of the knocker closest to your hand positively charged. As your negatively charged hand gets very close to the positively charged side of the metal, the air between your hand and the knocker also becomes electrically charged. This allows electrons to suddenly flow from your hand to the knocker. The sudden flow of electrons is static discharge. The discharge of electrons is the spark you see and the shock you feel. "
static electricity and static discharge,T_4897,"Another example of static discharge, but on a much larger scale, is lightning. You can see how it occurs in the following diagram (Figure 1.1). During a rainstorm, clouds develop regions of positive and negative charge due to the movement of air molecules, water drops, and ice particles. The negative charges are concentrated at the base of the clouds, and the positive charges are concentrated at the top. The negative charges repel electrons on the ground beneath them, so the ground below the clouds becomes positively charged. At first, the atmosphere prevents electrons from flowing away from areas of negative charge and toward areas of positive charge. As more charges build up, however, the air between the oppositely charged areas also becomes charged. When this happens, static electricity is discharged as bolts of lightning. "
surface wave,T_4900,"A surface wave is a wave that travels along the surface of a medium. The medium is the matter through which the wave travels. Ocean waves are the best-known examples of surface waves. They travel on the surface of the water between the ocean and the air. Q: What do you think causes ocean waves? A: Most ocean waves are caused by wind blowing across the water. Moving air molecules transfer some of their energy to molecules of ocean water. The energy travels across the surface of the water in waves. The stronger the winds are blowing, the larger the waves are and the more energy they have. "
surface wave,T_4901,"A surface wave is a combination of a transverse wave and a longitudinal wave. A transverse wave is a wave in which particles of the medium move up and down perpendicular to the direction of the wave. A longitudinal wave is a wave in which particles of the medium move parallel to the direction of the wave. In a surface wave, particles of the medium move up and down as well as back and forth. This gives them an overall circular motion. You can see how the particles move in the Figure 1.1. Click image to the left or use the URL below. URL: "
surface wave,T_4902,"In deep water, particles of water just move in circles. They dont actually move closer to shore with the energy of the waves. However, near the shore where the water is shallow, the waves behave differently. Look at the Figure 1.2. You can see how the waves start to drag on the bottom in shallow water. This creates friction that slows down the bottoms of the waves, while the tops of the waves keep moving at the same speed. The difference in speed causes the waves to get steeper until they topple over and break. The crashing waves carry water onto the shore as surf. Q: In this diagram of a wave breaking near shore, where do you think a surfer would try to catch the wave? A: The surfer would try to catch the wave where it starts to steepen and lean forward toward the shore. "
synthesis reactions,T_4903,"A synthesis reaction occurs when two or more reactants combine to form a single product. A synthesis reaction can be represented by the general equation: A+BC In this equation, the letters A and B represent the reactants that begin the reaction, and the letter C represents the product that is synthesized in the reaction. The arrow shows the direction in which the reaction occurs. Q: What is the chemical equation for the synthesis of nitrogen dioxide (NO2 ) from nitric oxide (NO) and oxygen (O2 )? A: The equation for this synthesis reaction is: 2NO + O2  2NO2 "
synthesis reactions,T_4904,"Another example of a synthesis reaction is the combination of sodium (Na) and chlorine (Cl) to produce sodium chloride (NaCl). This reaction is represented by the chemical equation: 2Na + Cl2  2NaCl Sodium is a highly reactive metal, and chlorine is a poisonous gas. Both elements are pictured in the Figure 1.1. The compound they synthesize has very different properties. Sodium chloride is commonly called table salt, which is neither reactive nor poisonous. In fact, salt is a necessary component of the human diet. "
technological design constraints,T_4905,"The development of new technologywhether its a simple kite or a complex machineis called technological design. The technological design process is a step-by-step approach to finding a solution to a problem. Often, the main challenge in technological design is finding a solution that works within the constraints, or limits, on the design. All technological designs have constraints. Q: Assume you want to design a kite. What might be some constraints on your design? A: Possible constraints might include the shape and size of the kite and the materials you use to make it. "
technological design constraints,T_4906,"Technological design constraints may be physical or social. Physical design constraints include factors such as natural laws and properties of materials. A kite, for example, will fly only if its shape allows air currents to lift it. Otherwise, gravity will keep it on the ground. Social design constraints include factors such as ease of use, safety, attractiveness, and cost. For example, a kite string should be easy to unwind as the wind carries the kite higher. "
technological design constraints,T_4907,"All technological designs have trade-offs because no design is perfect. For example, a design might be very good at solving a problem, but it might be too expensive to be practical. Or a design might be very attractive, but it might not be safe to use. Choosing the best design often involves weighing the pros and cons of different options and deciding which ones are most important. Q: What trade-offs might there be on the design of a kite? A: You might want to make a big kite, but if its too big it might be too heavy. Then it would fly only on very windy days. Or you might want to make a kite using a certain material that you really like, but the material might cost more than you can afford to spend. "
technology and society,T_4913,"Important new technologies such as the wheel have had a big impact on human society. Major advances in technol- ogy have influenced every aspect of life, including transportation, food production, manufacturing, communication, medicine, and the arts. Thats because technology has the goal of solving human problems, so new technologies usually make life better. They may make work easier, for example, or make people healthier. Sometimes, however, new technologies affect people in negative ways. For example, using a new product or process might cause human health problems or pollute the environment. Q: Can you think of a modern technology that has both positive and negative effects on people? A: Modern methods of transportation have both positive and negative effects on people. They help people and goods move quickly all over the world. However, most of them pollute the environment. For example, gasoline-powered cars and trucks add many pollutants to the atmosphere. The pollutants harm peoples health and contribute to global climate change. "
technology and society,T_4914,"Few technologies have impacted society as greatly as the powerful steam engine developed by Scottish inventor James Watt in 1775 (see Figure 1.1). Watts steam engine was soon being used to power all kinds of machines. It started a revolution in industry. For the first time in history, people did not have to rely on human or animal muscle, wind, or water for power. With the steam engine to power machines, new factories sprang up all over Britain. The Industrial Revolution began in Britain the late 1700s. It eventually spread throughout Western Europe, North America, Japan, and many other countries. It marked a major turning point in human history. Almost every aspect of daily life was influenced by it in some way. Average income and population both began to grow faster than ever before. People flocked to the new factories for jobs, and densely populated towns and cities grew up around the factories. The new towns and cities were crowded, and soot from the factories polluted the air. You can see an example of this in the Figure 1.2. This made living conditions very poor. Working conditions in the factories were also bad, with long hours and the pace set by machines. Even young children worked in the factories, damaging their health and giving them little opportunity for education or play. Q: In addition to factory machines, the steam engine was used to power farm machinery, trains, and ships. What effects might this have had on peoples lives? A: Farm machinery replaced human labor and allowed fewer people to produce more food. This is why many rural people migrated to the new towns and cities to look for work in factories. Steam-powered trains and ships made it easier for people to migrate. Food and factory goods could also be transported on steam-powered trains and ships, making them available to far more people. "
thermal conductors and insulators,T_4920,"Conduction is the transfer of thermal energy between particles of matter that are touching. Thermal conduction occurs when particles of warmer matter bump into particles of cooler matter and transfer some of their thermal energy to the cooler particles. Conduction is usually faster in certain solids and liquids than in gases. Materials that are good conductors of thermal energy are called thermal conductors. Metals are especially good thermal conductors because they have freely moving electrons that can transfer thermal energy quickly and easily. Besides the heating element inside a toaster, another example of a thermal conductor is a metal radiator, like the one in the Figure 1.1. When hot water flows through the coils of the radiator, the metal quickly heats up by conduction and then radiates thermal energy into the surrounding air. Q: Thermal conductors have many uses, but sometimes its important to prevent the transfer of thermal energy. Can you think of an example? A: One example is staying warm on a cold day. You will stay warmer if you can prevent the transfer of your own thermal energy to the outside air. "
thermal conductors and insulators,T_4921,"One way to retain your own thermal energy on a cold day is to wear clothes that trap air. Thats because air, like other gases, is a poor conductor of thermal energy. The particles of gases are relatively far apart, so they dont bump into each other or into other things as often as the more closely spaced particles of liquids or solids. Therefore, particles of gases have fewer opportunities to transfer thermal energy. Materials that are poor thermal conductors are called thermal insulators. Down-filled snowsuits, like those in the Figure 1.2, are good thermal insulators because their feather filling traps a lot of air. Another example of a thermal insulator is pictured in the Figure 1.3. The picture shows fluffy pink insulation inside the attic of a home. Like the down filling in a snowsuit, the insulation traps a lot of air. The insulation helps to prevent the transfer of thermal energy into the house on hot days and out of the house on cold days. Other materials that are thermal insulators include plastic and wood. Thats why pot handles and cooking utensils are often made of these materials. Notice that the outside of the toaster pictured in the opening image is made of plastic. The plastic casing helps prevent the transfer of thermal energy from the heating element inside to the outer surface of the toaster where it could cause burns. Q: Thermal insulators have many practical uses besides the uses mentioned above. Can you think of others? A: Thermal insulators are often used to keep food or drinks hot or cold. For example, Styrofoam coolers and thermos containers are used for these purposes. "
thermal energy,T_4922,"Why do the air and sand of Death Valley feel so hot? Its because their particles are moving very rapidly. Anything that is moving has kinetic energy, and the faster it is moving, the more kinetic energy it has. The total kinetic energy of moving particles of matter is called thermal energy. Its not just hot things such as the air and sand of Death Valley that have thermal energy. All matter has thermal energy, even matter that feels cold. Thats because the particles of all matter are in constant motion and have kinetic energy. "
thermal energy,T_4923,"Thermal energy and temperature are closely related. Both reflect the kinetic energy of moving particles of matter. However, temperature is the average kinetic energy of particles of matter, whereas thermal energy is the total kinetic energy of particles of matter. Does this mean that matter with a lower temperature has less thermal energy than matter with a higher temperature? Not necessarily. Another factor also affects thermal energy. The other factor is mass. Q: Look at the pot of soup and the tub of water in the Figure 1.1. Which do you think has greater thermal energy? A: The soup is boiling hot and has a temperature of 100  C, whereas the water in the tub is just comfortably warm, with a temperature of about 38  C. Although the water in the tub has a much lower temperature, it has greater thermal energy. The particles of soup have greater average kinetic energy than the particles of water in the tub, explaining why the soup has a higher temperature. However, the mass of the water in the tub is much greater than the mass of the soup in the pot. This means that there are many more particles of water than soup. All those moving particles give the water in the tub greater total kinetic energy, even though their average kinetic energy is less. Therefore, the water in the tub has greater thermal energy than the soup. Q: Could a block of ice have more thermal energy than a pot of boiling water? A: Yes, the block of ice could have more thermal energy if its mass was much greater than the mass of the boiling water. "
thermal radiation,T_4924,"The bonfire from the opening image has a lot of thermal energy. Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Thermal energy from the bonfire is transferred to the hands by thermal radiation. Thermal radiation is the transfer of thermal energy by waves that can travel through air or even through empty space, as shown in the Figure 1.1. When the waves of thermal energy reach objects, they transfer the energy to the objects, causing them to warm up. This is how the fire warms the hands of someone sitting near the bonfire. This is also how the suns energy reaches Earth and heats its surface. Without the energy radiated from the sun, Earth would be too cold to support life as we know it. Thermal radiation is one of three ways that thermal energy can be transferred. The other two ways are conduction and convection, both of which need matter to transfer energy. Radiation is the only way of transferring thermal energy that doesnt require matter. "
thermal radiation,T_4925,"You might be surprised to learn that everything radiates thermal energy, not just really hot things such as the sun or a fire. For example, when its cold outside, a heated home radiates some of its thermal energy into the outdoor environment. A home that is poorly insulated radiates more energy than a home that is well insulated. Special cameras can be used to detect radiated heat. In the Figure 1.2, you can see an image created by one of these cameras. The areas that are yellow are the areas where the greatest amount of thermal energy is radiating from the home. Even people radiate thermal energy. In fact, when a room is full of people, it may feel noticeably warmer because of all the thermal energy the people radiate! Q: Where is thermal radiation radiating from the home in the picture? A: The greatest amount of thermal energy is radiating from the window on the upper left. A lot of thermal energy is also radiating from the edges of the windows and door. "
thomsons atomic model,T_4926,"John Dalton discovered atoms in 1804. He thought they were the smallest particles of matter, which could not be broken down into smaller particles. He envisioned them as solid, hard spheres. It wasnt until 1897 that a scientist named Joseph John (J. J.) Thomson discovered that there are smaller particles within the atom. Thomson was born in England and studied at Cambridge University, where he later became a professor. In 1906, he won the Nobel Prize in physics for his research on how gases conduct electricity. This research also led to his discovery of the electron. You can see a picture of Thomson 1.1. "
thomsons atomic model,T_4927,"In his research, Thomson passed current through a cathode ray tube, similar to the one seen in the Figure 1.2. A cathode ray tube is a glass tube from which virtually all of the air has been removed. It contains a piece of metal called an electrode at each end. One electrode is negatively charged and known as a cathode. The other electrode is positively charged and known as an anode. When high-voltage electric current is applied to the end plates, a cathode ray travels from the cathode to the anode. What is a cathode ray? Thats what Thomson wanted to know. Is it just a ray of energy that travels in waves like a ray of light? That was one popular hypothesis at the time. Or was a cathode ray a stream of moving particles? That was the other popular hypothesis. Thomson tested these ideas by placing negative and positive plates along the sides of the cathode ray tube to see how the cathode ray would be affected. The cathode ray appeared to be repelled by the negative plate and attracted by the positive plate. This meant that the ray was negative in charge and that is must consist of particles that have mass. He called the particles corpuscles, but they were later renamed electrons. Thomson also measured the mass of the particles he had identified. He did this by determining how much the cathode rays were bent when he varied the voltage. He found that the mass of the particles was 2000 times smaller than the mass of the smallest atom, the hydrogen atom. In short, Thomson had discovered the existence of particles smaller than atoms. This disproved Daltons claim that atoms are the smallest particles of matter. From his discovery, Thomson also inferred that electrons are fundamental particles within atoms. Q: Atoms are neutral in electric charge. How can they be neutral if they contain negatively charged electrons? A: Atoms also contain positively charged particles that cancel out the negative charge of the electrons. However, these positive particles werent discovered until a couple of decades after Thomson discovered electrons. "
thomsons atomic model,T_4928,"Thomson also knew that atoms are neutral in electric charge, so he asked the same question: How can atoms contain negative particles and still be neutral? He hypothesized that the rest of the atom must be positively charged in order to cancel out the negative charge of the electrons. He envisioned the atom as being similar to a plum pudding, like the one pictured in the Figure 1.3mostly positive in charge (the pudding) with negative electrons (the plums) scattered through it. Q: How is our modern understanding of atomic structure different from Thomsons plum pudding model? A: Today we know that all of the positive charge in an atom is concentrated in a tiny central area called the nucleus, with the electrons swirling through empty space around it, as in the Figure 1.4. The nucleus was discovered just a few years after Thomson discovered the electron, so the plum pudding model was soon rejected. "
transfer of electric charge,T_4929,"The girl pictured above became negatively charged because electrons flowed from the van de Graaff generator to her. Whenever electrons are transferred between objects, neutral matter becomes charged. This occurs even with individual atoms. Atoms are neutral in electric charge because they have the same number of negative electrons as positive protons. However, if atoms lose or gain electrons, they become charged particles called ions. You can see how this happens in the Figure 1.1. When an atom loses electrons, it becomes a positively charged ion, or cation. When an atom gains electrons, it becomes a negative charged ion, or anion. "
transfer of electric charge,T_4930,"Like the formation of ions, the formation of charged matter in general depends on the transfer of electrons, either between two materials or within a material. Three ways this can occur are referred to as conduction, polarization, and friction. All three ways are described below. However, regardless of how electrons are transferred, the total charge always remains the same. Electrons move, but they arent destroyed. This is the law of conservation of charge. "
transfer of electric charge,T_4931,"The transfer of electrons from the van de Graaff generator to the man is an example of conduction. Conduction occurs when there is direct contact between materials that differ in their ability to give up or accept electrons. A van de Graff generator produces a negative charge on its dome, so it tends to give up electrons. Human hands are positively charged, so they tend to accept electrons. Therefore, electrons flow from the dome to the mans hand when they are in contact. You dont need a van de Graaff generator for conduction to take place. It may occur when you walk across a wool carpet in rubber-soled shoes. Wool tends to give up electrons and rubber tends to accept them. Therefore, the carpet transfers electrons to your shoes each time you put down your foot. The transfer of electrons results in you becoming negatively charged and the carpet becoming positively charged. "
transfer of electric charge,T_4932,"Assume that you have walked across a wool carpet in rubber-soled shoes and become negatively charged. If you then reach out to touch a metal doorknob, electrons in the neutral metal will be repelled and move away from your hand before you even touch the knob. In this way, one end of the doorknob becomes positively charged and the other end becomes negatively charged. This is called polarization. Polarization occurs whenever electrons within a neutral object move because of the electric field of a nearby charged object. It occurs without direct contact between the two objects. The Figure 1.2 models how polarization occurs. Q: What happens when the negatively charged plastic rod in the diagram is placed close to the neutral metal plate? A: Electrons in the plate are repelled by the positive charges in the rod. The electrons move away from the rod, causing one side of the plate to become positively charged and the other side to become negatively charged. "
transfer of electric charge,T_4933,"Did you ever rub an inflated balloon against your hair? You can see what happens in the Figure 1.3. Friction between the balloon and hair cause electrons from the hair to rub off on the balloon. Thats because a balloon attracts electrons more strongly than hair does. After the transfer of electrons, the balloon becomes negatively charged and the hair becomes positively charged. The individual hairs push away from each other and stand on end because like charges repel each other. The balloon and the hair attract each other because opposite charges attract. Electrons are transferred in this way whenever there is friction between materials that differ in their ability to give up or accept electrons. Q: If you rub a balloon against a wall, it may stick to the wall. Explain why. "
transition metals,T_4934,"Transition metals are all the elements in groups 3-12 of the periodic table. In the periodic table pictured in Figure known elements. In addition to copper (Cu), well known examples of transition metals include iron (Fe), zinc (Zn), silver (Ag), and gold (Au) (Copper (Cu) is pictured in its various applications in the opening image). Q: Transition metals have been called the most typical of all metals. What do you think this means? A: Unlike some other metals, transition metals have the properties that define the metals class. They are excellent conductors of electricity, for example, and they also have luster, malleability, and ductility. You can read more about these properties of transition metals below. "
transition metals,T_4935,"Transition metals are superior conductors of heat as well as electricity. They are malleable, which means they can be shaped into sheets, and ductile, which means they can be shaped into wires. They have high melting and boiling points, and all are solids at room temperature, except for mercury (Hg), which is a liquid. Transition metals are also high in density and very hard. Most of them are white or silvery in color, and they are generally lustrous, or shiny. The compounds that transition metals form with other elements are often very colorful. You can see several examples in the Figure 1.2. Some properties of transition metals set them apart from other metals. Compared with the alkali metals in group 1 and the alkaline Earth metals in group 2, the transition metals are much less reactive. They dont react quickly with water or oxygen, which explains why they resist corrosion. Q: How is the number of valence electrons typically related to the properties of elements? A: The number of valence electrons usually determines how reactive elements are as well as the ways in which they react with other elements. "
transition metals,T_4936,"Transition metals include the elements that are most often placed below the periodic table (the pink- and purple- shaded elements in the Figure 1.1). Those that follow lanthanum (La) are called lanthanides. They are all relatively reactive for transition metals. Those that follow actinium (Ac) are called actinides. They are all radioactive. This means that they are unstable, so they decay into different, more stable elements. Many of the actinides do not occur in nature but are made in laboratories. "
transverse wave,T_4937,"A transverse wave is a wave in which particles of the medium vibrate at right angles, or perpendicular, to the direction that the wave travels. Another example of a transverse wave is the wave that passes through a rope with you shake one end of the rope up and down, as in the Figure 1.1. The direction of the wave is down the length of the rope away from the hand. The rope itself moves up and down as the wave passes through it. Click image to the left or use the URL below. URL:  Q: When a guitar string is plucked, in what direction does the wave travel? In what directions does the string vibrate? A: The wave travels down the string to the end. The string vibrates up and down at right angles to the direction of the wave. "
transverse wave,T_4938,"A transverse wave is characterized by the high and low points reached by particles of the medium as the wave passes through. The high points are called crests, and the low points are called troughs. You can see both in the Figure below. "
transverse wave,T_4939,Transverse waves called S waves occur during earthquakes. The disturbance that causes an earthquake sends transverse waves through underground rocks in all directions away from the disturbance. S waves may travel for hundreds of miles. An S wave is modeled in the Figure 1.3. 
types of friction,T_4940,"Friction is the force that opposes motion between any surfaces that are in contact. There are four types of friction: static, sliding, rolling, and fluid friction. Static, sliding, and rolling friction occur between solid surfaces. Fluid friction occurs in liquids and gases. All four types of friction are described below. "
types of friction,T_4941,"Static friction acts on objects when they are resting on a surface. For example, if you are hiking in the woods, there is static friction between your shoes and the trail each time you put down your foot (see Figure 1.1). Without this static friction, your feet would slip out from under you, making it difficult to walk. In fact, thats exactly what happens if you try to walk on ice. Thats because ice is very slippery and offers very little friction. Q: Can you think of other examples of static friction? A: One example is the friction that helps the people climb the rock wall in the opening picture above. Static friction keeps their hands and feet from slipping. "
types of friction,T_4942,"Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. Thats why its easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil. The pencil lead slides easily over the paper, but theres just enough friction between the pencil and paper to leave a mark. Q: How does sliding friction help you ride a bike? A: There is sliding friction between the brake pads and bike rims each time you use your bikes brakes. This friction slows the rolling wheels so you can stop. "
types of friction,T_4943,"Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker than sliding friction or static friction. This explains why most forms of ground transportation use wheels, including bicycles, cars, 4-wheelers, roller skates, scooters, and skateboards. Ball bearings are another use of rolling friction. You can see what they look like in the Figure 1.2. They let parts of a wheel or other machine roll rather than slide over on another. The ball bearings in this wheel reduce friction between the inner and outer cylinders when they turn. "
types of friction,T_4944,"Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If youve ever tried to push your open hand through the water in a tub or pool, then youve experienced fluid friction. You can feel the resistance of the water against your hand. Look at the skydiver in the Figure 1.3. Hes falling toward Earth with a parachute. Resistance of the air against the parachute slows his descent. The faster or larger a moving object is, the greater is the fluid friction resisting its motion. Thats why there is greater air resistance against the parachute than the skydivers body. "
ultrasound,T_4945,"Ultrasound is sound that has a wave frequency higher than the human ear can detect. It includes all sound with wave frequencies higher than 20,000 waves per second, or 20,000 hertz (Hz). Although we cant hear ultrasound, it is very useful to humans and some other animals. Uses of ultrasound include echolocation, sonar, and ultrasonography. "
ultrasound,T_4946,"Animals such as bats and dolphins send out ultrasound waves and use their echoes, or reflected waves, to identify the locations of objects they cannot see. This is called echolocation. Animals use echolocation to find prey and avoid running into objects in the dark. You can see in the Figure 1.1 how a bat uses echolocation to find insect prey. "
ultrasound,T_4947,"Sonar uses ultrasound in a way that is similar to echolocation. Sonar stands for sound navigation and ranging. It is used to locate underwater objects such as submarines. Thats how the ship pictured in the Figure 1.2 is using it. A sonar device is both a sender and a receiver. It sends out ultrasound waves and detects the waves after they reflect from underwater objects. The distance to underwater objects can be calculated from the known speed of sound in water and the time it takes for the sound waves to travel to the object. The equation for distance traveled when speed and time are known is: Distance = Speed  Time Consider the ship and submarine pictured in the Figure 1.2. If an ultrasound wave travels from the ship to the submarine and back again in 2 seconds, what is the distance from the ship to the submarine? The sound wave travels from the ship to the submarine in just 1 second, or half the time it takes to make the round trip. The speed of sound waves through ocean water is 1437 m/s. Therefore, the distance from the ship to the submarine is: Q: Now assume that the sonar device on the ship sends an ultrasound wave to the bottom of the water. If the sound wave is reflected back to the device in 4 seconds, how deep is the water? A: The time it takes the wave to reach the bottom is 2 seconds. So the distance from the ship to the bottom of the water is: Distance = 1437 m/s  2 s = 2874 m "
ultrasound,T_4948,"Another use of ultrasound is to see inside the human body. This use of ultrasound is called ultrasonography. Harmless ultrasound waves are sent inside the body, and the reflected waves are used to create an image on a screen. This technology is used to examine internal organs and unborn babies without risk to the patient. You can see a doctor using ultrasound in the Figure 1.3. "
unsaturated hydrocarbons,T_4949,"Hydrocarbons are compounds that contain only carbon and hydrogen. The carbon atoms in hydrocarbons may share single, double, or triple covalent bonds. Unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms. They are classified on the basis of their bonds as alkenes, aromatic hydrocarbons, or alkynes. Q: Why do you suppose hydrocarbons with double or triple bonds are called unsaturated? A: A carbon atom always forms four covalent bonds. Carbon atoms with double or triple bonds are unable to bond with as many hydrogen atoms as they could if they were joined only by single bonds. This makes them unsaturated with hydrogen atoms. "
unsaturated hydrocarbons,T_4950,"Unsaturated hydrocarbons that contain one or more double bonds are called alkenes. The name of a specific alkene always ends in -ene and has a prefix indicating the number of carbon atoms. The structural formula in the Figure Ethene is produced by most fruits and vegetables. It speeds up ripening. The Figure 1.1 show the effects of ethene on bananas. Alkenes can have different shapes. They can form straight chains, branched chains, or rings. Alkenes with the same atoms but different shapes are called isomers. Smaller alkenes have relatively high boiling and melting points, so they are gases at room temperature. Larger alkenes have lower boiling and melting points, so they are liquids or waxy solids at room temperature. The bananas on the left were stored in a special bag that absorbs ethene. The bananas on the right were stored without a bag. "
unsaturated hydrocarbons,T_4951,"Unsaturated hydrocarbons called aromatic hydrocarbons are cyclic hydrocarbons that have double bonds. These compounds have six carbon atoms in a ring with alternating single and double bonds. The smallest aromatic hydrocarbon is benzene, which has just one ring. Its structural formula is shown in the Figure 1.2. Larger aromatic hydrocarbons consist of two or more rings, which are joined together by bonds between their carbon atoms. The name of aromatic hydrocarbons comes from their strong aroma, or scent. Thats why they are used in air fresheners and mothballs. A: Each carbon atom forms four covalent bonds. Carbon atoms always form four covalent bonds, regardless of the atoms to which it bonds. "
unsaturated hydrocarbons,T_4952,"Unsaturated hydrocarbons that contain one or more triple bonds are called alkynes. The names of specific alkynes always end in -yne and have a prefix for the number of carbon atoms. The structural formula in the Figure 1.3 represents the smallest alkyne, named ethyne, which has two carbon atoms and two hydrogen atoms (C2 H2 ). Ethyne is also called acetylene. It is burned in acetylene torches, like the one pictured in the Figure 1.4. The flame of an acetylene torch is so hot that it can melt metal. Cutting metal with an acetylene (ethyne) torch. Alkynes may form straight or branched chains. They rarely occur in ring shapes. In fact, alkynes of all shapes are relatively rare in nature. "
using earths magnetic field,T_4953,"Like a bar magnet, planet Earth has north and south magnetic poles and a magnetic field over which it exerts magnetic force. Earths magnetic field is called the magnetosphere. You can see it in the Figure 1.1. "
using earths magnetic field,T_4954,"The sun gives off radiation in solar winds. You can see solar winds in the Figure 1.1. Notice what happens to solar winds when they reach the magnetosphere. They are deflected almost completely by Earths magnetic field. Radiation in solar wind would wash over Earth and kill most living things were it not for the magnetosphere. It protects Earths organisms from radiation like an umbrella protects you from rain. Q: Now can you explain the northern lights? A: Energetic particles in solar wind collide with atoms in the atmosphere over the poles, and energy is released in the form of light. The swirling patterns of light follow lines of magnetic force in the magnetosphere. "
using earths magnetic field,T_4955,"Another benefit of Earths magnetic field is its use for navigation. People use compasses to detect Earths magnetic north pole and tell direction. Many animals have natural compasses that work just as well. For example, the loggerhead turtle in the Figure 1.2 senses the direction and strength of Earths magnetic field and uses it to navigate along migration routes. Many migratory bird species can also sense the magnetic field and use it for navigation. Recent research suggests that they may have structures in their eyes that let them see Earths magnetic field as a visual pattern. "
valence electrons,T_4956,"Valence electrons are the electrons in the outer energy level of an atom that can participate in interactions with other atoms. Valence electrons are generally the electrons that are farthest from the nucleus. As a result, they may be attracted as much or more by the nucleus of another atom than they are by their own nucleus. "
valence electrons,T_4957,"Because valence electrons are so important, atoms are often represented by simple diagrams that show only their valence electrons. These are called electron dot diagrams, and three are shown below. In this type of diagram, an elements chemical symbol is surrounded by dots that represent the valence electrons. Typically, the dots are drawn as if there is a square surrounding the element symbol with up to two dots per side. An element never has more than eight valence electrons, so there cant be more than eight dots per atom. Q: Carbon (C) has four valence electrons. What does an electron dot diagram for this element look like? A: An electron dot diagram for carbon looks like this: "
valence electrons,T_4958,"The number of valence electrons in an atom is reflected by its position in the periodic table of the elements (see the periodic table in the Figure 1.1). Across each row, or period, of the periodic table, the number of valence electrons in groups 1-2 and 13-18 increases by one from one element to the next. Within each column, or group, of the table, all the elements have the same number of valence electrons. This explains why all the elements in the same group have very similar chemical properties. For elements in groups 1-2 and 13-18, the number of valence electrons is easy to tell directly from the periodic table. This is illustrated in the simplified periodic table in the Figure 1.2. It shows just the numbers of valence electrons in each of these groups. For elements in groups 3-12, determining the number of valence electrons is more complicated. Q: Based on both periodic tables above (Figures 1.1 and 1.2), what are examples of elements that have just one valence electron? What are examples of elements that have eight valence electrons? How many valence electrons does oxygen (O) have? A: Any element in group 1 has just one valence electron. Examples include hydrogen (H), lithium (Li), and sodium (Na). Any element in group 18 has eight valence electrons (except for helium, which has a total of just two electrons). Examples include neon (Ne), argon (Ar), and krypton (Kr). Oxygen, like all the other elements in group 16, has six valence electrons. "
valence electrons,T_4959,"The table salt pictured in the Figure 1.3 contains two elements that are so reactive they are rarely found alone in nature. Instead, they undergo chemical reactions with other elements and form compounds. Table salt is the compound named sodium chloride (NaCl). It forms when an atom of sodium (Na) gives up an electron and an atom of chlorine (Cl) accepts it. When this happens, sodium becomes a positively charged ion (Na+ ), and chlorine becomes a negatively charged ion (Cl ). The two ions are attracted to each and join a matrix of interlocking sodium and chloride ions, forming a crystal of salt. Q: Why does sodium give up an electron? A: An atom of a group 1 element such as sodium has just one valence electron. It is eager to give up this electron in order to have a full outer energy level, because this will give it the most stable arrangement of electrons. You can see how this happens in the animation at the following URL and in the Figure 1.4. Group 2 elements with two valence electrons are almost as reactive as elements in group 1 for the same reason. Q: Why does chlorine accept the electron from sodium? A: An atom of a group 17 element such as chlorine has seven valence electrons. It is eager to gain an extra electron to fill its outer energy level and gain stability. Group 16 elements with six valence electrons are almost as reactive for the same reason. Atoms of group 18 elements have eight valence electrons (or two in the case of helium). These elements already have a full outer energy level, so they are very stable. As a result, they rarely if ever react with other elements. Elements in other groups vary in their reactivity but are generally less reactive than elements in groups 1, 2, 16, or 17. Q: Find calcium (Ca) in the periodic table (see Figure 1.1). Based on its position in the table, how reactive do you think calcium is? Name another element with which calcium might react. A: Calcium is a group 2 element with two valence electrons. Therefore, it is very reactive and gives up electrons in chemical reactions. It is likely to react with an element with six valence electrons that wants to gain two electrons. This would be an element in group 6, such as oxygen. Table salt (sodium chloride). "
valence electrons,T_4960,"Valence electrons also determine how wellif at allthe atoms of an element conduct electricity. The copper wires in the cable in the Figure 1.5 are coated with plastic. Copper is an excellent conductor of electricity, so it is used for wires that carry electric current. Plastic contains mainly carbon, which cannot conduct electricity, so it is used as insulation on the wires. Q: Why do copper and carbon differ in their ability to conduct electricity? A: Atoms of metals such as copper easily give up valence electrons. Their electrons can move freely and carry electric current. Atoms of nonmetals such as the carbon, on the other hand, hold onto their electrons. Their electrons cant move freely and carry current. A few elements, called metalloids, can conduct electricity, but not as well as metals. Examples include silicon and germanium in group 14. Both become better conductors at higher temperatures. These elements are called semiconductors. Q: How many valence electrons do atoms of silicon and germanium have? What happens to their valence electrons when the atoms are exposed to an electric field? A: Atoms of these two elements have four valence electrons. When the atoms are exposed to an electric field, the valence electrons move away from the atoms and allow current to flow. "
velocity,T_4961,"Speed tells you only how fast or slow an object is moving. It doesnt tell you the direction the object is moving. The measure of both speed and direction is called velocity. Velocity is a vector. A vector is measurement that includes both size and direction. Vectors are often represented by arrows. When using an arrow to represent velocity, the length of the arrow stands for speed, and the way the arrow points indicates the direction. Click image to the left or use the URL below. URL: "
velocity,T_4962,The arrows in the Figure 1.1 represent the velocity of three different objects. Arrows A and B are the same length but point in different directions. They represent objects moving at the same speed but in different directions. Arrow C is shorter than arrow A or B but points in the same direction as arrow A. It represents an object moving at a slower speed than A or B but in the same direction as A. 
velocity,T_4963,"Objects have the same velocity only if they are moving at the same speed and in the same direction. Objects moving at different speeds, in different directions, or both have different velocities. Look again at arrows A and B from the Figure 1.1. They represent objects that have different velocities only because they are moving in different directions. A and C represent objects that have different velocities only because they are moving at different speeds. Objects represented by B and C have different velocities because they are moving in different directions and at different speeds. Q: Jerod is riding his bike at a constant speed. As he rides down his street he is moving from east to west. At the end of the block, he turns right and starts moving from south to north, but hes still traveling at the same speed. Has his velocity changed? A: Although Jerods speed hasnt changed, his velocity has changed because he is moving in a different direction. Q: How could you use vector arrows to represent Jerods velocity and how it changes? A: The arrows might look like this: "
velocity,T_4964,"You can calculate the average velocity of a moving object that is not changing direction by dividing the distance the object travels by the time it takes to travel that distance. You would use this formula: velocity = distance time This is the same formula that is used for calculating average speed. It represents velocity only if the answer also includes the direction that the object is traveling. Lets work through a sample problem. Tonis dog is racing down the sidewalk toward the east. The dog travels 36 meters in 18 seconds before it stops running. The velocity of the dog is: distance time 36 m = 18 s = 2 m/s east velocity = Note that the answer is given in the SI unit for velocity, which is m/s, and it includes the direction that the dog is traveling. Q: What would the dogs velocity be if it ran the same distance in the opposite direction but covered the distance in 24 seconds? A: In this case, the velocity would be: distance time 36 m = 24 s = 1.5 m/s west velocity = "
velocity time graphs,T_4965,"The changing velocity of the sprinteror of any other moving person or objectcan be represented by a velocity- time graph like the one in the Figure 1.1 for the sprinter. A velocity-time graph shows how velocity changes over time. The sprinters velocity increases for the first 4 seconds of the race, it remains constant for the next 3 seconds, and it decreases during the last 3 seconds after she crosses the finish line. "
velocity time graphs,T_4966,"In a velocity-time graph, acceleration is represented by the slope, or steepness, of the graph line. If the line slopes upward, like the line between 0 and 4 seconds in the Figure 1.1, velocity is increasing, so acceleration is positive. If the line is horizontal, as it is between 4 and 7 seconds, velocity is constant and acceleration is zero. If the line slopes downward, like the line between 7 and 10 seconds, velocity is decreasing and acceleration is negative. Negative acceleration is called deceleration. Q: Assume that another sprinter is running the same race. The other runner reaches a top velocity of 9 m/s by 4 seconds after the start of the race. How would the first 4 seconds of the velocity-time graph for this runner be different from the Figure 1.1? A: The graph line for this runner during seconds 0-4 would be steeper (have a greater slope). This would show that acceleration is greater during this time period for the other sprinter. "
visible light and matter,T_4967,"Reflection of light occurs when light bounces back from a surface that it cannot pass through. Reflection may be regular or diffuse. If the surface is very smooth, like a mirror, the reflected light forms a very clear image. This is called regular, or specular, reflection. In the Figure 1.1, the smooth surface of the still water in the pond on the left reflects light in this way. When light is reflected from a rough surface, the waves of light are reflected in many different directions, so a clear image does not form. This is called diffuse reflection. In the Figure 1.1, the ripples in the water in the picture on the right cause diffuse reflection of the blooming trees. "
visible light and matter,T_4968,"Transmission of light occurs when light passes through matter. As light is transmitted, it may pass straight through matter or it may be refracted or scattered as it passes through. When light is refracted, it changes direction as it passes into a new medium and changes speed. The straw in the Figure 1.2 looks bent where light travels from water to air. Light travels more quickly in air than in water and changes direction. Scattering occurs when light bumps into tiny particles of matter and spreads out in all directions. In the Figure air, giving the headlights a halo appearance. Q: What might be another example of light scattering? A: When light passes through smoky air, it is scattered by tiny particles of soot. "
visible light and matter,T_4969,"Light may transfer its energy to matter rather than being reflected or transmitted by matter. This is called absorption. When light is absorbed, the added energy increases the temperature of matter. If you get into a car that has been sitting in the sun all day, the seats and other parts of the cars interior may be almost too hot to touch, especially if they are black or very dark in color. Thats because dark colors absorb most of the sunlight that strikes them. Q: In hot sunny climates, people often dress in light-colored clothes. Why is this a good idea? A: Light-colored clothes absorb less light and reflect more light than dark-colored clothes, so they keep people cooler. "
visible light and matter,T_4970,"Matter can be classified on the basis of its interactions with light. Matter may be transparent, translucent, or opaque. An example of each type of matter is pictured in the Figure 1.4. Transparent matter is matter that transmits light without scattering it. Examples of transparent matter include air, pure water, and clear glass. You can see clearly through transparent objects, such as the top panes of the window 1.4, because just about all of the light that strikes them passes through to the other side. Translucent matter is matter that transmits light but scatters the light as it passes through. Light passes through translucent objects but you cannot see clearly through them because the light is scattered in all directions. The frosted glass panes at the bottom of the window 1.4 are translucent. Opaque matter is matter that does not let any light pass through it. Matter may be opaque because it absorbs light, reflects light, or does some combination of both. Examples of opaque objects are objects made of wood, like the shutters in the Figure 1.5. The shutters absorb most of the light that strikes them and reflect just a few wavelengths of visible light. The glass mirror 1.5 is also opaque. Thats because it reflects all of the light that strikes it. "
vision and the eye,T_4971,"The human eye is an organ that is specialized to collect light and focus images. The structures of the human eye are shown in the Figure 1.1. Examine each structure in the diagram as you read about it below. The sclera, also known as the white of the eye, is an opaque outer covering that protects the eye. It keeps light out of the eye except at the center front of the eye. The cornea is a transparent outer covering of the front of the eye. It protects the eye and also acts as a convex lens. A convex lens is thicker in the middle than at the edges and makes rays of light converge, or meet at a point. The shape of the cornea helps focus light that enters the eye. The pupil is an opening in the front of the eye. It looks black because it doesnt reflect any light. All the light passes through it instead. The pupil controls the amount of light that enters the eye. It automatically gets bigger or smaller to let more or less light in as needed. The iris is the colored part of the eye. It controls the size of the pupil. The lens of the eye is a convex lens. It fine-tunes the focus so an image forms on the retina at the back of the eye. Tiny muscles control the shape of the lens to focus images of close or distant objects. The retina is a membrane lining the back of the eye. The retina has nerve cells called rods and cones that change images to electrical signals. Rods are good at sensing dim light but cant distinguish different colors of light. Cones can sense colors but not dim light. There are three different types of cones. Each type senses one of the three primary colors of light (red, green, or blue). The optic nerve carries electrical signals from the rods and cones to the brain. Q: The lens of the eye is a convex lens. How would vision be affected if the lens of the eye was concave instead of convex? A: A concave lens causes rays of light to diverge, or spread apart. It forms a virtual image on the same side of the lens at the object being viewed. Therefore, a concave lens would focus the image in front of the eye, not on the retina inside the eye. No signals would be sent to the brain so vision would not be possible. "
vision and the eye,T_4972,"The ability to see is called vision. This ability depends on more than healthy eyes. It also depends on certain parts of the brain, because the brain and eyes work together to allow us to see. The eyes collect and focus visible light. The lens and other structures of the eye work together to focus an image on the retina. The image is upside-down and reduced in size, as you can see in the Figure 1.2. Cells in the retina change the image to electrical signals that travel to the brain through the optic nerve. The brain interprets the electrical signals as shape, color, and brightness. It also interprets the image as though it were right-side up. The brain does this automatically, so what we see always appears right-side up. The brain also interprets what we are seeing. Q: The part of the brain that processes information from the eyes is the visual cortex. It is located at the back of the brain. How might an injury to the visual cortex affect vision? A: An injury to the visual cortex might cause abnormal vision or even blindness regardless of how well the eyes can gather and focus light. "
vision problems and corrective lenses,T_4973,"Many people have problems with their vision, or ability to see. Often, the problem is due to the shape of the eyes and how they focus light. Two of the most common vision problems are nearsightedness and farsightedness, which you can read about below. You may even have one of these vision problems yourself. Usually, the problems can be corrected with contact lenses or lenses in eyeglasses. In many people, they can also be corrected with laser surgery, which reshapes the outer layer of the eye. Click image to the left or use the URL below. URL: "
vision problems and corrective lenses,T_4974,"Nearsightedness, or myopia, is the condition in which nearby objects are seen clearly, but distant objects appear blurry. The Figure 1.1 shows how it occurs. The eyeball is longer (from front to back) than normal. This causes images to be focused in front of the retina instead of on the retina. Myopia can be corrected with concave lenses. The lenses focus images farther back in the eye, so they fall on the retina instead of in front of it. Q: Sometimes squinting the eyes can help someone see more clearly. Why do you think this works? A: Squinting may improve focus by slightly changing the shape of the eyes. When you squint, you tighten muscles around the eyes, putting pressure on the eyeballs. "
vision problems and corrective lenses,T_4975,"Farsightedness, or hyperopia, is the condition in which distant objects are seen clearly, but nearby objects appear blurry. It occurs when the eyeball is shorter than normal (see Figure 1.2). This causes images to be focused in a spot that would fall behind the retina (if light could pass through the retina). Hyperopia can be corrected with convex lenses. The lenses focus images farther forward in the eye, so they fall on the retina instead of behind it. Q: Joey has hyperopia. When is he more likely to need his glasses: when he reads a book or when he watches TV? A: With hyperopia, Joey is farsighted. He can probably see the TV more clearly than the words in a book because the TV is farther away. Therefore, he is more likely to need his glasses when he reads than when he watches TV. "
wave amplitude,T_4976,"Waves that travel through mattersuch as the fabric of a flagare called mechanical waves. The matter they travel through is called the medium. When the energy of a wave passes through the medium, particles of the medium move. The more energy the wave has, the farther the particles of the medium move. The distance the particles move is measured by the waves amplitude. "
wave amplitude,T_4977,"Wave amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through. The resting position of a particle of the medium is where the particle would be in the absence of a wave. The Figure 1.1 show the amplitudes of two different types of waves: transverse and longitudinal waves. In a transverse wave, particles of the medium move up and down at right angles to the direction of the wave. Wave amplitude of a transverse wave is the difference in height between the crest and the resting position. The crest is the highest point particles of the medium reach. The higher the crests are, the greater the amplitude of the wave. In a longitudinal wave, particles of the medium move back and forth in the same direction as the wave. Wave amplitude of a longitudinal wave is the distance between particles of the medium where it is compressed by the wave. The closer together the particles are, the greater the amplitude of the wave. Q: What do you think determines a waves amplitude? A: Wave amplitude is determined by the energy of the disturbance that causes the wave. "
wave amplitude,T_4978,A wave caused by a disturbance with more energy has greater amplitude. Imagine dropping a small pebble into a pond of still water. Tiny ripples will move out from the disturbance in concentric circles. The ripples are low- amplitude waves with very little energy. Now imagine throwing a big boulder into the pond. Very large waves will be generated by the disturbance. These waves are high-amplitude waves and have a great deal of energy. 
wave frequency,T_4979,"The number of waves that pass a fixed point in a given amount of time is wave frequency. Wave frequency can be measured by counting the number of crests (high points) of waves that pass the fixed point in 1 second or some other time period. The higher the number is, the greater the frequency of the waves. The SI unit for wave frequency is the hertz (Hz), where 1 hertz equals 1 wave passing a fixed point in 1 second. The Figure 1.1 shows high-frequency and low-frequency transverse waves. Q: The wavelength of a wave is the distance between corresponding points on adjacent waves. For example, it is the distance between two adjacent crests in the transverse waves in the diagram. Infer how wave frequency is related to wavelength. "
wave frequency,T_4980,"The frequency of a wave is the same as the frequency of the vibrations that caused the wave. For example, to generate a higher-frequency wave in a rope, you must move the rope up and down more quickly. This takes more energy, so a higher-frequency wave has more energy than a lower-frequency wave with the same amplitude. You can see examples of different frequencies in the Figure 1.2 (Amplitude is the distance that particles of the medium move when the energy of a wave passes through them.) "
wave interactions,T_4981,"Atoms are the building blocks of matter. Unlike blocks that we know, these building blocks are incredibly small. In fact, they are the smallest particles of an element. Atoms still have the same properties as the elements they make up. For example, an atom of gold has the same melting point as a gold coin. If we could see it, it would have the same color. Elements are also pure substances. This means they are not mixed with anything else. Pure substances such as nickel, hydrogen, and helium make up all kinds of matter. All the atoms of a given element are identical. Atoms of different elements are not physically the same. Think of something you might have made from LEGOs. You built some shape using the many different sized and shaped blocks. This is much like how atoms combine to become everything we know. If we took only one size and shape of block and put them together, we would make a pure substance. It would be an element. If you take apart anything that you have built, those individual parts are like the atoms. With those small parts, you build bigger things. Sometimes they are all the same type of block. Other times, they may be different kinds of blocks. We use these combinations of different blocks to make more complicated things. "
wave interactions,T_4982,"Unlike LEGO bricks, atoms are extremely small. The radius of an atom is well under 1 nanometer. Thats one- billionth of a meter. Such a number is hard to imagine. Consider this: trillions of atoms would fit inside the period at the end of this sentence. In other words, atoms are way too small to be seen with the naked eye. "
wave interactions,T_4983,"Although atoms are very tiny, they consist of even smaller particles. Atoms are made of protons, neutrons, and electrons: Protons have a positive charge. Electrons have a negative charge. Neutrons are neutral in charge. "
wave interactions,T_4984,"Figure below represents a simple model of an atom. Models help scientists make sense of things. Perhaps they are either too big or too small. Maybe they are just too complicated to make sense of. This simple model helps scientists think about the atom. Is this how the atom really looks? Not exactly! Remember, a model helps us make sense of things. They may not be an exact copy of the object. You will learn about more complex models of atoms in the coming years, but this model is a good place to start. "
wave interactions,T_4985,"At the center of an atom is the nucleus (plural, nuclei). The nucleus contains most of the atoms mass. However, in size, its just a tiny part of the atom. The model in Figure above is not to scale. If an atom were the size of a football stadium, the nucleus would be only about the size of a pea. The nucleus, in turn, consists of two types of particles, called protons and neutrons. These particles are tightly packed inside the nucleus. Constantly moving about the nucleus are other particles called electrons. "
wave interactions,T_4986,"A proton is a particle inside the nucleus of an atom. It has a positive electric charge. All protons are identical. It is all about the number of protons in the atoms. The number of protons is what gives the atoms of different elements their unique properties. Atoms of each type of element have a characteristic number of protons. For example, each atom of carbon has six protons (see Figure below ). No two elements have atoms with the same number of protons. "
wave interactions,T_4987,"A neutron is a particle inside the nucleus of an atom. It has no electric charge. Atoms of an element often have the same number of neutrons as protons. For example, most carbon atoms have six neutrons as well as six protons. This is also shown in Figure below . "
wave interactions,T_4988,"An electron is a particle outside the nucleus of an atom. It has a negative electric charge. The charge of an electron is opposite but equal to the charge of a proton. Atoms have the same number of electrons as protons. As a result, the negative and positive charges ""cancel out."" This makes atoms electrically neutral. For example, a carbon atom has six electrons that ""cancel out"" its six protons. "
wave interactions,T_4989,"By clicking a link below, you will leave the CK-12 site and open an external site in a new tab. This page will remain open in the original tab. "
wave interference,T_4990,"When two or more waves meet, they interact with each other. The interaction of waves with other waves is called wave interference. Wave interference may occur when two waves that are traveling in opposite directions meet. The two waves pass through each other, and this affects their amplitude. Amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through. How amplitude is affected by wave interference depends on the type of interference. Interference can be constructive or destructive. "
wave interference,T_4991,"Constructive interference occurs when the crests, or highest points, of one wave overlap the crests of the other wave. You can see this in the Figure 1.1. As the waves pass through each other, the crests combine to produce a wave with greater amplitude. "
wave interference,T_4992,"Destructive interference occurs when the crests of one wave overlap the troughs, or lowest points, of another wave. The Figure 1.2 shows what happens. As the waves pass through each other, the crests and troughs cancel each other out to produce a wave with zero amplitude. "
wave interference,T_4993,"Waves may reflect off an obstacle that they are unable to pass through. When waves are reflected straight back from an obstacle, the reflected waves interfere with the original waves and create standing waves. These are waves that appear to be standing still. Standing waves occur because of a combination of constructive and destructive interference. Q: How could you use a rope to produce standing waves? A: You could tie one end of the rope to a fixed object, such as doorknob, and move the other end up and down to generate waves in the rope. When the waves reach the fixed object, they are reflected back. The original waves and the reflected waves interfere to produce a standing wave. Try it yourself and see if the waves appear to stand still. "
wave particle theory,T_4994,"Electromagnetic radiation, commonly called light, is the transfer of energy by waves called electromagnetic waves. These waves consist of vibrating electric and magnetic fields. Where does electromagnetic energy come from? It is released when electrons return to lower energy levels in atoms. Electromagnetic radiation behaves like continuous waves of energy most of the time. Sometimes, however, electromagnetic radiation seems to behave like discrete, or separate, particles rather than waves. So does electromagnetic radiation consist of waves or particles? "
wave particle theory,T_4995,"This question about the nature of electromagnetic radiation was debated by scientists for more than two centuries, starting in the 1600s. Some scientists argued that electromagnetic radiation consists of particles that shoot around like tiny bullets. Other scientists argued that electromagnetic radiation consists of waves, like sound waves or water waves. Until the early 1900s, most scientists thought that electromagnetic radiation is either one or the other and that scientists on the other side of the argument were simply wrong. Q: Do you think electromagnetic radiation is a wave or a particle? A: Heres a hint: it may not be a question of either-or. Keep reading to learn more. "
wave particle theory,T_4996,"In 1905, the physicist Albert Einstein developed a new theory about electromagnetic radiation. The theory is often called the wave-particle theory. It explains how electromagnetic radiation can behave as both a wave and a particle. Einstein argued that when an electron returns to a lower energy level and gives off electromagnetic energy, the energy is released as a discrete packet of energy. We now call such a packet of energy a photon. According to Einstein, a photon resembles a particle but moves like a wave. You can see this in the Figure 1.1. The theory posits that waves of photons traveling through space or matter make up electromagnetic radiation. "
wave particle theory,T_4997,"A photon isnt a fixed amount of energy. Instead, the amount of energy in a photon depends on the frequency of the electromagnetic wave. The frequency of a wave is the number of waves that pass a fixed point in a given amount of time, such as the number of waves per second. In waves with higher frequencies, photons have more energy. "
wave particle theory,T_4998,"After Einstein proposed his theory, evidence was discovered to support it. For example, scientists shone laser light through two slits in a barrier made of a material that blocked light. You can see the setup of this type of experiment in the Figure 1.2. Using a special camera that was very sensitive to light, they took photos of the light that passed through the slits. The photos revealed tiny pinpoints of light passing through the double slits. This seemed to show that light consists of particles. However, if the camera was exposed to the light for a long time, the pinpoints accumulated in bands that resembled interfering waves. Therefore, the experiment showed that light seems to consist of particles that act like waves. "
wave speed,T_4999,"Wave speed is the distance a wave travels in a given amount of time, such as the number of meters it travels per second. Wave speed (and speed in general) can be represented by the equation: Speed = Distance Time "
wave speed,T_5000,"Wave speed is related to both wavelength and wave frequency. Wavelength is the distance between two correspond- ing points on adjacent waves. Wave frequency is the number of waves that pass a fixed point in a given amount of time. This equation shows how the three factors are related: Speed = Wavelength x Wave Frequency In this equation, wavelength is measured in meters and frequency is measured in hertz (Hz), or number of waves per second. Therefore, wave speed is given in meters per second, which is the SI unit for speed. Q: If you increase the wavelength of a wave, does the speed of the wave increase as well? A: Increasing the wavelength of a wave doesnt change its speed. Thats because when wavelength increases, wave frequency decreases. As a result, the product of wavelength and wave frequency is still the same speed. Click image to the left or use the URL below. URL: "
wave speed,T_5001,"The equation for wave speed can be used to calculate the speed of a wave when both wavelength and wave frequency are known. Consider an ocean wave with a wavelength of 3 meters and a frequency of 1 hertz. The speed of the wave is: Speed = 3 m x 1 wave/s = 3 m/s Q: Kim made a wave in a spring by pushing and pulling on one end. The wavelength is 0.1 m, and the wave frequency is 2 hertz. What is the speed of the wave? A: Substitute these values into the equation for speed: Speed = 0.1 m x 2 waves/s = 0.2 m/s "
wave speed,T_5002,"The equation for wave speed (above) can be rewritten as: Frequency = Speed Wavelength or Wavelength = Speed Frequency Therefore, if you know the speed of a wave and either the wavelength or wave frequency, you can calculate the missing value. For example, suppose that a wave is traveling at a speed of 2 meters per second and has a wavelength of 1 meter. Then the frequency of the wave is: Frequency = 2m/s 1m = 2 waves/s, or 2 Hz Q: A wave is traveling at a speed of 2 m/s and has a frequency of 2 Hz. What is its wavelength? A: Substitute these values into the equation for wavelength: Wavelength = 2m/s 2waves/s =1m "
wave speed,T_5003,"The speed of most waves depends on the medium, or the matter through which the waves are traveling. Generally, waves travel fastest through solids and slowest through gases. Thats because particles are closest together in solids and farthest apart in gases. When particles are farther apart, it takes longer for the energy of the disturbance to pass from particle to particle through the medium. Click image to the left or use the URL below. URL: "
wavelength,T_5004,"Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, and it is usually measured in meters. How it is measured is a little different for transverse and longitudinal waves. In a transverse wave, particles of the medium vibrate up and down at right angles to the direction that the wave travels. The wavelength of a transverse wave can be measured as the distance between two adjacent crests, or high points, as shown in the Figure 1.1. In a longitudinal wave, particles of matter vibrate back and forth in the same direction that the wave travels. The wavelength of a longitudinal wave can be measured as the distance between two adjacent compressions, as shown in the Figure 1.2. Compressions are the places where particles of the medium crowd close together as the energy of the wave passes through. "
wavelength,T_5005,The wavelength of a wave is related to the waves energy. Short-wavelength waves have more energy than long- wavelength waves of the same amplitude. (Amplitude is a measure of how far particles of the medium move up and down or back and forth when a wave passes through them.) You can see examples of transverse waves with shorter and longer wavelengths in the Figure 1.3. A: Violet light has the greatest energy because it has the shortest wavelength. 
wedge,T_5006,"A wedge is simple machine that consists of two inclined planes, giving it a thin end and thick end, as you can see in the Figure 1.1. A wedge is used to cut or split apart objects. Force is applied to the thick end of the wedge, and the wedge, in turn, applies force to the object along both of its sloping sides. This force causes the object to split apart. A knife is another example of a wedge. In the Figure 1.2, a knife is being used to chop tough pecans. The job is easy to do with the knife because of the wedge shape of the blade. The very thin edge of the blade easily enters and cuts through the pecans. "
wedge,T_5007,"The mechanical advantage of a simple machine is the factor by which it multiplies the force applied to the machine. It is the ratio of the output force to the input force. A wedge applies more force to the object (output force) than the user applies to the wedge (input force), so the mechanical advantage of a wedge is greater than 1. A longer, thinner wedge has a greater mechanical advantage than a shorter, wider wedge. With all wedges, the trade-off is that the output force is applied over a shorter distance, so force may need to be applied to the wedge repeatedly to push it through the object. Q: Which wedge in the Figure 1.3 do you think would do the same amount of work with less input force? A: The wedge on the left has a greater mechanical advantage, so it would do the same amount of work with less input force. "
wheel and axle,T_5008,"A wheel and axle is a simple machine that consists of two connected rings or cylinders, one inside the other. Both rings or cylinders turn in the same direction around a single center point. The inner ring or cylinder is called the axle, and the outer one is called the wheel. Besides the Ferris wheel, the doorknob in the Figure 1.1 is another example of a wheel and axle. In a wheel and axle, force may be applied either to the wheel or to the axle. This force is called the input force. A wheel and axle does not change the direction of the input force. However, the force put out by the machine, called the output force, is either greater than the input force or else applied over a greater distance. A: In a Ferris wheel, the force is applied to the axle by the Ferris wheels motor. In a doorknob, the force is applied to the wheel by a persons hand. "
wheel and axle,T_5009,"The mechanical advantage of a machine is the factor by which the machine changes the input force. It equals the ratio of the output force to the input force. A wheel and axle may either increase or decrease the input force, depending on whether the input force is applied to the axle or the wheel. When the input force is applied to the axle, as it is with a Ferris wheel, the wheel turns with less force. Because the output force is less than the input force, the mechanical advantage is less than 1. However, the wheel turns over a greater distance, so it turns faster than the axle. The speed of the wheel is one reason that the Ferris wheel ride is so exciting. When the input force is applied to the wheel, as it is with a doorknob, the axle turns over a shorter distance but with greater force, so the mechanical advantage is greater than 1. This allows you to turn the doorknob with relatively little effort, while the axle of the doorknob applies enough force to slide the bar into or out of the doorframe. "
why earth is a magnet,T_5010,"Like the real Earth, the globe pictured above is a magnet. A magnet is an object that has north and south magnetic poles and a magnetic field. The magnetic globe is a modern device, but the idea that Earth is a magnet is far from new. It was first proposed in 1600 by a British physician named William Gilbert. He used a spherical magnet to represent Earth. With a compass, he demonstrated that it the spherical magnet causes a compass needle to behave the same way that Earth causes a compass needle to behave. This showed that a spherical magnet is a good model for Earth and therefore that Earth is a magnet. Q: Can you describe Earths magnetic poles and magnetic field? A: Earth has north and south magnetic poles. The North Pole is located at about 80 degrees north latitude. The magnetic field is an area around Earth that is affected by its magnetic field. The field is strongest at the poles, and lines of magnetic force move from the north to the south magnetic pole. "
why earth is a magnet,T_5011,"Although the idea that Earth is a magnet is centuries old, the discovery of why Earth is a magnet is a relatively new. In the early 1900s, scientists started using seismographic data to learn about Earths inner structure. A seismograph detects and measure earthquake waves. Evidence from earthquakes showed that Earth has a solid inner core and a liquid outer core (see the Figure 1.1). The outer core consists of molten metals, mainly iron and nickel. Scientists think that Earths magnetic field is generated by the movement of charged particles through these molten metals in the outer core. The particles move as Earth spins on its axis. "
work,T_5014,"Work is defined differently in physics than in everyday language. In physics, work means the use of force to move an object. The teens who are playing basketball in the picture above are using force to move their bodies and the basketball, so they are doing work. The teen who is studying isnt moving anything, so she isnt doing work. Not all force that is used to move an object does work. For work to be done, the force must be applied in the same direction that the object moves. If a force is applied in a different direction than the object moves, no work is done. The Figure 1.1 illustrates this point. Q: If the box the man is carrying is very heavy, does he do any work as he walks across the room with it? A: Regardless of the weight of the box, the man does no work on it as he holds it while walking across the room. However, he does more work when he first lifts a heavier box to chest height. "
work,T_5015,"Work is directly related to both the force applied to an object and the distance the object moves. It can be represented by the equation: Work = Force  Distance This equation shows that the greater the force that is used to move an object or the farther the object is moved, the more work that is done. To see the effects of force and distance on work, compare the weight lifters in the Figure 1.2. The two weight lifters on the left are lifting the same amount of weight, but the one on the bottom is lifting the weight a greater distance. Therefore, this weight lifter is doing more work. The two weight lifters on the bottom right are both lifting the weight the same distance, but the weight lifter on the left is lifting a heavier weight, so she is doing more work. "
erosion and deposition by gravity,T_0058,The most destructive types of mass movement are landslides and mudslides. Both occur suddenly. 
erosion and deposition by gravity,T_0059,"A landslide happens when a large amount of soil and rock suddenly falls down a slope because of gravity. You can see an example in Figure 10.30. A landslide can be very destructive. It may bury or carry away entire villages. A landslide is more likely if the soil has become wet from heavy rains. The wet soil becomes slippery and heavy. Earthquakes often trigger landslides. The shaking ground causes soil and rocks to break loose and start sliding. If a landslide flows into a body of water, it may cause a huge wave called a tsunami. "
erosion and deposition by gravity,T_0060,"A mudslide is the sudden flow of mud down a slope because of gravity. Mudslides occur where the soil is mostly clay. Like landslides, mudslides usually occur when the soil is wet. Wet clay forms very slippery mud that slides easily. You can see an example of a mudslide in Figure 10.31. "
erosion and deposition by gravity,T_0061,"Two other types of mass movement are slump and creep. Both may move a lot of soil and rock. However, they usually arent as destructive as landslides and mudslides. "
erosion and deposition by gravity,T_0062,"Slump is the sudden movement of large blocks of rock and soil down a slope. You can see how it happens in Figure 10.32. All the material moves together in big chunks. Slump may be caused by a layer of slippery, wet clay underneath the rock and soil on a hillside. Or it may occur when a river undercuts a slope. Slump leaves behind crescent-shaped scars on the hillside. "
erosion and deposition by gravity,T_0063,"Creep is the very slow movement of rock and soil down a hillside. Creep occurs so slowly you cant see it happening. You can only see the effects of creep after years of movement. This is illustrated in Figure 10.33. The slowly moving ground causes trees, fence posts, and other structures on the surface to tilt downhill. Creep usually takes place where the ground freezes and thaws frequently. Soil and rock particles are lifted up when the ground freezes. When the ground thaws, the particles settle down again. Each time they settle down, they move a tiny bit farther down the slope because of gravity. "
the atmosphere,T_0194,"We are lucky to have an atmosphere on Earth. The atmosphere supports life, and is also needed for the water cycle and weather. The gases of the atmosphere even allow us to hear. "
the atmosphere,T_0195,"Most of the atmosphere is nitrogen, but it doesnt do much. Carbon dioxide and oxygen are the gases in the atmosphere that are needed for life. Plants need carbon dioxide for photosynthesis. They use sunlight to change carbon dioxide and water into food. The process releases oxygen. Without photosynthesis, there would be very little oxygen in the air. Other living things depend on plants for food. These organisms need the oxygen plants release to get energy out of the food. Even plants need oxygen for this purpose. "
the atmosphere,T_0196,The atmosphere protects living things from the Suns most harmful rays. Gases reflect or absorb the strongest rays of sunlight. Figure 15.1 models this role of the atmosphere. 
the atmosphere,T_0197,"Gases in the atmosphere surround Earth like a blanket. They keep the temperature in a range that can support life. The gases keep out some of the Suns scorching heat during the day. At night, they hold the heat close to the surface, so it doesnt radiate out into space. "
the atmosphere,T_0198,"Figure 15.2 shows the role of the atmosphere in the water cycle. Water vapor rises from Earths surface into the atmosphere. As it rises, it cools. The water vapor may then condense into water droplets and form clouds. If enough water droplets collect in clouds they may fall as rain. This how freshwater gets from the atmosphere back to Earths surface. "
the atmosphere,T_0199,"Without the atmosphere, there would be no clouds or rain. In fact, there would be no weather at all. Most weather occurs because the atmosphere heats up more in some places than others. "
the atmosphere,T_0200,"Weather makes life interesting. Weather also causes weathering. Weathering is the slow wearing down of rocks on Earths surface. Wind-blown sand scours rocks like sandpaper. Glaciers of ice scrape across rock surfaces like a file. Even gentle rain may seep into rocks and slowly dissolve them. If the water freezes, it expands. This eventually causes the rocks to crack. Without the atmosphere, none of this weathering would happen. "
the atmosphere,T_0201,"Sound is a form of energy that travels in waves. Sound waves cant travel through empty space, but they can travel through gases. Gases in the air allow us to hear most of the sounds in our world. Because of air, you can hear birds singing, horns tooting, and friends laughing. Without the atmosphere, the world would be a silent, eerie place. "
the atmosphere,T_0202,"Air is easy to forget about. We usually cant see it, taste it, or smell it. We can only feel it when it moves. But air is actually made of molecules of many different gases. It also contains tiny particles of solid matter. "
the atmosphere,T_0203,Figure 15.3 shows the main gases in air. Nitrogen and oxygen make up 99 percent of air. Argon and carbon dioxide make up much of the rest. These percentages are the same just about everywhere in the atmosphere. Air also includes water vapor. The amount of water vapor varies from place to place. Thats why water vapor isnt included in Figure 15.3. It can make up as much as 4 percent of the air. Ozone is a molecule made of three oxygen atoms. Ozone collects in a layer in the stratosphere. 
the atmosphere,T_0204,"Air includes many tiny particles. The particles may consist of dust, soil, salt, smoke, or ash. Some particles pollute the air and may make it unhealthy to breathe. But having particles in the air is very important. Tiny particles are needed for water vapor to condense on. Without particles, water vapor could not condense. Then clouds could not form and Earth would have no rain. "
the atmosphere,T_0205,"We usually cant sense the air around us unless it is moving. But air has the same basic properties as other matter. For example, air has mass, volume and, of course, density. "
the atmosphere,T_0206,"Density is mass per unit volume. Density is a measure of how closely molecules are packed together. The closer together they are, the greater the density. Since air is a gas, the molecules can pack tightly or spread out. The density of air varies from place to place. Air density depends on several factors. One is temperature. Like other materials, warm air is less dense than cool air. Since warmer molecules have more energy, they are more active. The molecules bounce off each other and spread apart. Another factor that affects the density of air is altitude. "
the atmosphere,T_0207,"Altitude is height above sea level. The density of air decreases with height. There are two reasons. At higher altitudes, there is less air pushing down from above. Also, gravity is weaker farther from Earths center. So at higher altitudes, air molecules can spread out more. Air density decreases. You can see this in Figure 15.4. "
the atmosphere,T_0208,"Because air is a gas, its molecules have a lot of energy. Air molecules move a lot and bump into things. For this reason, they exert pressure. Air pressure is defined as the weight of the air pressing against a given area. At sea level, the atmosphere presses down with a force of about 1 kilogram per square centimeter (14.76 pounds per square inch). If you are standing at sea level, you have more than a ton of air pressing against you. Why doesnt the pressure crush you? Air presses in all directions at once. Other molecules of air are pushing back. "
the atmosphere,T_0209,"Like density, the pressure of the air decreases with altitude. There is less air pressing down from above the higher up you go. Look at the bottle in Figure 15.5. It was drained by a hiker at the top of a mountain. Then the hiker screwed the cap on the bottle and carried it down to sea level. At the lower altitude, air pressure crushed it. Can you explain why? "
weather and water in the atmosphere,T_0248,"What do temperature, clouds, winds, and rain have in common? They are all part of weather. Weather refers to the conditions of the atmosphere at a given time and place. "
weather and water in the atmosphere,T_0249,"Weather occurs because of unequal heating of the atmosphere. The source of heat is the Sun. The general principles behind weather can be stated simply: The Sun heats Earths surface more in some places than others. Where it is warm, heat from the Sun warms the air close to the surface. If there is water at the surface, it may cause some of the water to evaporate. Warm air is less dense, so it rises. When this happens, more dense air flows in to take its place. The flowing surface air is wind. The rising air cools as it goes higher in the atmosphere. If it is moist, the water vapor may condense. Clouds may form, and precipitation may fall. "
weather and water in the atmosphere,T_0250,"The water cycle plays an important role in weather. When liquid water evaporates, it causes humidity. When water vapor condenses, it forms clouds and precipitation. Humidity, clouds, and precipitation are all important weather factors. "
weather and water in the atmosphere,T_0251,Humidity is the amount of water vapor in the air. High humidity increases the chances of clouds and precipitation. 
weather and water in the atmosphere,T_0252,Humidity usually refers to relative humidity. This is the percent of water vapor in the air relative to the total amount the air can hold. How much water vapor can the air hold? That depends on temperature. Warm air can hold more water vapor than cool air. You can see this in Figure 16.1. 
weather and water in the atmosphere,T_0253,"People often say, its not the heat but the humidity. Humidity can make a hot day feel even hotter. When sweat evaporates, it cools your body. But sweat cant evaporate when the air already contains as much water vapor as it can hold. The heat index is a measure of what the temperature feels like because of the humidity. You can see the heat index in Figure 16.2. "
weather and water in the atmosphere,T_0254,"Youve probably noticed dew on the grass on a summer morning. Why does dew form? Remember that the land heats up and cools down fairly readily. So when night comes, the land cools. Air that was warm and humid in the daytime also cools over night. As the air cools, it can hold less water vapor. Some of the water vapor condenses on the cool surfaces, such as blades of grass. The temperature at which water vapor condenses is called the dew point. If this temperature is below freezing, ice crystals of frost form instead of dew. As you can see in Figure 16.1, the dew point occurs at 100 percent relative humidity. Can you explain why? "
weather and water in the atmosphere,T_0255,Clouds form when air in the atmosphere reaches the dew point. Clouds may form anywhere in the troposphere. Clouds that form on the ground are called fog. 
weather and water in the atmosphere,T_0256,"Clouds form when water vapor condenses around particles in the air. The particles are specks of matter, such as dust or smoke. Billions of these tiny water droplets come together to make up a cloud. If the air is very cold, ice crystals form instead of liquid water. "
weather and water in the atmosphere,T_0257,"Clouds are classified on the basis of where and how they form. Three main types of clouds are cirrus, stratus, and cumulus. Figure 16.3 shows these and other types of clouds. Cirrus clouds form high in the troposphere. Because it is so cold they are made of ice crystals. They are thin and wispy. Cirrus clouds dont usually produce precipitation, but they may be a sign that wet weather is coming. Stratus clouds occur low in the troposphere. They form in layers that spread horizontally and may cover the entire sky like a thick blanket. Stratus clouds that produce precipitation are called nimbostratus. The prefix nimbo- means rain. Cumulus clouds are white and puffy. Convection currents make them grow upward and they may grow very tall. When they produce rain, they are called cumulonimbus. "
weather and water in the atmosphere,T_0258,"Clouds can affect the temperature on Earths surface. During the day, thick clouds block some of the Suns rays. This keeps the surface from heating up as much as it would on a clear day. At night, thick clouds prevent heat from radiating out into space. This keeps the surface warmer than it would be on a clear night. "
weather and water in the atmosphere,T_0259,"Clouds are needed for precipitation. This may fall as liquid water, or it may fall as frozen water, such as snow. "
weather and water in the atmosphere,T_0260,"Millions of water molecules in a cloud must condense to make a single raindrop or snowflake. The drop or flake falls when it becomes too heavy for updrafts to keep it aloft. As a drop or flake falls, it may collect more water and get larger. "
weather and water in the atmosphere,T_0261,"Why does it snow instead of rain? Air temperature determines which type of precipitation falls. Rain falls if the air temperature is above freezing (0 C or 32 F). Frozen precipitation falls if the air or ground is below freezing. Frozen precipitation may fall as snow, sleet, or freezing rain. You can see how the different types form in Figure Snow falls when water vapor condenses as ice crystals. The air temperature is below freezing all the way to the ground, so the ice crystals remain frozen. They fall as flakes. Sleet forms when snow melts as it falls through a layer of warm air and then refreezes. It turns into small, clear ice pellets as it passes through a cold layer near the ground. Freezing rain falls as liquid water. It freezes on contact with cold surfaces near the ground. It may cover everything with a glaze of ice. If the ice is thick, its weight may break tree branches and pull down power lines. Hail is another "
loss of soil,T_0354,"Runoff carved channels in the soil in Figure 19.1. Running water causes most soil erosion, but wind can carry soil away too. What humans do to soil makes it more or less likely to be eroded by wind or water. Human actions that can increase soil erosion are described below. "
loss of soil,T_0355,"The photos in Figure 19.2 show how farming practices can increase soil erosion. Plant roots penetrate the soil and keep it from eroding. Plowing turns over bare soil and cuts through plant roots. Bare soil is exposed to wind and water. In the past, farmers always plowed fields before planting. Some farmers now use no-till farming, which does not disturb the soil as much. The problem doesnt stop with plowing. Crops are usually planted in rows, with bare soil in between the rows. In places where crops grow only during part of the year, the land may be bare for a few months. "
loss of soil,T_0356,"As you can see in Figure 19.3, some grazing animals, especially sheep and goats, eat grass right down to the roots. They may even pull the grass entirely out of the ground. Grazing animals can kill the grass or thin it out so much that it offers little protection to the soil. If animals are kept in the same place too long, the soil may become completely bare. The bare soil is easily eroded by wind and water. "
loss of soil,T_0357,"Other human actions that put soil at risk include logging, mining, and construction. You can see examples of each in Figure 19.4. When forests are cut down, the soil is suddenly exposed to wind and rain. Without trees, there is no leaf litter to cover the ground and protect the soil. When leaf litter decays, it adds humus and nutrients to the soil. Mining and construction strip soil off the ground and leave the land bare. Paved roads and parking lots prevent rainwater from soaking into the ground. This increases runoff and the potential for soil erosion. "
loss of soil,T_0358,"Even things that people do for fun can expose soil to erosion. For example, overuse of hiking trails can leave bare patches of soil. Off-road vehicles cause even more damage. You can see examples of this in Figure 19.5. "
loss of soil,T_0359,"Soil is a renewable resource, but it can take thousands of years to form. Thats why people need to do what they can to prevent soil erosion. "
loss of soil,T_0360,The Dust Bowl taught people that soil could be lost by plowing and growing crops. This led to the development of new ways of farming that help protect the soil. Some of the methods are described in Figure 19.6. 
loss of soil,T_0361,"There are several other ways to help prevent soil loss. Some of them are shown in Figure 19.7. Prevent overgrazing. Frequently move animals from field to field. This gives the grass a chance to recover. Avoid logging steep hillsides. Cut only a few trees in any given place. Plant new trees to replace those that are cut down. Reclaim mine lands. Save the stripped topsoil and return it to the land. Once the soil is in place, plant trees and other plants to protect the bare soil. Use barriers to prevent runoff and soil erosion at construction sites. Plant grass to hold the soil in place. Develop paving materials that absorb water and reduce runoff. Restrict the use of off-road vehicles, especially in hilly areas. "
century tsunami,T_0449,"Not everyone had the same warning the people on Tillys beach had. The Boxing Day Tsunami of December 26, 2004 was by far the deadliest of all time (Figure 1.1). The tsunami was caused by the 2004 Indian Ocean Earthquake. With a magnitude of 9.2, it was the second largest earthquake ever recorded. The extreme movement of the crust displaced trillions of tons of water along the entire length of the rupture. Several tsunami waves were created with about 30 minutes between the peaks of each one. The waves that struck nearby Sumatra 15 minutes after the quake reached more than 10 meters (33 feet) in height. The size of the waves decreased with distance from the earthquake and were about 4 meters (13 feet) high in Somalia. The tsunami did so much damage because it traveled throughout the Indian Ocean. About 230,000 people died in eight countries. There were fatalities even as far away as South Africa, nearly 8,000 kilometers (5,000 miles) from the earthquake epicenter. More than 1.2 million people lost their homes and many more lost their ways of making a living. The countries that were most affected by the 2004 Boxing Day tsunami. "
century tsunami,T_0450,"The Japanese received a one-two punch in March 2011. The 2011 Tohoku earthquake offshore was a magnitude 9.0 and damage from the quake was extensive. People didnt have time to recover before massive tsunami waves hit the island nation. As seen in Figure 1.2, waves in some regions topped 9 meters (27 feet). The tsunami did much more damage than the massive earthquake (Figure 1.3). Worst was the damage done to nuclear power plants along the northeastern coast. Eleven reactors were automatically shut down. Power and backup power were lost at the Fukushima plant, leading to equipment failures, meltdowns, and the release of radioactive materials. Control and cleanup of the disabled plants will go on for many years. "
century tsunami,T_0451,"As a result of the 2004 tsunami, an Indian Ocean warning system was put into operation in June 2006. Prior to 2004, no one had thought a large tsunami was possible in the Indian Ocean. In comparison, a warning system has been in effect around the Pacific Ocean for more than 50 years. The system was used to warn of possible tsunami waves after the Tohoku earthquake, but most were too close to the quake to get to high ground in time. Further away, people were evacuated along many Pacific coastlines, but the waves were not that large. "
the universe,T_0633,"Hubble measured the distances to galaxies. He also studied the motions of galaxies. In doing these things, Hubble noticed a relationship. This is now called Hubbles Law: The farther away a galaxy is, the faster it is moving away from us. There was only one conclusion he could draw from this. The universe is expanding! Figure 26.15 shows a simple diagram of the expanding universe. Imagine a balloon covered with tiny dots. When you blow up the balloon, the rubber stretches. The dots slowly move away from each other as the space between them increases. In an expanding universe, the space between galaxies is expanding. We see this as the other galaxies moving away from us. We also see that galaxies farther away from us move away faster than nearby galaxies. "
the universe,T_0634,"About 13.7 billion years ago, the entire universe was packed together. Everything was squeezed into a tiny volume. Then there was an enormous explosion. After this big bang, the universe expanded rapidly (Figure 26.16). All of the matter and energy in the universe has been expanding ever since. Scientists have evidence this is how the universe formed. One piece of evidence is that we see galaxies moving away from us. If they are moving apart, they must once have been together. Also, there is energy left over from this explosion throughout the universe. The theory for the origin of the universe is called the Big Bang Theory. "
the universe,T_0635,"In the first few moments after the Big Bang, the universe was extremely hot and dense. As the universe expanded, it became less dense. It began to cool. First protons, neutrons, and electrons formed. From these particles came hydrogen. Nuclear fusion created helium atoms. Some parts of the universe had matter that was densely packed. Enormous clumps of matter were held together by gravity. Eventually this material became the gas clouds, stars, galaxies, and other structures that we see in the universe today. "
the universe,T_0636,"We see many objects out in space that emit light. This matter is contained in stars, and the stars are contained in galaxies. Scientists think that stars and galaxies make up only a small part of the matter in the universe. The rest of the matter is called dark matter. Dark matter doesnt emit light, so we cant see it. We know it is there because it affects the motion of objects around it. For example, astronomers measure how spiral galaxies rotate. The outside edges of a galaxy rotate at the same speed as parts closer to the center. This can only be explained if there is a lot more matter in the galaxy than we can see. What is dark matter? Actually, we dont really know. Dark matter could just be ordinary matter, like what makes up Earth. The universe could contain lots of objects that dont have enough mass to glow on their own. There might just be a lot of black holes. Another possibility is that the universe contains a lot of matter that is different from anything we know. If it doesnt interact much with ordinary matter, it would be very difficult or impossible to detect directly. Most scientists who study dark matter think it is a combination. Ordinary matter is part of it. That is mixed with some kind of matter that we havent discovered yet. Most scientists think that ordinary matter is less than half of the total matter in the universe. "
the universe,T_0637,"We know that the universe is expanding. Astronomers have wondered if it is expanding fast enough to escape the pull of gravity. Would the universe just expand forever? If it could not escape the pull of gravity, would it someday start to contract? This means it would eventually get squeezed together in a big crunch. This is the opposite of the Big Bang. Scientists may now have an answer. Recently, astronomers have discovered that the universe is expanding even faster than before. What is causing the expansion to accelerate? One hypothesis is that there is energy out in the universe that we cant see. Astronomers call this dark energy. We know even less about dark energy than we know about dark matter. Some scientists think that dark energy makes up more than half of the universe. "
minerals,T_0638,"To understand minerals, we must first understand matter. Matter is the substance that physical objects are made of. "
minerals,T_0639,"The basic unit of matter is an atom. At the center of an atom is its nucleus. Protons are positively charged particles in the nucleus. Also in the nucleus are neutrons with no electrical charge. Orbiting the nucleus are tiny electrons. Electrons are negatively charged. An atom with the same number of protons and electrons is electrically neutral. If the atom has more or less electrons to protons it is called an ion. An ion will have positive charge if it has more protons than electrons. It will have negative charge if it has more electrons than protons. An atom is the smallest unit of a chemical element. That is, an atom has all the properties of that element. All atoms of the same element have the same number of protons. "
minerals,T_0640,A molecule is the smallest unit of a chemical compound. A compound is a substance made of two or more elements. The elements in a chemical compound are always present in a certain ratio. Water is probably one of the simplest compounds that you know. A water molecule is made of two hydrogen atoms and one oxygen atom (Figure 3.2). All water molecules have the same ratio: two hydrogen atoms to one oxygen atom. 
minerals,T_0641,"A mineral is a solid material that forms by a natural process. A mineral can be made of an element or a compound. It has a specific chemical composition that is different from other minerals. One minerals physical properties differ from others. These properties include crystal structure, hardness, density and color. Each is made of different elements. Each has different physical properties. For example, silver is a soft, shiny metal. Salt is a white, cube- shaped crystal. Diamond is an extremely hard, translucent crystal. "
minerals,T_0642,"Minerals are made by natural processes. The processes that make minerals happen in or on the Earth. For example, when hot lava cools, mineral crystals form. Minerals also precipitate from water. Some minerals grow when rocks are exposed to high pressures and temperatures. Could something like a mineral be made by a process that was not natural? People make gemstones in a laboratory. Synthetic diamond is a common one. But that stone is not a mineral. It was not formed by a natural process. "
minerals,T_0643,"A mineral is an inorganic substance. It was not made by living organisms. Organic substances contain carbon. Some organic substances are proteins, carbohydrates, and oils. Everything else is inorganic. In a few cases, living organisms make inorganic materials. The calcium carbonate shells made by marine animals are inorganic. "
minerals,T_0644,"All minerals have a definite chemical makeup. A few minerals are made of only one kind of element. Silver is a mineral made only of silver atoms. Diamond and graphite are both made only of the element carbon. Minerals that are not pure elements are made of chemical compounds. For example, the mineral quartz is made of the compound silicon dioxide, or SiO2 . This compound has one atom of the element silicon for every two atoms of the element oxygen. Each mineral has its own unique chemical formula. For example, the mineral hematite has two iron atoms for every three oxygen atoms. The mineral magnetite has three iron atoms for every four oxygen atoms. Many minerals have very complex chemical formulas that include several elements. However, even in more complicated compounds, the elements occur in definite ratios. "
minerals,T_0645,"Minerals must be solid. For example, ice and water have the same chemical composition. Ice is a solid, so it is a mineral. Water is a liquid, so it is not a mineral. Some solids are not crystals. Glass, or the rock obsidian, are solid but not crystals. In a crystal, the atoms are arranged in a pattern. This pattern is regular and it repeats. Figure 3.3 shows how the atoms are arranged in halite (table salt). Halite contains atoms of sodium and chlorine in a pattern. Notice that the pattern goes in all three dimensions. The pattern of atoms in all halite is the same. Think about all of the grains of salt that are in a salt shaker. The atoms are arranged in the same way in every piece of salt. Sometimes two different minerals have the same chemical composition. But they are different minerals because they have different crystal structures. Diamonds are beautiful gemstones because they are very pretty and very hard. Graphite is the lead in pencils. Its not hard at all! Amazingly, both are made just of carbon. Compare the diamond with the pencil lead in Figure 3.4. Why are they so different? The carbon atoms in graphite bond to form layers. The bonds between each layer are weak. The carbon sheets can just slip past each other. The carbon atoms in diamonds bond together in all three directions. This strong network makes diamonds very hard. "
minerals,T_0646,"The patterns of atoms that make a mineral affect its physical properties. A minerals crystal shape is determined by the way the atoms are arranged. For example, you can see how atoms are arranged in halite in Figure 3.3. You can see how salt crystals look under a microscope in Figure 3.5. Salt crystals are all cubes whether theyre small or large. Other physical properties help scientists identify different minerals. They include: Color: the color of the mineral. Streak: the color of the minerals powder. Luster: the way light reflects off the minerals surface. Specific gravity: how heavy the mineral is relative to the same volume of water. Cleavage: the minerals tendency to break along flat surfaces. Fracture: the pattern in which a mineral breaks. Hardness: what minerals it can scratch and what minerals can scratch it. "
minerals,T_0647,"Imagine you are in charge of organizing more than 100 minerals for a museum exhibit. People can learn a lot more if they see the minerals together in groups. How would you group the minerals together in your exhibit? Mineralogists are scientists who study minerals. They divide minerals into groups based on chemical composition. Even though there are over 4,000 minerals, most minerals fit into one of eight mineral groups. Minerals with similar crystal structures are grouped together. "
minerals,T_0648,"About 1,000 silicate minerals are known. This makes silicates the largest mineral group. Silicate minerals make up over 90 percent of Earths crust! Silicates contain silicon atoms and oxygen atoms. One silicon atom is bonded to four oxygen atoms. These atoms form a pyramid (Figure 3.6). The silicate pyramid is the building block of silicate minerals. Most silicates contain other elements. These elements include calcium, iron, and magnesium. Silicate minerals are divided into six smaller groups. In each group, the silicate pyramids join together differently. The pyramids can stand alone. They can form into connected circles called rings. Some pyramids link into single and double chains. Others form large, flat sheets. Some join in three dimensions. Feldspar and quartz are the two most common silicates. In beryl, the silicate pyramids join together as rings. Biotite is mica. It can be broken apart into thin, flexible sheets. Compare the beryl and the biotite shown in Figure 3.7. "
minerals,T_0649,"Native elements contain only atoms of one type of element. They are not combined with other elements. There are very few examples of these types of minerals. Some native elements are rare and valuable. Gold, silver, sulfur, and diamond are examples. "
minerals,T_0650,"What do you guess carbonate minerals contain? If you guessed carbon, you would be right! All carbonates contain one carbon atom bonded to three oxygen atoms. Carbonates may include other elements. A few are calcium, iron, and copper. Carbonate minerals are often found where seas once covered the land. Some carbonate minerals are very common. Calcite contains calcium, carbon, and oxygen. Have you ever been in a limestone cave or seen a marble tile? Calcite is in both limestone and marble. Azurite and malachite are also carbonate minerals, but they contain copper instead of calcium. They are not as common as calcite. They are used in jewelry. You can see in Figure 3.8 that they are very colorful. "
minerals,T_0651,"Halide minerals are salts. They form when salt water evaporates. This mineral class includes more than just table salt. Halide minerals may contain the elements fluorine, chlorine, bromine, or iodine. Some will combine with metal elements. Common table salt is a halide mineral that contains the elements chlorine and sodium. Fluorite is a type of halide that contains fluorine and calcium. Fluorite can be found in many colors. If you shine an ultraviolet light on fluorite, it will glow! "
minerals,T_0652,"Earths crust contains a lot of oxygen. The oxygen combines with many other elements to create oxide minerals. Oxides contain one or two metal elements combined with oxygen. Oxides are different from silicates because they do not contain silicon. Many important metals are found as oxides. For example, hematite and magnetite are both oxides that contain iron. Hematite (Fe2 O3 ) has a ratio of two iron atoms to three oxygen atoms. Magnetite (Fe3 O4 ) has a ratio of three iron atoms to four oxygen atoms. Notice that the word magnetite contains the word magnet. Magnetite is a magnetic mineral. "
minerals,T_0653,"Phosphate minerals have a structure similar to silicates. In silicates, an atom of silicon is bonded to oxygen. In phosphates, an atom of phosphorus, arsenic, or vanadium is bonded to oxygen. There are many types of phosphate mineral, but still phosphate minerals are rare. The composition of phosphates is complex. For example, turquoise contains copper, aluminum, and phosphorus. The stone is rare and is used to make jewelry. "
minerals,T_0654,"Sulfate minerals contain sulfur atoms bonded to oxygen atoms. Like halides, they can form in places where salt water evaporates. Many minerals belong in the sulfate group, but there are only a few common sulfate minerals. Gypsum is a common sulfate mineral that contains calcium, sulfate, and water. Gypsum is found in various forms. For example, it can be pink and look like it has flower petals. However, it can also grow into very large white crystals. Gypsum crystals that are 11 meters long have been found. That is about as long as a school bus! Gypsum also forms at the Mammoth Hot Springs in Yellowstone National Park, shown in Figure 3.9. "
minerals,T_0655,Sulfides contain metal elements combined with sulfur. Sulfides are different from sulfates. They do not contain oxygen. Pyrite is a common sulfide mineral. It contains iron combined with sulfur. Pyrite is also known as fools gold. Gold miners have mistaken pyrite for gold because pyrite has a greenish gold color. 
identification of minerals,T_0656,"Imagine you were given a mineral sample similar to the one shown in Figure 3.10. How would you try to identify your mineral? You can observe some properties by looking at the mineral. For example, you can see that its color is beige. The mineral has a rose-like structure. But you cant see all mineral properties. You need to do simple tests to determine some properties. One common one is how hard the mineral is. You can use a minerals properties to identify it. The minerals physical properties are determined by its chemical composition and crystal structure. "
identification of minerals,T_0657,"Diamonds have many valuable properties. Diamonds are extremely hard and are used for industrial purposes. The most valuable diamonds are large, well-shaped and sparkly. Turquoise is another mineral that is used in jewelry because of its striking greenish-blue color. Many minerals have interesting appearances. Specific terms are used to describe the appearance of minerals. "
identification of minerals,T_0658,"Color is probably the easiest property to observe. Unfortunately, you can rarely identify a mineral only by its color. Sometimes, different minerals are the same color. For example, you might find a mineral that is a gold color, and so think it is gold. But it might actually be pyrite, or fools gold, which is made of iron and sulfide. It contains no gold atoms. A certain mineral may form in different colors. Figure 3.11 shows four samples of quartz, including one that is colorless and one that is purple. The purple color comes from a tiny amount of iron. The iron in quartz is a chemical impurity. Iron is not normally found in quartz. Many minerals are colored by chemical impurities. Other factors can also affect a minerals color. Weathering changes the surface of a mineral. Because color alone is unreliable, geologists rarely identify a mineral just on its color. To identify most minerals, they use several properties. "
identification of minerals,T_0659,"Streak is the color of the powder of a mineral. To do a streak test, you scrape the mineral across an unglazed porcelain plate. The plate is harder than many minerals, causing the minerals to leave a streak of powder on the plate. The color of the streak often differs from the color of the larger mineral sample, as Figure 3.12 shows. Streak is more reliable than color to identify minerals. The color of a mineral may vary. Streak does not vary. Also, different minerals may be the same color, but they may have a different color streak. For example, samples of hematite and galena can both be dark gray. They can be told apart because hematite has a red streak and galena has a gray streak. "
identification of minerals,T_0660,"Luster describes the way light reflects off of the surface of the mineral. You might describe diamonds as sparkly or pyrite as shiny. But mineralogists have special terms to describe luster. They first divide minerals into metallic and non-metallic luster. Minerals that are opaque and shiny, like pyrite, are said to have a metallic luster. Minerals with a non-metallic luster do not look like metals. There are many types of non-metallic luster. Six are described in Table 3.1. Non-Metallic Luster Adamantine Earthy Pearly Resinous Silky Vitreous Appearance Sparkly Dull, clay-like Pearl-like Like resins, such as tree sap Soft-looking with long fibers Glassy Can you match the minerals in Figure 3.13 with the correct luster from Table 3.1 without looking at the caption? "
identification of minerals,T_0661,"You are going to visit a friend. You fill one backpack with books so you can study later. You stuff your pillow into another backpack that is the same size. Which backpack will be easier to carry? Even though the backpacks are the same size, the bag that contains your books is going to be much heavier. It has a greater density than the backpack with your pillow. Density describes how much matter is in a certain amount of space. Substances that have more matter packed into a given space have higher densities. The water in a drinking glass has the same density as the water in a bathtub or swimming pool. All substances have characteristic densities, which does not depend on how much of a substance you have. Mass is a measure of the amount of matter in an object. The amount of space an object takes up is described by its volume. The density of an object depends on its mass and its volume. Density can be calculated using the following equation: Density = Mass/Volume Samples that are the same size, but have different densities, will have different masses. Gold has a density of about 19 g/cm3 . Pyrite has a density of only about 5 g/cm3 . Quartz is even less dense than pyrite, and has a density of 2.7 g/cm3 . If you picked up a piece of pyrite and a piece of quartz that were the same size, the pyrite would seem almost twice as heavy as the quartz. "
identification of minerals,T_0662,"Hardness is a minerals ability to resist being scratched. Minerals that are not easily scratched are hard. You test the hardness of a mineral by scratching its surface with a mineral of a known hardness. Mineralogists use the Mohs Hardness Scale, shown in Table 3.2, as a reference for mineral hardness. The scale lists common minerals in order of their relative hardness. You can use the minerals in the scale to test the hardness of an unknown mineral. "
identification of minerals,T_0663,"As you can see, diamond is a 10 on the Mohs Hardness Scale. Diamond is the hardest mineral; no other mineral can scratch a diamond. Quartz is a 7. It can be scratched by topaz, corundum, and diamond. Quartz will scratch minerals that have a lower number on the scale. Fluorite is one. Suppose you had a piece of pure gold. You find that calcite scratches the gold. Gypsum does not. Gypsum has a hardness of 2 and calcite is a 3. That means the hardness of gold is between gypsum and calcite. So the hardness of gold is about 2.5 on the scale. A hardness of 2.5 means that gold is a relatively soft mineral. It is only about as hard as your fingernail. Hardness 1 Mineral Talc "
identification of minerals,T_0664,"Different types of minerals break apart in their own way. Remember that all minerals are crystals. This means that the atoms in a mineral are arranged in a repeating pattern. This pattern determines how a mineral will break. When you break a mineral, you break chemical bonds. Because of the way the atoms are arranged, some bonds are weaker than other bonds. A mineral is more likely to break where the bonds between the atoms are weaker. "
identification of minerals,T_0665,"Cleavage is the tendency of a mineral to break along certain planes. When a mineral breaks along a plane it makes a smooth surface. Minerals with different crystal structures will break or cleave in different ways, as in Figure 3.14. Halite tends to form cubes with smooth surfaces. Mica tends to form sheets. Fluorite can form octahedrons. Minerals can form various shapes. Polygons are shown in Figure 3.15. The shapes form as the minerals are broken along their cleavage planes. Cleavage planes determine how the crystals can be cut to make smooth surfaces. People who cut gemstones follow cleavage planes. Diamonds and emeralds can be cut to make beautiful gemstones. "
identification of minerals,T_0666,"Fracture describes how a mineral breaks without any pattern. A fracture is uneven. The surface is not smooth and flat. You can learn about a mineral from the way it fractures. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, fracture to form smooth, curved surfaces. A mineral that broke forming a smooth, curved surface is shown in Figure 3.16. "
identification of minerals,T_0667,"Minerals have other properties that can be used for identification. For example, a minerals shape may indicate its crystal structure. Sometimes crystals are too small to see. Then a mineralogist may use a special instrument to find the crystal structure. Some minerals have unique properties. These can be used to the minerals. Some of these properties are listed in Table 3.3. An example of a mineral that has each property is also listed. Property Fluorescence Magnetism Radioactivity Reactivity Smell Description Mineral glows under ultraviolet light Mineral is attracted to a magnet Mineral gives off radiation that can be measured with Geiger counter Bubbles form when mineral is ex- posed to a weak acid Some minerals have a distinctive smell Example of Mineral Fluorite Magnetite Uraninite Calcite Sulfur (smells like rotten eggs) "
formation of minerals,T_0668,"You are on vacation at the beach. You take your flip-flops off so you can go swimming. The sand is so hot it hurts your feet. You have to run to the water. Now imagine if it were hot enough for the sand to melt. Some places inside Earth are so hot that rock melts. Melted rock inside the Earth is called magma. Magma can be hotter than 1,000C. When magma erupts onto Earths surface, it is known as lava, as Figure 3.17 shows. Minerals form when magma and lava cool. "
formation of minerals,T_0669,"Most water on Earth, like the water in the oceans, contains elements. The elements are mixed evenly through the water. Water plus other substances makes a solution. The particles are so small that they will not come out when you filter the water. But the elements in water can form solid mineral deposits. "
formation of minerals,T_0670,"Fresh water contains a small amount of dissolved elements. Salt water contains a lot more dissolved elements. Water can only hold a certain amount of dissolved substances. When the water evaporates, it leaves behind a solid layer of minerals, as Figure 3.18 shows. At this time, the particles come together to form minerals. These solids sink to the bottom. The amount of mineral formed is the same as the amount dissolved in the water. Seawater is salty enough for minerals to precipitate as solids. Some lakes, such as Mono Lake in California, or Utahs Great Salt Lake, can also precipitate salts. Salt easily precipitates out of water, as does calcite, as Figure 3.19 shows. The limestone towers in the figure are made mostly of the mineral calcite. The calcite was deposited in the salty and alkaline water of Mono Lake, in California. Calcium-rich spring water enters the bottom of the lake. The water bubbles up into the alkaline lake. The "
formation of minerals,T_0671,"Underground water can be heated by magma. The hot water moves through cracks below Earths surface. Hot water can hold more dissolved particles than cold water. The hot, salty solution has chemical reactions with the rocks around it. The water picks up more dissolved particles. As it flows through open spaces in rocks, the water deposits solid minerals. When a mineral fills cracks in rocks, the deposits are called veins. Figure 3.20 shows a white quartz vein. When the minerals are deposited in open spaces, large crystals grow. These rocks are called geodes. Figure 3.20 shows a geode that was formed when amethyst crystals grew in an open space in a rock. "
mining and using minerals,T_0672,A mineral deposit that contains enough minerals to be mined for profit is called an ore. Ores are rocks that contain concentrations of valuable minerals. The bauxite shown in the Figure 3.21 is a rock that contains minerals that are used to make aluminum. 
mining and using minerals,T_0673,"Ores have high concentrations of valuable minerals. Certain places on Earth are more likely to have certain ores. Geologists search for the places that might have ore deposits. Some of the valuable deposits may be hidden underground. To find an ore deposit, geologists will go to a likely spot. They then test the physical and chemical properties of soil and rocks. Ore deposits contain valuable minerals. They may also contain other chemical elements that indicate an ore deposit is nearby. After a mineral deposit is found, geologists determine how big it is. They outline the deposit and the surrounding geology on a map. The miners calculate the amount of valuable minerals they think they will get from the deposit. The minerals will only be mined if it is profitable. If it is profitable, they must then decide on the way it should be mined. The two main methods of mining are surface mining and underground mining. Placers are a type of surface deposit. "
mining and using minerals,T_0674,"Surface mining is used to obtain mineral ores that are near the surface. Blasting breaks up the soil and rocks that contain the ore. Enormous trucks haul the broken rocks to locations where the ores can be removed. Surface mining includes open-pit mining, quarrying, and strip mining. As the name suggests, open-pit mining creates a big pit from which the ore is mined. Figure 3.22 shows an open-pit diamond mine in Russia. The size of the pit grows as long as the miners can make a profit. Strip mines are similar to open-pit mines, but the ore is removed in large strips. A quarry is a type of open-pit mine that produces rocks and minerals that are used to make buildings and roads. "
mining and using minerals,T_0675,"Placer minerals collect in stream gravels. They can be found in modern rivers or ancient riverbeds. California was nicknamed the Golden State. This can be traced back to the discovery of placer gold in 1848. The amount of placer gold brought in miners from around the world. The gold formed in rocks in the Sierra Nevada Mountains. The rocks also contained other valuable minerals. The gold weathered out of the hard rock. It washed downstream and then settled in gravel deposits along the river. Currently, California has active gold and silver mines. California also has mines for non-metal minerals. For example, sand and gravel are mined for construction. "
mining and using minerals,T_0676,"If an ore is deep below Earths surface it may be too expensive to remove all the rock above it. These deposits are taken by underground mining. Underground mines can be very deep. The deepest gold mine in South Africa is more than 3,700 m deep (that is more than 2 miles)! There are various methods of underground mining. Underground mining is more expensive than surface mining. Tunnels must be blasted into the rock so that miners and equipment can get to the ore. Underground mining is dangerous work. Fresh air and lights must be brought in to the tunnels for the miners. The miners breathe in lots of particles and dust while they are underground. The ore is drilled, blasted, or cut away from the surrounding rock and taken out of the tunnels. Sometimes there are explosions as ore is being drilled or blasted. This can lead to a mine collapse. Miners may be hurt or killed in a mining accident. "
mining and using minerals,T_0677,"Most minerals are a combination of metal and other elements. The rocks that are taken from a mine are full of valuable minerals plus rock that isnt valuable. This is called waste rock. The valuable minerals must be separated from the waste rock. One way to do this is with a chemical reaction. Chemicals are added to the ores at very high temperatures. For example, getting aluminum from waste rock uses a lot of energy. This is because temperatures greater than 900o C are needed to separate out the aluminum. It also takes a huge amount of electricity. If you recycle just 40 aluminum cans, you will save the energy in one gallon of gasoline. We use over 80 billion cans each year. If all of these cans were recycled, we would save the energy in 2 billion gallons of gasoline! "
mining and using minerals,T_0678,"We rely on metals, such as aluminum, copper, iron, and gold. Look around the room. How many objects have metal parts? Metals are used in the tiny parts inside your computer, in the wires of anything that uses electricity, and to make the structure of a large building, such as the one shown in the Figure 3.23. "
mining and using minerals,T_0679,"Some minerals are valuable simply because they are beautiful. Jade has been used for thousands of years in China. Native Americans have been decorating items with turquoise since ancient times. Minerals like jade, turquoise, diamonds, and emeralds are gemstones. A gemstone is a material that is cut and polished to use in jewelry. Many gemstones, such as those shown in Figure 3.24, are minerals. Gemstones are beautiful, rare, and do not break or scratch easily. Generally, rarer gems are more valuable. If a gem Gemstones also have other uses. Most diamonds are actually not used as gemstones. Diamonds are used to cut and polish other materials, such as glass and metals, because they are so hard. The mineral corundum, which makes the gems ruby and sapphire, is used in products like sandpaper. Synthetic rubies and sapphires are also used in lasers. "
mining and using minerals,T_0680,"Metals and gemstones are often shiny, so they catch your eye. Many minerals that we use everyday are not so noticeable. For example, the buildings on your block could not have been built without minerals. The walls in your home might use the mineral gypsum for the sheetrock. The glass in your windows is made from sand, which is mostly the mineral quartz. Talc was once commonly used to make baby powder. The mineral halite is mined for rock salt. Diamond is commonly used in drill bits and saw blades to improve their cutting ability. Copper is used in electrical wiring, and the ore bauxite is the source for the aluminum in your soda can. "
mining and using minerals,T_0681,"Mining provides people with many resources they need, but mining can be hazardous to people and the environment. Miners should restore the mined region to its natural state. It is also important to use mineral resources wisely. Most ores are non-renewable resources. "
mining and using minerals,T_0682,"After the mining is finished, the land is greatly disturbed. The area around the mine needs to be restored to its natural state. This process of restoring the area is called reclamation. Native plants are planted. Pit mines may be refilled or reshaped so that they can become natural areas again. The mining company may be allowed to fill the pit with water to create a lake. The pits may be turned into landfills. Underground mines may be sealed off or left open as homes for bats. "
mining and using minerals,T_0683,Mining can cause pollution. Chemicals released from mining can contaminate nearby water sources. Figure 3.26 shows water that is contaminated from a nearby mine. The United States government has mining standards to protect water quality. 
mining and using minerals,T_0684,5. What are some disadvantages of underground mining? 6. What is the bottom line when it comes to deciding how what and how to mine? 7. How is land reclaimed after mining? Is it ever fully recovered? 8. How might the history of the Golden State been different if placers had not been found in its rivers? 
inside earth,T_0748,"If someone told you to figure out what is inside Earth, what would you do? How could you figure out what is inside our planet? How do scientists figure it out? "
inside earth,T_0749,Geologists study earthquake waves to see Earths interior. Waves of energy radiate out from an earthquakes focus. These are called seismic waves (Figure 6.1). Seismic waves change speed as they move through different materials. This causes them to bend. Some seismic waves do not travel through liquids or gases. Scientists use all of this information to understand what makes up the Earths interior. 
inside earth,T_0750,Scientists study meteorites to learn about Earths interior. Meteorites formed in the early solar system. These objects represent early solar system materials. Some meteorites are made of iron and nickel. They are thought to be very similar to Earths core (Figure 6.2). An iron meteorite is the closest thing to a sample of the core that scientists can hold in their hands! 
inside earth,T_0751,"Crust, mantle, and core differ from each other in chemical composition. Its understandable that scientists know the most about the crust, and less about deeper layers (Figure 6.3). Earths crust is a thin, brittle outer shell. The crust is made of rock. This layer is thinner under the oceans and much thicker in mountain ranges. "
inside earth,T_0752,"There are two kinds of crust. Oceanic crust is made of basalt lavas that flow onto the seafloor. It is relatively thin, between 5 to 12 kilometers thick (3 - 8 miles). The rocks of the oceanic crust are denser (3.0 g/cm3 ) than the rocks that make up the continents. Thick layers of mud cover much of the ocean floor. "
inside earth,T_0753,"Continental crust is much thicker than oceanic crust. It is 35 kilometers (22 miles) thick on average, but it varies a lot. Continental crust is made up of many different rocks. All three major rock types  igneous, metamorphic, and sedimentary  are found in the crust. On average, continental crust is much less dense (2.7 g/cm3) than oceanic crust. Since it is less dense, it rises higher above the mantle than oceanic crust. "
inside earth,T_0754,"Beneath the crust is the mantle. The mantle is made of hot, solid rock. Through the process of conduction, heat flows from warmer objects to cooler objects (Figure 6.4). The lower mantle is heated directly by conduction from the core. Hot lower mantle material rises upwards (Figure 6.5). As it rises, it cools. At the top of the mantle it moves horizontally. Over time it becomes cool and dense enough that it sinks. Back at the bottom of the mantle, it travels horizontally. Eventually the material gets to the location where warm mantle material is rising. The rising and sinking of warm and cooler material is convection. The motion described creates a convection cell. "
inside earth,T_0755,"The dense, iron core forms the center of the Earth. Scientists know that the core is metal from studying metallic meteorites and the Earths density. Seismic waves show that the outer core is liquid, while the inner core is solid. Movement within Earths outer liquid iron core creates Earths magnetic field. These convection currents form in the outer core because the base of the outer core is heated by the even hotter inner core. "
inside earth,T_0756,"Lithosphere and asthenosphere are layers based on physical properties. The outermost layer is the lithosphere. The lithosphere is the crust and the uppermost mantle. In terms of physical properties, this layer is rigid, solid, and brittle. It is easily cracked or broken. Below the lithosphere is the asthenosphere. The asthenosphere is also in the upper mantle. This layer is solid, but it can flow and bend. A solid that can flow is like silly putty. "
seafloor spreading,T_0764,"Before World War II, people thought the seafloor was completely flat and featureless. There was no reason to think otherwise. "
seafloor spreading,T_0765,"But during the war, battleships and submarines carried echo sounders. Their goal was to locate enemy submarines (Figure 6.9). Echo sounders produce sound waves that travel outward in all directions. The sound waves bounce off the nearest object, and then return to the ship. Scientists know the speed of sound in seawater. They then can calculate the distance to the object that the sound wave hit. Most of these sound waves did not hit submarines. They instead were used to map the ocean floor. "
seafloor spreading,T_0766,"Scientists were surprised to find huge mountains and deep trenches when they mapped the seafloor. The mid-ocean ridges form majestic mountain ranges through the deep oceans (Figure 6.10). Deep sea trenches are found near chains of active volcanoes. These volcanoes can be at the edges of continents or in the oceans. Trenches are the deepest places on Earth. The deepest trench is the Mariana Trench in the southwestern Pacific Ocean. This trench plunges about 11 kilometers (35,840 feet) beneath sea level. The ocean floor does have lots of flat areas. These abyssal plains are like the scientists had predicted. "
seafloor spreading,T_0767,Warships also carried magnetometers. They were also used to search for submarines. The magnetometers also revealed a lot about the magnetic properties of the seafloor. 
seafloor spreading,T_0768,"Indeed, scientists discovered something astonishing. Many times in Earths history, the magnetic poles have switched positions. North becomes south and south becomes north! When the north and south poles are aligned as they are now, geologists say it is normal polarity. When they are in the opposite position, they say that it is reversed polarity. "
seafloor spreading,T_0769,"Scientists were also surprised to discover a pattern of stripes of normal and reversed polarity. These stripes surround the mid-ocean ridges. There is one long stripe with normal magnetism at the top of the ridge. Next to that stripe are two long stripes with reversed magnetism. One is on either side of the normal stripe. Next come two normal stripes and then two reversed stripes, and so on across the ocean floor. The magnetic stripes end abruptly at the edges of continents. Sometimes the stripes end at a deep sea trench (Figure 6.11). "
seafloor spreading,T_0770,"The scientists used geologic dating techniques on seafloor rocks. They found that the youngest rocks on the seafloor were at the mid-ocean ridges. The rocks get older with distance from the ridge crest. The scientists were surprised to find that the oldest seafloor is less than 180 million years old. This may seem old, but the oldest continental crust is around 4 billion years old. Scientists also discovered that the mid-ocean ridge crest is nearly sediment free. The crust is also very thin there. With distance from the ridge crest, the sediments and crust get thicker. This also supports the idea that the youngest rocks are on the ridge axis and that the rocks get older with distance away from the ridge (Figure 6.12). Something causes the seafloor to be created at the ridge crest. The seafloor is also destroyed in a relatively short time. "
seafloor spreading,T_0771,"The seafloor spreading hypothesis brought all of these observations together in the early 1960s. Hot mantle material rises up at mid-ocean ridges. The hot magma erupts as lava. The lava cools to form new seafloor. Later, more lava erupts at the ridge. The new lava pushes the seafloor that is at the ridge horizontally away from ridge axis. The seafloor moves! In some places, the oceanic crust comes up to a continent. The moving crust pushes that continent away from the ridge axis as well. If the moving oceanic crust reaches a deep sea trench, the crust sinks into the mantle. The creation and destruction of oceanic crust is the reason that continents move. Seafloor spreading is the mechanism that Wegener was looking for! "
theory of plate tectonics,T_0772,"The Cold War helped scientists to learn more about our planet. They set up seismograph networks during the 1950s and early 1960s. The purpose was to see if other nations were testing atomic bombs. Of course, at the same time, the seismographs were recording earthquakes. "
theory of plate tectonics,T_0773,"The scientists realized that the earthquakes were most common in certain areas. In the oceans, they were found along mid-ocean ridges and deep sea trenches. Earthquakes and volcanoes were common all around the Pacific Ocean. They named this region the Pacific Ring of Fire (Figure 6.13). Earthquakes are also common in the worlds highest mountains, the Himalaya Mountains of Asia. The Mediterranean Sea also has many earthquakes. "
theory of plate tectonics,T_0774,"Earthquakes are used to identify plate boundaries (Figure 6.14). When earthquake locations are put on a map, they outline the plates. The movements of the plates are called plate tectonics. The lithosphere is divided into a dozen major and several minor plates. Each plate is named for the continent or ocean basin it contains. Some plates are made of all oceanic lithosphere. A few are all continental lithosphere. But "
theory of plate tectonics,T_0775,"Convection within the Earths mantle causes the plates to move. Mantle material is heated above the core. The hot mantle rises up towards the surface (Figure 6.16). As the mantle rises it cools. At the surface the material moves horizontally away from a mid-ocean ridge crest. The material continues to cool. It sinks back down into the mantle at a deep sea trench. The material sinks back down to the core. It moves horizontally again, completing a convection cell. "
theory of plate tectonics,T_0776,"Plate boundaries are where two plates meet. Most geologic activity takes place at plate boundaries. This activity includes volcanoes, earthquakes, and mountain building. The activity occurs as plates interact. How can plates interact? Plates can move away from each other. They can move toward each other. Finally, they can slide past each other. These are the three types of plate boundaries: Divergent plate boundaries: the two plates move away from each other. Convergent plate boundaries: the two plates move towards each other. Transform plate boundaries: the two plates slip past each other. The features that form at a plate boundary are determined by the direction of plate motion and by the type of crust at the boundary. "
theory of plate tectonics,T_0777,Plates move apart at divergent plate boundaries. This can occur in the oceans or on land. 
theory of plate tectonics,T_0778,"Plates move apart at mid-ocean ridges. Lava rises upward, erupts, and cools. Later, more lava erupts and pushes the original seafloor outward. This is seafloor spreading. Seafloor spreading forms new oceanic crust. The rising magma causes earthquakes. Most mid-ocean ridges are located deep below the sea. The island of Iceland sits right on the Mid-Atlantic ridge (Figure 6.17). "
theory of plate tectonics,T_0779,"A divergent plate boundary can also occur within a continent. This is called continental rifting (Figure 6.18). Magma rises beneath the continent. The crust thins, breaks, and then splits apart. This first produces a rift valley. The East African Rift is a rift valley. Eastern Africa is splitting away from the African continent. Eventually, as the continental crust breaks apart, oceanic crust will form. This is how the Atlantic Ocean formed when Pangaea broke up. "
theory of plate tectonics,T_0780,"A convergent plate boundary forms where two plates collide. That collision can happen between a continent and oceanic crust, between two oceanic plates, or between two continents. Oceanic crust is always destroyed in these collisions. "
theory of plate tectonics,T_0781,"Oceanic crust may collide with a continent. The oceanic plate is denser, so it undergoes subduction. This means that the oceanic plate sinks beneath the continent. This occurs at an ocean trench (Figure 6.19). Subduction zones are where subduction takes place. As you would expect, where plates collide there are lots of intense earthquakes and volcanic eruptions. The subducting oceanic plate melts as it reenters the mantle. The magma rises and erupts. This creates a volcanic mountain range near the coast of the continent. This range is called a volcanic arc. The Andes Mountains, along the western edge of South America, are a volcanic arc (Figure 6.20). "
theory of plate tectonics,T_0782,"Two oceanic plates may collide. In this case, the older plate is denser. This plate subducts beneath the younger plate. As the subducting plate is pushed deeper into the mantle, it melts. The magma this creates rises and erupts. This forms a line of volcanoes, known as an island arc (Figure 6.21). Japan, Indonesia, the Philippine Islands, and the Aleutian Islands of Alaska are examples of island arcs (Figure 6.22). "
theory of plate tectonics,T_0783,"Continental lithosphere is low in density and very thick. Continental lithosphere cannot subduct. So when two continental plates collide, they just smash together, just like if you put your hands on two sides of a sheet of paper and bring your hands together. The material has nowhere to go but up (Figure 6.23)! Earthquakes and metamorphic rocks result from the tremendous forces of the collision. But the crust is too thick for magma to get through, so there are no volcanoes. "
theory of plate tectonics,T_0784,Continent-continent convergence creates some of the worlds largest mountains ranges. The Himalayas (Figure are the remnants of a larger mountain range. This range formed from continent-continent collisions in the time of Pangaea. 
theory of plate tectonics,T_0785,"Two plates may slide past each other in opposite directions. This is called a transform plate boundary. These plate boundaries experience massive earthquakes. The worlds best known transform fault is the San Andreas Fault in California (Figure 6.25). At this fault, the Pacific and North American plates grind past each other. Transform plate boundaries are most common as offsets along mid-ocean ridges. Transform plate boundaries are different from the other two types. At divergent plate boundaries, new oceanic crust is formed. At convergent boundaries, old oceanic crust is destroyed. But at transform plate boundaries, crust is not created or destroyed. "
theory of plate tectonics,T_0786,Knowing where plate boundaries are helps explain the locations of landforms and types of geologic activity. The activity can be current or old. 
theory of plate tectonics,T_0787,"Western North America has volcanoes and earthquakes. Mountains line the region. California, with its volcanoes and earthquakes, is an important part of the Pacific Ring of Fire. This is the boundary between the North American and Pacific Plates. "
theory of plate tectonics,T_0788,Mountain ranges also line the eastern edge of North America. But there are no active volcanoes or earthquakes. Where did those mountains come from? These mountains formed at a convergent plate boundary when Pangaea came together. About 200 million years ago these mountains were similar to the Himalayas today (Figure 6.26)! There were also earthquakes. 
theory of plate tectonics,T_0789,"Scientists think that Pangaea was not the first supercontinent. There were others before it. The continents are now moving together. This is because of subduction around the Pacific Ocean. Eventually, the Pacific will disappear and a new supercontinent will form. This wont be for hundreds of millions of years. The creation and breakup of a supercontinent takes place about every 500 million years. "
theory of plate tectonics,T_0790,Most geological activity takes place at plate boundaries. But some activity does not. Much of this intraplate activity is found at hot spots. Hotspot volcanoes form as plumes of hot magma rise from deep in the mantle. 
theory of plate tectonics,T_0791,"A chain of volcanoes forms as an oceanic plate moves over a hot spot. This is how it happens. A volcano forms over the hotspot. Since the plate is moving, the volcano moves off of the hotspot. When the hotspot erupts again, a new volcano forms over it. This volcano is in line with the first. Over time, there is a line of volcanoes. The youngest is directly above the hot spot. The oldest is the furthest away (Figure 6.27). The Hawaii-Emperor chain of volcanoes formed over the Hawaiian Hotspot. The Hawaiian Islands formed most "
theory of plate tectonics,T_0792,"Hot spots are also found under the continental crust. Since it is more difficult for magma to make it through the thick crust, they are much less common. One exception is the Yellowstone hotspot (Figure 6.28). This hotspot is very active. In the past, the hotspot produced enormous volcanic eruptions. Now its activity is best seen in the regions famous geysers. "
nature of earthquakes,T_0803,"Almost all earthquakes occur at plate boundaries. All types of plate boundaries have earthquakes. Convection within the Earth causes the plates to move. As the plates move, stresses build. When the stresses build too much, the rocks break. The break releases the energy that was stored in the rocks. The sudden release of energy creates an earthquake. During an earthquake the rocks usually move several centimeters or rarely as much as a few meters. Elastic rebound theory describes how earthquakes occur (Figure 7.21). "
nature of earthquakes,T_0804,Where an earthquake takes place is described by its focus and epicenter. 
nature of earthquakes,T_0805,"The point where the rock ruptures is the earthquakes focus. The focus is below the Earths surface. A shallow earthquake has a focus less than 70 kilometers (45 miles). An intermediate-focus earthquake has a focus between 70 and 300 kilometers (45 to 200 miles). A deep-focus earthquake is greater than 300 kilometers (200 miles). About 75% of earthquakes have a focus in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage. This is because the focus is near the Earths surface, where people live. "
nature of earthquakes,T_0806,"The area just above the focus, on the land surface, is the earthquakes epicenter (Figure 7.22). The towns or cities near the epicenter will be strongly affected by the earthquake. "
nature of earthquakes,T_0807,"Nearly 95% of all earthquakes take place along one of the three types of plate boundaries. As you learned in the Plate Tectonics chapter, scientists use the location of earthquakes to draw plate boundaries. The region around the Pacific Ocean is called the Pacific Ring of Fire. This is due to the volcanoes that line the region. The area also has the most earthquakes. About 80% of all earthquakes strike this area. The Pacific Ring of Fire is caused by the convergent and transform plate boundaries that line the Pacific Ocean basin. About 15% of all earthquakes take place in the Mediterranean-Asiatic belt. The convergent plate boundaries in the region are shrinking the Mediterranean Sea. The convergence is also causing the Himalayas to grow. The remaining 5% of earthquakes are scattered around the other plate boundaries. A few earthquakes take place in the middle of a plate, away from plate boundaries. "
nature of earthquakes,T_0808,"Transform plate boundaries produce enormous and deadly earthquakes. These quakes at transform faults have shallow focus. This is because the plates slide past each other without moving up or down. The largest earthquake on the San Andreas Fault occurred in 1906 in San Francisco. Other significant earthquakes in California include the 1989 Loma Prieta earthquake near Santa Cruz (Figure 7.23) and the 1994 Northridge earthquake near Los Angeles. There are many other faults spreading off the San Andreas, which produce around 10,000 earthquakes a year (Figure "
nature of earthquakes,T_0809,"Convergent plate boundaries also produce strong, deadly earthquakes. Earthquakes mark the motions of colliding plates and the locations where plates plunge into the mantle. These earthquakes can be shallow, intermediate or deep focus. The Philippine plate and the Pacific plate subduct beneath Japan, creating as many as 1,500 earthquakes every year. In March 2011, the 9.0 magnitude Tohoku earthquake struck off of northeastern Japan. Damage from the quake was severe. More severe was the damage from the tsunami generated by the quake (Figure 7.25). In all, 25,000 people were known dead or missing. The Cascades Volcanoes line the Pacific Northwest of the United States. Here, the Juan de Fuca plate subducts beneath the North American plate. The Cascades volcanoes are active and include Mount Saint Helens. Major earthquakes occur here approximately every 300 to 600 years. The last was in 1700. Its magnitude was between 8.7 and 9.2. It has now been more than 300 years since that earthquake. The next massive earthquake could strike the Pacific Northwest at any time. "
nature of earthquakes,T_0810,"The collision of two continents also creates massive earthquakes. Many earthquakes happen in the region in and around the Himalayan Mountains. The 2001 Gujarat, India earthquake is responsible for about 20,000 deaths, with many more people injured or made homeless. "
nature of earthquakes,T_0811,"Earthquakes also occur at divergent plate boundaries. At mid-ocean ridges, these earthquakes tend to be small and shallow focus because the plates are thin, young, and hot. Earthquakes in the oceans are usually far from land, so they have little effect on peoples lives. On land, where continents are rifting apart, earthquakes are larger and stronger. "
nature of earthquakes,T_0812,"About 5% of earthquakes take place within a plate, away from plate boundaries. These intraplate earthquakes are caused by stresses within a plate. The plate moves over a spherical surface, creating zones of weakness. Intraplate earthquakes happen along these zones of weakness. A large intraplate earthquake occurred in 1812. A magnitude 7.5 earthquake struck near New Madrid, Missouri. This is a region not usually known for earthquakes. Because very few people lived here at the time, only 20 people died. The New Madrid Seismic Zone continues to be active (Figure 7.26). Many more people live here today. "
nature of earthquakes,T_0813,"Seismic waves are the energy from earthquakes. Seismic waves move outward in all directions away from their source. Each type of seismic wave travels at different speeds in different materials. All seismic waves travel through rock, but not all travel through liquid or gas. Geologists study seismic waves to learn about earthquakes and the Earths interior. "
nature of earthquakes,T_0814,Seismic waves are just one type of wave. Sound and light also travel in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The horizontal distance between waves from crest to crest (or trough to trough) is its wavelength (Figure 7.27). 
nature of earthquakes,T_0815,"There are two major types of seismic waves. Body waves travel through the Earths interior. Surface waves travel along the ground surface. In an earthquake, body waves are responsible for sharp jolts. Surface waves are responsible for rolling motions that do most of the damage in an earthquake. "
nature of earthquakes,T_0816,"Primary waves (P-waves) and secondary waves (S-waves) are the two types of body waves (Figure 7.28). Body waves move at different speeds through different materials. P-waves are faster. They travel at about 6 to 7 kilometers (about 4 miles) per second. Primary waves are so named because they are the first waves to reach a seismometer. P-waves squeeze and release rocks as they travel. The material returns to its original size and shape after the P-wave goes by. For this reason, P-waves are not the most damaging earthquake waves. P-waves travel through solids, liquids and gases. S-waves are slower than P-waves. They are the second waves to reach a seismometer. S-waves move up and down. They change the rocks shape as they travel. S-waves are about half as fast as P-waves, at about 3.5 km (2 miles) per second. S-waves can only move through solids. This is because liquids and gases dont resist changing shape. "
nature of earthquakes,T_0817,"Surface waves travel along the ground outward from an earthquakes epicenter. Surface waves are the slowest of all seismic waves. They travel at 2.5 km (1.5 miles) per second. There are two types of surface waves. Love waves move side-to-side, much like a snake. Rayleigh waves produce a rolling motion as they move up and backwards (Figure 7.29). Surface waves cause objects to fall and rise, while they are also swaying back and forth. These "
nature of earthquakes,T_0818,"Earthquakes can cause tsunami. These deadly ocean waves may result from any shock to ocean water. A shock could be a meteorite impact, landslide, or a nuclear explosion. An underwater earthquake creates a tsunami this way: The movement of the crust displaces water. The displacement forms a set of waves. The waves travel at jet speed through the ocean. Since the waves have low amplitudes and long wavelengths, they are unnoticed in deep water. As the waves reach shore they compress. They are also pushed upward by the shore. For these reasons, tsunami can grow to enormous wave heights. Tsunami waves can cause tremendous destruction and loss of life. Fortunately, few undersea earthquakes generate tsunami. "
nature of earthquakes,T_0819,"The Boxing Day Tsunami struck on December 26, 2004. This tsunami was by far the deadliest of all time (Figure registered magnitude 9.1. The quake struck near Sumatra, Indonesia, where the Indian plate is subducting beneath the Burma plate. It released about 550 million times the energy of the atomic bomb dropped on Hiroshima. Several tsunami waves were created. The tsunami struck eight countries around the Indian Ocean (Figure 7.31). About 230,000 people died. More than 1.2 million people lost their homes. Many more lost their way of making a living. Fishermen lost their boats, and businesspeople lost their restaurants and shops. Many marine animals washed onshore, including dolphins, turtles, and sharks. "
nature of earthquakes,T_0820,"Like other waves, a tsunami wave has a crest and a trough. When the wave hits the beach, the crest or the trough may come ashore first. When the trough comes in first, water is sucked out to sea. The seafloor just offshore from the beach is exposed. Curious people often walk out onto the beach to see the unusual sight. They drown when the wave crest hits. One amazing story from the Indian Ocean tsunami is that of Tilly Smith. Tilly was a 10-year-old British girl who was visiting Maikhao Beach in Thailand with her parents. Tilly had learned about tsunami in school two weeks before the earthquake. She knew that the receding water and the frothy bubbles at the sea surface meant a tsunami was coming. Tilly told her parents, who told other tourists and the staff at their hotel. The beach was evacuated and no one on Maikhao Beach died. Tilly is credited with saving nearly 100 people! "
nature of earthquakes,T_0821,"Most of the Indian Ocean tragedy could have been avoided if a warning system had been in place(Figure 7.32). As of June 2006, the Indian Ocean now has a warning system. Since tsunami are much more common in the Pacific, communities around the Pacific have had a tsunami warning system since 1948. Warning systems arent always helpful. People in communities very close to the earthquake do not have enough time to move inland or uphill. Farther away from the quake, evacuation of low-lying areas saves lives. "
measuring and predicting earthquakes,T_0822,"Seismic waves are measured on a seismograph. Seismographs contain a lot of information, and not just about earthquakes. "
measuring and predicting earthquakes,T_0823,"A seismograph is a machine that records seismic waves. In the past, seismographs produced a seismogram. A seismogram is a paper record of the seismic waves the seismograph received. Seismographs have a weighted pen suspended from a stationary frame. A drum of paper is attached to the ground. As the ground shakes in an earthquake, the pen remains stationary but the drum moves beneath it. This creates the squiggly lines that make up a seismogram (Figure 7.33). Modern seismographs record ground motions using electronic motion detectors. The data are recorded digitally on a computer. "
measuring and predicting earthquakes,T_0824,"Seismograms contain a lot of information about an earthquake: its strength, length and distance. Wave height used to determine the magnitude of the earthquake. The seismogram shows the different arrival times of the seismic waves (Figure 7.34). The first waves are P-waves since they are the fastest. S-waves come in next and are usually larger than P-waves. The surface waves arrive just after the S-waves. If the earthquake has a shallow focus, the surface waves are the largest ones recorded. A seismogram may record P-waves and surface waves, but not S-waves. This means that it was located more than halfway around the Earth from the earthquake. The reason is that Earths outer core is liquid. S-waves cannot travel "
measuring and predicting earthquakes,T_0825,"One seismogram indicates the distance to the epicenter. This is determined by the P-and S-wave arrival times. If a quake is near the seismograph, the S-waves arrive shortly after the P-waves. If a quake is far from the seismograph, the P-waves arrive long before the S-waves. The longer the time is between the P-and S-wave arrivals, the further away the earthquake was from the seismograph. First, seismologists calculate the arrival time difference. Then they know the distance to the epicenter from that seismograph. Next, the seismologists try to determine the location of the earthquake epicenter. To do this they need the distances to the epicenter from at least three seismographs. Lets say that they know that an earthquakes epicenter is 50 kilometers from Kansas City. They draw a circle with a 50 km radius around that seismic station. They do this twice more around two different seismic stations. The three circles intersect at a single point. This is the earthquakes epicenter (Figure 7.35). "
measuring and predicting earthquakes,T_0826,"The ways seismologists measure an earthquake have changed over the decades. Initially, they could only measure what people felt and saw, the intensity. Now they can measure the energy released during the quake, the magnitude. Early in the 20th century, earthquakes were described in terms of what people felt and the damage that was done to buildings. The Mercalli Intensity Scale describes earthquake intensity. There are many problems with the Mercalli scale. The damage from an earthquake is affected by many things. Different people experience an earthquake differently. Using this scale, comparisons between earthquakes were difficult to make. A new scale was needed. "
measuring and predicting earthquakes,T_0827,"Charles Richter developed the Richter magnitude scale in 1935. The Richter scale measures the magnitude of an earthquakes largest jolt of energy. This is determined by using the height of the waves recorded on a seismograph. Richter scale magnitudes jump from one level to the next. The height of the largest wave increases 10 times with each level. So the height of the largest seismic wave of a magnitude 5 quake is 10 times that of a magnitude 4 quake. A magnitude 5 is 100 times that of a magnitude 3 quake. With each level, thirty times more energy is released. A difference of two levels on the Richter scale equals 900 times more released energy. The Richter scale has limitations. A single sharp jolt measures higher on the Richter scale than a very long intense earthquake. Yet this is misleading because the longer quake releases more energy. Earthquakes that release more energy are likely to do more damage. As a result, another scale was needed. "
measuring and predicting earthquakes,T_0828,The moment magnitude scale is the favored method of measuring earthquake magnitudes. It measures the total energy released by an earthquake. Moment magnitude is calculated by two things. One is the length of the fault break. The other is the distance the ground moves along the fault. 
measuring and predicting earthquakes,T_0829,"Each year, more than 900,000 earthquakes are recorded. 150,000 of them are strong enough to be felt by people. About 18 each year are major, with a Richter magnitude of 7.0 to 7.9. Usually there is one earthquake with a magnitude of 8 to 8.9 each year. Earthquakes with a magnitude in the 9 range are rare. The United States Geological Survey lists five such earthquakes on the moment magnitude scale since 1900 (see Figure 7.36). All but one, the Great Indian Ocean Earthquake of 2004, occurred somewhere around the Pacific Ring of Fire. "
measuring and predicting earthquakes,T_0830,"Scientists are not able to predict earthquakes. Since nearly all earthquakes take place at plate boundaries, scientists can predict where an earthquake will occur (Figure 7.37). This information helps communities to prepare for an earthquake. For example, they can require that structures are built to be earthquake safe. Predicting when an earthquake will occur is much more difficult. Scientists can look at how often earthquakes have struck in the past. This does not allow an accurate prediction for the future. Small tremors, called foreshocks, often happen a short time before a major quake. The ground may also tilt as stress builds up in the rocks. Water levels in wells also change as groundwater moves through rock fractures. These do not usually allow accurate predictions. Folklore tells of animals behaving strangely just before an earthquake. Most people tell stories of these behaviors after the earthquake. Chinese scientists actively study the behavior of animals before earthquakes to see if there is a connection. So far nothing concrete has come of these studies. Once an earthquake has started, many actions must take place. Seismometers can detect P-waves a few seconds before more damaging S-waves and surface waves arrive. Although a few seconds is not much, computers can shut down gas mains and electrical transmission lines. They can initiate protective measures in chemical plants, nuclear power plants, mass transit systems, airports, and roadways. "
staying safe in earthquakes,T_0831,"Earthquake magnitude affects how much damage is done in an earthquake. A larger earthquake damages more buildings and kills more people than a smaller earthquake. But thats not the only factor that determines earthquake damage. The location of an earthquake relative to a large city is important. More damage is done if the ground shakes for a long time. The amount of damage also depends on the geology of the region. Strong, solid bedrock shakes less than soft or wet soils. Wet soils liquefy during an earthquake and become like quicksand. Soil on a hillside that is shaken loose can become a landslide. Hazard maps help city planners choose the best locations for buildings (Figure 7.38). For example, when faced with two possible locations for a new hospital, planners must build on bedrock rather than silt and clay. "
staying safe in earthquakes,T_0832,"The 1985 Mexico City earthquake measured magnitude 8.1. The earthquake killed at least 9,000 people, injured 30,000 more, and left 100,000 people homeless. It destroyed 416 buildings, and seriously damaged 3,000 other buildings. The intense destruction was due to the soft ground the city is built on. Silt and clay fill a basin made of solid rock. In an earthquake, seismic waves bounce back-and-forth off the sides and bottom of the rock basin. This amplifies the shaking. The wet clay converts to quicksand (Figure 7.39). Many buildings were not anchored to bedrock. They settled into the muck. This caused enormous damage. Water, sewer, and electrical systems were destroyed, resulting in fires. Acapulco was much closer to the epicenter, but since the city is built on bedrock it suffered little damage. "
staying safe in earthquakes,T_0833,"The amount of damage depends on the amount of development in the region. The 1964 Great Alaska Earthquake, near Anchorage, was the largest earthquake ever recorded in North America. The gigantic quake had a magnitude of 9.2. The earthquake lasted for several minutes and the ground slipped up to 11.5 meters (38 feet). An area of 100,000 square miles (250,000 square km) was affected. The ground liquefied, causing landslides (Figure 7.40). The earthquake occurred at a subduction zone, and large tsunami up to 70 meters (20 feet) high were created. Despite the intensity of the earthquake, only 131 people died. Most deaths were due to the tsunami. Property damage was just over $300 million ($1.8 billion in 2007 U.S. dollars). The reason there was such a small amount of damage is that very few people lived in the area (Alaska had only been a state for five years!). A similar earthquake today would affect many more people. "
staying safe in earthquakes,T_0834,Buildings must be specially built to withstand earthquakes. Skyscrapers and other large structures built on soft ground must be anchored to bedrock. Sometimes that bedrock is hundreds of meters below the ground surface! 
staying safe in earthquakes,T_0835,"Building materials need to be both strong and flexible. Small structures, like houses, should bend and sway. Wood and steel bend. Brick, stone, and adobe are brittle and will break. Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagonal steel beams can hold down sway. Buildings need strong, flexible connections where the walls meet the foundation. Earthquake-safe buildings are well connected (Figure Steel or wood can be added to older buildings to reinforce a buildings structure and its connections (Figure 7.42). Elevated freeways and bridges can also be reinforced so that they do not collapse. Important structures must be designed to survive intact. "
staying safe in earthquakes,T_0836,"One of the biggest problems caused by earthquakes is fire. Fires start because earthquakes rupture gas and electrical lines. Water mains may break. This makes it difficult to fight the fires. The shapes of pipes can make a big difference. Straight pipes will break in a quake. Zigzag pipes bend and flex when the ground shakes. In San Francisco, water and gas pipelines are separated by valves. Areas can be isolated if one segment breaks. "
staying safe in earthquakes,T_0837,"Strong, sturdy structures are expensive to build. Communities must decide how safe to make their buildings. They must weigh how great the hazard is, what different building strategies will cost, and how much risk they are willing to take. "
staying safe in earthquakes,T_0838,"If you live in an earthquake zone, there are many things you can do to protect yourself. You must protect your home. Your household must be ready to live independently for a few days. It may take emergency services that long to get to everyone. Before an Earthquake: Make sure the floor, walls, roof, and foundation are all well attached to each other. Have an engineer evaluate your house for structural integrity. Bracket or brace brick chimneys to the roof. Be sure that heavy objects are not stored in high places. Move them to low places so that they do not fall. Secure water heaters all around and at the top and bottom. Bolt heavy furniture onto walls with bolts, screws, or strap hinges. Replace halogen and incandescent light bulbs with fluorescent bulbs to lessen fire risk. Check to see that gas lines are made of flexible material so they do not rupture. Any equipment that uses gas should be well secured. Everyone in the household should know how to shut off the gas line. A wrench should be placed nearby for doing so. Prepare an earthquake kit with at least three days supply of water and food. Include a radio and batteries. Place flashlights all over the house so there is always one available. Place one in the glove box of your car. Keep several fire extinguishers around the house to fight any small fires that break out. Be sure to have a first aid kit. Everyone in the household who is capable should know basic first aid and CPR. Plan in advance how you will evacuate your property and where you will go. Do not plan on driving, as roadways will likely be damaged. During the Earthquake: If you are in a building, drop to the ground, get beneath a sturdy table or desk, cover your head, and hold on. Stay away from windows and mirrors since glass can break and fall on you. Stay away from large furniture that may fall on you. If the building is structurally unsound, get outside as fast as possible. Run into an open area away from buildings and power lines that may fall on you. If you are in a car, stay in the car and stay away from structures that might collapse like overpasses, bridges, or buildings. After the Earthquake: Be aware that aftershocks are likely. Avoid dangerous areas, like hillsides, that may experience a landslide. Turn off water, gas lines, and power to your home. Use your phone only if there is an emergency. Many people with urgent needs will be trying to get through to emergency services. Be prepared to wait for help or instructions. Assist others as necessary. "
volcanic activity,T_0839,Volcanoes rise where magma forms underground. Volcanoes are found at convergent plate boundaries and at hotspots. Volcanic activity is found at divergent plate boundaries. The map in Figure 8.1 shows where volcanoes are located. 
volcanic activity,T_0840,"There is a lot of volcanic activity at divergent plate boundaries in the oceans. As the plates pull away from each other, they create deep fissures. Molten lava erupts through these cracks. The East Pacific Rise is a divergent plate boundary in the Pacific Ocean (Figure 8.2). The Mid-Atlantic Ridge is a divergent plate boundary in the Atlantic Ocean. Continents can also rift apart. When mantle gets close enough to the surface, volcanoes form. Eventually, a rift valley will create a new mid-ocean ridge. "
volcanic activity,T_0841,Lots of volcanoes form along subduction plate boundaries. The edges of the Pacific Plate are a long subduction boundary. Lines of volcanoes can form at subduction zones on oceanic or continental crust. Japan is an example of a volcanic arc on oceanic crust. The Cascade Range and Andes Mountains are volcanic arcs on continental crust. 
volcanic activity,T_0842,"Some volcanoes form over active hot spots. Scientists count about 50 hot spots on the Earth. Hot spots may be in the middle of a tectonic plate. Hot spots lie directly above a column of hot rock called a mantle plume. Mantle plumes continuously bring magma up from the mantle towards the crust (Figure 8.3). As the tectonic plates move above a hot spot, they form a chain of volcanoes. The islands of Hawaii formed over a hot spot in the middle of the Pacific plate. The Hawaii hot spot has been active for tens of millions of years. The volcanoes of the Hawaiian Islands formed at this hot spot. Older volcanoes that formed at the hot spot have eroded below sea level. These are called the Emperor Seamounts. Loihi seamount is currently active beneath the water southeast of the Big Island of Hawaii. One day the volcano will rise above sea level and join the volcanoes of the island or create a new island (Figure 8.4). Hot spots may also be active at plate boundaries. This is especially common at mid-ocean ridges. Iceland is formed by a hot spot along the Mid-Atlantic Ridge. Hot spots are found within continents, but not as commonly as within oceans. The Yellowstone hot spot is a famous example of a continental hot spot. "
volcanic eruptions,T_0843,"All volcanoes share the same basic features. First, mantle rock melts. The molten rock collects in magma chambers that can be 160 kilometers (100 miles) beneath the surface. As the rock heats, it expands. The hot rock is less dense than the surrounding rock. The magma rises toward the surface through cracks in the crust. A volcanic eruption occurs when the magma reaches the surface. Lava can reach the surface gently or explosively. "
volcanic eruptions,T_0844,Eruptions can be explosive or non-explosive. Only rarely do gentle and explosive eruptions happen in the same volcano. 
volcanic eruptions,T_0845,"An explosive eruption produces huge clouds of volcanic ash. Chunks of the volcano fly high into the atmosphere. Explosive eruptions can be 10,000 times as powerful as an atomic bomb (Figure 8.6). Hot magma beneath the surface mixes with water. This forms gases. The gas pressure grows until it must be released. The volcano erupts in an enormous explosion. Ash and particles shoot many kilometers into the sky. The material may form a mushroom cloud, just like a nuclear explosion. Hot fragments of rock, called pyroclasts, fly up into the air at very high speeds. The pyroclasts cool in the atmosphere. Some ash may stay in the atmosphere for years. The ash may block out sunlight. This changes weather patterns and affects the temperature of the Earth. For a year or two after a large eruption, sunsets may be especially beautiful worldwide. Volcanic gases can form poisonous, invisible clouds. The poisonous gases may be toxic close to the eruption. The gases may cause environmental problems like acid rain and ozone destruction. Mt St. Helens was not a very large eruption for the Cascades. Mt. Mazama blew itself apart in an eruption about 42 times more powerful than Mount St. Helens in 1980. Today all that remains of that huge stratovolcano is Crater Lake (Figure 8.18). "
volcanic eruptions,T_0846,"Some volcanic eruptions are non-explosive (Figure 8.7). This happens when there is little or no gas. The lava is thin, fluid and runny. It flows over the ground like a river. People generally have a lot of warning before a lava flow like this reaches them, so non-explosive eruptions are much less deadly. They may still be destructive to property, though. Even when we know that a lava flow is approaching, there are few ways of stopping it! "
volcanic eruptions,T_0847,Great volcanic explosions and glowing red rivers of lava are fascinating. All igneous rock comes from magma or lava. Remember that magma is molten rock that is below Earths surface. Lava is molten rock at Earths surface. 
volcanic eruptions,T_0848,"Magma forms deep beneath the Earths surface. Rock melts below the surface under tremendous pressure and high temperatures. Molten rock flows like taffy or hot wax. Most magmas are formed at temperatures between 600o C and 1300o C (Figure 8.8). Magma collects in magma chambers beneath Earths surface. Magma chambers are located where the heat and pressure are great enough to melt rock. These locations are at divergent or convergent plate boundaries or at hotpots. The chemistry of a magma determines the type of igneous rock it forms. The chemistry also determines how the magma moves. Thicker magmas tend to stay below the surface or erupt explosively. When magma is fluid and runny, it often reaches the surface by flowing out in rivers of lava. "
volcanic eruptions,T_0849,"The way lava flows depends on what it is made of. Thick lava doesnt flow easily. It may block the vent of a volcano. If the lava traps a lot of gas, the pressure builds up. After the pressure becomes greater and greater, the volcano finally explodes. Ash and pyroclasts shoot up into the air. Pumice, with small holes in solid rock, shows where gas bubbles were when the rock was still molten. Fluid lava flows down mountainsides. The rock that the flow becomes depends on which type of lava it is and where it cools. The three types of flows are aa, pahoehoe, and pillow lava. Aa Lava Aa lava is the thickest of the non-explosive lavas. Aa forms a thick and brittle crust, which is torn into rough, rubbly pieces. The solidified surface is angular, jagged and sharp. Aa can spread over large areas as the lava continues to flow underneath. Pahoehoe Lava Pahoehoe lava is thinner than aa, and flows more readily. Its surface looks more wrinkly and smooth. Pahoehoe lava flows in a series of lobes that form strange twisted shapes and natural rock sculptures (Figure 8.9). Pahoehoe lava can form lava tubes. The outer layer of the lava flow cools and solidifies. The inner part of the flow remains fluid. The fluid lava flows through and leaves behind a tube (Figure 8.10). Pillow Lava Pillow lava is created from lava that enters the water. The volcanic vent may be underwater. The lava may flow over land and enter the water (Figure 8.11). Once in the water, the lava cools very quickly. The lava forms round rocks that resemble pillows. Pillow lava is particularly common along mid-ocean ridges. "
volcanic eruptions,T_0850,"Volcanic eruptions can be devastating, particularly to the people who live close to volcanoes. Volcanologists study volcanoes to be able to predict when a volcano will erupt. Many changes happen when a volcano is about to erupt. "
volcanic eruptions,T_0851,"Scientists study a volcanos history to try to predict when it will next erupt. They want to know how long it has been since it last erupted. They also want to know the time span between its previous eruptions. Volcanoes can be active, dormant, or extinct (Figure 8.12). An active volcano may be currently erupting. Alter- natively, it may be showing signs that it will erupt in the near future. A dormant volcano no longer shows signs of activity. But it has erupted in recent history and will probably erupt again. An extinct volcano is one that has not erupted in recent history. Scientists think that it will probably not erupt again. Scientists watch both active and dormant volcanoes closely for signs that show they might erupt. "
volcanic eruptions,T_0852,Earthquakes may take place every day near a volcano. But before an eruption the number and size of earthquakes increases. This is the result of magma pushing upward into the magma chamber. This motion causes stresses on neighboring rock to build up. Eventually the ground shakes. A continuous string of earthquakes may indicate that a volcano is about to erupt. Scientists use seismographs to record the length and strength of each earthquake. 
volcanic eruptions,T_0853,"All that magma and gas pushing upwards can make the volcanos slope begin to swell. Ground swelling may change the shape of a volcano or cause rock falls and landslides. Most of the time, the ground tilting is not visible. Scientists detect it by using tiltmeters, which are instruments that measure the angle of the slope of a volcano. "
volcanic eruptions,T_0854,"Scientists measure the gases that escape from a volcano to predict eruptions. Gases like sulfur dioxide (SO2 ), carbon dioxide (CO2 ), hydrochloric acid (HCl), and water vapor can be measured at the site. Gases may also be measured from satellites. The amounts of gases and the ratios of gases are calculated to help predict eruptions. "
volcanic eruptions,T_0855,Satellites can be used to monitor more than just gases (Figure 8.13). Satellites can look for high temperature spots or areas where the volcano surface is changing. This allows scientists to detect changes accurately and safely. 
volcanic eruptions,T_0856,"No scientist or government agency wants to announce an eruption and then be wrong. There is a very real cost and disruption to society during a large-scale evacuation. If the scientists are wrong, people would be less likely to evacuate the next time scientists predicted an eruption. But if scientists predict an eruption that does take place it could save many lives. "
types of volcanoes,T_0857,"A composite volcano forms the tall cone shape you usually think of when you think of a volcano. Shield volcanoes are huge, gently sloping volcanoes. Cinder cones are small, cone-shaped volcanoes. "
types of volcanoes,T_0858,"Figure 8.14 shows Mt. Fuji, a classic example of a composite volcano. Composite volcanoes have broad bases and steep sides. These volcanoes usually have a large crater at the top. The crater was created during the volcanos last eruption. Composite volcanoes are also called stratovolcanoes. This is because they are formed by alternating layers (strata) of magma and ash (Figure 8.15). The magma that creates composite volcanoes tends to be thick. The steep sides form because the lava cannot flow too far from the vent. The thick magma may also create explosive eruptions. Ash and pyroclasts erupt into the air. Much of this material falls back down near the vent. This creates the steep sides of stratovolcanoes. Composite volcanoes are common along convergent plate boundaries. When a tectonic plate subducts, it melts. This creates the thick magma needed for these eruptions. The Pacific Ring of Fire is dotted by composite volcanoes. "
types of volcanoes,T_0859,"Shield volcanoes look like a huge ancient warriors shield laid down. Figure 8.16 shows the Kilaeua Volcano. A shield volcano has a very wide base. It is much flatter on the top than a composite volcano. The lava that creates shield volcanoes is reltively thin. The thin lava spreads out. This builds a large, flat volcano layer by layer. Shield volcanoes are very large. For example, the Mauna Loa Volcano has a diameter of more than 112 kilometers (70 miles). The volcano forms a significant part of the island of Hawaii. The top of nearby Mauna Kea Volcano is more than ten kilometers (6 miles) from its base on the seafloor. Shield volcanoes often form along divergent plate boundaries. They also form at hot spots, like Hawaii. Shield volcano eruptions are non-explosive. "
types of volcanoes,T_0860,"Cinder cones are the smallest and most common type of volcano. Cinder cones have steep sides like composite volcanoes. But they are much smaller, rarely reaching even 300 meters in height. Cinder cones usually have a crater at the summit. Cinder cones are composed of small fragments of rock, called cinders. The cinders are piled on top of one another. These volcanoes usually do not produce streams of lava. Cinder cones often form near larger volcanoes. Most composite and shield volcanoes have nearby cinder cones. Cinder cones usually build up very rapidly. They only erupt for a short time. Many only produce one eruption. For this reason, cinder cones do not reach the sizes of stratovolcanoes or shield volcanoes (Figure 8.17). "
types of volcanoes,T_0861,"During a massive eruption all of the material may be ejected from a magma changer. Without support, the mountain above the empty chamber may collapse. This produces a huge caldera. Calderas are generally round, bowl-shaped formations like the picture in Figure 8.18. "
types of volcanoes,T_0862,"Supervolcanoes are the most dangerous type of volcano. During an eruption, enormous amounts of ash are thrown into the atmosphere. The ash encircles the globe. This blocks the Sun and lowers the temperature of the entire planet. The result is a volcanic winter. A supervolcano eruption took place at Lake Toba in northern Sumatra about 75,000 years ago (Figure 8.19). This was the largest eruption in the past 25 million years. As much as 2,800 cubic kilometers of material was ejected into the atmosphere. The result was a 6- to 10-year volcanic winter. Some scientists think that only 10,000 humans survived worldwide. The numbers of other mammals also plummeted. The most recent supervolcano eruption was in New Zealand. The eruption was less than 2000 years ago. For a supervolcano eruption it was small, about 100 cubic kilometers of material. A much larger super eruption in Colorado produced over 5,000 cubic kilometers of material. That eruption was 28 million years ago. It was 5000 times larger than the 1980 Mount St. Helens eruption. The largest potentially active supervolcano in North America is Yellowstone. The caldera has had three super eruptions at 2.1 million, 1.3 million and 640,000 years ago. The floor of the Yellowstone caldera is slowly rising upwards. Another eruption is very likely but no one knows when. The cause of supervolcano eruptions is being debated. Enormous magma chambers are filled with super hot magma. This enormous eruption leaves a huge hole. The ground collapses and creates a caldera. "
soils,T_0882,"We can think about soil as a living resource. Soil is an ecosystem all by itself! Soil is a complex mixture of different materials. Some of them are inorganic. Inorganic materials are made from non-living substances like pebbles and sand. Soil also contains bits of organic materials from plants and animals. In general, about half of the soil is made of pieces of rock and minerals. The other half is organic materials. In the spaces of soil are millions of living organisms. These include earthworms, ants, bacteria, and fungi. In some soils, the organic portion is entirely missing. This is true of desert sand. At the other extreme, a soil may be completely organic. Peat, found in a bog or swamp, is totally organic soil. Organic materials are necessary for a soil to be fertile. The organic portion provides the nutrients needed for strong plant growth. "
soils,T_0883,"Soil formation requires weathering. Where there is less weathering, soils are thinner. However, soluble minerals may be present. Where there is intense weathering, soils may be thick. Minerals and nutrients would have been washed out. Soil development takes a very long time. It may take hundreds or even thousands of years to form the fertile upper layer of soil. Soil scientists estimate that in the very best soil forming conditions, soil forms at a rate of about 1mm/year. In poor conditions, it may take thousands of years! How well soil forms and what type of soil forms depends on many factors. These include climate, the original rock type, the slope, the amount of time, and biological activity. "
soils,T_0884,"Climate is the most important factor in soil formation. The climate of a region is the result of its temperature and rainfall. We can identify different climates by the plants that grow there (Figure 9.6). Given enough time, a climate will produce a particular type of soil. The original rock type does not matter. The same rock type will form a different soil type in each different climate. Rainfall Rainfall in an area is important because it influences the rate of weathering. More rain means that more rainwater passes through the soil. The rainwater reacts chemically with the particles. The top layers of soil are in contact with the freshest water, so reactions are greatest there. High rainfall increases the amount of rock that experiences chemical reactions. High rainfall may also carry material away. This means that new surfaces are exposed. This increases the rate of weathering. Temperature The temperature of a region is the other important part of climate. The rate of chemical reactions increases with higher temperatures. The rate doubles for every 10 C increase in temperature. Plants and bacteria grow and multiply faster in warmer areas. "
soils,T_0885,"Soil formation increases with time. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration. The warmer the temperatures, the more rainfall, and the greater the amount of time, the thicker the soils will become. "
soils,T_0886,"The original rock is the source of the inorganic portion of the soil. Mechanical weathering breaks rock into smaller pieces. Chemical reactions change the rocks minerals. A transported soil forms from materials brought in from somewhere else. These soils form from sediments that were transported into the area and deposited. The rate of soil formation is faster for transported materials because they have already been weathered. A soil is a residual soil when it forms in place. Only about one third of the soils in the United States form this way. The material comes from the underlying bedrock. Residual soils form over many years since it takes a long time for solid rock to become soil. First, cracks break up the bedrock. This may happen due to ice wedging. Weathering breaks up the rock even more. Then plants, such as lichens or grasses, become established. They cause further weathering. As more time passes and more layers of material weather, the soil develops. "
soils,T_0887,"Biological activity produces the organic material in soil. Humus forms from the remains of plants and animals. It is an extremely important part of the soil. Humus coats the mineral grains. It binds them together into clumps that hold the soil together. This gives the soil its structure. Soils with high humus are better able to hold water. Soils rich with organic materials hold nutrients better and are more fertile. These soils are more easily farmed. The color of soil indicates its fertility. Black or dark brown soils are rich in nitrogen and contain a high percentage of organic materials. Soils that are nitrogen poor and low in organic material might be gray, yellow, or red. "
soils,T_0888,"The inorganic part of soil is made of different amounts of different size particles. This affects the characteristics of a soil. Water flows through soil more easily if the spaces between the particles are large enough and well connected. Sandy or silty soils are light soils because they drain water. Soils rich in clay are heavier. Clay particles allow only very small spaces between them, so clay-rich soils tend to hold water. Clay-rich soils are heavier and hold together more tightly. A soil that contains a mixture of grain sizes is called a loam. Soil scientists measure the percentage of sand, silt, and clay in soil. They plot this information on a triangular diagram, with each type of particle at one corner (Figure 9.7). The soil type is determined by where the soil falls on the diagram. At the top, the soil is clay rich. On the left corner, the soil is sandy. On the right corner, the soil is silty. "
soils,T_0889,"Soil develops over time and forms soil horizons. Soil horizons are different layers of soil with depth. The most weathering occurs in the top layer. This layer is most exposed to weather! It is where fresh water comes into contact with the soil. Each layer lower is weathered just a little bit less than the layer above. As water moves down through the layers, it is able to do less work to change the soil. If you dig a deep hole in the ground, you may see each of the different layers of soil. All together, the layers are a soil profile. Each horizon has its own set of characteristics (Figure 9.8). In the simplest soil profile, a soil has three horizons. "
soils,T_0890,The first horizon is the A horizon. It is more commonly called the topsoil. The topsoil is usually the darkest layer of the soil. It is the layer with the most organic material. Humus forms from all the plant and animal debris that falls to or grows on the ground. The topsoil is also the region with the most biological activity. Many organisms live within this layer. Plant roots stretch down into this layer. The roots help to hold the topsoil in place. Topsoil usually does not have very small particles like clay. Clay-sized particles are carried to lower layers as water seeps down into the ground. Many minerals dissolve in the fresh water that moves through the topsoil. These minerals are carried down to the lower layers of soil. 
soils,T_0891,"Below the topsoil is the B horizon. This is also called the subsoil. Soluble minerals and clays accumulate in the subsoil. Because it has less organic material, this layer is lighter brown in color than topsoil. It also holds more water due to the presence of iron and clay. There is less organic material in this layer. "
soils,T_0892,"The next layer down is the C horizon. This layer is made of partially altered bedrock. There is evidence of weathering in this layer. Still, it is possible to identify the original rock type from which this soil formed (Figure Not all climate regions develop soils. Arid regions are poor at soil development. Not all regions develop the same soil horizons. Some areas develop as many as five or six distinct layers. Others develop only a few. "
soils,T_0893,"For soil scientists, there are thousands of types of soil! Soil scientists put soils into very specific groups with certain characteristics. Each soil type has its own name. Lets consider a much simpler model, with just three types of soil. These types are based on climate. Just remember that there are many more than just these three types. "
soils,T_0894,"One important type of soil forms in a deciduous forest. In these forests, trees lose their leaves each winter. Deciduous trees need lots of rain  at least 65 cm of rainfall per year. Deciduous forests are common in the temperate, eastern United States. The type of soil found in a deciduous forest is a pedalfer (Figure 9.10). This type of soil is usually dark brown or black in color and very fertile. "
soils,T_0895,"Pedocal soil forms where grasses and brush are common (Figure 9.11). The climate is drier, with less than 65 cm of rain per year. With less rain, there is less chemical weathering. There is less organic material and the soils are slightly less fertile. "
soils,T_0896,"A third important type of soil is laterite. Laterite forms in tropical areas. Temperatures are warm and rain falls every day (Figure 9.12). So much rain falls that chemical weathering is intense. All soluble minerals are washed from the soil. Plant nutrients get leached or carried away. There is practically no humus. Laterite soils are often red in color from the iron oxides. If laterites are exposed to the Sun, they bake as hard as a brick. "
soils,T_0897,"Soil is a renewable resource. But it is only renewable if we take care of it. Natural events can degrade soil. These events include droughts, floods, insect plagues, or diseases that damage soil ecosystems. Human activities can also degrade soil. There are many ways in which people neglect or abuse this important resource. "
soils,T_0898,"People remove a lot of vegetation. They log forests or prepare the land for farming or construction. Even just walking or riding your bike over the same place can kill the grass. But plants help to hold the soil in place (Figure faster than it is forming. In these locations, soil is a non-renewable resource. Soils may also remain in place but become degraded. Soil is contaminated if too much salt accumulates. Soil can also be contaminated by pollutants. "
soils,T_0899,"There are many ways to protect soil. We can add organic material like manure or compost. This increases the soils fertility. Increased fertility improves the soils ability to hold water and nutrients. Inorganic fertilizers also increase fertility. These fertilizers are less expensive than natural fertilizers, but they do not provide the same long term benefits. Careful farming helps to keep up soil quality each season. One way is to plant different crops each year. Another is to alternate the crops planted in each row of the field. These techniques preserve and replenish soil nutrients. Planting nutrient rich cover crops helps the soil. Planting trees as windbreaks, plowing along contours of a field, or building terraces into steeper slopes all help to hold soil in place (Figure 9.14). No-till or low-till farming disturbs the ground as little as possible during planting. "
avoiding soil loss,T_0935,"Bad farming practices and a return to normal rainfall levels after an unusually wet period led to the Dust Bowl. In some regions more than 75% of the topsoil blew away. This is the most extreme example of soil erosion the United States has ever seen. Still, in many areas of the world, the rate of soil erosion is many times greater than the rate at which it is forming. Drought, insect plagues, or outbreaks of disease are natural cycles of events that can negatively impact ecosystems and the soil, but there are also many ways in which humans neglect or abuse this important resource. Soils can also be contaminated if too much salt accumulates in the soil or where pollutants sink into the ground. One harmful practice is removing the vegetation that helps to hold soil in place. Sometimes just walking or riding your bike over the same place will kill the grass that normally grows there. Land is also deliberately cleared or deforested for wood. The loose soils then may be carried away by wind or running water. A farmer and his sons walk through a dust storm in Cimarron County, Oklahoma in 1936. Click image to the left or use the URL below. URL: "
avoiding soil loss,T_0936,Soil is only a renewable resource if it is carefully managed. There are many practices that can protect and preserve soil resources. 
avoiding soil loss,T_0937,"Adding organic material to the soil in the form of plant or animal waste, such as compost or manure, increases the fertility of the soil and improves its ability to hold on to water and nutrients (Figure 1.2). Inorganic fertilizer can also temporarily increase the fertility of a soil and may be less expensive or time consuming, but it does not provide the same long-term improvements as organic materials. "
avoiding soil loss,T_0938,"Soil is a natural resource that is vitally important for sustaining natural habitats and for growing food. Although soil is a renewable resource, it is renewed slowly, taking hundreds or thousands of years for a good fertile soil to develop. Organic material can be added to soil to help increase its fertility. Most of the best land for farming is already being cultivated. With human populations continuing to grow, it is extremely important to protect our soil resources. Agricultural practices such as rotating crops, alternating the types of crops planted in each row, and planting nutrient-rich cover crops all help to keep soil more fertile as it is used season after season. Planting trees as windbreaks, plowing along contours of the field, or building terraces into steeper slopes will all help to hold soil in place (Figure 1.3). No-till or low-tillage farming helps to keep soil in place by disturbing the ground as little as possible when planting. Steep slopes can be terraced to make level planting areas and decrease surface water runoff and erosion. The rate of topsoil loss in the United States and other developed countries has decreased recently as better farming practices have been adopted. Unfortunately, in developing nations, soil is often not protected. Table 1.1 shows some steps that we can take to prevent erosion. Some are things that can be done by farmers or developers. Others are things that individual homeowners or community members can implement locally. Source of Erosion Strategies for Prevention Leave leaf litter on the ground in the winter. Grow cover crops, special crops grown in the winter to cover the soil. Plant tall trees around fields to buffer the effects of wind. Drive tractors as little as possible. Use drip irrigation that puts small amounts of water in the ground frequently. Avoid watering crops with sprinklers that make big water drops on the ground. Keep fields as flat as possible to avoid soil erod- ing down hill. Grazing Animals Move animals throughout the year, so they dont consume all the vegetation in one spot. Keep animals away from stream banks, where hills are especially prone to erosion. Logging and Mining Reduce the amount of land that is logged and mined. Reduce the number of roads that are built to access logging areas. Avoid logging and mining on steep lands. Cut only small areas at one time and quickly replant logged areas with new seedlings. Development Reduce the amount of land area that is developed into urban areas, parking lots, etc. Keep as much green space in cities as possible, such as parks or strips where plants can grow. Invest in and use new technologies for parking lots that make them permeable to water in order to reduce runoff of water. Recreational Activities Avoid using off-road vehicles on hilly lands. Stay on designated trails. Avoid building on steep hills. Grade surrounding land to distribute water rather than collecting it in one place. Where water collects, drain to creeks and rivers. Landscape with plants that minimize erosion. Click image to the left or use the URL below. URL: "
cenozoic plate tectonics,T_0971,"The Cenozoic began around 65.5 million years ago and continues today. Although it accounts for only about 1.5% of the Earths total history, as the most recent era it is the one scientists know the most about. Much of what has been discussed elsewhere in CK-12 Earth Science Concepts For High School describes the geological situation of the Cenozoic. A few highlights are mentioned here. "
cenozoic plate tectonics,T_0972,"The paleogeography of the era was very much like it is today. Early in the Cenozoic, blocks of crust uplifted to form the Rocky Mountains, which were later eroded away and then uplifted again. Subduction off of the Pacific Northwest formed the Cascades volcanic arc. The Basin and Range province that centers on Nevada is where crust is being pulled apart. "
cenozoic plate tectonics,T_0973,"The San Andreas Fault has grown where the Pacific and North American plates meet. The plate tectonic evolution of that plate boundary is complex and interesting (Figure 1.1). The Farallon Plate was subducting beneath the North American Plate 30 Ma. By 20 Ma the Pacific Plate and East Pacific Rise spreading center had started to subduct, splitting the Farallon Plate into two smaller plates. Transform motion where the Pacific and North American plates meet formed the San Andreas Fault. The fault moved inland and at present small sea floor spreading basins along with the transform motion of the San Andreas are splitting Baja California from mainland Mexico. This figure shows the evolution of the San Andreas Fault zone from 30 million years ago (bottom) to present (top). Although most plate tectonic activity involves continents moving apart, smaller regions are coming together. Africa collided with Eurasia to create the Alps. India crashed into Asia to form the Himalayas. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
cenozoic plate tectonics,T_0974,"As the continents moved apart, climate began to cool. When Australia and Antarctica separated, the Antarctic Circumpolar Current could then move the frigid water around Antarctica and spread it more widely around the planet. Antarctica drifted over the south polar region and the continent began to grow a permanent ice cap in the Oligocene. The climate warmed in the early Miocene but then began to cool again in the late Miocene and Pliocene when glaciers began to form. During the Pleistocene ice ages, which began 2.6 million years ago, glaciers advanced and retreated four times (Figure 1.2). During the retreats, the climate was often warmer than it is today. Glacial ice at its maximum during the Pleistocene. These continental ice sheets were extremely thick, like the Antarctic ice cap is today. The Pleistocene ice ages guided the evolution of life in the Cenozoic, including the evolution of humans. "
chemical weathering,T_0981,"Chemical weathering is the other important type of weathering. Chemical weathering may change the size of pieces of rock materials, but definitely changes the composition. So one type of mineral changes into a different mineral. Chemical weathering works through chemical reactions that cause changes in the minerals. "
chemical weathering,T_0982,"Most minerals form at high pressure or high temperatures deep in the crust, or sometimes in the mantle. When these rocks are uplifed onto Earths surface, they are at very low temperatures and pressures. This is a very different environment from the one in which they formed and the minerals are no longer stable. In chemical weathering, minerals that were stable inside the crust must change to minerals that are stable at Earths surface. "
chemical weathering,T_0983,Remember that the most common minerals in Earths crust are the silicate minerals. Many silicate minerals form in igneous or metamorphic rocks. The minerals that form at the highest temperatures and pressures are the least stable at the surface. Clay is stable at the surface and chemical weathering converts many minerals to clay (Figure 1.1). There are many types of chemical weathering because there are many agents of chemical weathering. Deforestation in Brazil reveals the under- lying clay-rich soil. 
chemical weathering,T_0984,"A water molecule has a very simple chemical formula, H2 O, two hydrogen atoms bonded to one oxygen atom. But water is pretty remarkable in terms of all the things it can do. Remember that water is a polar molecule. The positive side of the molecule attracts negative ions and the negative side attracts positive ions. So water molecules separate the ions from their compounds and surround them. Water can completely dissolve some minerals, such as salt. Weathered rock in Walnut Canyon near Flagstaff, Arizona. Hydrolysis is the name of the chemical reaction between a chemical compound and water. When this reaction takes place, water dissolves ions from the mineral and carries them away. These elements have been leached. Through hydrolysis, a mineral such as potassium feldspar is leached of potassium and changed into a clay mineral. Clay minerals are more stable at the Earths surface. "
chemical weathering,T_0985,"Carbon dioxide (CO2 ) combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. Carbonic acid is a very common in nature, where it works to dissolve rock. Pollutants, such as sulfur and nitrogen from fossil fuel burning, create sulfuric and nitric acid. Sulfuric and nitric acids are the two main components of acid rain, which accelerates chemical weathering (Figure 1.3). Acid rain is discussed in the chapter Human Impacts on Earths Systems. This statue at Washington Square Arch in New York City exhibits damage from acid rain. "
chemical weathering,T_0986,Oxidation is a chemical reaction that takes place when oxygen reacts with another element. Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create rust (Figure 1.4). Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. Iron oxide produces the red color in soils. 
chemical weathering,T_0987,"Now that you know what chemical weathering is, can you think of some other ways chemical weathering might occur? Chemical weathering can also be contributed to by plants and animals. As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration. "
chemical weathering,T_0988,"Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases Figure 1.5. With more surfaces exposed, there are more surfaces on which chemical weathering can occur. Mechanical weathering may increase the rate of chemical weathering. Click image to the left or use the URL below. URL: "
clouds,T_1006,"Humidity is the amount of water vapor in the air in a particular spot. We usually use the term to mean relative humidity, the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain. If the humidity today is 80%, it means that the air contains 80% of the total amount of water it can hold at that temperature. What will happen if the humidity increases to more than 100%? The excess water condenses and forms precipitation. Since warm air can hold more water vapor than cool air, raising or lowering temperature can change airs relative humidity (Figure 1.1). The temperature at which air becomes saturated with water is called the airs dew point. This term makes sense, because water condenses from the air as dew if the air cools down overnight and reaches 100% humidity. This diagram shows the amount of water air can hold at different temperatures. The temperatures are given in degrees Cel- sius. "
clouds,T_1007,"Water vapor is not visible unless it condenses to become a cloud. Water vapor condenses around a nucleus, such as dust, smoke, or a salt crystal. This forms a tiny liquid droplet. Billions of these water droplets together make a cloud. "
clouds,T_1008,"Clouds form when air reaches its dew point. This can happen in two ways: (1) Air temperature stays the same but humidity increases. This is common in locations that are warm and humid. (2) Humidity remains the same, but temperature decreases. When the air cools enough to reach 100% humidity, water droplets form. Air cools when it comes into contact with a cold surface or when it rises. Rising air creates clouds when it has been warmed at or near the ground level and then is pushed up over a mountain or mountain range or is thrust over a mass of cold, dense air. Click image to the left or use the URL below. URL: "
clouds,T_1009,"Clouds have a big influence on weather: by preventing solar radiation from reaching the ground. by absorbing warmth that is re-emitted from the ground. as the source of precipitation. When there are no clouds, there is less insulation. As a result, cloudless days can be extremely hot, and cloudless nights can be very cold. For this reason, cloudy days tend to have a lower range of temperatures than clear days. "
clouds,T_1010,"Clouds are classified in several ways. The most common classification used today divides clouds into four separate cloud groups, which are determined by their altitude (Figure 1.2). The four cloud types and where they are found in the atmosphere. High clouds form from ice crystals where the air is extremely cold and can hold little water vapor. Cirrus, cirrostratus, and cirrocumulus are all names of high clouds. Middle clouds, including altocumulus and altostratus clouds, may be made of water droplets, ice crystals or both, depending on the air temperatures. Thick and broad altostratus clouds are gray or blue-gray. They often cover the entire sky and usually mean a large storm, bearing a lot of precipitation, is coming. Low clouds are nearly all water droplets. Stratus, stratocumulus, and nimbostratus clouds are common low clouds. Nimbostratus clouds are thick and dark. They bring steady rain or snow. Vertical clouds, clouds with the prefix ""cumulo-,"" grow vertically instead of horizontally and have their bases at low altitude and their tops at high or middle altitude. Clouds grow vertically when strong air currents are rising upward. Precipitating clouds are nimbus clouds. "
clouds,T_1011,"Fog (Figure 1.3) is a cloud located at or near the ground . When humid air near the ground cools below its dew point, fog is formed. Each type of fog forms in a different way. Radiation fog forms at night when skies are clear and the relative humidity is high. As the ground cools, the bottom layer of air cools below its dew point. Tule fog is an extreme form of radiation fog found in some regions. San Francisco, California, is famous for its summertime advection fog. Warm, moist Pacific Ocean air blows over the cold California current and cools below its dew point. Sea breezes bring the fog onshore. Steam fog appears in autumn when cool air moves over a warm lake. Water evaporates from the lake surface and condenses as it cools, appearing like steam. Warm humid air travels up a hillside and cools below its dew point to create upslope fog. (a) Tule fog in the Central Valley of California. (b) Advection fog in San Francisco. (c) Steam fog over a lake. (d) Upslope fog in Terespolis city, Rio de Janeiro State, Brazil. Fog levels are declining along the California coast as climate warms. The change in fog may have big ecological changes for the state. Click image to the left or use the URL below. URL: "
composition of the atmosphere,T_1027,"Several properties of the atmosphere change with altitude, but the composition of the natural gases does not. The proportions of gases in the atmosphere are everywhere the same, with one exception. At about 20 km to 40 km above the surface, there is a greater concentration of ozone molecules than in other portions of the atmosphere. This is called the ozone layer. "
composition of the atmosphere,T_1028,"Nitrogen and oxygen together make up 99% of the planets atmosphere. Nitrogen makes up the bulk of the atmosphere, but is not involved in geological or biological processes in its gaseous form. Nitrogen fixing is described in the chapter Life on Earth. Oxygen is extremely important because it is needed by animals for respiration. The rest of the gases are minor components but sometimes are very important (Figure 1.1). Nitrogen and oxygen make up 99% of the atmosphere; carbon dioxide is a very important minor component. "
composition of the atmosphere,T_1029,"Humidity is the amount of water vapor in the air. Humidity varies from place to place and season to season. This fact is obvious if you compare a summer day in Atlanta, Georgia, where humidity is high, with a winter day in Phoenix, Arizona, where humidity is low. When the air is very humid, it feels heavy or sticky. Dry air usually feels more comfortable. When humidity is high, water vapor makes up only about 4% of the atmosphere. Where around the globe is mean atmospheric water vapor higher and where is it lower (Figure 1.2)? Why? Higher humidity is found around the equatorial regions because air temperatures are higher and warm air can hold more moisture than cooler air. Of course, humidity is lower near the polar regions because air temperature is lower. "
composition of the atmosphere,T_1030,"Remember that greenhouse gases trap heat in the atmosphere. Important natural greenhouse gases include carbon dioxide, methane, water vapor, and ozone. CFCs and some other man-made compounds are also greenhouse gases. "
composition of the atmosphere,T_1031,"Some of what is in the atmosphere is not gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash, and other solids make up a small percentage of the atmosphere and are called particulates. Particles provide starting points (or nuclei) for water vapor to condense on and form raindrops. Some particles are pollutants. Click image to the left or use the URL below. URL: "
dark matter,T_1041,"The things we observe in space are objects that emit some type of electromagnetic radiation. However, scientists think that matter that emits light makes up only a small part of the matter in the universe. The rest of the matter, about 80%, is dark matter. Dark matter emits no electromagnetic radiation, so we cant observe it directly. However, astronomers know that dark matter exists because its gravity affects the motion of objects around it. When astronomers measure how spiral galaxies rotate, they find that the outside edges of a galaxy rotate at the same speed as parts closer to the center. This can only be explained if there is a lot more matter in the galaxy than they can see. Gravitational lensing occurs when light is bent from a very distant bright source around a super-massive object (Figure 1.1). To explain strong gravitational lensing, more matter than is observed must be present. With so little to go on, astronomers dont really know much about the nature of dark matter. One possibility is that it could just be ordinary matter that does not emit radiation in objects such as black holes, neutron stars, and brown dwarfs  objects larger than Jupiter but smaller than the smallest stars. But astronomers cannot find enough of these types of objects, which they have named MACHOs (massive astrophyiscal compact halo object), to account for all the dark matter, so they are thought to be only a small part of the total. Another possibility is that the dark matter is very different from the ordinary matter we see. Some appear to be particles that have gravity, but dont otherwise appear to interact with other particles. Scientists call these theoretical particles WIMPs, which stands for Weakly Interactive Massive Particles. Most scientists who study dark matter think that the dark matter in the universe is a combination of MACHOs and some type of exotic matter, such as WIMPs. Researching dark matter is an active area of scientific research, and astronomers knowledge about dark matter is changing rapidly. "
dark matter,T_1042,"Astronomers who study the expansion of the universe are interested in knowing the rate of that expansion. Is the rate fast enough to overcome the attractive pull of gravity? If yes, then the universe will expand forever, although the expansion will slow down over time. If no, then the universe would someday start to contract, and eventually get squeezed together in a big crunch, the opposite of the Big Bang. Recently, astronomers have made a discovery that answers that question: the rate at which the universe is expanding is actually increasing. In other words, the universe is expanding faster now than ever before, and in the future it will expand even faster. So now astronomers think that the universe will keep expanding forever. But it also proposes a perplexing new question: what is causing the expansion of the universe to accelerate? One possible hypothesis involves a new, hypothetical form of energy called dark energy (Figure 1.2). Some scientists think that dark energy makes up as much as 71% of the total energy content of the universe. Today matter makes up a small percentage of the universe, but at the start of the universe it made up much more. Where did dark energy, if it even exists, come from? Other scientists have other hypotheses about why the universe is continuing to expand; the causes of the universes expansion is another unanswered question that scientists are researching. Click image to the left or use the URL below. URL: "
dark matter,T_1043,"Meet one of the three winners of the 2011 Nobel Prize in Physics, Lawrence Berkeley Lab astrophysicist Saul Perlmutter. He explains how dark energy, which makes up 70 percent of the universe, is causing our universe to expand. Click image to the left or use the URL below. URL: "
divergent plate boundaries,T_1057,"Were on a new trip now. We will start in Mexico, in the region surrounding the Gulf of California, where a divergent plate boundary is rifting Baja California and mainland Mexico apart. Then we will move up into California, where plates on both sides of a transform boundary are sliding past each other. Finally well end up off of the Pacific Northwest, where a divergent plate boundary is very near a subduction zone just offshore. In the Figure 1.1 a red bar where seafloor spreading is taking place. A long black line is a transform fault and a black line with hatch marks is a trench where subduction is taking place. Notice how one type of plate boundary transitions into another. "
divergent plate boundaries,T_1058,"A divergent plate boundary on land rips apart continents (Figure 1.2). In continental rifting, magma rises beneath the continent, causing it to become thinner, break, and ultimately split apart. New ocean crust erupts in the void, ultimately creating an ocean between continents. On either side of the ocean are now two different lithospheric plates. This is how continents split apart. These features are well displayed in the East African Rift, where rifting has begun, and in the Red Sea, where water is filling up the basin created by seafloor spreading. The Atlantic Ocean is the final stage, where rifting is now separating two plates of oceanic crust. "
divergent plate boundaries,T_1059,"Baja California is a state in Mexico just south of California. In the Figure 1.3, Baja California is the long, skinny land mass on the left. You can see that the Pacific Ocean is growing in between Baja California and mainland Mexico. This body of water is called the Gulf of California or, more romantically, the Sea of Cortez. Baja is on the Pacific Plate and the rest of Mexico is on the North American Plate. Extension is causing the two plates to move apart and will eventually break Baja and the westernmost part of California off of North America. The Gulf of California will expand into a larger sea. Rifting has caused volcanic activity on the Baja California peninsula as seen in the Figure 1.4. Can you relate what is happening at this plate boundary to what happened when Pangaea broke apart? "
divergent plate boundaries in the oceans,T_1060,"Iceland provides us with a fabulous view of a mid-ocean ridge above sea level (Figure 1.1) As you can see, where plates diverge at a mid-ocean ridge is a rift valley that marks the boundary between the two plates. Basalt lava erupts into that rift valley and forms new seafloor. Seafloor on one side of the rift is part of one plate and seafloor on the other side is part of another plate. Leif the Lucky Bridge straddles the divergent plate boundary. Look back at the photo at the top. You may think that the rock on the left side of the valley looks pretty much like the rock on the right side. Thats true - its all basalt and it even all has the same magnetic polarity. The rocks on both sides are extremely young. Whats different is that the rock one side of the bridge is the youngest rock of the North American Plate while the rock on the other side is the youngest rock on the Eurasian plate. This is a block diagram of a divergent plate boundary. Remember that most of these are on the seafloor and only in Iceland do we get such a good view of a divergent plate boundary in the ocean. "
divergent plate boundaries in the oceans,T_1061,"Remember that the mid-ocean ridge is where hot mantle material upwells in a convection cell. The upwelling mantle melts due to pressure release to form lava. Lava flows at the surface cool rapidly to become basalt, but deeper in the crust, magma cools more slowly to form gabbro. The entire ridge system is made up of igneous rock that is either extrusive or intrusive. The seafloor is also igneous rock with some sediment that has fallen onto it. Earthquakes are common at mid-ocean ridges since the movement of magma and oceanic crust results in crustal shaking. Click image to the left or use the URL below. URL: "
earthquake characteristics,T_1080,"An earthquake is sudden ground movement caused by the sudden release of energy stored in rocks. Earthquakes happen when so much stress builds up in the rocks that the rocks rupture. The energy is transmitted by seismic waves. Earthquakes can be so small they go completely unnoticed, or so large that it can take years for a region to recover. "
earthquake characteristics,T_1081,"The description of how earthquakes occur is called elastic rebound theory (Figure 1.1). Elastic rebound theory. Stresses build on both sides of a fault, causing the rocks to deform plastically (Time 2). When the stresses become too great, the rocks break and end up in a different location (Time 3). This releases the built up energy and creates an earthquake. Click image to the left or use the URL below. URL: "
earthquake characteristics,T_1082,"In an earthquake, the initial point where the rocks rupture in the crust is called the focus. The epicenter is the point on the land surface that is directly above the focus (Figure 1.2). In the vertical cross section of crust, there are two features labeled - the focus and the epicenter, which is directly above the focus. Click image to the left or use the URL below. URL: "
earthquake damage,T_1083,"We know that earthquakes kill lots of people. However, the ground shaking almost never kills people, and the ground does not swallow someone up. Fatalities depend somewhat on an earthquakes size and the type of ground people inhabit. But much of what determines the number of fatalities depends on the quality of structures. People are killed when structures fall on them. More damage is done and more people are killed by the fires that follow an earthquake than the earthquake itself. "
earthquake damage,T_1084,"Population density. The magnitude 9.2 Great Alaska Earthquake, near Anchorage, of 1964 resulted in only 131 deaths. At the time few people lived in the area (Figure 1.1). Not size. Only about 2,000 people died in the 1960 Great Chilean earthquake, the largest earthquake ever recorded. The Indian Ocean earthquake of 2004 was one of the largest ever, but most of the 230,000 fatalities were caused by the tsunami, not the earthquake itself. Ground type. Solid bedrock vibrates less than soft sediments, so there is less damage on bedrock. Sediments that are saturated with water undergo liquefaction and become like quicksand (Figure 1.2). Soil on a hillside may become a landslide. Liquefaction of sediments in Mexico City caused the collapse of many buildings in the 1985 earthquake. "
earthquake damage,T_1085,"In earthquake-prone areas, city planners try to reduce hazards. For example, in the San Francisco Bay Area, maps show how much shaking is expected for different ground types (Figure 1.3). This allows planners to locate new hospitals and schools more safely. The expected Modified Mercalli Intensity Scale for an earthquake of magnitude 7.1 on the northern portion of the Hayward Fault. Click image to the left or use the URL below. URL: "
earthquake safe structures,T_1086,"New construction can be made safer in many ways: Skyscrapers and other large structures built on soft ground must be anchored to bedrock, even if it lies hundreds of meters below the ground surface. The correct building materials must be used. Houses should bend and sway. Wood and steel are better than brick, stone, and adobe, which are brittle and will break. Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagonal steel beams are used to hold down sway. Large buildings can be placed on rollers so that they move with the ground. Buildings may be placed on layers of steel and rubber to absorb the shock of the waves. Connections, such as where the walls meet the foundation, must be made strong. In a multi-story building, the first story must be well supported (Figure 1.1). The first floor of this San Francisco build- ing is collapsing after the 1989 Loma Pri- eta earthquake. "
earthquake safe structures,T_1087,"To make older buildings more earthquake safe, retrofitting with steel or wood can reinforce a buildings structure and its connections. Elevated freeways and bridges can also be retrofitted so that they do not collapse. Steel trusses were built diagonally and horizontally across windows to retrofit a building at Stanford University in Palo Alto, California. The San Andreas Fault passes just west of the university. "
earthquake safe structures,T_1088,"Fires often cause more damage than the earthquake. Fires start because seismic waves rupture gas and electrical lines, and breaks in water mains make it difficult to fight the fires (Figure 1.3). Builders zigzag pipes so that they bend and flex when the ground shakes. In San Francisco, water and gas pipelines are separated by valves so that areas can be isolated if one segment breaks. "
earthquake safe structures,T_1089,"Why arent all structures in earthquakes zones constructed for maximum safety? Cost, of course. More sturdy structures are much more expensive to build. So communities must weigh how great the hazard is, what different In the 1906 San Francisco earthquake, fire was much more destructive than the ground shaking. building strategies cost, and make an informed decision. In 1868 marked the Hayward Fault erupted in what would be a disastrous earthquake today. Since the fault erupts every 140 years on average, East Bay residents and geologists are working to prepare for the inevitable event. Click image to the left or use the URL below. URL: "
earthquake zones,T_1090,"In a single year, on average, more than 900,000 earthquakes are recorded and 150,000 of them are strong enough to be felt. Each year about 18 earthquakes are major, with a Richter magnitude of 7.0 to 7.9, and on average one earthquake has a magnitude of 8 to 8.9. Magnitude 9 earthquakes are rare. The United States Geological Survey lists five since 1900 (see Figure 1.1 and Table 1.1). All but the Great Indian Ocean Earthquake of 2004 occurred somewhere around the Pacific Ocean basin. Location Valdivia, Chile Prince William Sound, Alaska Great Indian Ocean Earthquake Kamchatka, Alaska Tohoku, Japan Year 1960 1964 2004 1952 2011 Magnitude 9.5 9.2 9.1 9.0 9.0 The 1964 Good Friday Earthquake cen- tered in Prince William Sound, Alaska re- leased the second most amount of energy of any earthquake in recorded history. "
earthquake zones,T_1091,"Nearly 95% of all earthquakes take place along one of the three types of plate boundaries. About 80% of all earthquakes strike around the Pacific Ocean basin because it is lined with convergent and transform boundaries (Figure 1.2). About 15% take place in the Mediterranean-Asiatic Belt, where convergence is causing the Indian Plate to run into the Eurasian Plate. The remaining 5% are scattered around other plate boundaries or are intraplate earthquakes. Earthquake epicenters for magnitude 8.0 and greater events since 1900. The earthquake depth shows that most large quakes are shallow focus, but some sub- ducted plates cause deep focus quakes. "
earthquakes at convergent plate boundaries,T_1092,Earthquakes at convergent plate boundaries mark the motions of subducting lithosphere as it plunges through the mantle (Figure 1.1). Eventually the plate heats up enough deform plastically and earthquakes stop. Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin. 
earthquakes at convergent plate boundaries,T_1093,"Earthquakes in Japan are caused by ocean-ocean convergence. The Philippine Plate and the Pacific Plate subduct beneath oceanic crust on the North American or Eurasian plates. This complex plate tectonics situation creates a chain of volcanoes, the Japanese islands, and as many as 1,500 earthquakes annually. In March 2011 an enormous 9.0 earthquake struck off of Sendai in northeastern Japan. This quake, called the 2011 Tohoku earthquake, was the most powerful ever to strike Japan and one of the top five known in the world. Damage from the earthquake was nearly overshadowed by the tsunami it generated, which wiped out coastal cities and towns This cross section of earthquake epicen- ters with depth outlines the subducting plate with shallow, intermediate, and deep earthquakes. (Figure 1.2). Several months after the earthquake, about 22,000 people were dead or missing, and 190,000 buildings had been damaged or destroyed. Aftershocks, some as large as major earthquakes, have continued to rock the region. Destruction in Ofunato, Japan, from the 2011 Tohoku Earthquake. "
earthquakes at convergent plate boundaries,T_1094,"The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time. The subduction of three small plates beneath North America produces active volcanoes, the Cascades. As with an active subduction zone, there are also earthquakes. Surprisingly, large earthquakes only hit every 300 to 600 years. The last was in 1700, with an estimated magnitude of around 9. A quake of that magnitude today could produce an incredible amount of destruction and untold fatalities. "
earthquakes at convergent plate boundaries,T_1095,Massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge (Figure injured or homeless. Damage from the 2005 Kashmir earth- quake. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: 
earthquakes at transform plate boundaries,T_1096,Deadly earthquakes occur at transform plate boundaries. Transform faults have shallow focus earthquakes. Why do you think this is so? 
earthquakes at transform plate boundaries,T_1097,"As you learned in the chapter Plate Tectonics, the boundary between the Pacific and North American plates runs through much of California as the San Andreas Fault zone. As you can see in the (Figure 1.1), there is more than just one fault running through the area. There is really a fault zone. The San Andreas Fault runs from south to north up the peninsula, through San Francisco, gets through part of Marin north of the bay, and then goes out to sea. The other faults are part of the fault zone, and they too can be deadly. The faults along the San Andreas Fault zone produce around 10,000 earthquakes a year. Most are tiny, but occasion- ally one is massive. In the San Francisco Bay Area, the Hayward Fault was the site of a magnitude 7.0 earthquake in 1868. The 1906 quake on the San Andreas Fault had a magnitude estimated at about 7.9 (Figure 1.1). About 3,000 people died and 28,000 buildings were lost, mostly in the fire that followed the earthquake. (a) The San Andreas Fault zone in the San Francisco Bay Area. (b) The 1906 San Francisco earthquake is still the most costly natural disaster in California history. Recent California earthquakes occurred in: 1989: Loma Prieta earthquake near Santa Cruz, California. Magnitude 7.1 quake, 63 deaths, 3,756 injuries, 12,000+ people homeless, property damage about $6 billion. 1994: Northridge earthquake on a blind thrust fault near Los Angeles. Magnitude 6.7, 72 deaths, 12,000 injuries, damage estimated at $12.5 billion. In this video, the boundaries between three different tectonic plates and the earthquakes that result from their interactions are explored. Click image to the left or use the URL below. URL: "
earthquakes at transform plate boundaries,T_1098,"New Zealand also has a transform fault with strike-slip motion, causing about 20,000 earthquakes a year! Only a small percentage of those are large enough to be felt. A 6.3 quake in Christchurch in February 2011 killed about 180 people. "
earths crust,T_1100,"Earths outer surface is its crust, a cold, thin, brittle outer shell made of rock. The crust is very thin relative to the radius of the planet. There are two very different types of crust, each with its own distinctive physical and chemical properties, which are summarized in Table 1.1. Crust Oceanic Continental Thickness 5-12 km (3-8 mi) Avg. 35 km (22 mi) Density 3.0 g/cm3 2.7 g/cm3 Composition Mafic Felsic Rock types Basalt and gabbro All types "
earths crust,T_1101,"Oceanic crust is composed of mafic magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro (Figure 1.1). Gabbro from ocean crust. The gabbro is deformed because of intense faulting at the eruption site. Sediments, primarily mud and the shells of tiny sea creatures, coat the seafloor. Sediment is thickest near the shore, where it comes off the continents in rivers and on wind currents. The oceanic crust is relatively thin and lies above the mantle. The cross section of oceanic crust in the Figure 1.2 shows the layers that grade from sediments at the top to extrusive basalt lava, to the sheeted dikes that feed lava to the surface, to deeper intrusive gabbro, and finally to the mantle. "
earths crust,T_1102,"Continental crust is made up of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic rocks of the oceanic crust (Figure 1.3). Because it is thick and has relatively low density, continental crust rises higher on the mantle than oceanic crust, which sinks into the mantle to form basins. When filled with water, these basins form the planets oceans. Click image to the left or use the URL below. URL:  A cross-section of oceanic crust. "
earths layers,T_1112,"The layers scientists recognize are pictured below (Figure 1.1). Core, mantle, and crust are divisions based on composition: 1. The crust is less than 1% of Earth by mass. The two types are oceanic crust and continental crust.Continental crust is felsic and oceanic crust is mafic. 2. The mantle is hot, ultramafic rock. It represents about 68% of Earths mass. 3. The core is mostly iron metal. The core makes up about 31% of the Earth. "
earths layers,T_1113,"Lithosphere and asthenosphere are divisions based on mechanical properties: 1. The lithosphere is composed of both the crust and the portion of the upper mantle and behaves as a brittle, rigid solid. 2. The asthenosphere is partially molten upper mantle material and behaves plastically and can flow. A cross section of Earth showing the fol- lowing layers: (1) crust (2) mantle (3a) outer core (3b) inner core (4) lithosphere (5) asthenosphere (6) outer core (7) inner core. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
earths mantle,T_1116,"The two most important things about the mantle are: (1) it is made of solid rock, and (2) it is hot. "
earths mantle,T_1117,"Scientists know that the mantle is made of rock based on evidence from seismic waves, heat flow, and meteorites. The properties fit the ultramafic rock peridotite, which is made of the iron- and magnesium-rich silicate minerals (Figure 1.1). Peridotite is rarely found at Earths surface. "
earths mantle,T_1118,"Scientists know that the mantle is extremely hot because of the heat flowing outward from it and because of its physical properties. Heat flows in two different ways within the Earth: 1. Conduction: Heat is transferred through rapid collisions of atoms, which can only happen if the material is solid. Heat flows from warmer to cooler places until all are the same temperature. The mantle is hot mostly because of heat conducted from the core. Peridotite is formed of crystals of olivine (green) and pyroxene (black). 2. Convection: If a material is able to move, even if it moves very slowly, convection currents can form. Convection in the mantle is the same as convection in a pot of water on a stove. Convection currents within Earths mantle form as material near the core heats up. As the core heats the bottom layer of mantle material, particles move more rapidly, decreasing its density and causing it to rise. The rising material begins the convection current. When the warm material reaches the surface, it spreads horizontally. The material cools because it is no longer near the core. It eventually becomes cool and dense enough to sink back down into the mantle. At the bottom of the mantle, the material travels horizontally and is heated by the core. It reaches the location where warm mantle material rises, and the mantle convection cell is complete (Figure 1.2). Convection. "
earths tectonic plates,T_1120,"What portion of Earth makes up the plates in plate tectonics? Again, the answer came about in part due to war. In this case, the Cold War. During the 1950s and early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. These seismographs also recorded all of the earthquakes around the planet. The seismic records were used to locate an earthquakes epicenter, the point on Earths surface directly above the place where the earthquake occurs. Why is this relevant? It turns out that earthquake epicenters outline the plates. This is because earthquakes occur everywhere plates come into contact with each other. The lithosphere is divided into a dozen major and several minor plates (Figure 1.1). A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both. The movement of the plates over Earths surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, about the same rate fingernails grow. "
earths tectonic plates,T_1121,"If seafloor spreading drives the plates, what drives seafloor spreading? This goes back to Arthur Holmes idea of mantle convection. Picture two convection cells side by side in the mantle, similar to the illustration in Figure 1.2. 1. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust. 2. The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor. 3. The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches. 4. The material sinks to the core and moves horizontally. 5. The material heats up and reaches the zone where it rises again. "
earths tectonic plates,T_1122,"Plate boundaries are the edges where two plates meet. How can two plates move relative to each other? Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. The features found at these plate boundaries are the mid-ocean ridges, trenches, and large transform faults (Figure 1.3). Divergent plate boundaries: the two plates move away from each other. Convergent plate boundaries: the two plates move towards each other. Transform plate boundaries: the two plates slip past each other. The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there. We can visit each of these types of plate boundaries on land or at sea. "
effusive eruptions,T_1136,"Mafic magma creates gentler effusive eruptions. Although the pressure builds enough for the magma to erupt, it does not erupt with the same explosive force as felsic magma. Magma pushes toward the surface through fissures. Eventually, the magma reaches the surface and erupts through a vent (Figure 1.1). Effusive eruptions are common in Hawaii, where lavas are mafic. In effusive eruptions, lava flows readily, producing rivers of molten rock. A Quicktime movie with thermal camera of a lava stream within the vent of a Hawaiian volcano is seen here: "
effusive eruptions,T_1137,"Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in three types of lava flow coming from effusive eruptions. Aa lava forms a thick and brittle crust that is torn into rough and jagged pieces. Aa lava can spread over large areas as the lava continues to flow underneath the crusts surface. Pahoehoe lava forms lava tubes where fluid lava flows through the outer cooled rock crust. Pahoehoe lava is less viscous than aa lava, so its surface looks is smooth and ropy. Mafic lava that erupts underwater creates pillow lava. The lava cools very quickly, forming roughly spherical rocks. Pillow lava is common at mid-ocean ridges (Figure (a) Aa lava spread over large areas. (b) Pahoehoe lava tubes where at the Thurston Lava Tube in Hawaii Volcanoes National Park. (c) Pahoehoe lava is less viscous than aa lava so its surface looks is smooth and ropy. (d) Pillow lava. "
effusive eruptions,T_1138,"People can usually be evacuated before an effusive eruption, so they are much less deadly. Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building or road (Figure 1.3). "
evolution of simple cells,T_1149,"Simple organic molecules such as proteins and nucleic acids eventually became complex organic substances. Sci- entists think that the organic molecules adhered to clay minerals, which provided the structure needed for these substances to organize. The clays, along with their metal cations, catalyzed the chemical reactions that caused the molecules to form polymers. The first RNA fragments could also have come together on ancient clays. E. coli (Escherichia coli) is a primitive prokaryote that may resemble the earliest cells. For an organic molecule to become a cell, it must be able to separate itself from its environment. To enclose the molecule, a lipid membrane grew around the organic material. Eventually the molecules could synthesize their own organic material and replicate themselves. These became the first cells. "
evolution of simple cells,T_1150,"The earliest cells were prokaryotes (Figure 1.1). Although prokaryotes have a cell membrane, they lack a cell nucleus and other organelles. Without a nucleus, RNA was loose within the cell. Over time the cells became more complex. LUCA was a prokaryote but differed from the first living cells because its genetic code was based on DNA. The oldest fossils are tiny microbe-like objects that are 3.5 billion years old. Evidence for bacteria, the first single-celled life forms, goes back 3.5 billion years (Figure 1.2). "
evolution of simple cells,T_1151,"The earliest life forms did not have the ability to photosynthesize. Without photosynthesis what did the earliest cells eat? Most likely they absorbed the nutrients that floated around in the organic soup that surrounded them. After hundreds of millions of years, these nutrients would have become less abundant. Sometime around 3 billion years ago (about 1.5 billion years after Earth formed!), photosynthesis began. Photo- synthesis allowed organisms to use sunlight and inorganic molecules, such as carbon dioxide and water, to create chemical energy that they could use for food. To photosynthesize, a cell needs chloroplasts (Figure 1.3). A diagram of a bacterium. Chloroplasts are visible in these cells found within a moss. "
evolution of simple cells,T_1152,"In what two ways did photosynthesis make the planet much more favorable for life? 1. Photosynthesis allowed organisms to create food energy so that they did not need to rely on nutrients floating around in the environment. Photosynthesizing organisms could also become food for other organisms. 2. A byproduct of photosynthesis is oxygen. When photosynthesis evolved, all of a sudden oxygen was present in large amounts in the atmosphere. For organisms used to an anaerobic environment, the gas was toxic, and many organisms died out. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
evolution of simple cells,T_1153,"What were these organisms that completely changed the progression of life on Earth by changing the atmosphere from anaerobic to aerobic? The oldest known fossils that are from organisms known to photosynthesize are cyanobac- teria. Cyanobacteria were present by 2.8 billion years ago, and some may have been around as far back as 3.5 billion years. Cyanobacteria were the dominant life forms in the Archean. Why would such a primitive life-form have been dominant in the Precambrian? Many cyanobacteria lived in reef-like structures known as stromatolites (Figure These rocks in Glacier National Park, Montana may contain some of the oldest fossil microbes on Earth. Modern cyanobacteria are also called blue-green algae. These organisms may consist of a single or many cells and they are found in many different environments (Figure 1.5). Even now cyanobacteria account for 20% to 30% of photosynthesis on Earth. A large bloom of cyanobacteria is harmful to this lake. "
explosive eruptions,T_1162,"A large explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II, in which more than 40,000 people died. A large explosive volcanic eruption is 10,000 times as powerful. Explosive eruptions are found at the convergent plate boundaries that line parts of western North America, resulting in the Cascades in the Pacific Northwest and the Aleutians in Alaska. "
explosive eruptions,T_1163,"Explosive eruptions are caused by gas-rich, felsic magmas that churn within the magma chamber. When the pressure becomes too great the magma breaks through the rock above the chamber and explodes, just like when a cork is released from a bottle of champagne. Magma, rock, and ash burst upward in an enormous explosion (Figure 1.1). "
explosive eruptions,T_1164,"The erupted rock fragments are called tephra. Ash and gas also explode from the volcano. Scorching hot tephra, ash, and gas may speed down the volcanos slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic means fire rock (Figure 1.2). Left: An explosive eruption from the Mayon Volcano in the Philippines in 1984. Ash flies upward into the sky and pyroclastic flows pour down the mountainside. Right: The end of a pyroclastic flow at Mount St. Helens. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000 C (1,800 F). Blowdown of trees near Mount St. Helens shows the direction of the blast and pyro- clastic flow. "
explosive eruptions,T_1165,"Prior to the Mount St. Helens eruption in 1980, the Lassen Peak eruption on May 22, 1915, was the most recent Cascades eruption. A column of ash and gas shot 30,000 feet into the air. This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again. Mt. Shasta, the other active volcano in California, erupts every 600 to 800 years. An eruption would most likely create a large pyroclastic flow, and probably a lahar. Of course, Mt. Shasta could explode and collapse like Mt. Mazama in Oregon (Figure 1.4). Crater Lake fills the caldera of the col- lapsed Mt. Mazama, which erupted with 42 times more power than Mount St. He- lens in 1980. The bathymetry of the lake shows volcanic features such as cinder cones. "
explosive eruptions,T_1166,"Volcanic gases can form poisonous and invisible clouds in the atmosphere. These gases may contribute to environ- mental problems such as acid rain and ozone destruction. Particles of dust and ash may stay in the atmosphere for years, disrupting weather patterns and blocking sunlight (Figure 1.5). The ash plume from Eyjafjallajkull vol- cano in Iceland disrupted air travel across Europe for six days in April 2010. Click image to the left or use the URL below. URL: "
finding and mining ores,T_1174,Some minerals are very useful. An ore is a rock that contains minerals with useful elements. Aluminum in bauxite ore (Figure 1.1) is extracted from the ground and refined to be used in aluminum foil and many other products. The cost of creating a product from a mineral depends on how abundant the mineral is and how much the extraction and refining processes cost. Environmental damage from these processes is often not figured into a products cost. It is important to use mineral resources wisely. 
finding and mining ores,T_1175,Geologic processes create and concentrate minerals that are valuable natural resources. Geologists study geological formations and then test the physical and chemical properties of soil and rocks to locate possible ores and determine their size and concentration. A mineral deposit will only be mined if it is profitable. A concentration of minerals is only called an ore deposit if it is profitable to mine. There are many ways to mine ores. Aluminum is made from the aluminum- bearing minerals in bauxite. 
finding and mining ores,T_1176,"Surface mining allows extraction of ores that are close to Earths surface. Overlying rock is blasted and the rock that contains the valuable minerals is placed in a truck and taken to a refinery. As pictured in Figure 1.2, surface mining includes open-pit mining and mountaintop removal. Other methods of surface mining include strip mining, placer mining, and dredging. Strip mining is like open pit mining but with material removed along a strip. These different forms of surface mining are methods of extracting ores close to Earths surface. Placers are valuable minerals found in stream gravels. Californias nickname, the Golden State, can be traced back to the discovery of placer deposits of gold in 1848. The gold weathered out of hard metamorphic rock in the western Sierra Nevada, which also contains deposits of copper, lead, zinc, silver, chromite, and other valuable minerals. The "
finding and mining ores,T_1177,"Underground mining is used to recover ores that are deeper into Earths surface. Miners blast and tunnel into rock to gain access to the ores. How underground mining is approached  from above, below, or sideways  depends on the placement of the ore body, its depth, the concentration of ore, and the strength of the surrounding rock. Underground mining is very expensive and dangerous. Fresh air and lights must also be brought into the tunnels for the miners, and accidents are far too common. "
finding and mining ores,T_1178,"The ores journey to becoming a useable material is only just beginning when the ore leaves the mine (Figure separated out of the ore. A few methods for extracting ore are: heap leaching: the addition of chemicals, such as cyanide or acid, to remove ore. flotation: the addition of a compound that attaches to the valuable mineral and floats. smelting: roasting rock, causing it to segregate into layers so the mineral can be extracted. To extract the metal from the ore, the rock is melted at a temperature greater than 900o C, which requires a lot of energy. Extracting metal from rock is so energy-intensive that if you recycle just 40 aluminum cans, you will save the energy equivalent of one gallon of gasoline. "
intraplate activity,T_1333,"A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but within a plate instead. Mantle plumes are pipes of hot rock that rise through the mantle. The release of pressure causes melting near the surface to form a hotspot. Eruptions at the hotspot create a volcano. Hotspot volcanoes are found in a line (Figure 1.1). Can you figure out why? Hint: The youngest volcano sits above the hotspot and volcanoes become older with distance from the hotspot. "
intraplate activity,T_1334,"The first photo above is of a volcanic eruption in Hawaii. Hawaii is not in western North America, but is in the central Pacific ocean, near the middle of the Pacific Plate. The Hawaiian Islands are a beautiful example of a hotspot chain in the Pacific Ocean. Kilauea volcano lies above the Hawaiian hotspot. Mauna Loa volcano is older than Kilauea and is still erupting, but at a slower rate. The islands get progressively older to the northwest because they are further from the hotspot. This is because the Pacific Plate is moving toward the northwest over the hotspot. Loihi, the youngest volcano, is still below the sea surface. Since many hotspots are stationary in the mantle, geologists can use some hotspot chains to tell the direction and the speed a plate is moving (Figure 1.2). The Hawaiian chain continues into the Emperor Seamounts. The bend in the chain was caused by a change in the direction of the Pacific Plate 43 million years ago. Using the age and distance of the bend, geologists can figure out the speed of the Pacific Plate over the hotspot. The Hawaiian Islands have formed from volcanic eruptions above the Hawaii hotspot. "
intraplate activity,T_1335,"The second photo in the introduction is of a geyser at Yellowstone National Park in Wyoming. Yellowstone is in the western U.S. but is inland from the plate boundaries offshore. Hotspot magmas rarely penetrate through thick continental crust, so hotspot activity on continents is rare. One exception is the Yellowstone hotspot (Figure 1.3). Volcanic activity above the Yellowstone hotspot on can be traced from 15 million years ago to its present location on the North American Plate. The ages of volcanic activity attributed to the Yellowstone hotspot. Click image to the left or use the URL below. URL: "
intraplate earthquakes,T_1336,Intraplate earthquakes are the result of stresses caused by plate motions acting in solid slabs of lithosphere. The earthquakes take place along ancient faults or rift zones that have been weakened by activity that may have taken place hundreds of millions of years ago. 
intraplate earthquakes,T_1337,"In August 2011 the eastern seaboard of the U.S. was rocked by a magnitude 5.8 earthquake. While not huge, most of the residents had never experienced a quake and many didnt know what it was. Some people thought the shaking might have been the result of a terrorist attack. This region is no longer part of an active plate boundary. But if you went back in time to the late Paleozoic, you would find the region being uplifted into the ancestral Appalachian mountains as continent-continent convergence brought Pangaea together. The Piedmont Seismic Zone is an area of several hundred million year-old faults that sometimes reactivate. "
intraplate earthquakes,T_1338,"In 1812, a magnitude 7.5 earthquake struck near New Madrid, Missouri. The earthquake was strongly felt over approximately 50,000 square miles and altered the course of the Mississippi River. Because very few people lived there at the time, only 20 people died. Many more people live there today (Figure 1.1). A similar earthquake today would undoubtedly kill many people and cause a great deal of property damage. Like the Piedmont Seismic Zone, the New Madrid Seismic Zone is a set of reactivated faults. These faults are left from the rifting apart of the supercontinent Rodinia about 750 million years ago. The plates did not rift apart here but left a weakness in the lithosphere that makes the region vulnerable to earthquakes. Click image to the left or use the URL below. URL: "
landforms from erosion and deposition by gravity,T_1349,"Gravity shapes the Earths surface by moving weathered material from a higher place to a lower one. This occurs in a variety of ways and at a variety of rates, including sudden, dramatic events as well as slow, steady movements that happen over long periods of time. The force of gravity is constant and it is changing the Earths surface right now. "
landforms from erosion and deposition by gravity,T_1350,"Erosion by gravity is called mass wasting. Mass wasting can be slow and virtually imperceptible, or rapid, massive, and deadly. Weathered material may fall away from a cliff because there is nothing to keep it in place. Rocks that fall to the base of a cliff make a talus slope. Sometimes as one rock falls, it hits another rock, which hits another rock, and begins a landslide. "
landforms from erosion and deposition by gravity,T_1351,"Landslides are the most dramatic, sudden, and dangerous examples of Earth materials moved by gravity. Landslides are sudden falls of rock; by contrast, avalanches are sudden falls of snow. When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (Figure 1.1). Air trapped under the falling rocks acts as a cushion that keeps the rock from slowing down. Landslides can move as fast as 200 to 300 km/hour. This landslide in California in 2008 blocked Highway 140. Landslides are exceptionally destructive. Homes may be destroyed as hillsides collapse. Landslides can even bury entire villages. Landslides may create lakes when the rocky material dams a stream. If a landslide flows into a lake or bay, they can trigger a tsunami. Landslides often occur on steep slopes in dry or semi-arid climates. The California coastline, with its steep cliffs and years of drought punctuated by seasons of abundant rainfall, is prone to landslides. "
landforms from erosion and deposition by gravity,T_1352,"Added water creates natural hazards produced by gravity (Figure 1.2). On hillsides with soils rich in clay, little rain, and not much vegetation to hold the soil in place, a time of high precipitation will create a mudflow. Mudflows follow river channels, washing out bridges, trees, and homes that are in their path. A lahar is mudflow that flows down a composite volcano (Figure 1.3). Ash mixes with snow and ice melted by the eruption to produce hot, fast-moving flows. The lahar caused by the eruption of Nevado del Ruiz in Columbia in 1985 killed more than 23,000 people. "
landforms from erosion and deposition by gravity,T_1353,"Less dramatic types of downslope movement move Earth materials slowly down a hillside. Slump moves materials as a large block along a curved surface (Figure 1.4). Slumps often happen when a slope is undercut, with no support for the overlying materials, or when too much weight is added to an unstable slope. Mudflows are common in southern California. A lahar is a mudflow that forms from vol- canic ash and debris. Slump material moves as a whole unit, leaving behind a crescent shaped scar. The trunks of these trees near Mineral King, California, were bent by snow creeping downhill when the trees were saplings. Click image to the left or use the URL below. URL: "
landforms from erosion and deposition by gravity,T_1354,There are several factors that increase the chance that a landslide will occur. Some of these we can prevent and some we cannot. 
landforms from erosion and deposition by gravity,T_1355,"A little bit of water helps to hold grains of sand or soil together. For example, you can build a larger sand castle with slightly wet sand than with dry sand. However, too much water causes the sand to flow quickly away. Rapid snow melt or rainfall adds extra water to the soil, which increases the weight of the slope and makes sediment grains lose contact with each other, allowing flow. "
landforms from erosion and deposition by gravity,T_1356,"Layers of weak rock, such as clay, also allow more landslides. Wet clay is very slippery, which provides an easy surface for materials to slide over. "
landforms from erosion and deposition by gravity,T_1357,"If people dig into the base of a slope to create a road or a homesite, the slope may become unstable and move downhill. This is particularly dangerous when the underlying rock layers slope towards the area. When construction workers cut into slopes for homes or roads, they must stabilize the slope to help prevent a landslide (Figure 1.6). Tree roots or even grasses can bind soil together. It is also a good idea to provide drainage so that the slope does not become saturated with water. "
landforms from erosion and deposition by gravity,T_1358,"An earthquake, volcanic eruption, or even just a truck going by can shake unstable ground loose and cause a slide. Skiers and hikers may disturb the snow they travel over and set off an avalanche. "
landforms from erosion and deposition by gravity,T_1359,"Landslides cause $1 billion to $2 billion damage in the United States each year and are responsible for traumatic and sudden loss of life and homes in many areas of the world. Some at-risk communities have developed landslide warning systems. Around San Francisco Bay, the National Weather Service and the U.S. Geological Survey use rain gauges to monitor soil moisture. If soil becomes saturated, the weather service issues a warning. Earthquakes, which may occur on Californias abundant faults, can also trigger landslides. To be safe from landslides: Be aware of your surroundings and notice changes in the natural world. Look for cracks or bulges in hillsides, tilting of decks or patios, or leaning poles or fences when rainfall is heavy. Sticking windows and doors can indicate ground movement as soil pushes slowly against a house and knocks windows and doors out of alignment. Look for landslide scars because landslides are most likely to happen where they have occurred before. Plant vegetation and trees on the hillside around your home to help hold soil in place. Help to keep a slope stable by building retaining walls. Installing good drainage in a hillside may keep the soil from getting saturated. Hillside properties in the San Francisco Bay Area and elsewhere may be prone to damage from landslides. Geologists are studying the warning signs and progress of local landslides to help reduce risks and give people adequate warnings of these looming threats. "
lithosphere and asthenosphere,T_1370,"The lithosphere is composed of both the crust and the portion of the upper mantle that behaves as a brittle, rigid solid. The lithosphere is the outermost mechanical layer, which behaves as a brittle, rigid solid. The lithosphere is about 100 kilometers thick. How are crust and lithosphere different from each other? The definition of the lithosphere is based on how Earth materials behave, so it includes the crust and the uppermost mantle, which are both brittle. Since it is rigid and brittle, when stresses act on the lithosphere, it breaks. This is what we experience as an earthquake. Although we sometimes refer to Earths plates as being plates of crust, the plates are actually made of lithosphere. Much more about Earths plates follows in the chapter ""Plate Tectonics."" "
lithosphere and asthenosphere,T_1371,The asthenosphere is solid upper mantle material that is so hot that it behaves plastically and can flow. The lithosphere rides on the asthenosphere. 
locating earthquake epicenters,T_1380,"Here are the steps to finding an earthquake epicenter using three seismograms: 1. Determine the epicenter distance from three different seismographs. The longer the time between the arrival of the P-wave and S-wave, the farther away is the epicenter. So the difference in the P- and S-wave arrival times determines the distance between the epicenter and a seismometer. 2. Draw a circle with a radius equal to the distance from the epicenter for that seismograph. The epicenter is somewhere along that circle. Do this for three locations. Using data from two seismographs, the two circles will intercept at two points. A third circle will intercept the other two circles at a single point. This point is the earthquake epicenter (Figure 1.1). Of course, its been a long time since scientists drew circles to locate an earthquake epicenter. This is all done digitally now. but its a great way to learn the basics of how locating an epicenter works. Three circles drawn from three seismic stations each equal to the radius from the station to the epicenter of the quake will intercept at the actual epicenter. Click image to the left or use the URL below. URL: "
magma composition at volcanoes,T_1392,"There are as many types of volcanic eruptions as there are eruptions. Actually more since an eruption can change character as it progresses. Each volcanic eruption is unique, differing in size, style, and composition of erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines (1) the eruption style, (2) the type of volcanic cone that forms, and (3) the composition of rocks that are found at the volcano. Different minerals within a rock melt at different temperatures. The amount of partial melting and the composition of the original rock determine the composition of the magma. The words that describe composition of igneous rocks also describe magma composition. Mafic magmas are low in silica and contain more dark, magnesium- and iron-rich mafic minerals, such as olivine and pyroxene. Felsic magmas are higher in silica and contain lighter colored minerals such as quartz and orthoclase feldspar. The higher the amount of silica in the magma, the higher is its viscosity. Viscosity is a liquids resistance to flow. Viscosity determines what the magma will do. Mafic magma is not viscous and will flow easily to the surface. Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form igneous intrusive rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move, so it gets stuck. Dissolved gases become trapped by thick magma. The magma churns in the chamber and the pressure builds. Magma collects in magma chambers in the crust at 160 kilometers (100 miles) beneath the surface. Click image to the left or use the URL below. URL: "
materials humans use,T_1407,"People depend on natural resources for just about everything that keeps us fed and sheltered, as well as for the things that keep us entertained. Every person in the United States uses about 20,000 kilograms (40,000 pounds) of minerals every year for a wide range of products, such as cell phones, TVs, jewelry, and cars. Table 1.1 shows some common objects, the materials they are made from, and whether they are renewable or non-renewable. Common Object Natural Resources Used Cars 15 different metals, such as iron, lead, and chromium to make the body. Precious metals like gold, silver, and platinum. Gems like diamonds, rubies, emer- alds, turquoise. Jewelry Are These Resources Renewable or Non-Renewable? Non-renewable Non-renewable Common Object Natural Resources Used Electronic Appliances (TVs, com- puters, DVD players, cell phones, etc.) Clothing Many different metals, like copper, mercury, gold. Food Bottled Water Gasoline Household Electricity Paper Houses Soil to grow fibers such as cotton. Sunlight for the plants to grow. Animals for fur and leather. Soil to grow plants. Wildlife and agricultural animals. Water from streams or springs. Petroleum products to make plastic bottles. Petroleum drilled from wells. Coal, natural gas, solar power, wind power, hydroelectric power. Trees; Sunlight Soil. Trees for timber. Rocks and minerals for construc- tion materials, for example, granite, gravel, sand. Are These Resources Renewable or Non-Renewable? Non-renewable Renewable Renewable Non-renewable and Renewable Non-renewable Non-renewable and Renewable Renewable Non-renewable and Renewable Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
mesozoic plate tectonics,T_1424,"As heat builds up beneath a supercontinent, continental rifting begins. Basaltic lavas fill in the rift and eventually lead to seafloor spreading and the formation of a new ocean basin. This basalt province is where Africa is splitting apart and generating basalt lava. "
mesozoic plate tectonics,T_1425,"At the end of the Paleozoic there was one continent and one ocean. When Pangaea began to break apart about 180 million years ago, the Panthalassa Ocean separated into the individual but interconnected oceans that we see today on Earth. The Atlantic Ocean basin formed as Pangaea split apart. The seafloor spreading that pushed Africa and South America apart is continuing to enlarge the Atlantic Ocean (Figure 1.1). As the continents moved apart there was an intense period of plate tectonic activity. Seafloor spreading was so vig- orous that the mid-ocean ridge buoyed upwards and displaced so much water that there was a marine transgression. Later in the Mesozoic those seas regressed and then transgressed again. "
mesozoic plate tectonics,T_1426,"The moving continents collided with island arcs and microcontinents so that mountain ranges accreted onto the continents edges. The subduction of the oceanic Farallon plate beneath western North America during the late In the Afar Region of Ethiopia, Africa is splitting apart. Three plates are pulling away from a central point. Jurassic and early Cretaceous produced igneous intrusions and other structures. The intrusions have since been uplifted so that they are exposed in the Sierra Nevada Mountains (Figure 1.2). The snow-covered Sierra Nevada is seen striking SE to NW across the eastern third of the image. The mountain range is a line of uplifted batholiths from Mesozoic subduction. Click image to the left or use the URL below. URL: "
metabolism and replication,T_1427,"Organic molecules must also carry out the chemical work of cells; that is, their metabolism. Chemical reactions in a living organism allow that organism to live in its environment, grow, and reproduce. Metabolism gets energy from other sources and creates structures needed in cells. The chemical reactions occur in a sequence of steps known as metabolic pathways. The metabolic pathways are very similar between unicellular bacteria that have been around for billions of years and the most complex life forms on Earth today. This means that they evolved very early in Earths history. "
metabolism and replication,T_1428,"Living cells need organic molecules, known as nucleic acids, to store genetic information and pass it to the next generation. Deoxyribonucleic acid (DNA) is the nucleic acid that carries information for nearly all living cells today and did for most of Earths history. Ribonucleic acid (RNA) delivers genetic instructions to the location in a cell where protein is synthesized. "
metabolism and replication,T_1429,"Many scientists think that RNA was the first replicator. Since RNA catalyzes protein synthesis, most scientists think that RNA came before proteins. RNA can also encode genetic instructions and carry it to daughter cells, such as DNA. The idea that RNA is the most primitive organic molecule is called the RNA world hypothesis, referring to the possibility that the RNA is more ancient than DNA. RNA can pass along genetic instructions as DNA can, and some RNA can carry out chemical reactions like proteins can. Click image to the left or use the URL below. URL:  Pieces of many scenarios can be put together to come up with a plausible suggestion for how life began. Click image to the left or use the URL below. URL: "
mineral formation,T_1441,Minerals form in a variety of ways: crystallization from magma precipitation from ions in solution biological activity a change to a more stable state as in metamorphism precipitation from vapor 
mineral formation,T_1442,"Imagine a rock that becomes so hot it melts. Many minerals start out in liquids that are hot enough to melt rocks. Magma is melted rock inside Earth, a molten mixture of substances that can be hotter than 1,000 C. Magma cools slowly inside Earth, which gives mineral crystals time to grow large enough to be seen clearly (Figure 1.1). Granite is rock that forms from slowly cooled magma, containing the minerals quartz (clear), plagioclase feldspar (shiny white), potassium feldspar (pink), and bi- otite (black). When magma erupts onto Earths surface, it is called lava. Lava cools much more rapidly than magma. Crystals do not have time to form and are very small. The chemical composition between minerals that form rapidly or slowly is often the same, only their size differs. Existing rocks may be heated enough so that the molecules are released from their structure and can move around. The molecules may match up with different molecules to form new minerals as the rock cools. This occurs during metamorphism, which will be discussed in the ""Metamorphic Rocks"" concept. "
mineral formation,T_1443,"Water on Earth, such as the water in the oceans, contains chemical elements mixed into a solution. Various processes can cause these elements to combine to form solid mineral deposits. "
mineral formation,T_1444,"When water evaporates, it leaves behind a solid precipitate of minerals, as shown in Figure 1.2. When the water in glass A evaporates, the dissolved mineral particles are left behind. Water can only hold a certain amount of dissolved minerals and salts. When the amount is too great to stay dissolved in the water, the particles come together to form mineral solids, which sink. Halite easily precipitates out of water, as does calcite. Some lakes, such as Mono Lake in California (Figure 1.3) or The Great Salt Lake in Utah, contain many mineral precipitates. Tufa towers form when calcium-rich spring water at the bottom of Mono Lake bubbles up into the alkaline lake. The tufa towers appear when lake level drops. "
mineral formation,T_1445,"Magma heats nearby underground water, which reacts with the rocks around it to pick up dissolved particles. As the water flows through open spaces in the rock and cools, it deposits solid minerals. The mineral deposits that form when a mineral fills cracks in rocks are called veins (Figure 1.4). Quartz veins formed in this rock. When minerals are deposited in open spaces, large crystals form (Figure 1.5). Amethyst formed when large crystals grew in open spaces inside the rock. These special rocks are called geodes. "
mineral formation,T_1446,"In the last several years, many incredible discoveries have been made exploring how minerals behave under high pressure, like rocks experience inside the Earth. If a mineral is placed in a special machine and then squeezed, eventually it may convert into a different mineral. Ice is a classic example of a material that undergoes solid-solid ""phase transitions"" as pressure and/or temperature is changed. A ""phase diagram"" is a graph which plots the stability of phases of a compound as a function of pressure and temperature. A phase diagram for water (ice) is included in the Figure 1.6. The phase diagram is split up into 3 main areas for the solid crystalline phase (ice), the liquid phase (water), and the gas phase (water vapor). Notice that increasing pressure lowers the freezing point and raises the boiling point of water. What does that do to the stability conditions of the liquid phase? A sample phase diagram for water. Click image to the left or use the URL below. URL: "
mineral groups,T_1447,Minerals are divided into groups based on chemical composition. Most minerals fit into one of eight mineral groups. 
mineral groups,T_1448,"The roughly 1,000 silicate minerals make up over 90% of Earths crust. Silicates are by far the largest mineral group. Feldspar and quartz are the two most common silicate minerals. Both are extremely common rock-forming minerals. The basic building block for all silicate minerals is the silica tetrahedron, which is illustrated in Figure 1.1. To create the wide variety of silicate minerals, this pyramid-shaped structure is often bound to other elements, such as calcium, iron, and magnesium. Silica tetrahedrons combine together in six different ways to create different types of silicates (Figure 1.2). Tetrahe- drons can stand alone, form connected circles called rings, link into single and double chains, form large flat sheets of pyramids, or join in three dimensions. One silicon atom bonds to four oxygen atoms to form a silica tetrahedron. The different ways that silica tetrahedrons can join together cause these two minerals to look very different. "
mineral groups,T_1449,"Native elements contain atoms of only one type of element. Only a small number of minerals are found in this category. Some of the minerals in this group are rare and valuable. Gold (Figure 1.3), silver, sulfur, and diamond are examples of native elements. "
mineral groups,T_1450,"The basic carbonate structure is one carbon atom bonded to three oxygen atoms. Carbonates consists of some cation (like C, Fe, Cu, Mg, Ba, Sr, Pb) bonded to a carbonate molecule. Calcite (CaCO3 ) is the most common carbonate mineral (Figure 1.4). Calcite. "
mineral groups,T_1451,"Halide minerals are salts that form when salt water evaporates. Halite is a halide mineral, but table salt (see Figure bond with various metallic atoms to make halide minerals. All halides are ionic minerals, which means that they are typically soluble in water. Two carbonate minerals: (a) deep blue azurite and (b) opaque green malachite. Azurite and malachite are carbonates that contain copper instead of calcium. Beautiful halite crystal. "
mineral groups,T_1452,"Oxides contain one or two metal elements combined with oxygen. Many important metal ores are oxides. Hematite (Fe2 O3 ), with two iron atoms to three oxygen atoms, and magnetite (Fe3 O4 ) (Figure 1.7), with three iron atoms to four oxygen atoms, are both iron oxides. Magnetite is one of the most distinctive oxides since it is magnetic. "
mineral groups,T_1453,"Phosphate minerals are similar in atomic structure to the silicate minerals. In the phosphates, phosphorus bonds to oxygen to form a tetrahedron. As a mineral group they arent particularly common or important rock-forming minerals, but they are important for you and I. Apatite (Figure 1.8) is a phosphate (Ca5 (PO4 )3 (F,OH)) and is one of the major components of human bone! "
mineral groups,T_1454,"Sulfate minerals contain sulfur atoms bonded to four oxygen atoms, just like silicates and phosphates. Like halides, they form where salt water evaporates. The most common sulfate mineral is probably gypsum (CaSO4 (OH)2 ) (Figure 1.9). Some gigantic 11-meter gypsum crystals have been found (See opening image). That is about as long as a school bus! Gypsum. "
mineral groups,T_1455,"Sulfides are formed when metallic elements combine with sulfur in the absence of oxygen. Pyrite (Figure 1.10) (FeS2 ) is a common sulfide mineral colloquially known as ""fools gold"" because it has a golden metallic looking mineral. There are three easy ways to discriminate real gold from fools gold: real gold is extremely dense, real gold does not grow into perfect cubes, as pyrite commonly does, and pyrite smells like rotten eggs (because of the sulfur). Click image to the left or use the URL below. URL: "
mineral identification,T_1456,"There are a multitude of laboratory and field techniques for identifying minerals. While a mineralogist might use a high-powered microscope to identify some minerals, or even techniques like x-ray diffraction, most are recognizable using physical properties. The most common field techniques put the observer in the shoes of a detective, whose goal it is to determine, by process of elimination, what the mineral in question is. The process of elimination usually includes observing things like color, hardness, smell, solubility in acid, streak, striations and/or cleavage. Check out the mineral in the opening image. What is the minerals color? What is its shape? Are the individual crystals shiny or dull? Are there lines (striations) running across the minerals? In this concept, the properties used to identify minerals are described in more detail. "
mineral identification,T_1458,"Color may be the first feature you notice about a mineral, but color is not often important for mineral identification. For example, quartz can be colorless, purple (amethyst), or a variety of other colors depending on chemical impurities Figure 1.1. "
mineral identification,T_1459,"Streak is the color of a minerals powder, which often is not the same color as the mineral itself. Many minerals, such as the quartz in the Figure 1.1, do not have streak. Hematite is an example of a mineral that displays a certain color in hand sample (typically black to steel gray, sometimes reddish), and a different streak color (red/brown). "
mineral identification,T_1460,"Luster describes the reflection of light off a minerals surface. Mineralogists have special terms to describe luster. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. Minerals such as quartz have a non-metallic luster. Different types of non-metallic luster are described in Table 1.1. Luster Adamantine Earthy Pearly Resinous Silky Vitreous Appearance Sparkly Dull, clay-like Pearl-like Like resins, such as tree sap Soft-looking with long fibers Glassy The streak of hematite across an unglazed porcelain plate is red-brown. "
mineral identification,T_1461,"Density describes how much matter is in a certain amount of space: density = mass/volume. Mass is a measure of the amount of matter in an object. The amount of space an object takes up is described by its volume. The density of an object depends on its mass and its volume. For example, the water in a drinking glass has the same density as the water in the same volume of a swimming pool. Gold has a density of about 19 g/cm3 ; pyrite has a density of about 5 g/cm3 - thats another way to tell pyrite from gold. Quartz is even less dense than pyrite and has a density of 2.7 g/cm3 . The specific gravity of a substance compares its density to that of water. Substances that are more dense have higher specific gravity. "
mineral identification,T_1462,"Hardness is a measure of whether a mineral will scratch or be scratched. Mohs Hardness Scale, shown in Table Hardness 1 2 3 4 5 6 7 8 Mineral Talc Gypsum Calcite Fluorite Apatite Feldspar Quartz Topaz Hardness 9 10 Mineral Corundum Diamond With a Mohs scale, anyone can test an unknown mineral for its hardness. Imagine you have an unknown mineral. You find that it can scratch fluorite or even apatite, but feldspar scratches it. You know then that the minerals hardness is between 5 and 6. Note that no other mineral can scratch diamond. "
mineral identification,T_1463,"Breaking a mineral breaks its chemical bonds. Since some bonds are weaker than other bonds, each type of mineral is likely to break where the bonds between the atoms are weaker. For that reason, minerals break apart in characteristic ways. Cleavage is the tendency of a mineral to break along certain planes to make smooth surfaces. Halite (Figure 1.3) breaks between layers of sodium and chlorine to form cubes with smooth surfaces. Mica has cleavage in one direction and forms sheets (Figure 1.4). Minerals can cleave into polygons. Magnetite forms octahedrons (Figure 1.5). One reason gemstones are beautiful is that the cleavage planes make an attractive crystal shape with smooth faces. Fracture is a break in a mineral that is not along a cleavage plane. Fracture is not always the same in the same mineral because fracture is not determined by the structure of the mineral. Minerals may have characteristic fractures (Figure 1.6). Metals usually fracture into jagged edges. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, form smooth curved surfaces when they fracture. Sheets of mica. "
mineral identification,T_1464,"Some minerals have other unique properties, some of which are listed in Table 1.3. Can you name a unique property that would allow you to instantly identify a mineral thats been described quite a bit in this concept? (Hint: It is most likely found on your dinner table.) Chrysotile has splintery fracture. Property Fluorescence Magnetism Radioactivity Reactivity Smell Taste Description Mineral glows under ultraviolet light Mineral is attracted to a magnet Mineral gives off radiation that can be measured with Geiger counter Bubbles form when mineral is ex- posed to a weak acid Some minerals have a distinctive smell Some minerals taste salty Example of Mineral Fluorite Magnetite Uraninite Calcite Sulfur (smells like rotten eggs) Halite "
minerals,T_1465,"Minerals are everywhere! Scientists have identified more than 4,000 minerals in Earths crust, although the bulk of the planet is composed of just a few. A mineral possesses the following qualities: It must be solid. It must be crystalline, meaning it has a repeating arrangement of atoms. It must be naturally occurring. It must be inorganic. It must have a specific chemical composition. Minerals can be identified by their physical properties, such as hardness, color, luster (shininess), and odor. The most common laboratory technique used to identify a mineral is X-ray diffraction (XRD), a technique that involves shining an X-ray light on a sample, and observing how the light exiting the sample is bent. XRD is not useful in the field, however. The definition of a mineral is more restricted than you might think at first. For example, glass is made of sand, which is rich in the mineral quartz. But glass is not a mineral, because it is not crystalline. Instead, glass has a random assemblage of molecules. What about steel? Steel is made by mixing different metal minerals like iron, cobalt, chromium, vanadium, and molybdenum, but steel is not a mineral because it is made by humans and therefore is not naturally occurring. However, almost any rock you pick up is composed of minerals. Below we explore the qualities of minerals in more detail. "
minerals,T_1466,"Minerals are ""crystalline"" solids. A crystal is a solid in which the atoms are arranged in a regular, repeating pattern. Notice that in Figure 1.1 the green and purple spheres, representing sodium and chlorine, form a repeating pattern. In this case, they alternate in all directions. Sodium ions (purple balls) bond with chlo- ride ions (green balls) to make table salt (halite). All of the grains of salt that are in a salt shaker have this crystalline structure. "
minerals,T_1467,"Organic substances are the carbon-based compounds made by living creatures and include proteins, carbohydrates, and oils. Inorganic substances have a structure that is not characteristic of living bodies. Coal is made of plant and animal remains. Is it a mineral? Coal is a classified as a sedimentary rock, but is not a mineral. "
minerals,T_1468,"Minerals are made by natural processes, those that occur in or on Earth. A diamond created deep in Earths crust is a mineral, but a diamond made in a laboratory by humans is not. Be careful about buying a laboratory-made diamond for jewelry. It may look pretty, but its not a diamond and is not technically a mineral. "
minerals,T_1469,"Nearly all (98.5%) of Earths crust is made up of only eight elements - oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium - and these are the elements that make up most minerals. All minerals have a specific chemical composition. The mineral silver is made up of only silver atoms and diamond is made only of carbon atoms, but most minerals are made up of chemical compounds. Each mineral has its own chemical formula. Table salt (also known as halite), pictured in Figure 1.1, is NaCl (sodium chloride). Quartz is always made of two oxygen atoms (red) bonded to a silicon atom (grey), represented by the chemical formula SiO2 (Figure 1.2). Quartz is made of two oxygen atoms (red) bonded to a silicon atom (grey). In nature, things are rarely as simple as in the lab, and so it should not come as a surprise that some minerals have a range of chemical compositions. One important example in Earth science is olivine, which always has silicon and oxygen as well as some iron and magnesium, (Mg, Fe)2 SiO4 . "
minerals,T_1470,"Some minerals can be identified with little more than the naked eye. We do this by examining the physical properties of the mineral in question, which include: Color: the color of the mineral. Streak: the color of the minerals powder (this is often different from the color of the whole mineral). Luster: shininess. Density: mass per volume, typically reported in ""specific gravity,"" which is the density relative to water. Cleavage: the minerals tendency to break along planes of weakness. Fracture: the pattern in which a mineral breaks. Hardness: which minerals it can scratch and which minerals can scratch it. How physical properties are used to identify minerals is described in the concept ""Mineral Identification."" Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
mountain building,T_1476,"Converging plates create the worlds largest mountain ranges. Each combination of plate types  continent- continent, continent-ocean, and ocean-ocean  creates mountains. "
mountain building,T_1477,"Two converging continental plates smash upwards to create gigantic mountain ranges (Figure 1.1). Stresses from this uplift cause folds, reverse faults, and thrust faults, which allow the crust to rise upwards. As was stated previously there is currently no mountain range of this type in the western U.S., but we can find one where India is pushing into Eurasia. (a) The worlds highest mountain range, the Himalayas, is growing from the colli- sion between the Indian and the Eurasian plates. (b) The crumpling of the Indian and Eurasian plates of continental crust creates the Himalayas. "
mountain building,T_1478,"Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges. This happens on continental crust, as in the Andes Mountains (Figure 1.2), or on oceanic crust, as with the Aleutian Islands, which we visited earlier. The Cascades Mountains of the western U.S. are also created this way. The Andes Mountains are a chain of con- tinental arc volcanoes that build up as the Nazca Plate subducts beneath the South American Plate. "
mountain building,T_1479,"Amazingly, even divergence can create mountain ranges. When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults. The result is alternating mountains and valleys, known as a basin-and-range (Figure 1.3). In basin-and-range, some blocks are uplifted to form ranges, known as horsts, and some are down-dropped to form basins, known as grabens. (a) Horsts and grabens. (b) Mountains in Nevada are of classic basin-and-range form. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
plate tectonics through earth history,T_1550,"First, lets review plate tectonics theory. Plate tectonics theory explains why: Earths geography has changed over time and continues to change today. some places are prone to earthquakes while others are not. certain regions may have deadly, mild, or no volcanic eruptions. mountain ranges are located where they are. many ore deposits are located where they are. living and fossil species are found where they are. Plate tectonic motions affect Earths rock cycle, climate, and the evolution of life. "
plate tectonics through earth history,T_1551,"Remember that Wegener used the similarity of the mountains on the west and east sides of the Atlantic as evidence for his continental drift hypothesis. Those mountains rose at the convergent plate boundaries where the continents were smashing together to create Pangaea. As Pangaea came together about 300 million years ago, the continents were separated by an ocean where the Atlantic is now. The proto-Atlantic ocean shrank as the Pacific Ocean grew. The Appalachian mountains of eastern North America formed at a convergent plate boundary as Pangaea came together (Figure 1.1). About 200 million years ago, the they were probably as high as the Himalayas, but they have been weathered and eroded significantly since the breakup of Pangaea. Pangaea has been breaking apart since about 250 million years ago. Divergent plate boundaries formed within the continents to cause them to rift apart. The continents are still moving apart, since the Pacific is shrinking as the Atlantic is growing. If the continents continue in their current directions, they will come together to create a supercontinent on the other side of the planet in around 200 million years. If you go back before Pangaea there were earlier supercontinents, such as Rodinia, which existed 750 million to 1.1 billion years ago, and Columbia, at 1.5 to 1.8 billion years ago. This supercontinent cycle is responsible for most of the geologic features that we see and many more that are long gone (Figure 1.2). Scientists think that the creation and breakup of a supercontinent takes place about every 500 million years. The supercontinent before Pangaea was Rodinia. A new continent will form as the Pacific ocean disappears. Click image to the left or use the URL below. URL: "
precipitation,T_1563,Precipitation (Figure 1.1) is an extremely important part of weather. Water vapor condenses and usually falls to create precipitation. 
precipitation,T_1564,Some precipitation forms in place. Dew forms when moist air cools below its dew point on a cold surface. Frost is dew that forms when the air temperature is below freezing. 
precipitation,T_1565,"The most common precipitation comes from clouds. Rain or snow droplets grow as they ride air currents in a cloud and collect other droplets (Figure 1.2). They fall when they become heavy enough to escape from the rising air currents that hold them up in the cloud. One million cloud droplets will combine to make only one rain drop! If temperatures are cold, the droplet will hit the ground as snow. (a) Dew on a flower. (b) Hoar frost. (a) Rain falls from clouds when the temperature is fairly warm. (b) Snow storm in Helsinki, Finland. Other less common types of precipitation are sleet (Figure 1.3). Sleet is rain that becomes ice as it hits a layer of freezing air near the ground. If a frigid raindrop freezes on the frigid ground, it forms glaze. Hail forms in cumulonimbus clouds with strong updrafts. An ice particle travels until it finally becomes too heavy and it drops. (a) Sleet. (b) Glaze. (c) Hail. This large hail stone is about 6 cm (2.5 inches) in diameter. Click image to the left or use the URL below. URL: "
predicting earthquakes,T_1566,Scientists are a long way from being able to predict earthquakes. A good prediction must be detailed and accurate. Where will the earthquake occur? When will it occur? What will be the magnitude of the quake? With a good prediction authorities could get people to evacuate. An unnecessary evacuation is expensive and causes people not to believe authorities the next time an evacuation is ordered. 
predicting earthquakes,T_1567,Where an earthquake will occur is the easiest feature to predict. How would you predict this? Scientists know that earthquakes take place at plate boundaries and tend to happen where theyve occurred before (Figure 1.1). Fault segments behave consistently. A segment with frequent small earthquakes or one with infrequent huge earthquakes will likely do the same thing in the future. The probabilities of earthquakes striking along various faults in the San Francisco area between 2003 (when the work was done) and 2032. 
predicting earthquakes,T_1568,"When an earthquake will occur is much more difficult to predict. Since stress on a fault builds up at the same rate over time, earthquakes should occur at regular intervals (Figure 1.2). But so far scientists cannot predict when quakes will occur even to within a few years. Click image to the left or use the URL below. URL:  Around Parkfield, California, an earth- quake of magnitude 6.0 or higher occurs about every 22 years. So seismologists predicted that one would strike in 1993, but that quake came in 2004 - 11 years late. Click image to the left or use the URL below. URL: "
predicting earthquakes,T_1569,"Signs sometimes come before a large earthquake. Small quakes, called foreshocks, sometimes occur a few seconds to a few weeks before a major quake. However, many earthquakes do not have foreshocks, and small earthquakes are not necessarily followed by a large earthquake. Ground tilting, caused by the buildup of stress in the rocks, may precede a large earthquake, but not always. Water levels in wells fluctuate as water moves into or out of fractures before an earthquake. This is also an uncertain predictor of large earthquakes. The relative arrival times of P-waves and S-waves also decreases just before an earthquake occurs. Folklore tells of animals behaving erratically just before an earthquake. Mostly, these anecdotes are told after the earthquake. If indeed animals sense danger from earthquakes or tsunami, scientists do not know what it is they could be sensing, but they would like to find out. Earthquake prediction is very difficult and not very successful, but scientists are looking for a variety of clues in a variety of locations and to try to advance the field. "
predicting volcanic eruptions,T_1570,"Many pieces of evidence can mean that a volcano is about to erupt, but the time and magnitude of the eruption are difficult to pin down. This evidence includes the history of previous volcanic activity, earthquakes, slope deformation, and gas emissions. "
predicting volcanic eruptions,T_1571,"A volcanos history  how long since its last eruption and the time span between its previous eruptions  is a good first step to predicting eruptions. Active and dormant volcanoes are heavily monitored, especially in populated areas. "
predicting volcanic eruptions,T_1572,"Moving magma shakes the ground, so the number and size of earthquakes increases before an eruption. A volcano that is about to erupt may produce a sequence of earthquakes. Scientists use seismographs that record the length and strength of each earthquake to try to determine if an eruption is imminent. "
predicting volcanic eruptions,T_1573,"Magma and gas can push the volcanos slope upward. Most ground deformation is subtle and can only be detected by tiltmeters, which are instruments that measure the angle of the slope of a volcano. But ground swelling may sometimes create huge changes in the shape of a volcano. Mount St. Helens grew a bulge on its north side before its 1980 eruption. Ground swelling may also increase rock falls and landslides. "
predicting volcanic eruptions,T_1574,"Gases may be able to escape a volcano before magma reaches the surface. Scientists measure gas emissions in vents on or around the volcano. Gases, such as sulfur dioxide (SO2 ), carbon dioxide (CO2 ), hydrochloric acid (HCl), and even water vapor can be measured at the site (Figure 1.1) or, in some cases, from a distance using satellites. The amounts of gases and their ratios are calculated to help predict eruptions. Scientists monitoring gas emissions at Mount St. Helens. "
predicting volcanic eruptions,T_1575,"Some gases can be monitored using satellite technology (Figure 1.2). Satellites also monitor temperature readings and deformation. As technology improves, scientists are better able to detect changes in a volcano accurately and safely. "
predicting volcanic eruptions,T_1576,"Since volcanologists are usually uncertain about an eruption, officials may not know whether to require an evac- uation. If people are evacuated and the eruption doesnt happen, the people will be displeased and less likely to evacuate the next time there is a threat of an eruption. The costs of disrupting business are great. However, scientists continue to work to improve the accuracy of their predictions. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
scales that represent earthquake magnitude,T_1648,"People have always tried to quantify the size of and damage done by earthquakes. Since early in the 20th century, there have been three methods. What are the strengths and weaknesses of each? "
scales that represent earthquake magnitude,T_1649,Earthquakes are described in terms of what nearby residents felt and the damage that was done to nearby structures. What factors would go into determining the damage that was done and what the residents felt in a region? 
scales that represent earthquake magnitude,T_1650,"Developed in 1935 by Charles Richter, this scale uses a seismometer to measure the magnitude of the largest jolt of energy released by an earthquake. "
scales that represent earthquake magnitude,T_1651,This scale measures the total energy released by an earthquake. Moment magnitude is calculated from the area of the fault that is ruptured and the distance the ground moved along the fault. 
scales that represent earthquake magnitude,T_1652,The Richter scale and the moment magnitude scale are logarithmic scales. The amplitude of the largest wave increases ten times from one integer to the next. An increase in one integer means that thirty times more energy was released. These two scales often give very similar measurements. How does the amplitude of the largest seismic wave of a magnitude 5 earthquake compare with the largest wave of a magnitude 4 earthquake? How does it compare with a magnitude 3 quake? The amplitude of the largest seismic wave of a magnitude 5 quake is 10 times that of a magnitude 4 quake and 100 times that of a magnitude 3 quake. How does an increase in two integers on the moment magnitude scale compare in terms of the amount of energy released? Two integers equals a 900-fold increase in released energy. 
scales that represent earthquake magnitude,T_1653,"Which scale do you think is best? With the Richter scale, a single sharp jolt measures higher than a very long intense earthquake that releases more energy. The moment magnitude scale more accurately reflects the energy released and the damage caused. Most seismologists now use the moment magnitude scale. The way scientists measure earthquake intensity and the two most common scales, Richter and moment magnitude, are described in the video below. Click image to the left or use the URL below. URL: "
soil characteristics,T_1688,"Soil is a complex mixture of different materials. About half of most soils are inorganic materials, such as the products of weathered rock, including pebbles, sand, silt, and clay particles. About half of all soils are organic materials, formed from the partial breakdown and decomposition of plants and animals. The organic materials are necessary for a soil to be fertile. The organic portion provides the nutrients, such as nitrogen, needed for strong plant growth. In between the solid pieces, there are tiny spaces filled with air and water. Within the soil layer, important reactions between solid rock, liquid water, air, and living things take place. In some soils, the organic portion could be missing, as in desert sand. Or a soil could be completely organic, such as the materials that make up peat in a bog or swamp (Figure 1.1). "
soil characteristics,T_1689,"The inorganic portion of soil is made of many different size particles, and these different size particles are present in different proportions. The combination of these two factors determines some of the properties of the soil. A permeable soil allows water to flow through it easily because the spaces between the inorganic particles are large and well connected. Sandy or silty soils are considered ""light"" soils because they are permeable, water-draining types of soils. Soils that have lots of very small spaces are water-holding soils. For example, when clay is present in a soil, the soil is heavier, holds together more tightly, and holds water. When a soil contains a mixture of grain sizes, the soil is called a loam (Figure 1.2). A loam field. "
soil characteristics,T_1690,"When soil scientists want to precisely determine soil type, they measure the percentage of sand, silt, and clay. They plot this information on a triangular diagram, with each size particle at one corner (Figure 1.3). The soil type can then be determined from the location on the diagram. At the top, a soil would be clay; at the left corner, it would be sand; at the right corner, it would be silt. Soils in the lower middle with less than 50% clay are loams. Soil types by particle size. "
soil characteristics,T_1691,"Soil is an ecosystem unto itself. In the spaces of soil, there are thousands or even millions of living organisms. Those organisms could include earthworms, ants, bacteria, or fungi (Figure 1.4). "
soil erosion,T_1692,"The agents of soil erosion are the same as the agents of all types of erosion: water, wind, ice, or gravity. Running water is the leading cause of soil erosion, because water is abundant and has a lot of power. Wind is also a leading cause of soil erosion because wind can pick up soil and blow it far away. Activities that remove vegetation, disturb the ground, or allow the ground to dry are activities that increase erosion. What are some human activities that increase the likelihood that soil will be eroded? "
soil erosion,T_1693,"Agriculture is probably the most significant activity that accelerates soil erosion because of the amount of land that is farmed and how much farming practices disturb the ground (Figure 1.1). Farmers remove native vegetation and then plow the land to plant new seeds. Because most crops grow only in spring and summer, the land lies fallow during the winter. Of course, winter is also the stormy season in many locations, so wind and rain are available to wash soil away. Tractor tires make deep grooves, which are natural pathways for water. Fine soil is blown away by wind. The soil that is most likely to erode is the nutrient-rich topsoil, which degrades the farmland. (a) The bare areas of farmland are especially vulnerable to erosion. (b) Slash-and-burn agriculture leaves land open for soil erosion and is one of the leading causes of soil erosion in the world. "
soil erosion,T_1694,"Grazing animals (Figure 1.2) wander over large areas of pasture or natural grasslands eating grasses and shrubs. Grazers expose soil by removing the plant cover for an area. They also churn up the ground with their hooves. If too many animals graze the same land area, the animals hooves pull plants out by their roots. A land is overgrazed if too many animals are living there. Grazing animals can cause erosion if they are allowed to overgraze and remove too much or all of the vegetation in a pasture. "
soil erosion,T_1695,"Logging removes trees that protect the ground from soil erosion. The tree roots hold the soil together and the tree canopy protects the soil from hard falling rain. Logging results in the loss of leaf litter, or dead leaves, bark, and branches on the forest floor. Leaf litter plays an important role in protecting forest soils from erosion (Figure 1.3). Logging exposes large areas of land to erosion. Much of the worlds original forests have been logged. Many of the tropical forests that remain are currently the site of logging because North America and Europe have already harvested many of their trees (Figure 1.4). Soils eroded from logged forests clog rivers and lakes, fill estuaries, and bury coral reefs. Surface mining disturbs the land (Figure 1.5) and leaves the soil vulnerable to erosion. "
soil erosion,T_1696,"Constructing buildings and roads churns up the ground and exposes soil to erosion. In some locations, native landscapes, such as forest and grassland, are cleared, exposing the surface to erosion (in some locations the land that will be built on is farmland). Near construction sites, dirt, picked up by the wind, is often in the air. Completed construction can also contribute to erosion (Figure 1.6). "
soil erosion,T_1697,"Recreational activities may accelerate soil erosion. Off-road vehicles disturb the landscape and the area eventually develops bare spots where no plants can grow. In some delicate habitats, even hikers boots can disturb the ground, so its important to stay on the trail (Figure 1.7). Soil erosion is as natural as any other type of erosion, but human activities have greatly accelerated soil erosion. In some locations soil erosion may occur about 10 times faster than its natural rate. Since Europeans settled in North America, about one-third of the topsoil in the area that is now the United States has eroded away. Deforested swatches in Brazil show up as gray amid the bright red tropical rainfor- est. (a) Disturbed land at a coal mine pit in Germany. (b) This coal mine in West Virginia covers more than 10,000 acres (15.6 square miles). Some of the exposed ground is being reclaimed by planting trees. Click image to the left or use the URL below. URL:  (a) ATVS churn up the soil, accelerating erosion. (b) Hiking trails may become eroded. Click image to the left or use the URL below. URL: "
soil formation,T_1698,"How well soil forms and what type of soil forms depends on several different factors, which are described below. "
soil formation,T_1699,"Scientists know that climate is the most important factor determining soil type because, given enough time, different rock types in a given climate will produce a similar soil (Figure 1.1). Even the same rock type in different climates will not produce the same type of soil. This is true because most rocks on Earth are made of the same eight elements and when the rock breaks down to become soil, those elements dominate. The same factors that lead to increased weathering also lead to greater soil formation. More rain equals more chemical reactions to weather minerals and rocks. Those reactions are most efficient in the top layers of the soil, where the water is fresh and has not yet reacted with other materials. Increased rainfall increases the amount of rock that is dissolved as well as the amount of material that is carried away by moving water. As materials are carried away, new surfaces are exposed, which also increases the rate of weathering. Climate is the most important factor in determining the type of soil that will form in a particular area. Increased temperature increases the rate of chemical reactions, which also increases soil formation. In warmer regions, plants and bacteria grow faster, which helps to weather material and produce soils. In tropical regions, where temperature and precipitation are consistently high, thick soils form. Arid regions have thin soils. Soil type also influences the type of vegetation that can grow in the region. We can identify climate types by the types of plants that grow there. "
soil formation,T_1700,"The original rock is the source of the inorganic portion of the soil. The minerals that are present in the rock determine the composition of the material that is available to make soil. Soils may form in place or from material that has been moved. Residual soils form in place. The underlying rock breaks down to form the layers of soil that reside above it. Only about one-third of the soils in the United States are residual. Transported soils have been transported in from somewhere else. Sediments can be transported into an area by glaciers, wind, water, or gravity. Soils form from the loose particles that have been transported to a new location and deposited. "
soil formation,T_1701,"The steeper the slope, the less likely material will be able to stay in place to form soil. Material on a steep slope is likely to go downhill. Materials will accumulate and soil will form where land areas are flat or gently undulating. "
soil formation,T_1702,"Soils thicken as the amount of time available for weathering increases. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration. "
soil formation,T_1703,"The partial decay of plant material and animal remains produces the organic material and nutrients in soil. In soil, decomposing organisms breakdown the complex organic molecules of plant matter and animal remains to form simpler inorganic molecules that are soluble in water. Decomposing organisms also create organic acids that increase the rate of weathering and soil formation. Bacteria in the soil change atmospheric nitrogen into nitrates. The decayed remains of plant and animal life are called humus, which is an extremely important part of the soil. Humus coats the mineral grains. It binds them together into clumps that then hold the soil together, creating its structure. Humus increases the soils porosity and water-holding capacity and helps to buffer rapid changes in soil acidity. Humus also helps the soil to hold its nutrients, increasing its fertility. Fertile soils are rich in nitrogen, contain a high percentage of organic materials, and are usually black or dark brown in color. Soils that are nitrogen poor and low in organic material might be gray or yellow or even red in color. Fertile soils are more easily cultivated. Click image to the left or use the URL below. URL: "
soil horizons and profiles,T_1704,"A residual soil forms over many years, as mechanical and chemical weathering slowly change solid rock into soil. The development of a residual soil may go something like this. 1. 2. 3. 4. 5. The bedrock fractures because of weathering from ice wedging or another physical process. Water, oxygen, and carbon dioxide seep into the cracks to cause chemical weathering. Plants, such as lichens or grasses, become established and produce biological weathering. Weathered material collects until there is soil. The soil develops soil horizons, as each layer becomes progressively altered. The greatest degree of weather- ing is in the top layer. Each successive, lower layer is altered just a little bit less. This is because the first place where water and air come in contact with the soil is at the top. A cut in the side of a hillside shows each of the different layers of soil. All together, these are called a soil profile (Figure 1.1). The simplest soils have three horizons. Soil is an important resource. Each soil horizon is distinctly visible in this photo- graph. "
soil horizons and profiles,T_1705,"Called the A-horizon, the topsoil is usually the darkest layer of the soil because it has the highest proportion of organic material. The topsoil is the region of most intense biological activity: insects, worms, and other animals burrow through it and plants stretch their roots down into it. Plant roots help to hold this layer of soil in place. In the topsoil, minerals may dissolve in the fresh water that moves through it to be carried to lower layers of the soil. Very small particles, such as clay, may also get carried to lower layers as water seeps down into the ground. "
soil horizons and profiles,T_1706,The B-horizon or subsoil is where soluble minerals and clays accumulate. This layer is lighter brown and holds more water than the topsoil because of the presence of iron and clay minerals. There is less organic material. Figure A soil profile is the complete set of soil layers. Each layer is called a horizon. 
soil horizons and profiles,T_1707,"The C-horizon is a layer of partially altered bedrock. There is some evidence of weathering in this layer, but pieces of the original rock are seen and can be identified. Not all climate regions develop soils, and not all regions develop the same horizons. Some areas develop as many as five or six distinct layers, while others develop only very thin soils or perhaps no soils at all. Click image to the left or use the URL below. URL: "
staying safe in an earthquake,T_1725,"There are many things you can do to protect yourself before, during, and after an earthquake. "
staying safe in an earthquake,T_1726,"Have an engineer evaluate the house for structural integrity. Make sure the separate pieces  floor, walls, roof, and foundation  are all well-attached to each other. Bracket or brace brick chimneys to the roof. Be sure that heavy objects are not stored in high places. Secure water heaters all around and at the top and bottom. Bolt heavy furniture onto walls with bolts, screws, or strap hinges. Replace halogen and incandescent light bulbs with fluorescent bulbs to lessen fire risk. Check to see that gas lines are made of flexible material so that they do not rupture. Any equipment that uses gas should be well secured. Everyone in the household should know how to shut off the gas line. Prepare an earthquake kit with three days supply of water and food, a radio, and batteries. Place flashlights all over the house and in the glove box of your car. Keep several fire extinguishers around the house to fight small fires. Be sure to have a first aid kit. Everyone should know basic first aid and CPR. Plan in advance how you will evacuate and where you will go. Do not plan on driving, as roadways will likely be damaged. "
staying safe in an earthquake,T_1727,"If you are in a building, get beneath a sturdy table, cover your head, and hold on. Stay away from windows, mirrors, and large furniture. If the building is structurally unsound, get outside as fast as possible. If you are outside, run to an open area away from buildings and power lines that may fall. If you are in a car, stay in the car and stay away from structures that might collapse, such as overpasses, bridges, or buildings. "
staying safe in an earthquake,T_1728,Be aware that aftershocks are likely. Avoid dangerous areas like hillsides that may experience a landslide. Turn off water and power to your home. Use your phone only if there is an emergency. Many people will be trying to get through to emergency services. Be prepared to wait for help or instructions. Assist others as necessary. Click image to the left or use the URL below. URL: 
surface ocean currents,T_1745,Ocean water moves in predictable ways along the ocean surface. Surface currents can flow for thousands of kilometers and can reach depths of hundreds of meters. These surface currents do not depend on weather; they remain unchanged even in large storms because they depend on factors that do not change. Surface currents are created by three things: global wind patterns the rotation of the Earth the shape of the ocean basins Surface currents are extremely important because they distribute heat around the planet and are a major factor influencing climate around the globe. 
surface ocean currents,T_1746,"Winds on Earth are either global or local. Global winds blow in the same directions all the time and are related to the unequal heating of Earth by the Sun  that is, more solar radiation strikes the Equator than the polar regions and the rotation of the Earth  that is, the Coriolis effect. Coriolis was described in the chapter Earth as a Planet. The causes of the global wind patterns will be described in detail in the chapter Atmospheric Processes. Water in the surface currents is pushed in the direction of the major wind belts: trade winds: east to west between the Equator and 30o N and 30o S westerlies: west to east in the middle latitudes polar easterlies: east to west between 50o and 60o north and south of the Equator and the north and south pole "
surface ocean currents,T_1747,"When a surface current collides with land, the current must change direction. In the Figure 1.1, the Atlantic South Equatorial Current travels westward along the Equator until it reaches South America. At Brazil, some of it goes north and some goes south. Because of Coriolis effect, the water goes right in the Northern Hemisphere and left in the Southern Hemisphere. "
surface ocean currents,T_1748,You can see on the map of the major surface ocean currents that the surface ocean currents create loops called gyres (Figure 1.2). The Antarctic Circumpolar Current is unique because it travels uninhibited around the globe. Why is it the only current to go all the way around? Click image to the left or use the URL below. URL:  The major surface ocean currents. 
surface ocean currents,T_1749,The surface currents described above are all large and unchanging. Local surface currents are also found along shorelines (Figure 1.3). Two are longshore currents and rip currents. Rip currents are potentially dangerous currents that carry large amounts of water offshore quickly. Each summer in the United States at least a few people die when they are caught in rip currents. Longshore currents move water and sediment parallel to the shore in the direction of the prevailing local winds. 
temperature and heat in the atmosphere,T_1751,"Temperature is a measure of how fast the atoms in a material are vibrating. High temperature particles vibrate faster than low temperature particles. Rapidly vibrating atoms smash together, which generates heat. As a material cools down, the atoms vibrate more slowly and collide less frequently. As a result, they emit less heat. What is the difference between heat and temperature? Temperature measures how fast a materials atoms are vibrating. Heat measures the materials total energy. "
temperature and heat in the atmosphere,T_1752,"Heat energy is transferred between physical entities. Heat is taken in or released when an object changes state, or changes from a gas to a liquid, or a liquid to a solid. This heat is called latent heat. When a substance changes state, latent heat is released or absorbed. A substance that is changing its state of matter does not change temperature. All of the energy that is released or absorbed goes toward changing the materials state. For example, imagine a pot of boiling water on a stove burner: that water is at 100o C (212o F). If you increase the temperature of the burner, more heat enters the water. The water remains at its boiling temperature, but the additional energy goes into changing the water from liquid to gas. With more heat the water evaporates more rapidly. When water changes from a liquid to a gas it takes in heat. Since evaporation takes in heat, this is called evaporative cooling. Evaporative cooling is an inexpensive way to cool homes in hot, dry areas. Substances also differ in their specific heat, the amount of energy needed to raise the temperature of one gram of the material by 1.0o C (1.8o F). Water has a very high specific heat, which means it takes a lot of energy to change the temperature of water. Lets compare a puddle and asphalt, for example. If you are walking barefoot on a sunny day, which would you rather walk across, the shallow puddle or an asphalt parking lot? Because of its high specific heat, the water stays cooler than the asphalt, even though it receives the same amount of solar radiation. "
thunderstorms,T_1771,"Thunderstorms are extremely common. Worldwide there are 14 million per year  thats 40,000 per day! Most drop a lot of rain on a small area quickly, but some are severe and highly damaging. "
thunderstorms,T_1772,"Thunderstorms form when ground temperatures are high, ordinarily in the late afternoon or early evening in spring and summer. The two figures below show two stages of thunderstorm buildup (Figure 1.1). Click image to the left or use the URL below. URL:  (a) Cumulus and cumulonimbus clouds. (b) A thunderhead. "
thunderstorms,T_1773,"As temperatures increase, warm, moist air rises. These updrafts first form cumulus and then cumulonimbus clouds. Winds at the top of the troposphere blow the cloud top sideways to make the anvil shape that characterizes a cloud as a thunderhead. As water vapor condenses to form a cloud, the latent heat makes the air in the cloud warmer than the air outside the cloud. Water droplets and ice fly up through the cloud in updrafts. When these droplets get heavy enough, they fall. A mature thunderstorm with updrafts and downdrafts that reach the ground. This starts a downdraft, and soon there is a convection cell within the cloud. The cloud grows into a cumulonimbus "
thunderstorms,T_1774,"The downdrafts cool the air at the base of the cloud, so the air is no longer warm enough to rise. As a result, convection shuts down. Without convection, water vapor does not condense, no latent heat is released, and the thunderhead runs out of energy. A thunderstorm usually ends only 15 to 30 minutes after it begins, but other thunderstorms may start in the same area. "
thunderstorms,T_1775,"With severe thunderstorms, the downdrafts are so intense that when they hit the ground, warm air from the ground is sent upward into the storm. The warm air gives the convection cells more energy. Rain and hail grow huge before gravity pulls them to Earth. Severe thunderstorms can last for hours and can cause a lot of damage because of high winds, flooding, intense hail, and tornadoes. "
thunderstorms,T_1776,"Thunderstorms can form individually or in squall lines along a cold front. In the United States, squall lines form in spring and early summer in the Midwest, where the maritime tropical (mT) air mass from the Gulf of Mexico meets the continental polar (cP) air mass from Canada (Figure 1.3). Cold air from the Rockies collided with warm, moist air from the Gulf of Mexico to form this squall line. "
thunderstorms,T_1777,"So much energy collects in cumulonimbus clouds that a huge release of electricity, called lightning, may result (Figure 1.4). The electrical discharge may be between one part of the cloud and another, two clouds, or a cloud and the ground. Lightning heats the air so that it expands explosively. The loud clap is thunder. Light waves travel so rapidly that lightning is seen instantly. Sound waves travel much more slowly, so a thunderclap may come many seconds after the lightning is spotted. Lightning behind the town of Diamond Head, Hawaii. "
thunderstorms,T_1778,"Thunderstorms kill approximately 200 people in the United States and injure about 550 Americans per year, mostly from lightning strikes. Have you heard the common misconception that lightning doesnt strike the same place twice? In fact, lightning strikes the New York Citys Empire State Building about 100 times per year (Figure 1.5). Lightning strikes some places many times a year, such as the Eiffel Tower in Paris. "
transform plate boundaries,T_1787,"With transform plate boundaries, the two slabs of lithosphere are sliding past each other in opposite directions. The boundary between the two plates is a transform fault. "
transform plate boundaries,T_1788,"Transform faults on continents separate two massive plates of lithosphere. As they slide past each other, they may have massive earthquakes. The San Andreas Fault in California is perhaps the worlds most famous transform fault. Land on the west side is moving northward relative to land on the east side. This means that Los Angeles is moving northward relative to Palm Springs. The San Andreas Fault is famous because it is the site of many earthquakes, large and small. (Figure At the San Andreas Fault in California, the Pacific Plate is sliding northeast relative to the North American plate, which is moving southwest. At the northern end of the picture, the transform boundary turns into a subduction zone. Transform plate boundaries are also found in the oceans. They divide mid-ocean ridges into segments. In the diagram of western North America, the mid-ocean ridge up at the top, labeled the Juan de Fuca Ridge, is broken apart by a transform fault in the oceans. A careful look will show that different plates are found on each side of the ridge: the Juan de Fuca plate on the east side and the Pacific Plate on the west side. "
types of soils,T_1814,"Although soil scientists recognize thousands of types of soil - each with its own specific characteristics and name - lets consider just three soil types. This will help you to understand some of the basic ideas about how climate produces a certain type of soil, but there are many exceptions to what we will learn right now (Figure 1.1). Just some of the thousands of soil types. "
types of soils,T_1815,"Deciduous trees, the trees that lose their leaves each winter, need at least 65 cm of rain per year. These forests produce soils called pedalfers, which are common in many areas of the temperate, eastern part of the United States (Figure 1.2). The word pedalfer comes from some of the elements that are commonly found in the soil. The ""Al"" in pedalfer is the chemical symbol of the element aluminum, and the ""Fe"" in pedalfer is the chemical symbol for iron. Pedalfers are usually a very fertile, dark brown or black soil. Not surprisingly, they are rich in aluminum clays and iron oxides. Because a great deal of rainfall is common in this climate, most of the soluble minerals dissolve and are carried away, leaving the less soluble clays and iron oxides behind. A pedalfer is the dark, fertile type of soil that will form in a forested region. "
types of soils,T_1816,"Pedocal soils form in drier, temperate areas where grasslands and brush are the usual types of vegetation (Figure chemical weathering and less water to dissolve away soluble minerals, so more soluble minerals are present and fewer clay minerals are produced. It is a drier region with less vegetation, so the soils have lower amounts of organic material and are less fertile. A pedocal is named for the calcite enriched layer that forms. Water begins to move down through the soil layers, but before it gets very far, it begins to evaporate. Soluble minerals, like calcium carbonate, concentrate in a layer that marks the lowest place that water was able to reach. This layer is called caliche. A lizard on soil typical of an arid region in Mexico. "
types of soils,T_1817,"In tropical rainforests where it rains literally every day, laterite soils form (Figure 1.4). In these hot, wet, tropical regions, intense chemical weathering strips the soils of their nutrients. There is practically no humus. All soluble minerals are removed from the soil and all plant nutrients are carried away. All that is left behind are the least soluble materials, like aluminum and iron oxides. These soils are often red in color from the iron oxides. Laterite soils bake as hard as a brick if they are exposed to the Sun. A laterite is the type of thick, nutrient-poor soil that forms in the rainforest. Many climate types have not been mentioned here. Each produces a distinctive soil type that forms in the particular circumstances found there. Where there is less weathering, soils are thinner but soluble minerals may be present. Where there is intense weathering, soils may be thick but nutrient-poor. Soil development takes a very long time, it may take hundreds or even thousands of years for a good fertile topsoil to form. Soil scientists estimate that in the very best soil-forming conditions, soil forms at a rate of about 1mm/year. In poor conditions, soil formation may take thousands of years! "
types of volcanoes,T_1818,"A volcano is a vent through which molten rock and gas escape from a magma chamber. Volcanoes differ in many features, such as height, shape, and slope steepness. Some volcanoes are tall cones and others are just cracks in the ground (Figure 1.1). As you might expect, the shape of a volcano is related to the composition of its magma. "
types of volcanoes,T_1819,Composite volcanoes are constructed of felsic to intermediate rock. The viscosity of the lava means that eruptions at these volcanoes are often explosive. 
types of volcanoes,T_1820,"Viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopes of a composite volcano. In some eruptions the pressure builds up so much that the material explodes as ash and small rocks. The volcano is constructed layer by layer, as ash and lava solidify, one upon the other (Figure 1.3). The result is the classic cone shape of composite volcanoes. Mount St. Helens was a beautiful, classic, cone-shaped volcano. In May 1980 the volcano blew its top off in an explosive eruption, losing 1,300 feet off its summit. Mt. Fuji in Japan is one of the worlds most easily recognized composite volca- noes. "
types of volcanoes,T_1821,"Shield volcanoes get their name from their shape. Although shield volcanoes are not steep, they may be very large. Shield volcanoes are common at spreading centers or intraplate hot spots (Figure 1.4). Hawaii has some spectacular shield volcanoes including Mauna Kea, which is the largest mountain on Earth from base to top. The mountain stands 33,500 ft high, about 4,000 feet greater than the tallest mountain above sea level, Mt. Everest. A cross section of a composite volcano reveals alternating layers of rock and ash: (1) magma chamber, (2) bedrock, (3) pipe, (4) ash layers, (5) lava layers, (6) lava flow, (7) vent, (8) lava, (9) ash cloud. Frequently there is a large crater at the top from the last eruption. Mauna Kea on the Big Island of Hawaii is a classic shield volcano. "
types of volcanoes,T_1822,The lava that creates shield volcanoes is fluid and flows easily. The spreading lava creates the shield shape. Shield volcanoes are built by many layers over time and the layers are usually of very similar composition. The low viscosity also means that shield eruptions are non-explosive. 
types of volcanoes,T_1823,"Cinder cones are the most common type of volcano. A cinder cone has a cone shape, but is much smaller than a composite volcano. Cinder cones rarely reach 300 meters in height, but they have steep sides. Cinder cones grow rapidly, usually from a single eruption cycle. These volcanoes usually flank shield or composite volcanoes. Many cinder cones are found in Hawaii. A lava fountain erupts from Puu Oo, a cinder cone on Kilauea. "
types of volcanoes,T_1824,"Cinder cones are composed of small fragments of rock, such as pumice, piled on top of one another. The rock shoots up in the air and doesnt fall far from the vent. The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit. Most cinder cones are active only for a single eruption. Click image to the left or use the URL below. URL: "
volcanic landforms,T_1848,"The most obvious landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes. Eruptions also take place through other types of vents, commonly from fissures (Figure 1.1). The eruptions that created the entire ocean floor are essentially fissure eruptions. "
volcanic landforms,T_1849,"Viscous lava flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome. Because it is so thick, the lava does not flow far from the vent. (Figure 1.2). Lava flows often make mounds right in the middle of craters at the top of volcanoes, as seen in the Figure 1.3. A fissure eruption on Mauna Loa in Hawaii travels toward Mauna Kea on the Big Is- land. Lava domes are large, round landforms created by thick lava that does not travel far from the vent. Lava domes may form in the crater of composite volcanoes as at Mount St. He- lens. "
volcanic landforms,T_1850,"A lava plateau forms when large amounts of fluid lava flow over an extensive area (Figure 1.4). When the lava solidifies, it creates a large, flat surface of igneous rock. Layer upon layer of basalt have created the Columbia Plateau, which covers more than 161,000 square kilometers (63,000 square miles) in Washington, Oregon, and Idaho. "
volcanic landforms,T_1851,Lava creates new land as it solidifies on the coast or emerges from beneath the water (Figure 1.5). Lava flowing into the sea creates new land in Hawaii. Over time the eruptions can create whole islands. The Hawaiian Islands are formed from shield volcano eruptions that have grown over the last 5 million years (Figure 1.6). The island of Hawaii was created by hotspot volcanism. You can see some of the volcanoes (both active and extinct) in this mosaic of false-color composite satellite images. 
volcanic landforms,T_1852,Magma intrusions can create landforms. Shiprock in New Mexico is the neck of an old volcano that has eroded away (Figure 1.7). The volcanic neck is the remnant of the conduit the magma traveled up to feed an eruption. The aptly named Shiprock in New Mexico. 
volcano characteristics,T_1853,"A volcano is a vent from which the material from a magma chamber escapes. Volcanic eruptions can come from peaky volcanic cones, fractured domes, a vent in the ground, or many other types of structures. "
volcano characteristics,T_1854,"Volcanoes are a vibrant manifestation of plate tectonics processes. Volcanoes are common along convergent and di- vergent plate boundaries. Volcanoes are also found within lithospheric plates away from plate boundaries. Wherever mantle is able to melt, volcanoes may be the result. What is the geological reason for the locations of all the volcanoes in the figure? Does it resemble the map of earthquake epicenters? Are all of the volcanoes located along plate boundaries? Why are the Hawaiian volcanoes located away from any plate boundaries? World map of active volcanoes (red dots). "
volcano characteristics,T_1855,"Volcanoes erupt because mantle rock melts. This is the first stage in creating a volcano. Remember from the chapter Materials of Earths Crust that mantle may melt if temperature rises, pressure lowers, or water is added. Be sure to think about how and why melting occurs in the settings where there is volcanism mentioned in the next few concepts. "
volcano characteristics,T_1856,"Of all the volcanoes in the world, very few are erupting at any given time. Scientists question whether a volcano that is not erupting will ever erupt again and then describe it as active, dormant, or extinct. Active: currently erupting or showing signs of erupting soon. Dormant: no current activity, but has erupted recently. Extinct: no activity for some time; will probably not erupt again. Click image to the left or use the URL below. URL: "
volcanoes at hotspots,T_1857,"Although most volcanoes are found at convergent or divergent plate boundaries, intraplate volcanoes may be found in the middle of a tectonic plate. These volcanoes rise at a hotspot above a mantle plume. Melting at a hotspot is due to pressure release as the plume rises through the mantle. Earth is home to about 50 known hotspots. Most of these are in the oceans because they are better able to penetrate oceanic lithosphere to create volcanoes. But there are some large ones in the continents. Yellowstone is a good example of a mantle plume erupting within a continent. "
volcanoes at hotspots,T_1858,"The South Pacific has many hotspot volcanic chains. The hotspot is beneath the youngest volcano in the chain and older volcanoes are found to the northwest. A volcano forms above the hotspot, but as the Pacific Plate moves, that volcano moves off the hotspot. Without its source of volcanism, it no longer erupts. The crust gets cooler and the volcano erodes. The result is a chain of volcanoes and seamounts trending northwest from the hotspot. Prominent hotspots of the world. (a) The Society Islands formed above a hotspot that is now beneath Mehetia and two submarine volcanoes. (b) The satellite image shows how the islands become smaller and coral reefs became more developed as the volcanoes move off the hotspot and grow older. The most famous example of a hotspot in the oceans is the Hawaiian Islands. Forming above the hotspot are massive shield volcanoes that together create the islands. The lavas are mafic and have low viscosity. These lavas produce beautiful ropy flows of pahoehoe and clinkery flows of aa, which will be described in more detail in Effusive Eruptions. "
volcanoes at hotspots,T_1859,"The hotspots that are known beneath continents are extremely large. The reason is that it takes a massive mantle plume to generate enough heat to penetrate through the relatively thick continental crust. The eruptions that come from these hotspots are infrequent but massive, often felsic and explosive. All thats left at Yellowstone at the moment is a giant caldera and a very hot spot beneath. "
volcanoes at hotspots,T_1860,"How would you be able to tell hotspot volcanoes from island arc volcanoes? At island arcs, the volcanoes are all about the same age. By contrast, at hotspots the volcanoes are youngest at one end of the chain and oldest at the other. Click image to the left or use the URL below. URL: "
volcanoes at plate boundaries,T_1861,"Converging plates can be oceanic, continental, or one of each. If both are continental they will smash together and form a mountain range. If at least one is oceanic, it will subduct. A subducting plate creates volcanoes. In the chapter Plate Tectonics we moved up western North America to visit the different types of plate boundaries there. Locations with converging in which at least one plate is oceanic at the boundary have volcanoes. "
volcanoes at plate boundaries,T_1862,"Melting at convergent plate boundaries has many causes. The subducting plate heats up as it sinks into the mantle. Also, water is mixed in with the sediments lying on top of the subducting plate. As the sediments subduct, the water rises into the overlying mantle material and lowers its melting point. Melting in the mantle above the subducting plate leads to volcanoes within an island or continental arc. "
volcanoes at plate boundaries,T_1863,"Volcanoes at convergent plate boundaries are found all along the Pacific Ocean basin, primarily at the edges of the Pacific, Cocos, and Nazca plates. Trenches mark subduction zones, although only the Aleutian Trench and the Java Trench appear on the map in the previous concept, ""Volcano Characteristics."" The Cascades are a chain of volcanoes at a convergent boundary where an oceanic plate is subducting beneath a continental plate. Specifically the volcanoes are the result of subduction of the Juan de Fuca, Gorda, and Explorer Plates beneath North America. The volcanoes are located just above where the subducting plate is at the right depth in the mantle for there to be melting (Figure 1.1). The Cascades have been active for 27 million years, although the current peaks are no more than 2 million years old. The volcanoes are far enough north and are in a region where storms are common, so many are covered by glaciers. "
volcanoes at plate boundaries,T_1864,"At divergent plate boundaries hot mantle rock rises into the space where the plates are moving apart. As the hot mantle rock convects upward it rises higher in the mantle. The rock is under lower pressure; this lowers the melting temperature of the rock and so it melts. Lava erupts through long cracks in the ground, or fissures. "
volcanoes at plate boundaries,T_1865,"Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic ridge, where seafloor spreading creates new seafloor in the rift valleys. Where a hotspot is located along the ridge, such as at Iceland, volcanoes grow high enough to create islands (Figure 1.3). "
volcanoes at plate boundaries,T_1866,Eruptions are found at divergent plate boundaries as continents break apart. The volcanoes in Figure 1.4 are in the East African Rift between the African and Arabian plates. Remember from the chapter Plate Tectonics that Baja California is being broken apart from mainland Mexico as another example of continental rifting. Click image to the left or use the URL below. URL:  The Cascade Range is formed by volca- noes created from subduction of oceanic crust beneath the North American conti- nent. 
sponges and cnidarians,T_1983,"Sponges are aquatic invertebrates that make up Phylum Porifera. The word porifera means pore-bearing. As you can see from the close-up view in Figure 12.3, a sponge has a porous body with many small holes in it. There are at least 5000 living species of sponges. Almost all of them inhabit the ocean. Most live on coral reefs or the ocean floor. Adult sponges are unable to move from place to place. They have root-like projections that anchor them to surfaces. They may live in colonies of many sponges. You can visit the incredible world of sponges by watching this short video:  . MEDIA Click image to the left or use the URL below. URL: "
sponges and cnidarians,T_1984,"Sponges have several different types of specialized cells, although they lack tissues. You can see the basic sponge body plan and specialized cells in Figure 12.4. Some of the specialized cells grow short, sharp projections called spicules. Spicules make up the sponges internal skeleton, or endoskeleton. The endoskeleton helps to support and protect the sponge. Other specialized cells are involved in feeding. Sponges are filter feeders. They filter food out of the water as it flows through them. Sponges pump water into their body through specialized pore cells called porocytes. The water flows through a large central cavity. As it flows by, specialized cells called collar cells trap and digest food particles in the water. Specialized cells called amebocytes carry nutrients from the digested food to the rest of the cells in the sponge. As water flows through the sponge, oxygen diffuses from the water to the sponges cells. The cells also expel wastes into the water. The water then flows out of the sponge through an opening called the osculum. "
sponges and cnidarians,T_1985,"Sponges reproduce both asexually and sexually. Asexual reproduction occurs by budding. Sexual reproduction occurs by the production of eggs and sperm. Males release sperm into the water through the osculum. Sperm may enter a female sponge through a pore and fertilize her eggs. The resulting zygotes develop into larvae. Unlike sponge adults, sponge larvae can swim. They have cilia that propel them through the water. As larvae develop and grow, they become more similar to an adult sponge and lose their ability to swim. "
sponges and cnidarians,T_1986,"Many sponges live on coral reefs, like the one in Figure 12.5. Reef sponges typically have symbiotic relationships with other reef species. For example, the sponges provide shelter for algae, shrimp, and crabs. In return, they get nutrients from the metabolism of the organisms they shelter. Sponges are a source of food for many species of fish. Because sponges are anchored to a reef or rock, they cant run away from predators. However, their sharp spicules provide some defense. They also produce toxins that may poison predators that try to eat them. "
sponges and cnidarians,T_1987,"Cnidarians are invertebrates such as jellyfish and corals. They belong to Phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean. Cnidarians are a little more complex than sponges. Besides specialized cells, they have tissues and radial symmetry. There are more than 10,000 cnidarian species, see Figure 12.6. "
sponges and cnidarians,T_1988,"An interesting feature of all cnidarians is one or more stingers called nematocysts. You can see a nematocyst in the sketch of a hydra in Figure 12.7. The nematocyst is long and thin and has a poison barb on the end. When not in use, it lies coiled inside a special cell. Nematocysts are used to attack prey or defend against predators. Watch this awesome animation to see a nematocyst in action: http://commons.wikimedia.org/wiki/File:Nematocyst.gif Another interesting feature of many cnidarians is the ability to produce light. The production of light by living things is called bioluminescence. A more familiar example of bioluminescence is the light produced by fireflies. In cnidarians, bioluminescence may be used to startle predators or to attract prey or mates. Watch this short video to see an amazing light show put on by a jellyfish at the Monterey Aquarium in Monterey, California:  MEDIA Click image to the left or use the URL below. URL: "
sponges and cnidarians,T_1989,"Cnidarians have two basic body forms, called medusa and polyp: The medusa (medusae, plural) is a bell-shaped form. It is typically able to move. The polyp is a tubular form. It is usually attached to a surface and unable to move. As you can see in Figure 12.8, both body plans have radial symmetry. Some cnidarian species alternate between medusa and polyp forms. Other species exist in just one form or the other. "
sponges and cnidarians,T_1990,"Cnidarians have an incomplete digestive system with a single opening. The opening is surrounded by tentacles, which are covered with nematocyst cells and used to capture prey. Digestion takes place in the digestive cavity. Nutrients are absorbed and gases are exchanged through the cells lining this cavity. Fluid in the cavity supports and stiffens the cnidarian body. Cnidarians have a simple nervous system. It consists of a net of nerves that can sense touch. You can see a sketch of the nerve net in a hydra in Figure 12.9. Some cnidarians also have other sensory structures. For example, jellyfish have light-sensing structures and gravity-sensing structures. "
sponges and cnidarians,T_1991,"Cnidarians in the polyp form usually reproduce asexually. One type of asexual reproduction in polyps leads to the formation of new medusae. Medusae usually reproduce sexually with sperm and eggs. Fertilization forms a zygote. The zygote develops into a larva, and the larva develops into a polyp. There are many variations on this general life cycle. Obviously, species that exist only as polyps or medusae have a life cycle without the other form. "
sponges and cnidarians,T_1992,"Cnidarians can be found in almost all ocean habitats. A few species live in fresh water. Jellyfish spend most of their lives as medusae. They live virtually everywhere in the ocean. They prey on zooplankton, other invertebrates, and the eggs and larvae of fish. Corals form large colonies in shallow tropical water. They are confined to shallow water because they have a symbiotic relationship with algae that live inside of them. The algae need sunlight for photosynthesis, so they must stay relatively close to the surface of the water to get enough light. Corals exist only as polyps. They catch plankton with their tentacles. Many corals form a hard, mineral exoskeleton. Over time, this builds up to become a coral reef. A coral reef is pictured in Figure 12.10. Coral reefs provide food and shelter to many other ocean organisms. Watch this beautiful National Geographic video to learn more about corals, coral reefs, and coral reef life: http://video MEDIA Click image to the left or use the URL below. URL: "
characteristics of living organisms,T_2231,Five characteristics are used to define life. All living things share these characteristics. All living things: 1. 2. 3. 4. 5. are made of one or more cells. need energy to stay alive. respond to stimuli in their environment. grow and reproduce. maintain a stable internal environment. 
characteristics of living organisms,T_2232,"Cells are the basic building blocks of life. They are like tiny factories where virtually all life processes take place. Some living things, like the bacteria in Figure 2.1, consist of just one cell. They are called single-celled organisms. You can see other single-celled organisms in Figure 2.2. Some living things are composed of a few to many trillions of cells. They are called multicellular organisms. Your body is composed of trillions of cells. Regardless of the type of organism, all living cells share certain basic structures. For example, all cells are enclosed by a membrane. The cell membrane separates the cell from its environment. It also controls what enters or leaves the cell. "
characteristics of living organisms,T_2233,"Everything you do takes energy. Energy is the ability to change or move matter. Whether its reading these words or running a sprint, it requires energy. In fact, it takes energy just to stay alive. Where do you get energy? You probably know the answer. You get energy from food. Figure {{ref|MS-LS-SE-02-03-Food|below}] shows some healthy foods that can provide you with energy. Just like you, other living things need a source of energy. But they may use a different source. Organisms may be grouped on the basis of the source of energy they use. In which group do you belong? Producers such as the tree in Figure 2.1 use sunlight for energy to produce their own food. The process is called photosynthesis, and the food is sugar. Plants and other organisms use this food for energy. Consumers such as the raccoon in Figure 2.1 eat plantsor other consumers that eat plantsas a source of energy. Some consumers such as the mushroom in Figure 2.1 get their energy from dead organic matter. For example, they might consume dead leaves on a forest floor. "
characteristics of living organisms,T_2234,"When a living thing responds to its environment, it is responding to a stimulus. A stimulus (stimuli, plural) is something in the environment that causes a reaction in an organism. The reaction a stimulus produces is called a response. Imagine how you would respond to the following stimuli: Youre about to cross a street when the walk light turns red. You hear a smoke alarm go off in the kitchen. You step on an upturned tack with a bare foot. You smell the aroma of your favorite food. You taste something really sour. It doesnt take much imagination to realize that responding appropriately to such stimuli might help keep you safe. It might even help you survive. Like you, all other living things sense and respond to stimuli in their environment. In general, their responses help them survive or reproduce. Watch this amazing time-lapse video to see how a plant responds to the stimuli of light and gravity as it grows. Why do you think it is important for a plant to respond appropriately to these stimuli for proper growth? MEDIA Click image to the left or use the URL below. URL: "
characteristics of living organisms,T_2235,"Like plants, all living things have the capacity for growth. The ducklings in Figure 2.4 have a lot of growing to do to catch up in size to their mother. Multicellular organisms like ducks grow by increasing the size and number of their cells. Single-celled organisms just grow in size. As the ducklings grow, they will develop and mature into adults. By adulthood, they will be able to reproduce. Reproduction is the production of offspring. The ability to reproduce is another characteristic of living things. Many organisms reproduce sexually. In sexual reproduction, parents of different sexes mate to produce offspring. The offspring have some combination of the traits of the two parents. Ducks are examples of sexually reproducing organisms. Other organisms reproduce asexually. In asexual reproduction, a single parent can produce offspring alone. For example, a bacterial cell reproduces by dividing into two daughter cells. The daughter cells are identical to each other and to the parent cell. "
characteristics of living organisms,T_2236,"The tennis player in Figure 2.5 has really worked up a sweat. Do you know why we sweat? Sweating helps to keep us cool. When sweat evaporates from the skin, it uses up some of the bodys heat energy. Sweating is one of the ways that the body maintains a stable internal environment. It helps keep the bodys internal temperature constant. When the bodys internal environment is stable, the condition is called homeostasis. All living organisms have ways of maintaining homeostasis. They have mechanisms for controlling such factors as their internal temperature, water balance, and acidity. Homeostasis is necessary for normal life processes that take place inside cells. If an organism cant maintain homeostasis, normal life processes are disrupted. Disease or even death may result. "
classification of living things,T_2251,"Like you, scientists also group together similar organisms. The science of classifying living things is called taxon- omy. Scientists classify living things in order to organize and make sense of the incredible diversity of life. Modern scientists base their classifications mainly on molecular similarities. They group together organisms that have similar proteins and DNA. Molecular similarities show that organisms are related. In other words, they are descendants of a common ancestor in the past. "
classification of living things,T_2252,Carl Linnaeus (1707-1778) is called the father of taxonomy. You may already be familiar with the classification system Linnaeus introduced. 
classification of living things,T_2253,"You can see the main categories, or taxa (taxon, singular), of the Linnaean system in Figure 2.16. As an example, the figure applies the Linnaean system to classify our own species, Homo sapiens. Although the Linnaean system has been revised, it forms the basis of modern classification systems. The broadest category in the Linnaean system is the kingdom. Figure 2.16 shows the Animal Kingdom because Homo sapiens belongs to that kingdom. Other kingdoms include the Plant Kingdom, Fungus Kingdom, and Protist Kingdom. Kingdoms are divided, in turn, into phyla (phylum, singular). Each phylum is divided into classes, each class into orders, each order into families, and each family into genera (genus, singular). Each genus is divided into one or more species. The species is the narrowest category in the Linnaean system. A species is defined as a group of organisms that can breed and produce fertile offspring together. "
classification of living things,T_2254,"Linnaeus is also famous for his method of naming species, which is still used today. The method is called binomial nomenclature. Every species is given a unique two-word name. Usually written in Latin, it includes the genus name followed by the species name. Both names are always written in italics, and the genus name is always capitalized. For example, the human species is named Homo sapiens. The species of the family dog is named Canis familiaris. Coming up with a scientific naming method may not seem like a big deal, but it really is. Prior to Linnaeus, there was no consistent way to name species. Names given to organisms by scientists were long and cumbersome. Often, different scientists came up with different names for the same species. Common names also differed, generally from one place to another. A single, short scientific name for each species avoided a lot of mistakes and confusion. "
classification of living things,T_2255,"When Linnaeus was naming and classifying organisms in the 1700s, almost nothing was known of microorganisms. With the development of powerful microscopes, scientists discovered many single-celled organisms that didnt fit into any of Linnaeus kingdoms. As a result, a new taxon, called the domain, was added to the classification system. The domain is even broader than the kingdom, as you can see in Figure 2.17. Most scientists think that all living things can be classified in three domains: Archaea, Bacteria, and Eukarya. These domains are compared in Table 2.3. The Archaea Domain includes only the Archaea Kingdom, and the Bacteria Domain includes only the Bacteria Kingdom. The Eukarya Domain includes the Animal, Plant, Fungus, and Protist Kingdoms. Trait Multicellularity Archaea No Bacteria No Cell Wall Yes Without peptidoglycan Yes With peptidoglycan Cell Nucleus (DNA inside a membrane) No No Eukarya Yes except for many pro- tists Yes for plants, fungi, and some protists No for animals and other protists Yes Trait Cell Organelles structures membranes) (other inside Archaea No Bacteria No Eukarya Yes The Archaea and Bacteria Domains contain only single-celled organisms. Both Archaea and Bacteria have cells walls, but their cell walls are made of different materials. The cells of Archaea and Bacteria lack a nucleus. A nucleus is membrane-enclosed structure for holding a cells DNA. Some Eukarya are also single-celled, but many are multicellular. Some have a cell wall; others do not. However, the cells of all Eukarya have a nucleus and other organelles. Archaea and Bacteria may seem more similar to each other than either is to Eukarya. However, scientists think that Archaea may actually be more closely related to Eukarya than Bacteria are. This view is based on similarities in their DNA. "
classification of living things,T_2256,"This question was posed at the beginning of the chapter. Should viruses be placed in one of the three domains of life? Are viruses living things? Before considering these questions, you need to know the characteristics of viruses. A virus is nothing more than some DNA or RNA surrounded by a coat of proteins. A virus is not a cell. A virus cannot use energy, respond to stimuli, grow, or maintain homeostasis. A virus cannot reproduce on its own. However, a virus can reproduce by infecting the cell of a living host. Inside the host cell, the virus uses the cells structures, materials, and energy to make copies of itself. Because they have genetic material and can reproduce, viruses can evolve. Their DNA or RNA can change through time. The ability to evolve is a very lifelike attribute. Many scientists think that viruses should not be classified as living things because they lack most of the defining traits of living things. Other scientists arent so sure. They think that the ability of viruses to evolve and interact with living cells earns them special consideration. Perhaps a new category of life should be created for viruses. What do you think? "
first two lines of defense,T_2311,"Your bodys first line of defense is like a castles moat and walls. It keeps most pathogens out of your body. The first line of defense includes physical, chemical, and biological barriers. "
first two lines of defense,T_2312,"The skin is a very important barrier to pathogens. It is the bodys largest organ and the most important defense against disease. It forms a physical barrier between the body and the outside environment. The outer layer of the skin, called the epidermis, consists of dead cells filled with the protein keratin. These cells form a tough, waterproof covering on the body. It is very difficult for pathogens to get through the epidermis. The inside of the mouth and nose are lined with mucous membranes. Other organs that are exposed to substances from the environment are also lined with mucous membranes. These include the respiratory and digestive organs. Mucous membranes arent tough like skin, but they have other ways of keeping out pathogens. One way mucous membranes protect the body is by producing mucus. Mucus is a sticky, moist secretion that covers mucous membranes. The mucus traps pathogens and particles so they cant enter the body. Many mucous membranes are also covered with cilia. These are tiny, hair-like projections. Cilia move in waves and sweep mucus and trapped pathogens toward body openings. You can see this in the diagram in Figure 21.10. When you clear your throat or blow your nose, you remove mucus and pathogens from your body. "
first two lines of defense,T_2313,"In addition to mucus, your body releases a variety of fluids, including tears, saliva, and sweat. These fluids contain enzymes called lysozymes. Lysozymes break down the cell walls of bacteria and kill them. Your stomach contains a very strong acid, called hydrochloric acid. This acid kills most pathogens that enter the stomach in food or water. Urine is also acidic, so few pathogens are able to grow in it. "
first two lines of defense,T_2314,"Your skin is covered by millions of bacteria. Millions more live inside your body, mainly in your gastrointestinal tract. Most of these bacteria are helpful. For one thing, they help defend your body from pathogens. They do it by competing with harmful bacteria for food and space. They prevent the harmful bacteria from multiplying and making you sick. "
first two lines of defense,T_2315,"Did you ever get a splinter in your skin, like the one in Figure 21.11? It doesnt look like a serious injury, but even a tiny break in the skin may let pathogens enter the body. If bacteria enter through the break, for example, they could cause an infection. These bacteria would then face the bodys second line of defense. "
first two lines of defense,T_2316,"If bacteria enter the skin through a splinter or other wound, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. It occurs due to chemicals that are released when tissue is damaged. The chemicals cause nearby blood vessels to dilate, increasing blood flow to the area. The chemicals also attract white blood cells to the area. The white blood cells leak out of the blood vessels and into the damaged tissue. You can see an animation of the inflammatory response by watching this video: MEDIA Click image to the left or use the URL below. URL: "
first two lines of defense,T_2317,The white blood cells that go to a site of inflammation and leak into damaged tissue are called phagocytes. They start eating pathogens and dead cells by engulfing and destroying them. This process is called phagocytosis. You can see how it happens in Figure ??. You can see it in action in the animation at this link: http://commons.wikim 
first two lines of defense,T_2318,"Phagocytes also release chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6 F (37 C). Most bacteria and viruses that infect people reproduce quickly at this temperature. When the temperature rises higher, the pathogens cant reproduce as quickly. Therefore, a fever helps to limit the infection. A fever also causes the immune system to make more white blood cells to fight the infection. "
immune system defenses,T_2319,"The immune system is the body system that fights to protect the body from specific pathogens. It has a special response for each type of pathogen. The immune systems specific reaction to a pathogen is called an immune response. The immune system is shown in Figure 21.13. It includes several organs and a network of vessels that carry lymph. Lymph is a yellowish liquid that normally leaks out of tiny blood vessels into spaces between cells in tissues. When inflammation occurs, more lymph leaks into tissues, and the lymph is likely to contain pathogens. "
immune system defenses,T_2320,"Immune system organs include bone marrow, the thymus gland, the spleen, and the tonsils. Each organ has a different job in the immune system. Bone marrow is found inside many bones. Its role in the immune system is to produce white blood cells called lymphocytes. The thymus gland is in the chest behind the breast bone. It stores some types of lymphocytes while they mature. The spleen is in the abdomen below the lungs. Its job is to filter pathogens out of the blood. The two tonsils are located on either side of the throat. They trap pathogens that enter the body through the mouth or nose. "
immune system defenses,T_2321,"Lymph vessels make up a circulatory system that is similar to the blood vessels of the cardiovascular system. However, lymph vessels circulate lymph instead of blood, and the heart does not pump lymph through the vessels. Lymph that collects in tissues slowly passes into tiny lymph vessels. Lymph then travels from smaller to larger lymph vessels. Muscles around the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels also contract to help move the lymph along. Eventually, lymph reaches the main lymph vessels, which are located in the chest. From these vessels, lymph drains into two large veins of the cardiovascular system. This is how lymph returns to the blood. Before lymph reaches the bloodstream, it passes through small oval structures called lymph nodes, which are located along the lymph vessels. Figure 21.14 shows where some of the bodys many lymph nodes are concentrated. Lymph nodes act like filters and remove pathogens from lymph. "
immune system defenses,T_2322,"A lymphocyte is the type of white blood cell involved in an immune system response. You can see what a lymphocyte looks like, greatly magnified, in Figure 21.15. Lymphocytes make up about one quarter of all white blood cells, but there are trillions of them in the human body. Usually, fewer than half of the bodys lymphocytes are in the blood. The majority are in the lymph, lymph nodes, and lymph organs. There are two main types of lymphocytes, called B cells and T cells. Both types of lymphocytes are produced in bone marrow. They are named for the sites where they grow and mature. The B in B cells stands for bone marrow, where B cells mature. The T in T cells stands for thymus gland, where T cells mature. Both B cells and T cells must be switched on in order to fight a specific pathogen. Once this happens, they produce an army of cells that are ready to fight that particular pathogen. How can B and T cells recognize specific pathogens? Pathogens have unique antigens, often located on their cell surface. Antigens are proteins that the body recognizes either as self or nonself. Self antigens include those found on red blood cells that determine a persons blood type. Generally, the immune system doesnt respond to self antigens. Nonself antigens include those found on bacteria, viruses, and other pathogens. Nonself antigens are also found on other cells, such as pollen cells and cancer cells. It is these antigens that trigger an immune response. "
immune system defenses,T_2323,"There are two different types of immune responses. Both types involve lymphocytes. However, one type of response involves B cells. The other type involves T cells. "
immune system defenses,T_2324,"B cells respond to pathogens in the blood and lymph. Most B cells fight infections by making antibodies. An antibody is a large, Y-shaped molecule that binds to an antigen. Each antibody can bind with just one specific type of antigen. The antibody and antigen fit together like a lock and key. You can see how this works in Figure 21.16. The antibody in the figure can bind only with the type of antigen that is colored yellow. Once the antibody binds with the antigen, it signals a phagocyte to engulf and destroy them, along with the pathogen that carries the antigen on its surface. You can watch an animation of the antibody-antigen binding process at this link:  MEDIA Click image to the left or use the URL below. URL: "
immune system defenses,T_2325,"There are different types of T cells, including killer T cells and helper T cells. Killer T cells destroy infected, damaged, or cancerous body cells. Figure 21.17 shows how a killer T cells destroys an infected cell. When the killer T cell comes into contact with the infected cell, it releases toxins. The toxins make tiny holes in the infected cells membrane. This causes the cell to burst open. Both the infected cell and the pathogens inside it are destroyed. Helper T cells do not destroy infected, damaged, or cancerous body cells. However, they are still needed for an immune response. They help by releasing chemicals that control other lymphocytes. The chemicals released by helper T cells switch on B cells and killer T cells so they can recognize and fight specific pathogens. "
immune system defenses,T_2326,"Most B cells and T cells die after an infection has been brought under control. But some of them survive for many years. They may even survive for a persons lifetime. These long-lasting B and T cells are called memory cells Memory cells allow the immune system to remember a pathogen after the infection is over. If the pathogen invades the body again, the memory cells will start dividing in order to fight it. They will quickly produce a new army of B or T cells to fight the pathogen. They will begin a faster, stronger attack than the first time the pathogen invaded the body. As a result, the immune system will be able to destroy the pathogen before it can cause an infection. Being able to fight off and resist a pathogen in this way is called immunity. You dont have to suffer through an infection to gain immunity to some diseases. Immunity can also come about by vaccination. Vaccination is the process of exposing a person to pathogens on purpose so the person will develop immunity to them. In vaccination, the pathogens are usually injected under the skin. Only part of the pathogens are injected, or else weakened or dead pathogens are used. This causes an immune response without causing the disease. Diseases you are likely to have been vaccinated against include measles, mumps, and chicken pox. "
lifes building blocks,T_2455,Cells were first discovered in the mid-1600s. The cell theory came about some 200 years later. You can see a re- enactment of some of the discoveries that led to the cell theory in this video:  MEDIA Click image to the left or use the URL below. URL: 
lifes building blocks,T_2456,"British scientist Robert Hooke first discovered cells in 1665. He was one of the earliest scientists to study living things under a microscope. He saw that cork was divided into many tiny compartments, like little rooms. (Do the cells in Figure 3.1 look like little rooms to you too?) Hooke called these little rooms cells. Cork comes from trees, so what Hooke observed was dead plant cells. In the late 1600s, Dutch scientist Anton van Leeuwenhoek made more powerful microscopes. He used them to observe cells of other organisms. For example, he saw human blood cells and bacterial cells. Over the next century, microscopes were improved and more cells were observed. "
lifes building blocks,T_2457,"By the early 1800s, scientists had seen cells in many different types of organisms. Every organism that was examined was found to consist of cells. From all these observations, German scientists Theodor Schwann and Matthias Schleiden drew two major conclusions about cells. They concluded that: cells are alive. all living things are made of cells. Around 1850, a German doctor named Rudolf Virchow was observing living cells under a microscope. As he was watching, one of the cells happened to divide. Figure 3.2 shows a cell dividing, like the cell observed by Virchow. This was an aha moment for Virchow. He realized that living cells produce new cells by dividing. This was evidence that cells arise from other cells. The work of Schwann, Schleiden, and Virchow led to the cell theory. This is one of the most important theories in life science. The cell theory can be summed up as follows: All organisms consist of one or more cells. Cells are alive and the site of all life processes. All cells come from pre-existing cells. "
lifes building blocks,T_2458,"All cells have certain parts in common. These parts include the cell membrane, cytoplasm, DNA, and ribosomes. The cell membrane is a thin coat of phospholipids that surrounds the cell. Its like the skin of the cell. It forms a physical boundary between the contents of the cell and the environment outside the cell. It also controls what enters and leaves the cell. The cell membrane is sometimes called the plasma membrane. Cytoplasm is the material inside the cell membrane. It includes a watery substance called cytosol. Besides water, cytosol contains enzymes and other substances. Cytoplasm also includes other cell structures suspended in the cytosol. DNA is a nucleic acid found in cells. It contains genetic instructions that cells need to make proteins. Ribosomes are structures in the cytoplasm where proteins are made. They consist of RNA and proteins. These four components are found in all cells. They are found in the cells of organisms as different as bacteria and people. How did all known organisms come to have such similar cells? The answer is evolution. The similarities show that all life on Earth evolved from a common ancestor. "
lifes building blocks,T_2459,"Besides the four parts listed above, many cells also have a nucleus. The nucleus of a cell is a structure enclosed by a membrane that contains most of the cells DNA. Cells are classified in two major groups based on whether or not they have a nucleus. The two groups are prokaryotic cells and eukaryotic cells. "
lifes building blocks,T_2460,"Prokaryotic cells are cells that lack a nucleus. The DNA in prokaryotic cells is in the cytoplasm, rather than enclosed within a nuclear membrane. All the organisms in the Bacteria and Archaea Domains have prokaryotic cells. No other organisms have this type of cell. Organisms with prokaryotic cells are called prokaryotes. They are all single-celled organisms. They were the first type of organisms to evolve. They are still the most numerous organisms today. You can see a model of a prokaryotic cell in Figure 3.3. The cell in the figure is a bacterium. Notice how it contains a cell membrane, cytoplasm, ribosomes, and several other structures. However, the cell lacks a nucleus. The cells DNA is circular. It coils up in a mass called a nucleoid that floats in the cytoplasm. "
lifes building blocks,T_2461,"Eukaryotic cells are cells that contain a nucleus. They are larger than prokaryotic cells. They are also more complex. Living things with eukaryotic cells are called eukaryotes. All of them belong to the Eukarya Domain. This domain includes protists, fungi, plants, and animals. Many protists consist of a single cell. However, most eukaryotes have more than one cell. You can see a model of a eukaryotic cell in Figure 3.4. The cell in the figure is an animal cell. The nucleus is an example of an organelle. An organelle is any structure inside a cell that is enclosed by a membrane. Eukaryotic cells may contain many different organelles. Each does a special job. For example, the mitochondrion is an organelle that provides energy to the cell. You can see a mitochondrion and several other organelles in the animal cell in Figure 3.4. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can. "
lifes building blocks,T_2462,"All living cells have certain things in common. Besides having the basic parts described above, all cells can perform the same basic functions. For example, all cells can use energy, respond to their environment, and reproduce. However, cells may also have special functions. Multicellular organisms such as you have many different types of specialized cells. Each specialized cell has a particular job. Cells with special functions generally have a shape that suits them for that job. Figure 3.5 shows four examples of specialized cells. Each type of cell in the figure has a different function. It also has a shape that helps it perform that function. The function of a nerve cell is to carry messages to other cells. It has many long arms that extend outward from the cell. The ""arms"" let the cell pass messages to many other cells at once. The function of a red blood cell is to carry oxygen to other cells. A red blood cell is small and smooth. This helps it slip through small blood vessels. A red blood cell is also shaped like a fattened disc. This gives it a lot of surface area for transferring oxygen. The function of a sperm cell is to swim through fluid to an egg cell. A sperm cell has a long tail that helps it swim. The function of a pollen cell is to pollinate flowers. The pollen cells in the figure have tiny spikes that help them stick to insects such as bees. The bees then carry the pollen cells to other flowers for pollination. "
lifes building blocks,T_2463,"Cells and organelles are made of biochemical molecules. These include nucleic acids and proteins. Molecules, in turn, are made of atoms. Figure 3.6 shows these different levels of organization in living things. As you can see in Figure 3.6, living things also have levels of organization higher than the cell. These higher levels are found only in multicellular organisms with specialized cells. Specialized cells may be organized into tissues. A tissue is a group of cells of the same kind that performs the same function. For example, muscle cells are organized into muscle tissue. The function of muscle tissue is to contract in order to move the body or its parts. Tissues may be organized into organs. An organ is a structure composed of two or more types of tissue that work together to do a specific task. For example, the heart is an organ. It consists of muscle, nerve, and other types of tissues. Its task is to pump blood. Organs may be organized into organ systems. An organ system is a group of organs that work together to do the same job. For example, the heart is part of the cardiovascular system. This system also includes blood vessels and blood. The job of the cardiovascular system is to transport substances in blood to and from cells throughout the body. Organ systems are organized into the organism. The different organ systems work together to carry out all the life functions of the individual. For example, cardiovascular and respiratory systems work together to provide the individual with oxygen and rid it of carbon dioxide. "
lifes building blocks,T_2464,"Cells with different functions often vary in shape. They may also vary in size. However, all cells are very small. Even the largest organisms have microscopic cells. Cells are so small that their diameter is measured in micrometers. A micrometer is just one-millionth of a meter. Use the sliding scale at the following link to see how small cells and cell parts are compared with other objects. Why are cells so small? The answer has to do with the surface area and volume of cells. To carry out life processes, a cell must be able to pass substances into and out of the cell. For example, it must be able to pass nutrients into the cell and waste products out of the cell. Anything that enters or leaves a cell has to go through the cell membrane on the surface of the cell. A bigger cell needs more nutrients and creates more wastes. As the size of a cell increases, its volume increases more quickly that its surface area. If the volume of a cell becomes too great, it wont have enough surface area to transfer all of its nutrients and wastes. "
cell structures,T_2465,The cell membrane is like the bag holding the Jell-O. It encloses the cytoplasm of the cell. It forms a barrier between the cytoplasm and the environment outside the cell. The function of the cell membrane is to protect and support the cell. It also controls what enters or leaves the cell. It allows only certain substances to pass through. It keeps other substances inside or outside the cell. 
cell structures,T_2466,"The structure of the cell membrane explains how it can control what enters and leaves the cell. The membrane is composed mainly of two layers of phospholipids. Figure 3.8 shows how the phospholipids are arranged in the cell membrane. Each phospholipid molecule has a head and two tails. The heads are water loving (hydrophilic), and the tails are water fearing (hydrophobic). The water-loving heads are on the outer surfaces of the cell membrane. They point toward the watery cytoplasm within the cell or the watery fluid that surrounds the cell. The water-fearing tails are in the middle of the cell membrane. "
cell structures,T_2467,Hydrophobic molecules like to be near other hydrophobic molecules. They fear being near hydrophilic molecules. The opposite is true of hydrophilic molecules. They like to be near other hydrophilic molecules. They fear being near hydrophobic molecules. These likes and fears explain why some molecules can pass through the cell membrane while others cannot. Hydrophobic molecules can pass through the cell membrane. Thats because they like the hydrophobic interior of the membrane and fear the hydrophilic exterior of the membrane. Hydrophilic molecules cant pass through the cell membrane. Thats because they like the hydrophilic exterior of the membrane and fear the hydrophobic interior of the membrane. You can see how this works in the video at this link:  . MEDIA Click image to the left or use the URL below. URL: 
cell structures,T_2468,"Cytoplasm is everything inside the cell membrane (except the nucleus if there is one). It includes the watery, gel-like cytosol. It also includes other structures. The water in the cytoplasm makes up about two-thirds of the cells weight. It gives the cell many of its properties. "
cell structures,T_2469,Why does a cell have cytoplasm? Cytoplasm has several important functions. These include: suspending cell organelles. pushing against the cell membrane to help the cell keep its shape. providing a site for many of the biochemical reactions of the cell. 
cell structures,T_2470,"Crisscrossing the cytoplasm is a structure called the cytoskeleton. It consists of thread-like filaments and tubules. The cytoskeleton is like a cellular skeleton. It helps the cell keep its shape. It also holds cell organelles in place within the cytoplasm. Figure 3.9 shows several cells. In the figure, the filaments of their cytoskeletons are colored green. The tubules are colored red. The blue dots are the cell nuclei. "
cell structures,T_2471,Eukaryotic cells contain a nucleus and several other types of organelles. These structures carry out many vital cell functions. 
cell structures,T_2472,"The nucleus is the largest organelle in a eukaryotic cell. It contains most of the cells DNA. DNA, in turn, contains the genetic code. This code tells the cell which proteins to make and when to make them. You can see a diagram of a cell nucleus in Figure 3.10. Besides DNA, the nucleus contains a structure called a nucleolus. Its function is to form ribosomes. The membrane enclosing the nucleus is called the nuclear envelope. The envelope has tiny holes, or pores, in it. The pores allow substances to move into and out of the nucleus. "
cell structures,T_2473,"The mitochondrion (mitochondria, plural) is an organelle that makes energy available to the cell. Its like the power plant of a cell. It uses energy in glucose to make smaller molecules called ATP (adenosine triphosphate). ATP packages energy in smaller amounts that cells can use. Think about buying a bottle of water from a vending machine. The machine takes only quarters, and you have only dollar bills. The dollar bills wont work in the vending machine. Glucose is like a dollar bill. It contains too much energy for cells to use. ATP is like a quarter. It contains just the right amount of energy for use by cells. "
cell structures,T_2474,"A ribosome is a small organelle where proteins are made. Its like a factory in the cell. It gathers amino acids and joins them together into proteins. Unlike other organelles, the ribosome is not surrounded by a membrane. As a result, some scientists do not classify it as an organelle. Ribosomes may be found floating in the cytoplasm. Some ribosomes are located on the surface of another organelle, the endoplasmic reticulum. "
cell structures,T_2475,The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Its made of folded membranes. Bits of membrane can pinch off to form tiny sacs called vesicles. The vesicles carry proteins or lipids away from the ER. There are two types of endoplasmic reticulum. They are called rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Both types are shown in Figure 3.11. NOTE: Crop to include only part a of the original image.] 
cell structures,T_2476,The Golgi apparatus is a large organelle that sends proteins and lipids where they need to go. Its like a post office. It receives molecules from the endoplasmic reticulum. It packages and labels the molecules. Then it sends them where they are needed. Some molecules are sent to different parts of the cell. Others are sent to the cell membrane for transport out of the cell. Small bits of membrane pinch off the Golgi apparatus to enclose and transport the proteins and lipids. You can see a Golgi apparatus at work in this animation: 
cell structures,T_2477,Both vesicles and vacuoles are sac-like organelles. They store and transport materials in the cell. They are like movable storage containers. Some vacuoles are used to isolate materials that are harmful to the cell. Other vacuoles are used to store needed substances such as water. Vesicles are much smaller than vacuoles and have a variety of functions. Some vesicles pinch off from the membranes of the endoplasmic reticulum and Golgi apparatus. These vesicles store and transport proteins and lipids. Other vesicles are used as chambers for biochemical reactions. 
cell structures,T_2478,A lysosome is an organelle that recycles unneeded molecules. It uses enzymes to break down the molecules into their components. Then the components can be reused to make new molecules. Lysosomes are like recycling centers. 
cell structures,T_2479,Centrioles are organelles that are found only in animal cells. They are located near the nucleus. They help organize the DNA in the nucleus before cell division takes place. They ensure that the DNA divides correctly when the cell divides. 
cell structures,T_2480,"All but one of the structures described above are found in plant cells as well as animal cells. The only exception is centrioles, which are not found in plant cells. Plant cells have three additional structures that are not found in animals cells. These include a cell wall, large central vacuole, and organelles called plastids. You can see these structures in the model of a plant cell in Figure 3.12. You can also see them in the interactive plant cell at this link: "
cell structures,T_2481,"The cell wall is a rigid layer that surrounds the cell membrane of a plant cell. Its made mainly of the complex carbohydrate called cellulose. The cell wall supports and protects the cell. The cell wall isnt solid like a brick wall. It has tiny holes in it called pores. The pores let water, nutrients, and other substances move into and out of the cell. "
cell structures,T_2482,"Most plant cells have a large central vacuole. It can make up as much as 90 percent of a plant cells total volume. The central vacuole is like a large storage container. It may store substances such as water, enzymes, and salts. It may have other roles as well. For example, the central vacuole helps stems and leaves hold their shape. It may also contain pigments that give flowers their colors. "
cell structures,T_2483,"Plastids are organelles in plant cells that may have various jobs. The main types of plastids are chloroplasts, chromoplasts, and leucoplasts. Chloroplasts are plastids that contain chlorophyll. Chlorophyll is a green pigment. It gives plants their green color. Photosynthesis takes place in chloroplasts. They capture sunlight and use its energy to make glucose. Chromoplasts are plastids that contain other pigments. These other pigments give flowers and fruits their colors. Leucoplasts are plastids that make or store other molecules. For example, some leucoplasts make amino acids. Other leucoplasts store starch or oil. "
transport,T_2484,"Youve probably blown soap bubbles like the child in Figure 4.1. In some ways, the thin film of soap molecules that forms a bubble resembles the cell membrane. Like the soap film, the cell membrane consists of a thin skin of molecules. You can see a model of the cell membrane in Figure below. The molecules that make up the cell membrane are mainly phospholipids. There are two layers of phospholipids. They are arranged so the lipid tails are on the inside of the membrane. They make the interior of the membrane hydrophobic, or ""water fearing"". The lipid heads point toward the outside of the membrane. The make the outer surfaces of the membrane hydrophilic, or ""water loving"". Different types of proteins are embedded in the lipid layers. The proteins are needed to help transport many substances across the membrane. The passage of a substance through a cell membrane is called transport. There are two basic ways that transport can occur: passive transport and active transport. For a good video introduction to passive and active transport, click on this link:  . MEDIA Click image to the left or use the URL below. URL: "
transport,T_2485,"Passive transport occurs when a substance passes through the cell membrane without needing any energy to pass through. This happens when a substance moves from an area where it is more concentrated to an area where it is less concentrated. Concentration is the number of particles of a substance in a given volume. Lets say you dissolve a teaspoon of salt in a cup of water. Then you dissolve two teaspoons of salt in another cup of water. The second solution will have a higher concentration of salt. Why does passive transport require no energy? A substance naturally moves from an area of higher to lower concentration. This is known as moving down the concentration gradient. The process is called diffusion. Its a little like a ball rolling down a hill. The ball naturally rolls from a higher to lower position without any added energy. You can see diffusion if you place a few drops of food coloring in a pan of water. Even without shaking or stirring, the food coloring gradually spreads throughout the water in the pan. Some substances can also diffuse through a cell membrane. This can occur in two ways: simple diffusion or facilitated diffusion. "
transport,T_2486,"Simple diffusion occurs when a substance diffuses through a cell membrane without any help from other molecules. The substance simply passes through tiny spaces in the membrane. It moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. You can see how this happens in Figure 4.2. Substances that cross cell membranes by simple diffusion must squeeze between the lipid molecules in the mem- brane. As a result, the diffusing molecules must be very small. Oxygen (O2 ) and carbon dioxide (CO2 ) are examples of molecules that can cross cell membranes this way. When you breathe in, oxygen is more concentrated in the air in your lungs than it is in your blood. So oxygen diffuses from your lungs to your blood. The reverse happens with carbon dioxide. Carbon dioxide is more concentrated in your blood than it is in the air in your lungs. So carbon dioxide diffuses out of your blood to your lungs. "
transport,T_2487,"Hydrophilic molecules and very large molecules cant pass through the cell membrane by simple diffusion. They need help to pass through the membrane. The help is provided by proteins called transport proteins. This process is known as facilitated diffusion. There are two types of transport proteins: channel proteins and carrier proteins. They work in different ways. You can see how they work in Figure 4.3. A channel protein forms a tiny hole called a pore in the cell membrane. This allows water or hydrophilic molecules to bypass the hydrophobic interior of the membrane. A carrier protein binds with a diffusing molecule. This causes the carrier protein to change shape. As it does, it carries the molecule across the membrane. This allows large molecules to pass through the cell membrane. "
transport,T_2488,"Osmosis is the special case of the diffusion of water. Its an important means of transport in cells because the fluid inside and outside cells is mostly water. Water can pass through the cell membrane by simple diffusion, but it can happen more quickly with the help of channel proteins. Water moves in or out of a cell by osmosis until its concentration is the same on both sides of the cell membrane. "
transport,T_2489,"Active transport occurs when a substance passes through the cell membrane with the help of extra energy. This happens when a substance moves from an area where it is less concentrated to an area where it is more concentrated. This is the opposite of diffusion. The substance moves up, instead of down, the concentration gradient. Like rolling a ball uphill, this requires an input of energy. The energy comes from the molecule named ATP (adenosine triphosphate). The energy allows special transport proteins called pumps to move substances to areas of higher concentration. An example is the sodium-potassium pump. "
transport,T_2490,"Sodium and potassium are two of the most important elements in living things. They are present mainly as positively charged ions dissolved in water. The sodium-potassium pump moves sodium ions (Na+ ) out of the cell and potassium ions (K+ ) into the cell. In both cases, the ions are moving from an area of lower to higher concentration. Energy in ATP is needed for this ""uphill"" process. Figure 4.4 shows how this pump works. Trace these steps from left to right in the figure: 1. Three sodium ions inside the cell bind with a carrier protein in the cell membrane. 2. The carrier protein receives a phosphate from ATP. This forms ADP (adenosine diphosphate) and releases energy. 3. The energy causes the carrier protein to change shape. As it does, it pumps the three sodium ions out of the cell. 4. Two potassium ions outside the cell next bind with the carrier protein. Then the process reverses, and the two potassium ions are pumped into the cell. "
transport,T_2491,"Some substances are too big to be pumped across the cell membrane. They may enter or leave the cell by vesicle transport. This takes energy, so its another form of active transport. You can see how vesicle transport occurs in Figure 4.5. Vesicle transport out of the cell is called exocytosis. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then the vesicle fuses with the cell membrane and releases the substance outside the cell. You can watch this happening in this very short animation:  MEDIA Click image to the left or use the URL below. URL:  Vesicle transport into the cell is called endocytosis. The cell membrane engulfs the substance. Then a vesicle pinches off from the membrane and carries the substance into the cell. "
cell division,T_2513,"DNA stands for deoxyribonucleic acid. It is a very large molecule. It consists of two strands of smaller molecules called nucleotides. Before learning how DNA is copied, its a good idea to review its structure. "
cell division,T_2514,"As you can see in Figure 5.1, each nucleotide includes a sugar, a phosphate, and a nitrogen base. The sugar in DNA is called deoxyribose. There are four different nitrogen bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Chemical bonds between the bases hold the two strands of DNA together. Adenine always bonds with thymine, and cytosine always bonds with guanine. These pairs of bases are called complementary base pairs. "
cell division,T_2515,"As a cell prepares to divide, its DNA first forms one or more structures called chromosomes. A chromosome consists of DNA and protein molecules coiled into a definite shape. Chromosomes are circular in prokaryotes and rodlike in eukaryotes. You can see an example of a human chromosome in Figure below. The rest of the time, DNA looks like a tangled mass of strings. In this form, it would be very difficult to copy and divide. "
cell division,T_2516,"The process in which DNA is copied is called DNA replication. You can see how it happens in Figure 5.3. An enzyme breaks the bonds between the two DNA strands. Another enzyme pairs new, complementary nucleotides with those in the original chains. Two daughter DNA molecules form. Each contains one new chain and one original chain. "
cell division,T_2517,"How cell division proceeds depends on whether a cell has a nucleus. Prokaryotic cells lack a nucleus. Their DNA is in the cytoplasm. It forms just one circular chromosome. Eukaryotic cells have a nucleus holding their DNA. Their DNA forms multiple rodlike chromosomes, like the one in Figure 5.2. Eukaryotic cells also have other organelles. For these reasons, cell division is more complex in eukaryotic cells. "
cell division,T_2518,"You can see how a prokaryotic cell divides in Figure 5.4. This type of cell division is called binary fission. The cell simply splits into two equal halves. Binary fission occurs in bacteria and other prokaryotes. It takes place in three continuous steps: 1. The cells chromosome is copied to form two identical chromosomes. This is DNA replication. 2. The copies of the chromosome separate from each other. They move to opposite poles, or ends, of the cell. This is called chromosome segregation. 3. The cell wall grows toward the center of the cell. The cytoplasm splits apart, and the cell pinches in two. This is called cytokinesis. "
cell division,T_2519,"Before a eukaryotic cell divides, the nucleus and other organelles must be copied. Only then will each daughter cell have all the needed structures. 1. The first step in eukaryotic cell division, as it is in prokaryotic cell division, is DNA replication. As you can see in Figure 5.5, each chromosome then consists of two identical copies. The two copies are called sister chromatids. They are attached to each other at a point called the centromere. 2. The second step in eukaryotic cell division is division of the cells nucleus. This includes division of the chromosomes. This step is called mitosis. It is a complex process that occurs in four phases. The phases of mitosis are described below. 3. The third step is the division of the rest of the cell. This is called cytokinesis, as it is in a prokaryotic cell. During this step, the cytoplasm divides, and two daughter cells form. These three steps are shown in Figure 5.6. "
cell division,T_2520,"Mitosis, or division of the nucleus, occurs only in eukaryotic cells. By the time mitosis occurs, the cells DNA has already replicated. Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase. You can see what happens in each phase in Figure below. The phases are described below. You can also learn more about the phases of mitosis by watching this video:  . MEDIA Click image to the left or use the URL below. URL:  1. Prophase: Chromosomes form, and the nuclear membrane breaks down. In animal cells, the centrioles near the nucleus move to opposite poles of the cell. Fibers called spindles form between the centrioles. 2. Metaphase: Spindle fibers attach to the centromeres of the sister chromatids. The sister chromatids line up at the center of the cell. 3. Anaphase: Spindle fibers shorten, pulling the sister chromatids toward the opposite poles of the cell. This gives each pole a complete set of chromosomes. 4. Telophase: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. "
cell division,T_2521,Cell division is just one of the stages that a cell goes through during its lifetime. All of the stages that a cell goes through make up the cell cycle. 
cell division,T_2522,"The cell cycle of a prokaryotic cell is simple. The cell grows in size, its DNA replicates, and the cell divides. "
cell division,T_2523,"In eukaryotes, the cell cycle is more complicated. The diagram in Figure 5.7 shows the stages that a eukaryotic cell goes through in its lifetime. There are two main stages: interphase and mitotic phase. They are described below. You can watch a eukaryotic cell going through the phases of the cell cycle at this link:  Interphase is longer than mitotic phase. Interphase, in turn, is divided into three phases: Mitotic phase is when the cell divides. It includes mitosis (M) and cytokinesis (C). "
reproduction,T_2524,"Asexual reproduction is simpler than sexual reproduction. It involves just one parent. The offspring are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of asexual reproduction. They include binary fission, fragmentation, and budding. "
reproduction,T_2525,"Binary fission occurs when a parent cell simply splits into two daughter cells. This method is described in detail in the lesson ""Cell Division."" Bacteria reproduce this way. You can see a bacterial cell reproducing by binary fission in Figure 5.9. "
reproduction,T_2526,"Fragmentation occurs when a piece breaks off from a parent organism. Then the piece develops into a new organism. Sea stars, like the one in Figure 5.10, can reproduce this way. In fact, a new sea star can form from a single arm. "
reproduction,T_2527,Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent while it grows and develops. It breaks away from the parent only after it is fully formed. Yeasts can reproduce this way. You can see two yeast cells budding in Figure 5.11. 
reproduction,T_2528,Sexual reproduction is more complicated. It involves two parents. Special cells called gametes are produced by the parents. A gamete produced by a female parent is generally called an egg. A gamete produced by a male parent is usually called a sperm. An offspring forms when two gametes unite. The union of the two gametes is called fertilization. You can see a human sperm and egg uniting in Figure 5.12. The initial cell that forms when two gametes unite is called a zygote. 
reproduction,T_2529,"In species with sexual reproduction, each cell of the body has two copies of each chromosome. For example, human beings have 23 different chromosomes. Each body cell contains two of each chromosome, for a total of 46 chromosomes. You can see the 23 pairs of human chromosomes in Figure 5.13. The number of different types of chromosomes is called the haploid number. In humans, the haploid number is 23. The number of chromosomes in normal body cells is called the diploid number. The diploid number is twice the haploid number. In humans, the diploid number is two times 23, or 46. "
reproduction,T_2530,"The two members of a given pair of chromosomes are called homologous chromosomes. We get one of each homologous pair, or 23 chromosomes, from our father. We get the other one of each pair, or 23 chromosomes, from our mother. A gamete must have the haploid number of chromosomes. That way, when two gametes unite, the zygote will have the diploid number. How are haploid cells produced? The answer is meiosis. "
reproduction,T_2531,"Meiosis is a special type of cell division. It produces haploid daughter cells. It occurs when an organism makes gametes. Meiosis is basically mitosis times two. The original diploid cell divides twice. The first time is called meiosis I. The second time is called meiosis II. However, the DNA replicates only once. It replicates before meiosis I but not before meiosis II. This results in four haploid daughter cells. Meiosis I and meiosis II occurs in the same four phases as mitosis. The phases are prophase, metaphase, anaphase, and telophase. However, meiosis I has an important difference. In meiosis I, homologous chromosomes pair up and then separate. As a result, each daughter cell has only one chromosome from each homologous pair. Figure 5.14 is a simple model of meiosis. It shows both meiosis I and II. You can read more about the stages below. You can also learn more about them by watching this video:  . MEDIA Click image to the left or use the URL below. URL: "
reproduction,T_2532,"After DNA replicates during interphase, the nucleus of the cell undergoes the four phases of meiosis I: 1. Prophase I: Chromosomes form, and the nuclear membrane breaks down. Centrioles move to opposite poles of the cell. Spindle fibers form between the centrioles. Heres whats special about meiosis: Homologous chromosomes pair up! You can see this in Figure below. 2. Metaphase I: Spindle fibers attach to the centromeres of the paired homologous chromosomes. The paired chromosomes line up at the center of the cell. 3. Anaphase I: Spindle fibers shorten, pulling apart the chromosome pairs. The chromosomes are pulled toward opposite poles of the cell. One of each pair goes to one pole. The other of each pair goes to the opposite pole. 4. Telophase I: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. Phases of meiosis I Meiosis I is followed by cytokinesis. Thats when the cytoplasm of the cell divides. Two haploid daughter cells result. Both of these cells go on to meiosis II. "
reproduction,T_2533,"Meiosis II is just like mitosis. 1. Prophase II: Chromosomes form. The nuclear membrane breaks down. Centrioles move to opposite poles. Spindle fibers form. 2. Metaphase II: Spindle fibers attach to the centromeres of sister chromatids. Sister chromatids line up at the center of the cell. 3. Anaphase II: Spindle fibers shorten. They pull the sister chromatids to opposite poles. 4. Telophase II: The chromosomes uncoil. The spindle fibers break down. New nuclear membranes form. Meiosis II is also followed by cytokinesis. This time, four haploid daughter cells result. Thats because both daughter cells from meiosis I have gone through meiosis II. The four daughter cells must continue to develop before they become gametes. For example, in males, the cells must develop tails, among other changes, in order to become sperm. "
reproduction,T_2534,Both types of reproduction have certain advantages. 
reproduction,T_2535,"Asexual reproduction can happen very quickly. It doesnt require two parents to meet and mate. Under ideal conditions, 100 bacteria can divide to produce millions of bacteria in just a few hours! Most bacteria dont live under ideal conditions. Even so, rapid reproduction may allow asexual organisms to be very successful. They may crowd out other species that reproduce more slowly. "
reproduction,T_2536,"Sexual reproduction is typically slower. However, it also has an advantage. Sexual reproduction results in offspring that are all genetically different. This can be a big plus for a species. The variation may help it adapt to changes in the environment. How does genetic variation arise during sexual reproduction? It happens in three ways: crossing over, independent assortment, and the random union of gametes. Crossing over occurs during meiosis I. It happens when homologous chromosomes pair up during prophase I. The paired chromosomes exchange bits of DNA. This recombines their genetic material. You can see where crossing over has occurred in Figures 5.15 and 5.16. Independent assortment occurs when chromosomes go to opposite poles of the cell in anaphase I. Which chromosomes end up together at each pole is a matter of chance. You can see this in Figures 5.15 and 5.16 as well. In sexual reproduction, two gametes unite to produce an offspring. Which two gametes is a matter of chance. The union of gametes occurs randomly. Due to these sources of variation, each human couple has the potential to produce more than 64 trillion unique offspring. No wonder we are all different! "
introduction to prokaryotes,T_2634,Prokaryotes are currently placed in two domains. A domain is the highest taxon in the classification of living things. Its even higher than the kingdom. 
introduction to prokaryotes,T_2635,"The prokaryote domains are the Bacteria Domain and Archaea Domain, shown in Figure 8.2. All other living things are eukaryotes and placed in the domain Eukarya. (Unlike prokaryotes, eukaryotes have a nucleus in their cells.) "
introduction to prokaryotes,T_2636,"Prokaryotes were the first living things to evolve on Earth, probably around 3.8 billion years ago. They were the only living things until the first eukaryotic cells evolved about 2 billion years ago. Prokaryotes are still the most numerous organisms on Earth. Its not certain how the three domains of life are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share. How can this be explained? One hypothesis is that the first Eukarya formed when an archaean cell fused with a bacterial cell. By fusing, the two prokaryotic cells became the nucleus and cytoplasm of a new eukaryotic cell. If this hypothesis is correct, both prokaryotic domains are ancestors of Eukarya. "
introduction to prokaryotes,T_2637,All prokaryotes consist of just one cell. They share a number of other traits as well. Watch this entertaining video from the Amoeba Sisters to see how prokaryotes differ in structure from eukaryotes:  MEDIA Click image to the left or use the URL below. URL: 
introduction to prokaryotes,T_2638,"Most prokaryotic cells are much smaller than eukaryotic cells. Prokaryotic cells are typically only 0.2-2.0 microm- eter in diameter. Eukaryotic cells are about 50 times as big. Prokaryotic cells have a variety of different cell shapes. Figure 8.3 shows three of the most common shapes: spirals (helices), spheres, and rods. Bacteria may be classified by their shape. "
introduction to prokaryotes,T_2639,"Most prokaryotes have one or more long, thin ""whips"" called flagella (flagellum, plural). You can see flagella in Figure 8.4. Flagella help prokaryotes move toward food or away from toxins. Each flagellum spins around a fixed base. This causes the cell to roll and tumble. "
introduction to prokaryotes,T_2640,"The cells of prokaryotes have two or three outer layers. Like all other living cells, prokaryotes have a cell membrane. It controls what enters and leaves the cell. Its also the site of many metabolic reactions. For example, cellular respiration takes place in the cell membrane. Most prokaryotes also have a cell wall. It lies just outside the cell membrane. It makes the cell stronger and more rigid. Many prokaryotes have another layer, called a capsule, outside the cell wall. The capsule protects the cell from chemicals and drying out. It also allows the cell to stick to surfaces and to other cells. You can see a model of a prokaryotic cell in Figure 8.5. Find the cell membrane, cell wall, and capsule in the figure. "
introduction to prokaryotes,T_2641,"Several other prokaryotic cell structures are also shown in Figure 8.5. They include: cytoplasm. Like all other cells, prokaryotic cells are filled with cytoplasm. It includes watery cytosol and other structures. ribosomes. This is the site where proteins are made. cytoskeleton. This is a network of fibers and tubules that crisscrosses the cytoplasm. The cytoskeleton helps the cell keep its shape. pili. These are hair-like projections from the surface of the cell. They help the cell hold on to surfaces or do other jobs for the cell. "
introduction to prokaryotes,T_2642,"All prokaryotic cells contain DNA, as you can see in Figure 8.6. Most of the DNA is in the form of a single large loop. This DNA coils up in the cytoplasm to form a structure called a nucleoid. There is no membrane surrounding it. Most prokaryotes also have one or more small loops of DNA. They are called plasmids. "
introduction to prokaryotes,T_2643,"Some prokaryotes form structures consisting of many individual cells, like the cells in Figure 8.7. This is called a biofilm. A biofilm is a colony of prokaryotes that is stuck to a surface. The surface might be a rock or a hosts tissues. The sticky plaque that collects on your teeth between brushings is a biofilm. It consists of millions of prokaryotic cells. "
introduction to prokaryotes,T_2644,"Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways and in a range of habitats. "
introduction to prokaryotes,T_2645,"Prokaryotes may have just about any type of metabolism. They may get energy from light or from chemical compounds. They may get carbon from carbon dioxide or from other living things. Most prokaryotes get both energy and carbon from other living things. Many of them are decomposers. They break down wastes and remains of dead organisms. In this way, they help to recycle carbon and nitrogen through ecosystems. Some prokaryotes use energy in sunlight to make food from carbon dioxide. They do this by the process of photosynthesis. They are important producers in aquatic ecosystems. Look at the green streaks on the lake in Figure 8.8. They consist of billions of photosynthetic bacteria called cyanobacteria. "
introduction to prokaryotes,T_2646,"Prokaryotes live in a wide range of habitats. For example, they may live in habitats with or without oxygen. Prokaryotes that need oxygen are described as aerobic. They use oxygen for cellular respiration. Examples include the prokaryotes that live on your skin. Prokaryotes that dont need oxygen or are poisoned by it are described as anaerobic. They use fermentation or other anaerobic processes rather than cellular respiration. Examples include many of the prokaryotes that live inside your body. Like most other living things, prokaryotes have a temperature range that they ""like"" best. Thermophiles are prokaryotes that prefer a temperature above 45 C (113 F). They might be found in a compost pile. Mesophiles are prokaryotes that prefer a temperature of about 37 C (98 C). They might be found inside the body of an animal such as you. Psychrophiles are prokaryotes that prefer a temperature below 20 C (68 F). They might be found deep in the ocean. "
introduction to prokaryotes,T_2647,"Prokaryotes reproduce asexually. This can happen by binary fission or budding. In binary fission, a cell splits in two. First, the large circular chromosome is copied. Then the cell divides to form two new daughter cells. Each has a copy of the parent cells chromosome. In budding, a new cell grows from a bud on the parent cell. It only breaks off to form a new cell when it is fully formed. "
introduction to prokaryotes,T_2648,"For natural selection to take place, organisms must vary in their traits. Asexual reproduction results in offspring that are all the same. They are also identical to the parent cell. So how can prokaryotes increase genetic variation? They can exchange plasmids. This is called genetic transfer. It may happen by direct contact between cells. Or a ""bridge"" may form between cells. Genetic transfer mixes the genes of different cells. It creates new combinations of alleles. "
protists,T_2666,"Protists are placed in the Protist Kingdom. This kingdom is one of four kingdoms in the Eukarya domain. The other three Eukarya kingdoms are the Fungi, Plant, and Animal Kingdoms. "
protists,T_2667,"The Protist Kingdom is hard to define. It includes many different types of organisms. You can see some examples of protists in Figure 9.1. The Protist Kingdom includes all eukaryotes that dont fit into one of the other three eukaryote kingdoms. For that reason, its sometimes called the trash can kingdom. The number of species in the Protist Kingdom is unknown. It could range from as few as 60,000 to as many as 200,000 species. For a beautiful introduction to the amazing world of protists, watch this video:  MEDIA Click image to the left or use the URL below. URL: "
protists,T_2668,"Scientists think that protists are the oldest eukaryotes. If so, they must have evolved from prokaryotes. How did this happen? How did cells without organelles acquire them? What was the origin of mitochondria, chloroplasts, and other organelles? The most likely way organelles evolved is shown in Figure 9.2. First, smaller prokaryotic cells invaded, or were engulfed by, larger prokaryotic cells. The smaller cells benefited by getting nutrients and a safe place to live. The larger cells benefited by getting some of the organic molecules or energy released by the smaller cells. Eventually, the smaller cells evolved into organelles in the larger cells. After that, neither could live without the other. "
protists,T_2669,"Despite the diversity of protists, they do share some traits. The cells of all protists have a nucleus. They also have other membrane-bound organelles. For example, all of them have mitochondria, and some of them have chloroplasts. Most protists consist of a single cell. Some are multicellular but they lack specialized cells. Most protists live in wet places. They are found in oceans, lakes, swamps, or damp soils. Many protists can move. Most protists also have a complex life cycle. The life cycle of an organism is the cycle of phases it goes through until it returns to the starting phase. The protist life cycle includes both sexual and asexual reproduction. Why reproduce both ways? Each way has benefits. Asexual reproduction is fast. It allows rapid population growth when conditions are stable. Sexual reproduction increases genetic variation. This helps ensure that some organisms will survive if conditions change. "
protists,T_2670,"Protists are classified based on traits they share with other eukaryotes. There are animal-like, plant-like, and fungus- like protists. The three groups differ mainly in how they get carbon and energy. "
protists,T_2671,"Animal-like protists are called protozoa (protozoan, singular). Most protozoa consist of a single cell. Protozoa are probably ancestors of animals. Protozoa are like animals in two ways: 1. Protozoa are heterotrophs. Heterotrophs get food by eating other organisms. Some protozoa prey on bacteria. Some are parasites of animals. Others graze on algae. Still others are decomposers that break down dead organic matter. 2. Almost all protozoa can move. They have special appendages for this purpose. You can see different types in Figure 9.3. Cilia (cilium, singular) are short, hair-like projections. Pseudopods are temporary extensions of the cytoplasm. Flagella are long, whip-like structures. Flagella are also found in most prokaryotes. "
protists,T_2672,"Plant-like protists are commonly called algae (alga, singular). Some algae consist of single cells. They are called diatoms. Other algae are multicellular. An example is seaweed. Seaweed called kelp can grow as large as trees. You can see both a diatom and kelp in Figure 9.4. Algae are probably ancestors of plants. Algae are like plants mainly because they contain chloroplasts. This allows them to make food by photosynthesis. Algae are important producers in water-based ecosystems such as the ocean. On the other hand, algae lack other plant structures. For example, they dont have roots, stems, or leaves. Also unlike plants, some algae can move. They may move with pseudopods or flagella. "
protists,T_2673,"Fungus-like protists include slime molds and water molds, both shown in Figure 9.5. They exist as individual cells or as many cells that form a blob-like colony. They are probably ancestors of fungi. Like fungi, many fungus-like protists are decomposers. They absorb nutrients from dead logs, compost, and other organic remains Slime molds are commonly found on rotting organic matter such as compost. Swarms of cells move very slowly over the surface. They digest and absorb nutrients as they go. Water molds are commonly found in moist soil and surface water. Many water molds are plant pathogens or fish parasites. "
protists,T_2674,"Many human diseases are caused by protists. Most of them are caused by protozoa. They are parasites that invade and live in the human body. The parasites get a place to live and nutrients from the human host. In return, they make the host sick. Examples of human diseases caused by protozoa include giardiasis and malaria. Protozoa that cause giardiasis are spread by contaminated food or water. They live inside the intestine. They may cause abdominal pain, fever, and diarrhea. Protozoa that cause malaria are spread by a vector. They enter the blood through the bite of an infected mosquito. They live inside red blood cells. They cause overall body pain, fever, and fatigue. Malaria kills several million people each year. Most of the deaths occur in children. "
fungi,T_2675,"Fungi (fungus, singular) are relatively simple eukaryotic organisms. They are placed in their own kingdom, the Fungus Kingdom. Most fungi are multicellular organisms. These fungi are called molds. However, some fungi exist as single cells. These fungi are called yeasts. You can see examples of different types of fungi in Figure 9.7. For a funny, fast-paced overview of fungi, watch this video:  . MEDIA Click image to the left or use the URL below. URL: "
fungi,T_2676,"For a long time, scientists classified fungi as members of the Plant Kingdom. Fungi share several obvious traits with plants. For example, both fungi and plants lack the ability to move. Both grow in soil, and both have cell walls. Some fungi even look like plants. "
fungi,T_2677,"Today, fungi are no longer classified as plants. We now know that they have important traits that set them apart from plants. Thats why they are placed in their own kingdom. How do fungi differ from plants? The cell walls of fungi are made of chitin. Chitin is a tough carbohydrate that also makes up the outer skeleton of insects. The cell walls of plants are made of cellulose. Fungi are heterotrophs that absorb food from other organisms. Plants are autotrophs that make their own food. The Fungus Kingdom is large and diverse. It may contain more than a million species. However, fewer than 100,000 species of fungi have been identified. "
fungi,T_2678,"The earliest fungi evolved about 600 million years ago. They lived in the water. Fungi colonized the land around the same time as plants. That was probably between 400 and 500 million years ago. After that, fungi became very abundant on land. By 250 million years ago, they may have been the dominant life forms on land. "
fungi,T_2679,"Yeasts grow as single cells. Other fungi grow into multicellular, thread-like structures. These structures are called hyphae (hypha, singular). You can see a photo of hyphae in Figure 9.8. They resemble plant roots. Each hypha consists of a group of cells surrounded by a tubular cell wall. A mass of hyphae make up the body of a fungus. The body is called the mycelium (mycelia, plural). A mycelium may range in size from microscopic to very large. In fact, the largest living thing on Earth is the mycelium of a single fungus. Nicknamed the humongous fungus, it grows in a forest in Oregon. A small part of the fungus is pictured in Figure 9.9. The giant fungus covers an area of 2384 acres. Thats about the size of 1,665 football fields! The fungus is estimated to be at least 2400 years old, but it could be much older. "
fungi,T_2680,"Most fungi reproduce both asexually and sexually. In both types of reproduction, they produce spores. A spore is a special reproductive cell. When fungi reproduce asexually, they can spread quickly. This is good when conditions are stable. They can increase their genetic variation by sexual reproduction. This is beneficial when conditions are changing. Variation helps ensure that at least some organisms survive the changing conditions. Figure 9.10 shows how asexual and sexual reproduction occur in fungi. Refer to the figure as you read about each of them below. "
fungi,T_2681,"During asexual reproduction, fungi produce haploid spores by mitosis of a haploid parent cell. A haploid cell has just one of each pair of chromosomes. The haploid spores are genetically identical to the parent cell. Spores may be spread by moving water, wind, or other organisms. Wherever the spores land, they will develop into new hyphae only when conditions are suitable for growth. Yeasts are an exception. They reproduce asexually by budding instead of by producing spores. An offspring cell forms on a parent cell. After it grows and develops, it buds off to form a new cell. The offspring cell is genetically identical to the parent cell. You can see yeast cells budding in Figure 9.11. "
fungi,T_2682,"Sexual reproduction also occurs in most fungi. It happens when two haploid hyphae mate. During mating, two haploid parent cells fuse. The single fused cell that results is a diploid spore. It is genetically different from both parents. The spore undergoes meiosis to form haploid daughter cells. These haploid cells develop into new hyphae. "
fungi,T_2683,"Most fungi grow on moist soil or rotting vegetation such as dead logs. Some fungi live in water. Others live in or on other organisms. Fungi get their nutrition by absorbing organic compounds from other organisms. The other organisms may be dead or alive, depending on the fungus. "
fungi,T_2684,Most fungi get organic compounds from dead organisms. Fungi use their hyphae to penetrate deep into decaying organic matter. They produce enzymes at the tips of their hyphae. The enzymes digest the organic matter so the fungal cells can absorb it. Fungi are the main decomposers in forests. They are the only decomposers that can break down cellulose and wood. They have special enzymes for this purpose. 
fungi,T_2685,Many fungi get organic compounds from living organisms. They have close relationships with other species. A close relationship between two species is called a symbiotic relationship. Two symbiotic relationships in fungi are mycorrhiza and lichen. These relationships are beneficial for both species. Mycorrhiza is a relationship between a fungus and a plant. The fungus grows in or on the plants roots. The fungus benefits from easy access to food made by the plant. The plant benefits because the fungal hyphae absorb water and nutrients from the soil that the plant needs. Lichen is a relationship between a fungus and cyanobacteria or green algae. The fungus grows around the bacterial or algal cells. The fungus benefits by getting some of the food made by the photosynthetic cells. The bacteria or algae benefit by getting some of the water and nutrients absorbed by the fungus. You can see a picture of lichen in Figure 9.12. Some fungi have a different kind of relationship with plants. They are plant parasites. They get food from the plants and cause harm to the plants in return. Fungi are the major causes of disease in agricultural crops. They may eventually kill their plant hosts. Some fungi are animal parasites. The wasp in Figure 9.13 is infected with a fungus. The fungus is the white fuzzy matter on the dark brown moth. 
fungi,T_2686,"Fungi may cause disease in people as well as other organisms. On the other hand, people have been using fungi for thousands of years. "
fungi,T_2687,"One way we use fungi is by eating them. Many species of mushrooms are edible. Yeasts are used for break making. Other fungi are used to ferment foods, such as soy sauce and cheeses. You can see the fungus growing through the blue cheese in Figure 9.14. The fungus gives the cheese its distinctive appearance and taste. People also use fungi: to produce antibiotics. to produce human hormones such as insulin. as natural pesticides. as model research organisms. "
fungi,T_2688,"Several common human diseases are caused by fungi. They include ringworm and athletes foot, both shown in Figure 9.15. Ringworm isnt caused by a worm. Its a skin infection by a fungus that leads to a ring-shaped rash. The rash may occur on the head, neck, trunk, arms, or legs. Athletes foot is caused by the same fungus as ringworm. But in athletes foot, the fungus infects the skin between the toes. Athletes foot is the second most common skin disease in the U.S. "
active transport,T_2689,"During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called ""active"" because this type of transport requires energy to move molecules. ATP is the most common source of energy for active transport. As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called ""pumps"" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport. The sodium-potassium pump ( Figure 1.1) is an example of an active transport pump. The sodium-potassium pump uses ATP to move three sodium (Na+ ) ions and two potassium (K+ ) ions to where they are already highly concentrated. Sodium ions move out of the cell, and potassium ions move into the cell. How do these ions then return to their original positions? As the ions now can flow down their concentration gradients, facilitated diffusion returns the ions to their original positions either inside or outside the cell. "
archaea,T_2724,"For many years, archaea were classified as bacteria. Like the bacteria, archaea lacked a nucleus and membrane- bound organelles and, therefore, were prokaryotic cells. However, when scientists compared the DNA of the two prokaryotes, they found that there were distinct differences. They concluded that there must be two distinct types of prokaryotes, which they named archaea and bacteria. Even though the two groups might seem similar, archaea have many features that distinguish them from bacteria: 1. The cell walls of archaea are distinct from those of bacteria. While bacteria have cell walls made up of the polymer peptidoglycan, most archaea do not have peptidoglycan in their cell walls. 2. The plasma membranes of the archaea are also made up of lipids that are distinct from those in bacteria. 3. The ribosomal proteins of the archaea are similar to those in eukaryotic cells, not those in bacteria. Although archaea and bacteria share some fundamental differences, they are still similar in many ways: 1. They both are single-celled, microscopic organisms that can come in a variety of shapes ( Figure 1.1). 2. Both archaea and bacteria have a single circular chromosome of DNA and lack membrane-bound organelles. 3. Like bacteria, archaea can have flagella to assist with movement. Archaea shapes can vary widely, but some are bacilli (rod-shaped). "
archaea,T_2725,"Most archaea are chemotrophs and derive their energy and nutrients from breaking down molecules in their envi- ronment. A few species of archaea are photosynthetic and capture the energy of sunlight. Unlike bacteria, which can be parasites and are known to cause a variety of diseases, there are no known archaea that act as parasites. Some archaea do live within other organisms. But these archea form mutualistic relationships with their host, where both the archaea and the host benefit. In other words, they assist the host in some way, for example by helping to digest food. "
archaea,T_2726,"Like bacteria, reproduction in archaea is asexual. Archaea can reproduce through binary fission, where a parent cell divides into two genetically identical daughter cells. Archaea can also reproduce asexually through budding and fragmentation, where pieces of the cell break off and form a new cell, also producing genetically identical organisms. "
asexual vs. sexual reproduction,T_2730,"Animals and other organisms cannot live forever. They must reproduce if their species is to survive. But what does it mean to reproduce? Reproduction is the ability to make the next generation, and it is one of the basic characteristics of life. Two methods of reproduction are: 1. Asexual reproduction, the process of forming a new individual from a single parent. 2. Sexual reproduction, the process of forming a new individual from two parents. There are advantages and disadvantages to each method, but the result is always the same: a new life begins. "
asexual vs. sexual reproduction,T_2731,"When humans reproduce, there are two parents involved. DNA must be passed from both the mother and father to the child. Humans cannot reproduce with just one parent; humans can only reproduce sexually. But having just one parent is possible in other eukaryotic organisms, including some insects, fish, and reptiles. These organisms can reproduce asexually, meaning the offspring (""children"") have a single parent and share the exact same genetic material as the parent. This is very different from reproduction in humans. Bacteria, being a prokaryotic, single- celled organism, must reproduce asexually. The advantage of asexual reproduction is that it can be very quick and does not require the meeting of a male and female organism. The disadvantage of asexual reproduction is that organisms do not receive a mix of traits from both parents. An organism that is born through asexual reproduction only has the DNA from the one parent. In fact, the offspring is genetically an exact copy of the parent. This can cause problems for the individual. For example, if the parent has a gene that causes a particular disease, the offspring will also have the gene that causes that disease. Organisms produced sexually may or may not inherit the disease gene because they receive a mix of their parents genes. Types of organisms that reproduce asexually include: 1. Prokaryotic organisms, like bacteria. Bacteria reproduce through binary fission, where they grow and divide in half ( Figure 1.1). First, their chromosome replicates and the cell enlarges. The cell then divides into two cells as new membranes form to separate the two cells. After cell division, the two new cells each have one identical chromosome. This simple process allows bacteria to reproduce very rapidly. 2. Flatworms, an invertebrate animal species. Flatworms divide in two, then each half regenerates into a new flatworm identical to the original, a process called fragmentation. 3. Different types of insects, fish, and lizards. These organisms can reproduce asexually through a process called parthenogenesis. Parthenogenesis happens when an unfertilized egg cell grows into a new organism. The resulting organism has half the amount of genetic material of the parent. Parthenogenesis is common in honeybees. In a hive, the sexually produced eggs become workers, while the asexually produced eggs become drones. "
asexual vs. sexual reproduction,T_2732,"During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. "
b and t cell response,T_2736,"Some defenses, like your skin and mucous membranes, are not designed to ward off a specific pathogen. They are just general defenders against disease. Your body also has defenses that are more specialized. Through the help of your immune system, your body can generate an army of cells to kill that one specific pathogen. There are two different types of specific immune responses. One type involves B cells. The other type involves T cells. Recall that B cells and T cells are types of white blood cells that are key in the immune response. Whereas the immune systems first and second line of defense are more generalized or non-specific, the immune response is specific. It can be described as a specific response to a specific pathogen, meaning it uses methods to target just one pathogen at a time. These methods involve B and T cells. "
b and t cell response,T_2737,"B cells respond to pathogens and other cells from outside the body in the blood and lymph. Most B cells fight infections by making antibodies. An antibody is a large, Y-shaped protein that binds to an antigen, a protein that is recognized as foreign. Antigens are found on the outside of bacteria, viruses and other foreign microorganisms. Each antibody can bind with just one specific type of antigen ( Figure 1.1). They fit together like a lock and key. Once an antigen and antibody bind together, they signal for a phagocyte to destroy them. Phagocytes are white blood cells that engulf targeted antigens by phagocytosis. As the antigen is on the outside of a pathogen, the pathogen is destroyed by this process. At any one time the average human body contains antibodies that can react with about 100,000,000 different antigens. This means that there can be 100,000,000 different antibody proteins in the body. "
b and t cell response,T_2738,"There are different types of T cells, including killer T cells and helper T cells. Killer T cells destroy infected, damaged, or cancerous body cells ( Figure 1.2). When the killer T cell comes into contact with the infected cell, it releases poisons. The poisons make tiny holes in the cell membrane of the infected cell. This causes the cell to burst open. Both the infected cell and the pathogens inside it are destroyed. Helper T cells do not destroy infected or damaged body cells. But they are still necessary for an immune response. They help by releasing chemicals that control other lymphocytes. The chemicals released by helper T cells switch on both B cells and killer T cells so they can recognize and fight specific pathogens. "
bacteria characteristics,T_2739,"Bacteria are the most successful organisms on the planet. They lived on this planet for two billion years before the first eukaryotes and, during that time, evolved into millions of different species. "
bacteria characteristics,T_2740,"Bacteria are so small that they can only be seen with a microscope. When viewed under the microscope, they have three distinct shapes ( Figure 1.1). Bacteria can be identified and classified by their shape: 1. Bacilli are rod-shaped. 2. Cocci are sphere-shaped. 3. Spirilli are spiral-shaped. Bacteria come in many different shapes. Some of the most common shapes are bacilli (rods), cocci (spheres), and spirilli (spirals). Bacteria can be identified and classified by their shape. "
bacteria characteristics,T_2741,"Like eukaryotic cells, bacterial cells have: 1. 2. 3. 4. Cytoplasm, the fluid inside the cell. A plasma or cell membrane, which acts as a barrier around the cell. Ribosomes, in which proteins are put together. DNA. By contrast though, bacterial DNA is contained in a large, circular strand. This single chromosome is located in a region of the cell called the nucleoid. The nucleoid is not an organelle, but a region within the cytoplasm. Many bacteria also have additional small rings of DNA known as plasmids. See bacterial cell pictured below ( Figure 1.2). The structure of a bacterial cell is dis- tinctive from a eukaryotic cell because of features such as an outer cell wall, the circular DNA of the nucleoid, and the lack of membrane-bound organelles. "
bacteria characteristics,T_2742,"Bacteria lack many of the structures that eukaryotic cells contain. For example, they dont have a nucleus. They also lack membrane-bound organelles, such as mitochondria or chloroplasts. The DNA of a bacterial cell is also different from a eukaryotic cell. Bacterial DNA is contained in one circular chromosome, located in the cytoplasm. Eukaryotes have several linear chromosomes. Bacteria also have two additional unique features: a cell wall and flagella. Some bacteria also have a capsule outside the cell wall. "
bacteria characteristics,T_2743,"Bacteria are surrounded by a cell wall consisting of peptidoglycan. This complex molecule consists of sugars and amino acids. The cell wall is important for protecting bacteria. The cell wall is so important that some antibiotics, such as penicillin, kill bacteria by preventing the cell wall from forming. Some bacteria depend on a host organism for energy and nutrients. These bacteria are known as parasites. If the host starts attacking the parasitic bacteria, the bacteria release a layer of slime that surrounds the cell wall. This slime offers an extra layer of protection. "
bacteria characteristics,T_2744,"Some bacteria also have tail-like structures called flagella ( Figure 1.3). Flagella help bacteria move. As the flagella rotate, they spin the bacteria and propel them forward. It is often said the flagella looks like a tiny whip, propelling the bacteria forward. Though some eukaryotic cells do have a flagella, a flagella in eukaryotes is rare. The flagella facilitate movement in bacte- ria. Bacteria may have one, two, or many flagellaor none at all. "
bacteria reproduction,T_2752,"Bacteria, being single-celled prokaryotic organisms, do not have a male or female version. Bacteria reproduce asexually. In asexual reproduction, the ""parent"" produces a genetically identical copy of itself. "
bacteria reproduction,T_2753,"Bacteria reproduce through a process called binary fission. During binary fission, the chromosome copies itself, forming two genetically identical copies. Then, the cell enlarges and divides into two new daughter cells. The two daughter cells are identical to the parent cell. Binary fission can happen very rapidly. Some species of bacteria can double their population in less than ten minutes! This process makes it possible for a tremendous bacterial colony to start from a single cell. "
bacteria reproduction,T_2754,"Are there male and female bacteria? Of course the answer is no. So, sexual reproduction does not occur in bacteria. But not all new bacteria are clones. This is because bacteria can acquire new DNA. This process occurs in three different ways: 1. Conjugation: In conjugation, DNA passes through an extension on the surface of one bacterium and travels to another bacterium ( Figure 1.1). Bacteria essential exchange DNA via conjugation. 2. Transformation: In transformation, bacteria pick up pieces of DNA from their environment. 3. Transduction: In transduction, viruses that infect bacteria carry DNA from one bacterium to another. "
blood diseases,T_2767,"Problems can occur with red blood cells, white blood cells, platelets, and other parts of the blood. Many blood disorders are genetic, meaning they are inherited from a parent. Some blood diseases are caused by not getting enough of a certain nutrient, while others are cancers of the blood. "
blood diseases,T_2768,"Anemia is a disease that occurs when there is not enough hemoglobin in the blood to carry oxygen to body cells. Hemoglobin is the blood protein that normally carries oxygen from the lungs to the tissues. Anemia leads to a lack of oxygen in organs. Anemia is usually caused by one of the following: A loss of blood from a bleeding wound or a slow leak of blood. The destruction of red blood cells. A lack of red blood cell production. Anemia may not have any symptoms. Some people with anemia feel weak or tired in general or during exercise. They also may have poor concentration. People with more severe anemia often get short of breath during times of activity. Iron-deficiency anemia is the most common type of anemia. It occurs when the body does not receive enough iron. Since there is not enough iron, hemoglobin, which needs iron to bind oxygen, cannot function properly. In the United States, 20% of all women of childbearing age have iron-deficiency anemia, compared with only 2% of adult men. The most common cause of iron-deficiency anemia in young women is blood lost during menstruation. Iron deficiency anemia can be avoided by getting the recommended amount of iron in ones diet. Anemia is often treated or prevented by taking iron supplements. Boys and girls between the ages of 9 and 13 should get 9 mg of iron every day. Girls between the ages of 14 and 18 should get 15 mg of iron every day. Boys between the ages of 14 and 18 should get 11 mg of iron every day. Pregnant women need the most iron27 mg daily. Good sources of iron include shellfish, such as clams and oysters. Red meats, such as beef, are also a good source of iron. Non-animal sources of iron include seeds, nuts, and legumes. Breakfast cereals often have iron added to them in a process called fortification. Some good sources of iron are listed below ( Table 1.1). Eating vitamin C along with iron-containing food increases the amount of iron that the body can absorb. Food Canned clams, drained, 3 oz. Fortified dry cereals, about 1 oz. Roasted pumpkin and squash seeds, 1 oz. Cooked lentils, 12 cup Cooked fresh spinach, 21 cup Cooked ground beef, 3 oz. Cooked sirloin beef, 3 oz. Milligrams (mg) of Iron 23.8 1.8 to 21.1 4.2 3.3 3.2 2.2 2.0 "
blood diseases,T_2769,"Sickle-cell anemia is a blood disease that is caused by an abnormally shaped hemoglobin protein in red blood cells. Many of the red blood cells of a person with sickle-cell anemia are long and curved (sickle-shaped) ( Figure 1.1). The long, sickle shape of the cells can cause them to get stuck in narrow blood vessels. This clotting means that oxygen cannot reach the cells. People with sickle-cell anemia are most often well but can occasionally have painful attacks. The disease is not curable, but it can be treated with medicines. The red blood cells of a person with sickle-cell anemia (left) are long and pointed, rather than straight, like normal cells (right). The abnormal cells cannot carry oxygen properly and can get stuck in capillaries. "
blood diseases,T_2770,"Blood cancers affect the production and function of your blood cells. Most of these cancers start in your bone marrow where blood is produced. In most blood cancers, the normal production of blood cells is replaced by uncontrolled growth of an abnormal type of blood cell. These abnormal blood cells are cancerous cells, and prevent your blood from performing many of its functions, like fighting off infections or preventing serious bleeding. Leukemia is a cancer of the blood or bone marrow. It is characterized by an abnormal production of blood cells, usually white blood cells. Lymphoma is a cancer of a type of white blood cell called lymphocytes. There are many types of lymphoma. "
blood diseases,T_2771,"Hemophilia is the name of a group of hereditary diseases that affect the bodys ability to control blood clotting. Hemophilia is caused by a lack of clotting factors in the blood. Clotting factors are normally released by platelets. Since people with hemophilia cannot produce clots, any cut can put a person at risk of bleeding to death. The risk of internal bleeding is also increased in hemophilia, especially into muscles and joints. This disease affected the royal families of Europe. "
cell biology,T_2801,"A cell is the smallest structural and functional unit of an organism. Some organisms, like bacteria, consist of only one cell. Big organisms, like humans, consist of trillions of cells. Compare a human to a banana. On the outside, they look very different, but if you look close enough youll see that their cells are actually very similar. "
cell biology,T_2802,"Most cells are so small that you cannot see them without the help of a microscope. It was not until 1665 that English scientist Robert Hooke invented a basic light microscope and observed cells for the first time, by looking at a piece of cork. You may use light microscopes in the classroom. You can use a light microscope to see cells ( Figure 1.1). But many structures in the cell are too small to see with a light microscope. So, what do you do if you want to see the tiny structures inside of cells? In the 1950s, scientists developed more powerful microscopes. A light microscope sends a beam of light through a specimen, or the object you are studying. A more powerful microscope, called an electron microscope, passes a beam of electrons through the specimen. Sending electrons through a cell allows us to see its smallest parts, even the parts inside the cell ( Figure 1.2). Without electron microscopes, we would not know what the inside of a cell looked like. The outline of onion cells are visible under a light microscope. "
cell biology,T_2803,"In 1858, after using microscopes much better than Hookes first microscope, Rudolf Virchow developed the hypoth- esis that cells only come from other cells. For example, bacteria, which are single-celled organisms, divide in half (after they grow some) to make new bacteria. In the same way, your body makes new cells by dividing the cells you already have. In all cases, cells only come from cells that have existed before. This idea led to the development of one of the most important theories in biology, the cell theory. Cell theory states that: 1. All organisms are composed of cells. 2. Cells are alive and the basic living units of organization in all organisms. 3. All cells come from other cells. As with other scientific theories, many hundreds, if not thousands, of experiments support the cell theory. Since Virchow created the theory, no evidence has ever been identified to contradict it. "
cell biology,T_2804,"Although cells share many of the same features and structures, they also can be very different ( Figure 1.3). Each cell in your body is designed for a specific task. In other words, the cells function is partly based on the cells structure. For example: Red blood cells are shaped with a pocket that traps oxygen and brings it to other body cells. Nerve cells are long and stringy in order to form a line of communication with other nerve cells, like a wire. Because of this shape, they can quickly send signals, such as the feeling of touching a hot stove, to your brain. Skin cells are flat and fit tightly together to protect your body. As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms have many types of specialized cells in their bodies. Red blood cells (left) are specialized to carry oxygen in the blood. Neurons (cen- ter ) are shaped to conduct electrical im- pulses to many other nerve cells. These epidermal cells (right) make up the skin of plants. Note how the cells fit tightly together. "
cell biology,T_2805,"While cells are the basic units of an organism, groups of cells can perform a job together. These cells are called specialized because they have a special job. Specialized cells can be organized into tissues. For example, your liver cells are organized into liver tissue. Your liver tissue is further organized into an organ, your liver. Organs are formed from two or more specialized tissues working together to perform a job. All organs, from your heart to your liver, are made up of an organized group of tissues. These organs are part of a larger system, the organ systems. For example, your brain works together with your spinal cord and other nerves to form the nervous system. This organ system must be organized with other organ systems, such as the circulatory system and the digestive system, for your body to work. Organ systems work together to form the entire organism. There are many levels of organization in living things ( Figure 1.4). Levels of organization, from the atom (smallest) to the organism (largest). Notice that organelles are inside a cell, and organs are inside an organ- ism. "
cell cycle,T_2806,"The process of cell division in eukaryotic cells is carefully controlled. The cell cycle ( Figure 1.1) is the life cycle of an eukaryotic cell, with cell division at the end of the cycle. Like a human life cycle, which is made up of different phases, like childhood, adolescence, and adulthood, the cell cycle also occurs in a series of phases. The first cell cycle begins with the formation of a zygote from the fusion of a male and female sex cell ( gamete). The steps of the cell cycle can be divided into two main components: interphase and the mitotic phase. Interphase is the stage when the cell mostly performs its everyday functions. For example, it is when a kidney cell does what a kidney cell is supposed to do. The cell also gets ready to divide during this time. The cell divides during the mitotic phase, which consists of mitosis and cytokinesis. Most of the cell cycle consists of interphase, the time between cell divisions. Interphase can be divided into three stages: 1. The first growth phase (G1): During the G1 stage, the cell doubles in size and doubles the number of organelles. 2. The synthesis phase (S): The DNA is replicated during this phase. In other words, an identical copy of all the cells DNA is made. This ensures that each new cell has a set of genetic material identical to that of the parental cell. This process is called DNA replication. 3. The second growth phase (G2): Proteins are synthesized that will help the cell divide. At the end of interphase, the cell is ready to enter mitosis. Shown is the cell cycle. Notice that most of the cell cycle is spent in Inter- phase (G1, S, and G2). Mitosis and cy- tokinesis occur during the Mitotic phase. Some cells may enter a resting phase dur- ing which progression through the cycle stops. During mitosis, the nucleus divides as the chromosomes are equally separated. One nucleus becomes two nuclei, each with an identical set of chromosomes. Mitosis is followed by cytokinesis, when the cytoplasm divides, "
cell division,T_2807,"Imagine the first stages of life. In humans and other animals, a sperm fertilizes an egg, forming the first cell. But humans are made up of trillions of cells, so where do the new cells come from? Remember that according to the cell theory, all cells come from existing cells. Once a sperm and egg cell unite and the first cell, called a zygote, forms, an entire baby will develop. And each cell in that baby will be genetically identical, meaning that each cell will have exactly the same DNA. How does a new life go from one cell to so many? The cell divides in half, creating two cells. Then those two cells divide, for a total of four cells. The new cells continue to divide and divide. One cell becomes two, then four, then eight, and so on ( Figure 1.1). This continual process of a cell dividing and creating two new cells is known as cell division. Cell division is part of a cycle of cellular growth and division known as the cell cyclecells must grow before they divide. The cell cycle describes the ""life"" of a eukayrotic cell. In addition to cell division, the cell cycle includes the division of the nucleus and the cytoplasm. Most cell division produces genetically identical cells, meaning they have the same DNA. The process of mitosis, which specifically is the division of the nucleus, ensures that each cell has the same DNA. During mitosis, the chromosomes equally separate, thus making sure each nucleus in each resulting cell after cell division is genetically identical. A special form of cell division, called meiosis, produces cells with half as much DNA as the parent cell. These cells are used for reproduction. In prokaryotic organisms, cell division is how those organisms reproduce. Besides the development of a baby, there are many other reasons that cell division is necessary for life: Cells divide repeatedly to produce an em- bryo. Previously the one-celled zygote (the first cell of a new organism) divided to make two cells (a). Each of the two cells divides to yield four cells (b), then the four cells divide to make eight cells (c), and so on. Through cell division, an entire embryo forms from one initial cell. 1. To grow and develop, you must form new cells. Imagine how often your cells must divide during a growth spurt. Growing just an inch requires countless cell divisions. Your body must produce new bone cells, new skin cells, new cells in your blood vessels and so on. 2. Cell division is also necessary to repair damaged cells. Imagine you cut your finger. After the scab forms, it will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come from? Some of your existing skin cells divide and produce new cells. 3. Your cells can also simply wear out. Over time you must replace old and worn-out cells. Cell division is essential to this process. "
cell membrane,T_2808,"If the outside environment of a cell is water-based, and the inside of the cell is also mostly water, something has to make sure the cell stays intact in this environment. What would happen if a cell dissolved in water, like sugar does? Obviously, the cell could not survive in such an environment. So something must protect the cell and allow it to survive in its water-based environment. All cells have a barrier around them that separates them from the environment and from other cells. This barrier is called the plasma membrane, or cell membrane. "
cell membrane,T_2809,"The plasma membrane ( Figure 1.1) is made of a double layer of special lipids, known as phospholipids. The phospholipid is a lipid molecule with a hydrophilic (""water-loving"") head and two hydrophobic (""water-hating"") tails. Because of the hydrophilic and hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern as only certain parts of the molecule can physically be in contact with water. Remember that there is water outside the cell, and the cytoplasm inside the cell is mostly water as well. So the phospholipids are arranged in a double layer (a bilayer) to keep the cell separate from its environment. Lipids do not mix with water (recall that oil is a lipid), so the phospholipid bilayer of the cell membrane acts as a barrier, keeping water out of the cell, and keeping the cytoplasm inside the cell. The cell membrane allows the cell to stay structurally intact in its water-based environment. The function of the plasma membrane is to control what goes in and out of the cell. Some molecules can go through the cell membrane to enter and leave the cell, but some cannot. The cell is therefore not completely permeable. ""Permeable"" means that anything can cross a barrier. An open door is completely permeable to anything that wants to enter or exit through the door. The plasma membrane is semipermeable, meaning that some things can enter the cell, and some things cannot. "
cell membrane,T_2810,"The inside of all cells also contain a jelly-like substance called cytosol. Cytosol is composed of water and other molecules, including enzymes, which are proteins that speed up the cells chemical reactions. Everything in the cell sits in the cytosol, like fruit in a jello mold. The term cytoplasm refers to the cytosol and all of the organelles, the specialized compartments of the cell. The cytoplasm does not include the nucleus. As a prokaryotic cell does not have a nucleus, the DNA is in the cytoplasm. "
cell nucleus,T_2811,"The nucleus is only found in eukaryotic cells. It contains most of the genetic material (the DNA) of the cell. The genetic material of the nucleus is like a set of instructions. These instructions tell the cell how to build molecules needed for the cell to function properly. That is, the DNA tells the cell how to build molecules needed for life. The nucleus is surrounded by the nuclear envelope, a double membrane (two phospholipid bilayers) that controls what goes in and out of the nucleus. The nucleus also has holes embedded in the nuclear envelope. These holes are known as nuclear pores, and they allow things to flow in and out of the nucleus. "
cell nucleus,T_2812,"Inside of the nucleus, you will find the chromosomes. Chromosomes are strands of DNA wrapped around proteins. They contain genes, or small units of genetic material (DNA) that contains the code for the creation of a protein. Human cells have 46 chromosomes (23 pairs). There are hundreds to thousands of genes on each chromosome. "
cell nucleus,T_2813,The nucleus of many cells also contains a central region called the nucleolus. The job of the nucleolus is to build ribosomes. These ribosomes flow out the nuclear pores into the cytoplasm. Ribosomes are organelles that make proteins in the cytoplasm. See the composition of the nucleus pictured below ( Figure 1.1). 
cell transport,T_2814,"Cells are found in all different types of environments, and these environments are constantly changing. For example, one-celled organisms, like bacteria, can be found on your skin, in the ground, or in all different types of water. Therefore, cells need a way to protect themselves. This job is done by the cell membrane, which is also known as the plasma membrane. "
cell transport,T_2815,"The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can pass through the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as the outside of the cell. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining a stable internal environment. For example, if your body cells have a temperature of 98.6 F, and it is freezing outside, your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop with the outside temperature. How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell? The composition of the cell membrane helps to control what can pass through it. "
cell transport,T_2816,"Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer of phospholipids (a ""bilayer"") and proteins ( Figure below). Recall that phospholipids, being lipids, do not mix with water. It is this quality that allows them to form the outside barrier of the cell. A single phospholipid molecule has two parts: 1. A polar head that is hydrophilic, or water-loving. 2. A fatty acid tail that is hydrophobic, or water-fearing. The cell membrane is made up of a phos- pholipid bilayer, two layers of phospho- lipid molecules. Notice the polar head group of the phospholipid is attached to the phosphate, and the tails are two fatty acid chains. The head group and tails are attached by a glycerol backbone. There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving, and they want to be near water, the heads face the inside and outside of the cell where water is found. The water-fearing, hydrophobic tails face each other in the middle of the cell membrane, because water is not found in this space. The phospholipid bilayer allows the cell to stay intact in a water-based environment. An interesting quality of the plasma membrane is that it is very ""fluid"" and constantly moving, like a soap bubble. This fluid nature of the membrane is important in maintaining homeostasis. It allows the proteins in the membrane to float to areas where they are needed. Due to the composition of the cell membrane, small molecules such as oxygen and carbon dioxide can pass freely through the membrane, but other molecules, especially large molecules, cannot easily pass through the plasma membrane. These molecules need assistance to get across the membrane. That assistance will come in the form of transport proteins. "
characteristics of life,T_2828,"How do you define a living thing? What do mushrooms, daisies, cats, and bacteria have in common? All of these are living things, or organisms. It might seem hard to think of similarities among such different organisms, but they actually have many properties in common. Living organisms are similar to each other because all organisms evolved from the same common ancestor that lived billions of years ago. All living organisms: 1. Need energy to carry out life processes. 2. Are composed of one or more cells. 3. Respond to their environment. 4. Grow and reproduce. 5. Maintain a stable internal environment. "
characteristics of life,T_2829,"Why do you eat everyday? To get energy. Energy is the ability to do work. Without energy, you could not do any ""work."" Though not doing any ""work"" may sound nice, the ""work"" fueled by energy includes everyday activities, such as walking, writing, and thinking. But you are not the only one who needs energy. In order to grow and reproduce and carry out the other process of life, all living organisms need energy. But where does this energy come from? The source of energy differs for each type of living thing. In your body, the source of energy is the food you eat. Here is how animals, plants, and fungi obtain their energy: All animals must eat in order to obtain energy. Animals also eat to obtain building materials. Animals eat plants and other animals. Plants dont eat. Instead, they use energy from the sun to make their ""food"" through the process of photosyn- thesis. Mushrooms and other fungi obtain energy from other organisms. Thats why you often see fungi growing on a fallen tree; the rotting tree is their source of energy ( Figure 1.1). Since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the energy of living things initially comes from the sun. Orange bracket fungi on a rotting log in the Oak Openings Preserve in Ohio. Fungi obtain energy from breaking down dead organisms, such as this rotting log. "
characteristics of life,T_2830,"If you zoom in very close on a leaf of a plant, or on the skin on your hand, or a drop of blood, you will find cells, you will find cells ( Figure 1.2). Cells are the smallest structural and functional unit of all living organisms. Most cells are so small that they are usually visible only through a microscope. Some organisms, like bacteria, plankton that live in the ocean, or the Paramecium, shown in Figure 1.3, are unicellular, made of just one cell. Other organisms have millions, billions, or trillions of cells. All cells have at least some structures in common, such as ribosomes, which are the sites where proteins are made. All cells also have DNA and proteins. The nucleus is clearly visible in the blood cells ( Figure 1.2). The nucleus can be described as the ""information center,"" containing the instructions (DNA) for making all the proteins in a cell, as well as how much of each protein to make. The nucleus is also the main distinguishing feature between the two general categories of cell, with cells known as prokaryotic lacking a nucleus. Although the cells of different organisms are built differently, they all have certain general functions. Every cell must get energy from food, be able to grow and divide, and respond to its environment. More about cell structure and function will be discussed in additional concepts. "
characteristics of life,T_2831,"All living organisms are able to react to something important or interesting in their external environment. For example, living organisms constantly respond to their environment. They respond to changes in light, heat, sound, and chemical and mechanical contact. Organisms have means for receiving information, such as eyes, ears, taste buds, or other structures. "
characteristics of life,T_2832,"All living things reproduce to make the next generation. Organisms that do not reproduce will go extinct. As a result, there are no species that do not reproduce ( Figure 1.4). Some organisms reproduce asexually ( asexual reproduction), especially single-celled organisms, and make identical copies (or clones) of themselves. Other organisms reproduce sexually ( sexual reproduction), combining genetic information from two parents to make genetically unique offspring. "
characteristics of life,T_2833,"When you are cold, what does your body do to keep warm? You shiver to warm up your body. When you are too warm, you sweat to release heat. When any living organism gets thrown off balance, its body or cells help it return to normal. In other words, living organisms have the ability to keep a stable internal environment. Maintaining a balance inside the body or cells of organisms is known as homeostasis. Like us, many animals have evolved behaviors that control their internal temperature. A lizard may stretch out on a sunny rock to increase its internal temperature, and a bird may fluff its feathers to stay warm ( Figure 1.5). A bird fluffs its feathers to stay warm and to maintain homeostasis. "
cloning,T_2854,"Cloning is the process of creating an exact genetic replica of an organism. The clones DNA is exactly the same as the parents DNA. Bacteria and other single-celled organisms have long been able to clone themselves through asexual reproduction. Plants can also reproduce asexually. In animals, however, cloning does not happen naturally. In 1997, that all changed when a sheep named Dolly was the first large mammal ever to be successfully cloned. Other animals can now also be cloned in a laboratory. The process of producing an animal like Dolly starts with a single cell from the animal that is going to be cloned. Below are the steps involved in the process of cloning: 1. In the case of Dolly, cells from the mammary glands were taken from the adult that was to be cloned. But other somatic cells can be used. Somatic cells come from the body and are not gametes like sperm or egg. 2. The nucleus is removed from this cell. 3. The nucleus is placed in a donor egg that has had its nucleus removed. The nucleus must be removed from the donor egg to maintain the appropriate chromosome number. 4. The new cell is stimulated with an electric shock and embryo development begins, as if it were a normal zygote. The zygote is the first cell of a new organism. 5. The resulting embryo is implanted into a mother sheep, where it continue its development ( Figure 1.1). To clone an animal, a nucleus from the animals cells are fused with an egg cell (from which the nucleus has been re- moved) from a donor, creating a new zy- gote. "
cloning,T_2855,"Cloning is not always successful. Most of the time, this cloning process does not result in a healthy adult animal. The process has to be repeated many times until it works. In fact, 277 tries were needed to produce Dolly. This high failure rate is one reason that human cloning is banned in the United States. In order to produce a cloned human, many attempts would result in the surrogate mothers experiencing miscarriages, stillbirths, or deformities in the infant. There are also many additional ethical considerations related to human cloning. Can you think of reasons why people are for or against cloning? "
components of blood,T_2860,"Did you know that blood is a tissue? Blood is a fluid connective tissue that is made up of red blood cells, white blood cells, platelets, and plasma. The cells that make up blood are pictured below ( Figure 1.1). The different parts of blood have different roles. A scanning electron microscope (SEM) image of human blood cells. Red blood cells are the flat, bowl-shaped cells, the tiny disc-shaped pieces are platelets, and white blood cells are the round cells shown in the center. "
components of blood,T_2861,"If you were to filter out all the cells in blood, a golden-yellow liquid would be left behind. Plasma is this fluid part of the blood. Plasma is about 90% water and about 10% dissolved proteins, glucose, ions, hormones, and gases. Blood is made up mostly of plasma. "
components of blood,T_2862,"Red blood cells (RBCs) are flattened, disk-shaped cells that carry oxygen. They are the most common blood cell in the blood. There are about 4 to 6 million RBCs per cubic millimeter of blood. Each RBC has about 200 million molecules of hemoglobin. Hemoglobin is the protein that carries oxygen. Hemoglobin also gives the red blood cells their red color. Red blood cells ( Figure 1.2) are made in the red marrow of long bones, rib bones, the skull, and vertebrae. Each red blood cell lives for only 120 days (about four months). After this time, they are destroyed in the liver and spleen. Mature red blood cells do not have a nucleus or other organelles. Lacking these components allows the cells to have more hemoglobin and carry more oxygen. The flattened shape of red blood cells helps them carry more oxygen than if they were rounded. "
components of blood,T_2863,"White blood cells (WBCs) are usually larger than red blood cells. They do not have hemoglobin and do not carry oxygen. White blood cells make up less than one percent of the bloods volume. Most WBCs are made in the bone marrow, and some mature in the lymphatic system. There are different WBCs with different jobs. WBCs defend the body against infection by bacteria, viruses, and other pathogens. WBCs do have a nucleus and other organelles. Neutrophils are WBCs that can squeeze through capillary walls and swallow particles such as bacteria and parasites. Macrophages are large WBCs that can also swallow and destroy old and dying cells, bacteria, or viruses. Below, a macrophage is attacking and swallowing two particles, possibly disease-causing pathogens ( Figure Lymphocytes are WBCs that fight infections caused by viruses and bacteria. Some lymphocytes attack and kill cancer cells. Lymphocytes called B-cells make antibodies. A type of white blood cell, called a macrophage, is attacking a cancer cell. "
components of blood,T_2864,"Platelets ( Figure 1.4) are very small, but they are very important in blood clotting. Platelets are not cells. They are sticky little pieces of larger cells. Platelets bud off large cells that stay in the bone marrow. When a blood vessel gets cut, platelets stick to the injured areas. They release chemicals called clotting factors, which cause proteins to form over the wound. This web of proteins catches red blood cells and forms a clot. This clot stops more blood from leaving the body through the cut blood vessel. The clot also stops bacteria from entering the body. Platelets survive in the blood for ten days before they are removed by the liver and spleen. "
diffusion,T_2884,"Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance ( Figure 1.1). The amount of a substance in relation to the total volume is the concentration. During diffusion, molecules are said to flow down their concentration gradient, flowing from an area of high concentration to an area of low concentration. Molecules flowing down a concentration gradient is a natural process and does not require energy. Diffusion can occur across a semipermeable membrane, such as the cell membrane, as long as a concentration gradient exists. Molecules will continue to flow in this manner until equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration, and molecules flow equally in both directions across the semipermeable membrane. At equilibrium, equal amounts of a molecule are entering and leaving a cell. Diffusion is the movement of a substance from an area of a higher amount toward an area of lower amount. A concentra- tion gradient initially exists across the cell membrane. Equilibrium is reached when there is an equal amount of the substance on both sides of the membrane. "
diffusion,T_2885,"The diffusion of water across a membrane because of a difference in concentration is called osmosis. Lets explore three different situations and analyze the flow of water. 1. A hypotonic solution means the environment outside of the cell has a lower concentration of dissolved material than the inside of the cell. If a cell is placed in a hypotonic solution, water will move into the cell. This causes the cell to swell, and it may even burst. 2. A hypertonic solution means the environment outside of the cell has more dissolved material than inside of the cell. If a cell is placed in a hypertonic solution, water will leave the cell. This can cause a cell to shrink and shrivel. 3. An isotonic solution is a solution in which the amount of dissolved material is equal both inside and outside of the cell. Water still flows in both directions, but an equal amount enters and leaves the cell. "
diffusion,T_2886,"How do marine animals keep their cells from shrinking? How do you keep your blood cells from bursting? Both of these questions have to do with the cell membrane and osmosis. Marine animals live in salt water, which is a hypertonic environment; there is more salt in the water than in their cells. To prevent losing too much water from their bodies, these animals intake large quantities of salt water and then secrete the excess salt. Red blood cells can be kept from bursting or shriveling if put in a solution that is isotonic to the blood cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so water would enter the blood cells, and they would swell and burst ( Figure 1.2). Osmosis causes these red blood cells to change shape by losing or gaining water. "
dna structure and replication,T_2900,"DNA must replicate (copy) itself so that each resulting cell after mitosis and cell division has the same DNA as the parent cell. All these cells, the parent cell and the two new daughter cells, are genetically identical. DNA replication occurs during the S phase (the Synthesis phase) of the cell cycle, before mitosis and cell division. The base pairing rules are crucial for the process of replication. DNA replication occurs when DNA is copied to form an identical molecule of DNA. The general steps involved in DNA replication are as follows: 1. The DNA helix unwinds like a zipper as the bonds between the base pairs are broken. The enzyme DNA Helicase is involved in breaking these bonds. 2. The two single strands of DNA then each serve as a template for a new stand to be created. Using DNA as a template means that on the new strand, the bases are placed in the correct order because of the base pairing rules. Recall that A and T are complementary bases, as are G and C. As a template strand is read, the new strand is created. If ATGCCA is on the ""template strand,"" then TACGGT will be on the new DNA strand. The enzyme DNA Polymerase reads the template and builds the new strand of DNA. 3. The new set of nucleotides then join together to form a new strand of DNA. The process results in two DNA molecules, each with one old strand and one new strand of DNA. This process is known as semiconservative replication because one strand is conserved (kept the same) in each new DNA molecule ( Figure 1.1). DNA replication occurs when the DNA strands unzip, and the original strands of DNA serve as a template for new nucleotides to join and form a new strand. "
domains of life,T_2905,"Lets explore the domain, the least specific category of classification. All of life can be divided into three domains, based on the type of cell of the organism: 1. Bacteria: cells do not contain a nucleus. 2. Archaea: cells do not contain a nucleus; they have a different cell wall from bacteria. 3. Eukarya: cells do contain a nucleus. "
domains of life,T_2906,"The Archaea and Bacteria domains ( Figure 1.1) are both entirely composed of small, single-celled organisms and seem very similar, but they also have significant differences. Both are composed of prokaryotic cells, which are cells without a nucleus. In addition, both domains are composed of species that reproduce asexually ( asexual reproduction) by dividing in two. Both domains also have species with cells surrounded by a cell wall, however, the cell walls are made of different materials. Bacterial cell walls contain the polysaccharide peptidoglycan. Lastly, Archaea often live in extreme environments including hot springs, geysers, and salt flats. Bacteria do not live in these environments. The Group A Streptococcus organism (left) is in the domain Bacteria, one of the three domains of life. The Halobacterium (right) is in the domain Archaea, another one of the three domains. "
domains of life,T_2907,"All of the cells in the domain Eukarya keep their genetic material, or DNA, inside the nucleus. The domain Eukarya is made up of four kingdoms: 1. Plantae: Plants, such as trees and grasses, survive by capturing energy from the sun, a process called photo- synthesis. 2. Fungi: Fungi, such as mushrooms and molds, survive by ""eating"" other organisms or the remains of other organisms. These organisms absorb their nutrients from other organisms. 3. Animalia: Animals also survive by eating other organisms or the remains of other organisms. Animals range from tiny ants to the largest whales, and include arthropods, fish, amphibians, reptiles, and mammals ( Figure 4. Protista: Protists are not all descended from a single common ancestor in the way that plants, animals, and fungi are. Protists are all the eukaryotic organisms that do not fit into one of the other three kingdoms. They include many kinds of microscopic one-celled (unicellular) organisms, such as algae and plankton, but also giant seaweeds that can grow to be 200 feet long. Plants, animals, fungi, and protists might seem very different, but remember that if you look through a microscope, you will find similar cells with a membrane-bound nucleus in all of them. These are eukaryotic cells. These cells also have membrane-bound organelles, which prokaryotic cells lack. The main characteristics of the three domains of life are summarized in Table 1.1. Multicelluar Cell wall Nucleus (Membrane- Enclosed DNA) Membrane-Bound Organelles Archaea No Yes, without peptidogly- can Bacteria No Yes, with peptidoglycan No No Eukarya Yes Varies. Plants and fungi have a cell wall; animals do not. Yes No No Yes Diversity of Animals. These photos give just an inkling of the diversity of organisms that belong to the animal kingdom. (A) Sponge, (B) Flatworm, (C) Flying Insect, (D) Frog, (E) Tiger, (F) Gorilla. "
fertilization,T_2933,"The sperm and egg dont look anything like a human baby ( Figure 1.1). After they come together, they will develop into a human being. How does a single cell become a complex organism made up of billions of cells? Keep reading to find out. Sexual reproduction happens when a sperm and an egg cell combine together. This is called fertilization. Sperm are released into the vagina during sexual intercourse. They swim through the uterus and enter a fallopian tube. This is where fertilization normally takes place. A sperm that is about to enter an egg is pictured below ( Figure 1.1). If the sperm breaks through the eggs membrane, it will immediately cause changes in the egg that keep other sperm out. This ensures that only a single sperm can penetrate an egg. It will also cause the egg to go through meiosis. Recall that meiosis, cell division that creates the egg, begins long before an egg is released from an ovary. In fact, it begins prior to birth. The sperm and egg each have only half the number of chromosomes as other cells in the body. These cells are haploid, with a single set of chromosomes. This is because when they combine together, they form a cell with the full number of chromosomes. The cell they form is called a zygote. The zygote is diploid, with two sets of chromosomes, one from each parent. A human zygote has two sets of 23 chromosomes, for a total of 46 chromosomes (23 pairs). The zygote slowly travels down the fallopian tube to the uterus. As it travels, it divides by mitosis many times. It forms a hollow ball of cells. After the ball of cells reaches the uterus, it fixes itself to the side of the uterus. This is called implantation. It usually happens about a week after fertilization. Now the implanted ball of cells is ready to continue its development into a baby boy or girl. "
fungi structure,T_2962,"The most important body parts of fungi include: 1. Cell wall: A layer around the cell membrane of fungi cells made largely of chitin and other polysaccharides. It is similar to that found in plant cells, though the plant cell wall contains the polysaccharide cellulose. 2. Hyphae: These are thread-like strands which interconnect and bunch up into a mycelium ( Figure 1.1). Ever see mold on a damp wall or on old bread? The things that you are seeing are really mycelia. The hyphae and mycelia help the fungi absorb nutrients from other organisms. Most of the mycelium is hidden from view deep within the fungal food source, such as rotting matter in the soil, leaf litter, rotting wood, or dead animals. Fungi produce enzymes to digest cellulose and various other materials found in rotting matter, helping with the decaying process. 3. Specialized structures for reproduction: One example is a fruiting body. Just like a fruit is involved in the reproduction of a fruiting plant, a fruiting body is involved in the reproduction of a fungus. A mushroom is a fruiting body, which is the part of the fungus that produces spores ( Figure 1.2). The spores are the basic reproductive units of fungi. The mycelium remains hidden until it develops one or more fruiting bodies. The fruiting bodies are usually produced at the surface of the food source, rather than hidden within it. This allows the reproductive spores to be easily shed and carried away by the wind, water, or animals. The fruiting bodies are usually the only indication that a fungus is present. Like icebergs, the fruiting bodies represent only a tiny fraction of the whole fungus, with most of the fungus hidden from view. Hyphae of a Penicillium mold. The little trees are specialized hyphae on which spores are produced. A mushroom is a fruiting body. "
fungus like protists,T_2963,"Fungus-like protists share many features with fungi. Like fungi, they are heterotrophs, meaning they must obtain food outside themselves. They also have cell walls and reproduce by forming spores, just like fungi. Fungus-like protists usually do not move, but a few develop movement at some point in their lives. Two major types of fungus- like protists are slime molds and water molds. "
fungus like protists,T_2964,"Slime molds usually measure about one or two centimeters, but a few slime molds are as big as several meters. They often have bright colors, such as a vibrant yellow ( Figure 1.1). Others are brown or white. Stemonitis is a kind of slime mold which forms small brown bunches on the outside of rotting logs. Physarum polycephalum lives inside rotting logs and is a gooey mesh of yellow ""threads"" that are several centimeters long. Fuligo, sometimes called vomit mold, is a yellow slime mold found in decaying wood. An example of a slime mold. "
fungus like protists,T_2965,"Water molds mostly live in water or moist soil. They can be parasites of plants and animals, getting their nutrients from these organisms and also from decaying organisms. They are a common problem for farmers since they cause a variety of plant diseases. One of the most famous of these diseases was the fungus that caused the Irish potato famine in the 1800s. At this time, potatoes were the main source of food for many of the Irish people. The failure of the potato crop meant that many people in Ireland died of starvation or migrated to other countries. "
gene therapy,T_2966,"Gene therapy is the insertion of genes into a persons cells to cure a genetic disorder. Could gene therapy be the cure for AIDS? No, AIDS is caused by a virus. Gene therapy only works to fix disorders caused by a faulty gene. The patient would have had this disorder from birth. Though gene therapy is still in experimental stages, the common use of this therapy may occur during your lifetime. There are two main types of gene therapy: 1. One done inside the body ( in vivo). 2. One done outside the body ( ex vivo). Both types of gene therapy use a vector, or carrier molecule for the gene. The vector helps incorporate the desired gene into the patients DNA. Usually this vector is modified viral DNA in which the viral genes have been removed. Dont worry, the virus used in gene therapy has been deactivated. "
gene therapy,T_2967,"During in vivo gene therapy, done inside the body, the vector with the gene of interest is introduced directly into the patient and taken up by the patients cells ( Figure 1.1). For example, cystic fibrosis gene therapy is targeted at the respiratory system, so a solution with the vector can be sprayed into the patients nose. Recently, in vivo gene therapy was also used to partially restore the vision of three young adults with a rare type of eye disease. In ex vivo gene therapy, done outside the body, cells are removed from the patient and the proper gene is inserted using a virus as a vector. The modified cells are placed back into the patient. One of the first uses of this type of gene therapy was in the treatment of a young girl with a rare genetic disease, adenosine deaminase deficiency, or ADA deficiency. People with this disorder are missing the ADA enzyme, which breaks down a toxin called deoxyadenosine. If the toxin is not broken down, it accumulates and destroys immune cells. As a result, individuals with ADA deficiency do not have a healthy immune system to fight off infections. In the gene therapy treatment for this disorder, bone marrow stem cells were taken from the girls body, and the missing gene was inserted into these cells outside the body. Then the modified cells were put back into her bloodstream. This treatment successfully restored the function of her immune system, but only with repeated treatments. "
human egg cells,T_3022,"When a baby girl is born, her ovaries contain all of the eggs they will ever produce. But these eggs are not fully developed. They develop only after she starts having menstrual periods at about age 12 or 13. Just one egg develops each month. A woman will release an egg once each month until she is in her 40s. A girl is born with over a million eggs. They die off and by puberty about 40,000 remain. "
human egg cells,T_3023,"Eggs are very big cells. In fact, they are the biggest cells in the human female body. (How many egg cells are in the human male body?) An egg is about 30 times as wide as a sperm cell! You can even see an egg cell without a microscope. Like a sperm cell, the egg contains a nucleus with half the number of chromosomes as other body cells. Unlike a sperm cell, the egg contains a lot of cytoplasm, the contents of the cell, which is why it is so big. The egg also does not have a tail. "
human egg cells,T_3024,"Egg production takes place in the ovaries. It takes several steps to make an egg: 1. Before birth, special cells in the ovaries go through mitosis (cell division), producing identical cells. 2. The daughter cells then start to divide by meiosis. But they only go through the first of the two cell divisions of meiosis at that time. They go through the second stage of cell division after the female goes through puberty. 3. In a mature female, an egg develops in an ovary about once a month. The drawing below shows how this happens ( Figure 1.1). As you can see from the figure, the egg rests in a nest of cells called a follicle. The follicle and egg grow larger and go through other changes. The follicle protects the egg as it matures in the ovary. After a couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This is called ovulation, which usually occurs at the midpoint of a monthly cycle. In a 28 day cycle, ovulation would occur around day 14. The moving fingers of the nearby fallopian tube then sweep the egg into the tube. At this time, if sperm are present the egg can be fertilized. Fertilization occurs if a sperm enters the egg while it is passing through the fallopian tube. When this happens, the egg finally completes meiosis. This results in two daughter cells that are different in size. The smaller cell is called a polar body. It contains very little cytoplasm. It soon breaks down and disappears. The larger cell is the egg. It contains most of the cytoplasm. This will develop into a child. "
human sperm,T_3033,"Sperm ( Figure 1.1), the male reproductive cells, are tiny. In fact, they are the smallest cells in the human body. What do you think a sperm cell looks like? Some people think that it looks like a tadpole. Do you agree? "
human sperm,T_3034,"A sperm has three main parts: 1. The head of the sperm contains the nucleus. The nucleus holds the DNA of the cell. The head also contains enzymes that help the sperm break through the cell membrane of an egg. 2. The midpiece of the sperm is packed with mitochondria. Mitochondria are organelles in cells that produce energy. Sperm use the energy in the midpiece to move. 3. The tail of the sperm moves like a propeller, around and around. This tail is a long flagella that pushes the sperm forward. A sperm can travel about 30 inches per hour. This may not sound very fast, but dont forget how small a sperm is. For its size, a sperm moves about as fast as you do when you walk briskly. This drawing of a sperm shows its main parts. What is the role of each part? How do you think the shape of the sperm might help it swim? "
human sperm,T_3035,"To make sperm, cells start in the testes and end in the epididymis. It takes up to two months to make sperm. The steps are explained below: 1. Special cells in the testes go through mitosis (cell division) to make identical copies of themselves. 2. The copies of the original cells divide by meiosis, producing cells called spermatids. The spermatids have half the number of chromosomes as the original cell. The spermatids are immature and cannot move on their own. 3. The spermatids are transported from the testes to the epididymis. Involuntary muscular contraction moves the spermatids along. 4. In the epididymis, spermatids slowly grow older and mature. They grow a tail. They also lose some of the cytoplasm from the head. It is here that the spermatids mature, becoming sperm cells. 5. When sperm are mature, they can swim. The mature sperm are stored in the epididymis until it is time for them to leave the body. Sperm leave the epididymis through the vas deferens. As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with liquids from these glands, forming semen. The semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as 500 million sperm! "
immunity,T_3047,"In previous concepts, you learned about B and T cells, special types of white blood cells that help your body to fight off a specific pathogen. They are necessary when the body is fighting off an infection. But what happens to them after the pathogen has been destroyed? Most B and T cells die after an infection has been brought under control. But some of them survive for many years. They may even survive for a persons lifetime. These long-lasting B and T cells are called memory cells. They allow the immune system to remember the pathogen after the infection is over. If the pathogen invades the body again, the memory cells will start dividing in order to fight the pathogen or disease. These dividing cells will quickly produce a new army of B or T cells to fight the pathogen. They will begin a faster, stronger attack than the first time the pathogen invaded the body. As a result, the immune system will be able to destroy the pathogen before it can cause an infection. Being able to attack the pathogen in this way is called immunity. Immunity can also be caused by vaccination. Vaccination is the process of exposing a person to a pathogen on purpose in order to develop immunity. In vaccination, a modified pathogen is usually injected under the skin by a shot. Only part of the pathogen is injected, or a weak or dead pathogen is used. It sounds dangerous, but the shot prepares your body for fighting the pathogen without causing the actual illness. Vaccination triggers an immune response against the injected antigen. The body prepares ""memory"" cells for use at a later time, in case the antigen is ever encountered again. Essentially, a vaccine imitates an infection, triggering an immune response, without making a person sick. In many countries, children receive their first vaccination at birth with the Hepatitis B shot, which protects infants from Hepatitis B, a serious liver disease. Before vaccines, many children died from diseases that vaccines now prevent, such as whooping cough, measles, and polio. Those same germs exist today, but because babies are now protected by vaccines, we do not see these diseases nearly as often. Diseases you have probably been vaccinated against include measles, mumps, and chicken pox. How does a vaccine work? See How a Vaccine Works at  and The History of Vaccines at  . Click image to the left or use the URL below. URL: "
inflammatory response,T_3090,"The little girl pictured below ( Figure 1.1) has a scraped knee. A scrape is a break in the skin that may let pathogens enter the body. If bacteria enter through the scrape, they could cause an infection. These bacteria would then face the bodys second line of defense. The second line of defense is also nonspecific, fighting many types of pathogens. "
inflammatory response,T_3091,"The bodys second line of defense against pathogens includes the inflammatory response. If bacteria enter the skin through a scrape, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. Inflammation is caused by chemicals that are released when skin or other tissues are damaged. The chemicals cause nearby blood vessels to dilate, or expand. This increases blood flow to the damaged area, which makes the area red and slightly warm. The chemicals also attract white blood cells called neutrophils to the wound and cause them to leak out of blood vessels into the damaged tissue. This little girl just got her first scraped knee. It doesnt seem to hurt, but the break in her skin could let pathogens enter her body. Thats why scrapes should be kept clean and protected until they heal. "
inflammatory response,T_3092,"What do these white blood cells do at the site of inflammation? The main role of white blood cells is to fight pathogens in the body. There are actually several different kinds of white blood cells. Some white blood cells have very specific functions. They attack only certain pathogens. Other white blood cells attack any pathogen they find. These white blood cells travel to areas of the body that are inflamed. They are called phagocytes, which means eating cells. Neutrophils are a type of phagocyte. In addition to pathogens, phagocytes eat dead cells. They surround the pathogens and destroy them. Sometimes it is said that the phagocyte engulfs the pathogen, and then destroys it. This process is called phagocytosis. White blood cells also make chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6 F (37 C). Most bacteria and viruses that infect people reproduce fastest at this temperature. When the temperature is higher, the pathogens cannot reproduce as fast, so the body raises the temperature to kill them. A fever also causes the immune system to make more white blood cells. In these ways, a fever helps the body fight infection. "
lymphatic system,T_3152,"If pathogens get through the bodys first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. It is called an immune response, and is a specific type of response. The immune system has a special response for each type of pathogen. The immune system ( Figure 1.1) is also part of the lymphatic systemnamed for lymphocytes, which are the type of white blood cells involved in an immune response. They include several lymph organs, lymph vessels, lymph, and lymph nodes. This diagram shows the parts of the im- mune system. The immune system in- cludes several organs and a system of vessels that carry lymph. Lymph nodes are located along the lymph vessels. "
lymphatic system,T_3153,"The lymph organs are the red bone marrow, tonsils, spleen, and thymus gland. They are described below ( Figure Each lymph organ has a different job in the immune system. "
lymphatic system,T_3154,"Lymph vessels make up a circulatory system that is similar to the cardiovascular system, which you can read about in a previous concept. Lymph vessels are like blood vessels, except they move lymph instead of blood. Lymph is a yellowish liquid that leaks out of tiny blood vessels into spaces between cells in tissues. Where there is more inflammation, there is usually more lymph in tissues. This lymph may contain many pathogens. The lymph that collects in tissues slowly passes into tiny lymph vessels. It then travels from smaller to larger lymph vessels. Lymph is not pumped through lymph vessels like blood is pumped through blood vessels by the heart. Instead, muscles around the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels also contract to help move the lymph along. The lymph finally reaches the main lymph vessels in the chest. Here, the lymph drains into two large veins. This is how the lymph returns to the bloodstream. Before lymph reaches the bloodstream, pathogens are removed from it at lymph nodes. Lymph nodes are small, oval structures located along the lymph vessels. They act like filters. Any pathogens filtered out of the lymph at lymph nodes are destroyed by lymphocytes in the nodes. "
lymphatic system,T_3155,"Lymphocytes ( Figure 1.3), a type of white blood cell, are the key cells of an immune response. There are trillions of lymphocytes in the human body. They make up about one quarter of all white blood cells. Usually, fewer than half of the bodys lymphocytes are in the blood. The rest are in the lymph, lymph nodes, and lymph organs. There are two main types of lymphocytes: 1. B cells. This image of a lymphocyte was made with an electron microscope. The lym- phocyte is shown 10,000 times its actual size. 2. T cells. Both types of lymphocytes are produced in the red bone marrow. They are named for the sites where they grow larger. The ""B"" in B cells stands for bone. B cells grow larger in red bone marrow. The ""T"" in T cells stands for thymus. T cells mature in the thymus gland. B and T cells must be switched on in order to fight a specific pathogen. Once this happens, they produce an army of cells ready to fight that particular pathogen. How can B and T cells recognize specific pathogens? Pathogens have proteins, often located on their cell surface. These proteins are called antigens. An antigen is any protein that causes an immune response, because it is unlike any protein that the body makes. Antigens are found on bacteria, viruses, and other pathogens. Your body sees these as foreign, meaning they do not belong in your body. "
meiosis,T_3162,"Sexual reproduction combines gametes from two parents. Gametes are reproductive cells, such as sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote must contain genetic information from the mother and from the father, so the gametes must contain half of the chromosomes found in normal body cells. When two gametes come together at fertilization, the normal amount of chromosomes results. Gametes are produced by a special type of cell division known as meiosis. Meiosis contains two rounds of cell division without DNA replication in between. This process reduces the number of chromosomes by half. Human cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromo- some. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. Alleles are alternate forms of genes found on chromosomes. Homologous chromosomes have the same genes, though they may have different alleles. So, though homologous chromosomes are very similar, they are not identical. The homologous chromosomes are separated when gametes are formed. Therefore, gametes have only 23 chromosomes, not 23 pairs. "
meiosis,T_3163,"A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent. "
meiosis,T_3164,"Before meiosis begins, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Meiosis ( Figure 1.1) is divided into two divisions: Meiosis I and Meiosis II. Each division can be divided into the same phases: prophase, metaphase, anaphase, and telophase. Cytokinesis follows telophase each time. Between the two cell divisions, DNA replication does not occur. Through this process, one diploid cell will divide into four haploid cells. Overview of Meiosis. During meiosis, four haploid cells are created from one diploid parent cell. "
meiosis,T_3165,"During meiosis I, the pairs of homologous chromosomes are separated from each other. This requires that they line up in their homologous paris during metaphase I. The steps are outlined below: 1. Prophase I: The homologous chromosomes line up together. During this time, a process that only happens in meiosis can occur. This process is called crossing-over ( Figure 1.2), which is the exchange of DNA between homologous chromosomes. Crossing-over forms new combinations of alleles on the resulting chromosome. Without crossing-over, the offspring would always inherit all of the alleles on one of the homologous chromo- somes. Also during prophase I, the spindle forms, the chromosomes condense as they coil up tightly, and the nuclear envelope disappears. 2. Metaphase I: The homologous chromosomes line up in their pairs in the middle of the cell. Chromosomes from the mother or from the father can each attach to either side of the spindle. Their attachment is random, so all of the chromosomes from the mother or father do not end up in the same gamete. The gamete will contain some chromosomes from the mother and some chromosomes from the father. 3. Anaphase I: The homologous chromosomes are separated as the spindle shortens, and begin to move to opposite sides (opposite poles) of the cell. 4. Telophase I: The spindle fibers dissolves, but a new nuclear envelope does not need to form. This is because, after cytokinesis, the nucleus will immediately begin to divide again. No DNA replication occurs between meiosis I and meiosis II because the chromosomes are already duplicated. After cytokinesis, two haploid cells result, each with chromosomes made of sister chromatids. Since the separation of chromosomes into gametes is random during meiosis I, this process results in different combinations of chromosomes (and alleles) in each gamete. With 23 pairs of chromosomes, there is a possibility of over 8 million different combinations of chromosomes (223 ) in a human gamete. During crossing-over, segments of DNA are exchanged between non-sister chro- matids of homologous chromosomes. Notice how this can result in an allele (A) on one chromatid being moved onto the other non-sister chromatid. "
meiosis,T_3166,"During meiosis II, the sister chromatids are separated and the gametes are generated. This cell division is similar to that of mitosis, but results in four genetically unique haploid cells. The steps are outlined below: 1. Prophase II: The chromosomes condense. 2. Metaphase II: The chromosomes line up one on top of each other along the middle of the cell, similar to how they line up in mitosis. The spindle is attached to the centromere of each chromosome. 3. Anaphase II: The sister chromatids separate as the spindle shortens and move to opposite ends of the cell. 4. Telophase II: A nuclear envelope forms around the chromosomes in all four cells. This is followed by cytokinesis. After cytokinesis, each cell has divided again. Therefore, meiosis results in four haploid genetically unique daughter cells, each with half the DNA of the parent cell ( Figure 1.3). In human cells, the parent cell has 46 chromosomes (23 pairs), so the cells produced by meiosis have 23 chromosomes. These cells will become gametes. "
mitosis and cytokinesis,T_3180,"The genetic information of the cell, or DNA, is stored in the nucleus. During mitosis, two nuclei (plural for nucleus) must form, so that one nucleus can be in each of the new cells after the cell divides. In order to create two genetically identical nuclei, DNA inside of the nucleus must be copied or replicated. This occurs during the S phase of the cell cycle. During mitosis, the copied DNA is divided into two complete sets, so that after cytokinesis, each cell has a complete set of genetic instructions. "
mitosis and cytokinesis,T_3181,"To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes. Each organism has a unique number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. Replicated DNA forms a chromosome made from two identical sister chromatids, forming an ""X"" shaped molecule ( Figure 1.1). The two chromatids are held together on the chromosome by the centromere. The centromere is also where spindle fiber microtubules attach during mitosis. The spindles separate sister chromatids from each other. "
mitosis and cytokinesis,T_3182,"During mitosis, the two sister chromatids must be divided. This is a precise process that has four individual phases to it. After the sister chromatids separate, each separate chromatid is now known as a chromosome. Each resulting chromosome is made of DNA from just one chromatid. So, each chromosome after this separation is made of ""1/2 of the X."" Through this process, each daughter cell receives one copy of each chromosome. The four phases of mitosis are prophase, metaphase, anaphase and telophase ( Figure 1.2). 1. Prophase: The chromatin, which is unwound DNA, condenses forming chromosomes. The DNA becomes so tightly wound that you can see them under a microscope. The membrane around the nucleus, called the nuclear envelope, disappears. Spindles also form and attach to chromosomes to help them move. 2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The chromosomes line up in a row, one on top of the next. 3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull the chromatids apart, resulting in two sets of identical chromosomes. 4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells. An overview of the cell cycle and mito- sis: during prophase the chromosomes condense, during metaphase the chromo- somes line up, during anaphase the sister chromatids are pulled to opposite sides of the cell, and during telophase the nuclear envelope forms. This is a representation of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis? "
mitosis vs. meiosis,T_3183,"Mitosis, meiosis, and sexual reproduction are discussed at  . Click image to the left or use the URL below. URL:  Both mitosis and meiosis result in eukaryotic cells dividing. So what is the difference between mitosis and meiosis? The primary difference is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell, meaning the new cells have exactly the same DNA as the parent cell. Mitosis happens when you want to grow, for example. You want all your new cells to have the same DNA as the previous cells. The goal of meiosis, however, is to produce sperm or eggs, also known as gametes. The resulting gametes are not genetically identical to the parent cell. Gametes are haploid cells, with only half the DNA present in the diploid parent cell. This is necessary so that when a sperm and an egg combine at fertilization, the resulting zygote has the correct amount of DNAnot twice as much as the parents. The zygote then begins to divide through mitosis. Pictured below is a comparison between binary fission (Figure 1.1), which is cell division of prokaryotic organisms, mitosis, and meiosis. Mitosis and meiosis are also compared in the table that follows (Table 1.1). A comparison between binary fission, mi- tosis, and meiosis. Purpose Number of Cells Produced Rounds of Cell Division Haploid or Diploid Daughter cells identical to parent cells? Daughter cells identical to each other? Mitosis To produce new cells 2 1 Diploid Yes Meiosis To produce gametes 4 2 Haploid No Yes No "
nerve cells and nerve impulses,T_3206,"The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages is called a neuron ( Figure 1.1). The messages carried by neurons are called nerve impulses. Nerve impulses can travel very quickly because they are electrical impulses. Think about flipping on a light switch when you enter a room. When you flip the switch, the electricity flows to the light through wires inside the walls. The electricity may have to travel many meters to reach the light, but the light still comes on as soon as you flip the switch. Nerve impulses travel just as fast through the network of nerves inside the body. The axons of many neurons, like the one shown here, are covered with a fatty layer called myelin sheath. The sheath covers the axon, like the plastic covering on an electrical wire, and allows nerve impulses to travel faster along the axon. The node of Ranvier, shown in this diagram, is any gap in the myelin sheath; it allows faster transmission of a signal. "
nerve cells and nerve impulses,T_3207,"A neuron has a special shape that lets it pass signals from one cell to another. A neuron has three main parts ( Figure 1. The cell body. 2. Many dendrites. 3. One axon. The cell body contains the nucleus and other organelles. Dendrites and axons connect to the cell body, similar to rays coming off of the sun. Dendrites receive nerve impulses from other cells. Axons pass the nerve impulses on to other cells. A single neuron may have thousands of dendrites, so it can communicate with thousands of other cells but only one axon. The axon is covered with a myelin sheath, a fatty layer that insulates the axon and allows the electrical signal to travel much more quickly. The node of Ranvier is any gap within the myelin sheath exposing the axon, and it allows even faster transmission of a signal. "
nerve cells and nerve impulses,T_3208,"Neurons are usually classified based on the role they play in the body. Two main types of neurons are sensory neurons and motor neurons. Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous system. Motor neurons carry nerve impulses from the central nervous system to organs, glands, and musclesthe opposite direction. Both types of neurons work together. Sensory neurons carry information about the environment found inside or outside of the body to the central nervous system. The central nervous system uses the information to send messages through motor neurons to tell the body how to respond to the information. "
nerve cells and nerve impulses,T_3209,"The place where the axon of one neuron meets the dendrite of another is called a synapse. Synapses are also found between neurons and other types of cells, such as muscle cells. The axon of the sending neuron does not actually touch the dendrite of the receiving neuron. There is a tiny gap between them, the synaptic cleft ( Figure 1.2). The following steps describe what happens when a nerve impulse reaches the end of an axon. 1. 2. 3. 4. When a nerve impulse reaches the end of an axon, the axon releases chemicals called neurotransmitters. Neurotransmitters travel across the synapse between the axon and the dendrite of the next neuron. Neurotransmitters bind to the membrane of the dendrite. The binding allows the nerve impulse to travel through the receiving neuron. Did you ever watch a relay race? After the first runner races, he or she passes the baton to the next runner, who takes over. Neurons are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron. Examples of neurotransmitters are chemicals such as serotonin, dopamine, and adrenaline. You can watch an animation of nerve impulses and neurotransmitters at  Some people have low levels of the neurotransmitter called serotonin in their brain. Scientists think that this is one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives. "
non mendelian inheritance,T_3214,"In all of Mendels experiments, he worked with traits where a single gene controlled the trait. Each also had one allele that was always dominant over the recessive allele. But this is not always true. There are exceptions to Mendels rules, and these exceptions usually have something to do with the dominant allele. If you cross a homozygous red flower with a homozygous white flower, according to Mendels laws, what color flower should result from the cross? Either a completely red or completely white flower, depending on which allele is dominant. But since Mendels time, scientists have discovered this is not always the case. "
non mendelian inheritance,T_3215,"One allele is NOT always completely dominant over another allele. Sometimes an individual has a phenotype between the two parents because one allele is not dominant over another. This pattern of inheritance is called incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) ( Figure 1.1) as both alleles are expressed. Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents. Pink snapdragons are an example of in- complete dominance. Another example of incomplete dominance is with sickle cell anemia, a disease in which a blood protein called hemoglobin is produced incorrectly. This causes the red blood cells to have a sickle shape, making it difficult for these misshapen cells to pass through the smallest blood vessels. A person that is homozygous recessive (ss) for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant (SS) will have normal red blood cells. What type of blood cells do you think a person who is heterozygous (Ss) for the trait will have? They will have some misshapen cells and some normal cells ( Figure 1.2). Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits. Sickle cell anemia causes red blood cells to become misshapen and curved unlike normal, rounded red blood cells. "
non mendelian inheritance,T_3216,"Another exception to Mendels laws is a phenomenon called codominance. For example, our blood type shows codominance. Do you know what your blood type is? Are you A? O? AB? Those letters actually represent alleles. Unlike other traits, your blood type has three alleles, instead of two! The ABO blood types ( Figure 1.3) are named for the protein attached to the outside of the blood cell. In this case, two alleles are dominant and completely expressed (IA and IB ), while one allele is recessive (i). The IA allele encodes for red blood cells with the A antigen, while the IB allele encodes for red blood cells with the B antigen. The recessive allele (i) does not encode for any proteins. Therefore a person with two recessive alleles (ii) has type O blood. As no dominant (IA and IB ) allele is present, the person cannot have type A or type B blood. What are the genotypes of a person with type A or type B blood? An example of codominant inheritance is ABO blood types. There are two possible genotypes for type A blood, homozygous (IA IA ) and heterozygous (IA i), and two possible genotypes for type B blood, (IB IB and IB i). If a person is heterozygous for both the IA and IB alleles, they will express both and have type AB blood with both proteins on each red blood cell. This pattern of inheritance is significantly different than Mendels rules for inheritance, because both alleles are expressed completely, and one does not mask the other. "
organelles,T_3222,"Eukaryotic cells have many specific functions, so it can be said that a cell is like a factory. A factory has many machines and people, and each has a specific role. Just like a factory, the cell is made up of many different parts. Each part has a special role. The different parts of the cell are called organelles, which means ""small organs."" All organelles are found in eukaryotic cells. Prokaryotic cells are ""simpler"" than eukaryotic cells. Though prokaryotic cells still have many functions, they are not as specialized as eukaryotic cells, lacking membrane-bound organelles. Thus, most organelles are not found in prokaryotic cells. Below are the main organelles found in eukaryotic cells ( Figure 1.1): 1. The nucleus of a cell is like a safe containing the factorys trade secrets. The nucleus contains the genetic material (DNA), the information needed to build thousands of proteins. 2. The mitochondria are the powerhouses of the cell. Mitochondria are the organelles where cellular energy is produced, providing the energy needed to power chemical reactions. This process, known as cellular respiration, produces energy is in the form of ATP (adenosine triphosphate). Cells that use a lot of energy may have thousands of mitochondria. 3. Vesicles are small membrane bound sacs that transport materials around the cell and to the cell membrane. 4. The vacuoles are like storage centers. Plant cells have larger vacuoles than animal cells. Plants store water and nutrients in their large central vacuoles. 5. Lysosomes are like the recycling trucks that carry waste away from the factory. Lysosomes have digestive enzymes that break down old molecules into parts that can be recycled. 6. In both eukaryotes and prokaryotes, ribosomes are the non-membrane bound organelles where proteins are made. Ribosomes are like the machines in the factory that produce the factorys main product. Proteins are the main product of the cell. 7. Some ribosomes can be found on folded membranes called the endoplasmic reticulum (ER), others float freely in the cytoplasm. If the ER is covered with ribosomes, it looks bumpy like sandpaper, and is called the rough endoplasmic reticulum. If the ER does not contain ribosomes, it is smooth and called the smooth endoplasmic reticulum. Many proteins are made on the ribosomes on the rough ER. These proteins immedi- ately enter the ER, where they are modified, packaged into vesicles and sent to the Golgi apparatus. Lipids are made in the smooth ER. 8. The Golgi apparatus works like a mail room. The Golgi apparatus receives proteins from the rough ER and puts ""shipping addresses"" on them. The Golgi then packages the proteins into vesicles and sends them to the right place in the cell or to the cell membrane. Some of these proteins are secreted from the cell (they exit the cell); others are placed into the cell membrane. Also, the cytoskeleton gives the cell its shape, and the flagella helps the cell to move. Prokaryotic cells may also have flagella. "
passive transport,T_3247,"Recall that the cell membrane is semipermeable. It does not allow everything to pass through. Some molecules can pass easily through your cell membranes, while others have more difficulty. Sometimes molecules need the help of special transport proteins to move across the cell membrane. Some molecules even need an input of energy to help get them across the cell membrane. The movement of molecules across a membrane without the input of energy is known as passive transport. When energy (ATP) is needed, the movement is known as active transport. Active transport moves molecules against their concentration gradient, from an area of low concentration to an area of high concentration. "
passive transport,T_3248,"One example of passive transport is diffusion, when molecules move from an area of high concentration (large amount) to an area of low concentration (low amount). Molecules are said to naturally flow down their concentration gradient. This type of diffusion proceeds without an input of energy. In simple diffusion, molecules that are small and uncharged can freely diffuse across a cell membrane. They simply flow through the cell membrane. Simple diffusion does not require energy or need the assistance of a transport protein. Other larger or charged molecules that diffuse across a membrane may need assistance from a protein. Oxygen is a molecule that can freely diffuse across a cell membrane. For example, oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood. Oxygen moves from the high concentration of oxygen in your lungs to the low concentration of oxygen in your bloodstream. Carbon dioxide, which is exhaled, moves in the opposite direction - from a high concentration in your bloodstream to a low concentration in your lungs. "
passive transport,T_3249,"Sometimes, molecules cannot move through the cell membrane on their own. These molecules need special transport proteins to help them move across the membrane, a process known as facilitative diffusion. These special proteins are called channel proteins or carrier proteins ( Figure 1.1), and they are attached to the cell membrane. In fact, they go through the cell membrane, from the inside of the cell to the outside. Channel proteins provide an open channel or passageway through the cell membrane for molecules to move across. Many channel proteins allow the diffusion of ions. Ions are charged atoms. The charge makes it difficult to cross the cell membrane without assistance. Channel proteins are specific for the molecule they transport. For example a sodium ion crosses the membrane through a channel protein specific for sodium ions. Carrier proteins bind and carry the molecules across the cell membrane. These proteins bind a molecule on one side of the membrane, change shape as they carry the molecule across the membrane, and deposit the molecule on the other side of the membrane. Even though a protein is involved in both these methods of transport, neither method requires energy. Therefore these are still types of passive transport. "
plant cell structures,T_3261,"Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells ( Figure organelles of photosynthesis. Photosynthesis converts the suns solar energy into chemical energy. This chemical energy, which is the carbohydrate glucose, serves as ""food"" for the plant. "
plant cell structures,T_3262,"First, plant cells have a large central vacuole that holds a mixture of water, nutrients, and wastes. A plant cells vacuole can make up 90% of the cells volume. The large central vacuole essentially stores water. In animal cells, vacuoles are much smaller. A plant cell has several features that make it different from an animal cell, including a cell wall, huge vacuoles, and chloroplasts, which photosynthesize. "
plant cell structures,T_3263,"Second, plant cells have a cell wall, while animal cells do not ( Figure 1.2). The cell wall surrounds the plasma membrane but does not keep substances from entering or leaving the cell. A cell wall gives the plant cell strength and protection. In this photo of plant cells taken with a light microscope, you can see green chloroplasts, as well as a cell wall around each cell. "
plant cell structures,T_3264,"A third difference between plant and animal cells is that plants have several kinds of organelles called plastids. And there are several different kinds of plastids in plant cells. For example, Chloroplasts are needed for photosynthesis, leucoplasts can store starch or oil, and brightly colored chromoplasts give some flowers and fruits their yellow, orange, or red color. It is the presence of chloroplasts and the ability to photosynthesize, that is one of the defining features of a plant. No animal or fungi can photosynthesize, and only some protists are able to. The photosynthetic protists are the plant-like protists, represented mainly by the unicellular algae. "
plant reproduction and life cycle,T_3275,"The life cycle of a plant is very different from the life cycle of an animal. Humans are made entirely of diploid cells (cells with two sets of chromosomes, referred to as 2n). Our only cells that are haploid cells (cells with one set of chromosomes, n) are sperm and egg cells. Plants, however, can live when they are are at the stage of having haploid cells or diploid cells. If a plant has a haploid chromosome number of 20, what is the diploid chromosome number? If the diploid chromosome number is 20, what is the haploid number? Plants alternate between diploid-cell plants and haploid-cell plants. This is called alternation of generations, because the plant type alternates from generation to generation. In alternation of generations, the plant alternates between a sporophyte that has diploid cells and a gametophyte that has haploid cells. Alternation of generations can be summarized in the following four steps: follow along in the Figure 1.1 as you read through the steps. 1. The haploid gametophyte produces the gametes, or sperm and egg, by mitosis. Remember, gametes are haploid, having one set of chromosomes. 2. Then, the sperm fertilizes the egg, producing a diploid zygote that develops into the sporophyte, which of course, is diploid. 3. The diploid sporophyte produces haploid spores by meiosis. 4. The haploid spores go through mitosis, developing into the haploid gametophyte. As we will see in additional Plants concepts, the generation in which the plant spends most of its life cycle is different between various plants. In the plants that first evolved, the gametophyte takes up the majority of the life cycle of the plant. During the course of evolution, the sporophyte became the major stage of the life cycle of the plant. In ferns, the sporophyte is dominant and produces spores that germinate into a heart-shaped gametophyte. "
prokaryotic and eukaryotic cells,T_3307,"There are two basic types of cells, prokaryotic cells and eukaryotic cells. The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a nucleus. The nucleus is where cells store their DNA, which is the genetic material. The nucleus is surrounded by a membrane. Prokaryotic cells do not have a nucleus. Instead, their DNA floats around inside the cell. Organisms with prokaryotic cells are called prokaryotes. All prokaryotes are single-celled (unicellular) organisms. Bacteria and Archaea are the only prokaryotes. Organisms with eukaryotic cells are called eukaryotes. Animals, plants, fungi, and protists are eukaryotes. All multicellular organisms are eukaryotes. Eukaryotes may also be single-celled. Both prokaryotic and eukaryotic cells have structures in common. All cells have a plasma membrane, ribosomes, cytoplasm, and DNA. The plasma membrane, or cell membrane, is the phospholipid layer that surrounds the cell and protects it from the outside environment. Ribosomes are the non-membrane bound organelles where proteins are made, a process called protein synthesis. The cytoplasm is all the contents of the cell inside the cell membrane, not including the nucleus. "
prokaryotic and eukaryotic cells,T_3308,"Eukaryotic cells usually have multiple chromosomes, composed of DNA and protein. Some eukaryotic species have just a few chromosomes, others have close to 100 or more. These chromosomes are protected within the nucleus. In addition to a nucleus, eukaryotic cells include other membrane-bound structures called organelles. Organelles allow eukaryotic cells to be more specialized than prokaryotic cells. Pictured below are the organelles of eukaryotic cells ( Figure 1.1), including the mitochondria, endoplasmic reticulum, and Golgi apparatus. These will be discussed in additional concepts. DNA (chromatin) is stored. Organelles give eukaryotic cells more functions than prokaryotic cells. "
prokaryotic and eukaryotic cells,T_3309,"Prokaryotic cells ( Figure 1.2) are usually smaller and simpler than eukaryotic cells. They do not have a nucleus or other membrane-bound organelles. In prokaryotic cells, the DNA, or genetic material, forms a single large circle that coils up on itself. The DNA is located in the main part of the cell. Nucleus DNA Membrane-Bound Organelles Examples Prokaryotic Cells No Single circular piece of DNA No Bacteria Eukaryotic Cells Yes Multiple chromosomes Yes Plants, animals, fungi "
protein synthesis and gene expression,T_3310,"A monomer is a molecule that can bind to other monomers to form a polymer. Amino acids are the monomers of a protein. The DNA sequence contains the instructions to place amino acids into a specific order. When the amino acid monomers are assembled in that specific order, proteins are made, a process called protein synthesis. In short, DNA contains the instructions to create proteins. But DNA does not directly make the proteins. Proteins are made on the ribosomes in the cytoplasm, and DNA (in an eukaryotic cell) is in the nucleus. So the cell uses an RNA intermediate to produce proteins. Each strand of DNA has many separate sequences that code for a specific protein. Insulin is an example of a protein made by your cells ( Figure 1.1). Units of DNA that contain code for the creation of a protein are called genes. "
protein synthesis and gene expression,T_3311,"There are about 22,000 genes in every human cell. Does every human cell have the same genes? Yes. Does every human cell make the same proteins? No. In a multicellular organism, such as us, cells have specific functions because they have different proteins. They have different proteins because different genes are expressed in different cell types (which is known as gene expression). Imagine that all of your genes are ""turned off."" Each cell type only ""turns on"" (or expresses) the genes that have the code for the proteins it needs to use. So different cell types ""turn on"" different genes, allowing different proteins to be made. This gives different cell types different functions. Once a gene is expressed, the protein product of that gene is usually made. For this reason, gene expression and protein synthesis are often considered the same process. "
sponges,T_3408,"Sponges ( Figure 1.1) are classified in the phylum Porifera, from the Latin words meaning ""having pores."" These pores allow the movement of water into the sponges sac-like bodies. Sponges must pump water through their bodies in order to eat. Because sponges are sessile, meaning they cannot move, they filter water to obtain their food. They are, therefore, known as filter feeders. Filter feeders must filter the water to separate out the organisms and nutrients they want to eat from those they do not. You might think that sponges dont look like animals at all. They dont have a head or legs. Internally, they do not have brains, stomachs, or other organs. This is because sponges evolved much earlier than other animals. In fact, sponges do not even have true tissues. Instead, their bodies are made up of specialized cells (cell-level organization) that do specific jobs. Other animals, including humans, have tissue-level organization because they have tissues with specific functions. Sponge cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals. For example, some cells control the flow of water, in and out of the sponge, by increasing or decreasing the size of the pores. The sponges often have tube-like bodies with many tiny pores. There are roughly 5,000 sponge species. Sponges are characterized by a feeding system unique among animals. As sponges dont have mouths, they must feed by some other method. Sponges have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter food from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella, which line the surface of chambers connected by a series of canals. Sponges reproduce by both asexual and sexual means. Sponges that reproduce asexually produce buds or, more often, structures called gemmules, which are packets of several cells of various types inside a protective covering. Freshwater sponges often produce gemmules prior to winter, which then develop into adult sponges beginning the following spring. Most sponges that reproduce sexually are hermaphroditic and produce eggs and sperm at different times. Sperm are frequently released into the water, where they are captured by sponges of the same species. The sperm are then transported to eggs, fertilization occurs and the zygotes develop into larvae. Some sponges release their larvae, where others retain them for some time. Once the larvae are in the water, they settle and develop into juvenile sponges. "
viruses,T_3484,"We have all heard of viruses. The flu, the common cold, and many other diseases are caused by viruses. But what is a virus? Do you think viruses are living? Which domain do they belong to? Bacteria? Archaea? Eukarya? "
viruses,T_3485,"The answer is actually no. A virus is essentially DNA or RNA surrounded by a coat of protein ( Figure 1.1). It is not made of a cell, and cannot maintain a stable internal environment ( homeostasis). Recall that a cell is the basic unit of living organisms. So if a virus is not made of at least one cell, can it be living? Viruses also cannot reproduce on their ownthey need to infect a host cell to reproduce. So a virus is very different from any of the organisms that fall into the three domains of life. Though viruses are not considered living, they share two important traits with living organisms. They have genetic material like all cells do (though they are not made of cells), and they can evolve. The genetic material of a virus can change (mutate), altering the traits of the virus. As the process of evolution has resulted in all life on the planet today, the classification of viruses has been controversial. It calls into question the very definition of life. "
viruses,T_3486,"Viruses infect a variety of organisms, including plants, animals, and bacteria, injecting its genetic material into a cell of the host organism. Once inside the host cell, they use the cells own ATP (energy), ribosomes, enzymes, and other cellular parts to make copies of themselves. The host cell makes a copy of the viral DNA and produces viral proteins. These are then packaged into new viruses. So viruses cannot replicate or reproduce on their own; they rely on a host cell to make additional viruses. "
viruses,T_3487,"Viruses cause many human diseases. In addition to the flu and the common cold, viruses cause rabies, diarrheal diseases, AIDS, cold sores, and many other diseases ( Figure 1.2). Viral diseases range from mild to fatal. Cold sores are caused by a herpes virus. "
introduction to solutions,T_3508,"A solution forms when one substance dissolves in another. The substance that dissolves is called the solute. The substance it dissolves in is called the solvent. For example, ocean water is a solution in which the solute is salt and the solvent is water. In this example, a solid (salt) is dissolved in a liquid (water). However, matter in any state can be the solute or solvent in a solution. Solutions may be gases, liquids, or solids. In Table 10.1 and the video at the URL below, you can learn about solutions involving other states of matter. Solution Gas dissolved in gas Example: Earths atmosphere Gas dissolved in liquid Example: carbonated water Liquid dissolved in gas Example: moist air Solute oxygen (and other gases) Solvent nitrogen carbon dioxide water water air Solution Liquid dissolved in liquid Example: vinegar Solid dissolved in liquid Example: sweet tea Solid dissolved in solid Example: bronze Solute acetic acid Solvent water sugar tea copper tin When a solute dissolves in a solvent, it changes to the same state as the solvent. For example, when solid salt dissolves in liquid water, it becomes part of the liquid solution, salt water. If the solute and solvent are already in the same state, the substance present in greater quantity is considered to be the solvent. For example, nitrogen is the solvent in Earths atmosphere because it makes up 78 percent of air. "
introduction to solutions,T_3509,"When a solute dissolves, it separates into individual particles that spread evenly throughout the solvent. Exactly how this happens depends on the type of bonds the solute contains. Solutes with ionic bonds, such as table salt (NaCl), separate into individual ions (Na+ and Cl ). Solutes with covalent bonds, such as glucose (H6 C12 O6 ), separate into individual molecules. In either case, the individual ions or molecules spread apart and are surrounded by molecules of the solvent. This is illustrated in Figure 10.1 and in the videos at the URLs below. MEDIA Click image to the left or use the URL below. URL:  MEDIA Click image to the left or use the URL below. URL: "
introduction to solutions,T_3510,"When you add sugar to a cold drink, you may stir it to help the sugar dissolve. If you dont stir, the sugar may eventually dissolve, but it will take much longer. Stirring is one of several factors that affect how fast a solute dissolves in a solvent. Temperature is another factor. A solid solute dissolves faster at a higher temperature. For example, sugar dissolves faster in hot tea than in ice tea. A third factor that affects the rate of dissolving is the surface area of the solute. For example, if you put granulated sugar in a glass of ice tea, it will dissolve more quickly than the same amount of sugar in a cube. Thats because granulated sugar has much more surface area than a cube of sugar. You can see videos of all three factors at these URLs: MEDIA Click image to the left or use the URL below. URL:  MEDIA Click image to the left or use the URL below. URL: "
introduction to solutions,T_3511,"Water is a polar compound. This means it has positively and negatively charged ends. This is why it is so good at dissolving ionic compounds such as salt and polar covalent compounds such as sugar. Solutes that can dissolve in a given solvent, such as water, are said to be soluble in that solvent. So many solutes are soluble in water that water is called the universal solvent. However, there are substances that dont dissolve in water. Did you ever try to clean a paintbrush after painting with an oil-based paint? It doesnt work. Oil-based paint is nonpolar, so it doesnt dissolve in water. In other words, it is insoluble in water. Instead, a nonpolar solvent such as paint thinner must be used to dissolve nonpolar paint. You can see a video about soluble and insoluble solutes at this URL:  (1:51). MEDIA Click image to the left or use the URL below. URL: "
introduction to solutions,T_3512,"When a solute dissolves in a solvent, it changes the physical properties of the solvent. Two properties that change when a solute is added are the freezing and boiling points. Generally, solutes lower the freezing point and raise the boiling point of solvents. You can see some examples of this in Figure below. To see why solutes change the freezing and boiling points of solvents, watch this video:  (14:00). MEDIA Click image to the left or use the URL below. URL:  In each of these examples, a solute changes the freezing and/or boiling points of a solvent. "
solubility and concentration,T_3513,"Solubility is the amount of solute that can dissolve in a given amount of solvent at a given temperature. Some solutes have greater solubility than others in a given solvent. For example, table sugar is much more soluble in water than is baking soda. You can dissolve much more sugar than baking soda in a given amount of water. Compare the solubility of these and other solutes in Figure 10.2. For a video about solubility, go to this URL:  MEDIA Click image to the left or use the URL below. URL: "
solubility and concentration,T_3514,"There is a limit on the amount of solute that can dissolve in a given solvent. Tanya found this out with her baking soda mixture. But even sugar, which is very soluble, has an upper limit. The maximum amount of table sugar that will dissolve in 1 L of water at 20C is about 2000 g. If you add more sugar than this, the extra sugar wont dissolve. A solution that contains as much solute as can dissolve at a given temperature is called a saturated solution. A solution that contains less solute than can dissolve at a given temperature is called an unsaturated solution. A solution of 2000 grams of sugar in 1 L of 20C water is saturated. Thats all the sugar the solution can hold. Any solution containing less than 2000 g of sugar is unsaturated. It can hold more sugar. To learn more about saturated and unsaturated solutions, watch the video at this URL:  . You Try It! Problem: A solution contains 249 grams of Epsom salt in 1 L of water at 20C. Is the solution saturated or unsaturated? Problem: Give an example of an unsaturated solution of table salt in 1 L of 20C water. "
solubility and concentration,T_3515,"Certain factors can change the solubility of a solute. Temperature is one such factor. How temperature affects solubility depends on the state of the solute, as you can see in Figure 10.3. If a solute is a solid or liquid, increasing the temperature increases its solubility. For example, more sugar can dissolve in hot tea than in iced tea. If a solute is a gas, increasing the temperature decreases its solubility. For example, less carbon dioxide can dissolve in warm ocean water than in cold ocean water. The solubility of gases is also affected by pressure. Pressure is the amount of force pushing against a given area. Increasing the pressure on a gas increases its solubility. Did you ever open a can of soda and notice how it fizzes out of the can? Soda contains carbon dioxide. Opening the can reduces the pressure on the gas so it is less soluble. As a result, some of the carbon dioxide comes out of solution and rushes into the air. Do you wonder why temperature and pressure affect solubility in these ways? If so, watch the video at the URL below. It explains why. "
solubility and concentration,T_3516,"The concentration of a solution is the amount of solute in a given amount of solution. A solution with little dissolved solute has a low concentration. It is called a dilute solution. A solution with a lot of dissolved solute has a high concentration. It is called a concentrated solution. Concentration is often expressed as a percent. You can calculate the concentration of a solution using this formula: Concentration = Mass (or Volume) of Solute 100% Mass (or Volume) of Solution For example, if a 100 g solution of salt water contains 3 g of salt, then its concentration is: Concentration = 3g 100% = 3% 100 g For some problems that are more challenging, go to these URLs: MEDIA Click image to the left or use the URL below. URL:  MEDIA Click image to the left or use the URL below. URL:  You Try It! Problem: A 1 L container of juice drink, called brand A, contains 250 mL of juice. The rest of the drink is water. How concentrated is brand A juice drink? Problem: A 600 mL container of another juice drink, called brand B, contains 200 mL of juice. Which brand of juice drink is more concentrated, brand A or brand B? "
electric charge,T_3848,"Electric charge is a physical property of particles or objects that causes them to attract or repel each other without touching. All electric charge is based on the protons and electrons in atoms. A proton has a positive electric charge, and an electron has a negative electric charge (see Figure 23.2). When it comes to electric charges, opposites attract. In other words, positive and negative particles are attracted to each other. Like charges, on the other hand, repel each other, so two positive or two negative charges push apart from each other. The force of attraction or repulsion between charged particles is called electric force. It is illustrated in Figure 23.3. The strength of electric force depends on the amount of electric charge and the distance between the charged particles. The larger the charge or the closer together the charges are, the greater is the electric force. "
electric charge,T_3849,"Electric force is exerted over a distance, so charged particles do not have to be in contact in order to exert force over each other. Thats because each charged particle is surrounded by an electric field. An electric field is a space around a charged particle where the particle exerts electric force on other particles. Electric fields surrounding positively and negatively charged particles are illustrated in Figure 23.4 and at the URL below. When charged particles exert force on each other, their electric fields interact. This is also illustrated in Figure 23.4. "
electric charge,T_3850,"Atoms are neutral in electric charge because they have the same number of electrons as protons. However, atoms may transfer electrons and become charged ions, as illustrated in Figure 23.5. Positively charged ions, or cations, form when atoms give up electrons. Negatively charged ions, or anions, form when atoms gain electrons. Like the formation of ions, the formation of charged matter in general depends on the transfer of electrons either between two materials or within a material. Three ways this can occur are friction, conduction, and polarization. In all cases, the total charge remains the same. Electrons move, but they arent destroyed. This is the law of conservation of charge. "
electric charge,T_3851,"Did you ever rub an inflated balloon against your hair? You can see what happens in Figure 23.6. Friction between the rubber of the balloon and the babys hair results in electrons from the hair ""rubbing off"" onto the balloon. Thats because rubber attracts electrons more strongly than hair does. After the transfer of electrons, the balloon becomes negatively charged and the hair becomes positively charged. As a result, the individual hairs repel each other and the balloon and the hair attract each other. Electrons are transferred in this way whenever there is friction between materials that differ in their ability to give up or accept electrons. "
electric charge,T_3852,"Another way electrons may be transferred is through conduction. This occurs when there is direct contact between materials that differ in their ability to give up or accept electrons. For example, wool tends to give up electrons and rubber tends to accept them. Therefore, when you walk across a wool carpet in rubber-soled shoes, electrons transfer from the carpet to your shoes. You become negatively charged, while the carpet becomes positively charged. Another example of conduction is pictured in Figure 23.7. The device this girl is touching is called a van de Graaff generator. The dome on top is negatively charged. When the girl places her hand on the dome, electrons are transferred to her, so she becomes negatively charged as well. Even the hairs on her head become negatively charged. As a result, individual hairs repel each other, causing them to stand on end. You can see a video demonstration of a van de Graff generator at this URL:  . "
electric charge,T_3853,"Polarization is the movement of electrons within a neutral object due to the electric field of a nearby charged object. It occurs without direct contact between the two objects. You can see how it happens in Figure 23.8. When the negatively charged plastic rod in the figure is placed close to the neutral metal plate, electrons in the plate are repelled by the positive charges in the rod. The electrons move away from the rod, causing one side of the plate to become positively charged and the other side to become negatively charged. Polarization may also occur after you walk across a wool carpet in rubber-soled shoes and become negatively charged. If you reach out to touch a metal doorknob, electrons in the neutral metal will be repelled and move away from your hand before you even touch the knob. In this way, one end of the doorknob becomes positively charged and the other end becomes negatively charged. "
electric charge,T_3854,"Polarization leads to the buildup of electric charges on objects. This buildup of charges is known as static electricity. Once an object becomes charged, it is likely to remain charged until another object touches it or at least comes very close to it. Thats because electric charge cannot travel easily through air, especially if the air is dry. Consider again the example of your hand and the metal doorknob. When your negatively charged hand gets very close to the positively charged doorknob, the air between your hand and the knob may become electrically charged. If that happens, it allows electrons to suddenly flow from your hand to the knob. This is the electric shock you feel when you reach for the knob. You may even see a spark as the electrons jump from your hand to the metal. This sudden flow of electrons is called static discharge. Another example of static discharge, on a much larger scale, is lightning. You can see how it occurs in Figure 23.9. At the URL below, you can watch a slow-motion lightning strike. Be sure to wait for the real-time lightning strike at the very end of the video. "
electric current,T_3855,"Electric current is a continuous flow of electric charges. Current is measured as the amount of charge that flows past a given point in a certain amount of time. The SI unit for electric current is the ampere (A), or amp. Electric current may flow in just one direction, or it may keep reversing direction. When current flows in just one direction, it is called direct current (DC). The current that flows through a battery-powered flashlight is direct current. When current keeps reversing direction, it is called alternating current (AC). The current that runs through the wires in your home is alternating current. Graphs of both types of current are shown in Figure 23.10. You can watch an animation of both types at this URL: MEDIA Click image to the left or use the URL below. URL: "
electric current,T_3856,"Why do charges flow in an electric current? The answer has to do with electric potential energy. Potential energy is stored energy that an object has due to its position or shape. An electric charge has potential energy because of its position in an electric field. For example, when two negative charges are close together, they have potential energy because they repel each other and have the potential to push apart. If the charges move apart, their potential energy decreases. Electric charges always move spontaneously from a position where they have higher potential energy to a position where their potential energy is lower. This is similar to water falling over a dam from an area of higher to lower potential energy due to gravity. In general, for an electric charge to move from one position to another, there must be a difference in electric potential energy between the two positions. The difference in electric potential energy is called potential difference, or voltage. Voltage is measured in an SI unit called the volt (V). For example, the terminals of the car battery in Figure 23.11 have a potential difference of 12 volts. This difference in voltage results in a spontaneous flow of charges, or electric current. "
electric current,T_3857,"Batteries like the one in Figure 23.11 are one of several possible sources of voltage needed to produce electric current. Sources of voltage include generators, chemical cells, and solar cells. Generators change the kinetic energy of a spinning turbine to electrical energy in a process called electromag- netic induction. You can read about generators and how they work in the chapter ""Electromagnetism."" Chemical and solar cells are devices that change chemical or light energy to electrical energy. You can read about both types of cells and how they work below. "
electric current,T_3858,"Chemical cells are found in batteries. They produce voltage by means of chemical reactions. A chemical cell has two electrodes, which are strips made of different materials, such as zinc and carbon (see Figure 23.12). The electrodes are suspended in an electrolyte. An electrolyte is a substance containing free ions that can carry electric current. The electrolyte may be either a paste, in which case the cell is called a dry cell, or a liquid, in which case the cell is called a wet cell. Flashlight batteries contain dry cells. Car batteries contain wet cells. Animations at the URL below show how batteries work. Both dry and wet cells work the same basic way. The electrodes react chemically with the electrolyte, causing one electrode to give up electrons and the other electrode to accept electrons. In the case of zinc and carbon electrodes, the zinc electrode attracts electrons and becomes negatively charged, while the carbon electrode gives up electrons and becomes positively charged. Electrons flow through the electrolyte from the negative to positive electrode. If wires are used to connect the two electrodes at their terminal ends, electric current will flow through the wires and can be used to power a light bulb or other electric device. "
electric current,T_3859,Solar cells convert the energy in sunlight to electrical energy. They contain a material such as silicon that absorbs light energy and gives off electrons. The electrons flow and create electric current. Figure 23.13 and the animation at the URL below show how a solar cell uses light energy to produce electric current and power a light bulb. Many calculators and other devices are also powered by solar cells. 
electric current,T_3860,"Electric current cannot travel through empty space. It needs a material through which to travel. However, when current travels through a material, the flowing electrons collide with particles of the material, and this creates resistance. "
electric current,T_3861,"Resistance is opposition to the flow of electric charges that occurs when electric current travels through matter. The SI unit of resistance is the ohm (named for the scientist Georg Ohm, whom you can read about below). Resistance is caused by electrons in a current bumping into electrons and ions in the matter through which the current is flowing. Resistance is similar to the friction that resists the movement of one surface as it slides over another. Resistance reduces the amount of current that can travel through the material because some of the electrical energy is converted to other forms of energy. For example, when electric current flows through the tungsten wire inside an incandescent light bulb, the tungsten resists the flow of electric charge, and some of the electrical energy is converted to light and thermal energy. "
electric current,T_3862,"Some materials resist the flow of electric current more or less than other materials do. Materials that have low resistance to electric current are called electric conductors. Many metalsincluding copper, aluminum, and steelare good conductors of electricity. Water that has even a tiny amount of impurities in it is an electric conductor as well. Materials that have high resistance to electric current are called electric insulators. Wood, rubber, and plastic are examples of electric insulators. Dry air is also an electric insulator. You probably know that electric wires are made of metal and coated with rubber or plastic (see Figure 23.14). Now you know why. Metals are good electric conductors, so they offer little resistance and allow most of the current to pass through. Rubber and plastic are good insulators, so they offer a lot of resistance and allow little current to pass through. When more than one material is available for electric current to flow through, the current always travels through the material with the least resistance. Thats why all the current passes through a metal wire and none flows through its rubber or plastic coating. "
electric current,T_3863,"For a given material, three properties of the material determine how resistant it is to electric current: length, width, and temperature. Consider an electric wire like one of the wires in Figure 23.14. A longer wire has more resistance. Current must travel farther, so there are more chances for it to collide with particles of wire. A wider wire has less resistance. A given amount of current has more room to flow through a wider wire. A cooler wire has less resistance than a warmer wire. Cooler particles have less kinetic energy, so they move more slowly. Current is less likely to collide with slowly moving particles. Materials called superconductors have virtually no resistance when they are cooled to extremely low temperatures. "
electric current,T_3864,"Voltage, or a difference in electric potential energy, is needed for electric current to flow. As you might have guessed, greater voltage results in more current. Resistance, on the other hand, opposes the flow of electric current, so greater resistance results in less current. These relationships between current, voltage, and resistance were first demonstrated by a German scientist named Georg Ohm in the early 1800s, so they are referred to as Ohms law. Ohms law can be represented by the following equation. Current (amps) = Voltage (volts) Resistance (ohms) "
electric current,T_3865,"You may have a better understanding of Ohms law if you compare current flowing through a wire from a battery to water flowing through a garden hose from a tap. Increasing voltage is like opening the tap wider. When the tap is opened wider, more water flows through the hose. This is like an increase in current. Stepping on the hose makes it harder for the water to pass through. This is like increasing resistance, which causes less current to flow through a material. Still not sure about the relationship among voltage, current, and resistance? Watch the video at this URL: MEDIA Click image to the left or use the URL below. URL: "
electric current,T_3866,"You can use the equation for current (above) to calculate the amount of current flowing through a material when voltage and resistance are known. Consider an electric wire that is connected to a 12-volt battery. If the wire has a resistance of 3 ohms, how much current is flowing through the wire? Current = 12 volts = 4 amps 3 ohms You Try It! Problem: A 120-volt voltage source is connected to a wire with 20 ohms of resistance. How much current flows through the wire? "
electric circuits,T_3867,"A closed loop through which current can flow is called an electric circuit. In homes in the U.S., most electric circuits have a voltage of 120 volts. The amount of current (amps) a circuit carries depends on the number and power of electrical devices connected to the circuit. But home circuits generally have a safe upper limit of about 20 or 30 amps. "
electric circuits,T_3868,"All electric circuits have at least two parts: a voltage source and a conductor. The voltage source of the circuit in Figure 23.16 is a battery. In a home circuit, the source of voltage is an electric power plant, which may supply electric current to many homes and businesses in a community or even to many communities. The conductor in most circuits consists of one or more wires. The conductor must form a closed loop from the source of voltage and back again. In Figure 23.16, the wires are connected to both terminals of the battery, so they form a closed loop. The circuit in Figure 23.16 also has two other parts: a light bulb and a switch. Most circuits have devices such as light bulbs that convert electric energy to other forms of energy. In the case of a light bulb, electricity is converted to light and thermal energy. Many circuits have switches to control the flow of current through the circuit. When the switch is turned on, the circuit is closed and current can flow through it. When the switch is turned off, the circuit is open and current cannot flow through it. "
electric circuits,T_3869,"When a contractor builds a new home, she uses a set of plans called blueprints that show her how to build the house. The blueprints include circuit diagrams that show how the wiring and other electrical components are to be installed in order to supply current to appliances, lights, and other electrical devices in the home. You can see an example of a very simple circuit diagram in Figure 23.17. Different parts of the circuit are represented by standard symbols, as defined in the figure. An ammeter measures the flow of current through the circuit, and a voltmeter measures the voltage. A resistor is any device that converts some of the electricity to other forms of energy. It could be a light bulb, doorbell, or similar device. "
electric circuits,T_3870,"There are two basic types of electric circuits, called series and parallel circuits. They differ in the number of loops through which current can flow. You can see an example of each type of circuit in Figure 23.18. A series circuit has only one loop through which current can flow. If the circuit is interrupted at any point in the loop, no current can flow through the circuit and no devices in the circuit will work. In the series circuit in Figure 23.18, if one light bulb burns out the other light bulb will not work because it wont receive any current. Series circuits are commonly used in flashlights. You can see an animation of a series circuit at this URL: http://regentsprep.org/regents/physics/phys03/bsercir/default.htm . A parallel circuit has two (or more) loops through which current can flow. If the circuit is interrupted in one of the loops, current can still flow through the other loop(s). For example, if one light bulb burns out in the parallel circuit in Figure 23.18, the other light bulb will still work because current can by-pass the burned-out bulb. The wiring in a house consists of parallel circuits. You can see an animation of a parallel circuit at this URL: http://regentsprep.org/regents/physics/phys03/bsercir/default.htm . "
electric circuits,T_3871,"We use electricity for many purposes. Devices such as lights, stoves, and stereos all use electricity and convert it to energy in other forms. However, devices may vary in how quickly they change electricity to other forms of energy. "
electric circuits,T_3872,"The rate at which a device changes electric current to another form of energy is called electric power. The SI unit of powerincluding electric poweris the watt. A watt equals 1 joule of energy per second. High wattages are often expressed in kilowatts, where 1 kilowatt equals 1000 watts. The power of an electric device, such as a microwave, can be calculated if you know the current and voltage of the circuit. This equation shows how power, current, and voltage are related: Power (watts) = Current (amps)  Voltage (volts) Consider a microwave that is plugged into a home circuit. Assume the microwave is the only device connected to the circuit. If the voltage of the circuit is 120 volts and it carries 10 amps of current, then the power of the microwave is: Power = 120 volts  10 amps = 1200 watts, or 1.2 kilowatts You Try It! Problem: A hair dryer is connected to a 120-volt circuit that carries 12 amps of current. What is the power of the hair dryer in kilowatts? "
electric circuits,T_3873,"Did you ever wonder how much electrical energy it takes to use an appliance such as a microwave or hair dryer? Electrical energy use depends on the power of the appliance and how long it is used. It can be represented by the equation: Electrical Energy = Power  Time 1 Suppose you use a 1.2-kilowatt microwave for 5 minutes ( 12 hour). Then the energy used would be: Electrical Energy = 1.2 kilowatts 1 hour = 0.1 kilowatt-hours 12 Electrical energy use is typically expressed in kilowatt-hours, as in this example. How much energy is this? One kilowatt-hour equals 3.6 million joules of energy. Therefore, the 0.1 kilowatt-hours used by the microwave equals 0.36 million joules of energy. You Try It! Problem: A family watches television for an average of 2 hours per day. The television has 0.12 kilowatts of power. How much electrical energy does the family use watching television each day? "
electric circuits,T_3874,Electricity is dangerous. Contact with electric current can cause severe burns and even death. Electricity can also cause serious fires. A common cause of electric hazards and fires is a short circuit. 
electric circuits,T_3875,"An electric cord contains two wires. One wire carries current from the outlet to the appliance or other electric device, and one wire carries current back to the outlet. Did you ever see an old appliance with a damaged cord, like the one in Figure 23.19? A damaged electric cord can cause a severe shock if it allows current to pass from the cord to a person who touches it. A damaged cord can also cause a short circuit. A short circuit occurs when electric current follows a shorter path than the intended loop of the circuit. For example, if the two wires in a damaged cord come into contact with each other, current flows from one wire to the other and bypasses the appliance. This may cause the wires to overheat and start a fire. "
electric circuits,T_3876,"Because electricity can be so dangerous, safety features are built into electric circuits and devices. They include three-prong plugs, circuit breakers, and GFCI outlets. Each feature is described and illustrated in Table 23.1. You can learn more about electric safety features in the home by watching the video at this URL:  Electric Safety Feature Three-Prong Plug Circuit Breaker Description A three-prong plug is generally used on metal appli- ances. The two flat prongs carry current to and from the appliance. The round prong is for safety. It connects with a wire inside the outlet that goes down into the ground. If any stray current leaks from the circuit or if there is a short circuit, the ground wire carries the current into the ground, which harmlessly absorbs it. A circuit breaker is a switch that automatically opens a circuit if too much current flows through it. This could happen if too many electric devices are plugged into the circuit or if there is an electric short. Once the problem is resolved, the circuit breaker can be switched back on to close the circuit. Circuit breakers are generally found in a breaker box that controls all the circuits in a building. Electric Safety Feature GFCI Outlet Description GFCI stands for ground-fault circuit interrupter. GFCI outlets are typically found in bathrooms and kitchens where the use of water poses a risk of shock (because water is a good electric conductor). A GFCI outlet contains a device that monitors the amounts of current leaving and returning to the outlet. If less current is returning than leaving, this means that current is escaping. When this occurs, a tiny circuit breaker in the outlet opens the circuit. The breaker can be reset by pushing a button on the outlet cover. "
electric circuits,T_3877,"Even with electric safety features, electricity is still dangerous if it is not used safely. Follow the safety rules below to reduce the risk of injury or fire from electricity. Never mix electricity and water. Dont turn on or plug in electric lights or appliances when your hands are wet, you are standing in water, or you are in the shower or bathtub. The current could flow through the waterand youbecause water is a very good electric conductor. Never overload circuits. Avoid plugging too many devices into one outlet or extension cord. The more devices that are plugged in, the more current the circuit carries. Too much current can overheat a circuit and start a fire. Never use devices with damaged cords or plugs. They can cause shocks, shorts, and fires. Never put anything except plugs into electric outlets. Plugging in other objects is likely to cause a serious shock that could be fatal. Never go near fallen electric lines. They could be carrying a lot of current. Report fallen lines to the electric company as soon as possible. "
electronics,T_3878,"Did you ever make a secret code? One way to make a code is to represent each letter of the alphabet by a different number. Then you can send a coded message by writing words as strings of digits. This is similar to how information is encoded using an electric current. The voltage of the current is changed rapidly and repeatedly to encode a message, called an electronic signal. There are two different types of electronic signals: analog signals and digital signals. Both are illustrated in Figure 23.20. A digital signal consists of pulses of voltage, created by repeatedly switching the current off and on. This type of signal encodes information as a string of 0s (current off) and 1s (current on). This is called a binary (""two-digit"") code. DVDs, for example, encode sounds and pictures as digital signals. An analog signal consists of continuously changing voltage in a circuit. For example, microphones encode sounds as analog signals. "
electronics,T_3879,Electronic components are the parts used in electronic devices such as computers. The components transmit and change electric current. They are made of materials called semiconductors. 
electronics,T_3880,"A semiconductor is a solid crystalusually consisting mainly of siliconthat can conduct current better than an electric insulator but not as well as an electric conductor. Very small amounts of other elements, such as boron or phosphorus, are added to the silicon so it can conduct current. A semiconductor is illustrated in Figure 23.21. There are two different types of semiconductors: n-type and p-type. An n-type semiconductor consists of silicon and an element such as phosphorus that gives the silicon crystal extra electrons. An n-type semiconductor is like the negative terminal in a chemical cell. A p-type semiconductor consists of silicon and an element such as boron that gives the silicon positively charged holes where electrons are missing. A p-type semiconductor is like the positive terminal in a chemical cell. "
electronics,T_3881,"Electronic components contain many semiconductors. Types of components include diodes, transistors, and inte- grated circuits. Each type is described in Table 23.2. Electronic Component Diode Transistor Integrated Circuit (Microchip) Description A diode consists of a p-type and an n-type semicon- ductor placed side by side. When a diode is connected by leads to a source of voltage, electrons flow from the n-type to the p-type semiconductor. This is the only direction that electrons can flow in a diode. This makes a diode useful for changing alternating current to direct current. A transistor consists of three semiconductors, either p- n-p or n-p-n. Current cant flow through a transistor unless a small amount of current is applied to the center semiconductor (through the base). Then a much larger current can flow through the transistor from end to end (from collector to emitter). This means that a transmitter can be used as a switch, with pulses of a small current turning a larger current on and off. A transistor can also be used to increase the amount of current flowing through a circuit. You can learn more about transistors and how they work at this URL: http An integrated circuitalso called a microchipis a tiny, flat piece of silicon that consists of layers of elec- tronic components such as transistors. An integrated circuit as small as a fingernail can contain millions of electronic components. Current flows extremely rapidly in an integrated circuit because it doesnt have far to travel. You can learn how microprocessors are made at this URL: "
electronics,T_3882,"Many of the devices you commonly use are electronic. Electronic devices include computers, mobile phones, TV remotes, DVD and CD players, game systems, MP3 players, and digital cameras. All of these devices use electric current to encode, analyze, or transmit information. Consider the computer as an example of an electronic device. A computer contains microchips with millions of tiny electronic components. Information is encoded as 0s and 1s and transmitted as electrical pulses. One digit (either 0 or 1) is called a bit, which stands for ""binary digit."" Each group of eight digits is called a byte. A gigabyte is a billion bytes  thats 8 billion 0s and 1s! Because a computers circuits are so tiny and close together, the computer can be very fast and capable of many complex tasks while remaining small. The parts of a computer that transmit, process, or store digital signals are pictured in Figure 23.22 and described below. They include the CPU, hard drive, ROM, and RAM. The motherboard ties all these parts of the computer together. The CPU, or central processing unit, carries out program instructions. You can learn more about CPUs and how they work by watching the video at this URL:  . The hard drive is a magnetic disc that provides long-term storage for programs and data. ROM (read-only memory) is a microchip that provides permanent storage. It stores important information such as start-up instructions. This memory remains even after the computer is turned off. RAM (random-access memory) is a microchip that temporarily stores programs and data that are currently being used. Anything stored in RAM is lost when the computer is turned off. The motherboard is connected to the CPU, hard drive, ROM, and RAM. It allows all these parts of the computer to receive power and communicate with one another. "
electricity and magnetism,T_3897,"In 1820, a physicist in Denmark, named Hans Christian Oersted, discovered how electric currents and magnetic fields are related. However, it was just a lucky accident. Oersted, who is pictured in Figure 25.1, was presenting a demonstration to his students. Ironically, he was trying to show that electricity and magnetism are not related. He placed a wire with electric current flowing through it next to a magnet, and nothing happened. After class, a student held the wire near the magnet again, but in a different direction. To Oersteds surprise, the pointer of the magnet swung toward the wire so it was no longer pointing to Earths magnetic north pole. Oersted was intrigued. He turned off the current in the wire to see what would happen to the magnet. The pointer swung back to its original position, pointing north again. Oersted had discovered that an electric current creates a magnetic field. The magnetic field created by the current was strong enough to attract the pointer of the nearby compass. Oersted wanted to learn more about the magnetic field created by a current, so he placed a magnet at different locations around a wire with current flowing through it. You can see some of his results in Figure 25.2. They show that the magnetic field created by a current has field lines that circle around the wire. You can learn more about Oersteds investigations of current and magnetism at the URL below. MEDIA Click image to the left or use the URL below. URL: "
electricity and magnetism,T_3898,"The magnetic field created by a current flowing through a wire actually surrounds the wire in concentric circles. This magnetic field is stronger if more current is flowing through the wire. The direction of the magnetic field also depends on the direction that the current is flowing through the wire. A simple rule, called the right hand rule, makes it easy to find the direction of the magnetic field if the direction of the current is known. The right hand rule is illustrated in Figure 25.3. When the thumb of the right hand is pointing in the same direction as the current, the fingers of the right hand curl around the wire in the direction of the magnetic field. You can see the right hand rule in action at this URL:  . "
using electromagnetism,T_3899,"A solenoid is a coil of wire with electric current flowing through it, giving it a magnetic field (see Figure 25.5). Recall that current flowing through a straight wire produces a weak electromagnetic field that circles around the wire. Current flowing through a coil of wire, in contrast, produces a magnetic field that has north and south poles like a bar magnet. The magnetic field around a coiled wire is also stronger than the magnetic field around a straight wire because each turn of the wire has its own magnetic field. Adding more turns increases the strength of the field, as does increasing the amount of current flowing through the coil. You can see an actual solenoid with a compass showing its magnetic north pole at this URL:  . "
using electromagnetism,T_3900,"Solenoids are the basis of electromagnets. An electromagnet is a solenoid wrapped around a bar of iron or other ferromagnetic material (see Figure 25.6). The electromagnetic field of the solenoid magnetizes the iron bar by aligning its magnetic domains. The combined magnetic force of the magnetized iron bar and the wire coil makes an electromagnet very strong. In fact, electromagnets are the strongest magnets made. Some of them are strong enough to lift a train. The maglev train described earlier, in the lesson ""Electricity and Magnetism,"" contains permanent magnets. Strong electromagnets in the track repel the train magnets, causing the train to levitate above the track. Like a solenoid, an electromagnet is stronger if there are more turns in the coil or more current is flowing through it. A bigger bar or one made of material that is easier to magnetize also increases an electromagnets strength. You can see how to make a simple electromagnet at this URL:  (4:57). MEDIA Click image to the left or use the URL below. URL: "
using electromagnetism,T_3901,"Many common electric devices contain electromagnets. Some examples include hair dryers, fans, CD players, telephones, and doorbells. Most electric devices that have moving parts contain electric motors. You can read below how doorbells and electric motors use electromagnets. "
using electromagnetism,T_3902,"Figure 25.7 shows a diagram of a simple doorbell. Like most doorbells, it has a button located by the front door. Pressing the button causes two electric contacts to come together and complete an electric circuit. In other words, the button is a switch. The circuit is also connected to a voltage source, an electromagnet, and the clapper of a bell. When current flows through the circuit, the electromagnet turns on, and its magnetic field attracts the clapper. This causes the clapper to hit the bell, making it ring. Because the clapper is part of the circuit, when it moves to strike the bell, it breaks the circuit. Without current flowing through the circuit, the electromagnet turns off. The clapper returns to its original position, which closes the circuit again and turns the electromagnet back on. The electromagnet again attracts the clapper, which hits the bell once more. This sequence of events keeps repeating as long as the button by the front door is being pressed. "
using electromagnetism,T_3903,"An electric motor is a device that uses an electromagnet to change electrical energy to kinetic energy. Figure 25.8 shows a simple diagram of an electric motor. The motor contains an electromagnet that is connected to a shaft. When current flows through the motor, the electromagnet turns, causing the shaft to turn as well. The rotating shaft moves other parts of the device. Why does the motors electromagnet turn? Notice that the electromagnet is located between the north and south poles of two permanent magnets. When current flows through the electromagnet, it becomes magnetized, and its poles are repelled by the like poles of the permanent magnets. This causes the electromagnet to turn toward the unlike poles of the permanent magnets. A device called a commutator then changes the direction of the current so the poles of the electromagnet are reversed. The reversed poles are once again repelled by the like poles of the permanent magnets. This causes the electromagnet to continue to turn. These events keep repeating, so the electromagnet rotates continuously. You can make a very simple electric motor with a battery, wire, and magnet following instructions at this URL:  . "
generating and using electricity,T_3904,"Just about a decade after Oersted discovered that electric current produces a magnetic field, an English scientist named Michael Faraday discovered that the reverse is also true. A magnetic field produces an electric current, as long as the magnetic field is changing. This is called Faradays law. "
generating and using electricity,T_3905,"The process of generating electric current with a changing magnetic field is called electromagnetic induction. It occurs whenever a magnetic field and an electric conductor, such as a coil of wire, move relative to one another. As long as the conductor is part of a closed circuit, current will flow through it whenever it crosses magnetic field lines. One way this can happen is pictured in Figure 25.9. It shows a magnet moving inside a wire coil. Another way is for the coil to move instead of the magnet. You can watch an animated version of Figure 25.9 at this URL: http://jsticca.wordpress.com/2009/09/01/the-magn "
generating and using electricity,T_3906,"The device in the circuit in Figure 25.9 is an ammeter. It measures the current that flows through the wire. The faster the magnet or coil moves, the greater the amount of current that is produced. If more turns were added to the coil, this would increase the strength of the magnetic field as well. If the magnet were moved back and forth repeatedly, the current would keep changing direction. In other words, alternating current would be produced. This is illustrated in Figure 25.10. "
generating and using electricity,T_3907,Two important devices depend on electromagnetic induction: electric generators and electric transformers. Both devices play critical roles in producing and regulating the electric current we depend on in our daily lives. 
generating and using electricity,T_3908,"An electric generator is a device that changes kinetic energy to electrical energy through electromagnetic induction. A simple diagram of an electric generator is shown in Figure 25.11. In a generator, some form of energy is applied to turn a shaft. This causes a coil of wire to rotate between opposite poles of a magnet. Because the coil is rotating in a magnetic field, electric current is generated in the wire. If the diagram in Figure 25.11 looks familiar to you, thats because a generator is an electric motor in reverse. Look back at the electric motor in Figure 25.8. If you were to mechanically turn the shaft of the motor (instead of using electromagnetism to turn it), the motor would generate electricity just like an electric generator. You can learn how to make a very simple electric generator by watching the video at the URL below. Making your own generator will help you understand how a generator works. Generators may be set up to produce either alternating or direct current. Generators in cars and most power plants produce alternating current. A car generator produces electricity with some of the kinetic energy of the turning crankshaft. The electricity is used to run the cars lights, power windows, radio, and other electric devices. Some of the electricity is stored in the cars battery to provide electrical energy when the car isnt running. A power plant generator produces electricity with the kinetic energy of a turning turbine. The energy to turn the turbine may come from burning fuel, falling water, or some other energy source. You can see how falling water is used to generate electricity in Figure 25.12 and in the video at this URL: "
generating and using electricity,T_3909,"An electric transformer is a device that uses electromagnetic induction to change the voltage of electric current. A transformer may either increase or decrease voltage, but it only works with alternating current. You can see the components of an electric transformer in Figure 25.13. As you can see in Figure 25.13, a transformer consists of two wire coils wrapped around an iron core. When alternating primary current passes through coil P, it magnetizes the iron core. Because the current is alternating, the magnetic field of the iron core keeps reversing. This changing magnetic field induces alternating current in coil S, which is part of another circuit. In Figure 25.13, coil P and coil S have the same number of turns of wire. In this case, the voltages of the primary and secondary currents are the same. However, when the two coils have different numbers of turns, the voltage of the secondary current is different than the voltage of the primary current. Both cases are illustrated in Figure 25.14. When coil S has more turns of wire than coil P, the voltage in the secondary current is greater than the voltage in the primary current. This type of transformer is called a step-up transformer. When coil S has fewer turns of wire than coil P, the voltage in the secondary current is less than the voltage in the primary current. This type of transformer is called a step-down transformer. For an animation of a transformer, go to this URL:  . "
generating and using electricity,T_3910,"Power plant generators produce high-voltage electric current. Many power plants also use step-up transformers to increase the voltage of the current even more (see Figure 25.15). By increasing the voltage, the amount of current is decreased, so less power is lost as the electricity travels through the power lines. However, the voltage in power lines is too high to be safe for home circuits. The voltage in power lines may be as great as 750,000 volts, whereas most home circuits are 240 or 120 volts. One or more step-down transformers decrease the voltage of current before it enters the home. Other step-down transformers within the home lower the voltage of some of the homes circuits. For an overview of electric power generation, transmission, and distribution in the U.S., go to this URL: http://w "
properties of matter,T_3911,"Both you and the speck of dust consist of atoms of matter. So does the ground beneath your feet. In fact, everything you can see and touch is made of matter. The only things that arent matter are forms of energy, such as light and sound. Although forms of energy are not matter, the air and other substances they travel through are. So what is matter? Matter is defined as anything that has mass and volume. "
properties of matter,T_3912,"Mass is the amount of matter in a substance or object. Mass is commonly measured with a balance. A simple mechanical balance is shown in Figure 3.1. It allows an object to be matched with other objects of known mass. SI units for mass are the kilogram, but for smaller masses grams are often used instead. "
properties of matter,T_3913,"The more matter an object contains, generally the more it weighs. However, weight is not the same thing as mass. Weight is a measure of the force of gravity pulling on an object. It is measured with a scale, like the kitchen- scale in Figure 3.2. The scale detects how forcefully objects in the pan are being pulled downward by the force of gravity. The SI unit for weight is the newton (N). The common English unit is the pound (lb). With Earths gravity, a mass of 1 kg has a weight of 9.8 N (2.2 lb). Problem Solving Problem: At Earths gravity, what is the weight in newtons of an object with a mass of 10 kg? Solution: At Earths gravity, 1 kg has a weight of 9.8 N. Therefore, 10 kg has a weight of (10  9.8 N) = 98 N. You Try It! Problem: If you have a mass of 50 kg on Earth, what is your weight in newtons? An object with more mass is pulled by gravity with greater force, so mass and weight are closely related. However, the weight of an object can change if the force of gravity changes, even while the mass of the object remains constant. Look at the photo of astronaut Edwin E. Aldrin Jr taken by fellow astronaut Neil Armstrong, the first human to walk on the moon, in Figure 3.3. An astronaut weighed less on the moon than he did on Earth because the moons gravity is weaker than Earths. The astronauts mass, on the other hand, did not change. He still contained the same amount of matter on the moon as he did on Earth. The amount of space matter takes up is its volume. How the volume of matter is measured depends on its state. The volume of liquids is measured with measuring containers. In the kitchen, liquid volume is usually measured with measuring cups or spoons. In the lab, liquid volume is measured with containers such as graduated cylinders. Units in the metric system for liquid volume include liters (L) and milliliters (mL). The volume of gases depends on the volume of their container. Thats because gases expand to fill whatever space is available to them. For example, as you drink water from a bottle, air rushes in to take the place of the water. An ""empty"" liter bottle actually holds a liter of air. How could you find the volume of air in an ""empty"" room? The volume of regularly shaped solids can be calculated from their dimensions. For example, the volume of a rectangular solid is the product of its length, width, and height (l  w  h). For solids that have irregular shapes, the displacement method is used to measure volume. You can see how it works in Figure 3.4 and in the video below. The SI unit for solid volumes is cubic meters (m3 ). However, cubic centimeters (cm3 ) are often used for smaller volume measurements. "
properties of matter,T_3914,"Matter has many properties. Some are physical properties. Physical properties of matter are properties that can be measured or observed without matter changing to a different substance. For example, whether a given substance normally exists as a solid, liquid, or gas is a physical property. Consider water. It is a liquid at room temperature, but if it freezes and changes to ice, it is still water. Generally, physical properties are things you can see, hear, smell, or feel with your senses. "
properties of matter,T_3915,"Physical properties include the state of matter and its color and odor. For example, oxygen is a colorless, odorless gas. Chlorine is a greenish gas with a strong, sharp odor. Other physical properties include hardness, freezing and boiling points, the ability to dissolve in other substances, and the ability to conduct heat or electricity. These properties are demonstrated in Figure 3.5. Can you think of other physical properties? "
properties of matter,T_3916,"Density is an important physical property of matter. It reflects how closely packed the particles of matter are. Density is calculated from the amount of mass in a given volume of matter, using the formula: Density (D) = Mass (M) Volume (V ) Problem Solving Problem: What is the density of a substance that has a mass of 20 g and a volume of 10 mL? Solution: D = 20 g/10 mL = 2.0 g/mL You Try It! Problem: An object has a mass of 180 kg and a volume of 90 m3 . What is its density? To better understand density, think about a bowling ball and a volleyball. The bowling ball feels heavy. It is solid all the way through. It contains a lot of tightly packed particles of matter. In contrast, the volleyball feels light. It is full of air. It contains fewer, more widely spaced particles of matter. Both balls have about the same volume, but the bowling ball has a much greater mass. Its matter is denser. "
properties of matter,T_3917,"It looks like frozen smoke, and its the lightest solid material on the planet. Aerogel insulates space suits, makes tennis rackets stronger and could be used one day to clean up oil spills. Lawrence Livermore National Laboratory scientist Alex Gash shows us some remarkable properties of this truly unique substance. For more information on aerogel, see http://science.kqed.org/quest/video/quest-lab-aerogel/ . MEDIA Click image to the left or use the URL below. URL: "
properties of matter,T_3918,Some properties of matter can be measured or observed only when matter undergoes a change to become an entirely different substance. These properties are called chemical properties. They include flammability and reactivity. 
properties of matter,T_3919,"Flammability is the ability of matter to burn. Wood is flammable; iron is not. When wood burns, it changes to ashes, carbon dioxide, water vapor, and other gases. After burning, it is no longer wood. "
properties of matter,T_3920,"Reactivity is the ability of matter to combine chemically with other substances. For example, iron is highly reactive with oxygen. When it combines with oxygen, it forms the reddish powder called rust (see Figure 3.6). Rust is not iron but an entirely different substance that consists of both iron and oxygen. "
changes in matter,T_3932,"A physical change in matter is a change in one or more of matters physical properties. Glass breaking is just one example of a physical change. Some other examples are shown in Figure 3.16 and in the video below. In each example, matter may look different after the change occurs, but its still the same substance with the same chemical properties. For example, smaller pieces of wood have the ability to burn just as larger logs do. MEDIA Click image to the left or use the URL below. URL:  Because the type of matter remains the same with physical changes, the changes are often easy to undo. For example, braided hair can be unbraided again. Melted chocolate can be put in a fridge to re-harden. Dissolving salt in water is also a physical change. How do you think you could undo it? "
changes in matter,T_3933,"Did you ever make a ""volcano,"" like the one in Figure 3.17, using baking soda and vinegar? What happens when the two substances combine? They produce an eruption of foamy bubbles. This happens because of a chemical change. A chemical change occurs when matter changes chemically into an entirely different substance with different chemical properties. When vinegar and baking soda combine, they form carbon dioxide, a gas that causes the bubbles. Its the same gas that gives soft drinks their fizz. Not all chemical changes are as dramatic as this ""volcano."" Some are slower and less obvious. Figure 3.18 and the video below show other examples of chemical changes. MEDIA Click image to the left or use the URL below. URL: "
changes in matter,T_3934,"How can you tell whether a chemical change has occurred? Often, there are clues. Several are demonstrated in Figures 3.17 and 3.18 and in the video below. MEDIA Click image to the left or use the URL below. URL:  To decide whether a chemical change has occurred, look for these signs: Gas bubbles are released. (Example: Baking soda and vinegar mix and produce bubbles.) Something changes color. (Example: Leaves turn from green to other colors.) An odor is produced. (Example: Logs burn and smell smoky.) A solid comes out of a solution. (Example: Eggs cook and a white solid comes out of the clear liquid part of the egg.) "
changes in matter,T_3935,"Because chemical changes produce new substances, they often cannot be undone. For example, you cant change a fried egg back to a raw egg. Some chemical changes can be reversed, but only by other chemical changes. For example, to undo the tarnish on copper pennies, you can place them in vinegar. The acid in the vinegar reacts with the tarnish. This is a chemical change that makes the pennies bright and shiny again. You can try this yourself at home to see how well it works. "
changes in matter,T_3936,"If you build a campfire, like the one in Figure 3.19, you start with a large stack of sticks and logs. As the fire burns, the stack slowly shrinks. By the end of the evening, all thats left is a small pile of ashes. What happened to the matter that you started with? Was it destroyed by the flames? It may seem that way, but in fact, the same amount of matter still exists. The wood changed not only to ashes but also to carbon dioxide, water vapor, and other gases. The gases floated off into the air, leaving behind just the ashes. Assume you had measured the mass of the wood before you burned it. Assume you had also trapped the gases released by the burning wood and measured their mass and the mass of the ashes. What would you find? The ashes and gases combined have the same mass as the wood you started with. This example illustrates the law of conservation of mass. The law states that matter cannot be created or destroyed. Even when matter goes through physical or chemical changes, the total mass of matter always remains the same. (In the chapter Nuclear Chemistry, you will learn about nuclear reactions, in which mass is converted into energy. But other than that, the law of conservation of mass holds.) For a fun challenge, try to apply the law of conservation of mass to a scene from a Harry Potter film at this link:  . "
solids liquids gases and plasmas,T_3937,"Ice is an example of solid matter. A solid is matter that has a fixed volume and a fixed shape. Figure 4.3 shows examples of matter that are usually solids under Earth conditions. In the figure, salt and cellulose are examples of crystalline solids. The particles of crystalline solids are arranged in a regular repeating pattern. The steaks and candle wax are examples of amorphous (""shapeless"") solids. Their particles have no definite pattern. "
solids liquids gases and plasmas,T_3938,"Ocean water is an example of a liquid. A liquid is matter that has a fixed volume but not a fixed shape. Instead, a liquid takes the shape of its container. If the volume of a liquid is less than the volume of its container, the top surface will be exposed to the air, like the oil in the bottles in Figure 4.4. Two interesting properties of liquids are surface tension and viscosity. Surface tension is a force that pulls particles at the exposed surface of a liquid toward other liquid particles. Surface tension explains why water forms droplets, like those in Figure 4.5. Viscosity is a liquids resistance to flowing. Thicker liquids are more viscous than thinner liquids. For example, the honey in Figure 4.5 is more viscous than the vinegar. You can learn more about surface tension and viscosity at these URLs: http://io9.com/5668221/an-experiment-with-soap-water-pepper-and-surface-tension http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Viscosity-840.html  (1:40) MEDIA Click image to the left or use the URL below. URL: "
solids liquids gases and plasmas,T_3939,"Water vapor is an example of a gas. A gas is matter that has neither a fixed volume nor a fixed shape. Instead, a gas takes both the volume and the shape of its container. It spreads out to take up all available space. You can see an example in Figure 4.6. "
solids liquids gases and plasmas,T_3940,"Youre probably less familiar with plasmas than with solids, liquids, and gases. Yet, most of the universe consists of plasma. Plasma is a state of matter that resembles a gas but has certain properties that a gas does not have. Like a gas, plasma lacks a fixed volume and shape. Unlike a gas, plasma can conduct electricity and respond to magnetism. Thats because plasma contains charged particles called ions. This gives plasma other interesting properties. For example, it glows with light. Where can you find plasmas? Two examples are shown in Figure 4.7. The sun and other stars consist of plasma. Plasmas are also found naturally in lightning and the polar auroras (northern and southern lights). Artificial plasmas are found in fluorescent lights, plasma TV screens, and plasma balls like the one that opened this chapter. You can learn more about plasmas at this URL:  (2:58). MEDIA Click image to the left or use the URL below. URL: "
solids liquids gases and plasmas,T_3941,"Why do different states of matter have different properties? Its because of differences in energy at the level of atoms and molecules, the tiny particles that make up matter. "
solids liquids gases and plasmas,T_3942,"Energy is defined as the ability to cause changes in matter. You can change energy from one form to another when you lift your arm or take a step. In each case, energy is used to move matter  you. The energy of moving matter is called kinetic energy. "
solids liquids gases and plasmas,T_3943,The particles that make up matter are also constantly moving. They have kinetic energy. The theory that all matter consists of constantly moving particles is called the kinetic theory of matter. You can learn more about it at the URL below. 
solids liquids gases and plasmas,T_3944,"Particles of matter of the same substance, such as the same element, are attracted to one another. The force of attraction tends to pull the particles closer together. The particles need a lot of kinetic energy to overcome the force of attraction and move apart. Its like a tug of war between opposing forces. The kinetic energy of individual particles is on one side, and the force of attraction between different particles is on the other side. The outcome of the ""war"" depends on the state of matter. This is illustrated in Figure 4.8 and in the animation at this URL: http://w In solids, particles dont have enough kinetic energy to overcome the force of attraction between them. The particles are packed closely together and cannot move around. All they can do is vibrate. This explains why solids have a fixed volume and shape. In liquids, particles have enough kinetic energy to partly overcome the force of attraction between them. They can slide past one another but not pull completely apart. This explains why liquids can change shape but have a fixed volume. In gases, particles have a lot of kinetic energy. They can completely overcome the force of attraction between them and move apart. This explains why gases have neither a fixed volume nor a fixed shape. "
changes of state,T_3950,"What causes clouds to form? And in general, how does matter change from one state to another? As you may have guessed, changes in energy are involved. "
changes of state,T_3951,"Changes of state are physical changes in matter. They are reversible changes that do not involve changes in matters chemical makeup or chemical properties. Common changes of state include melting, freezing, sublimation, deposition, condensation, and vaporization. These changes are shown in Figure 4.18. Each is described in detail below. "
changes of state,T_3952,"Energy is always involved in changes of state. Matter either loses or absorbs energy when it changes from one state to another. For example, when matter changes from a liquid to a solid, it loses energy. The opposite happens when matter changes from a solid to a liquid. For a solid to change to a liquid, matter must absorb energy from its surroundings. The amount of energy in matter can be measured with a thermometer. Thats because a thermometer measures temperature, and temperature is the average kinetic energy of the particles of matter. You can learn more about energy, temperature, and changes of state at this URL: http://hogan.chem.lsu.edu/matter/chap26/animate3/an2 "
changes of state,T_3953,Think about how you would make ice cubes in a tray. First you would fill the tray with water from a tap. Then you would place the tray in the freezer compartment of a refrigerator. The freezer is very cold. What happens next? 
changes of state,T_3954,"The warmer water in the tray loses heat to the colder air in the freezer. The water cools until its particles no longer have enough energy to slide past each other. Instead, they remain in fixed positions, locked in place by the forces of attraction between them. The liquid water has changed to solid ice. Another example of liquid water changing to solid ice is pictured in Figure 4.19. The process in which a liquid changes to a solid is called freezing. The temperature at which a liquid changes to a solid is its freezing point. The freezing point of water is 0C (32F). Other types of matter may have higher or lower freezing points. For example, the freezing point of iron is 1535C. The freezing point of oxygen is -219C. "
changes of state,T_3955,"If you took ice cubes out of a freezer and left them in a warm room, the ice would absorb energy from the warmer air around it. The energy would allow the particles of frozen water to overcome some of the forces of attraction holding them together. They would be able to slip out of the fixed positions they held as ice. In this way, the solid ice would turn to liquid water. The process in which a solid changes to a liquid is called melting. The melting point is the temperature at which a solid changes to a liquid. For a given type of matter, the melting point is the same as the freezing point. What is the melting point of ice? What is the melting point of iron, pictured in Figure 4.20? "
changes of state,T_3956,"If you fill a pot with cool tap water and place the pot on a hot stovetop, the water heats up. Heat energy travels from the stovetop to the pot, and the water absorbs the energy from the pot. What happens to the water next? "
changes of state,T_3957,"If water gets hot enough, it starts to boil. Bubbles of water vapor form in boiling water. This happens as particles of liquid water gain enough energy to completely overcome the force of attraction between them and change to the gaseous state. The bubbles rise through the water and escape from the pot as steam. The process in which a liquid boils and changes to a gas is called vaporization. The temperature at which a liquid boils is its boiling point. The boiling point of water is 100C (212F). Other types of matter may have higher or lower boiling points. For example, the boiling point of table salt is 1413C. The boiling point of nitrogen is -196C. "
changes of state,T_3958,A liquid can also change to a gas without boiling. This process is called evaporation. It occurs when particles at the exposed surface of a liquid absorb just enough energy to pull away from the liquid and escape into the air. This happens faster at warmer temperatures. Look at the puddle in Figure 4.21. It formed in a pothole during a rain shower. The puddle will eventually evaporate. It will evaporate faster if the sun comes out and heats the water than if the sky remains cloudy. 
changes of state,T_3959,"If you take a hot shower in a closed bathroom, the mirror is likely to ""fog"" up. The ""fog"" consists of tiny droplets of water that form on the cool surface of the mirror. Why does this happen? Some of the hot water from the shower evaporates, so the air in the bathroom contains a lot of water vapor. When the water vapor contacts cooler surfaces, such as the mirror, it cools and loses energy. The cooler water particles no longer have enough energy to overcome the forces of attraction between them. They come together and form droplets of liquid water. The process in which a gas changes to a liquid is called condensation. Other examples of condensation are shown in Figure 4.22. A gas condenses when it is cooled below its boiling point. At what temperature does water vapor condense? "
changes of state,T_3960,"Solids that change to gases generally first pass through the liquid state. However, sometimes solids change directly to gases and skip the liquid state. The reverse can also occur. Sometimes gases change directly to solids. "
changes of state,T_3961,"The process in which a solid changes directly to a gas is called sublimation. It occurs when the particles of a solid absorb enough energy to completely overcome the force of attraction between them. Dry ice (solid carbon dioxide, CO2 ) is an example of a solid that undergoes sublimation. Figure 4.23 shows a chunk of dry ice changing directly to carbon dioxide gas. Sometimes snow undergoes sublimation as well. This is most likely to occur on sunny winter days when the air is very dry. What gas does snow become? "
changes of state,T_3962,"The opposite of sublimation is deposition. This is the process in which a gas changes directly to a solid without going through the liquid state. It occurs when gas particles become very cold. For example, when water vapor in the air contacts a very cold windowpane, the water vapor may change to tiny ice crystals on the glass. The ice crystals are called frost. You can see an example in Figure 4.24. "
atoms,T_4146,Identify the conditions that will speed up or slow down the dissolving process. 
atoms,T_4147,"Did you ever drink the tea before all the sugar has dissolved? Did you ever notice that some of the sugar is sitting at the bottom of the glass? Q: What could you do to dissolve the sugar faster? A: The rate of dissolving is caused by several factors. These factors include stirring, temperature, and the size of the particles. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
atoms,T_4148,"What would happen if you added sugar to iced tea and did not stir the liquid? Thats right, most of the sugar you added would fall to the bottom of the glass. Like most people, when you add sugar to a liquid, you stir it, but why? For most of us, it is automatic. How many times have you added something to a liquid and immediately grabbed our spoon and started to stir. Have you ever thought about why we stir? So, why do we stir liquids when we add other ingredients? Stirring a liquid while you are mixing in another ingredient speeds up the rate of dissolving. This is because it helps distribute the particles that are being dissolved. What happens when you add sugar (the solute) to iced tea (the solvent) and then stir the tea? The obvious answer is that the sugar will dissolve. The more quickly you stir, the faster the sugar will dissolve. What if you dont stir the tea? Will the sugar still dissolve? It may eventually dissolve, but it will take much longer. You can think of stirring like adding energy to the process. What are other ways to add energy? "
atoms,T_4149,"What do you think will happen when you add the same amount of sugar to cups of hot and cold tea? Will the sugar dissolve at the same rate? Is that why people start with warm water when they make iced tea? The temperature of the solvent is an important factor in how fast something dissolves. Temperature affects how fast a solute dissolves. Generally, a solute dissolves faster in a warmer solvent. It dissolves more slowly in a cooler solvent. Think about that next time you make iced tea. "
atoms,T_4150,"There is another factor that affects the rate of dissolving. The particle size of solute particles affects the rate. Smaller particles have greater surface area. Think of a large block of Legos. When all the blocks are stuck together you can measure their surface area. Now take all the blocks apart and measure their individual surface areas. Which has more? Greater surface area provides more contact between the particles and the solvent. For example, if you put granulated sugar in a glass of iced tea, it will dissolve more quickly. If you put a sugar cube in a glass of iced tea, it will dissolve more slowly. Thats because all those tiny particles of granulated sugar have greater surface area than a single sugar cube. "
atoms,T_4151,1. List three factors that affect the rate at which a solute dissolves in a solvent. 2. Gina is trying to dissolve bath salts in her bathwater. How could she speed up the rate of dissolving? 
atoms,T_4152,"By clicking a link below, you will leave the CK-12 site and open an external site in a new tab. This page will remain open in the original tab. "
boiling,T_4173,"Steam actually consists of tiny droplets of liquid water. What you cant see in the picture is the water vapor that is also present in the air above the spring. Water vapor is water in the gaseous state. It constantly rises up from the surface of boiling hot water. Why? At high temperatures, particles of a liquid gain enough energy to completely overcome the force of attraction between them, so they change to a gas. The gas forms bubbles that rise to the surface of the liquid because gas is less dense than liquid. The bubbling up of the liquid is called boiling. When the bubbles reach the surface, the gas escapes into the air. The entire process in which a liquid boils and changes to a gas that escapes into the air is called vaporization. Q: Why does steam form over the hot spring pictured above? A: Steam forms when some of the water vapor from the boiling water cools in the air and condenses to form droplets of liquid water. "
boiling,T_4174,"Vaporization is easily confused with evaporation, but the two processes are not the same. Evaporation also changes a liquid to a gas, but it doesnt involve boiling. Instead, evaporation occurs when particles at the surface of a liquid gain enough energy to escape into the air. This happens without the liquid becoming hot enough to boil. "
boiling,T_4175,"The temperature at which a substance boils and changes to a gas is called its boiling point. Boiling point is a physical property of matter. The boiling point of pure water is 100 C. Other substances may have higher or lower boiling points. Several examples are listed in the Table 1.1. Pure water is included in the table for comparison. Substance Hydrogen Nitrogen Carbon dioxide Ammonia Pure water Salty ocean water Petroleum Olive oil Sodium chloride Boiling Point ( C) -253 -196 -79 -36 100 101 210 300 1413 Q: Assume you want to get the salt (sodium chloride) out of salt water. Based on information in the table, how could you do it? A: You could heat the salt water to 101 C. The water would boil and vaporize but the salt would not. Instead, the salt would be left behind as solid particles. Q: Oxygen is a gas at room temperature (20 C). What does this tell you about its boiling point? A: The boiling point of oxygen must be lower than 20 C. Otherwise, it would be a liquid at room temperature. "
calculating derived quantities,T_4190,"Derived quantities are quantities that are calculated from two or more measurements. Derived quantities cannot be measured directly. They can only be computed. Many derived quantities are calculated in physical science. Three examples are area, volume, and density. "
calculating derived quantities,T_4191,"The area of a surface is how much space it covers. Its easy to calculate the area of a surface if it has a regular shape, such as the blue rectangle in the sketch below. You simply substitute measurements of the surface into the correct formula. To find the area of a rectangular surface, use this formula: Area (rectangular surface) = length  width (l  w) Q: What is the area of the blue rectangle? A: Substitute the values for the rectangles length and width into the formula for area: Area = 9 cm  5 cm = 45 cm2 Q: Can you use this formula to find the area of a square surface? A: Yes, you can. A square has four sides that are all the same length, so you would substitute the same value for both length and width in the formula for the area of a rectangle. "
calculating derived quantities,T_4192,"The volume of a solid object is how much space it takes up. Its easy to calculate the volume of a solid if it has a simple, regular shape, such as the rectangular solid pictured in the sketch below. To find the volume of a rectangular solid, use this formula: Volume (rectangular solid) = length  width  height (l  w  h) Q: What is the volume of the blue rectangular solid? A: Substitute the values for the rectangular solids length, width, and height into the formula for volume: Volume = 10 cm  3 cm  5 cm = 150 cm3 "
calculating derived quantities,T_4193,"Density is a quantity that expresses how much matter is packed into a given space. The amount of matter is its mass, and the space it takes up is its volume. To calculate the density of an object, then, you would use this formula: Density = mass volume Q: The volume of the blue rectangular solid above is 150 cm3 . If it has a mass of 300 g, what is its density? A: The density of the rectangular solid is: Density = 300 g = 2 g/cm3 150 cm3 Q: Suppose you have two boxes that are the same size but one box is full of feathers and the other box is full of books. Which box has greater density? A: Both boxes have the same volume because they are the same size. However, the books have greater mass than the feathers. Therefore, the box of books has greater density. "
calculating derived quantities,T_4194,"A given derived quantity, such as area, is always expressed in the same type of units. For example, area is always expressed in squared units, such as cm2 or m2 . If you calculate area and your answer isnt in squared units, then you have made an error. Q: What units are used to express volume? A: Volume is expressed in cubed units, such as cm3 or m3 . Q: A certain derived quantity is expressed in the units kg/m3 . Which derived quantity is it? A: The derived quantity is density, which is mass (kg) divided by volume (m3 ). "
changes of state,T_4214,"The water droplets of fog form from water vapor in the air. Fog disappears when the water droplets change back to water vapor. These changes are examples of changes of state. A change of state occurs whenever matter changes from one state to another. Common states of matter on Earth are solid, liquid, and gas. Matter may change back and forth between any two of these states. Changes of state are physical changes in matter. They are reversible changes that do not change matters chemical makeup or chemical properties. For example, when fog changes to water vapor, it is still water and can change back to liquid water again. "
changes of state,T_4215,"Several processes are involved in common changes of state. They include melting, freezing, sublimation, deposition, condensation, and evaporation. The Figure 1.1 shows how matter changes in each of these processes. Q: Which two processes result in matter changing to the solid state? A: The processes are deposition, in which matter changes from a gas to a solid, and freezing, in which matter changes from a liquid to a solid. "
changes of state,T_4216,"Suppose that you leave some squares of chocolate candy in the hot sun. A couple of hours later, you notice that the chocolate has turned into a puddle like the one pictured in the Figure 1.2. Q: What happened to the chocolate? A: The chocolate melted. It changed from a solid to a liquid. In order for solid chocolate to melt and change to a liquid, the particles of chocolate must gain energy. The chocolate pictured in the Figure 1.2 gained energy from sunlight. Energy is the ability to cause changes in matter, and it is always involved in changes of state. When matter changes from one state to another, it either absorbs energyas when chocolate meltsor loses energy. For example, if you were to place the melted chocolate in a refrigerator, it would lose energy to the cold air inside the refrigerator. As a result, the liquid chocolate would change to a solid Q: Why is energy always involved in changes of state? A: The energy of particles of matter determines the matters state. Particles of a gas have more energy than particles of a liquid, and particles of a liquid have more energy than particles of a solid. Therefore, in order for matter to change from a solid to a liquid or from a liquid to a gas, particles of matter must absorb energy. In order for matter to change from a gas to a liquid or from a liquid to a solid, particles of matter must lose energy. "
chemical and solar cells,T_4218,"Chemical cells are found in batteries. They produce voltage by means of chemical reactions. Chemical cells have two electrodes, which are strips of different materials, such as zinc and carbon. The electrodes are suspended in an electrolyte. This is a substance that contains free ions, which can carry electric current. The electrolyte may be either a paste, in which case the cell is called a dry cell, or a liquid, in which case the cell is called a wet cell. Flashlight batteries contain dry cells. Car batteries contain wet cells. The Figure 1.1 shows how a battery works. The diagram represents the simplest type of battery, one that contains a single chemical cell. Both dry and wet cells work the same basic way. The electrodes react chemically with the electrolyte, causing one electrode to give up electrons and the other electrode to accept electrons. Electrons flow through the electrolyte from the negative to positive electrode. The electrodes extend out of the battery for the attachment of wires that carry the current. The current can be used to power a light bulb or other electric device. "
chemical and solar cells,T_4219,"Solar cells convert the energy in sunlight to electrical energy. Solar cells are also called photovoltaic (PV) cells because they use light (photo-) to produce voltage (-voltaic). Solar cells contain a material such as silicon that absorbs light energy. The energy knocks electrons loose so they can flow freely and produce a difference in electric potential energy, or voltage. The flow of electrons creates electric current. Solar cells have positive and negative contacts, like the terminals in chemical cells. If the contacts are connected with wire, current flows from the negative to positive contact. The Figure 1.2 shows how a solar cell works. "
chemical change,T_4223,"A chemical change occurs whenever matter changes into an entirely different substance with different chemical properties. A chemical change is also called a chemical reaction. Many complex chemical changes occur to produce the explosions of fireworks. An example of a simpler chemical change is the burning of methane. Methane is the main component of natural gas, which is burned in many home furnaces. During burning, methane combines with oxygen in the air to produce entirely different chemical substances, including the gases carbon dioxide and water vapor. Click image to the left or use the URL below. URL: "
chemical change,T_4224,"Most chemical changes are not as dramatic as exploding fireworks, so how can you tell whether a chemical change has occurred? There are usually clues. You just need to know what to look for. A chemical change has probably occurred if bubbles are released, there is a change of color, or an odor is produced. Other clues include the release of heat, light, or loud sounds. Examples of chemical changes that produce these clues are shown in the Figure 1.1. Q: In addition to iron rusting, what is another example of matter changing color? Do you think this color change is a sign that a new chemical substance has been produced? A: Another example of matter changing color is a penny changing from reddish brown to greenish brown as it becomes tarnished. The color change indicates that a new chemical substance has been produced. Copper on the surface of the penny has combined with oxygen in the air to produce a different substance called copper oxide. Q: Besides food spoiling, what is another change that produces an odor? Is this a chemical change? A: When wood burns, it produces a smoky odor. Burning is a chemical change. Q: Which signs of chemical change do fireworks produce? A: Fireworks produce heat, light, and loud sounds. These are all signs of chemical change. "
chemical change,T_4225,"Because chemical changes produce new substances, they often cannot be undone. For example, you cant change ashes from burning logs back into wood. Some chemical changes can be reversed, but only by other chemical changes. For example, to undo tarnish on copper pennies, you can place them in vinegar. The acid in the vinegar combines with the copper oxide of the tarnish. This changes the copper oxide back to copper and oxygen, making the pennies reddish brown again. You can try this at home to see how well it works. "
chemical properties of matter,T_4232,"Chemical properties are properties that can be measured or observed only when matter undergoes a change to become an entirely different kind of matter. For example, the ability of iron to rust can only be observed when iron actually rusts. When it does, it combines with oxygen to become a different substance called iron oxide. Iron is very hard and silver in color, whereas iron oxide is flakey and reddish brown. Besides the ability to rust, other chemical properties include reactivity and flammability. "
chemical properties of matter,T_4233,"Reactivity is the ability of matter to combine chemically with other substances. Some kinds of matter are extremely reactive; others are extremely unreactive. For example, potassium is very reactive, even with water. When a pea- sized piece of potassium is added to a small amount of water, it reacts explosively. You can observe this reaction in the video below. (Caution: Dont try this at home!) In contrast, noble gases such as helium almost never react with any other substances. Click image to the left or use the URL below. URL: "
chemical properties of matter,T_4234,"Flammability is the ability of matter to burn. When matter burns, it combines with oxygen and changes to different substances. Wood is an example of flammable matter, as seen in Figure 1.1. Q: How can you tell that wood ashes are a different substance than wood? A: Ashes have different properties than wood. For example, ashes are gray and powdery, whereas wood is brown and hard. Q: What are some other substances that have the property of flammability? A: Substances called fuels have the property of flammability. They include fossil fuels such as coal, natural gas, and petroleum, as well as fuels made from petroleum, such as gasoline and kerosene. Substances made of wood, such as paper and cardboard, are also flammable. "
condensation,T_4266,"The drops of water on the spider web are dewdrops. They formed overnight when warm moist air came into contact with the cooler spider web. Contact with the cooler web cooled the air. When air cools, it can hold less water vapor, so some of the water vapor in the air changed to liquid water. The process in which water vaporor another gaschanges to a liquid is called condensation. Another common example of condensation is pictured in the Figure 1.1. "
condensation,T_4267,"When air is very humid, it doesnt have to cool very much for water vapor in the air to start condensing. The temperature at which condensation occurs is called the dew point. The dew point varies depending on air temperature and moisture content. It is always less than or equal to the actual air temperature, but warmer air and moister air have dew points closer to the actual air temperature. Thats why glasses of cold drinks sweat more on a hot, humid day than they do on a cool, dry day. Q: What happens when air temperature reaches the dew point? A: When air temperature reaches the dew point, water vapor starts condensing. It may form dew (as on the spider web in the opening image), clouds, or fog. Dew forms on solid objects on the ground. Clouds form on tiny particles in the air high above the ground. Fog is a cloud that forms on tiny particles in the air close to the ground. "
condensation,T_4268,"The water cycle continuously recycles Earths water. Condensation plays an important role in this cycle. Find condensation in the water cycle Figure 1.3. It changes water vapor in the atmosphere to liquid water that can fall to Earth again. Without condensation, the water cycle would be interrupted and Earths water could not recycle. Q: In the water cycle, what happens to water after it condenses? A: After water condenses, it may form clouds that produce precipitation such as rain. "
conservation of mass,T_4271,"It may seem as though burning destroys matter, but the same amount, or mass, of matter still exists after a campfire as before. Look at the sketch in Figure 1.1. It shows that when wood burns, it combines with oxygen and changes not only to ashes but also to carbon dioxide and water vapor. The gases float off into the air, leaving behind just the ashes. Suppose you had measured the mass of the wood before it burned and the mass of the ashes after it burned. Also suppose you had been able to measure the oxygen used by the fire and the gases produced by the fire. What would you find? The total mass of matter after the fire would be the same as the total mass of matter before the fire. Q: What can you infer from this example? A: You can infer that burning does not destroy matter. It just changes matter into different substances. "
conservation of mass,T_4272,"This burning campfire example illustrates a very important law in science: the law of conservation of mass. This law states that matter cannot be created or destroyed. Even when matter goes through a physical or chemical change, the total mass of matter always remains the same. Q: How could you show that the mass of matter remains the same when matter changes state? A: You could find the mass of a quantity of liquid water. Then you could freeze the water and find the mass of the ice. The mass before and after freezing would be the same, showing that mass is conserved when matter changes state. "
density,T_4306,"Density is an important physical property of matter. It reflects how closely packed the particles of matter are. When particles are packed together more tightly, matter has greater density. Differences in density of matter explain many phenomena, not just why helium balloons rise. For example, differences in density of cool and warm ocean water explain why currents such as the Gulf Stream flow through the oceans. Click image to the left or use the URL below. URL:  To better understand density, think about a bowling ball and volleyball, pictured in the Figure 1.1. Imagine lifting each ball. The two balls are about the same size, but the bowling ball feels much heavier than the volleyball. Thats because the bowling ball is made of solid plastic, which contains a lot of tightly packed particles of matter. The volleyball, in contrast, is full of air, which contains fewer, more widely spaced particles of matter. In other words, the matter inside the bowling ball is denser than the matter inside the volleyball. Q: If you ever went bowling, you may have noticed that some bowling balls feel heavier than others even though they are the same size. How can this be? A: Bowling balls that feel lighter are made of matter that is less dense. "
density,T_4307,"The density of matter is actually the amount of matter in a given space. The amount of matter is measured by its mass, and the space matter takes up is measured by its volume. Therefore, the density of matter can be calculated with this formula: Density = mass volume Assume, for example, that a book has a mass of 500 g and a volume of 1000 cm3 . Then the density of the book is: Density = 500 g = 0.5 g/cm3 1000 cm3 Q: What is the density of a liquid that has a volume of 30 mL and a mass of 300 g? A: The density of the liquid is: Density = 300 g = 10 g/mL 30 mL "
deposition,T_4308,"Deposition refers to the process in which a gas changes directly to a solid without going through the liquid state. For example, when warm moist air inside a house comes into contact with a freezing cold windowpane, water vapor in the air changes to tiny ice crystals. The ice crystals are deposited on the glass, often in beautiful patterns like the leaves on the window above. Be aware that deposition has a different meaning in Earth science than in chemistry. In Earth science, deposition refers to the dropping of sediments by wind or water, rather than to a change of state. "
deposition,T_4309,"Deposition as a change of state often occurs in nature. For example, when warm moist air comes into contact with very cold surfacessuch as the ground or objects on the groundice crystals are deposited on them. These ice crystals are commonly called frost. Look at the dead leaf and blades of grass in the Figure 1.1. They are covered with frost. If you look closely, you can see the individual crystals of ice. You can watch a demonstration of frost forming on the side of a very cold can at the URL below. (Click on the mulitmedia choice Ice on a Can.). The ice in the can has been cooled to a very low temperature by adding salt to it. Q: In places with very cold winters, why might frost be more likely to form on the ground in the fall than in the winter? A: Frost forms when the air is warmer than the ground. This is more likely to be the case in the fall. In the winter, the air is likely to be as cold as the ground. Deposition also occurs high above the ground when water vapor in the air changes to ice crystals. In the atmosphere, the ice crystals are deposited on tiny dust particles. These ice crystals form clouds, generally cirrus clouds, which are thin and wispy. You can see cirrus clouds in the Figure 1.2. Q: Cirrus clouds form only at altitudes of 6 kilometers or higher above sea level. Do you know why? A: At this altitude, the atmosphere is always very cold. Unless the air is cold, water vapor will condense to form water droplets instead of ice crystals. "
direct and alternating current,T_4313,"When current flows in just one direction, it is called direct current (DC). The diagram below shows how direct current flows through a simple circuit. An example of direct current is the current that flows through a battery- powered flashlight. In addition to batteries, solar cells and fuel cells can also provide direct current. "
direct and alternating current,T_4314,"When current keeps reversing direction, it is called alternating current (AC). You can see how it works in the two diagrams below. The current that comes from a power plant and supplies electricity to homes and businesses is alternating current. The current changes direction 60 times per second. It happens so quickly that the light bulb doesnt have a chance to stop glowing when the reversals occur. Q: Which type of current flows through the wires in your home? A: Alternating current from a power plant flows through the wires in a home. "
discovery of electromagnetism,T_4318,"Magnetism produced by electricity is called electromagnetism. Today, electromagnetism is used in many electric devices. However, until electromagnetism was discovered, scientists thought that electricity and magnetism were unrelated. A Danish scientist named Hans Christian Oersted (pictured in the Figure 1.1) changed all that. He made the important discovery that electric current creates a magnetic field. But like many other important discoveries in science, Oersteds discovery was just a lucky accident. "
discovery of electromagnetism,T_4319,"In 1820, Oersted was presenting a demonstration to some science students. Ironically, he was trying to show them that electricity and magnetism are not related. He placed a wire with electric current flowing through it next to a compass, which has a magnetic needle. As he expected, the needle of the compass didnt move. It just kept pointing toward Earths north magnetic pole. After the demonstration, a curious student held the wire near the compass again, but in a different direction. To Oersteds surprise, the needle of the compass swung toward the wire so it was no longer pointing north. Oersted was intrigued. He turned off the current in the wire to see what would happen to the compass needle. The needle swung back to its original position, pointing north once again. Oersted had discovered that an electric current creates a magnetic field. The magnetic field created by the current was strong enough to attract the needle of the nearby compass. "
discovery of electromagnetism,T_4320,Oersted wanted to learn more about the magnetic field created by a current. He placed a compass at different locations around a wire with current flowing through it. You can see what he found in the Figure 1.2. The lines of magnetic force circle around the wire in a counterclockwise direction. 
discovery of electromagnetism,T_4321,"Just about a decade after Oersted discovered that electric current can produce a magnetic field, an English scientist named Michael Faraday discovered that the opposite is also true. A magnetic field can produce an electric current. This is known as Faradays law. The process by which a magnetic field produces current is called electromagnetic induction. It occurs when a conductor, such as a wire, crosses lines of force in a magnetic field. This can happen when a wire is moving relative to a magnet or a magnet is moving relative to a wire. "
electric charge and electric force,T_4338,"Electric charge is a physical property of particles or objects that causes them to attract or repel each other without touching. All electric charge is based on the protons and electrons in atoms. A proton has a positive electric charge, and an electron has a negative electric charge. In the Figure 1.1, you can see that positively charged protons (+) are located in the nucleus of the atom, while negatively charged electrons (-) move around the nucleus. "
electric charge and electric force,T_4339,"When it comes to electric charges, opposites attract, so positive and negative particles attract each other. You can see this in the Figure 1.2. This attraction explains why negative electrons keep moving around the positive nucleus of the atom. Like charges, on the other hand, repel each other, so two positive or two negative charges push apart. This is also shown in the diagram. The attraction or repulsion between charged particles is called electric force. The strength of electric force depends on the amount of electric charge on the particles and the distance between them. Larger charges or shorter distances result in greater force. Q: How do positive protons stay close together inside the nucleus of the atom if like charges repel each other? A: Other, stronger forces in the nucleus hold the protons together. "
electric circuits,T_4340,"A closed loop through which current can flow is called an electric circuit. In homes in the U.S., most electric circuits have a voltage of 120 volts. The amount of current (amps) a circuit carries depends on the number and power of electrical devices connected to the circuit. Home circuits generally have a safe upper limit of about 20 or 30 amps. "
electric circuits,T_4341,"All electric circuits have at least two parts: a voltage source and a conductor. They may have other parts as well, such as light bulbs and switches, as in the simple circuit seen in the Figure 1.1. The voltage source of this simple circuit is a battery. In a home circuit, the source of voltage is an electric power plant, which may supply electric current to many homes and businesses in a community or even to many communities. The conductor in most circuits consists of one or more wires. The conductor must form a closed loop from the source of voltage and back again. In the Figure 1.1, the wires are connected to both terminals of the battery, so they form a closed loop. Most circuits have devices such as light bulbs that convert electrical energy to other forms of energy. In the case of a light bulb, electrical energy is converted to light and thermal energy. Many circuits have switches to control the flow of current. When the switch is turned on, the circuit is closed and current can flow through it. When the switch is turned off, the circuit is open and current cannot flow through it. "
electric circuits,T_4342,"When a contractor builds a new home, she uses a set of plans called blueprints that show her how to build the house. The blueprints include circuit diagrams. The diagrams show how the wiring and other electrical components are to be installed in order to supply current to appliances, lights, and other electric devices. You can see an example of a very simple circuit in the Figure 1.2. Different parts of the circuit are represented by standard circuit symbols. An ammeter measures the flow of current through the circuit, and a voltmeter measures the voltage. A resistor is any device that converts some of the electricity to other forms of energy. For example, a resistor might be a light bulb or doorbell. A: The battery symbol (or a symbol for some other voltage source) must be included in every circuit. Without a source of voltage, there is no electric current. "
electric conductors and insulators,T_4343,"Electrical energy is transmitted by moving electrons in an electric current. In order to travel, electric current needs matter. It cannot pass through empty space. However, matter resists the flow of electric current. Thats because flowing electrons in current collide with particles of matter, which absorb their energy. Some types of matter offer more or less resistance to electric current than others. "
electric conductors and insulators,T_4344,"Materials that have low resistance to electric current are called electric conductors. Many metalsincluding copper, aluminum, and steelare good conductors of electricity. The outer electrons of metal atoms are loosely bound and free to move, allowing electric current to flow. Water that has even a tiny amount of impurities is an electric conductor as well. Q: What do you think lightning rods are made of? A: Lightning rods are made of metal, usually copper or aluminum, both of which are excellent conductors of electricity. "
electric conductors and insulators,T_4345,"Materials that have high resistance to electric current are called electric insulators. Examples include most non- metallic solids, such as wood, rubber, and plastic. Their atoms hold onto their electrons tightly, so electric current cannot flow freely through them. Dry air is also an electric insulator. Q: You may have heard that rubber-soled shoes will protect you if you are struck by lightning. Do you think this is true? Why or why not? A: It isnt true. Rubber is an electric insulator, but a half-inch layer on the bottom of a pair of shoes is insignificant when it comes to lightning. The average lightning bolt has 100 million volts and can burn through any insulator, even the insulators on high-voltage power lines. "
electric conductors and insulators,T_4346,"Look at the electric wires in the Figure 1.1. They are made of copper and coated with plastic. Copper is very good conductor, and plastic is a very good insulator. When more than one material is available for electric current to flow through, the current always travels through the material with the least resistance. Thats why all the current passes through the copper wire and none flows through its plastic coating. "
electric current,T_4347,"Electric current is a continuous flow of electric charges (electrons). Current is measured as the amount of charge that flows past a given point in a certain amount of time. The SI unit for electric current is the ampere (A), or amp. Electric current may flow in just one direction (direct current), or it may keep reversing direction (alternating current). Q: Why do you think charges flow in an electric current? A: Electric charges flow when they have electric potential energy. Potential energy is stored energy that an object has due to its position or shape. "
electric current,T_4348,"Electric potential energy comes from the position of a charged particle in an electric field. For example, when two negative charges are close together, they have potential energy because they repel each other and have the potential to push apart. If the charges actually move apart, their potential energy decreases. Electric charges always move spontaneously from a position where they have higher potential energy to a position where they have lower potential energy. This is like water falling over a dam from an area of higher to lower potential energy due to gravity. "
electric current,T_4349,"For an electric charge to move from one position to another, there must be a difference in electric potential energy between the two positions. A difference in electric potential energy is called voltage. The SI unit for voltage is the volt (V). Look at the Figure 1.1. It shows a simple circuit. The source of voltage in the circuit is a 1.5-volt battery. The difference of 1.5 volts between the two battery terminals results in a spontaneous flow of charges, or electric current, between them. Notice that the current flows from the negative terminal to the positive terminal, because electric current is a flow of electrons. Q: You might put a 1.5-volt battery in a TV remote. The battery in a car is a 12-volt battery. How do you think the current of a 12-volt battery compares to the current of a 1.5-volt battery? A: Greater voltage means a greater difference in potential energy, so the 12-volt battery can produce more current than the 1.5-volt battery. "
electric fields,T_4350,"An electric field is a space around a charged particle where the particle exerts electric force on other charged particles. Because of their force fields, charged particles can exert force on each other without actually touching. Electric fields are generally represented by arrows, as you can see in the Figure 1.1. The arrows show the direction of electric force around a positive particle and a negative particle. "
electric fields,T_4351,"When charged particles are close enough to exert force on each other, their electric fields interact. This is illustrated in the Figure 1.2. The lines of force bend together when particles with different charges attract each other. The lines bend apart when particles with like charges repel each other. Q: What would the lines of force look like around two negative particles? A: They would look like the lines around two positive particles, except the arrows would point toward, rather than away from, the negative particles. "
electric generators,T_4352,"You have already learned about the physical properties of matter. You may recall that physical properties can be measured and observed. You are able to use your senses to observe and measure them. You can easily tell if something is a certain color. You can tell what state it is in, whether solid, liquid, or gas. You can run tests to see if it conducts electricity. Also, physical changes occur without matter becoming something else. If you tear a piece of paper, each piece is still paper. Do you think this holds true for chemical properties? Chemical properties can also be measured or observed. This is where the similarity ends. You can only see chemical properties when matter undergoes a change. This change results in an entirely different kind of matter. For example, the ability of iron to rust can only be observed after it rusts. The shiny piece of metal gives no clue to whether it will rust or not until it does. But what is rust? When iron rusts, it combines with oxygen. Iron and oxygen is new substance, called iron oxide. It is no longer just iron. It has undergone a change. It is now a different substance. Iron is very hard and silver in color. In contrast, iron oxide is flakey and reddish brown. The ability to rust is only one type of chemical property. "
electric generators,T_4353,"Reactivity is another type of chemical property. Reactivity is the ability of matter to combine chemically with other substances. Some kinds of matter are extremely reactive. Others kinds of matter are extremely unreactive. Have you ever mixed baking soda with vinegar in your science class? If you have, you have seen an interesting reaction. Please do not try this at home without supervision. Vinegar and baking soda both have the chemical property that causes them to react with each other. Other substances are very unreactive. "
electric generators,T_4354,"Have you ever seen a symbol that says ""Flammable""? You might see such a symbol on a gasoline can. Gasoline is highly flammable. That is why there are signs at the gas station that say, ""NO SMOKING."" Flammability is the ability of matter to burn. When matter burns, it combines with oxygen. When it does, it changes to different substances. Wood is an example of flammable matter, as seen in Figure 1.1. Q: How can you tell that wood ashes are a different substance than wood? A: Ashes have different properties than wood. For example, ashes are gray and powdery. Wood is brown and hard. Q: What are some other substances that have the property of flammability? A: Substances called fuels have the property of flammability. They include fossil fuels such as coal, natural gas, and petroleum. For example, gasoline is used in our cars because it is flammable. This property enables car engines to run. Substances made of wood, such as paper and cardboard, are also flammable. "
electric generators,T_4355,1. What is a chemical property? 2. Define the chemical property called reactivity. 3. What is flammability? Identify examples of flammable matter. 
electric power and electrical energy use,T_4356,"The rate at which a device changes electric current to another form of energy is called electric power. The SI unit for powerincluding electric poweris the watt. A watt equals 1 joule of energy per second. High wattages are often expressed in kilowatts, where 1 kilowatt equals 1000 watts. The power of an electric device, such as a hair dryer, can be calculated if you know the voltage of the circuit and how much current the device receives. The following equation is used: Power (watts) = Current (amps)  Voltage (volts) Assume that Mirandas hair dryer is the only electric device in a 120-volt circuit that carries 15 amps of current. Then the power of her hair dryer in kilowatts is: Power = 15 amps  120 volts = 1800 watts, or 1.8 kilowatts Q: If a different hair dryer is plugged into a 120-volt circuit that carries 10 amps of current. What is the power of the other hair dryer? A: Substitute these values in the power equation: Power = 10 amps  120 volts = 1200 watts, or 1.2 kilowatts "
electric power and electrical energy use,T_4357,"Did you ever wonder how much electrical energy it takes to use an appliance such as a hair dryer? Electrical energy use depends on the power of the appliance and how long it is used. It can be calculated with this equation: Electrical Energy = Power  Time If Miranda uses her 1.8-kilowatt hair dryer for 0.2 hours, how much electrical energy does she use? Electrical Energy = 1.8 kilowatts  0.2 hours = 0.36 kilowatt-hours Electrical energy use is typically expressed in kilowatt-hours, as in this example. How much energy is this? One kilowatt-hour equals 3.6 million joules of energy. Q: Suppose Miranda were to use a 1.2-kilowatt hair dryer for 0.2 hours. How much electrical energy would she use then? A: She would use: Electrical Energy = 1.2 kilowatts  0.2 hour = 0.24 kilowatt-hours "
electric resistance,T_4358,"In physics, resistance is opposition to the flow of electric charges in an electric current as it travels through matter. The SI unit for resistance is the ohm. Resistance occurs because moving electrons in current bump into atoms of matter. Resistance reduces the amount of electrical energy that is transferred through matter. Thats because some of the electrical energy is absorbed by the atoms and changed to other forms of energy, such as heat. Q: In the rugby analogy to resistance in physics, what do the players on each team represent? A: The player on the blue and black team represents a moving electron in an electric current. The players on the red and blue team represent particles of matter through which the current is flowing. "
electric resistance,T_4359,"How much resistance a material has depends on several factors: the type of material, its width, its length, and its temperature. All materials have some resistance, but certain materials resist the flow of electric current more or less than other materials do. Materials such as plastics have high resistance to electric current. They are called electric insulators. Materials such as metals have low resistance to electric current. They are called electric conductors. A wide wire has less resistance than a narrow wire of the same material. Electricity flowing through a wire is like water flowing through a hose. More water can flow through a wide hose than a narrow hose. In a similar way, more current can flow through a wide wire than a narrow wire. A longer wire has more resistance than a shorter wire. Current must travel farther through a longer wire, so there are more chances for it to collide with particles of matter. A cooler wire has less resistance than a warmer wire. Cooler particles have less kinetic energy, so they move more slowly. Therefore, they are less likely to collide with moving electrons in current. Materials called superconductors have virtually no resistance when they are cooled to extremely low temperatures. "
electric resistance,T_4360,"Resistance can be helpful or just a drain on electrical energy. If the aim is to transmit electric current through a wire from one place to another, then resistance is a drawback. It reduces the amount of electrical energy that is transmitted because some of the current is absorbed by particles of matter. On the other hand, if the aim is to use electricity to produce heat or light, then resistance is useful. When particles of matter absorb electrical energy, they change it to heat or light. For example, when electric current flows through the tungsten wire inside an incandescent light bulb like the one in the Figure 1.1, the tungsten resists the flow of electric charge. It absorbs electrical energy and converts some of it to light and heat. Q: The tungsten wire inside a light bulb is extremely thin. How does this help it do its job? A: The extremely thin wire has more resistance than a wider wire would. This helps the wire resist electric current and change it to light. "
electric safety,T_4361,"Did you ever see an old appliance with a damaged cord, like the old shown in the Figure 1.1? A damaged electric cord can cause a severe shock if it allows current to pass from the cord to a person who touches it. A damaged cord can also cause a short circuit. A short circuit occurs when electric current follows a shorter path than the intended loop of the circuit. An electric cord contains two wires: one that carries current from the outlet to the appliance and one that carries current from the appliance back to the outlet. If the two wires in a damaged cord come into contact with each other, current flows from one wire to the other and bypasses the appliance. This may cause the wires to overheat and start a fire. "
electric safety,T_4362,"Because electricity can be so dangerous, safety features are built into modern electric circuits and devices. They include three-prong plugs, circuit breakers, and GFCI outlets. You can read about these three safety features in the Figure 1.2. GFCI Outlet: GFCI stands for ground- fault circuit interrupter. GFCI outlets are typically found in bathrooms and kitchens where the use of water poses a risk of shock (because water is a good electric conductor). A GFCI outlet contains a device that monitors the amount of current leaving and returning to the outlet. If less current is returning than leaving, this means that current is escaping. When this occurs, a tiny circuit breaker in the outlet interrupts the circuit. The breaker can be reset by pushing a button on the outlet cover. Q: Can you think of any other electric safety features? A: One safety feature is the label on a lamp that warns the user of the maximum safe wattage for light bulbs. Another safety feature is double insulation on many electric devices. Not only are the electric wires insulated with a coating "
electric safety,T_4363,"Even with electric safety features, electricity is still dangerous if it is misused. Follow the safety rules below to reduce the risk of injury or fire from electricity. Never mix electricity and water. Dont plug in or turn on electric lights or appliances when your hands are wet, you are standing in water, or you are in the shower or bathtub. The current could flow through the waterand youbecause water is a good conductor of electricity. Never overload circuits. Avoid plugging too many devices into one outlet or extension cord. The more devices that are plugged in, the more current the circuit carries. Too much current can overheat a circuit and start a fire. Never use devices with damaged cords or plugs. They can cause shocks, shorts, and fires. Never put anything except plugs into electric outlets. Putting any other object into an outlet is likely to cause a serious shock that could be fatal. Never go near fallen electric lines. They could have very high voltage. Report fallen lines to the electric company as soon as possible. "
electric transformers,T_4364,"An electric transformer is a device that uses electromagnetic induction to change the voltage of electric current. Electromagnetic induction is the process of generating current with a magnetic field. It occurs when a magnetic field and electric conductor, such as a coil of wire, move relative to one another. A transformer may either increase or decrease voltage. You can see the basic components of an electric transformer in the Figure 1.1. Click image to the left or use the URL below. URL:  The transformer in the diagram consists of two wire coils wrapped around an iron core. Each coil is part of a different circuit. When alternating current passes through coil P, it magnetizes the iron core. Because the current is alternating, the magnetic field of the iron core keeps reversing. This is where electromagnetic induction comes in. The changing magnetic field induces alternating current in coil S of the other circuit. "
electric transformers,T_4365,"Notice that coil P and coil S in the Figure 1.1 have the same number of turns of wire. In this case, the voltages of the primary and secondary currents are the same. Usually, the two coils of a transformer have different numbers of turns. In that case, the voltages of the two currents are different. When coil S has more turns than coil P, the voltage in the secondary current is greater than the voltage in the primary current (see Figure 1.2). This type of transformer is called a step-up transformer. Thats because it steps up, or increases, the voltage. When coil S has fewer turns of wire than coil P, the voltage in the secondary current is less than the voltage in the primary current (see Figure 1.3). This type of transformer is called a step-down transformer because it steps down, or decreases, the voltage. Q: Both step-up and step-down transformers are used in the electrical grid that carries electricity from a power plant to your home. Where in the grid do you think step-down transformers might be used? A: One place that step-down transformers are used is on the electric poles that supply current to homes. They reduce the voltage of the electric current before it enters home circuits. "
electrical grid,T_4366,"An electrical grid is the entire electrical system that generates, transmits, and distributes electric power throughout a region or country. A very simple electrical grid is sketched in the Figure 1.1. The grid includes a power plant, transmission lines, and electric substations, all of which work together to provide alternating current to customers. "
electrical grid,T_4367,"Electricity originates in power plants. They have electric generators that produce electricity by electromagnetic induction. In this process, a changing magnetic field is used to generate electric current. The generators convert kinetic energy to electrical energy. The kinetic energy may come from flowing water, burning fuel, wind, or some other energy source. "
electrical grid,T_4368,Transmission lines on big towerslike those in the opening photo abovecarry high-voltage electric current from power plants to electric substations. Smaller towers and individual power poles carry lower-voltage current from electric substations to homes and businesses. 
electrical grid,T_4369,"Electric substations have several functions. Many substations distribute electricity from a few high-voltage lines to several lower-voltage lines. They have electric transformers, which use electromagnetic induction to change the voltage of the current. Some transformers increase the voltage; others decrease the voltage. In the Figure 1.2, you can see how both types of transformers are used in an electrical grid. A step-up transformer increases the voltage of the current as it leaves the power plant. After the voltage has been increased, less current travels through the high-voltage power lines. This reduces the amount of power that is lost due to resistance of the power lines. A step-down transformer decreases the voltage of the current so it can be distributed safely to businesses and homes. A high-voltage power line may have 750,000 volts, whereas most home circuits have a maximum of 240 volts. Therefore, one or more step-down transformers are needed to decrease the voltage of current before it enters homes. Q: Assume that a home needs a 14-volt circuit for a light and a 120-volt circuit for a microwave oven. If the main power line entering home has 240 volts, what can you infer about the homes electrical system? A: The homes electrical system must have step-down transformers that lower the voltage for some of the homes circuits. "
electromagnet,T_4370,An electromagnet is a solenoid wrapped around a bar of iron or other ferromagnetic material. A solenoid is a coil of wire with electric current flowing through it. This gives the coil north and south magnetic poles and a magnetic field. The magnetic field of the solenoid magnetizes the iron bar by aligning its magnetic domains. You can see this in the Figure 1.1. 
electromagnet,T_4371,"The combined magnetic force of the magnetized wire coil and iron bar makes an electromagnet very strong. In fact, electromagnets are the strongest magnets made. An electromagnet is stronger if there are more turns in the coil of wire or there is more current flowing through it. A bigger bar or one made of material that is easier to magnetize also increases an electromagnets strength. "
electromagnet,T_4372,"Besides their strength, another pro of electromagnets is the ability to control them by controlling the electric current. Turning the current on or off turns the magnetic field on or off. The amount of current flowing through the coil can also be changed to control the strength of the electromagnet. Q: Why might it be useful to be able to turn an electromagnet on and off? A: Look back at the electromagnet hanging from the crane in the opening photo. It is useful to turn on its magnetic field so it can pick up the metal car parts. It is also useful to turn off its magnetic field so it can drop the parts into the train car. "
electromagnetic devices,T_4373,"Many common electric devices contain electromagnets. An electromagnet is a coil of wire wrapped around a bar of iron or other ferromagnetic material. When electric current flows through the wire, it causes the coil and iron bar to become magnetized. An electromagnet has north and south magnetic poles and a magnetic field. Turning off the current turns off the electromagnet. To understand how electromagnets are used in electric devices, well focus on two common devices: doorbells and electric motors like the one that turns the blades of a fan. Q: Besides doorbells and fans, what are some other devices that contain electromagnets? A: Any device that has an electric motor contains electromagnets. Some other examples include hairdryers, CD players, power drills, electric saws, and electric mixers. "
electromagnetic devices,T_4374,"The Figure 1.1 represents a simple doorbell. Like most doorbells, it has a button located by the front door. Pressing the button causes two electric contacts to come together and complete an electric circuit. In other words, the button is a switch. The circuit is also connected to a source of current, an electromagnet, and a clapper that strikes a bell. What happens when current flows through the doorbell circuit? The electromagnet turns on, and its magnetic field attracts the clapper. This causes the clapper to hit the bell, making it ring. Because the clapper is part of the circuit, when it moves to strike the bell, it breaks the circuit. Without current flowing through the circuit, the electromagnet turns off, and the clapper returns to its original position. When the clapper moves back to its original position, this closes the circuit again and turns the electromagnet back on. The electromagnet again attracts the clapper, which hits the bell once more. This sequence of events keeps repeating. Q: How can you stop the sequence of events so the doorbell will stop ringing? A: Stop pressing the button! This interrupts the circuit so no current can flow through it. "
electromagnetic devices,T_4375,"An electric motor is a device that uses an electromagnet to change electrical energy to kinetic energy. You can see a simple diagram of an electric motor in the Figure 1.2. The motor contains an electromagnet that is connected to a shaft. When current flows through the motor, the electromagnet rotates, causing the shaft to rotate as well. The rotating shaft moves other parts of the device. For example, in an electric fan, the rotating shaft turns the blades of the fan. Why does the motors electromagnet rotate? The electromagnet is located between the north and south poles of two permanent magnets. When current flows through the electromagnet, it becomes magnetized, and its poles are repelled by the like poles of the permanent magnets. This causes the electromagnet to rotate toward the unlike poles of the permanent magnets. A device called a commutator then changes the direction of the current so the poles of the electromagnet are reversed. The reversed poles are again repelled by the poles of the permanent magnets, which have not reversed. This causes the electromagnet to continue to rotate. These events keep repeating, so the electromagnet rotates continuously. "
electromagnetic induction,T_4376,"Electromagnetic induction is the process of generating electric current with a magnetic field. It occurs whenever a magnetic field and an electric conductor, such as a coil of wire, move relative to one another. As long as the conductor is part of a closed circuit, current will flow through it whenever it crosses lines of force in the magnetic field. One way this can happen is illustrated in the Figure 1.1. The sketch shows a magnet moving through a wire coil. Q: What is another way that a coil of wire and magnet can move relative to one another and generate an electric current? A: The coil of wire could be moved back and forth over the magnet. "
electromagnetic induction,T_4377,"The device with the pointer in the Figure 1.1 is an ammeter. It measures the current that flows through the wire. The faster the magnet or coil moves, the greater the amount of current that is produced. If more turns were added to the coil or a stronger magnet were used, this would produce more current as well. The Figure 1.2 shows the direction of the current that is generated by a moving magnet. If the magnet is moved back and forth repeatedly, the current keeps changing direction. In other words, alternating current (AC) is produced. Alternating current is electric current that keeps reversing direction. "
electromagnetic induction,T_4378,"Two important devices depend on electromagnetic induction: electric generators and electric transformers. Both devices play critical roles in producing and regulating the electric current we depend on in our daily lives. Electric generators use electromagnetic induction to change kinetic energy to electrical energy. They produce electricity in power plants. Electric transformers use electromagnetic induction to change the voltage of electric current. Some transformers increase voltage and other decrease voltage. Q: How do you think the girl on the exercise bike in the opening photo is using electromagnetic induction? A: As she pedals the bike, the kinetic energy of the turning pedals is used to move a conductor through a magnetic field. This generates electric current by electromagnetic induction. "
electromagnetism,T_4387,"Electromagnetism is magnetism produced by an electric current. When electric current flows through a wire, it creates a magnetic field that surrounds the wire in circles. You can see this in the diagram below. Note that electric current is conventionally shown moving from positive to negative electric potential, as in this diagram. However, electrons in current actually flow in the opposite direction, from negative to positive potential. Q: If more current flows through a wire, how might this affect the magnetic field surrounding the wire? A: With more current, the magnetic field is stronger. "
electromagnetism,T_4388,"The direction of the magnetic field created when current flows through a wire depends on the direction of the current. A simple rule, called the right hand rule, makes it easy to find the direction of the magnetic field if the direction of the current is known. The rule is illustrated in the Figure 1.1. When the thumb of the right hand is pointing in the same direction as the current, the fingers of the right hand curl around the wire in the direction of the magnetic field. Click image to the left or use the URL below. URL: "
electromagnetism,T_4389,"Electromagnetism is used not only in a doorbells but in many other electric devices as well, such as electric motors and loudspeakers. It is also used to store information on computer disks. An important medical use of electromagnetism is magnetic resonance imaging (MRI). This is a technique for making images of the inside of the body in order to diagnose diseases or injuries. Magnetism created with electric current is so useful because it can be turned on or off simply by turning the current on or off. The strength of the magnetic field is also easy to control by changing the amount of current. You cant do either of these things with a regular magnet. "
electronic component,T_4393,"Electronic components are the parts used in electronic devices such as computers. The components change electric current so it can carry information. Types of electronic components include diodes, transistors, and integrated circuits, all of which you can read about below. However, to understand how these components work, you first need to know about semiconductors. Thats because electronic components consist of semiconductorssometimes millions of them! "
electronic component,T_4394,"A semiconductor is a solid crystal, consisting mainly of silicon. It gets its name from the fact that it can conduct current better than an electric insulator but not as well as an electric conductor. As you can see in the Figure 1.1, each silicon atom has four valence electrons that it shares with other silicon atoms in the crystal. A semiconductor is formed by replacing a few silicon atoms with other atoms, such as phosphorus or boron, which have more or less valence electrons than silicon. This is called doping, and its what allows the semiconductor to conduct electric current. Q: Why wouldnt a pure silicon crystal be able to conduct electric current? A: Electric current is a flow of electrons. All of the valence electrons of silicon atoms in a pure crystal are shared with other silicon atoms, so they are not free to move and carry current. There are two different types of semiconductors: n-type and p-type. An n-type (negative-type) semiconductor consists of silicon and an element such as phosphorus that gives the silicon crystal extra electrons. You can see this in the Figure 1.1. An n-type semiconductor is like the negative terminal of a battery. A p-type (positive-type) semiconductor consists of silicon and an element such as boron that gives the silicon positively charged holes where electrons are missing. This is also shown in the Figure 1.1. A p-type semiconductor is like the positive terminal of a battery. "
electronic component,T_4395,"A diode is an electronic component that consists of a p-type and an n-type semiconductor placed side by side, as shown in the Figure 1.2. When a diode is connected by leads to a source of voltage, electrons flow from the n-type to the p-type semiconductor. This is the only direction that electrons can flow in a diode. This makes a diode useful for changing alternating current to direct current. "
electronic component,T_4396,"A transistor consists of three semiconductors, either p-n-p or n-p-n. Both arrangements are illustrated in the Figure (through the base). Then a much larger current can flow through the transistor from end to end (from collector to emitter). This means that a transmitter can be used as a switch, with pulses of a small current turning a larger current on and off. A transistor can also be used to increase the amount of current flowing through a circuit. "
electronic component,T_4397,"An integrated circuitalso called a microchipis a tiny, flat piece of silicon that consists of layers of many electronic components such as transistors. You can see an integrated circuit in the Figure 1.4. Look how small it is compared with the finger its resting on. Although the integrated circuit is tiny, it may contain millions of smaller electronic components. Current flows extremely rapidly in an integrated circuit because it doesnt have far to travel. Integrated circuits are used in virtually all modern electronic devices to carry out specific tasks. "
electronic device,T_4398,"Many of the devices people commonly use today are electronic devices. Electronic devices use electric current to encode, analyze, or transmit information. In addition to computers, they include mobile phones, TV remotes, DVD and CD players, and digital cameras, to name just a few. Q: Can you think of other electronic devices that you use? A: Other examples include game systems and MP3 players. "
electronic device,T_4399,"Lets take a close look at the computer as an example of an electronic device. A computer contains integrated circuits, or microchips, that consist of millions of tiny electronic components. Information is encoded in digital electronic signals. Rapid pulses of voltage switch electric current on and off, producing long strings of 1s (current on) and 0s (current off). The 1s and 0s are the letters of the code, and a huge number of them are needed. One digit (either 0 or 1) is called a bit, which stands for binary digit. Each group of eight digits is called a byte, and a billion bytes is called a gigabyte. Because a computers circuits are so tiny and close together, the computer can be very fast and capable of many complex tasks while remaining small. The parts of a computer that transmit, process, or store digital signals are pictured and described in the Figure 1.1. They include the CPU, hard drive, ROM, and RAM. The motherboard ties all these parts of the computer together. The CPU, or central processing unit, carries out program instructions. The hard drive is a magnetic disc that provides long-term storage for programs and data. ROM (read-only memory) is a microchip that provides permanent storage. It stores important information such as start-up instructions. This memory remains even after the computer is turned off. RAM (random-access memory) is a microchip that temporarily stores programs and data that are currently being used. Anything stored in RAM is lost when the computer is turned off. The motherboard is connected to the CPU, hard drive, ROM, and RAM. It allows all these parts of the computer to receive power and communicate with one another. Q: Which part(s) of a computer are you using when you type a school report? A: You are using the RAM to store the word processing program and your document as you type it. You are using the CPU to carry out instructions in the word processing program, and you are probably using the hard drive to save your document. "
electronic signal,T_4400,"Electric devices, such as lights and household appliances, change electric current to other forms of energy. For example, an electric stove changes electric current to thermal energy. Other common devices, such as mobile phones and computers, use electric current for another purpose: to encode information. A message encoded this way is called an electronic signal, and the use of electric current for this purpose is called electronics. To encode a message with electric current, the voltage is changed rapidly, over and over again. Voltage is a difference in electric potential energy that is needed in order for electric current to flow. There are two different ways voltage can be changed, resulting in two different types of electronic signals, called analog signals and digital signals. "
electronic signal,T_4401,"Analog signals consist of continuously changing voltage in an electric circuit. The Figure 1.1 represents analog signals. These were the first electronic signals to be invented. They were used in early computers and other early electronic devices. Analog signals are subject to distortion and noise, so they arent used as often anymore. They are used mainly in microphones and some mobile phones to encode sounds as electronic signals. "
electronic signal,T_4402,"Today, most electronic signals are digital signals. Digital signals consist of rapid pulses of voltage that repeatedly switch the current off and on. The Figure 1.2 represents digital signals. This type of signal encodes information as a string of 0s (current off) and 1s (current on). This is called a binary (two-digit) code. The majority of modern electronic devices, including computers and many mobile phones, encode data as digital signals. Compared with analog signals, digital signals are easier to transmit and more accurate. "
evaporation,T_4432,"Evaporation explains why clothes dry on a clothesline. Evaporation is the process in which a liquid changes to a gas without becoming hot enough to boil. It occurs when individual liquid particles at the exposed surface of the liquid absorb just enough energy to overcome the force of attraction with other liquid particles. If the surface particles are moving in the right direction, they will pull away from the liquid and move into the air. This is illustrated in the Figure 1.1. "
evaporation,T_4433,Many factors influence how quickly a liquid evaporates. They include: temperature of the liquid. A cup of hot water will evaporate more quickly than a cup of cold water. exposed surface area of the liquid. The same amount of water will evaporate more quickly in a wide shallow bowl than in a tall narrow glass. presence or absence of other substances in the liquid. Pure water will evaporate more quickly than salt water. air movement. Clothes on a clothesline will dry more quickly on a windy day than on a still day. concentration of the evaporating substance in the air. Clothes will dry more quickly when air contains little water vapor. 
evaporation,T_4434,"Did you ever notice that moving air cools you down when youre hot and sweaty? For example, if you sit in front of a fan, you feel cooler. Thats because moving air helps to evaporate the sweat on your skin. But why does the evaporation of sweat cool you down? When a liquid such as sweat evaporates, energetic particles on the surface of the liquid escape into the air. After these particles leave, the remaining liquid has less energy, so it is cooler. This is called evaporative cooling. Q: On a hot day, high humidity makes you feel even hotter. Can you explain why? A: Humidity is a measure of the amount of water vapor in the air. When humidity is high, sweat evaporates more slowly because there is already a lot of water vapor in the air. The slower evaporation rate reduces the potential for evaporative cooling. "
freezing,T_4450,"You dont have to be an ice climber to enjoy ice. Skating and fishing are two other sports that are also done on ice. What is ice? Its simply water in the solid state. The process in which water or any other liquid changes to a solid is called freezing. Freezing occurs when a liquid cools to a point at which its particles no longer have enough energy to overcome the force of attraction between them. Instead, the particles remain in fixed positions, crowded closely together, as shown in the Figure 1.1. "
freezing,T_4451,"The temperature at which a substance freezes is known as its freezing point. Freezing point is a physical property of matter. The freezing point of pure water is 0 C. Below this temperature, water exists as ice. Above this temperature, it exists as liquid water or water vapor. Many other substances have much lower or higher freezing points than water. You can see some examples in the Table 1.1. The freezing point of pure water is included in the table for comparison. Substance Helium Oxygen Nitrogen Pure Water Lead Iron Carbon Freezing Point ( C) -272 -222 -210 0 328 1535 3500 Q: What trend do you see in this table? A: Substances in the table with freezing points lower than water are gases. Substances in the table with freezing points higher than water are solids. Q: Sodium is a solid at room temperature. Given this information, what can you infer about its freezing point? A: You can infer that the freezing point of sodium must be higher than room temperature, which is about 20 C. The freezing point of sodium is actually 98 C. "
liquids,T_4584,"Water is the most common substance on Earth, and most of it exists in the liquid state. A liquid is one of four well-known states of matter, along with solid, gas, and plasma states. The particles of liquids are in close contact with each other but not as tightly packed as the particles in solids. The particles can slip past one another and take the shape of their container. However, they cannot pull apart and spread out to take the volume of their container, as particles of a gas can. If the volume of a liquid is less than the volume of its container, the top surface of the liquid will be exposed to the air, like the vinegar in the bottle pictured in the Figure 1.1. Q: Why does most water on Earths surface exist in a liquid state? In what other states does water exist on Earth? A: Almost 97 percent of water on Earths surface is found as liquid salt water in the oceans. The temperature over most of Earths surface is above the freezing point (0 C) of water, so relatively little water exists as ice. Even near the poles, most of the water in the oceans is above the freezing point. And in very few places on Earths surface do temperatures reach the boiling point (100 C) of water. Although water exists in the atmosphere in a gaseous state, water vapor makes up less than 1 percent of Earths total water. "
liquids,T_4585,"Two unique properties of liquids are surface tension and viscosity. Surface tension is a force that pulls particles at the exposed surface of a liquid toward other liquid particles. Surface tension explains why water forms droplets, like the water droplet that has formed on the leaky faucet pictured in the Figure 1.2. Water drips from a leaky faucet. Viscosity is a liquids resistance to flowing. You can think of it as friction between particles of liquid. Thicker liquids are more viscous than thinner liquids. For example, the honey pictured in the Figure 1.3 is more viscous than the vinegar. Q: Which liquid do you think is more viscous: honey or chocolate syrup? "
matter mass and volume,T_4593,"Matter is all the stuff that exists in the universe. Everything you can see and touch is made of matter, including you! The only things that arent matter are forms of energy, such as light and sound. In science, matter is defined as anything that has mass and volume. Mass and volume measure different aspects of matter. "
matter mass and volume,T_4594,"Mass is a measure of the amount of matter in a substance or an object. The basic SI unit for mass is the kilogram (kg), but smaller masses may be measured in grams (g). To measure mass, you would use a balance. In the lab, mass may be measured with a triple beam balance or an electronic balance, but the old-fashioned balance pictured below may give you a better idea of what mass is. If both sides of this balance were at the same level, it would mean that the fruit in the left pan has the same mass as the iron object in the right pan. In that case, the fruit would have a mass of 1 kg, the same as the iron. As you can see, however, the fruit is at a higher level than the iron. This means that the fruit has less mass than the iron, that is, the fruits mass is less than 1 kg. Q: If the fruit were at a lower level than the iron object, what would be the mass of the fruit? A: The mass of the fruit would be greater than 1 kg. Mass is commonly confused with weight. The two are closely related, but they measure different things. Whereas mass measures the amount of matter in an object, weight measures the force of gravity acting on an object. The force of gravity on an object depends on its mass but also on the strength of gravity. If the strength of gravity is held constant (as it is all over Earth), then an object with a greater mass also has a greater weight. Q: With Earths gravity, an object with a mass of 1 kg has a weight of 2.2 lb. How much does a 10 kg object weigh on Earth? A: A 10 kg object weighs ten times as much as a 1 kg object: 10  2.2 lb = 22 lb "
matter mass and volume,T_4595,"Volume is a measure of the amount of space that a substance or an object takes up. The basic SI unit for volume is the cubic meter (m3 ), but smaller volumes may be measured in cm3 , and liquids may be measured in liters (L) or milliliters (mL). How the volume of matter is measured depends on its state. The volume of a liquid is measured with a measuring container, such as a measuring cup or graduated cylinder. The volume of a gas depends on the volume of its container: gases expand to fill whatever space is available to them. The volume of a regularly shaped solid can be calculated from its dimensions. For example, the volume of a rectangular solid is the product of its length, width, and height. The volume of an irregularly shaped solid can be measured by the displacement method. You can read below how this method works. Click image to the left or use the URL below. URL:  Q: How could you find the volume of air in an otherwise empty room? A: If the room has a regular shape, you could calculate its volume from its dimensions. For example, the volume of a rectangular room can be calculated with the formula: Volume = length  width  height If the length of the room is 5.0 meters, the width is 3.0 meters, and the height is 2.5 meters, then the volume of the room is: Volume = 5.0 m  3.0 m  2.5 m = 37.5 m3 Q: What is the volume of the dinosaur in the diagram above? A: The volume of the water alone is 4.8 mL. The volume of the water and dinosaur together is 5.6 mL. Therefore, the volume of the dinosaur alone is 5.6 mL - 4.8 mL = 0.8 mL. "
melting,T_4603,"The process in which rocks or other solids change to liquids is called melting. Melting occurs when particles of a solid absorb enough energy to partly overcome the force of attraction holding them together. This allows them to move out of their fixed positions and slip over one another. Melting, like other changes of state, is a physical change in matter, so it doesnt change the chemical makeup or chemical properties of matter. Q: The molten rock that erupts from a volcano comes from deep underground. How is this related to its liquid state? A: It is always very hot deep underground where molten rock originates. The high temperatures give rock enough energy to melt and remain in a molten state. Underground rock in this state is called magma. Q: What happens to magma after it erupts and starts flowing over the surface of the ground? A: After magma erupts, it is called lava. On the surface, lava eventually cools and hardens to form solid rock. Other substances that are normally solids on Earth can also be heated until they melt. You can see an example in the Figure 1.1. The photo shows molten gold being poured into a mold. When the gold cools, it will harden into a solid gold bar that has the same shape as the mold. "
melting,T_4604,"The temperature at which a substance melts is called its melting point. Melting point is a physical property of matter. The gold pictured in the Figure 1.1, for example, has a melting point of 1064 C. This is a high melting point, and most other metals also have high melting points. The melting point of ice, in comparison, is much lower at 0 C. Many substances have even lower melting points. For example, the melting point of oxygen is -222 C. "
melting,T_4605,"Because of global climate change, temperatures all over Earth are rising. However, the melting points of Earths substances, including ice, are constant. The result? Glaciers are melting at an alarming rate. Melting glaciers cause rising sea levels and the risk of dangerous river flooding on land. Click image to the left or use the URL below. URL: "
mixtures,T_4625,"A mixture is a combination of two or more substances in any proportion. This is different from a compound, which consists of substances in fixed proportions. The substances in a mixture also do not combine chemically to form a new substance, as they do in a compound. Instead, they just intermingle and keep their original properties. The lemonade pictured above is a mixture because it doesnt have fixed proportions of ingredients. It could have more or less lemon juice, for example, or more or less sugar, and it would still be lemonade. Q: What are some other examples of mixtures? A: Other examples of liquid mixtures include salt water and salad dressing. Air is a mixture of gases, mainly nitrogen and oxygen. The rock pictured in the Figure 1.1 is a solid mixture. This rock is a mixture of smaller rocks and minerals. "
mixtures,T_4626,"The lemonade in the opening picture is an example of a homogeneous mixture. A homogeneous mixture has the same composition throughout. Another example of a homogeneous mixture is salt water. If you analyzed samples of ocean water in different places, you would find that the proportion of salt in each sample is the same: 3.5 percent. The rock in Figure 1.1 is an example of a heterogeneous mixture. A heterogeneous mixture varies in its composition. The black nuggets, for example, are not distributed evenly throughout the rock. "
mixtures,T_4627,"Mixtures have different properties depending on the size of their particles. Three types of mixtures based on particle size are solutions, suspensions, and colloids, all of which are described in Table 1.1. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL:  Type of Mixture Solutions Description A solution is a homogeneous mixture with tiny parti- cles. The particles are too small to see and also too small to settle or be filtered out of the mixture. When the salt is thoroughly mixed into the water in this glass, it will form a solution. The salt will no longer be visible in the water, and it wont settle to the bottom of the glass. Colloids A colloid is a homogeneous mixture with medium- sized particles. The particles are large enough to see but not large enough to settle or be filtered out of the mixture. The gelatin in this dish is a colloid. It looks red because you can see the red gelatin particles in the mixture. However, the particles are too small to settle to the bottom of the dish. A suspension is a heterogeneous mixture with large particles. The particles are large enough to see and also to settle or be filtered out of the mixture. The salad dressing in this bottle is a suspension. It contains oil, vinegar, herbs, and spices. If the bottle sits undisturbed for very long, the mixture will separate into its component parts. Thats why you should shake it before you use it. Suspensions Q: If you buy a can of paint at a paint store, a store employee may put the can on a shaker machine to mix up the paint in the can. What type of mixture is the paint? A: The paint is a suspension. Some of the components of the paint settle out of the mixture when it sits undisturbed for a long time. This explains why you need to shake (or stir) the paint before you use it. Q: The milk you buy in the supermarket has gone through a process called homogenization. This process breaks up the cream in the milk into smaller particles. As a result, the cream doesnt separate out of the milk no matter how long it sits on the shelf. Which type of mixture is homogenized milk? A: Homogenized milk is a colloid. The particles in the milk are large enough to seethats why milk is white instead of clear like water, which is the main component of milk. However, the particles are not large enough to settle out of the mixture. "
mixtures,T_4628,"The components of a mixture keep their own identity when they combine, so they retain their physical properties. Examples of physical properties include boiling point, ability to dissolve, and particle size. When components of mixtures vary in physical properties such as these, processes such as boiling, dissolving, or filtering can be used to separate them. Look at the Figure 1.2 of the Great Salt Lake in Utah. The water in the lake is a solution of salt and water. Do you see the white salt deposits near the shore? How did the salt separate from the salt water? Water has a lower boiling point than salt, and it evaporates in the heat of the sun. With its higher boiling point, the salt doesnt get hot enough to evaporate, so it is left behind. Q: Suppose you have a mixture of salt and pepper. What properties of the salt and pepper might allow you to separate them? A: Salt dissolves in water but pepper does not. If you mix salt and pepper with water, only the salt will dissolve, leaving the pepper floating in the water. You can separate the pepper from the water by pouring the mixture through a filter, such as a coffee filter. Q: After you separate the pepper from the salt water, how could you separate the salt from the water? A: You could heat the water until it boils and evaporates. The salt would be left behind. "
ohms law,T_4688,"For electric current to flow through a wire, there must be a source of voltage. Voltage is a difference in electric potential energy. As you might have guessed, greater voltage results in more current. As electric current flows through matter, particles of matter resist the moving charges. This is called resistance, and greater resistance results in less current. These relationships between electric current, voltage, and resistance were first demonstrated in the early 1800s by a German scientist named Georg Ohm, so they are referred to as Ohms law. Ohms law can be represented by the following equation. Current(amps) = Voltage(volts) Resistance(ohms) "
ohms law,T_4689,Ohms law may be easier to understand with an analogy. Current flowing through a wire is like water flowing through a hose. Increasing voltage with a higher-volt battery increases the current. This is like opening the tap wider so more water flows through the hose. Increasing resistance reduces the current. This is like stepping on the hose so less water can flow through it. 
ohms law,T_4690,"You can use the equation for current (above) to calculate the amount of current flowing through a circuit when the voltage and resistance are known. Consider an electric wire that is connected to a 12-volt battery. If the wire has a resistance of 2 ohms, how much current is flowing through the wire? Current = 12 volts 2 ohms = 6 amps Q: If a 120-volt voltage source is connected to a wire with 10 ohms of resistance, how much current is flowing through the wire? A: Substitute these values into the equation for current: Current = 120 volts 10 ohms = 12 amps "
physical change,T_4709,"A physical change is a change in one or more physical properties of matter without any change in chemical properties. In other words, matter doesnt change into a different substance in a physical change. Examples of physical change include changes in the size or shape of matter. Changes of statefor example, from solid to liquid or from liquid to gasare also physical changes. Some of the processes that cause physical changes include cutting, bending, dissolving, freezing, boiling, and melting. Four examples of physical change are pictured in the Figure Click image to the left or use the URL below. URL:  Q: In the Figure 1.1, what physical changes are occurring? A: The paper is being cut into smaller pieces, which is changing its size and shape. The ice cubes are turning into a puddle of liquid water because they are melting. This is a change of state. The tablet is disappearing in the glass of water because it is dissolving into particles that are too small to see. The lighthouse is becoming coated with ice as ocean spray freezes on its surface. This is another change of state. "
physical change,T_4710,"When matter undergoes physical change, it doesnt become a different substance. Therefore, physical changes are often easy to reverse. For example, when liquid water freezes to form ice, it can be changed back to liquid water by heating and melting the ice. Q: Salt dissolving in water is a physical change. How could this change be reversed? A: The salt water could be boiled until the water evaporates, leaving behind the salt. Water vapor from the boiling water could be captured and cooled. The water vapor would condense and change back to liquid water. "
physical properties of matter,T_4711,"Physical properties of matter are properties that can be measured or observed without matter changing to an entirely different substance. Physical properties are typically things you can detect with your senses. For example, they may be things that you can see, hear, smell, or feel. Q: What differences between snow and sand can you detect with your senses? A: You can see that snow and sand have a different color. You can also feel that snow is softer than sand. Both color and hardness are physical properties of matter. "
physical properties of matter,T_4712,"In addition to these properties, other physical properties of matter include the state of matter. States of matter include liquid, solid, and gaseous states. For example at 20 C, coal exists as a solid and water exists as a liquid. Additional examples of physical properties include: odor boiling point ability to conduct heat ability to conduct electricity ability to dissolve in other substances Some of these properties are illustrated in the Figures 1.1, 1.2, 1.3, and 1.4. Click image to the left or use the URL below. URL:  The strong smell of swimming pool water is the odor of chlorine, which is added to the water to kill germs and algae. In con- trast, bottled spring water, which contains no chlorine, does not have an odor. Coolant is added to the water in a car radiator to keep the water from boiling and evaporating. Coolant has a higher boiling point than water and adding it to the water increases the boiling point of the solution. Q: The coolant that is added to a car radiator also has a lower freezing point than water. Why is this physical property useful? A: When coolant is added to water in a car radiator, it lowers the freezing point of the water. This prevents the water in the radiator from freezing when the temperature drops below 0 C, which is the freezing point of pure water. Q: Besides being able to conduct electricity, what other physical property of copper makes it well suited for electric wires? A: Copper, like other metals, is ductile. This means that it can be rolled and stretched into long thin shapes such as wires. This teakettle is made of aluminum except for its handle, which is made of plastic. Aluminum is a good conductor of heat. It conducts heat from the flames on the range to the water inside the kettle, so the water heats quickly. Plastic, on the other hand, is not a good conductor of heat. It stays cool enough to touch even when the rest of the teakettle becomes very hot. "
plasma,T_4715,"Compare and contrast the basic properties of matter, such as mass and volume. "
plasma,T_4716,"Here is a riddle for you to ponder: What do you and a tiny speck of dust in outer space have in common? Think you know the answer? Both you and the speck of dust consist of matter. So does the ground beneath your feet. In fact, everything you can see and touch is made of matter. The only things that are not matter are forms of energy. This would include things such as light and sound. Although forms of energy are not matter, the air and other substances they travel through are. So what is matter? Matter is defined as anything that has mass and volume. You may recall that atoms are the building blocks of matter. Even things as small as atoms have mass and volume. The more atoms, or matter, the more mass and volume are present. Different types of atoms have different amounts of mass and volume. So, its not enough to know the count of atoms to determine the mass. You must also know the type of atoms an item is made of. Mass and volume are just two ways to describe the physical property of a substance. Physical properties are all determined by the amounts and type of atoms that compose items. "
plasma,T_4717,"Mass refers to the amount of matter. Mass is usually measured with a balance. A balance allows an object to be matched with other objects of known mass. The SI unit for mass is the kilogram. For smaller masses, grams are often used instead. You may have a balance in your classroom. The balance may be either a triple-beam balance or an electronic balance. The figure below of the old-fashioned balance may give you a better idea of what mass is. What does it mean when both sides of the balance are at the same level? Thats correct, it would mean the masses of each object are equal. In that case, the fruit would have a mass of 1 kg. It would have the same mass as the iron. As you can see, the fruit is at a higher level than the iron. This means the fruit has less mass than the 1 kg iron object. Q: What If the fruit were at a lower level than the iron object? A: The mass of the fruit would be greater than 1 kg. "
plasma,T_4718,"Mass is often confused with weight. The two are closely related, but they are not the same. Mass is the amount of matter. Weight is a measure of the force of gravity acting on the mass. On Earth, the force of gravity is constant. If we are comparing objects on Earth, objects with a greater mass also have a greater weight. Weight is measured with a device called a scale. Remember, mass is measured with a balance. You might find an example of a scale in your kitchen or bathroom. Scales detect how forcefully objects are being pulled downward by gravity. The SI unit for weight is the newton (N). A mass of 10 kg has a weight of 100 newtons (N). "
plasma,T_4719,"At Earths gravity, what is the weight in newtons of an object with a mass of 10 kg? At Earths gravity, 1 kg has a weight of 10 N. Therefore, 10 kg has a weight of (10 kg x 10 m/s2 ) = 100 N. "
plasma,T_4720,"If you have a mass of 50 kg on Earth, what is your weight in newtons? An object with more mass is pulled by gravity with greater force. Mass and weight are closely related. However, the weight of an object can change if the force of gravity changes. On Earth, the force of gravity is the same everywhere. So how does the force of gravity change? It doesnt if you stay on Earth. What if we travel to another planet or moon in our solar system? Look at the photo of astronaut Edwin E. Aldrin Jr. taken by fellow astronaut Neil Armstrong in the Figure ??. They were the first humans to walk on the moon. An astronaut weighs less on the moon than he would on Earth. This is because the moons gravity is weaker than Earths. The astronauts mass, on the other hand, did not change. He still contained the same amount of matter on the moon as he did on Earth. If the astronaut weighed 175 pounds on Earth, he would have weighed only 29 pounds on the moon. If his mass on Earth was 80 kg, what would his mass have been on the moon? [Figure 3] "
plasma,T_4721,"The amount of space matter takes up is its volume. How the volume of matter is measured depends on its state. The volume of liquids is measured with measuring containers. In the kitchen, liquid volume is usually measured with measuring cups or spoons. In the lab, liquid volume is measured with containers such as graduated cylinders. Units in the metric system for liquid volume include liters (L) and milliliters (mL). The volume of gases depends on the volume of their container. Thats because gases expand to fill whatever space is available to them. For example, as you drink water from a bottle, air rushes in to take the place of the water. An ""empty"" liter bottle actually holds a liter of air. How could you find the volume of air in an ""empty"" room? The volume of regularly shaped solids can be calculated from their dimensions. For example, the volume of a rectangular solid is the product of its length, width, and height (l x w x h). For solids that have irregular shapes, the displacement method is used. You can see how it works in the video below. The SI unit for solid volumes is cubic meters (m3 ). However, cubic centimeters (cm3 ) are often used for smaller volume measurements. The displacement method is used to find the volume of irregularly shaped objects. It measures the amount of water that the object displaces, or moves out of the way. What is the volume of the toy dinosaur in mL? [See Figure ??] Click image to the left or use the URL below. URL:  Q: How could you find the volume of air in an otherwise empty room? A: If the room has a regular shape, you could calculate its volume from its dimensions. For example, the volume of a rectangular room can be calculated with this formula: Volume = length  width  height If the length of the room is 5.0 meters, the width is 3.0 meters, and the height is 2.5 meters, then the volume of the room is: Volume = 5.0 m  3.0 m  2.5 m = 37.5 m3 Q: What is the volume of the dinosaur in the diagram above? A: The volume of the water alone is 4.8 mL. The volume of the water and dinosaur together is 5.6 mL. Therefore, the volume of the dinosaur alone is 5.6 mL - 4.8 mL = 0.8 mL. "
plasma,T_4722,"Density is also an important physical property of matter. The concept of density combines what we know about an objects mass and volume. Density reflects how closely packed the particles of matter are. When particles are packed together more tightly, matter is more dense. Differences in density of matter explain many phenomena. It explains why helium balloons rise. It explains why currents such as the Gulf Stream flow through the oceans. It explains why some things float in or sink. You can see this in action by pouring vegetable oil into water. You can see a colorful demonstration in this video. Click image to the left or use the URL below. URL:  To better understand density, think about a bowling ball and volleyball, pictured in the Figure 1.1. Imagine lifting each ball. The two balls have about the same volume. The bowling ball feels much heavier than the volleyball, but why? It is because the bowling ball is made of solid plastic. Plastic contains a lot of tightly packed particles of matter. In contrast, the volleyball is full of a gas (air). The air atoms are further apart than in the solid bowling ball. Therefore, the matter inside the bowling ball is more dense than the matter inside the volleyball. Q: If you ever went bowling, you may have noticed that all the bowling balls are the same size. This means they have the same volume. Even though they are the same size, some bowling balls feel heavier than others. How can this be? A: Bowling balls that feel lighter are made of matter that is less dense. "
plasma,T_4723,"The density of matter is actually the amount of matter in a given space. The amount of matter is measured by its mass. The space matter takes up is measured by its volume. Therefore, the density of matter can be calculated with this formula: Density = mass volume Assume, for example, that a book has a mass of 500 g and a volume of 1000 cm3 . Then the density of the book is: Density = 500 g = 0.5 g/cm3 1000 cm3 Q: What is the density of a liquid that has a volume of 30 mL and a mass of 300 g? A: The density of the liquid is: Density = 300 g = 10 g/mL 30 mL "
plasma,T_4724,"By clicking a link below, you will leave the CK-12 site and open an external site in a new tab. This page will remain open in the original tab. "
properties of solutions,T_4756,"Salt water in the ocean is a solution. In a solution, one substance, called the solute, dissolves in another substance, called the solvent. In ocean water, salt is the solute and water is the solvent. When a solute dissolves in a solvent, it changes the physical properties of the solvent. In particular, the solute generally lowers the freezing point of the solvent, which is called freezing point depression, and raises the boiling point of the solvent, which is called boiling point elevation. For example, adding either salt to water lowers the freezing point and raises the boiling point of the water. "
properties of solutions,T_4757,"Pure water freezes at 0  C, but the salt water in the ocean freezes at -2.2  C because of freezing point depression. We take advantage of the freezing point depression of salt in water by putting salt on ice to melt it. Thats why the truck in the Figure 1.1 is spreading salt on an icy road. Did you ever see anyone add a fluid to their car radiator? The fluid might be antifreeze, like in the Figure 1.2. Antifreeze lowers the temperature of the water in the car radiator so it wont freeze, even when the temperature falls far below 0  C. For example, a 50 percent antifreeze solution wont freeze unless the temperature goes below -37 C. "
properties of solutions,T_4758,"Antifreeze could also be called antiboil because it also raises the boiling point of the water in a car radiator. Hot weather combined with a hot engine can easily raise the temperature of the water in the radiator above 100  C, which is the boiling point of pure water. If the water boils, it could cause the engine to overheat and become seriously damaged. However, if antifreeze has been added to the water, the boiling point is much higher. For example a 50 percent antifreeze solution has a boiling point of 129  C. Unless the water gets hotter than this, it wont boil and ruin the engine. "
rate of dissolving,T_4782,"Did you ever get impatient and start drinking a sweetened drink before all the sugar has dissolved? As you drink the last few drops, you notice that some of the sugar is sitting on the bottom of the container. Q: What could you do to dissolve the sugar faster? A: The rate of dissolving is influenced by several factors, including stirring, temperature of solvent, and size of solute particles. "
rate of dissolving,T_4783,"Stirring a solute into a solvent speeds up the rate of dissolving because it helps distribute the solute particles throughout the solvent. For example, when you add sugar to iced tea and then stir the tea, the sugar will dissolve faster. If you dont stir the iced tea, the sugar may eventually dissolve, but it will take much longer. "
rate of dissolving,T_4784,"The temperature of the solvent is another factor that affects how fast a solute dissolves. Generally, a solute dissolves faster in a warmer solvent than it does in a cooler solvent because particles have more energy of movement. For example, if you add the same amount of sugar to a cup of hot tea and a cup of iced tea, the sugar will dissolve faster in the hot tea. "
rate of dissolving,T_4785,"A third factor that affects the rate of dissolving is the size of solute particles. For a given amount of solute, smaller particles have greater surface area. With greater surface area, there can be more contact between particles of solute and solvent. For example, if you put granulated sugar in a glass of iced tea, it will dissolve more quickly than the same amount of sugar in a cube (see Figure 1.1). Thats because all those tiny particles of granulated sugar have greater total surface area than a single sugar cube. "
refraction,T_4791,"Physical properties of matter can be measured and observed. Physical properties can be detected with your senses . For example, they may be things that you can see, hear, smell, feel, or even taste. Q: What are some differences between snow and sand? Which senses could you use to find out the differences? A: You can see that snow and sand have a different color . You can also feel that snow is softer than sand. Both color and hardness are physical properties of matter . You can notice that ice will melt at room temperature. Sand will remain a solid at room temperature. "
refraction,T_4792,"Physical properties include the state of matter. We know these states as solid, liquid, or gas. Properties can also include color and odor. For example, oxygen is a gas. It is a major part of the air we breathe. It is colorless and odorless. Chlorine is also a gas. In contrast to oxygen, chlorine is greenish in color. It has a strong, sharp odor. Have you ever smelled cleaning products used around your home? If so, you have probably smelled chlorine. Another place you might smell chlorine is at a public swimming pool. The chlorine is used to kill bacteria that may grow in the water. Other physical properties include hardness, freezing, and boiling points. Some substances have the ability to dissolve in other substances. Some substances cannot be dissolved. For example, salt easily dissolves in water. Oil does not dissolve in water. Some substances may have the ability to conduct heat or electricity. Some substances resist the flow of electricity and heat. These properties are demonstrated in Figure 1.1. Can you think of other physical properties? The video below compares physical properties. "
refraction,T_4793,"By clicking a link below, you will leave the CK-12 site and open an external site in a new tab. This page will remain open in the original tab. "
saturation,T_4809,"The maximum amount of sugar that will dissolve in a liter of 20  C water is 2000 grams. A sugar-water solution that contains 1 liter of water and 2000 grams of sugar is said to be saturated. A saturated solution is a solution that contains as much solute as can dissolve in a given solvent at a given temperature. If you add more than 2000 grams of sugar to a liter of 20  C water, the extra sugar wont dissolve. On the other hand, a solution containing less than 2000 g of sugar in 1 liter of 20  C water can hold more sugar. A solution that contains less solute than can dissolve at a given temperature is called an unsaturated solution. You can learn more about saturated and unsaturated solutions by watching the video at this URL:  . "
saturation,T_4810,"The Figure 1.1 shows the amounts of several different solutes that will dissolve in a liter of water at 20  C. As you can see from the graph, solutes vary greatly in how soluble they are in water. For example, you can dissolve almost 20 times as much sugar as baking soda in the same amount of water at this temperature. Q: Assume that a solution contains 150 grams of Epsom salt in 1 liter of water at 20  C. Is the solution saturated or unsaturated? A: A saturated solution of Epsom salt in 1 liter of 20  C water would contain 250 grams of Epsom salt. Therefore, this solution is unsaturated. It can hold another 100 grams of Epsom salt. Q: What do you think would happen if you added more than 250 grams of Epsom salt to a liter of 20  C water? A: Any Epsom salt over 250 grams would not dissolve in the solution. "
scientific measuring devices,T_4825,"Youve probably been using a ruler to measure length since you were in elementary school. But you may have made most of the measurements in English units of length, such as inches and feet. In science, length is most often measured in SI units, such as millimeters and centimeters. Many rulers have both types of units, one on each edge. The ruler pictured below has only SI units. It is shown here bigger than it really is so its easier to see the small lines, which measure millimeters. The large lines and numbers stand for centimeters. Count the number of small lines from the left end of the ruler (0.0). You should count 10 lines because there are 10 millimeters in a centimeter. Q: What is the length in millimeters of the red line above the metric ruler? A: The length of the red line is 32 mm. Q: What is the length of the red line in centimeters? A: The length of the red line is 3.2 cm. "
scientific measuring devices,T_4826,"Mass is the amount of matter in an object. Scientists often measure mass with a balance. A type of balance called a triple beam balance is pictured in Figure 1.1. To use this type of balance, follow these steps: 1. Place the object to be measured on the pan at the left side of the balance. 2. Slide the movable masses to the right until the right end of the arm is level with the balance mark. Start by moving the larger masses and then fine tune the measurement by moving the smaller masses as needed. 3. Read the three scales to determine the values of the masses that were moved to the right. Their combined mass is equal to the mass of the object. The Figure 1.2 is an enlarged version of the scales of the triple beam balance in Figure 1.1. It allows you to read the scales. The middle scale, which measures the largest movable mass, reads 300 grams. This is followed by the top scale, which reads 30 grams. The bottom scale reads 5.1 grams. Therefore, the mass of the object in the pan is 335.1 grams (300 grams + 30 grams + 5.1 grams). Q: What is the maximum mass this triple beam balance can measure? A: The maximum mass it can measure is 610 grams (500 grams + 100 grams + 10 grams). Q: What is the smallest mass this triple beam balance can measure? A: The smallest mass it can measure is one-tenth (0.1) of a gram. To measure very small masses, scientists use electronic balances, like the one in the Figure 1.3. This type of balance also makes it easier to make accurate measurements because mass is shown as a digital readout. In the picture, the balance is being used to measure the mass of a white powder on a plastic weighing tray. The mass of the tray alone would have to be measured first and then subtracted from the mass of the tray and powder together. The difference between the two masses is the mass of the powder alone. "
scientific measuring devices,T_4827,"At home, you might measure the volume of a liquid with a measuring cup. In science, the volume of a liquid might be measured with a graduated cylinder, like the one sketched below. The cylinder in the picture has a scale in milliliters (mL), with a maximum volume of 100 mL. Follow these steps when using a graduated cylinder to measure the volume of a liquid: 1. Place the cylinder on a level surface before adding the liquid. 2. After adding the liquid, move so your eyes are at the same level as the top of the liquid in the cylinder. 3. Read the mark on the glass that is at the lowest point of the curved surface of the liquid. This is called the meniscus. Q: What is the volume of the liquid in the graduated cylinder pictured above? A: The volume of the liquid is 67 mL. Q: What would the measurement be if you read the highest point of the curved surface of the liquid by mistake? A: The measurement would be 68 mL. "
series and parallel circuits,T_4844,"An electric circuit consists of at least one closed loop through which electric current can flow. Every circuit has a voltage source such as a battery and a conductor such as metal wire. A circuit may have other parts as well, such as lights and switches. In addition, a circuit may consist of one loop or two loops. "
series and parallel circuits,T_4845,"A circuit that consists of one loop is called a series circuit. You can see a simple series circuit below. If a series circuit is interrupted at any point in its single loop, no current can flow through the circuit and no devices in the circuit will work. In the series circuit below, if one light bulb burns out, the other light bulb wont work because it wont receive any current. Series circuits are commonly used in flashlights. Q: If one light bulb burns out in this series circuit, how can you tell which bulb it is? A: It may not be obvious, because neither bulb will light if one is burned out. You can tell which one it is only by replacing first one bulb and then the other to see which replacement results in both bulbs lighting up. "
series and parallel circuits,T_4846,"A circuit that has two loops is called a parallel circuit. A simple parallel circuit is sketched below. If one loop of a parallel circuit is interrupted, current can still flow through the other loop. In the parallel circuit below, if one light bulb burns out, the other light bulb will still work because current can bypass the burned-out bulb. The wiring in a house consists of parallel circuits. "
solenoid,T_4857,"A solenoid is a coil of wire with electric current flowing through it. You can see a solenoid in the Figure 1.1. Current flowing through the coil produces a magnetic field that has north and south poles. Click image to the left or use the URL below. URL:  Q: How is a solenoid like a bar magnet? A: Like a bar magnet, a solenoid has north and south magnetic poles and is surrounded by a magnetic field. "
solenoid,T_4858,"Any wire with current flowing through it has a magnetic field. However, the magnetic field around a coiled wire is stronger than the magnetic field around a straight wire. Thats because each turn of the wire in the coil has its own magnetic field. Adding more turns to the coil of wire increases the strength of the field. Increasing the amount of current flowing through the coil also increases the strength of the magnetic field. "
solenoid,T_4859,"A solenoid is generally used to convert electromagnetic energy into motion. Solenoids are often used in devices that need a sudden burst of power to move a specific part. In addition to paintball markers, you can find solenoids in machines ranging from motor vehicles to electric dishwashers. Another device that uses solenoids is pictured in the Figure 1.2. "
solids,T_4860,"A snowflake is made of ice, or water in the solid state. A solid is one of four well-known states of matter. The other three states are liquid, gas, and plasma. Compared with these other states of matter, solids have particles that are much more tightly packed together. The particles are held rigidly in place by all the other particles around them so they cant slip past one another or move apart. This gives solids a fixed shape and a fixed volume. "
solids,T_4861,"Not all solids are alike. Some are crystalline solids; others are amorphous solids. Snowflakes are crystalline solids. Particles of crystalline solids are arranged in a regular repeating pattern, as you can see in the sketch in Figure chloride). Crystals of table salt are pictured in the Figure 1.1. Amorphous means shapeless. Particles of amorphous solids are arranged more-or-less at random and do not form crystals, as you can see in the Figure 1.2. An example of an amorphous solid is cotton candy, also shown in the Figure 1.2. Q: Look at the quartz rock and plastic bag pictured in the Figure 1.3. Which type of solid do you think each of them is? A: The quartz is a crystalline solid. Rocks are made of minerals and minerals form crystals. You can see their geometric shapes. The bag is an amorphous solid. It is made of the plastic and most plastics do not form crystals. "
solubility,T_4862,"Solubility is the amount of solute that can dissolve in a given amount of solvent at a given temperature. In a solution, the solute is the substance that dissolves, and the solvent is the substance that does the dissolving. For a given solvent, some solutes have greater solubility than others. For example, sugar is much more soluble in water than is salt. But even sugar has an upper limit on how much can dissolve. In a half liter of 20  C water, the maximum amount is 1000 grams. If you add more sugar than this, the extra sugar wont dissolve. You can compare the solubility of sugar, salt, and some other solutes in the Table 1.1. Solute Baking Soda Epsom salt Table salt Table sugar Grams of Solute that Will Dissolve in 0.5 L of Water (20  C) 48 125 180 1000 Q: How much salt do you think Rhonda added to the half-liter of water in her experiment? A: The solubility of salt is 180 grams per half liter of water at 20  C. If Rhonda had added less than 180 grams of salt to the half-liter of water, then all of it would have dissolved. Because some of the salt did not dissolve, she must have added more than 180 grams of salt to the water. "
solubility,T_4863,"Certain factors can change the solubility of a solute. Temperature is one such factor. How temperature affects solubility depends on the state of the solute, as you can see in the Figure 1.1. If a solute is a solid or liquid, increasing the temperature increases its solubility. For example, more sugar can dissolve in hot water than in cold water. If a solute is a gas, increasing the temperature decreases its solubility. For example, less carbon dioxide can dissolve in warm water than in cold water. The solubility of gases is also affected by pressure. Pressure is the force pushing against a given area. Increasing the pressure on a gas increases its solubility. Did you ever open a can of soda and notice how it fizzes out of the can? Soda contains dissolved carbon dioxide. Opening the can reduces the pressure on the gas in solution, so it is less soluble. As a result, some of the carbon dioxide comes out of solution and rushes into the air. Q: Which do you think will fizz more when you open it, a can of warm soda or a can of cold soda? A: A can of warm soda will fizz more because increasing the temperature decreases the solubility of a gas. Therefore, less carbon dioxide can remain dissolved in warm soda than in cold soda. "
solute and solvent,T_4864,"A solution forms when one substance is dissolved by another. The substance that dissolves is called the solute. The substance that dissolves it is called the solvent. The solute is present in a lesser amount that the solvent. When the solute dissolves, it separates into individual particles, which spread throughout the solvent. Q: In bronze, what are the solute and solvent? A: Because bronze consists mainly of copper, copper is the solvent and tin is the solute. The two metals are combined in a hot, molten state, but they form a solid solution at room temperature. "
solute and solvent,T_4865,"In the example of bronze, a solid (tin) is dissolved in another solid (copper). However, matter in any state can be the solute or solvent in a solution. For example, in a saltwater solution, a solid (salt) is dissolved in a liquid (water). The Table 1.1 describes examples of solutions consisting of solutes and solvents in various states of matter. Type of Solution: Example Gas dissolved in gas: dry air Gas dissolved in liquid: carbonated water Liquid dissolved in gas: moist air Liquid dissolved in liquid: vinegar Solid dissolved in liquid: sweet tea Solute oxygen carbon dioxide Solvent nitrogen water water acetic acid sugar air water tea "
solute and solvent,T_4866,"Salt isnt the only solute that dissolves in water. In fact, so many things dissolve in water that water is sometimes called the universal solvent. Water is such a good solvent because it is a very polar compound. A polar compound has positively and negatively charged ends. Solutes that are also charged are attracted to the oppositely charged ends of water molecules. This allows the water molecules to pull the solute particles apart. On the other hand, there are some substances that dont dissolve in water. Did you ever try to clean a paintbrush with water after painting with an oil-based paint? It doesnt work. Oil-based paint is nonpolar, so its particles arent charged. As a result, oil-based paint doesnt dissolve in water. (You can see how to dissolve oil-based paint in the Figure 1.1.) "
solute and solvent,T_4867,"These examples illustrate a general rule about solutes and solvents: like dissolves like. In other words, polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. You can see below a students video demonstrating solutes that do and solutes that dont dissolve in water. Click image to the left or use the URL below. URL: "
solute and solvent,T_4868,"All solutes separate into individual particles when they dissolve, but the particles are different for ionic and covalent compounds. Ionic solutes separate into individual ions. Covalent solutes separate into individual molecules. Salt, or sodium chloride (NaCl), is an ionic compound. When it dissolves in water, it separates into positive sodium ions (Na+ ) and negative chloride ions (Cl ). You can see how this happens in the Figure 1.2. The negative oxygen ends of water molecules attract the positive sodium ions, and the positive hydrogen ends of water molecules attract the negative chloride ions. These forces of attraction pull the ions apart. The sugar glucose is a covalent compound. When sugar dissolves in water, it forms individual glucose molecules (C6 H12 O6 ). You can see how this happens in the Figure 1.3. Sugar is polar like water, so sugar molecules also have positive and negative ends. Forces of attraction between oppositely charged ends of water and sugar molecules pull individual sugar molecules away from the sugar crystal. Little by little, the sugar molecules are separated from the crystal and surrounded by water. Click image to the left or use the URL below. URL: "
solution concentration,T_4869,"A solution is a mixture of two or more substances in which dissolved particles are distributed evenly throughout the solution. The substance that dissolves in a solution is called the solute, and the substance that does the dissolving is called the solvent. The concentration of a solution is the amount of solute in a given amount of solution. A solution with a lot of dissolved solute has a high concentration and is called a concentrated solution. A solution with little dissolved solute has a low concentration and is called a dilute solution. "
solution concentration,T_4870,"The concentration of a solution represents the percentage of the solution that is the solute. You can calculate the concentration of a solution using this formula: Concentration = Mass (or volume) of Solute Mass (or volume) of Solution 100% For example, if a 100-gram solution of salt water contains 3 grams of salt, then its concentration is: Concentration = 3g 100g 100% = 3% Q: A 1000 mL container of brand A juice drink contains 250 mL of juice and 750 mL of water. A 600 mL container of brand B juice drink contains 200 mL of juice and 400 mL of water. Which brand of juice drink is more concentrated, brand A or brand B? 250 mL 1000 mL  100% = 25% 200 mL 600 mL  100% = 33% A: Concentration(A) = Concentration(B) = You can conclude that brand B is more concentrated. "
solutions,T_4871,"A solution is a mixture of two or more substances, but its not just any mixture. A solution is a homogeneous mixture. In a homogeneous mixture, the dissolved particles are spread evenly through the mixture. The particles of the solution are also too small to be seen or to settle out of the mixture. Click image to the left or use the URL below. URL: "
solutions,T_4872,"All solutions have two parts: the solute and the solvent. The solute is the substance that dissolves, and the solvent is the substance that dissolves the solute. Particles of solvent pull apart particles of solute, and the solute particles spread throughout the solvent. Salt water, such as the ocean water in the Figure 1.1, is an example of a solution. In a saltwater solution, salt is the solute and water is the solvent. Q: A scientist obtained a sample of water from the Atlantic Ocean and determined that the sample was about 3.5 percent dissolved salt. Predict the percent of dissolved salt in a sample of water from the Pacific Ocean. A: As a solution, ocean water is a homogeneous mixture. Therefore, no matter where the water sample is obtained, its composition will be about 3.5 percent dissolved salt. "
solutions,T_4873,"Not only salt, but many other solutes can dissolve in water. In fact, so many solutes can dissolve in water that water has been called the universal solvent. Even rocks can dissolve in water, which explains the cave that opened this article. A solute that can dissolve in a given solvent, such as water, is said to be soluble in that solvent. Conversely, a solute that cannot dissolve in a given solvent is said to be insoluble in that solvent. Although most solutes can dissolve in water, some solutes are insoluble in water. Oil is an example. Did you ever try to mix oil with water? No matter how well you mix the oil into the water, after the mixture stands for a while, the oil separates from the water and rises to the top. You can see how oil floats on ocean water in the Figure 1.2. "
solutions,T_4874,"Like salt water in the ocean, many solutions are normally in the liquid state. However, matter in any state can form a solution. An alloy, which is a mixture of a metal with one or more other substances, is a solid solution at room temperature. For example, the alloy bronze is a solution of copper and tin. Matter in the gaseous state can also form solutions. Q: What is an example of a gaseous solution? A: Air in the atmosphere is a gaseous solution. It is a mixture that contains mainly nitrogen and oxygen gases, with very small amounts of several other gases. The circle graph in the Figure 1.3 shows the composition of air. Oil from an oil spill floats on ocean water. Because air is a solution, it is homogeneous. In other words, no matter where you go, the air always contains the same proportion of gases that are shown in the graph. "
specific heat,T_4883,Specific heat is a measure of how much energy it takes to raise the temperature of a substance. It is the amount of energy (in joules) needed to raise the temperature of 1 gram of the substance by 1  C. Specific heat is a property that is specific to a given type of matter. Thats why its called specific. 
specific heat,T_4884,"The Table 1.1 compares the specific heat of four different substances. Metals such as iron have low specific heat. It doesnt take much energy to raise their temperature. Thats why a metal spoon heats up quickly when placed in a cup of hot coffee. Sand also has a relatively low specific heat. Water, on the other hand, has a very high specific heat. It takes a lot more energy to increase the temperature of water than sand. This explains why the sand on a beach gets hot while the water stays cool. Differences in the specific heat of water and land even affect climate. Substance iron sand wood Specific Heat (joules) 0.45 0.67 1.76 Q: Metal cooking pots and pans often have wooden handles. Can you explain why? A: Wood has a higher specific heat than metal, so it takes more energy to heat a wooden handle than a metal handle. As a result, a wooden handle would heat up more slowly and be less likely to burn your hand when you touch it. "
states of matter,T_4892,"The photo above represents water in three common states of matter. States of matter are different phases in which any given type of matter can exist. There are actually four well-known states of matter: solid, liquid, gas, and plasma. Plasma isnt represented in the iceberg photo, but the other three states of matter are. The iceberg itself consists of water in the solid state, and the lake consists of water in the liquid state. Q: Where is water in the gaseous state in the above photo? A: You cant see the gaseous water, but its there. It exists as water vapor in the air. Q: Water is one of the few substances that commonly exist on Earth in more than one state. Many other substances typically exist only in the solid, liquid, or gaseous state. Can you think of examples of matter that usually exists in just one of these three states? A: Just look around you and you will see many examples of matter that usually exists in the solid state. They include soil, rock, wood, metal, glass, and plastic. Examples of matter that usually exist in the liquid state include cooking oil, gasoline, and mercury, which is the only metal that commonly exists as a liquid. Examples of matter that usually exists in the gaseous state include oxygen and nitrogen, which are the chief gases in Earths atmosphere. "
states of matter,T_4893,"A given kind of matter has the same chemical makeup and the same chemical properties regardless of its state. Thats because state of matter is a physical property. As a result, when matter changes state, it doesnt become a different kind of substance. For example, water is still water whether it exists as ice, liquid water, or water vapor. "
states of matter,T_4894,"The most common states of matter on Earth are solids, liquids, and gases. How do these states of matter differ? Their properties are contrasted in the Figure 1.1. Click image to the left or use the URL below. URL:  Properties of matter in different states. Q: The Figure 1.2 shows that a liquid takes the shape of its container. How could you demonstrate this? A: You could put the same volume of liquid in containers with different shapes. This is illustrated below with a beaker (left) and a graduated cylinder (right). The shape of the liquid in the beaker is short and wide like the beaker, while the shape of the liquid in the graduated cylinder is tall and narrow like that container, but each container holds the same volume of liquid. Q: How could you show that a gas spreads out to take the volume as well as the shape of its container? A: You could pump air into a bicycle tire. The tire would become firm all over as air molecules spread out to take the shape of the tire and also to occupy the entire volume of the tire. "
sublimation,T_4898,"Solid carbon dioxide is also called dry ice. Thats because when it gets warmer and changes state, it doesnt change to a liquid by melting. Instead, it changes directly to a gas without going through the liquid state. The process in which a solid changes directly to a gas is called sublimation. It occurs when energy is added to a solid such as dry ice. Click image to the left or use the URL below. URL:  Q: Alyssas mom put some mothballs in her closet in the spring to keep moths away from her wool clothes. By autumn, the mothballs were much smaller. What happened to them? A: Mothballs are made of naphthalene, a substance that undergoes sublimation at room temperature. The solid mothballs slowly changed to a gas during the summer months, explaining why they were much smaller by autumn. "
sublimation,T_4899,"Snow and ice may also undergo sublimation under certain conditions. This is most likely to happen where there is intense sunlight, very cold temperatures, and dry winds. These conditions are often found on mountain peaks. As snow sublimates, it gradually shrinks without any runoff of liquid water. Click image to the left or use the URL below. URL: "
temperature,T_4917,"No doubt you already have a good idea of what temperature is. You might say that its how warm or cool something feels. In physics, temperature is defined as the average kinetic energy of the particles of matter. When particles of matter move more quickly, they have more kinetic energy, so their temperature is higher. With a higher temperature, matter feels warmer. When particles move more slowly, they have less kinetic energy on average, so their temperature is lower. With a lower temperature, matter feels cooler. "
temperature,T_4918,"Many thermometers measure temperature with a liquid that expands when it gets warmer and contracts when it gets cooler. Look at the common household thermometer pictured in the Figure 1.1. The red liquid rises or falls in the glass tube as the temperature changes. Temperature is read off the scale at the height of the liquid in the tube. Q: Why does the liquid in the thermometer expand and contract when temperature changes? A: When the temperature is higher, particles of the liquid have greater kinetic energy, so they move about more and spread apart. This causes the liquid to expand. The opposite happens when the temperature is lower and particles of liquid have less kinetic energy. The particles move less and crowd closer together, causing the liquid to contract. "
temperature,T_4919,"The thermometer pictured in the Figure 1.1 measures temperature on two different scales: Celsius (C) and Fahrenheit (F). Although some scientists use the Celsius scale, the SI scale for measuring temperature is the Kelvin scale. If you live in the U.S., you are probably most familiar with the Fahrenheit scale. The Table 1.1 compares all three temperature scales. Each scale uses as reference points the freezing and boiling points of water. Notice that temperatures on the Kelvin scale are not given in degrees ( ). Scale Kelvin Celsius Fahrenheit Freezing Point of Water 273 K 0 C 32  F Boiling Point of Water 373 K 100  C 212  F Because all three temperature scales are frequently used, its useful to know how to convert temperatures from one scale to another. Its easy to convert temperatures between the Kelvin and Celsius scales. Each 1-degree change on the Kelvin scale is equal to a 1-degree change on the Celsius scale. Therefore, to convert a temperature from Celsius to Kelvin, just add 273 to the Celsius temperature. For example, 10  C equals 283 Kelvin. Q: How would you convert a temperature from Kelvin to Celsius? A: You would subtract 273 from the Kelvin temperature. For example, a temperature of 300 Kevin equals 27  C. Converting between Celsius and Fahrenheit is more complicated. The following conversion factors can be used: Celsius  Fahrenheit: ( C  1.8) + 32 =  F Fahrenheit  Celsius: ( F - 32)  1.8 =  C 3. Assume that the temperature outside is 293 Kelvin but youre familiar only with the Fahrenheit scale. Do you need to wear a hat and gloves when you go outside? To find out, convert the Kelvin temperature to Fahrenheit. (Hint: Convert the Kelvin temperature to Celsius first.) "
the nature of science,T_0001,"The scientific method is a set of steps that help us to answer questions. When we use logical steps and control the number of things that can be changed, we get better answers. As we test our ideas, we may come up with more questions. The basic sequence of steps followed in the scientific method is illustrated in Figure 1.1. "
the nature of science,T_0002,Asking a question is one really good way to begin to learn about the natural world. You might have seen something that makes you curious. You might want to know what to change to produce a better result. Lets say a farmer is having an erosion problem. She wants to keep more soil on her farm. The farmer learns that a farming method called no-till farming allows farmers to plant seeds without plowing the land. She wonders if planting seeds without plowing will reduce the erosion problem and help keep more soil on her farmland. Her question is this: Will using the no-till method of farming help me to lose less soil on my farm? (Figure 1.2). 
the nature of science,T_0003,"Before she begins, the farmer needs to learn more about this farming method. She can look up information in books and magazines in the library. She may also search the Internet. A good way for her to learn is to talk to people who have tried this way of farming. She can use all of this information to figure out how she is going to test her question about no-till farming. Farming machines are shown in the Figure 1.3. "
the nature of science,T_0004,"After doing the research, the farmer will try to answer the question. She might think, If I dont plow my fields, I will lose less soil than if I do plow the fields. Plowing disrupts the soil and breaks up roots that help hold soil in place. This answer to her question is a hypothesis. A hypothesis is a reasonable explanation. A hypothesis can be tested. It may be the right answer, it may be a wrong answer, but it must be testable. Once she has a hypothesis, the next step is to do experiments to test the hypothesis. A hypothesis can be proved or disproved by testing. If a hypothesis is repeatedly tested and shown to be true, then scientists call it a theory. "
the nature of science,T_0005,"When we design experiments, we choose just one thing to change. The thing we change is called the independent variable. In the example, the farmer chooses two fields and then changes only one thing between them. She changes how she plows her fields. One field will be tilled and one will not. Everything else will be the same on both fields: the type of crop she grows, the amount of water and fertilizer that she uses, and the slope of the fields she plants on. The fields should be facing the same direction to get about the same amount of sunlight. These are the experimental controls. If the farmer only changes how she plows her fields, she can see the impact of the one change. After the experiment is complete, scientists then measure the result. The farmer measures how much soil is lost from each field. This is the dependent variable. How much soil is lost from each field depends on the plowing method. "
the nature of science,T_0006,"During an experiment, a scientist collects data. The data might be measurements, like the farmer is taking in Figure labeled. Labeling helps the scientist to know what each number represents. A scientist may also write descriptions of what happened during the experiment. At the end of the experiment the scientist studies the data. The scientist may create a graph or drawing to show the data. If the scientist can picture the data the results may be easier to understand. Then it is easier to draw logical conclusions. Even if the scientist is really careful it is possible to make a mistake. One kind of mistake is with the equipment. For example, an electronic balance may always measure one gram high. To fix this, the balance should be adjusted. If it cant be adjusted, each measurement should be corrected. A mistake can come if a measurement is hard to make. For example, the scientist may stop a stopwatch too soon or too late. To fix this, the scientist should run the experiment many times and make many measurements. The average of the measurements will be the accurate answer. Sometimes the result from one experiment is very different from the other results. If one data point is really different, it may be thrown out. It is likely a mistake was made in that experiment. "
the nature of science,T_0007,"The scientist must next form a conclusion. The scientist must study all of the data. What statement best explains the data? Did the experiment prove the hypothesis? Sometimes an experiment shows that a hypothesis is correct. Other times the data disproves the hypothesis. Sometimes its not possible to tell. If there is no conclusion, the scientist may test the hypothesis again. This time he will use some different experiments. No matter what the experiment shows the scientist has learned something. Even a disproved hypothesis can lead to new questions. The farmer grows crops on the two fields for a season. She finds that 2.2 times as much soil was lost on the plowed field as compared to the unplowed field. She concludes that her hypothesis was correct. The farmer also notices some other differences in the two plots. The plants in the no-till plots are taller. The soil moisture seems higher. She decides to repeat the experiment. This time she will measure soil moisture, plant growth, and the total amount of water the plants consume. From now on she will use no-till methods of farming. She will also research other factors that may reduce soil erosion. "
the nature of science,T_0008,"When scientists have the data and conclusions, they write a paper. They publish their paper in a scientific journal. A journal is a magazine for the scientists who are interested in a certain field. Before the paper is printed, other scientists look at it to try to find mistakes. They see if the conclusions follow from the data. This is called peer review. If the paper is sound it is printed in the journal. Other papers are published on the same topic in the journal. The evidence for or against a hypothesis is discussed by many scientists. Sometimes a hypothesis is repeatedly shown to be true and never shown to be false. The hypothesis then becomes a theory. Sometimes people say they have a theory when what they have is a hypothesis. In science, a theory has been repeatedly shown to be true. A theory is supported by many observations. However, a theory may be disproved if conflicting data is discovered. Many important theories have been shown to be true by many observations and experiments and are extremely unlikely to be disproved. These include the theory of plate tectonics and the theory of evolution. "
the nature of science,T_0009,"Scientists use models to help them understand and explain ideas. Models explain objects or systems in a more simple way. Models often only show only a part of a system. The real situation is more complicated. Models help scientists to make predictions about complex systems. Some models are something that you can see or touch. Other types of models use an idea or numbers. Each type is useful in certain ways. Scientists create models with computers. Computers can handle enormous amounts of data. This can more accu- rately represent the real situation. For example, Earths climate depends on an enormous number of factors. Climate models can predict how climate will change as certain gases are added to the atmosphere. To test how good a model is, scientists might start a test run at a time in the past. If the model can predict the present it is probably a good model. It is more likely to be accurate when predicting the future. "
the nature of science,T_0010,"A physical model is a representation of something using objects. It can be three-dimensional, like a globe. It can also be a two-dimensional drawing or diagram. Models are usually smaller and simpler than the real object. They most likely leave out some parts, but contain the important parts. In a good model the parts are made or drawn to scale. Physical models allow us to see, feel and move their parts. This allows us to better understand the real system. An example of a physical model is a drawing of the layers of Earth (Figure 1.5). A drawing helps us to understand the structure of the planet. Yet there are many differences between a drawing and the real thing. The size of a model is much smaller, for example. A drawing also doesnt give good idea of how substances move. Arrows showing the direction the material moves can help. A physical model is very useful but it cant explain the real Earth perfectly. "
the nature of science,T_0011,"Some models are based on an idea that helps scientists explain something. A good idea explains all the known facts. An example is how Earth got its Moon. A Mars-sized planet hit Earth and rocky material broke off of both bodies (Figure 1.6). This material orbited Earth and then came together to form the Moon. This is a model of something that happened billions of years ago. It brings together many facts known from our studies of the Moons surface. It accounts for the chemical makeup of rocks from the Moon, Earth, and meteorites. The physical properties of Earth and Moon figure in as well. Not all known data fits this model, but much does. There is also more information that we simply dont yet know. "
the nature of science,T_0012,"Models may use formulas or equations to describe something. Sometimes math may be the only way to describe it. For example, equations help scientists to explain what happened in the early days of the universe. The universe formed so long ago that math is the only way to describe it. A climate model includes lots of numbers, including temperature readings, ice density, snowfall levels, and humidity. These numbers are put into equations to make a model. The results are used to predict future climate. For example, if there are more clouds, does global temperature go up or down? Models are not perfect because they are simple versions of the real situation. Even so, these models are very useful to scientists. These days, models of complex things are made on computers. "
the nature of science,T_0013,"Accidents happen from time to time in everyday life. Since science involves an adventure into the unknown, it is natural that accidents can happen. Therefore, we must be careful and use proper equipment to prevent accidents (Figure 1.7). We must also be sure to treat any injury or accident appropriately. "
the nature of science,T_0014,"If you work in the science lab, you may come across dangerous materials or situations. Sharp objects, chemicals, heat, and electricity are all used at times in science laboratories. With proper protection and precautions, almost all accidents can be prevented (Figure 1.8). If an accident happens, it can be dealt with appropriately. Below is a list of safety guidelines to follow when doing labs: Follow directions at all times. A science lab is not a play area. Be sure to obey all safety guidelines given in lab instructions or by the lab supervisor. Be sure to use the correct amount of each material. Tie back long hair. Wear closed shoes with flat heels. Shirts should have no hanging sleeves, hoods, or drawstrings. Use gloves, goggles, or safety aprons as instructed. Be very careful when you use sharp or pointed objects, such as knives. Clean up broken glass quickly with a dust pan and broom. Never touch broken glass with your bare hands. Never eat or drink in the science lab. Table tops and counters could have dangerous substances on them. Keep your work area neat and clean. A messy work area can lead to spills and breakage. Completely clean materials like test tubes and beakers. Leftover substances could interact with other sub- stances in future experiments. If you are using flames or heat plates, be careful when you reach. Be sure your arms and hair are kept far away from heat sources. Use electrical appliances and burners as instructed. Know how to use an eye wash station, fire blanket, fire extinguisher, and first aid kit. Alert the lab supervisor if anything unusual occurs. Fill out an accident report if someone is hurt. The lab supervisor must know if any materials are damaged or discarded. "
the nature of science,T_0015,"Many Earth science investigations are conducted in the field (Figure 1.9). Field work needs some additional precautions: Be sure to wear appropriate clothing. Hiking requires boots, long pants, and protection from the Sun, for example. Bring sufficient supplies like food and water, even for a short trip. Dehydration can occur rapidly. Take along first aid supplies. Let others know where you are going, what you will be doing, and when you will be returning. Take a map with you if you dont know the area and leave a copy of the map with someone at home. Try to have access to emergency services and some way to communicate. Beware that cell phones may not have coverage in all locations. Be sure that you are accompanied by a person familiar with the area or is familiar with field work. "
erosion and deposition by wind,T_0045,"Dust storms like the one in Figure 10.20 are more common in dry climates. The soil is dried out and dusty. Plants may be few and far between. Dry, bare soil is more easily blown away by the wind than wetter soil or soil held in place by plant roots. "
erosion and deposition by wind,T_0046,"Like flowing water, wind picks up and transports particles. Wind carries particles of different sizes in the same ways that water carries them. You can see this in Figure 10.21. Tiny particles, such as clay and silt, move by suspension. They hang in the air, sometimes for days. They may be carried great distances and rise high above the ground. Larger particles, such as sand, move by saltation. The wind blows them in short hops. They stay close to the ground. Particles larger than sand move by traction. The wind rolls or pushes them over the surface. They stay on the ground. "
erosion and deposition by wind,T_0047,Did you ever see workers sandblasting a building to clean it? Sand is blown onto the surface to scour away dirt and debris. Wind-blown sand has the same effect. It scours and polishes rocks and other surfaces. Wind-blown sand may carve rocks into interesting shapes. You can see an example in Figure 10.22. This form of erosion is called abrasion. It occurs any time rough sediments are blown or dragged over surfaces. Can you think of other ways abrasion might occur? 
erosion and deposition by wind,T_0048,"Like water, when wind slows down it drops the sediment its carrying. This often happens when the wind has to move over or around an obstacle. A rock or tree may cause wind to slow down. As the wind slows, it deposits the largest particles first. Different types of deposits form depending on the size of the particles deposited. "
erosion and deposition by wind,T_0049,"When the wind deposits sand, it forms small hills of sand. These hills are called sand dunes. For sand dunes to form, there must be plenty of sand and wind. Sand dunes are found mainly in deserts and on beaches. You can see examples of sand dunes in Figure 10.23. "
erosion and deposition by wind,T_0050,"What causes a sand dune to form? It starts with an obstacle, such as a rock. The obstacle causes the wind to slow down. The wind then drops some of its sand. As more sand is deposited, the dune gets bigger. The dune becomes the obstacle that slows the wind and causes it to drop its sand. The hill takes on the typical shape of a sand dune, shown in Figure 10.24. "
erosion and deposition by wind,T_0051,"Once a sand dune forms, it may slowly migrate over the land. The wind moves grains of sand up the gently sloping side of the dune. This is done by saltation. When the sand grains reach the top of the dune, they slip down the steeper side. The grains are pulled by gravity. The constant movement of sand up and over the dune causes the dune to move along the ground. It always moves in the same direction that the wind usually blows. Can you explain why? "
erosion and deposition by wind,T_0052,"When the wind drops fine particles of silt and clay, it forms deposits called loess. Loess deposits form vertical cliffs. Loess can become a thick, rich soil. Thats why loess deposits are used for farming in many parts of the world. You can see an example of loess in Figure 10.25. "
erosion and deposition by wind,T_0053,"Its very important to control wind erosion of soil. Good soil is a precious resource that takes a long time to form. Covering soil with plants is one way to reduce wind erosion. Plants and their roots help hold the soil in place. They also help the soil retain water so it is less likely to blow away. Planting rows of trees around fields is another way to reduce wind erosion. The trees slow down the wind, so it doesnt cause as much erosion. Fences like the one in Figure 10.26 serve the same purpose. The fence in the figure is preventing erosion and migration of sand dunes on a beach. "
history of earths life forms,T_0113,There are over 1 million species of plants and animals living on Earth today. Scientists think that there are millions more that have not yet been discovered. 
history of earths life forms,T_0114,"Each organism has the ability to survive in a specific environment. Dry desert environments are difficult to live in. Desert plants have special stems and leaves to conserve water. Animals have other ways to live in the desert. The Namib Desert receives only 1.5 inches of rainfall each year. The Namib Desert beetle lives there. How do the beetles get enough water to survive? Early morning fog deposits water droplets. The droplets collect on a beetles wings and back. The beetle tilts its rear end up. When the droplet is heavy enough, it slides forward. It lands in the beetles mouth. There are many other environments that need unique approaches for survival (Figure 12.10). "
history of earths life forms,T_0115,"Organisms must be able to get food and avoid being food. Hummingbirds have long, thin beaks that help them drink nectar from flowers. Some flowers are tubular to fit hummingbird beaks. The battle between needing food and being food plays out in the drama between lions and zebras. When a herd of zebras senses a lion, the animals run away. The zebras dark stripes confuse the lions. It becomes hard for them to focus on just one zebra. The zebras may get away. But lions are swift and agile. A lion may be able to get a zebra, maybe one thats old or sick. "
history of earths life forms,T_0116,"Every organism is different from every other organism. Every organisms genes are different, too. "
history of earths life forms,T_0117,"There are variations in the traits of a population. For example, there are lots of variations in the color of human hair. Hair can be blonde, brown, black, or even red. Hair color is a trait determined by genes. "
history of earths life forms,T_0118,"At some point, the variation probably came from a mutation. A mutation is a random change in an organisms genes. Mutations are natural. Some are harmful, but many are neutral. If the trait from the mutation is beneficial, that organism may have a better chance to survive. An organism that survives is likely to have offspring. If it does, it may pass the mutation on to its offspring. The offspring may be more likely to survive. "
history of earths life forms,T_0119,"Some of the characteristics an organism has may help it survive. These characteristics are called adaptations. Some adaptations are better than others. Adaptations develop this way. Think about a population of oak trees. Imagine that a fungus has arrived from Asia to North America. Most of the North American are killed by the fungus. But a few oak trees have a mutation that allows them to survive the fungus. Those oak trees are better adapted to the new environment than the others. Those trees have a better chance of surviving. They will probably reproduce. The trees may pass on the favorable mutation to their offspring. The other trees will die. Eventually, the population of oak trees will change. Most of the trees will have the trait to survive the fungus. This is an adaptation. Over time, traits that help an organism survive become more common. Traits that hinder survival eventually disappear. "
history of earths life forms,T_0120,"Adaptations in a species add up. If the environment is stable, the species wont change. But if the environment is changing, the species will need to adapt. Many adaptations may be necessary. In time, the species may change a lot. The descendants will be very different from their ancestors. They may even become a new species. This process is called evolution. Evolution happens as a species changes over time. Organisms alive today evolved from earlier life forms. We can learn about this from fossils. For example, horse fossils from 60 million years ago are very different from modern horses. Ancient horses were much smaller than they are today (Figure 12.12). The horses teeth and hooves have also changed. The horses evolved because of changes in their environment. "
history of earths life forms,T_0121,"Most of the organisms that once lived on Earth are now extinct. Earths environment has changed many times. Many organisms could not adapt to the changes. They died out. The organisms that did survive passed traits on to their offspring. The changes added up, eventually producing the species we see today. We study fossils to see the organisms that lived at certain times. We can see how those organisms changed with time. We can see how they evolved. "
history of earths life forms,T_0122,"The Phanerozoic Eon is divided into three eras the Paleozoic, the Mesozoic, and the Cenozoic (Table 12.1). They span from about 540 million years ago to the present. We live now in the Cenozoic Era. Earths climate changed numerous times during the Phanerozoic Eon. Just before the beginning of the Phanerozoic Eon, much of the Earth was covered with glaciers. As the Phanerozoic Eon began, the climate became a warm and humid tropical climate. During the Phanerozoic, Earths climate has gone through at least 4 major cycles between times of cold glaciers and times of warm tropical seas. Some organisms survived environmental changes in the climate; others became extinct when the climate changed beyond their capacity to cope with it. "
history of earths life forms,T_0123,"The warm, humid climate of the early Cambrian allowed life to expand and diversify. This brought the Cambrian Explosion. Life exploded both in diversity and in quantity! By the beginning of the Paleozoic, organisms had developed shells. Shells could hold their soft tissues together. They could protect the organisms from predators and from drying out. Some organisms evolved external skeletons, called exoskeletons. Organisms with hard parts also make good fossils. Fossils from the Cambrian are much more abundant than fossils from the Precambrian. There was much more diversity, so complex ecosystems could develop (Figure 12.14). All of this was in the seas. "
history of earths life forms,T_0124,"Paleozoic life was most diverse in the oceans. Paleozoic seas were full of worms, snails, clams, trilobites, sponges, and brachiopods. Organisms with shells were common. The first fish were simple, armored, jawless fish. Fish have internal skeletons. Some, like sharks, skates, and rays, have skeletons of cartilage. More advanced fish have skeletons of bones. Fish evolved jaws and many other adaptations for ocean life. Figure 12.13 shows some of the diversity of Earths oceans. "
history of earths life forms,T_0125,"An organism that lives in water is supported by the water. It does not need strong support structures. It also does not need to be protected against drying out. This is not true of land. Moving from the seas to land required many adaptations. Algae had covered moist land areas for millions of years. By about 450 million years ago, plants began to appear on land. Once there were land plants, animals had a source of food and shelter. To move to land, animals needed strong skeletons. They needed protection from drying out. They needed to be able to breathe air. Eventually they had skeletons, lungs and the other the adaptations they needed moved onto the land. One group of fish evolved into amphibians. Insects and spiders were already land dwellers by the time amphibians appeared. "
history of earths life forms,T_0126,"The Mesozoic Era is the age of reptiles. Mostly we think of it as the age of dinosaurs. Earth was populated by an enormous diversity of reptiles. Some were small and some were tremendously large. Some were peaceful plant eaters. Some were extremely frightening meat eaters. Some dinosaurs developed protection, such as horns, spikes, tail clubs, and shielding plates. These adaptations were defense against active predators. Most dinosaurs lived on land. Still, pterosaurs flew the skies. Plesiosaurs and ichthyosaurs swam in the oceans (Figure 12.15). Feathered dinosaurs gave rise to birds. "
history of earths life forms,T_0127,"The Cenozoic Era is the age of mammals. The Cenozoic began with the extinction of every land creature larger than a dog. The most famous victims were the dinosaurs. Mammals have the ability to regulate body temperature. This is an advantage, as Earths climate went through sudden and dramatic changes. Mastodons, saber tooth tigers, hoofed mammals, whales, primates and eventually humans all lived during the Cenozoic Era (Figure 12.16). Table 12.1 shows some of the life forms that developed during the Phanerozoic Eon. Life gradually became more diverse and new species appeared. Most modern organisms evolved from species that are now extinct. Era Cenozoic Millions of Years Ago 0.2 (200,000 years ago) 35 Mesozoic 130 150 200 Major Forms of Life First humans First grasses; grasslands begin to dominate the land First plants with flowers First birds on Earth First mammals on Earth Paleozoic 300 360 400 475 First reptiles on Earth First amphibians on Earth First insects on Earth First plants and fungi begin growing on land First fish on Earth 500 "
history of earths life forms,T_0128,"The eras of the Phanerozoic Eon are separated by mass extinctions. During these events, large numbers of organisms became extinct very rapidly. There have been several extinctions in the Phanerozoic but two stand out more than the others. "
history of earths life forms,T_0129,"Between the Paleozoic Era and the Mesozoic Era was the largest mass extinction known. At the end of the Permian, nearly 95% of all marine species died off. In addition, 70% of land species became extinct. No one knows the cause of this extinction. Some scientists blame an asteroid impact. Other scientists think it was a gigantic volcanic eruption. "
history of earths life forms,T_0130,"The most famous mass extinction was 65 million years ago. Between the Mesozoic Era and the Cenozoic Era, about 50% of all animal species died off. This mass extinction is when the dinosaurs became extinct. Most scientists think that the extinction was caused by a giant meteorite that struck Earth. The impact heated the atmosphere until it became as hot as a kitchen oven. Animals roasted. Dust flew into the atmosphere and blocked sunlight for a year or more. This caused a deep freeze and ended photosynthesis. Sulfur from the impact mixed with water in the atmosphere. The result was acid rain. The rain dissolved the shells of the tiny marine plankton that form the base of the food chain. With little food being produced, animals starved. "
air movement,T_0240,"Air movement takes place in the troposphere. This is the lowest layer of the atmosphere. Air moves because of differences in heating. These differences create convection currents and winds. Figure 15.19 shows how this happens. Air in the troposphere is warmer near the ground. The warm air rises because it is light. The light, rising air creates an area of low air pressure at the surface. The rising air cools as it reaches the top of the troposphere. The air gets denser, so it sinks to the surface. The sinking, heavy air creates an area of high air pressure near the ground. Air always flows from an area of higher pressure to an area of lower pressure. Air flowing over Earths surface is called wind. The greater the difference in pressure, the stronger the wind blows. "
air movement,T_0241,"Local winds are winds that blow over a limited area. They are influenced by local geography. Nearness to an ocean, lake or mountain range can affect local winds. Some examples are found below. "
air movement,T_0242,Ocean water is slower to warm up and cool down than land. So the sea surface is cooler than the land in the daytime. It is also cooler than the land in the summer. The opposite is also true. The water stays warmer than the land during the night and the winter. These differences in heating cause local winds known as land and sea breezes. Land and sea breezes are illustrated in Figure 15.20. A sea breeze blows from sea to land during the day or in summer. Thats when air over the land is warmer than air over the water. The warm air rises. Cool air from over the water flows in to take its place. A land breeze blows from land to sea during the night or in winter. Thats when air over the water is warmer than air over the land. The warm air rises. Cool air from the land flows out to take its place. 
air movement,T_0243,"Monsoons are like land and sea breezes, but on a larger scale. They occur because of seasonal changes in the temperature of land and water. In the winter, they blow from land to water. In the summer, they blow from water to land. In regions that experience monsoons, the seawater offshore is extremely warm. The hot air absorbs a lot of the moisture and carries it over the land. Summer monsoons bring heavy rains on land. Monsoons occur in several places around the globe. The most important monsoon in the world is in southern Asia, as shown in Figure 15.21. These monsoons are important because they carry water to the many people who live there. "
air movement,T_0244,"Global winds are winds that occur in belts that go all around the planet. You can see them in Figure 15.22. Like local winds, global winds are caused by unequal heating of the atmosphere. "
air movement,T_0245,"Earth is hottest at the equator and gets cooler toward the poles. The differences in heating create huge convection currents in the troposphere. At the equator, for example, warm air rises up to the tropopause. It cant rise any higher, so it flows north or south. By the time the moving air reaches 30 N or S latitude, it has cooled. The cool air sinks to the surface. Then it flows over the surface back to the equator. Other global winds occur in much the same way. There are three enormous convection cells north of the equator and three south of the equator. "
air movement,T_0246,"Earth is spinning as air moves over its surface. This causes the Coriolis effect. Winds blow on a diagonal over the surface, instead of due north or south. From which direction do the northern trade winds blow? Without Coriolis Effect the global winds would blow north to south or south to north. But Coriolis makes them blow northeast to southwest or the reverse in the Northern Hemisphere. The winds blow northwest to southeast or the reverse in the southern hemisphere. The wind belts have names. The Trade Winds are nearest the equator. The next belt is the westerlies. Finally are the polar easterlies. The names are the same in both hemispheres. "
air movement,T_0247,"Jet streams are fast-moving air currents high in the troposphere. They are also the result of unequal heating of the atmosphere. Jet streams circle the planet, mainly from west to east. The strongest jet streams are the polar jets. The northern polar jet is shown in Figure 15.23. "
changing weather,T_0262,"An air mass is a large body of air that has about the same conditions throughout. For example, an air mass might have cold dry air. Another air mass might have warm moist air. The conditions in an air mass depend on where the air mass formed. "
changing weather,T_0263,"Most air masses form over polar or tropical regions. They may form over continents or oceans. Air masses are moist if they form over oceans. They are dry if they form over continents. Air masses that form over oceans are called maritime air masses. Those that form over continents are called continental air masses. Figure 16.6 shows air masses that form over or near North America. An air mass takes on the conditions of the area where it forms. For example, a continental polar air mass has cold dry air. A maritime polar air mass has cold moist air. Which air masses have warm moist air? Where do they form? "
changing weather,T_0264,"When a new air mass goes over a region it brings its characteristics to the region. This may change the areas temperature and humidity. Moving air masses cause the weather to change when they contact different conditions. For example, a warm air mass moving over cold ground may cause an inversion. Why do air masses move? Winds and jet streams push them along. Cold air masses tend to move toward the equator. Warm air masses tend to move toward the poles. Coriolis effect causes them to move on a diagonal. Many air masses move toward the northeast over the U.S. This is the same direction that global winds blow. "
changing weather,T_0265,"When cold air masses move south from the poles, they run into warm air masses moving north from the tropics. The boundary between two air masses is called a front. Air masses usually dont mix at a front. The differences in temperature and pressure cause clouds and precipitation. Types of fronts include cold, warm, occluded, and stationary fronts. "
changing weather,T_0266,"A cold front occurs when a cold air mass runs into a warm air mass. This is shown in Figure 16.7. The cold air mass moves faster than the warm air mass and lifts the warm air mass out of its way. As the warm air rises, its water vapor condenses. Clouds form, and precipitation falls. If the warm air is very humid, precipitation can be heavy. Temperature and pressure differences between the two air masses cause winds. Winds may be very strong along a cold front. As the fast-moving cold air mass keeps advancing, so does the cold front. Cold fronts often bring sudden changes in the weather. There may be a thin line of storms right at the front that moves as it moves. In the spring and summer, these storms may be thunderstorms and tornadoes. In the late fall and winter, snow storms may occur. After a cold front passes, the cold air mass behind it brings cooler temperatures. The air is likely to be less humid as well. Can you explain why? "
changing weather,T_0267,"When a warm air mass runs into a cold air mass it creates a warm front. This is shown in Figure 16.8. The warm air mass is moving faster than the cold air mass, so it flows up over the cold air mass. As the warm air rises, it cools, resulting in clouds and sometimes light precipitation. Warm fronts move slowly and cover a wide area. After a warm front passes, the warm air mass behind it brings warmer temperatures. The warm air is also likely to be more humid. "
changing weather,T_0268,"With an occluded front, a warm air mass becomes trapped between two cold air masses. The warm air is lifted up above the cold air as in Figure 16.9. Cloudy weather and precipitation along the front are typical. "
changing weather,T_0269,Sometimes two air masses stop moving when they meet. These stalled air masses create a stationary front. Such a front may bring clouds and precipitation to the same area for many days. 
changing weather,T_0270,"Cold air is dense, so it sinks. This creates a center of high pressure. Warm air is less dense so it rises. This creates a center of low pressure. Air always flows from higher to lower pressure. As the air flows, Earths surface rotates below it causing Coriolis effect. So while the wind blows into the low pressure, it revolves in a circular pattern. This wind pattern forms a cyclone. The same happens while the wind blows out of a high pressure. This forms an anticyclone. Both are shown in Figure 16.10. A cyclone is a system of winds that rotates around a center of low pressure. Cyclones bring cloudy, wet weather. An anticyclone is a system of winds that rotates around a center of high pressure. Anticyclones bring fair, dry weather. "
storms,T_0271,"A storm is an episode of severe weather caused by a major disturbance in the atmosphere. Storms can vary a lot in the time they last and in how severe they are. A storm may last for less than an hour or for more than a week. It may affect just a few square kilometers or thousands. Some storms are harmless and some are disastrous. The size and strength of a storm depends on the amount of energy in the atmosphere. Greater differences in temperature and air pressure produce stronger storms. Types of storms include thunderstorms, tornadoes, hurricanes, and winter storms such as blizzards. "
storms,T_0272,"Thunderstorms are are known for their heavy rains and lightning. In strong thunderstorms, hail and high winds are also likely. Thunderstorms are very common. Worldwide, there are about 14 million of them each year! In the U.S., they are most common and strongest in the Midwest. "
storms,T_0273,"Thunderstorms occur when the air is very warm and humid. The warm air rises rapidly to create strong updrafts. When the rising air cools, its water vapor condenses. The updrafts create tall cumulonimbus clouds called thunder- heads. You can see one in Figure 16.12. "
storms,T_0274,"During a thunderstorm, some parts of a thunderhead become negatively charged. Other parts become positively charged. The difference in charge creates lightning. Lightning is a huge release of electricity. Lightning can jump between oppositely charged parts of the same cloud, between one cloud and another, or between a cloud and the ground. You can see lightning in Figure 16.13. Lightning blasts the air with energy. The air heats and expands so quickly that it explodes. This creates the loud sound of thunder. Do you know why you always hear the boom of thunder after you see the flash of lightning? Its because light travels faster than sound. If you count the seconds between seeing lightning and hearing thunder, you can estimate how far away the lightning was. A lapse of 5 seconds is equal to about a mile. "
storms,T_0275,"Severe thunderstorms have a lot of energy and strong winds. This allows them to produce tornadoes. A tornado is a funnel-shaped cloud of whirling high winds. You can see a tornado in Figure 16.14. The funnel moves along the ground, destroying everything in its path. As it moves it loses energy. Before this happens it may have gone up to 25 kilometers (16 miles). Fortunately, tornadoes are narrow. They may be only 150 meters (500 feet) wide. "
storms,T_0276,"The winds of a tornado can reach very high speeds. The faster the winds blow, the greater the damage they cause. Wind speed and damage are used to classify tornadoes. Table 16.1 shows how. F Scale F0 (km/hr) 64-116 (mph) 40-72 F1 117-180 73-112 Damage Light - tree branches fall and chimneys may col- lapse Moderate - mobile homes, autos pushed aside F Scale F2 (km/hr) 181-253 (mph) 113-157 F3 254-332 158-206 F4 333-419 207-260 F5 420-512 261-318 F6 >512 >318 Damage Considerable - roofs torn off houses, large trees up- rooted Severe - houses torn apart, trees uprooted, cars lifted Devastating - houses lev- eled, cars thrown Incredible - structures fly, cars become missiles Maximum tornado wind speed "
storms,T_0277,Look at the map in Figure 16.15. It shows where the greatest number of tornadoes occur in the U.S. Tornadoes can happen almost anywhere in the U.S. but only this area is called tornado alley. Why do so many tornadoes occur here? This is where warm air masses from the south run into cold air masses from the north. 
storms,T_0278,"Tornadoes may also come from hurricanes. A hurricane is an enormous storm with high winds and heavy rains. Hurricanes may be hundreds of kilometers wide. They may travel for thousands of kilometers. The storms wind speeds may be greater than 251 kilometers (156 miles) per hour. Hurricanes develop from tropical cyclones. Hurricanes form over warm very ocean water. This water gives them their energy. As long as a hurricane stays over the warm ocean, it keeps growing stronger. However, if it goes ashore or moves over cooler water, it is cut off from the hot water energy. The storm then loses strength and slowly fades away. "
storms,T_0279,At the center of a hurricane is a small area where the air is calm and clear. This is the eye of the hurricane. The eye forms at the low-pressure center of the hurricane. You can see the eye of a hurricane in Figure 16.16. 
storms,T_0280,"Like tornadoes, hurricanes are classified on the basis of wind speed and damage. Table 16.2 shows how. Category 1 (weak) Kph 119-153 Mph 74-95 2 (moderate) 154-177 96-110 3 (strong) 178-209 111-130 Damage Above normal; no real damage to structures Some roofing, door, and window damage, consid- erable damage to vegeta- tion, mobile homes, and piers Some buildings damaged; mobile homes destroyed Category 4 (very strong) Kph 210-251 Mph 131-156 5 (devastating) >251 >156 Damage Complete roof failure on small residences; major erosion of beach areas; major damage to lower floors of structures near shore Complete roof failure on many residences and in- dustrial buildings; some complete building failures "
storms,T_0281,"Some of the damage from a hurricane is caused by storm surge. Storm surge is very high water located in the low pressure eye of the hurricane. The very low pressure of the eye allows the water level to rise above normal sea level. Storm surge can cause flooding when it reaches land. You can see this in Figure 16.17. High winds do a great deal of damage in hurricanes. High winds can also create very big waves. If the large waves are atop a storm surge, the high water can flood the shore. If the storm happens to occur at high tide, the water will rise even higher. "
storms,T_0282,"Like hurricanes, winter storms develop from cyclones. But in the case of winter storms, the cyclones form at higher latitudes. In North America, cyclones often form when the jet stream dips south in the winter. This lets dry polar air pour south. At the same time, warm moist air from the Gulf of Mexico flows north. When the two air masses meet, the differences in temperature and pressure cause strong winds and heavy precipitation. Two types of winter storms that occur in the U.S. are blizzards and lake-effect snow storms. "
storms,T_0283,"A blizzard is a snow storm that has high winds. To be called a blizzard, a storm must have winds greater than 56 kilometers (35 miles) per hour and visibility of 14 mile or less because of wind-blown snow. You can see a blizzard in Figure 16.18. Blizzards are dangerous storms. The wind may blow the snow into deep drifts. Along with the poor visibility, the snow drifts make driving risky. The wind also makes cold temperatures more dangerous. The greater the wind speed, the higher the windchill. Windchill is what the temperature feels like when the wind is taken into account. It depends on air temperature and wind speed, as you can see in Figure 16.19. Higher windchill will cause a person to suffer frostbite and other harmful effects of cold sooner than if the wind isnt blowing. "
storms,T_0284,"Some places receive very heavy snowfall just about every winter. If they are near a lake, they may be getting lake- effect snow. Figure 16.20 shows how lake-effect snow occurs. Winter winds pick up moisture as they pass over the relatively warm waters of a large lake. When the winds reach the cold land on the other side, the air cools. Since there was so much moisture in the air it can drop a lot of snow. More than 254 centimeters (100 inches) of snow may fall in a single lake-effect storm! "
weather forecasting,T_0285,"Weather is very difficult to predict. Thats because its very complex and many factors are involved. Slight changes in even one factor can cause a big change in the weather. Still, certain rules of thumb generally apply. These rules help meteorologists forecast the weather. For example, low pressure is likely to bring stormy weather. So if a center of low pressure is moving your way, you can expect a storm. "
weather forecasting,T_0286,Predicting the weather requires a lot of weather data. Technology is used to gather the data and computers are used to analyze the data. Using this information gives meteorologists the best chance of predicting the weather. 
weather forecasting,T_0287,"Weather instruments measure weather conditions. One of the most important conditions is air pressure, which is measured with a barometer. Figure 16.23 shows how a barometer works. There are also a number of other commonly used weather instruments (see Figure 16.24): A thermometer measures temperature. An anemometer measures wind speed. A rain gauge measures the amount of rain. A hygrometer measures humidity. A wind vane shows wind direction. A snow gauge measures the amount of snow. "
weather forecasting,T_0288,"Weather instruments collect data from all over the world at thousands of weather stations. Many are on land but some float in the oceans on buoys. You can see what a weather station looks like in Figure 16.25. Theres probably at least one weather station near you. Other weather devices are needed to collect weather data in the atmosphere. They include weather balloons, satellites, and radar. You can read about them in Figure 16.25. Weather stations contain many instruments for measuring weather conditions. The weather balloon in Figure "
weather forecasting,T_0289,What do meteorologists do with all that weather data? They use it in weather models. The models analyze the data and predict the weather. The models require computers. Thats because so many measurements and calculations are involved. 
weather forecasting,T_0290,You may have seen weather maps like the one in Figure 16.26. A weather map shows weather conditions for a certain area. The map may show the actual weather on a given day or it may show the predicted weather for some time in the future. Some weather maps show many weather conditions. Others show a single condition. 
weather forecasting,T_0291,The weather map in Figure 16.26 shows air pressure. The lines on the map connect places that have the same air pressure. Air pressure is measured in a unit called the millibar. Isobars are the lines that connect the points with the same air pressure. The map also shows low- and high-pressure centers and fronts. Find the cold front on the map. This cold front is likely to move toward the northeast over the next couple of days. How could you use this information to predict what the weather will be on the East Coast? 
weather forecasting,T_0292,"Instead of air pressure, weather maps may show other weather conditions. For example, a temperature map might show the high and low temperatures of major cities. The map may have isotherms, lines that connect places with the same temperature. "
climate and its causes,T_0293,Climate is the average weather of a place over many years. It includes average temperatures. It also includes average precipitation. The timing of precipitation is part of climate as well. What determines the climate of a place? Latitude is the main factor. A nearby ocean or mountain range can also play a role. 
climate and its causes,T_0294,"Latitude is the distance north or south of the equator. Its measured in degrees, from 0 to 90 . Several climate factors vary with latitude. "
climate and its causes,T_0295,"Temperature changes with latitude. You can see how in Figure 17.2 At the equator, the Suns rays are most direct. Temperatures are highest. At higher latitudes, the Suns rays are less direct. The farther an area is from the equator, the lower is its temperature. At the poles, the Suns rays are least direct. Much of the area is covered with ice and snow, which reflect a lot of sunlight. Temperatures are lowest here. "
climate and its causes,T_0296,Global air currents affect precipitation. How they affect it varies with latitude. You can see why in Figure 17.3. 
climate and its causes,T_0297,"Global air currents cause global winds. Figure 17.4 shows the direction that these winds blow. Global winds are the prevailing, or usual, winds at a given latitude. The winds move air masses, which causes weather. The direction of prevailing winds determines which type of air mass usually moves over an area. For example, a west wind might bring warm moist air from over an ocean. An east wind might bring cold dry air from over a mountain range. Which wind prevails has a big effect on the climate. What if the prevailing winds are westerlies? What would the climate be like? "
climate and its causes,T_0298,"When a place is near an ocean, the water can have a big effect on the climate. "
climate and its causes,T_0299,"Even places at the same latitude may have different climates if one is on a coast and one is inland. On the coast, the climate is influenced by warm moist air from the ocean. A coastal climate is usually mild. Summers arent too hot, and winters arent too cold. Precipitation can be high due to the moisture in the air. Farther inland, the climate is influenced by cold or hot air from the land. This air may be dry because it comes from over land. An inland climate is usually more extreme. Winters may be very cold, and summers may be very hot. Precipitation can be low. "
climate and its causes,T_0300,Ocean currents carry warm or cold water throughout the worlds oceans. They help to even out the temperatures in the oceans. This also affects the temperature of the atmosphere and the climate around the world. Currents that are near shore have a direct impact on climate. They may make the climate much colder or warmer. You can see examples of this in Figure 17.5. 
climate and its causes,T_0301,Did you ever hike or drive up a mountain? Did you notice that it was cooler near the top? Climate is not just different on a mountain. Just having a mountain range nearby can affect the climate. 
climate and its causes,T_0302,"Air temperature falls at higher altitudes. You can see this in Figure 17.6. Why does this happen? Since air is less dense at higher altitudes, its molecules are spread farther apart than they are at sea level. These molecules have fewer collisions, so they produce less heat. Look at the mountain in Figure 17.7. The peak of Mount Kilimanjaro, Tanzania (Africa, 3 south latitude) is 6 kilometers (4 miles) above sea level. At 3 S its very close to the equator. At the bottom of the mountain, the temperature is high year round. How can you tell that its much cooler at the top? "
climate and its causes,T_0303,Mountains can also affect precipitation. Mountains and mountain ranges can cast a rain shadow. As winds rise up a mountain range the air cools and precipitation falls. On the other side of the range the air is dry and it sinks. So there is very little precipitation on the far (leeward) side of a mountain range. Figure 17.8 shows how this happens. 
ecosystems,T_0324,"An ecosystem is a group of living things and their environment. The word ecosystem is short for ecological system. Like any system, an ecosystem is a group of parts that work together. You can see examples of ecosystems in Figure 18.1. The forest pictured is a big ecosystem. Besides trees, what living things do you think are part of the forest ecosystem? The dead tree stump in the same forest is a small ecosystem. It includes plants, mosses, and fungi. It also includes insects and worms. "
ecosystems,T_0325,"Abiotic factors are the nonliving parts of ecosystems. They include air, sunlight, soil, water, and minerals. These are all things that are needed for life. They determine which living things  and how many of them  an ecosystem can support. Figure 18.2 shows an ecosystem and its abiotic factors. "
ecosystems,T_0326,Biotic factors are the living parts of ecosystems. They are the species of living things that reside together. 
ecosystems,T_0327,"A species is a unique type of organism. Members of a species can interbreed and produce offspring that can breed (they are fertile). Organisms that are not in the same species cannot do this. Examples of species include humans, lions, and redwood trees. Can you name other examples? Each species has a particular way of making a living. This is called its niche. You can see the niche of a lion in Figure 18.3. A lion makes its living by hunting and eating other animals. Each species also has a certain place where it is best suited to live. This is called its habitat. The lions habitat is a grassland. Why is a lion better off in a grassland than in a forest? "
ecosystems,T_0328,All the members of a species that live in the same area form a population. Many different species live together in an ecosystem. All their populations make up a community. What populations live together in the grassland in Figure 
ecosystems,T_0329,All ecosystems have living things that play the same basic roles. Some organisms must be producers. Others must be consumers. Decomposers are also important. 
ecosystems,T_0330,"Producers are living things that use energy to make food. Producers make food for themselves and other living things. There are two types of producers: By far the most common producers use the energy in sunlight to make food. This is called photosynthesis. Producers that photosynthesize include plants and algae. These organisms must live where there is plenty of sunlight. Which living things are producers in Figure 18.3? Other producers use the energy in chemicals to make food. This is called chemosynthesis. Only a very few producers are of this type, and all of them are microbes. These producers live deep under the ocean where there is no sunlight. You can see an example in Figure 18.4. "
ecosystems,T_0331,"Consumers cant make their own food. Consumers must eat producers or other consumers. Figure 18.5 lists the three main types of consumers. Which type are you? Consumers get their food in different ways Figure 18.6. Grazers feed on living organisms without killing them. A rabbit nibbles on leaves and a mosquito sucks a drop of blood. Predators, like lions, capture and kill animals for food. The animals they eat are called prey. Even some plants are consumers. Pitcher plants trap insects in their sticky fluid in their pitchers. The insects are their prey. Scavengers eat animals that are already dead. This hyena is eating the remains of a lions prey. Decomposers break down dead organisms and the wastes of living things. This dung beetle is rolling a ball of dung (animal waste) back to its nest. The beetle will use the dung to feed its young. The mushrooms pictured are growing on a dead log. They will slowly break it down. This releases its nutrients to the soil. "
ecosystems,T_0332,"All living things need energy. They need it to power the processes of life. For example, it takes energy to grow. It also takes energy to produce offspring. In fact, it takes energy just to stay alive. Remember that energy cant be created or destroyed. It can only change form. Energy changes form as it moves through ecosystems. "
ecosystems,T_0333,"Most ecosystems get their energy from the Sun. Only producers can use sunlight to make usable energy. Producers convert the sunlight into chemical energy or food. Consumers get some of that energy when they eat producers. They also pass some of the energy on to other consumers when they are eaten. In this way, energy flows from one living thing to another. "
ecosystems,T_0334,"A food chain is a simple diagram that shows one way energy flows through an ecosystem. You can see an example of a food chain in Figure 18.7. Producers form the base of all food chains. The consumers that eat producers are called primary consumers. The consumers that eat primary consumers are secondary consumers. This chain can continue to multiple levels. At each level of a food chain, a lot of energy is lost. Only about 10 percent of the energy passes to the next level. Where does that energy go? Some energy is given off as heat. Some energy goes into animal wastes. Energy also goes into growing things that another consumer cant eat, like fur. Its because so much energy is lost that most food chains have just a few levels. Theres not enough energy left for higher levels. "
ecosystems,T_0335,Food chains are too simple to represent the real world. They dont show all the ways that energy flows through an ecosystem. A more complex diagram is called a food web. You can see an example in Figure 18.8. A food web consists of many overlapping food chains. Can you identify the food chains in the figure? How many food chains include the mouse? 
ecosystems,T_0336,"Living things need nonliving matter as well as energy. What do you think matter is used for? One thing is to build bodies. They also need it to carry out the processes of life. Any nonliving matter that living things need is called a nutrient. Carbon and nitrogen are examples of nutrients. Unlike energy, matter is recycled in ecosystems. You can see how in Figure 18.9. Decomposers release nutrients when they break down dead organisms. The nutrients are taken up by plants through their roots. The nutrients pass to primary consumers when they eat the plants. The nutrients pass to higher level consumers when they eat lower level consumers. When living things die, the cycle repeats. "
air masses,T_0914,"An air mass is a batch of air that has nearly the same temperature and humidity (Figure 1.1). An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days or longer, it picks up the distinct temperature and humidity characteristics of that region. "
air masses,T_0915,"Air masses form over a large area; they can be 1,600 km (1,000 miles) across and several kilometers thick. Air masses form primarily in high pressure zones, most commonly in polar and tropical regions. Temperate zones are ordinarily too unstable for air masses to form. Instead, air masses move across temperate zones, so the middle latitudes are prone to having interesting weather. The source regions of air masses found around the world. Symbols: (1) origin over a continent (c) or an ocean (m, for maritime); (2) arctic (A), polar (P,) tropical (T), and equatorial (E); (3) properties relative to the ground it moves over: k, for colder, w for warmer. What does an air mass with the symbol cPk mean? The symbol cPk is an air mass with a continental polar source region that is colder than the region it is now moving over. "
air masses,T_0916,"Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it. "
air masses,T_0917,"Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms. How would a moving air mass form an inversion? When a warmer air mass travels over colder ground, the bottom layer of air cools and, because of its high density, is trapped near the ground. "
air masses,T_0918,"In general, cold air masses tend to flow toward the Equator and warm air masses tend to flow toward the poles. This brings heat to cold areas and cools down areas that are warm. It is one of the many processes that act to balance out the planets temperatures. Click image to the left or use the URL below. URL: "
biological communities,T_0946,A population consists of all individuals of a single species that exist together at a given place and time. A species is a single type of organism that can interbreed and produce fertile offspring. All of the populations living together in the same area make up a community. 
biological communities,T_0947,"An ecosystem is made up of the living organisms in a community and the nonliving things, the physical and chemical factors, that they interact with. The living organisms within an ecosystem are its biotic factors (Figure 1.1). Living things include bacteria, algae, fungi, plants, and animals, including invertebrates, animals without backbones, and vertebrates, animals with backbones. (a) The horsetail Equisetum is a primitive plant. (b) Insects are among the many different types of invertebrates. (c) A giraffe is an example of a vertebrate. Physical and chemical features are abiotic factors. Abiotic factors include resources living organisms need, such as light, oxygen, water, carbon dioxide, good soil, and nitrogen, phosphorous, and other nutrients. Nutrients cycle through different parts of the ecosystem and can enter or leave the ecosystem at many points. Abiotic factors also include environmental features that are not materials or living things, such as living space and the right temperature range. Energy moves through an ecosystem in one direction. Click image to the left or use the URL below. URL: "
biological communities,T_0948,"Organisms must make a living, just like a lawyer or a ballet dancer. This means that each individual organism must acquire enough food energy to live and reproduce. A species way of making a living is called its niche. An example of a niche is making a living as a top carnivore, an animal that eats other animals, but is not eaten by any other animals (Figure 1.2). Every species fills a niche, and niches are almost always filled in an ecosystem. The top carnivore niche is filled by lions on the savanna. Click image to the left or use the URL below. URL: "
biological communities,T_0949,"An organisms habitat is where it lives (Figure 1.3). The important characteristics of a habitat include climate, the availability of food, water, and other resources, and other factors, such as weather. "
blizzards,T_0950,A blizzard is distinguished by certain conditions: Temperatures below -7 C (20 F); -12 C (10 F) for a severe blizzard. Winds greater than 56 kmh (35 mph); 72 kmh (45 mph) for a severe blizzard. Snow so heavy that visibility is 2/5 km (1/4 mile) or less for at least three hours; near zero visibility for a severe blizzard. 
blizzards,T_0951,"Blizzards happen across the middle latitudes and toward the poles, usually as part of a mid-latitude cyclone. Bliz- zards are most common in winter, when the jet stream has traveled south and a cold, northern air mass comes into contact with a warmer, semitropical air mass (Figure 1.2). The very strong winds develop because of the pressure gradient between the low-pressure storm and the higher pressure west of the storm. Snow produced by the storm gets caught in the winds and blows nearly horizontally. Blizzards can also produce sleet or freezing rain. A blizzard obscures the Capitol in Wash- ington, DC. Blizzard snows blanket the East Coast of the United States in February 2010. "
blizzards,T_0952,"In winter, a continental polar air mass travels down from Canada. As the frigid air travels across one of the Great Lakes, it warms and absorbs moisture. When the air mass reaches the leeward side of the lake, it is very unstable and it drops tremendous amounts of snow. This lake-effect snow falls on the snowiest metropolitan areas in the United States: Buffalo and Rochester, New York (Figure 1.3). Click image to the left or use the URL below. URL:  Frigid air travels across the Great Lakes and dumps lake-effect snow on the lee- ward side. "
branches of earth science,T_0953,"Geology is the study of the Earths solid material and structures and the processes that create them. Some ideas geologists might consider include how rocks and landforms are created or the composition of rocks, minerals, or various landforms. Geologists consider how natural processes create and destroy materials on Earth, and how humans can use Earth materials as resources, among other topics. Geologists study rocks in the field to learn what they can from them. "
branches of earth science,T_0954,"Oceanography is the study of everything in the ocean environment, which covers about 70% of the Earths surface. Recent technology has allowed people and probes to venture to the deepest parts of the ocean, but much of the ocean remains unexplored. Marine geologists learn about the rocks and geologic processes of the ocean basins. "
branches of earth science,T_0955,"Meteorology includes the study of weather patterns, clouds, hurricanes, and tornadoes. Using modern technology such as radars and satellites, meteorologists are getting more accurate at forecasting the weather all the time. Climatology is the study of the whole atmosphere, taking a long-range view. Climatologists can help us better understand how and why climate changes (Figure 1.2). Carbon dioxide released into the atmo- sphere is causing the global climate to change. "
branches of earth science,T_0956,"Environmental scientists study the effects people have on their environment, including the landscape, atmosphere, water, and living things. Climate change is part of climatology or environmental science. "
branches of earth science,T_0957,Astronomy is the study of outer space and the physical bodies beyond the Earth. Astronomers use telescopes to see things far beyond what the human eye can see. Astronomers help to design spacecraft that travel into space and send back information about faraway places or satellites (Figure 1.3). The Hubble Space Telescope. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: 
collecting weather data,T_1018,"To make a weather forecast, the conditions of the atmosphere must be known for that location and for the surrounding area. Temperature, air pressure, and other characteristics of the atmosphere must be measured and the data collected. "
collecting weather data,T_1019,"Thermometers measure temperature. In an old-style mercury thermometer, mercury is placed in a long, very narrow tube with a bulb. Because mercury is temperature sensitive, it expands when temperatures are high and contracts when they are low. A scale on the outside of the thermometer matches up with the air temperature. Some modern thermometers use a coiled strip composed of two kinds of metal, each of which conducts heat differently. As the temperature rises and falls, the coil unfolds or curls up tighter. Other modern thermometers measure infrared radiation or electrical resistance. Modern thermometers usually produce digital data that can be fed directly into a computer. "
collecting weather data,T_1020,"Meteorologists use barometers to measure air pressure. A barometer may contain water, air, or mercury, but like thermometers, barometers are now mostly digital. A change in barometric pressure indicates that a change in weather is coming. If air pressure rises, a high pressure cell is on the way and clear skies can be expected. If pressure falls, a low pressure cell is coming and will likely bring storm clouds. Barometric pressure data over a larger area can be used to identify pressure systems, fronts, and other weather systems. "
collecting weather data,T_1021,"Weather stations contain some type of thermometer and barometer. Other instruments measure different characteris- tics of the atmosphere, such as wind speed, wind direction, humidity, and amount of precipitation. These instruments are placed in various locations so that they can check the atmospheric characteristics of that location (Figure 1.1). Weather stations are located on land, the surface of the sea, and in orbit all around the world. According to the World Meteorological Organization, weather information is collected from 15 satellites, 100 stationary buoys, 600 drifting buoys, 3,000 aircraft, 7,300 ships, and some 10,000 land-based stations. "
collecting weather data,T_1022,"Radiosondes measure atmospheric characteristics, such as temperature, pressure, and humidity as they move through the air. Radiosondes in flight can be tracked to obtain wind speed and direction. Radiosondes use a radio to communicate the data they collect to a computer. Radiosondes are launched from about 800 sites around the globe twice daily to provide a profile of the atmosphere. Radiosondes can be dropped from a balloon or airplane to make measurements as they fall. This is done to monitor storms, for example, since they are dangerous places for airplanes to fly. "
collecting weather data,T_1023,"Radar stands for Radio Detection and Ranging (Figure 1.2). A transmitter sends out radio waves that bounce off the nearest object and then return to a receiver. Weather radar can sense many characteristics of precipitation: its location, motion, intensity, and the likelihood of future precipitation. Doppler radar can also track how fast the precipitation falls. Radar can outline the structure of a storm and can be used to estimate its possible effects. Radar view of a line of thunderstorms. "
collecting weather data,T_1024,"Weather satellites have been increasingly important sources of weather data since the first one was launched in 1952. Weather satellites are the best way to monitor large-scale systems, such as storms. Satellites are able to record long-term changes, such as the amount of ice cover over the Arctic Ocean in September each year. Weather satellites may observe all energy from all wavelengths in the electromagnetic spectrum. Visible light images record storms, clouds, fires, and smog. Infrared images record clouds, water and land temperatures, and features of the ocean, such as ocean currents (Figure 1.3). Click image to the left or use the URL below. URL:  Infrared data superimposed on a satellite image shows rainfall patterns in Hurricane Ernesto in 2006. "
development of theories,T_1052,"Scientists seek evidence that supports or refutes a hypothesis. If there is no significant evidence to refute the hypothesis and there is an enormous amount of evidence to support it, the idea is accepted. It may become a theory. A scientific theory is strongly supported by many different lines of evidence. A theory has no major inconsistencies. A theory must be constantly tested and revised. A theory provides a model of reality that is simpler than the phenomenon itself. Scientists can use a theory to offer reliable explanations and make accurate predictions. A theory can be revised or thrown out if conflicting data is discovered. However, a longstanding theory that has lots of evidence to back it up is less likely to be overthrown than a newer theory. But science does not prove anything beyond a shadow of a doubt. Click image to the left or use the URL below. URL: "
development of theories,T_1053,"Many people think that any idea that is completely accepted in science is a law. In science, a law is something that always applies under the same conditions. If you hold something above the ground and let go it will fall. This phenomenon is recognized by the law of gravity. A law explains a simpler phenomenon or set of phenomena than does a theory. But a theory tells you why something happens and a law only tells you that it happens. Amazingly, scientific laws may have exceptions. Even the law of gravity does not always hold! If water is in an enclosed space between a hillside and a glacier, the weight of the glacier at the bottom of the hill may force the water to flow uphill - against gravity! That doesnt mean that gravity is not a law. A law always applies under the right circumstances. Click image to the left or use the URL below. URL: "
effect of continental position on climate,T_1125,"When a particular location is near an ocean or large lake, the body of water plays an extremely important role in affecting the regions climate. A maritime climate is strongly influenced by the nearby sea. Temperatures vary a relatively small amount seasonally and daily. For a location to have a true maritime climate, the winds must most frequently come off the sea. A continental climate is more extreme, with greater temperature differences between day and night and between summer and winter. The oceans influence in moderating climate can be seen in the following temperature comparisons. Each of these cities is located at 37o N latitude, within the westerly winds (Figure 1.1). The climate of San Francisco is influenced by the cool California current and offshore upwelling. Wichita has a more extreme continental climate. Virginia Beach, though, is near the Atlantic Ocean. Why is the climate there less influenced by the ocean than is the climate in San Francisco? Hint: Think about the direction the winds are going at that latitude. The weather in San Francisco comes from over the Pacific Ocean while much of the weather in Virginia comes from the continent. How does the ocean influence the climate of these three cities? "
effect of continental position on climate,T_1126,"The temperature of the water offshore influences the temperature of a coastal location, particularly if the winds come off the sea. The cool waters of the California Current bring cooler temperatures to the California coastal region. Coastal upwelling also brings cold, deep water up to the ocean surface off of California, which contributes to the cool coastal temperatures. Further north, in southern Alaska, the upwelling actually raises the temperature of the surrounding land because the ocean water is much warmer than the land. The important effect of the Gulf Stream on the climate of northern Europe is described in the chapter Water on Earth. "
effects of air pollution on the environment,T_1132,All air pollutants cause some damage to living creatures and the environment. Different types of pollutants cause different types of harm. 
effects of air pollution on the environment,T_1133,"Particulates reduce visibility. In the western United States, people can now ordinarily see only about 100 to 150 kilometers (60 to 90 miles), which is one-half to two-thirds the natural (pre-pollution) range on a clear day. In the East, people can only see about 40 to 60 kilometers (25-35 miles), about one-fifth the distance they could see without any air pollution (Figure 1.1). Particulates reduce the amount of sunshine that reaches the ground, which may reduce photosynthesis. Since particulates form the nucleus for raindrops, snowflakes, or other forms of precipitation, precipitation may increase Smog in New York City. when particulates are high. An increase in particles in the air seems to increase the number of raindrops, but often decreases their size. By reducing sunshine, particulates can also alter air temperature as mentioned above. Imagine how much all of the sources of particulates combine to reduce temperatures. What affect might this have on global warming? "
effects of air pollution on the environment,T_1134,"Ozone damages some plants. Since ozone effects accumulate, plants that live a long time show the most damage. Some species of trees appear to be the most susceptible. If a forest contains ozone-sensitive trees, they may die out and be replaced by species that are not as easily harmed. This can change an entire ecosystem, because animals and plants may not be able to survive without the habitats created by the native trees. Some crop plants show ozone damage (Figure 1.2). When exposed to ozone, spinach leaves become spotted. Soybeans and other crops have reduced productivity. In developing nations, where getting every last bit of food energy out of the agricultural system is critical, any loss is keenly felt. "
effects of air pollution on the environment,T_1135,"Oxide air pollutants also damage the environment. NO2 is a toxic, orange-brown colored gas that gives air a distinctive orange color and an unpleasant odor. Nitrogen and sulfur-oxides in the atmosphere create acids that fall as acid rain. Lichen get a lot of their nutrients from the air so they may be good indicators of changes in the atmosphere such as increased nitrogen. In Yosemite National Park, this could change the ecosystem of the region and lead to fires and other problems. The spots on this leaf are caused by ozone damage. Click image to the left or use the URL below. URL: "
evolution plate tectonics and climate change,T_1154,"Scientific theories are sometimes thrown out when the data shows them to be wrong. Before plate tectonics theory was accepted, people thought that fossil organisms had spread around using land bridges. Although a land bridge across the Atlantic seemed a bit far-fetched, there was no better idea. Most scientists were relieved when they could toss that theory out. But some theories account for so many phenomena and are so broadly supported by so many lines of evidence that they are unlikely ever to be disproved. Additional scientific evidence may reveal problems and scientists may need to modify the theories. But there is so much evidence to support them and nothing major to refute them that they have become essential to their fields of science. "
evolution plate tectonics and climate change,T_1155,"Darwins theory of evolution has been under attack ever since Darwin proposed it. But nearly all biologists accept the theory and recognize that everything they learn about life on Earth supports the theory. Evolution is seen in the fossil record, in the developmental paths of organisms, in the geographic distribution of organisms, and in the genetic codes of living organisms. Evolution has a mechanism, called natural selection. People often refer to natural selection as the survival of the fittest. With natural selection, the organism that is best adapted to its environment will be most likely to survive and produce offspring, thus spreading its genes to the next generation. The theory of evolution maintains that modern humans evolved from ape-like ancestors. "
evolution plate tectonics and climate change,T_1156,"The theory of plate tectonics is the most important theory in much of earth science. Plate tectonics explains why much geological activity happens where it does, why many natural resources are found where they are, and can be used to determine what was happening long ago in Earths history. The theory of plate tectonics will be explored in detail in later concepts. "
evolution plate tectonics and climate change,T_1157,"The theory of climate change is a much newer theory than the previous two. We know that average global tempera- tures are rising. We even know why: Carbon dioxide is released into the atmosphere when fossil fuels are burned. Carbon dioxide is a greenhouse gas. In the atmosphere, greenhouse gases trap heat. This is like putting an extra blanket over Earth. Since more heat is being trapped, global temperature is rising. There is very little information that contradicts the theory that climate is changing due in large part to human activities. Unless some major discrepancy is discovered about how the atmosphere works, the theory is very likely to stand. So far, the evidence that is being collected supports the idea and global warming can be used to predict future events, which are already taking place. This idea will be explored in detail in later concepts. "
extinction and radiation of life,T_1167,"Most of the species that have lived have also gone extinct. There are two ways to go extinct: besides the obvious way of dying out completely, a species goes extinct if it evolves into a different species. Extinction is a normal part of Earths history. But sometimes large numbers of species go extinct in a short amount of time. This is a mass extinction. The causes of different mass extinctions are different: collisions with comets or asteroids, massive volcanic eruptions, or rapidly changing climate are all possible causes of some of these disasters (Figure 1.1). "
extinction and radiation of life,T_1168,"After a mass extinction, many habitats are no longer inhabited by organisms because they have gone extinct. With new habitats available, some species will adapt to the new environments. Evolutionary processes act rapidly during An extinct Tyrannosaurus rex. This fossil resembles a living organism. these times and many new species evolve to fill those available habitats. The process in which many new species evolve in a short period of time to fill available niches is called adaptive radiation. At the end of this period of rapid evolution the life forms do not look much like the ones that were around before the mass extinction. For example, after the extinction of the dinosaurs, mammals underwent adaptive radiation and became the dominant life form. "
flow of matter in ecosystems,T_1184,The flow of matter in an ecosystem is not like energy flow. Matter enters an ecosystem at any level and leaves at any level. Matter cycles freely between trophic levels and between the ecosystem and the physical environment (Figure 
flow of matter in ecosystems,T_1185,"Nutrients are ions that are crucial to the growth of living organisms. Nutrients such as nitrogen and phosphorous are important for plant cell growth. Animals use silica and calcium to build shells and skeletons. Cells need nitrates and phosphates to create proteins and other biochemicals. From nutrients, organisms make tissues and complex molecules such as carbohydrates, lipids, proteins, and nucleic acids. What are the sources of nutrients in an ecosystem? Rocks and minerals break down to release nutrients. Some enter the soil and are taken up by plants. Nutrients can be brought in from other regions, carried by wind or water. When one organism eats another organism, it receives all of its nutrients. Nutrients can also cycle out of an ecosystem. Decaying leaves may be transported out of an ecosystem by a stream. Wind or water carries nutrients out of an ecosystem. Nutrients cycle through ocean food webs. Decomposers play a key role in making nutrients available to organisms. Decomposers break down dead organisms into nutrients and carbon dioxide, which they respire into the air. If dead tissue would remain as it is, eventually nutrients would run out. Without decomposers, life on Earth would have died out long ago. "
global wind belts,T_1233,"Global winds blow in belts encircling the planet. Notice that the locations of these wind belts correlate with the atmospheric circulation cells. Air blowing at the base of the circulation cells, from high pressure to low pressure, creates the global wind belts. The global wind belts are enormous and the winds are relatively steady (Figure 1.1). "
global wind belts,T_1234,"Lets look at the global wind belts in the Northern Hemisphere. In the Hadley cell air should move north to south, but it is deflected to the right by Coriolis. So the air blows from northeast to the southwest. This belt is the trade winds, so called because at the time of sailing ships they were good for trade. In the Ferrel cell air should move south to north, but the winds actually blow from the southwest. This belt is the westerly winds or westerlies. In the Polar cell, the winds travel from the northeast and are called the polar easterlies. The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. These names hold for the winds in the wind belts of the Southern Hemisphere as well. Click image to the left or use the URL below. URL: "
global wind belts,T_1235,The high and low pressure areas created by the six atmospheric circulation cells also determine in a general way the amount of precipitation a region receives. Rain is common in low pressure regions due to rising air. Air sinking in high pressure areas causes evaporation; these regions are usually dry. These features have a great deal of influence on climate. 
global wind belts,T_1236,"The polar front is the junction between the Ferrell and Polar cells. At this low pressure zone, relatively warm, moist air of the Ferrell Cell runs into relatively cold, dry air of the Polar cell. The weather where these two meet is extremely variable, typical of much of North America and Europe. "
global wind belts,T_1237,"The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast- flowing river of air at the boundary between the troposphere and the stratosphere. Jet streams form where there is a large temperature difference between two air masses. This explains why the polar jet stream is the worlds most powerful (Figure 1.2). A cross section of the atmosphere with major circulation cells and jet streams. The polar jet stream is the site of extremely turbulent weather. Jet streams move seasonally just as the angle of the Sun in the sky moves north and south. The polar jet stream, known as the jet stream, moves south in the winter and north in the summer between about 30 N and 50 to 75 N. Click image to the left or use the URL below. URL: "
history of cenozoic life,T_1263,"The extinction of so many species at the end of the Mesozoic again left many niches available to be filled. Although we call the Cenozoic the age of mammals, birds are more common and more diverse. Early in the era, terrestrial crocodiles lumbered around along with large, primitive mammals and prehistoric birds. "
history of cenozoic life,T_1264,"Their adaptations have allowed mammals to spread to even more environments than reptiles. The success of mammals is due to several of their unique traits. Mammals are endothermic and have fur, hair, or blubber for warmth. Mammals can swim, fly, and live in nearly all terrestrial environments. Mammals initially filled the forests that covered many early Cenozoic lands. Over time, the forests gave way to grasslands, which created more niches for mammals to fill. "
history of cenozoic life,T_1265,"As climate cooled during the ice ages, large mammals were able to stand the cold weather, so many interesting megafauna developed. These included giant sloths, saber-toothed cats, wooly mammoths, giant condors, and many other animals that are now extinct (Figure 1.1). Many of the organisms that made up the Pleistocene megafauna went extinct as conditions warmed. Some may have been driven to extinction by human activities. Imagine a vast grassy plain covered with herds of elephants, bison and camels stretching as far as the eye can see. Lions, tigers, wolves and later, humans, hunt the herds on their summer migration. This was the San Francisco Bay Area at the close of the last Ice Age. Click image to the left or use the URL below. URL: "
history of mesozoic life,T_1266,"With most niches available after the mass extinction, a great diversity of organisms evolved. Mostly these niches were filled with reptiles. Climate alternated between cool, warm, and tropical, but overall the planet was much warmer than today. These conditions were good for reptiles. Surprisingly, there was more oxygen in the Mesozoic atmosphere than there is today. "
history of mesozoic life,T_1267,"Tiny phytoplankton arose to become the base of the marine food web. At the beginning of the Mesozoic, Pangaea began to break apart, so more beaches and continental shelf areas were available for colonization by new species of marine organisms. Marine reptiles colonized the seas and diversified. Some became huge, filling the niches that are filled by large marine mammals today. "
history of mesozoic life,T_1268,"On land, seed plants and trees diversified and spread widely. Ferns were common at the time of the dinosaurs (Figure "
history of mesozoic life,T_1269,"Of course the most famous Mesozoic reptiles were the dinosaurs (Figure 1.2). Dinosaurs reigned for 160 million years and had tremendous numbers and diversity. Species of dinosaurs filled all the niches that are currently filled by mammals. Dinosaurs were plant eaters, meat eaters, bipedal, quadrupedal, endothermic (warm-blooded), exothermic (cold-blooded), enormous, small, and some could swim or fly. Scientists now think that some dinosaurs were endotherms (warm-blooded) due to the evidence that has been collected over the decades. There are still some scientists who do not agree, but the amount of evidence makes it likely. Some dinosaurs lived in polar regions where animals that needed sunlight for warmth could not survive in winter. Dinosaurs bones had canals, similar to those of birds, indicating that they grew fast and were very active. Fast growth usually indicates an active metabolism typical of endotherms. Dinosaurs had erect posture and large brains, both correlated with endothermy. The earliest known fossil of a flowering plant is this 125 million year old Creta- ceous fossil. "
history of mesozoic life,T_1270,"Mammals appeared near the end of the Triassic, but the Mesozoic is known as the age of the reptiles. In a great advance over amphibians, which must live near water, reptiles developed adaptations for living away from water. Their thick skin keeps them from drying out, and the evolution of the amniote egg allowed them to lay their eggs on dry land. The amniote egg has a shell and contains all the nutrients and water required for the developing embryo (Figure 1.3). "
history of mesozoic life,T_1271,"Between the Mesozoic and the Cenozoic, 65 million years ago, about 50% of all animal species, including the dinosaurs, became extinct. Although there are other hypotheses, most scientists think that this mass extinction took place when a giant meteorite struck Earth with 2 million times the energy of the most powerful nuclear weapon (Figure 1.4). The impact kicked up a massive dust cloud, and when the particles rained back onto the surface they heated the atmosphere until it became as hot as a kitchen oven. Animals roasted. Dust that remained in the atmosphere blocked sunlight for a year or more, causing a deep freeze and temporarily ending photosynthesis. Sulfur from the impact mixed with water in the atmosphere to form acid rain, which dissolved the shells of the tiny marine plankton that form the base of the food chain. With little food being produced by land plants and plankton, animals starved. Carbon dioxide was also released from the impact and eventually caused global warming. Life forms could not survive the dramatic temperature swings. You may be surprised to know that dinosaurs in one form survived the mass extinctions and live all over the world today. Birds evolved from theropod dinosaurs, and these creatures not only survived the asteroid impact and its aftermath, but they have also diversified into some of the most fantastic creatures we know (Figure 1.5). "
history of paleozoic life,T_1272,The Paleozoic saw the evolution a tremendous diversity of life throughout the seas and onto land. 
history of paleozoic life,T_1273,"The Cambrian began with the most rapid and far-reaching evolution of life forms ever in Earths history. Evolving to inhabit so many different habitats resulted in a tremendous diversification of life forms. Shallow seas covered the lands, so every major marine organism group, including nearly all invertebrate animal phyla, evolved during this time. With the evolution of hard body parts, fossils are much more abundant and better preserved from this period than from the Precambrian. The Burgess shale formation in the Rocky Mountains of British Columbia, Canada, contains an amazing diversity of middle Cambrian life forms, from about 505 million years ago. Paleontologists do not agree on whether the Burgess shale fossils can all be classified into modern groups of organisms or whether many represent lines that have gone completely extinct. "
history of paleozoic life,T_1274,"Throughout the Paleozoic, seas transgressed and regressed. When continental areas were covered with shallow seas, the number and diversity of marine organisms increased. During regressions the number shrank. Arthropods, fish, amphibians and reptiles all originated in the Paleozoic. Simple plants began to colonize the land during the Ordovician, but land plants really flourished when seeds evolved during the Carboniferous (Figure 1.2). The abundant swamps became the coal and petroleum deposits that are the source of much of our fossil fuels today. During the later part of the Paleozoic, land animals and insects greatly increased in numbers and diversity. A modern rainforest has many seed- bearing plants that are similar to those that were common during the Carbonifer- ous. "
history of paleozoic life,T_1275,"Large extinction events separate the periods of the Paleozoic. After extinctions, new life forms evolved (Figure ). For example, after the extinction at the end of the Ordovician, fish and the first tetrapod animals appeared. Tetrapods are four legged vertebrates, but the earliest ones did not leave shallow, brackish water. "
history of paleozoic life,T_1276,"The largest mass extinction in Earths history occurred at the end of the Permian period, about 250 million years ago. In this catastrophe, it is estimated that more than 95% of marine species on Earth went extinct. Marine species with calcium carbonate shells and skeletons suffered worst. About 70% of terrestrial vertebrate species (land animals) suffered the same fate. This was the only known mass extinction of insects. This mass extinction appears to have taken place in three pulses, with three separate causes. Gradual environmental change, an asteroid impact, intense volcanism, or changes in the composition of the atmosphere may all have played a role. Click image to the left or use the URL below. URL: "
hurricanes,T_1292,"Hurricanes  called typhoons in the Pacific  are also cyclones. They are cyclones that form in the tropics and so they are also called tropical cyclones. By any name, they are the most damaging storms on Earth. "
hurricanes,T_1293,"Hurricanes arise in the tropical latitudes (between 10o and 25o N) in summer and autumn when sea surface temper- ature are 28o C (82o F) or higher. The warm seas create a large humid air mass. The warm air rises and forms a low pressure cell, known as a tropical depression. Thunderstorms materialize around the tropical depression. If the temperature reaches or exceeds 28o C (82o F), the air begins to rotate around the low pressure (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). As the air rises, water vapor condenses, releasing energy from latent heat. If wind shear is low, the storm builds into a hurricane within two to three days. Hurricanes are huge and produce high winds. The exception is the relatively calm eye of the storm, where air is rising upward. Rainfall can be as high as 2.5 cm (1"") per hour, resulting in about 20 billion metric tons of water released daily in a hurricane. The release of latent heat generates enormous amounts of energy, nearly the total annual electrical power consumption of the United States from one storm. Hurricanes can also generate tornadoes. A cross-sectional view of a hurricane. Hurricanes move with the prevailing winds. In the Northern Hemisphere, they originate in the trade winds and move to the west. When they reach the latitude of the westerlies, they switch direction and travel toward the north or northeast. Hurricanes may cover 800 km (500 miles) in one day. Click image to the left or use the URL below. URL: "
hurricanes,T_1294,"Hurricanes are assigned to categories based on their wind speed. The categories are listed on the Saffir-Simpson hurricane scale (Table 1.1). Category 1 (weak) Kph 119-153 Mph 74-95 2 (moderate) 154-177 96-110 3 (strong) 178-209 111-130 Estimated Damage Above normal; no real damage to structures Some roofing, door, and window damage, consid- erable damage to vegeta- tion, mobile homes, and piers Some buildings damaged; mobile homes destroyed Category 4 (very strong) Kph 210-251 Mph 131-156 5 (devastating) >251 >156 Estimated Damage Complete roof failure on small residences; major erosion of beach areas; major damage to lower floors of structures near shore Complete roof failure on many residences and in- dustrial buildings; some complete building failures "
hurricanes,T_1295,"Damage from hurricanes comes from the high winds, rainfall, and storm surge. Storm surge occurs as the storms low pressure center comes onto land, causing the sea level to rise unusually high. A storm surge is often made worse by the hurricanes high winds blowing seawater across the ocean onto the shoreline. Flooding can be devastating, especially along low-lying coastlines such as the Atlantic and Gulf Coasts. Hurricane Camille in 1969 had a 7.3 m (24 foot) storm surge that traveled 125 miles (200 km) inland. "
hurricanes,T_1296,"Hurricanes typically last for 5 to 10 days. The winds push them to the northwest and then to the northeast. Eventually a hurricane will end up over cooler water or land. At that time the hurricanes latent heat source shut downs and the storm weakens. When a hurricane disintegrates, it is replaced with intense rains and tornadoes. There are about 100 hurricanes around the world each year, plus many smaller tropical storms and tropical depres- sions. As people develop coastal regions, property damage from storms continues to rise. However, scientists are becoming better at predicting the paths of these storms and fatalities are decreasing. There is, however, one major exception to the previous statement: Hurricane Katrina. "
hurricanes,T_1297,"The 2005 Atlantic hurricane season was the longest, costliest, and deadliest hurricane season so far. Total damage from all the storms together was estimated at more than $128 billion, with more than 2,280 deaths. Hurricane Katrina was both the most destructive hurricane and the most costly (Figure 1.2). "
local winds,T_1372,Local winds result from air moving between small low and high pressure systems. High and low pressure cells are created by a variety of conditions. Some local winds have very important effects on the weather and climate of some regions. 
local winds,T_1373,"Since water has a very high specific heat, it maintains its temperature well. So water heats and cools more slowly than land. If there is a large temperature difference between the surface of the sea (or a large lake) and the land next to it, high and low pressure regions form. This creates local winds. Sea breezes blow from the cooler ocean over the warmer land in summer. Where is the high pressure zone and where is the low pressure zone (Figure 1.1)? Sea breezes blow at about 10 to 20 km (6 to 12 miles) per hour and lower air temperature much as 5 to 10o C (9 to 18o F). Land breezes blow from the land to the sea in winter. Where is the high pressure zone and where is the low pressure zone? Some warmer air from the ocean rises and then sinks on land, causing the temperature over the land to become warmer. How do sea and land breezes moderate coastal climates? Land and sea breezes create the pleasant climate for which Southern California is known. The effect of land and sea breezes are felt only about 50 to 100 km (30 to 60 miles) inland. This same cooling and warming effect occurs to a smaller degree during day and night, because land warms and cools faster than the ocean. "
local winds,T_1374,"Monsoon winds are larger scale versions of land and sea breezes; they blow from the sea onto the land in summer and from the land onto the sea in winter. Monsoon winds occur where very hot summer lands are next to the sea. Thunderstorms are common during monsoons (Figure 1.2). In the southwestern United States rela- tively cool moist air sucked in from the Gulf of Mexico and the Gulf of California meets air that has been heated by scorch- ing desert temperatures. The most important monsoon in the world occurs each year over the Indian subcontinent. More than two billion residents of India and southeastern Asia depend on monsoon rains for their drinking and irrigation water. Back in the days of sailing ships, seasonal shifts in the monsoon winds carried goods back and forth between India and Africa. "
local winds,T_1375,"Temperature differences between mountains and valleys create mountain and valley breezes. During the day, air on mountain slopes is heated more than air at the same elevation over an adjacent valley. As the day progresses, warm air rises and draws the cool air up from the valley, creating a valley breeze. At night the mountain slopes cool more quickly than the nearby valley, which causes a mountain breeze to flow downhill. "
local winds,T_1376,"Katabatic winds move up and down slopes, but they are stronger mountain and valley breezes. Katabatic winds form over a high land area, like a high plateau. The plateau is usually surrounded on almost all sides by mountains. In winter, the plateau grows cold. The air above the plateau grows cold and sinks down from the plateau through gaps in the mountains. Wind speeds depend on the difference in air pressure over the plateau and over the surroundings. Katabatic winds form over many continental areas. Extremely cold katabatic winds blow over Antarctica and Greenland. "
local winds,T_1377,"Chinook winds (or Foehn winds) develop when air is forced up over a mountain range. This takes place, for example, when the westerly winds bring air from the Pacific Ocean over the Sierra Nevada Mountains in California. As the relatively warm, moist air rises over the windward side of the mountains, it cools and contracts. If the air is humid, it may form clouds and drop rain or snow. When the air sinks on the leeward side of the mountains, it forms a high pressure zone. The windward side of a mountain range is the side that receives the wind; the leeward side is the side where air sinks. The descending air warms and creates strong, dry winds. Chinook winds can raise temperatures more than 20o C (36o F) in an hour and they rapidly decrease humidity. Snow on the leeward side of the mountain melts quickly. If precipitation falls as the air rises over the mountains, the air will be dry as it sinks on the leeward size. This dry, sinking air causes a rainshadow effect (Figure 1.3), which creates many of the worlds deserts. "
local winds,T_1378,"Santa Ana winds are created in the late fall and winter when the Great Basin east of the Sierra Nevada cools, creating a high pressure zone. The high pressure forces winds downhill and in a clockwise direction (because of Coriolis). The air pressure rises, so temperature rises and humidity falls. The winds blow across the Southwestern deserts and then race downhill and westward toward the ocean. Air is forced through canyons cutting the San Gabriel and San Bernardino mountains. (Figure 1.4). The winds are especially fast through Santa Ana Canyon, for which they are named. Santa Ana winds blow dust and smoke westward over the Pacific from Southern California. The Santa Ana winds often arrive at the end of Californias long summer drought season. The hot, dry winds dry out the landscape even more. If a fire starts, it can spread quickly, causing large-scale devastation (Figure 1.5). In October 2007, Santa Ana winds fueled many fires that together burned 426,000 acres of wild land and more than 1,500 homes in Southern California. "
local winds,T_1379,"High summer temperatures on the desert create high winds, which are often associated with monsoon storms. Desert winds pick up dust because there is not as much vegetation to hold down the dirt and sand. (Figure 1.6). A haboob forms in the downdrafts on the front of a thunderstorm. Dust devils, also called whirlwinds, form as the ground becomes so hot that the air above it heats and rises. Air flows into the low pressure and begins to spin. Dust devils are small and short-lived, but they may cause damage. "
mid latitude cyclones,T_1436,"Cyclones can be the most intense storms on Earth. A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a low pressure center. The swirling air rises and cools, creating clouds and precipitation. Mid-latitude cyclones form at the polar front when the temperature difference between two air masses is large. These air masses blow past each other in opposite directions. Coriolis effect deflects winds to the right in the Northern Hemisphere, causing the winds to strike the polar front at an angle. Warm and cold fronts form next to each other. Most winter storms in the middle latitudes, including most of the United States and Europe, are caused by mid-latitude cyclones (Figure 1.1). The warm air at the cold front rises and creates a low pressure cell. Winds rush into the low pressure and create a rising column of air. The air twists, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Since the rising air is moist, rain or snow falls. Mid-latitude cyclones form in winter in the mid-latitudes and move eastward with the westerly winds. These two- to five-day storms can reach 1,000 to 2,500 km (625 to 1,600 miles) in diameter and produce winds up to 125 km (75 miles) per hour. "
mid latitude cyclones,T_1437,"Mid-latitude cyclones are especially fierce in the mid-Atlantic and New England states, where they are called noreasters because they come from the northeast. About 30 noreasters strike the region each year. (Figure A hypothetical mid-latitude cyclone affect- ing the United Kingdom. The arrows point the wind direction and its relative temper- ature; L is the low pressure area. Notice the warm, cold, and occluded fronts. The 1993 Storm of the Century was a noreaster that covered the entire eastern seaboard of the United States. "
modern biodiversity,T_1471,There are more than 1 million species of plants and animals known to be currently alive on Earth (Figure 1.1) and many millions more that have not been discovered yet. The tremendous variety of creatures is due to the tremendous numbers of habitats that organisms have evolved to fill. 
modern biodiversity,T_1472,"Many adaptations protect organisms from the external environment (Figure 1.2). Other adaptations help an organism move or gather food. Reindeer have sponge-like hoofs that help them walk on snowy ground without slipping and falling. Hummingbirds have long, thin beaks that help them drink nectar from flowers. Organisms have special features that help them avoid being eaten. When a herd of zebras run away from lions, the zebras dark stripes confuse the predators so that they have difficulty focusing on just one zebra during the chase. Some plants have poisonous or foul-tasting substances in them that keep animals from eating them. Their brightly colored flowers serve as a warning. There is an amazing diversity of organisms on Earth. How do the organisms in this picture each make their living? Cacti have thick, water- retaining bodies that help them conserve water. Poison dart frogs have toxins in their skin. Their bright colors warn potential predators not to take a bite! Thousands of northern elephant seals  some weighing up to 4,500 pounds  make an annual migration to breed each winter at Ao Nuevo State Reserve in California. Marine biologists are using high-tech tools to explore the secrets of these amazing creatures. Click image to the left or use the URL below. URL: "
observations and experiments,T_1499,If we were doing a scientific investigation we need to gather the information to test the hypotheses ourselves. We would do this by making observations or running experiments. 
observations and experiments,T_1500,"Observations of Earths surface may be made from the land surface or from space. Many important observations are made by orbiting satellites, which have a birds eye view of how the planet is changing (for example, see Figure Often, observation is used to collect data when it is not possible for practical or ethical reasons to perform experi- ments. Scientists may send devices to make observations for them when it is too dangerous or impractical for them to make the observations directly. They may use microscopes to explore tiny objects or telescopes to learn about the universe (see Figure 1.2). Artists concept of the Juno orbiter circling Jupiter. The mission is ongoing. "
observations and experiments,T_1501,"Answering some questions requires experiments. An experiment is a test that may be performed in the field or in a laboratory. An experiment must always done under controlled conditions. The goal of an experiment is to verify or falsify a hypothesis. In an experiment, it is important to change only one factor. All other factors must be kept the same. Independent variable: The factor that will be manipulated. Dependent variable: The factors that depend on the independent variable. An experiment must have a control group. The control group is not subjected to the independent variable. For example, if you want to test if Vitamin C prevents colds, you must divide your sample group up so that some receive Vitamin C and some do not. Those who do not receive the Vitamin C are the control group. "
observations and experiments,T_1502,"Scientists often make many measurements during experiments. As in just about every human endeavor, errors are unavoidable. In a scientific experiment, this is called experimental error. Systematic errors are part of the experimental setup, so that the numbers are always skewed in one direction. For example, a scale may always measure one-half of an ounce high. Random errors occur because a measurement is not made precisely. For example, a stopwatch may be stopped too soon or too late. To correct for this, many measurements are taken and then averaged. Experiments always have a margin of error associated with them. In an experiment, if a result is inconsistent with the results from other samples and many tests have been done, it is likely that a mistake was made in that experiment. The inconsistent data point can be thrown out. Click image to the left or use the URL below. URL: "
predicting weather,T_1577,"The most accurate weather forecasts are made by advanced computers, with analysis and interpretation added by experienced meteorologists. These computers have up-to-date mathematical models that can use much more data and make many more calculations than would ever be possible by scientists working with just maps and calculators. Meteorologists can use these results to give much more accurate weather forecasts and climate predictions. In Numerical Weather Prediction (NWP), atmospheric data from many sources are plugged into supercomputers running complex mathematical models (Figure 1.1). The models then calculate what will happen over time at various altitudes for a grid of evenly spaced locations. The grid points are usually between 10 and 200 kilometers apart. Using the results calculated by the model, the program projects weather further into the future. It then uses these results to project the weather still further into the future, as far as the meteorologists want to go. Once a forecast is made, it is broadcast by satellites to more than 1,000 sites around the world. NWP produces the most accurate weather forecasts, but as anyone knows, even the best forecasts are not always right. Weather prediction is extremely valuable for reducing property damage and even fatalities. If the proposed track of a hurricane can be predicted, people can try to secure their property and then evacuate (Figure 1.2). A weather forecast using numerical weather prediction. "
pressure and density of the atmosphere,T_1578,"The atmosphere has different properties at different elevations above sea level, or altitudes. "
pressure and density of the atmosphere,T_1579,"The air density (the number of molecules in a given volume) decreases with increasing altitude. This is why people who climb tall mountains, such as Mt. Everest, have to set up camp at different elevations to let their bodies get used to the decreased air density (Figure 1.1). Why does air density decrease with altitude? Gravity pulls the gas molecules towards Earths center. The pull of gravity is stronger closer to the center, at sea level. Air is denser at sea level, where the gravitational pull is greater. Click image to the left or use the URL below. URL: "
pressure and density of the atmosphere,T_1580,"Gases at sea level are also compressed by the weight of the atmosphere above them. The force of the air weighing down over a unit of area is known as its atmospheric pressure, or air pressure. Why are we not crushed? The molecules inside our bodies are pushing outward to compensate. Air pressure is felt from all directions, not just from above. This bottle was closed at an altitude of 3,000 meters where air pressure is lower. When it was brought down to sea level, the higher air pressure caused the bottle to collapse. At higher altitudes the atmospheric pressure is lower and the air is less dense than at lower altitudes. Thats what makes your ears pop when you change altitude. Gas molecules are found inside and outside your ears. When you change altitude quickly, like when an airplane is descending, your inner ear keeps the density of molecules at the original altitude. Eventually the air molecules inside your ear suddenly move through a small tube in your ear to equalize the pressure. This sudden rush of air is felt as a popping sensation. Click image to the left or use the URL below. URL: "
roles in an ecosystem,T_1631,"There are many different types of ecosystems. Climate conditions determine which ecosystems are found in a particular location. A biome encompasses all of the ecosystems that have similar climate and organisms. Different organisms live in different types of ecosystems because they are adapted to different conditions. Lizards thrive in deserts, but no reptiles are found in any polar ecosystems. Amphibians cant live too far from the water. Large animals generally do better in cold climates than in hot climates. Despite this, every ecosystem has the same general roles that living creatures fill. Its just the organisms that fill those niches that are different. For example, every ecosystem must have some organisms that produce food in the form of chemical energy. These organisms are primarily algae in the oceans, plants on land, and bacteria at hydrothermal vents. "
roles in an ecosystem,T_1632,"The organisms that produce food are extremely important in every ecosystem. Organisms that produce their own food are called producers. There are two ways of producing food energy: Photosynthesis: plants on land, phytoplankton in the surface ocean, and some other organisms. Chemosynthesis: bacteria at hydrothermal vents. Organisms that use the food energy that was created by producers are named consumers. There are many types of consumers: Herbivores eat producers directly. These animals break down the plant structures to get the materials and energy they need. Carnivores eat animals; they can eat herbivores or other carnivores. Omnivores eat plants and animals as well as fungi, bacteria, and organisms from the other kingdoms. "
roles in an ecosystem,T_1633,"There are many types of feeding relationships (Figure 1.2) between organisms. A predator is an animal that kills and eats another animal, known as its prey. Scavengers are animals, such as vultures and hyenas, that eat organisms that are already dead. Decomposers break apart dead organisms or the waste material of living organisms, returning the nutrients to the ecosystem. (a) Predator and prey; (b) Scavengers; (c) Bacteria and fungi, acting as decomposers. "
roles in an ecosystem,T_1634,"Species have different types of relationships with each other. Competition occurs between species that try to use the same resources. When there is too much competition, one species may move or adapt so that it uses slightly different resources. It may live at the tops of trees and eat leaves that are somewhat higher on bushes, for example. If the competition does not end, one species will die out. Each niche can only be inhabited by one species. Some relationships between species are beneficial to at least one of the two interacting species. These relationships are known as symbiosis and there are three types: In mutualism, the relationship benefits both species. Most plant-pollinator relationships are mutually benefi- cial. What does each get from the relationship? In commensalism, one organism benefits and the other is not harmed. In parasitism, the parasite species benefits and the host is harmed. Parasites do not usually kill their hosts because a dead host is no longer useful to the parasite. Humans host parasites, such as the flatworms that cause schistosomiasis. Choose which type of relationship is described by each of the images and captions below (Figure 1.3). Click image to the left or use the URL below. URL:  (a) The pollinator gets food; the plants pollen gets caught in the birds feathers so it is spread to far away flowers. (b) The barnacles receive protection and get to move to new locations; the whale is not harmed. (c) These tiny mites are parasitic and consume the insect called a harvestman. Click image to the left or use the URL below. URL: "
scientific community,T_1654,"A hypothesis will not be fully accepted unless it is supported by the work of many scientists. Although a study may take place in a single laboratory, a scientist must present her work to the community of scientists in her field. Initially, she may present her data and conclusions at a scientific conference where she will talk with many other scientists. Later, she will write a paper to be published in a scientific journal. After she submits the paper, several scientists will review the paper - a process called peer review - to suggest further investigations or changes in interpretation to make the paper stronger. The scientists will then recommend or deny the paper for publication. Once it is published, other scientists incorporate the results into their own research. If they cannot replicate her results, her work will be thrown out! Scientific ideas are advanced after many papers on a topic are published. "
scientific community,T_1655,"There scientific community controls the quality and type of research that is done by project funding. Most scientific research is expensive, so scientists must write a proposal to a funding agency, such as the National Science Foun- dation or the National Aeronautics and Space Administration (NASA), to pay for equipment, supplies, and salaries. Scientific proposals are reviewed by other scientists in the field and are evaluated for funding. In many fields, the funding rate is low and the money goes only to the most worthy research projects. The scientific community monitors scientific integrity. During their training, students learn how to conduct good scientific experiments. They learn not to fake, hide, or selectively report data, and they learn how to fairly evaluate data and the work of other scientists. Scientists who do not have scientific integrity are strongly condemned by the scientific community. Nothing is perfect, but considering all the scientific research that is done, there are few incidences of scientific dishonesty. Yet when they do occur, they are often reported with great vehemence by the media. Often this causes the public to mistrust scientists in ways that are unwarranted. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
scientific explanations and interpretations,T_1656,"Scientists usually begin an investigation with facts. A fact is a bit of information that is true. Facts come from data collected from observations or from experiments that have already been run. Data is factual information that is not subject to opinion or bias. What is a fact? Look at the following list and identify if the statement is a fact (from observation or prior experi- ments), an opinion, or a combination. Can you be sure from the photo that Susan has a cold? 1. 2. 3. 4. 5. 6. 7. Susan has long hair. Susan is sneezing and has itchy eyes. She is not well. She has a cold. Colds are caused by viruses. Echinacea is an herb that prevents colds. Bill Gates is the smartest man in the United States. People born under the astrological sign Leo are fiery, self-assured, and charming. Average global temperature has been rising at least since 1960. "
scientific explanations and interpretations,T_1657,"The following is an analysis of the statements above: 1. This is a fact made from observation. 2. The first part is from observations. The second is a fact drawn from the prior observations. The third is an opinion, since she might actually have allergies or the flu. Tests could be done to see what is causing her illness. 3. This is a fact. Many, many scientific experiments have shown that colds are caused by viruses. 4. While that sounds like a fact, the scientific evidence is mixed. One reputable study published in 2007 showed a decrease of 58%, but several other studies have shown no beneficial effect. 5. Bill Gates is the wealthiest man in the United States; thats a fact. But theres no evidence that hes also the smartest man, and chances are hes not. This is an opinion. 6. This sounds like a fact, but it is not. It is easy to test. Gather together a large number of subjects, each with a friend. Have the friends fill out a questionnaire describing the subject. Match the traits against the persons astrological sign to see if the astrological predictions fit. Are Leos actually more fiery, self assured, and charming? Tests like this have not supported the claims of astrologers, yet astrologers have not modified their opinions. 7. This is a fact. The Figure 1.2 shows the temperature anomaly since 1880. Theres no doubt that temperature has risen overall since 1880 and especially since the late 1970s. Global Average Annual Temperatures are Rising. This graph shows temperature anomaly relative to the 1951-1980 aver- age (the average is made to be 0). The green bars show uncertainty. "
scientific method,T_1658,"The goal of science is to answer questions about the natural world. Scientific questions must be testable. Which of these two questions is a good scientific question and which is not? What is the age of our planet Earth? How many angels can dance on the head of a pin? The first is a good scientific question that can be answered by radiometrically dating rocks among other techniques. The second cannot be answered using data, so it is not a scientific question. "
scientific method,T_1659,"Scientists use the scientific method to answer questions. The scientific method is a series of steps that help to investigate a question. Often, students learn that the scientific method is a linear process that goes like this: Ask a question. The question is based on one or more observations or on data from a previous experiment. Do some background research. Create a hypothesis. Do experiments or make observations to test the hypothesis. Gather the data. Formulate a conclusion. The process doesnt always go in a straight line. A scientist might ask a question, then do some background research and discover that the question needed to be asked a different way, or that a different question should be asked. Click image to the left or use the URL below. URL: "
scientific method,T_1660,"Now, lets ask a scientific question. Remember that it must be testable. We learned above that average global temperature has been rising since record keeping began in 1880. We know that carbon dioxide is a greenhouse gas. Greenhouse gases trap heat in the atmosphere. This leads us to a question: Question: Is the amount of carbon dioxide in Earths atmosphere changing? This is a good scientific question because it is testable. How has carbon dioxide in the atmosphere changed over those 50-plus years (see Figure 1.1)? About how much has atmospheric CO2 risen between 1958 and 2011 in parts per million? "
scientific method,T_1661,"So weve answered the question using data from research that has already been done. If scientists had not been monitoring CO2 levels over the years, wed have had to start these measurements now. Because this question can be answered with data, it is testable. Click image to the left or use the URL below. URL: "
temperature of the atmosphere,T_1753,"The atmosphere is layered, corresponding with how the atmospheres temperature changes with altitude. By under- standing the way temperature changes with altitude, we can learn a lot about how the atmosphere works. "
temperature of the atmosphere,T_1754,"Why does warm air rise (Figure 1.1)? Gas molecules are able to move freely, and if they are uncontained, as they are in the atmosphere, they can take up more or less space. When gas molecules are cool, they are sluggish and do not take up as much space. With the same number of molecules in less space, both air density and air pressure are higher. When gas molecules are warm, they move vigorously and take up more space. Air density and air pressure are lower. Warmer, lighter air is more buoyant than the cooler air above it, so it rises. The cooler air then sinks down, because it is denser than the air beneath it. This is convection, which was described in the chapter Plate Tectonics. "
temperature of the atmosphere,T_1755,"The property that changes most strikingly with altitude is air temperature. Unlike the change in pressure and density, which decrease with altitude, changes in air temperature are not regular. A change in temperature with distance is called a temperature gradient. "
temperature of the atmosphere,T_1756,"The atmosphere is divided into layers based on how the temperature in that layer changes with altitude, the layers temperature gradient (Figure 1.2). The temperature gradient of each layer is different. In some layers, temperature increases with altitude and in others it decreases. The temperature gradient in each layer is determined by the heat source of the layer (See opening image). The four main layers of the atmosphere have different temperature gradients, cre- ating the thermal structure of the atmo- sphere. This video is very thorough in its discussion of the layers of the atmosphere. Remember that the chemical composi- tion of each layer is nearly the same except for the ozone layer that is found in the stratosphere. Click image to the left or use the URL below. URL: "
tornadoes,T_1782,"Tornadoes, also called twisters, are fierce products of severe thunderstorms (Figure 1.1). As air in a thunderstorm rises, the surrounding air races in to fill the gap. This forms a tornado, a funnel-shaped, whirling column of air extending downward from a cumulonimbus cloud. A tornado lasts from a few seconds to several hours. The average wind speed is about 177 kph (110 mph), but some winds are much faster. A tornado travels over the ground at about 45 km per hour (28 miles per hour) and goes about 25 km (16 miles) before losing energy and disappearing (Figure 1.2). The formation of this tornado outside Dimmit, Texas, in 1995 was well studied. This tornado struck Seymour, Texas, in 1979. "
tornadoes,T_1783,"An individual tornado strikes a small area, but it can destroy everything in its path. Most injuries and deaths from tornadoes are caused by flying debris (Figure 1.3). In the United States an average of 90 people are killed by tornadoes each year. The most violent two percent of tornadoes account for 70% of the deaths by tornadoes. "
tornadoes,T_1784,"Tornadoes form at the front of severe thunderstorms. Lines of these thunderstorms form in the spring where where maritime tropical (mT) and continental polar (cP) air masses meet. Although there is an average of 770 tornadoes annually, the number of tornadoes each year varies greatly (Figure 1.4). "
tornadoes,T_1785,"In late April 2011, severe thunderstorms pictured in the satellite image spawned the deadliest set of tornadoes in more than 25 years. In addition to the meeting of cP and mT mentioned above, the jet stream was blowing strongly Tornado damage at Ringgold, Georgia in April 2011. The frequency of F3, F4, and F5 torna- does in the United States. The red region that starts in Texas and covers Oklahoma, Nebraska, and South Dakota is called Tornado Alley because it is where most of the violent tornadoes occur. in from the west. The result was more than 150 tornadoes reported throughout the day (Figure 1.5). The entire region was alerted to the possibility of tornadoes in those late April days. But meteorologists can only predict tornado danger over a very wide region. No one can tell exactly where and when a tornado will touch down. Once a tornado is sighted on radar, its path is predicted and a warning is issued to people in that area. The exact path is unknown because tornado movement is not very predictable. "
tornadoes,T_1786,"The intensity of tornadoes is measured on the Fujita Scale (see Table 1.1), which assigns a value based on wind speed and damage. F Scale F0 (km/hr) 64-116 (mph) 40-72 F1 117-180 73-112 F2 181-253 113-157 F3 254-333 158-206 F4 333-419 207-260 F5 420-512 261-318 F6 >512 >318 Damage Light - tree branches fall and chimneys may col- lapse Moderate - mobile homes, autos pushed aside Considerable - roofs torn off houses, large trees up- rooted Severe - houses torn apart, trees uprooted, cars lifted Devastating - houses lev- eled, cars thrown Incredible - structures fly, cars become missiles Maximum tornado wind speed Click image to the left or use the URL below. URL: "
types of marine organisms,T_1806,"The smallest and largest animals on Earth live in the oceans. Why do you think the oceans can support large animals? Marine animals breathe air or extract oxygen from the water. Some float on the surface and others dive into the oceans depths. There are animals that eat other animals, and plants generate food from sunlight. A few bizarre creatures break down chemicals to make food! The following section divides ocean life into seven basic groups. "
types of marine organisms,T_1807,"Plankton are organisms that cannot swim but that float along with the current. The word ""plankton"" comes from the Greek for wanderer. Most plankton are microscopic, but some are visible to the naked eye (Figure 1.1). Phytoplankton are tiny plants that make food by photosynthesis. Because they need sunlight, phytoplankton live in the photic zone. Phytoplankton are responsible for about half of the total primary productivity (food energy) on Earth. Like other plants, phytoplankton release oxygen as a waste product. Microscopic diatoms are a type of phyto- plankton. Zooplankton, or animal plankton, eat phytoplankton as their source of food (Figure 1.2). Some zooplankton live as plankton all their lives and others are juvenile forms of animals that will attach to the bottom as adults. Some small invertebrates live as zooplankton. Copepods are abundant and so are an important food source for larger animals. "
types of marine organisms,T_1808,"The few true plants found in the oceans include salt marsh grasses and mangrove trees. Although they are not true plants, large algae, which are called seaweed, also use photosynthesis to make food. Plants and seaweeds are found in the neritic zone, where the light they need penetrates so that they can photosynthesize (Figure 1.3). Kelp grows in forests in the neritic zone. Otters and other organisms depend on the kelp-forest ecosystem. "
types of marine organisms,T_1809,"The variety and number of invertebrates, animals without a backbone, is truly remarkable (Figure 1.4). Marine invertebrates include sea slugs, sea anemones, starfish, octopuses, clams, sponges, sea worms, crabs, and lobsters. Most of these animals are found close to the shore, but they can be found throughout the ocean. Jellies are otherworldly creatures that glow in the dark, without brains or bones, some more than 100 feet long. Along with many other ocean areas, they live just off Californias coast. Click image to the left or use the URL below. URL: "
types of marine organisms,T_1810,"Fish are vertebrates; they have a backbone. What are some of the features fish have that allows them to live in the oceans? All fish have most or all of these traits: Fins with which to move and steer. Scales for protection. Gills for extracting oxygen from the water. A swim bladder that lets them rise and sink to different depths. (a) Mussels; (b) Crown of thorns sea star; (c) Moon jelly; (d) A squid. Ectothermy (cold-bloodedness), so that their bodies are the same temperature as the surrounding water. Bioluminescence, or light created from a chemical reaction that can attract prey or mates in the dark ocean. Included among the fish are sardines, salmon, and eels, as well as the sharks and rays (which lack swim bladders) (Figure 1.5). "
types of marine organisms,T_1811,"Only a few types of reptiles live in the oceans and they live in warm water. Why are reptiles so restricted in their ability to live in the sea? Sea turtles, sea snakes, saltwater crocodiles, and marine iguana that are found only at the Galapagos Islands sum up the marine reptile groups (Figure 1.6). Sea snakes bear live young in the ocean, but turtles, crocodiles, and marine iguanas all lay their eggs on land. The Great White Shark is a fish that preys on other fish and marine mammals. Sea turtles are found all over the oceans, but their numbers are diminishing. "
types of marine organisms,T_1812,"Many types of birds are adapted to living in the sea or on the shore. With their long legs for wading and long bills for digging in sand for food, shorebirds are well adapted for the intertidal zone. Many seabirds live on land but go to sea to fish, such as gulls, pelicans, and frigate birds. Some birds, like albatross, spend months at sea and only come on shore to raise chicks (Figure 1.7). "
types of marine organisms,T_1813,"What are the common traits of mammals? Mammals are endothermic (warm-blooded) vertebrates that give birth to live young, feed them with milk, and have hair, ears, and a jaw bone with teeth. What traits might mammals have to be adapted to life in the ocean? (a) Shorebirds; (b) Seabirds; (c) Albatross. For swimming: streamlined bodies, slippery skin or hair, fins. For warmth: fur, fat, high metabolic rate, small surface area to volume, specialized blood system. For salinity: kidneys that excrete salt, impervious skin. The five types of marine mammals are pictured here: (Figure 1.8). (a) Cetaceans: whales, dolphins, and porpoises. (b) Sirenians: manatee and the dugong. (c) Mustelids: Sea otters (terrestrial members are skunks, badgers and weasels). (d) Pinnipeds: Seals, sea lions, and walruses. (e) Polar bear. "
weather fronts,T_1877,"Two air masses meet at a front. At a front, the two air masses have different densities and do not easily mix. One air mass is lifted above the other, creating a low pressure zone. If the lifted air is moist, there will be condensation and precipitation. Winds are common at a front. The greater the temperature difference between the two air masses, the stronger the winds will be. Fronts are the main cause of stormy weather. There are four types of fronts, three moving and one stationary. With cold fronts and warm fronts, the air mass at the leading edge of the front gives the front its name. In other words, a cold front is right at the leading edge of moving cold air and a warm front marks the leading edge of moving warm air. "
weather fronts,T_1878,"At a stationary front the air masses do not move (Figure 1.1). A front may become stationary if an air mass is stopped by a barrier, such as a mountain range. A stationary front may bring days of rain, drizzle, and fog. Winds usually blow parallel to the front, but in opposite directions. After several days, the front will likely break apart. "
weather fronts,T_1879,"When a cold air mass takes the place of a warm air mass, there is a cold front (Figure 1.2). The map symbol for a stationary front has red domes for the warm air mass and blue triangles for the cold air mass. Imagine that you are standing in one spot as a cold front approaches. Along the cold front, the denser, cold air pushes up the warm air, causing the air pressure to decrease (Figure 1.2). If the humidity is high enough, some types of cumulus clouds will grow. High in the atmosphere, winds blow ice crystals from the tops of these clouds to create cirrostratus and cirrus clouds. At the front, there will be a line of rain showers, snow showers, or thunderstorms with blustery winds (Figure 1.3). A squall line is a line of severe thunderstorms that forms along a cold front. Behind the front is the cold air mass. This mass is drier, so precipitation stops. The weather may be cold and clear or only partly cloudy. Winds may continue to blow into the low pressure zone at the front. The weather at a cold front varies with the season. Spring and summer: the air is unstable so thunderstorms or tornadoes may form. Spring: if the temperature gradient is high, strong winds blow. Autumn: strong rains fall over a large area. Winter: the cold air mass is likely to have formed in the frigid arctic, so there are frigid temperatures and heavy snows. "
weather fronts,T_1880,"At a warm front, a warm air mass slides over a cold air mass (Figure 1.4). When warm, less dense air moves over the colder, denser air, the atmosphere is relatively stable. Imagine that you are on the ground in the wintertime under a cold winter air mass with a warm front approaching. The transition from cold air to warm air takes place over a long distance, so the first signs of changing weather appear long before the front is actually over you. Initially, the air is cold: the cold air mass is above you and the warm air mass is above it. High cirrus clouds mark the transition from one air mass to the other. Warm air moves forward to take over the position of colder air. Over time, cirrus clouds become thicker and cirrostratus clouds form. As the front approaches, altocumulus and altostratus clouds appear and the sky turns gray. Since it is winter, snowflakes fall. The clouds thicken and nimbostratus clouds form. Snowfall increases. Winds grow stronger as the low pressure approaches. As the front gets closer, the cold air mass is just above you but the warm air mass is not too far above that. The weather worsens. As the warm air mass approaches, temperatures rise and snow turns to sleet and freezing rain. Warm and cold air mix at the front, leading to the formation of stratus clouds and fog (Figure 1.5). Cumulus clouds build at a warm front. "
weather fronts,T_1881,"An occluded front usually forms around a low pressure system (Figure 1.6). The occlusion starts when a cold front catches up to a warm front. The air masses, in order from front to back, are cold, warm, and then cold again. The map symbol for an occluded front is mixed cold front triangles and warm front domes. Coriolis effect curves the boundary where the two fronts meet towards the pole. If the air mass that arrives third is colder than either of the first two air masses, that air mass slip beneath them both. This is called a cold occlusion. If the air mass that arrives third is warm, that air mass rides over the other air mass. This is called a warm occlusion (Figure 1.7). The weather at an occluded front is especially fierce right at the occlusion. Precipitation and shifting winds are typical. The Pacific Coast has frequent occluded fronts. An occluded front with the air masses from front to rear in order as cold, warm, cold. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
weather maps,T_1882,"Weather maps simply and graphically depict meteorological conditions in the atmosphere. Weather maps may display only one feature of the atmosphere or multiple features. They can depict information from computer models or from human observations. On a weather map, important meteorological conditions are plotted for each weather station. Meteorologists use many different symbols as a quick and easy way to display information on the map (Figure 1.1). Once conditions have been plotted, points of equal value can be connected by isolines. Weather maps can have many types of connecting lines. For example: Explanation of some symbols that may appear on a weather map. Lines of equal temperature are called isotherms. Isotherms show temperature gradients and can indicate the location of a front. In terms of precipitation, what does the 0o C (32o F) isotherm show? Isobars are lines of equal average air pressure at sea level (Figure 1.2). Closed isobars represent the locations of high and low pressure cells. Isotachs are lines of constant wind speed. Where the minimum values occur high in the atmosphere, tropical cyclones may develop. The highest wind speeds can be used to locate the jet stream. Surface weather analysis maps are weather maps that only show conditions on the ground (Figure 1.3). Surface analysis maps may show sea level mean pressure, temperature, and amount of cloud cover. Click image to the left or use the URL below. URL: "
weather versus climate,T_1883,"All weather takes place in the atmosphere, virtually all of it in the lower atmosphere. Weather describes what the atmosphere is like at a specific time and place. A locations weather depends on: air temperature air pressure fog humidity cloud cover precipitation wind speed and direction All of these characteristics are directly related to the amount of energy that is in the system and where that energy is. The ultimate source of this energy is the Sun. Weather is the change we experience from day to day. Weather can change rapidly. "
weather versus climate,T_1884,"Although almost anything can happen with the weather, climate is more predictable. The weather on a particular winter day in San Diego may be colder than on the same day in Lake Tahoe, but, on average, Tahoes winter climate is significantly colder than San Diegos (Figure 1.1). Climate is the long-term average of weather in a particular spot. Good climate is why we choose to vacation in Hawaii in February, even though the weather is not guaranteed to be good! A locations climate can be described by its air temperature, humidity, wind speed and direction, and the type, quantity, and frequency of precipitation. The climate for a particular place is steady, and changes only very slowly. Climate is determined by many factors, including the angle of the Sun, the likelihood of cloud cover, and the air pressure. All of these factors are related to the amount of energy that is found in that location over time. The climate of a region depends on its position relative to many things. These factors are described in the next sections. Click image to the left or use the URL below. URL:  Click image to the left or use the URL below. URL: "
wind power,T_1890,"Energy from the Sun also creates wind, which can be used as wind power. The Sun heats different locations on Earth by different amounts. Air that becomes warm rises and then sucks cooler air into that spot. The movement of air from one spot to another along the ground creates wind. Since wind is moving, it has kinetic energy. Wind power is the fastest growing renewable energy source in the world. Windmills are now seen in many locations, either individually or, more commonly, in large fields. "
wind power,T_1891,"Wind is the source of energy for wind power. Wind has been used for power for centuries. For example, windmills were used to grind grain and pump water. Sailing ships traveled by wind power long before ships were powered by fossil fuels. Wind can be used to generate electricity, as the moving air spins a turbine to create electricity (Figure Click image to the left or use the URL below. URL: "
wind power,T_1892,"Wind power has many advantages. It does not burn, so it does not release pollution or carbon dioxide. Also, wind is plentiful in many places. Wind, however, does not blow all of the time, even though power is needed all of the time. Just as with solar power, engineers are working on technologies that can store wind power for later use. Windmills are expensive and wear out quickly. A lot of windmills are needed to power a region, so nearby residents may complain about the loss of a nice view if a wind farm is built. Coastlines typically receive a lot of wind, but wind farms built near beaches may cause unhappiness for local residents and tourists. The Cape Wind project off of Cape Cod, Massachusetts has been approved but is generating much controversy. Opponents are in favor of green power but not at that location. Proponents say that clean energy is needed and the project would supply 75% of the electricity needed for Cape Cod and nearby islands (Figure 1.2). California was an early adopter of wind power. Windmills are found in mountain passes, where the cooler Pacific Ocean air is sucked through on its way to warmer inland valleys. Large fields of windmills can be seen at Altamont Pass in the eastern San Francisco Bay Area, San Gorgonio Pass east of Los Angeles, and Tehachapi Pass at the southern end of the San Joaquin Valley. "
scientific ways of thinking,T_1898,"Most people think of science as a collection of facts or a body of knowledge. For example, you may have memorized the processes of the water cycle. As shown in Figure 1.1, the processes include evaporation and precipitation. Such knowledge of the natural world is only part of what science is. Science is as much about doing as knowing. Science is a way of learning about the natural world that depends on evidence, reasoning, and repeated testing. Scientists explain the world based on their observations. If they develop new ideas about the way the world works, they set up ways to test these new ideas. Scientific knowledge keeps changing because scientists are always doing science. "
scientific ways of thinking,T_1899,"When Miranda and Jeanny wondered whether bacteria might decompose plastic, they were thinking like a scientist. What does it mean to think like a scientist? A scientist is observant. Miranda and Jeanny observed all the plastic trash when they visited a landfill. They also saw a lot of plastic trash along a local river. A scientist wonders and asks questions. Miranda and Jeanny wondered if any bacteria could help break down plastic. They asked: Can some bacteria consume chemicals in plastic for food? A scientist tries to find answers using evidence and logic. Often, a scientist does experiments to gather more evidence and test ideas. Miranda and Jeanny did a lot of online research to find out what other scientists had already learned. Then they did their own experiments. They gathered and tested bacteria. For example, they grew bacteria on gel like the red gel in Figure 1.2. You can learn the details of their research and their amazing results by watching this video: A scientist is skeptical. Claims must be backed by adequate evidence. Miranda and Jeanny repeated their experiments so they were confident in their results. Only then did they draw conclusions. A scientist has an open mind. Scientific knowledge is always evolving as new evidence comes in. Miranda and Jeanny made an important contribution with the evidence they gathered. They discovered two species of bacteria that could consume a harmful chemical in plastic. "
scientific ways of thinking,T_1900,"Some knowledge in science gains the status of a theory. Scientists use the term theory differently than it is used in everyday language. You might say, I think my dad is late because he got stuck in traffic, but its just a theory. In other words, its just one of many possible explanations for why hes late. In science, a theory is much more than that. A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence. Scientific theories are tested and confirmed repeatedly. Because theories are broad explanations, they generally help explain many different observations. An example in life science is the theory of evolution by natural selection. It explains how living things change through time as they adapt to their environment. This theory is supported by a huge amount of evidence. The evidence ranges from DNA to fossils like the ones in Figure 1.3. Another sort of scientific knowledge is called a law. A scientific law is a description of what always occurs under certain conditions in nature. In other words, it describes many observations but doesnt explain them. Examples of scientific laws in life science include Mendels laws of inheritance. These laws describe how traits are passed from parents to their offspring. "
what is life science,T_1901,"Life is complex, and there are millions of species alive today. Many millions more lived in the past and then went extinct. Organisms include microscopic, single-celled organisms. They also include complex, multicellular animals such as you. Clearly, life science is a huge science. Thats why a life scientist usually specializes in just one field within life science. Dr. Smith, for example, specializes in ecology. You can see the focus of ecology and several other life science fields in Table 1.1. Click on the links provided if you want to learn about careers in these fields. Field Ecology Focus of Study interactions of organisms with each other and their environment Botany Zoology plants animals Microbiology microorganisms such as bacteria Entomology insects Cell biology cells of living things Physiology tissues and organs and how they function genes, traits, and inheritance Genetics Epidemiology Paleontology causes of diseases and how they spread fossils and evolution Learn about a Career in this Field "
what is life science,T_1902,"Each field of life science has its own specific body of knowledge and relevant theories. However, two theories are basic to all of the life sciences. They form the foundation of every life science field. They are the cell theory and the theory of evolution by natural selection. Both theories have been tested repeatedly. Both are supported by a great deal of evidence. "
what is life science,T_1903,"According to the cell theory, all organisms are made up of one or more cells. Cells are the sites where all life processes take place. Cells come only from pre-existing cells. New cells forms when existing cells divide. Most cells are too small to see without a microscope. If you were to look at a drop of your blood under a microscope, Figure 1.5 shows two types of cells you might see. You can learn more about cells and the cell theory in the chapter Cells and Their Structures. "
what is life science,T_1904,"The theory of evolution by natural selection explains how populations of organisms can change over time. As environments change, so must the traits of organisms if they are to survive in the new conditions. Evolution by natural selection explains how this happens. It also explains why there are so many different species of organisms on Earth today. You can see examples of the incredible diversity of living animals in Figure 1.6. You can read more about the theory of evolution in the chapter Evolution. "
what is life science,T_1905,"Most scientific theories were developed by scientists doing basic scientific research. Like other sciences, life science may be either basic or applied science. "
what is life science,T_1906,"The aim of basic science is to discover new knowledge. It leads to a better understanding of the natural world. It doesnt necessarily have any practical use. An example of basic research in life science is studying how yeast cells grow and divide. Yeasts are single-celled organisms that are easy to study. By studying yeast cells, life scientists discovered the series of events called the cell cycle. The cell cycle works not only in yeasts but in all other organisms with similar cells. Therefore, this basic research made a major contribution to our understanding of living things. Watch the following animation to learn more about the basic yeast research and the cell cycle. You can also see yeast cells dividing. "
what is life science,T_1907,"Knowledge gained by this basic research on yeast cells has been applied to practical problems. Scientists have developed drugs to treat cancer based on knowledge of the cell cycle. Cancer is a disease in which cells divide out of control. The new drugs interfere with the cell cycle of cancer cells, so the cells stop dividing. This is an example of applied science. The aim of applied science is to find solutions to practical problems. Applied science generally rests on knowledge gained by basic science. "
the scientific method,T_1908,A life scientist would carry out a scientific investigation to try to answer this question. A scientific investigation follows a general plan called the scientific method. The scientific method is a series of logical steps for testing a possible answer to a question. The steps are shown in the flow chart in Figure 1.8. 
the scientific method,T_1909,"The steps of the scientific method are described in greater detail below. Note that these steps are meant as a guide, not a rigid sequence. Steps may be followed in a somewhat different order, for example, or steps may be repeated or skipped. 1. Make observations. Observations refer to anything detected with one or more senses. The senses include sight, hearing, touch, smell, and taste. 2. Ask a question raised by the observations. 3. Form a hypothesis. A hypothesis is a potential, testable answer to a scientific question. Testable means that if the hypothesis is false, its possible to find evidence showing that its false. This step usually requires some research. You have to find out what other investigators have already learned about the observations. For example, has anyone already tried to answer the question? What other hypotheses have been proposed? 4. Test the hypothesis. Make predictions based on the hypothesis and then determine if they are correct. This may involve carrying out an experiment. An experiment is a controlled scientific test that often takes place in a lab. It investigates the effects of one factor, called the independent variable, on another factor, called the dependent variable. Experimental controls are other factors that might affect the dependent variable. Controls are kept constant so they will not affect the results of the experiment. 5. Analyze the results of the test and draw a conclusion. Do the results agree with the predictions? If so, they provide support for the hypothesis. If not, they disprove the hypothesis. 6. Communicate results. One way is by presenting a poster at a scientific conference, like the scientists in Figure are communicated, scientists should describe their hypothesis and how it was tested in addition to the results of the test. This allows other scientists to repeat the investigation to see whether they get the same results. This is called replication. Replication is important because it adds weight to the findings. The results are more likely to be reliable if they can be repeated. "
the scientific method,T_1910,"You can apply the scientific method to the question that was raised above about athletic ability. Assume you are a life scientist. You observe variation in athletic abilities. Some athletes tend to build more muscle mass. Others tend to develop greater endurance. You ask, Is there a gene that might explain these differences? You research the problem on the Internet. You learn about a gene named ACE that might affect how people respond to athletic training. Based on all of your research, you develop a hypothesis. You hypothesize that people with different versions (D or I) of the ACE gene will respond differently to the same athletic training program. People with D genes will increase their muscle mass but not their endurance. People with I genes will increase their endurance but not their muscle mass. How can you test your hypothesis? You can see how actual life science researchers did it by watching this video: MEDIA Click image to the left or use the URL below. URL: "
safety in life science research,T_1916,"A science lab has many potential dangers. Thats why lab procedures and equipment are often labeled with safety symbols, like the ones in Figure 1.14. These symbols warn of specific hazards, such as flames or broken glass. Learn the symbols so you can recognize the dangers. Then learn how to avoid them. The best way to avoid lab dangers is to follow the lab safety rules listed below. Following the rules can help prevent accidents. Watch this funny student video to see just how important some of these rules are:  MEDIA Click image to the left or use the URL below. URL: "
safety in life science research,T_1917,"Wear long sleeves and shoes that completely cover your feet. If your hair is long, tie it back or cover it with a hair net. Protect your eyes, skin, and clothing by wearing safety goggles, an apron, and gloves. Use hot mitts to handle hot objects. Never work alone in the lab. Never engage in horseplay in the lab. Never eat or drink in the lab. Never do experiments without your teachers approval. Always add acid to water, never the other way around. Add the acid slowly to avoid splashing. Take care to avoid knocking over Bunsen burners. Keep them away from flammable materials such as paper. Use your hand to fan vapors toward your nose rather than smelling substances directly. Never point the open end of a test tube toward anyoneincluding you! Clean up any spills immediately. Dispose of lab wastes according to your teachers instructions. Wash glassware and counters when you finish your work. Wash your hands with soap and water before leaving the lab. "
safety in life science research,T_1918,"Many of the lab safety rules are common-sense precautions. Common-sense should also prevail in the field. Be aware, however, that field research may have its own unique dangers. Therefore, other safety rules may apply when you work in the field. The rules will depend on the particular field setting and its specific risks. Consider the field botanist in Figure 1.13. There may be microorganisms in the water that could make her sick. She might come into contact with plants that cause an allergic reaction. The water or shore might be strewn with dangerous objects such as broken glass that could cause serious injury. To stay safe in the field, she needs to be aware of these risks and take steps to avoid them. If you work in the field or take a science fieldtrip, you should do the sameand always follow your teachers instructions. "
safety in life science research,T_1919,"Even when you follow the rules, accidents can happen. Immediately alert your teacher if an accident occurs. Report all accidents, whether or not you think they are serious. "
introduction to plants,T_1920,"Plants are multicellular eukaryotes that are placed in the Plant Kingdom. Plant cells have cell walls that are made of cellulose. Plant cells also have chloroplasts. They allow plants to make food by photosynthesis. In addition, plants have specialized reproductive organs that produce gametes. Male reproductive organs produce sperm. Female reproductive organs produce eggs. Male and female reproductive organs may be on the same plant or on different plants. "
introduction to plants,T_1921,"Plants are somewhat limited by temperature in terms of where they can grow. They need temperatures above freezing while they are actively growing. They also need light, carbon dioxide, and water. These substances are required for photosynthesis. Like most other living things, plants need oxygen. Oxygen is required for cellular respiration. In addition, plants need minerals. The minerals are required to make proteins and other organic molecules. "
introduction to plants,T_1922,"Life as we know it would not be possible without plants. Why are plants so important? Plants supply food to nearly all land organisms, including people. We mainly eat either plants or other living things that eat plants. Plants produce oxygen during photosynthesis. Oxygen is needed by all aerobic organisms. Plants absorb carbon dioxide during photosynthesis. This helps control the greenhouse effect and global warming. Plants recycle matter in ecosystems. For example, they are an important part of the water cycle. They take up liquid water from the soil through their roots. They release water vapor to the air from their leaves. This is called transpiration. Plants provide many products for human use. They include timber, medicines, dyes, oils, and rubber. Plants provide homes for many other living things. For example, a single tree may provide food and shelter to many species of animals, like the birds in Figure 10.2. "
introduction to plants,T_1923,"A tissue is a group of specialized cells of the same kind that perform the same function. Modern plants have three major types of tissues. Theyre called dermal, ground, and vascular tissues. "
introduction to plants,T_1924,Dermal tissue covers the outside of a plant. Its like the plants skin. Cells of dermal tissue secrete a waxy substance called cuticle. Cuticle helps prevent water loss and damage to the plant. 
introduction to plants,T_1925,Ground tissue makes up much of the inside of a plant. The cells of ground tissue carry out basic metabolic functions and other biochemical reactions. Ground tissue may also store food or water. 
introduction to plants,T_1926,"Vascular tissue runs through the ground tissue inside a plant. It transports fluids throughout the plant. Vascular tissue actually consists of two types of tissues, called xylem and phloem. The two types of vascular tissue are packaged together in bundles. You can see them in the celery in Figure 10.3. Xylem carries water and dissolved minerals from the roots upward to the leaves. Phloem carries water and dissolved sugar from the leaves to other parts of the plant. "
introduction to plants,T_1927,"An organ is a structure composed of two or more types of tissues that work together to do a specific task. Most modern plants have several organs that help them survive and reproduce in a variety of habitats. Major organs of most plants include roots, stems, and leaves. These and other plant organs generally contain all three major tissue types. "
introduction to plants,T_1928,"Roots are important organs in most modern plants. There are two types of roots: primary roots, which grow downward; and secondary roots, which branch out to the sides. Together, all the roots of a plant make up the plants root system. Figure 10.4 shows two different types of plant root systems. A taproot system has a very long primary root, called a taproot. A fibrous root system has many smaller roots and no large, primary root. The roots of plants have three major jobs: absorbing water and minerals, anchoring and supporting the plant, and storing food. Roots are covered with thin-walled dermal cells and tiny root hairs. These features are well suited to absorb water and dissolved minerals from the soil. Root systems help anchor plants to the ground. They allow plants to grow tall without toppling over. A tough covering may replace the dermal cells in older roots. This makes them ropelike and even stronger. In many plants, ground tissue in roots stores food produced by the leaves during photosynthesis. "
introduction to plants,T_1929,"Stems are organs that hold plants upright. They allow plants to get the sunlight and air they need. Stems also bear leaves, flowers, cones, and smaller stems. These structures grow at points called nodes. The stem between nodes is called an internode. (See Figure 10.5.) Stems are needed for transport and storage. Their vascular tissue carries water and minerals from roots to leaves. It carries dissolved sugar from the leaves to the rest of the plant. Without this connection between roots and leaves, plants could not survive high above the ground in the air. In many plants, ground tissue in stems also stores food or water during cold or dry seasons. "
introduction to plants,T_1930,"Leaves are the keys not only to plant life but to virtually all life on land. The primary role of leaves is to collect sunlight and make food by photosynthesis. Leaves vary in size, shape, and how they are arranged on stems. You can see examples of different types of leaves in Figure 10.6. Each type of leaf is well suited for the plants environment. It maximizes light exposure while conserving water, reducing wind resistance, or benefiting the plant in some other way in its particular habitat. For example, some leaves are divided into many smaller leaflets. This reduces wind resistance and water loss. Leaves are basically factories for photosynthesis. A factory has specialized machines to produce a product. In a leaf, the ""machines"" are the chloroplasts. A factory is connected to a transportation system that supplies it with raw materials and carries away the finished product. In a leaf, transport is carried out by veins containing vascular tissue. Veins carry water and minerals to the cells of leaves. They carry away dissolved sugar. A factory has bricks, siding, or other external protection. A leaf is covered with dermal cells. They secrete waxy cuticle to prevent evaporation of water from the leaf. A factory has doors and windows to let some materials enter and leave. The surface of the leaf has tiny pores called stomata (stoma, singular). They can open and close to control the movement of gases between the leaves and the air. You can see a close-up of a stoma in Figure 10.7. "
introduction to plants,T_1931,"Most plants continue to grow throughout their lives. Like other multicellular organisms, plants grow through a combination of cell growth and cell division. Cell growth increases cell size. Cell division increases the number of cells. As plant cells grow, they also become specialized into different cell types. Once cells become specialized, they can no longer divide. So how do plants grow after that? The key to continued growth is meristem. Meristem is a type of plant tissue consisting of undifferentiated cells that can continue to divide. Meristem at the tips of roots and stems allows them to grow in length. This is called primary growth. The stem (trunk) of the giant sequoia tree in Figure 10.8 has achieved amazing growth in length during its many years of life. Meristem within and around vascular tissues allows growth in width. This is called secondary growth. The rings in the tree stump in Figure 10.8 show secondary growth in a tree. Each ring represents one year of growth. "
introduction to plants,T_1932,All plants have a life cycle that includes alternation of generations. You can see a general plant life cycle in Figure MEDIA Click image to the left or use the URL below. URL: 
introduction to plants,T_1933,Plants alternate between haploid and diploid generations. Haploid cells have one of each pair of chromosomes. Diploid cells have two of each pair. Plants in the haploid generation are called gametophytes. They form from haploid spores. They have male and/or female reproductive organs and reproduce sexually. They produce haploid gametes by mitosis. Fertilization of gametes produces diploid zygotes. Zygotes develop into the diploid generation. Plants in the diploid generation are called sporophytes. They form from the fertilization of gametes. They reproduce asexually. They have a structure called a sporangium that produces haploid spores by meiosis. Spores develop into the haploid generation. Then the cycle repeats. 
introduction to plants,T_1934,"One of the two generations of a plants life cycle is usually dominant. Individuals in the dominant generation generally live longer and grow larger. They are the organisms that you would recognize as a fern, tree, or other plant. Individuals in the nondominant generation tend to be smaller and shorter-lived. They often live in or on the dominant plant. They may go unnoticed. Early plants spent most of their life cycle as gametophytes. Some modern plants such as mosses still have this type of life cycle. However, almost all modern plants spend most of their life cycle as sporophytes. "
evolution and classification of plants,T_1935,"The first plants were probably similar to the stoneworts in Figure 10.11. Stoneworts are green algae. Like stoneworts, the first plants were aquatic. They may have had stalks but not stems. They also may have had hair-like structures called rhizoids but not roots. The first plants probably had male and female reproductive organs and needed water to reproduce. In stoneworts, sperm need at least a thin film of moisture to swim to eggs. "
evolution and classification of plants,T_1936,"By the time the earliest plants evolved, animals were already the dominant living things in the water. Plants were also limited to the upper layer of water. Only near the top of the water column is there enough sunlight for photosynthesis. So plants never became dominant aquatic organisms. "
evolution and classification of plants,T_1937,"All that changed when plants moved from water to land. This may have happened by 500 million years ago or even earlier. On land, everything was wide open. There were no other living things. Without plants, there was nothing for other organisms to eat. Land could not be colonized by other organisms until land plants became established. The earliest land plants may have resembled the modern liverworts in Figure 10.12. "
evolution and classification of plants,T_1938,"Moving to the land was a huge step in plant evolution. Until then, virtually all life had evolved in water. Dry land was a very different kind of place. The biggest problem was the dryness. Simply absorbing enough water to stay alive was a huge challenge. It kept early plants small and low to the ground. Water was also needed for sexual reproduction, so sperm could swim to eggs. There were other hardships on land besides dryness. For example, sunlight on land was strong and dangerous. Solar radiation put land organisms at high risk of mutations. "
evolution and classification of plants,T_1939,"After they left the water, plants evolved adaptations that helped them withstand the harsh conditions on land. One of the earliest and most important adaptations to evolve was vascular tissue. For a fast-paced introduction to vascular plants and their successes, watch this video:  . MEDIA Click image to the left or use the URL below. URL: "
evolution and classification of plants,T_1940,"Vascular tissue forms a plants ""plumbing system."" It carries water and dissolved minerals from the soil to all the other cells of the plant. It also carries food (sugar dissolved in water) from photosynthetic cells to other cells in the plant for growth or storage. The evolution of vascular tissue revolutionized the plant kingdom. Vascular tissue greatly improved the ability of plants to absorb and transfer water. This allowed plants to grow larger and taller. They could also liver in drier habitats and survive periods of drought. Early vascular plants probably resembled the fern in Figure 10.13. "
evolution and classification of plants,T_1941,"Other early adaptations to life on land included the evolution of true leaves and roots. Leaves allowed plants to take better advantage of sunlight for photosynthesis. Roots helped plants absorb water and minerals from soil. Early land plants also evolved a dominant sporophyte generation. Sporophytes are diploid, so they have two copies of each gene. This gives them a ""back-up"" copy in case of mutation. This was important for coping with the strong solar radiation and higher risk of mutations on land. "
evolution and classification of plants,T_1942,"With all these adaptations, its easy to see why vascular plants were very successful. They spread quickly and widely on land. As vascular plants spread, many nonvascular plants went extinct. Vascular plants became and remain the dominant land plants on Earth. "
evolution and classification of plants,T_1943,"Early vascular plants still needed moisture. They needed it in order to reproduce. Sperm had to swim from male to female reproductive organs for fertilization. Even spores needed some water to grow and often to disperse as well. In addition, dryness and other harsh conditions made it very difficult for tiny new offspring plants to survive. With the evolution of seeds in vascular plants, all that changed. Seed plants evolved a number of adaptations that made it possible to reproduce without water. Seeds also nourished and protected tiny new offspring. As a result, seed plants were wildly successful. They exploded into virtually all of Earths habitats. "
evolution and classification of plants,T_1944,"A seed is a reproductive structure that contains an embryo and a food supply, called endosperm. Both the embryo and endosperm are enclosed within a tough outer coating, called a hull (or shell). You can see these parts of a seed in Figure 10.14. An embryo is a zygote that has already started to develop and grow. Early growth and development of a plant embryo inside a seed is called germination. The seed protects and nourishes the embryo and gives it a huge head start in the ""race"" of life. Both a parent plant and its offspring are better off if they dont grow too closely together. That way, they will not need to compete for resources. Many seeds have structures that help them travel away from the parent plant. You can see some examples in Figure 10.15. Some seeds can also wait to germinate until conditions are favorable for growth. This increases the offsprings chances of surviving even more. "
evolution and classification of plants,T_1945,"Seed plants also evolved other reproductive structures. These included ovules, pollen, and pollen tubes. An ovule is a female reproductive structure in seed plants. It contains a tiny female gametophyte. The gametophyte produces an egg cell. After the egg is fertilized by sperm, the ovule develops into a seed. Pollen is a tiny male gametophyte enclosed in a tough capsule. Pollen carries sperm to an ovule while preventing the sperm from drying out. Pollen grains cant swim, but they are very light, so the wind can carry them. Therefore, they can travel through air instead of water. Pollen also evolved the ability to grow a tube, called a pollen tube. Sperm could be transferred through the tube directly from the pollen grain to the egg. This allowed sperm to reach an egg without swimming through a film of water. It finally freed plants from depending on moisture to reproduce. "
evolution and classification of plants,T_1946,"The first seed plants formed seeds in cones, like the cone in Figure 10.16. Cones are reproductive structures made of overlapping scales. Scales are modified leaves. Male cones contain pollen. Female cones contain eggs. They are also where seeds develop. The seeds in cones are ""naked."" They arent protected inside an ovary, which was a later adaptation of seed plants. "
evolution and classification of plants,T_1947,Some seed plants evolved another major adaptation. This was the formation of seeds in flowers. Flowers are plant structures that contain male and/or female reproductive organs. 
evolution and classification of plants,T_1948,"You can see the parts of a typical flower in Figure 10.17. The male reproductive organ in a flower is the stamen. It has a stalk-like filament that ends in an anther. The anther is where pollen forms. The female reproductive organ in a flower is the pistil. It consists of a stigma, style, and ovary. The stigma is the top of the pistil. It is sticky to help it ""catch"" pollen. The style connects the stigma to the ovary. The ovary is where eggs form and seeds develop. As seeds develop, the ovary turns into a fruit. The fruit protects the seeds. It also attracts animals that may eat the fruit and help disperse the seeds. Petals are usually the most visible parts of a flower. They may be large and showy and are often brightly colored. Leaf-like green sepals protect the flower while it is still a bud. "
evolution and classification of plants,T_1949,"The showy petals of flowers evolved to help attract pollinators. Wind-blown pollen might land just anywhere and be wasted. A pollinator is an animal that picks up pollen on its body and carries it directly to another flower of the same species. This helps ensure that pollination occurs. Pollinators are usually small animals such as bees, butterflies, and bats. You can see an example in Figure 10.18. "
evolution and classification of plants,T_1950,"The most basic division of modern plants is between nonvascular and vascular plants. Vascular plants are further divided into those that reproduce without seeds and those that reproduce with seeds. Seed plants, in turn, are divided into those that produce naked seeds in cones and those that produce seeds in the ovaries of flowers. "
evolution and classification of plants,T_1951,"Modern nonvascular plants are called bryophytes. There are about 17,000 bryophyte species. They include liver- worts, hornworts, and mosses. Mosses are the most numerous group of bryophytes. You can see an example of moss in Figure 10.19. Like the moss in the figure, most bryophytes are small. They lack not only vascular tissues. They also lack true roots, leaves, seeds, and flowers. Bryophytes live in moist habitats. Without the adaptations of vascular plants, bryophytes are not very good at absorbing water. They also need water to reproduce. "
evolution and classification of plants,T_1952,Todays vascular plants are called tracheophytes. Their vascular tissue is specialized to transport fluid. This allows them to grow tall and take advantage of sunlight high up in the air. It also allows them to live in drier habitats. Most modern plants are tracheophytes. There are hundreds of thousands of species of them. Seedless vascular plants include plants such as ferns. You can see a fern in Figure 10.20. Ferns reproduce with spores instead of seeds. The black dots on the back of the fern leaf in Figure 10.20 are spores. 
evolution and classification of plants,T_1953,"Seed plants are vascular plants that reproduce with seeds. Modern seed plants are called spermatophytes. Seeds allow the plants to reproduce without water. Most vascular plants today are seed plants. Modern seed plants include gymnosperms and angiosperms. Gymnosperms are seed plants that produce naked seeds in cones. There are about 1000 species of gym- nosperms. Conifers are the most common group of gymnosperms. The spruce tree in Figure 10.21 is an example of a conifer. Angiosperms are seed plants that produce seeds in the ovaries of flowers. Today, they are by far the most diverse type of seed plants. In fact, the vast majority of all modern plants are angiosperms. There are hundreds of thousands of species of them. The apple tree in Figure 10.21 is an example of a common angiosperm. "
plant responses and special adaptations,T_1954,"Instead of fleeing, a plants primary way of responding is to change how it is growing. One way is by tropisms. "
plant responses and special adaptations,T_1955,"A tropism is a turning toward, or away from, a stimulus in the environment. Examples of tropisms in plants include gravitropism and phototropism. You can see both tropisms in action in this amazing time-lapse video:  MEDIA Click image to the left or use the URL below. URL:  Gravitropism is a response to gravity. Plant roots always grow downward because of the pull of Earths gravity. Specialized cells in the tips of plant roots detect and respond to gravity in this way. Phototropism is a response to light. Plant stems and leaves grow toward a light source. The house plant in Figure 10.23 shows the effects of phototropism. The plant receives light mainly from the left so it grows in that direction. "
plant responses and special adaptations,T_1956,"Plants also detect and respond to the daily cycle of light and darkness. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme coldness and dryness of winter. Part of this response causes the leaves of many trees to change color and then fall off (see Figure 10.24). Dormancy ensures that plants will grow and produce seeds only when conditions are favorable. "
plant responses and special adaptations,T_1957,"Plants dont have an immune system, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens. For example, willow trees, like the one in Figure Exciting new research suggests that plants may even produce chemicals that warn other, nearby plants of threats to their health. The warnings allow nearby plants to prepare for their own defense. As these and other responses show, plants may be rooted in place, but they are far from helpless. "
plant responses and special adaptations,T_1958,"Plants live just about everywhere on Earth. To live in so many different habitats, they have evolved adaptations that allow them to survive and reproduce under a diversity of conditions. Some plants have evolved special adaptations that let them live in extreme environments. "
plant responses and special adaptations,T_1959,"All plants are adapted to live on land. Or are they? All living plants today have land-plant ancestors. But some plants now live in the water. They have had to evolve new adaptations for their watery habitat. Modern plants that live in water are called aquatic plants. Living in water has certain advantages for plants. One advantage is, well, the water. Theres plenty of it and its all around. Therefore, most aquatic plants do not need adaptations for absorbing, transporting, and conserving water. They can save energy and matter by not growing extensive root systems, vascular tissues, or thick cuticle on leaves. Support is also less of a problem because of the buoyancy of water. As a result, adaptations such as strong woody stems and deep anchoring roots are not necessary for most aquatic plants. Living in water does present challenges to plants, however. For one thing, pollination by wind or animals isnt feasible under water. Sunlight also cant penetrate very far below the water surface. Thats why some aquatic plants have adaptations that help them keep their flowers and leaves above water. An example is the water lily, shown in Figure 10.26. The water lily has bowl-shaped flowers and broad, flat leaves that float. Plants that live in moving water, such as streams or rivers, may have different adaptations. For example, the cattails shown in Figure 10.26 have narrow, strap-like leaves that reduce their resistance to moving water. "
plant responses and special adaptations,T_1960,"Plants that live in extremely dry environments have the opposite problem: how to get and keep water. Plants that are adapted to very dry environments are called xerophytes. Their adaptations may help them increase water intake, decrease water loss, or store water when its available. The saguaro cactus pictured in Figure 10.27 has adapted in all three ways. When it was still a very small plant, just a few inches high, its shallow roots already reached out as much as 2 meters (7 feet) from the base of the stem. By now, its root system is much more widespread. It allows the cactus to gather as much moisture as possible from rare rainfalls. The saguaro doesnt have any leaves to lose water by transpiration. It also has a large, barrel-shaped stem that can store a lot of water. Thorns protect the stem from thirsty animals that might try to get at the water inside. "
plant responses and special adaptations,T_1961,"Plants called epiphytes grow on other plants. They obtain moisture from the air instead of the soil. Most epiphytes are ferns or orchids that live in rainforests. Host trees provide support for the plants. They allow epiphytes to get air and sunlight high above the forest floor. This lets the plants get out of the shadows on the forest floor so they can get enough light for photosynthesis. Being elevated may also reduce the risk of being eaten by herbivores. In addition, it may increase the chances of pollination by wind. "
plant responses and special adaptations,T_1962,"Carnivorous plants are plants that get some or most of their nutrients (but not energy or carbon compounds) from other organisms. They trap and digest insects or other small animals or protozoa. However, they still need sunlight in order to make food by photosynthesis. Carnivorous plants have adapted to grow in places where the soil is thin or poor in nutrients. They are found in places such as bogs and rock outcroppings. Venus fly traps, like those in Figure in action:  . MEDIA Click image to the left or use the URL below. URL: "
what are animals,T_1963,"Animals are multicellular eukaryotes in the Animal Kingdom. All animals are heterotrophs. They eat other living things because they cant make their own food. All animals also have specialized cells that can do different jobs. Most animals have higher levels of organization as well. They may have specialized tissues, organs, and even organ systems. Having higher levels of organization allows animals to perform many complex functions. For a visual introduction to what makes a living thing an animal, watch this short video:  MEDIA Click image to the left or use the URL below. URL: "
what are animals,T_1964,"Like the cells of all eukaryotes, animal cells have a nucleus and other membrane-bound organelles. Unlike the cells of eukaryotes in the Plant and Fungus Kingdoms, animal cells lack a cell wall. This gives animal cells flexibility. It lets them take on different shapes. This in turn allows them to become specialized for particular jobs. The human nerve cell in Figure 11.2 is a good example of a specialized animal cell. Its shape suits it for its function of sending nerve signals to other cells. A nerve cell couldnt take this shape if it were surrounded by a rigid cell wall. "
what are animals,T_1965,"With their specialized cells and higher levels of organization, animals can do several things that other eukaryotes cannot. Animals can detect and quickly respond to a variety of stimuli. They have specialized nerve cells that can detect light, sound, touch, or other stimuli. Most animals also have a nervous system that can direct the body to respond to the stimuli. All animals can move, at least during some stage of their life cycle. Specialized muscle and nerve tissues work together to allow movement. Being able to move lets animals actively search for food and mates. It also helps them escape from predators and other dangers. Virtually all animals have internal digestion of food. Animals consume other organisms and may use special tissues and organs to digest them. (Other heterotrophs, such as fungi, absorb nutrients directly from the environment.) "
what are animals,T_1966,"Many animals have a relatively simple life cycle. A general animal life cycle is shown in Figure 11.3. Most animals spend the majority of their life as diploid organisms. Just about all animals reproduce sexually. Diploid adults undergo meiosis to produce haploid sperm or eggs. Fertilization occurs when a sperm and an egg fuse. The diploid zygote that forms develops into an embryo. The embryo eventually develops into an adult, often going through one or more larval stages on the way. A larva (larvae, plural) is a distinct juvenile form that many animals go through before becoming an adult. The larval form may be very different from the adult form. For example, a caterpillar is the larval form of an insect that becomes a butterfly as an adult. "
what are animals,T_1967,"The Animal Kingdom is one of four kingdoms in the Eukarya Domain. The Animal Kingdom, in turn, is divided into almost 40 phyla. Table 11.1 lists the 9 animal phyla that contain the largest numbers of species. Each phylum in the table has at least 10,000 species. Phylum Porifera Animals It Includes sponges Cnidaria jellyfish, corals Platyhelminthes flatworms, tapeworms, flukes Nematoda roundworms Mollusca snails, clams, squids Phylum Annelida Animals It Includes earthworms, leeches, marine worms Arthropoda insects, spiders, crustaceans, cen- tipedes Echinodermata sea stars, sea urchins, sand dollars, sea cucumbers Chordata tunicates, lancelets, fish, amphib- ians, reptiles, birds, mammals One basic way to divide animals is between invertebrates and vertebrates. Invertebrates are animals that lack a vertebral column, or backbone. All the phyla in Table 11.1, except the Phylum Chordata, consist only of invertebrates. Even the Phylum Chordata includes some invertebrate taxa. Invertebrates make up about 95 percent of all animal species. Vertebrates are animals that have a backbone. All of them are placed in the Phylum Chordata. Modern vertebrates include fish, amphibians, reptiles, birds, and mammals. Only about 5 percent of animal species are vertebrates. "
how animals evolved,T_1968,"The partial geologic time scale in Figure 11.5 shows when some of the major events in animal evolution took place. The oldest animal fossils are about 630 million years old, so presumably animals evolved around that time or somewhat earlier. The earliest animals were aquatic invertebrates. The first vertebrates evolved around 550 million years ago. By 500 million years ago, most modern phyla of animals had evolved. The first terrestrial animals evolved about 50 million years after that. "
how animals evolved,T_1969,Animals evolved many important traits that set them apart from other eukaryotes. The traitsand the order in which they evolvedinclude: multicellularity and cell specialization; tissues and higher levels of organization; body symmetry; third embryonic cell layer (mesoderm); digestive system; fluid-filled body cavity (coelom); segmented body; and notochord. Each of these traits is described below. All of them evolved in invertebrates. Each major trait to evolve led to a new stage in animal evolution. The phyla in Table 11.1 represent modern animals at each of these major stages. Refer back to the table as you read about the evolution of these traits. 
how animals evolved,T_1970,"The first animal trait to evolve was multicellularity. This is the presence of multiple cells in a single organism. Scientists think that the earliest animals with multiple cells evolved from animal-like protists that lived in colonies. Some of the cells in the colonies became specialized for different jobs. After a while, the specialized cells came to need each other for survival. Thus, the first multicellular animals evolved. Multicellularity was highly adaptive. Multiple cells could do different jobs. They could evolve special adaptations that allowed them to do a particular job really well. Modern animals that represent this stage of animal evolution are sponges. They are placed in Phylum Porifera (see Table 11.1). They have multiple specialized cells, but their cells are not organized into tissues. "
how animals evolved,T_1971,"The next major stage of animal evolution was the evolution of tissues. It was the first step in the evolution of organs and organ systems. At first, invertebrates developed tissues from just two embryonic cell layers. There was an outer cell layer called ectoderm and an inner cell layer called endoderm. The two cell layers allowed different types of tissues to form. Modern animals that represent this stage of evolution include jellyfish. They are placed in Phylum Cnidaria. "
how animals evolved,T_1972,"Another trait that evolved early was symmetry. A symmetrical organism can be divided into two identical halves. Both the coral and the beetle in Figure 11.6 have symmetry, while the sponge lacks symmetry. There are two types of symmetry: radial and bilateral. Radial symmetry is demonstrated by the coral in Figure 11.6. It can be divided into identical halves along any diameter, just like a circular pie. Radial symmetry was the first type of symmetry to evolve. Animals with radial symmetry, such as cnidarians, have no sense of left or right. This makes controlled movement in these directions impossible. Bilateral symmetry is demonstrated by the beetle in Figure 11.6. It can be divided into identical halves just down the middle from top to bottom. Bilateral symmetry could come about only after animals evolved a distinctive head region where nerve tissue was concentrated. The concentration of nerve tissue in the head region was the first step in the evolution of a brain. Animals with bilateral symmetry can tell left from right. This gives them better control over the direction of their movements. "
how animals evolved,T_1973,"The next major trait to evolve was mesoderm. This is a third embryonic layer of cells between the ectoderm and the endoderm. Modern animals that represent this stage of evolution are the flatworms. They are placed in Phylum Platyhelminthes. You can see the mesoderm in a flatworm in Figure 11.7. Evolution of this new cell layer allowed animals to develop new types of tissues, such as muscle tissue. "
how animals evolved,T_1974,"Even early invertebrates had a digestive system. However, the earliest digestive system was incomplete. There was just one opening for food to enter the body and waste to leave the body. In other words, the same opening was both mouth and anus. A modern jellyfish has this type of digestive system, as shown in Figure 11.8. Eventually a complete digestive system with two body openings evolved, as shown in Figure 11.8. With a separate mouth and anus, food could move through the body in just one direction. This made digestion more efficient. An animal could keep eating while digesting food and getting rid of waste. Different parts of the digestive tract could also become specialized for different digestive functions. This led to the evolution of digestive organs. Modern animals that represent this stage of evolution are roundworms. They are placed in Phylum Nematoda. "
how animals evolved,T_1975,"The next major animal trait to evolve was a body cavity filled with fluid. At first, this was just a partial body cavity, called a pseudocoelom. A pseudocoelom isnt completely enclosed by mesoderm. However, it still allows room for internal organs to develop. The fluid in the cavity also cushions the internal organs. The pressure of the fluid provides stiffness as well. It gives the body internal support. Modern invertebrates with a pseudocoelom include roundworms. Flatworms lack this trait. This difference explains why roundworms are round whereas flatworms are flat. Later, a true coelom evolved. This is a fluid-filled body cavity that is completely enclosed by mesoderm. The coelom lies between the digestive cavity and body wall. You can see it in the invertebrate in Figure 11.9. Modern invertebrates with a coelom include mollusks (Phylum Mollusca) and annelids (Phylum Annelida). "
how animals evolved,T_1976,"Segmentation evolved next. Segmentation is the division of the body into multiple parts, or segments. Both the earthworm (Phylum Annelida) in Figure 11.10 and ant (Phylum Arthropoda) in Figure 11.11 have segmented bodies. The earthworm has many small segments. The ant has three larger segments. Segmentation increases an animals flexibility. It allows a wider range of motion. Different segments can also be specialized for different functions. All modern annelids and arthropods are segmented. Arthropods also evolved jointed appendages. For example, they evolved jointed legs for walking and jointed feelers (antennae) for sensing. Notice the ants jointed legs and antennae in Figure 11.11 . "
how animals evolved,T_1977,Some invertebrates evolved a rigid rod along the length of their body. This rod is called a notochord. You can see the notochord in the tunicates in Figure 11.12. The notochord gives the body support and shape. It also provides a place for muscles to attach. It can counterbalance the pull of the muscles when they contract. Animals with a notochord are called chordates. All of them are placed in Phylum Chordata. Some early chordates eventually evolved into vertebrates. 
how animals evolved,T_1978,"The earliest vertebrates evolved around 550 million years ago. It happened when some chordates evolved a backbone to replace the notochord after the embryo stage. They also evolved a cranium, or bony skull. The cranium enclosed and protected the brain. The earliest vertebrates probably looked like the hagfish in Figure 11.13. "
how animals evolved,T_1979,"Invertebrates were the first animals to colonize the land. The move to land occurred about 450 million years ago. It required new adaptations. For example, animals needed a way to keep their body from drying out. They also needed a way to support their body on dry land without the buoyancy of water. "
how animals evolved,T_1980,"One way early land invertebrates solved these problems was with an exoskeleton. This is a non-bony skeleton that forms on the outside of the body. It supports the body and helps it retain water. As the organism grows, it sheds its old exoskeleton and grows a new one. Figure 11.14 shows the discarded exoskeleton of a dragonfly. "
how animals evolved,T_1981,"The first vertebrates moved onto land about 365 million years ago. They were early amphibians. They were the first animals to have true lungs and limbs for life on land. However, they still had to return to the water to reproduce. Thats because their eggs lacked a waterproof covering and would dry out on land. "
how animals evolved,T_1982,"The first vertebrates to live fully on land were amniotes. Amniotes are animals that produce eggs with waterproof membranes. The membranes let gases but not water pass through. They allow embryos to breathe without drying out. Amniotic eggs were the first eggs that could be laid on land. The earliest amniotes evolved about 350 million years ago. Amniotes would eventually evolve into modern reptiles, mammals, and birds. "
insects and other arthropods,T_2010,"Arthropods are invertebrates in Phylum Arthropoda. There are more than a million known species of arthropods. However, scientists estimate that only about a tenth of all arthropod species have been identified. In addition to insects, arthropods include animals such as spiders, centipedes, and lobsters. You can see why arthropods were successful both in the water and on land, by watching these excellent videos: MEDIA Click image to the left or use the URL below. URL:  MEDIA Click image to the left or use the URL below. URL:  There are several traits shared by all arthropods. Arthropods have a complete digestive system. They also have a circulatory system and a nervous system. In addition, they have special organs for breathing and excreting wastes. Other traits of arthropods include: segmented body; hard exoskeleton; and jointed appendages. "
insects and other arthropods,T_2011,"Most arthropods have three body segments. The segments are the head, thorax, and abdomen. You can see the three segments in a range of arthropods in Figure 12.21. In some arthropods, the head and thorax are joined together. "
insects and other arthropods,T_2012,"The exoskeleton (or external skeleton) of an arthropod consists of several layers of cuticle. The exoskeleton prevents water loss. It also protects and supports the body. In addition, it acts as a counterforce for the contraction of muscles. The exoskeleton doesnt grow larger as the animal grows. Eventually, it must be shed and replaced with a new one. This happens periodically throughout an arthropods life. The shedding of the exoskeleton is called molting. You can see a time-lapse video of an insect molting at this link: http://commons.wikimedia.org/wiki/File:Cicada_moltin "
insects and other arthropods,T_2013,"Because arthropod appendages are jointed, they can bend. This makes them flexible. Jointed appendages on the body are usually used as legs for walking or jumping. Jointed appendages on the head may be modified for other purposes. Head appendages often include upper and lower jaws. Jaws are used for eating and may also be used for defense. Sensory organs such as eyes and antennae are also found on the head. You can see some of these head appendages on the bee in Figure 12.22. "
insects and other arthropods,T_2014,"Arthropods reproduce sexually. Male and female adults produce gametes. If fertilization occurs, eggs hatch into offspring. After hatching, most arthropods go through one or more larval stages before reaching adulthood. The larvae may look very different from the adults. They change into the adult form in a process called metamorphosis. During metamorphosis, the arthropod is called a pupa. It may or may not spend this stage inside a special container called a cocoon. A familiar example of arthropod metamorphosis is the transformation of a caterpillar (larva) into a butterfly (adult) (see Figure 12.23). Distinctive life stages and metamorphosis are highly adaptive. They allow functions to be divided among different life stages. Each life stage can evolve adaptations to suit it for its specific functions without affecting the adaptations of the other stages. In some arthropods, newly hatched offspring look like small adults. These arthropods dont go through larval stages. They just grow larger until they reach adult size. This type of life cycle is called incomplete metamorphosis. You can see incomplete metamorphosis in a grasshopper in Figure 12.24. "
insects and other arthropods,T_2015,"The majority of arthropods are insects (Class Insecta). In fact, more than half of all known organisms are insects. There may be more than 10 million insect species in the world, although most of them have not yet been identified. In terms of their numbers and diversity, insects clearly are the dominant animals in the world. "
insects and other arthropods,T_2016,"Like other arthropods, insects have three body segments and many jointed appendages. The abdomen contains most of the internal organs. Six legs are attached to the thorax. There are several appendages on the insects head: The head has a pair of antennae. Insects use their antennae to smell and taste chemicals. Some insects can also use their antennae to hear sounds. The head generally has several simple eyes and a pair of compound eyes. Simple eyes have a single lens, like the human eye. Compound eyes have many lenses. For feeding, the insect head contains one pair of lower jaws and two pairs of upper jaws. Insects have also evolved a wide range of specialized mouthparts for eating certain foods. You can see some examples in Figure "
insects and other arthropods,T_2017,"The main reason that insects have been so successful is their ability to fly. Insects are the only invertebrates that can fly. They were also the first animals to evolve flight. The ability to fly is highly adaptive. Its a guaranteed means of escape from nonflying predators. Its also useful for finding food and mates. Insects that fly have wings, like the dragonfly in Figure 12.26. Insects generally have two pairs of wings. They are attached to the thorax. The wings form from the exoskeleton. You can learn how insects flyand how scientists study insect flightby watching this short video:  MEDIA Click image to the left or use the URL below. URL: "
insects and other arthropods,T_2018,"Most humans interact with insects every day. Many of these interactions are harmless and often go unnoticed. However, insects can also cause humans a lot of harm. Some insects are vectors for human diseases. The mosquito in Figure 12.27 is a vector for malaria. Malaria kills millions of people each year. Many other insects feed on food crops. Growers may need to apply chemical pesticides to control them. On the other hand, without insects to pollinate them, many flowering plants, including important food crops, could not reproduce. "
echinoderms and invertebrate chordates,T_2019,"Echinoderms are invertebrates in Phylum Echinodermata. All of them are ocean dwellers. They can be found in marine habitats from the equator to the poles. They live at all depths of water. There are about 6000 living species of echinoderms. Besides sea urchins and sea cucumbers, they include sea stars (starfish), feather stars, and sand dollars. Learn more about the amazing world of echinoderms and why they are called the ultimate animal by watching this video: http://shapeoflife.org/video/echinoderms-ultimate-animal MEDIA Click image to the left or use the URL below. URL: "
echinoderms and invertebrate chordates,T_2020,"The term echinoderm means spiny skin. An echinoderms spines arent actually made of skin. They are part of the animals endoskeleton and just covered with a thin layer of skin. Most adult echinoderms have radial symmetry. This is clear from the sea star pictured in Figure 12.29. However, echinoderms evolved from an ancestor with bilateral symmetry. You can tell because echinoderm larvae have bilateral symmetry and only develop radial symmetry as adults. Another unique trait of echinoderms is a network of internal canals. Most of the canals have projections called tube feet. The end of each tube foot has a sucker. The suckers can stick to surfaces and help the animal crawl. The suckers can also be used to pry open the shells of prey. You can see suckers on the sea star in Figure 12.29. Although echinoderms have a well-developed coelom and complete digestive system, they lack a centralized nervous system and do not have a heart. Some echinoderms have simple eyes that can sense light. Like annelids, echinoderms can regrow a missing body part. In fact, a complete starfish can regrow from a single arm. "
echinoderms and invertebrate chordates,T_2021,"Some echinoderms can reproduce asexually by fission. However, most echinoderms reproduce sexually. They generally have separate sexes that produce sperm and eggs. Fertilization typically occurs outside the body in the water. Eggs hatch into larvae that have bilateral symmetry and can swim. The larvae undergo metamorphosis to change into the adult form. During metamorphosis, their bilateral symmetry changes to radial symmetry. "
echinoderms and invertebrate chordates,T_2022,"Chordates are animals in Phylum Chordata. They are animals that have a notochord and certain other traits. The notochord is a rigid rod that runs down the back of the body. Phylum Chordata is a large and diverse phylum. It includes at least 60,000 species, including the human species. For a visual introduction to chordates, watch this video: http://video.about.com/animals/What-Is-Phylum-Chordata-.htm "
echinoderms and invertebrate chordates,T_2023,"Chordates have three embryonic cell layers: endoderm, mesoderm, and ectoderm. They also have a segmented body with a complete coelom and bilateral symmetry. In addition, chordates have a complete digestive system, central nervous system, and circulatory system. "
echinoderms and invertebrate chordates,T_2024,"There are four traits that are unique to chordates and define Phylum Chordata. The four traits are a post-anal tail, dorsal hollow nerve cord, notochord, and pharyngeal slits. You can see the four traits in Figure 12.30. 1. The post-anal tail is at the end of the organism opposite the head. It extends beyond the anus. 2. The hollow nerve cord runs along the top (dorsal) side of the animal. (In nonchordate animals, the nerve cord is solid and runs along the bottom side.) 3. The notochord lies between the dorsal nerve cord and the digestive tract. It provides stiffness to counterbalance the pull of muscles. 4. The pharyngeal slits are located in the pharynx. The pharynx is the tube that joins the mouth to the digestive and respiratory tracts. In some chordates, all four of these defining traits last throughout life and have important functions. For example, in some chordates, pharyngeal slits are used to filter food out of water. In many chordates, however, including humans, all four traits are present only in the embryo. After that, some of the traits disappear or develop into other structures. For example, in humans, pharyngeal slits are present in the embryo but later develop into parts of the ear. "
echinoderms and invertebrate chordates,T_2025,"Living chordates are mainly vertebrates. In vertebrates, the notochord develops into a backbone, or vertebral column, after the embryonic stage. A small percentage of chordates are invertebrates. Their notochord never develops into a backbone. Invertebrate chordates include tunicates and lancelets. Both groups of animals are small and relatively primitive. They are probably similar to the earliest chordates that evolved more than 500 million years ago. "
echinoderms and invertebrate chordates,T_2026,"Tunicates are invertebrate chordates that lose some of the four defining chordate traits by adulthood. Tunicates are also called sea squirts. There are about 3000 living species of tunicates. All are ocean dwellers and live in shallow water. You can see examples of tunicates in Figure 12.31. As larvae, tunicates can swim freely to find food. As adults, tunicates lack a post-anal tail and notochord, and they can no longer swim. Instead, they remain in one place and are filter feeders. Tunicates can reproduce both sexually and asexually. The same adults produce sperm and eggs. However, fertilization always involves gametes from different parents. Asexual reproduction is by budding. "
echinoderms and invertebrate chordates,T_2027,"Lancelets are invertebrate chordates that retain all four defining chordate traits as adults. There are only about 25 species of living lancelets. Lancelets resemble tunicates in several ways. For example: lancelets live in shallow ocean water; lancelet larvae can swim to find food; and lancelet adults are filter feeders that can no longer swim. Adult lancelets spend most of their time buried in sand on the ocean floor. Lancelets reproduce sexually, with separate sexes producing sperm and eggs. "
amphibians,T_2047,"Amphibians are vertebrates that live part of the time in fresh water and part of the time on land. They were the first vertebrates to evolve four legs and colonize the land. They most likely evolved from lobe-finned fish. Modern amphibians include frogs, toads, salamanders, newts, and caecilians. They are ectotherms, so they have little control over their body temperature. This allows them to be active in warm weather, but they become sluggish when the temperature cools. "
amphibians,T_2048,"Amphibians have moist skin without scales. The skin is kept moist by mucus, which is secreted by mucous glands. In some species, the mucous glands also secrete toxins that make the animal poisonous to predators. The blue poison-dart frogs in Figure 13.12 are a good example. The toxin in their mucus is used by native people in South America to poison the tips of their hunting arrows. Amphibian skin contains keratin, a protein that is also found in the outer covering of most other four-legged vertebrates. The keratin in amphibians is not too tough to allow gases and water to pass through their skin. Most amphibians breathe with gills as larvae and with lungs as adults. However, extra oxygen is absorbed through the skin. "
amphibians,T_2049,"All amphibians have digestive, excretory, and reproductive systems. All three of these organ systems use a single body cavity, called the cloaca. Wastes enter the cloaca from the digestive and excretory systems. Gametes enter the cloaca from the reproductive system. A single external opening in the cloaca allows the wastes and gametes to exit the body. (Many other four legged vertebrates also have a cloaca.) Amphibians have relatively complex circulatory and nervous systems. They have sensory organs for smelling and tasting, as well as eyes and ears. Frogs also have a larynx, or voice box, that allows them to make sounds. The purpose of frog calls varies. Some calls are used to attract mates, some are used to scare off other frogs, and some are signals of distress.You can hear a collection of frog calls at this link: http://animaldiversity.ummz.umich.edu/co "
amphibians,T_2050,Amphibians reproduce sexually. Fertilization may take place inside or outside the body. Amphibians are oviparous. Embryos develop in eggs outside the mothers body. 
amphibians,T_2051,"Amphibians do not produce amniotic eggs with waterproof membranes. Therefore, they must lay their eggs in water. The eggs are usually covered with a jelly-like substance that helps keep them moist and offers some projection from predators. You can see a mass of frog eggs in jelly in Figure 13.13. Amphibians generally lay large numbers of eggs. Often, many adults lay eggs in the same place at the same time. This helps ensure that the eggs will be fertilized. Once eggs are laid, amphibian parents typically provide no parental care. "
amphibians,T_2052,"Most amphibians go through a larval stage that is different from the adult form. In frogs, for example, the early larval stage resembles a fish, as you can see in Figure 13.14. Frogs at this stage of development are called tadpoles. Tadpoles live in the water. They lack legs and have a long tail that helps them swim. They also have gills, which absorb oxygen from the water. During metamorphosis, the tadpole changes to the form of an adult frog. It grows legs, loses its tail, and develops lungs. All of these changes prepare it to live on the land. In Figure 13.15, you can see how a frog larva looks as it changes to the adult form. "
amphibians,T_2053,"There are only about 6200 known species of amphibians. They are placed in three orders: frogs, salamanders, and caecilians. Table 13.4 shows a picture of an amphibian in each order. It also provides additional information about the orders. Class Frogs Distinguishing Traits The frog order also includes toads. Unlike other amphibians, frogs and toads lack a tail by adulthood. Their back legs are also longer because they are specialized for jumping. Frogs can jump as far as 20 times their body length. Thats like you jumping more than the length of a basketball court! Example red-eyed tree frog Class Salamanders Caecilians Distinguishing Traits The salamander order also includes newts. Salamanders and newts keep their tails as adults. They have a long body with short legs. They are adapted for walking and swim- ming rather than jumping. Unlike other vertebrates, salamanders can regrow legs or other body parts if they are bitten off by a predator. The caecilian order is the amphib- ian order with the fewest species. Caecilians are closely related to salamanders. They have a long, worm-like body. They are the only amphibians without legs. Caecil- ians evolved from a four-legged an- cestor but lost their legs later in their evolution. As adults, they often bur- row into the soil. Thats one reason why Caecilians tend to be less well known than other amphibians. Example smooth newt microcaecilia "
amphibians,T_2054,Amphibians live in freshwater and moist-soil habitats throughout the world. The only continent that lacks amphib- ians is Antarctica. Amphibians are especially common in temperate lakes and ponds and in tropical rainforests. 
amphibians,T_2055,"Amphibians are the prey of many other vertebrates, including birds, snakes, raccoons, and fish. Amphibians are also important predators. As larvae, they may eat water insects and algae. As adults, they typically eat invertebrates, including worms, snails, and insects. You can watch a frog catching an invertebrate in the slow-motion video at the following link. At its real speed, you would barely see it because it happens so quickly. MEDIA Click image to the left or use the URL below. URL: "
amphibians,T_2056,"Why are so many amphibian species threatened by extinction, and why should you care? The second question is easy. Amphibians control pests, may be a source of new medicines, and help feed many other animals. The nature of amphibian skin may help explain why so many amphibian species are at risk. Their skin easily absorbs substances from the environment, such as pollutants in water or air. Therefore, they may suffer from poor environmental quality before other animals do. As such, they may provide an early-warning system of environmental damage. What can you do to help save amphibians? Protect the natural environment. For example, reduce your use of energy to curb greenhouse gases and global warming. Avoid the use of garden pesticides. Poisoned insects may be eaten by amphibians that are also harmed by the poison. Make a backyard habitat. A small pond surrounded by native vegetation provides a place for amphibians to live. Help raise awareness. Start a letter-writing campaign to politicians, asking them to support conservation activities for amphibians. For more ideas about what you can do to help save amphibians, check out this website: "
reptiles,T_2057,"Reptiles are ectothermic, four-legged vertebrates that produce amniotic eggs. The reptile class is one of the largest classes of vertebrates. Besides turtles, it includes crocodiles, alligators, lizards, and snakes. Although some turtles and other reptiles now live mainly in the water, reptiles evolved many adaptations for life on land. For an amusing overview of reptiles, watch this Bill Nye the Science Guy reptile video:  MEDIA Click image to the left or use the URL below. URL: "
reptiles,T_2058,"Reptiles were the first vertebrates to lay amniotic eggs. This freed them from returning to the water to reproduce. In addition to amniotic eggs, reptiles have several other adaptations for living on land. For example, reptile skin is covered with scales. You can see how the scales overlap and cover the snake in Figure 13.17. Reptile scales are made of very tough keratin. They help protect reptiles from injury as well as loss of water. Because of their tough scales, reptiles cant absorb oxygen through their skin as amphibians can. However, reptiles have more efficient lungs for breathing air. They also have various ways of moving air into and out of their lungs. For example, their chest muscles contract to push air out of the lungs. The muscles relax to allow air to rush into the lungs. Another muscle, called the diaphragm, which lies below the lungs, also helps move air into and out of the lungs. (Mammals also have a diaphragm for breathing air.) "
reptiles,T_2059,"Reptiles have a circulatory system with a heart that pumps blood. Reptiles also have a centralized nervous system with a brain. Their brain is relatively small, but the parts of the brain that control the senses and learning are larger than in amphibians. Reptiles have good senses of sight and smell. They use their tongue to smell scents. Thats what the blue-tongued lizard in Figure 13.18 is doing. Some reptiles also have a heat-sensing organ that helps them locate the warm bodies of prey animals such as birds and small mammals. "
reptiles,T_2060,"Most reptiles have sexual reproduction with internal fertilization. Reptiles have a body cavity called a cloaca that is involved in reproduction. Sperm or eggs are released into an adult reptiles cloaca. Males have one or two penises that pass sperm from their cloaca to the eggs in the cloaca of a female, where fertilization takes place. In most reptile species, once fertilized the eggs leave the body through an opening in the cloaca. These reptiles are oviparous. Eggs develop and hatch outside the mothers body. Young reptiles, like the baby alligator in Figure 13.19, look like smaller versions of the adults. They dont have a larval stage as most amphibians do. Baby reptiles are able to move and search for food but are at high risk of predation. Adult reptiles rarely provide any care for their offspring once the eggs are laid. The only exceptions are female alligators and crocodiles. They defend their eggs and hatchlings from predators and help them reach the water. "
reptiles,T_2061,"There are over 8200 living species of reptiles. They are classified in four orders, called Crocodilia, Sphenodontia, Squamata, and Testudines. Table 13.4 shows a picture of a reptile in each order. It also provides additional information about the orders. For an online gallery of amazing photos of reptiles, go to this link: http://video MEDIA Click image to the left or use the URL below. URL:  Class Crocodilia Distinguishing Traits Reptiles in the Crocodilia Order are called crocodilians. They include crocodiles, alligators, caimans, and gharils. They have four sprawling legs that allow them to run surpris- ingly fast. They have strong jaws and replace their teeth throughout life. Crocodilians have relatively complex brains and greater intelli- gence than other reptiles. Example crocodile Class Sphenodontia Distinguishing Traits The Sphenodontia Order includes only tuataras like the one in this photo. They resemble lizards but are the least specialized of all living reptiles. Their brain is similar to the amphibian brain. Example tuatara Squamata The Squamata order includes lizards and snakes. Lizards have four legs for running or climbing, and they can also swim. Many change their color when threatened. Snakes do not have legs, although they evolved from a four-legged ancestor. They have a very flexible jaw for swallowing large prey whole. Some inject poison into their prey through fangs. The Testudines Order includes tur- tles, tortoises, and terrapins. They have four legs for walking. They have a hard shell covering most of their body. lizard Testudines terrapin "
reptiles,T_2062,Modern reptiles live in many different habitats. They can be found on every continent except Antarctica. 
reptiles,T_2063,"Many turtles are aquatic. They may live in the ocean or in fresh water. Other turtles are terrestrial and live on land. All lizards are terrestrial. Their habitats may range from deserts to rainforests. They may live in a range of places, from underground burrows to the tops of trees. Most snakes are terrestrial, but some are aquatic. Crocodilians live in and around swamps or bodies of water. The water may be fresh or salty, depending on the species of crocodilian. "
reptiles,T_2064,"All reptiles are heterotrophs, and the majority eats other animals. Heterotrophs that eat only or mainly animals are called carnivores. Large carnivorous reptiles such as crocodilians are the top predators in their ecosystems. They prey on large birds, fish, deer, turtles, and sometimes farm livestock. Their powerful jaws are strong enough to crush bones and turtle shells. Smaller carnivorous reptilesincluding tuataras, snakes, and many lizardsare lower-level predators. They prey on small animals such as insects, frogs, birds, and mice. Most terrestrial turtles eat plants. Heterotrophs that eat only or mainly plants are called herbivores. Herbivorous turtles graze on grasses, leaves, flowers, and fruits. Marine turtles and some lizards feed on both plants and animals. Heterotrophs that eat a variety of foods including both plants and animals are called omnivores. "
birds,T_2065,"Birds are four-limbed, endothermic vertebrates. The upper pair of limbs are wings that most birds use for flying. The lower pair of limbs are legs with feet that birds use for walking. Because birds walk on two legs, they are called bipedal. (Humans are bipedal too.) Birds also have feathers and beaks, and they produce amniotic eggs. Of all vertebrate classes, birds are the most numerous, even though they evolved most recently. Why have birds been so successful? The answer is flight. Being able to fly opened up a whole new world to birds: the world of the air above the land and water. Other than insects, virtually no other animals can inhabit the airy world. Flying is a sure-fire way to escape from all but the quickest nonflying predators. Flying also gives birds a good view for finding food and mates. "
birds,T_2066,"Wings and feathers are two adaptations for flight that evolved in birds. Both are clearly displayed in the flying gull in Figure 14.2. Wings evolved from the front limbs of a four-legged ancestor. The wings are controlled by large flight muscles in the chest. Feathers also help birds fly. They provide air resistance and lift. In addition, they provide insulation and serve other roles. "
birds,T_2067,"To keep their flight muscles well supplied with oxygen, birds evolved specialized respiratory and circulatory systems. Birds have special air sacs for storing extra air and pumping it into the lungs. They also have a relatively large heart and a rapid heart rate. These adaptations keep plenty of oxygenated blood circulating to the flight muscles. "
birds,T_2068,"Birds have relatively big brains for their body size. This is reflected in their high level of intelligence and complex behavior. Some birds, including crows, are more intelligent than many mammals. They are smart enough to use tools to solve problems. You can see this in the video below. However, the part of the brain that is most developed in birds is the part that controls flying. This is another adaptation for flight. MEDIA Click image to the left or use the URL below. URL: "
birds,T_2069,"Birds reproduce sexually and have separates sexes. Fertilization occurs internally, so males and females must mate. Many bird species have special behaviors, such as unique songs or visual displays, for attracting mates. These special behaviors are called courtship. The white peacock in Figure 14.3 is putting on a stunning display of his amazing tail feathers to court a mate. "
birds,T_2070,"After mating and fertilization occur, eggs are laid, usually in a nest. Most birds build nests for their eggs and hatchlings, and each species has a certain way of doing it. You can see examples of different types of bird nests in Figure 14.4. Nests range from little more than a depression in the ground (killdeer) to elaborately built structures (weaver bird). You can see how skillful a weaver bird is at weaving its nest by watching this video:  MEDIA Click image to the left or use the URL below. URL: "
birds,T_2071,"In most species, one or both parents take care of the eggs. They sit on the eggs to keep them warm until they hatch. This is called incubation. After the eggs hatch, the parents generally continue their care. They feed the hatchlings until they are big enough to feed on their own. This is usually at a younger age in ground-nesting birds such as ducks than in tree-nesting birds such as robins. "
birds,T_2072,"There are about 10,000 living species of birds. Almost all of them can fly. Very few birds are flightless. "
birds,T_2073,"Birds that can fly are classified in 29 orders. Birds in the different orders vary in their physical traits and how they behave. You can see seven of the most common orders of flying birds in Table 14.1. The majority of flying birds are perching birds, described in the last row of the table. There are more species in this order than in all other bird orders combined. Many perching birds are familiar songbirds such as the mockingbird. You can hear a mockingbirds amazing and complex song in this video: http://youtu.be/NNNX3f3_svo Order Landfowl: pheasants Description They are large in size; they spend most of their time on the ground; they usually have a thick neck and short, rounded wings; their flight tends to be brief and close to the ground. Example turkey Waterfowl: ducks, geese, swans They are large in size; they spend most of their time on the water sur- face; they have webbed feet and are good swimmers; most are strong flyers. ducks Shorebirds: puffins, gulls, plovers They range from small to large; most live near the water, and some are sea birds; they have webbed feet and are good swimmers; most are strong flyers. puffin Diurnal Raptors: hawks, falcons, eagles They range from small to large; they are active during the day and sleep during the night; they have a sharp, hooked beak and strong legs with clawed feet; they hunt by sight and have excellent vision. hawk Nocturnal Raptors: burrowing owls, barn owls, horned owls They range from small to large; they are active during the night and sleep during the day; they have a sharp, hooked beak and strong legs with clawed feet; they have large, forward-facing eyes; they have ex- cellent hearing and can hunt with their sense of hearing alone. burrowing owl turkeys, chickens, Order Parrots: cockatoos, parrots, para- keets Description They range from small to large; they are found in tropical regions; they have a strong, curved bill; they stand upright on strong legs with clawed feet; many are brightly col- ored; they are very intelligent. Example cockatoo Perching Birds: honeyeaters, spar- rows, crows They are small in size; they perch above the ground in trees and on buildings and wires; they have four toes for grasping a perch; many are songbirds. honeyeater "
birds,T_2074,"Some birds lost the ability to fly during their evolution. They include the ostrich, pictured above in Figure 14.1, as well as the kiwi, rhea, cassowary, and moa. All of these birds have long legs that are adapted for running. Penguins, like the one pictured in Figure 14.5, are also flightless, but they have a very different body shape. They are adapted for swimming instead of running. "
birds,T_2075,"Birds are endothermic. They can maintain a warm body temperature even in a cold climate. Therefore, they can live in a wider range of habitats than ectothermic vertebrates such as amphibians and reptiles. "
birds,T_2076,"Birds live and breed in most terrestrial habitats on Earth. They can be found on all seven continents, from the Arctic to Antarctica. However, the majority of bird species are native to tropical areas of the planet. "
birds,T_2077,"Birds may be specialists or generalists in terms of what they eat. Generalists are organisms that eat many different types of food. Birds that are generalists include the red-winged blackbird in Figure 14.6. It has a basic beak that can eat many different foods. Red-winged blackbirds are omnivores. They may eat a wide variety of seeds as well as insects and other small animals such as snails and frogs. Specialists are organisms that eat just one type of food. Birds that are specialists include ospreys, which eat only live fish. You can see an osprey in Figure 14.7. The ospreys feet are very well-adapted for catching fish. Its eyes are also well-adapted for seeing fish under the water. Its beak is well suited for gripping and ripping into fish flesh. Ospreys are so well-adapted to catching fish that they cant catch anything else! "
mammals,T_2078,"Mammals are endothermic vertebrates with four limbs. Examples of mammals include bats, whales, mice, and humans. Clearly, mammals are a very diverse group. Nonetheless, they share several traits that set them apart from other vertebrates. "
mammals,T_2079,"Two traits are used to define the mammal class. They are fur or hair and mammary glands in females. All mammals have fur or hair on their skin. It provides insulation and helps keep the body warm. It also can be used for sensing. For example, cats can feel with their whiskers. All female mammals have mammary glands. Mammary glands are glands that produce milk after the birth of offspring. Producing milk for offspring is called lactation. The colt in Figure 14.9 is getting milk from its mother. "
mammals,T_2080,"Most mammals share several other traits. These include: a large, complex brain and relatively great intelligence; ears with specialized structures that make them extremely good at hearing; four different types of teeth (reptiles have just one type), allowing them to eat a wide range of foods; tiny air sacs called alveoli (alveolus, singular) in the lungs for enhanced gas exchange; and glands in the skin that produce sweat, a salty fluid that helps cool down the body. "
mammals,T_2081,"Mammals are noted for the many ways they can move. Some mammals are well known for their speed. The fastest land animal is a mammal, the cheetah. It can race at speeds of up to 112 kilometers (70 miles) per hour. The limbs of most mammals are specialized for a particular way of moving. They may be specialized for running, jumping, climbing, flying, gliding, or swimming. The limbs of some mammals are even specialized for swinging through tree tops. You can see mammals with some of these specializations in Figure 14.10. "
mammals,T_2082,Mammals have a variety of ways to keep their body temperature stable. 
mammals,T_2083,"Mammals stay warm in cool weather in two general ways. One way is by generating more heat. The other way is by conserving the heat that is generated. Mammals generate heat mainly by maintaining a high rate of metabolism. Compared with the cells of other animals, the cells of mammals have more mitochondria. Mitochondria are the cell organelles that generate energy. Mammals may also produce little bursts of heat by shivering. Shivering occurs when many muscles all contract slightly at the same time. The muscle contractions generate a small amount of heat. Mammals conserve heat with their hair or fur. It works like the layer of insulation in the walls of a house. It traps warm air next to the skin so it cant escape into the environment. Like the squirrel in Figure 14.11, most mammals can make their hair or fur stand up from the skin. This makes it a better insulator. Mammals also have a layer of insulating fat beneath their skin. Other vertebrates lack this layer of fat. "
mammals,T_2084,"In hot weather, mammals may need to lose excess body heat. One way they do this is by increasing blood flow to the body surface. The increased blood flow warms the skin, which gives off heat to the environment. Most mammals also sweat to lose excess heat. Sweating wets the skin. Evaporation of the sweat requires heat. The heat comes from the body and cools it down. Animals with fur, like the dogs in Figure 14.12, may pant instead of sweat to lose body heat. Water evaporates from the tongue and other moist surfaces of the mouth, using heat from the body. Watch this video to learn about some unique ways that elephants lose excess heat:  MEDIA Click image to the left or use the URL below. URL: "
mammals,T_2085,"Generating body heat to stay warm takes a lot of energy. Mammals are heterotrophs that get their energy by eating other organisms. Mammals eat a wide range of different foods. Except for leaf litter and wood, almost any kind of organic matter is consumed by some type of mammal. The organic matter typically comes from plants, other animals, or some mix of these sources. "
mammals,T_2086,"Many mammals are herbivores. Herbivores are heterotrophs that eat only or mainly plant foods (or algae). Depend- ing on the species of mammals, they may eat leaves, shoots, stems, roots, seeds, nuts, fruits, flowers, and/or grasses. Some mammals even eat conifer needles or tree bark. Mammals that are herbivores include rabbits, mice, sheep, zebras, deer, kangaroos, and monkeys. The manatee in Figure 14.13 is also a herbivorous mammal. It eats mainly kelp (seaweed). "
mammals,T_2087,"Some mammals are carnivores. Carnivores are heterotrophs that eat only or mainly animal foods. Depending on their species, carnivorous mammals may eat other mammals, birds, reptiles, amphibians, fish, mollusks, worms, and/or insects. Mammals that are carnivores include anteaters, whales, hyenas, wolves, and seals. The bat in Figure "
mammals,T_2088,"Some mammals are omnivores. Omnivores are heterotrophs that eat a mix of plant and animal foods. Mammals that are omnivores include bears, foxes, rats, pigs, and human beings. The chimpanzees in Figure 14.15 are also omnivorous mammals. In the wild, they eat mainly plant foods, but they supplement plants with birds, bird eggs, insects, small monkeys, and other small mammals. Their favorite and most common food, however, is fruit. Animals that eat mainly fruit are called frugivores. "
mammals,T_2089,"Mammals have separate sexes and reproduce sexually. They produce eggs or sperm and must mate in order for fertilization to occur. A few mammals are oviparous. They lay eggs, which later hatch. These mammals are called monotremes. Most mammals are viviparous and give birth to live young. These mammals are either placental mammals or marsupials. Placental mammals give birth to relatively large and well-developed fetuses. Marsupials give birth to smaller, less-developed embryos. In both placental and marsupial mammals, the young grow and develop inside the mothers body in an organ called the uterus. At birth, they pass through a tube-like organ called the birth canal, or vagina. "
mammals,T_2090,"Placental mammals get their name from the placenta. This is a spongy structure that develops during pregnancy only in placental mammals. You can see where a human placenta forms in Figure 14.16. The placenta sustains the fetus while it grows inside the mothers uterus. It consists of membranes and blood vessels from both mother and fetus. It allows substances to pass between the mothers blood and that of the fetus. The fetus gets oxygen and nutrients from the mother. It passes carbon dioxide and other wastes to the mother. The placenta permits a long period of fetal growth. As a result, the fetus can become relatively large and mature before birth. This increases its chances of survival. On the other hand, supporting a growing fetus may be difficult for the mother. She has to eat more while pregnant and may become less mobile as the fetus grows larger. Giving birth to a large infant is also risky. "
mammals,T_2091,"By giving birth to tiny embryos, marsupial mothers are at less risk. However, the tiny newborn marsupial may be less likely to survive than a newborn placental mammal. The marsupial embryo completes its growth and development outside the mothers body in a pouch. It gets milk by sucking on a nipple in the pouch. There are very few living species of marsupials. They include kangaroos, koalas, and opossums. You can see a baby koala peeking out of its mothers pouch in Figure 14.17. "
mammals,T_2092,"There are very few living species of monotremes, or egg-laying mammals. They include the echidna and platypus, both pictured in Figure 14.18. Monotremes are found only in Australia and the nearby island of New Guinea. Female monotremes lack a uterus and vagina. Instead, they have a cloaca with one external opening, like the cloaca of reptiles and birds. The opening is used to excrete wastes as well as lay eggs. The eggs of monotremes have a leathery shell, like the eggs of reptiles. Female monotremes have mammary glands but not nipples. They secrete milk to feed their young from a patch on their belly. This form of reproduction is least risky for the mother but most risky for the offspring. "
mammals,T_2093,"Mammals are a class in Phylum Chordata. Monotremes, marsupials, and placental mammals are subclasses of mammals. Almost all living mammals are placental mammals. Placental mammals, in turn, are divided into many orders. Some of the larger orders are described in Table 14.2. Order Insectivora Example mole Sample Trait small sharp teeth Chiroptera bat digits support membranous wings Order Carnivora Example coyote Sample Trait long pointed canine teeth Rodentia mouse incisor teeth grow continuously Lagomorpha rabbit chisel-like incisor teeth Artiodactyla deer even-toed hooves Cetacea whale paddle-like forelimbs Primates monkey five digits on hands and feet The orders in Table 14.2 are still widely used, but ideas about mammal classification are constantly changing. Traditional classifications are based on similarities and differences in physical traits. More recent classifications are based on similarities and differences in DNA. The latter are more useful for determining how mammals evolved. "
primates,T_2094,"A primate is a mammal in the Primate Order of placental mammals. In addition to human beings, this order consists of lemurs, tarsiers, monkeys, and apes. It includes mammals that range in size from the tiny mouse lemur, which weighs only 30 g (about an ounce), to the majestic gorilla, an ape that may weigh as much as 200 kg (440 lb). Both a mouse lemur and gorilla are pictured in Figure 14.19. "
primates,T_2095,"Primates are generally divided into prosimian and non-prosimian primates. Primates called prosimians are generally smaller. There are also far fewer of them. Prosimians include lemurs, such as the mouse lemur in Figure 14.19, and lorises. Prosimians are thought to be more similar to the earliest primates. All other primates are non-prosimian primates. They are placed in groups that include tarsiers, New World (Central and South America) monkeys, Old World (Africa and Asia) monkeys, apes, and humans. You can see examples of non-prosimian primates in Figure 14.20. "
primates,T_2096,"A number of traits set primates apart from other orders of placental mammals. Primates evolved from tree-living, or arboreal, ancestors. As a result, many primate traits are adaptations for life in the trees. Living in trees requires good grasping ability. Being able to judge distances is also important. Primates have five digits (fingers or toes) on each extremity. Unlike the hooves of horses or the paddles of whales, the digits of primates are relatively unspecialized. Therefore, they can be used to do a variety of tasks, including grasping branches and holding tools. Most primates have opposable thumbs. An opposable thumb can be brought into opposition with the other fingers of the same hand. This allows the hand to grasp and hold things. Primates usually rely more on the sense of vision rather than the sense of smell, which is the dominant sense in many other mammals. The importance of vision in primates is reflected by the bony socket that surrounds and protects the primate eye. Primates have widely spaced eyes in the same plane that give them stereoscopic (3-D) vision, needed for judging distances. Some primates, including humans, have also evolved color vision. Primates tend to have bigger brains for their body size than other mammals. This is reflected in their relatively high level of intelligence and their ability to learn new behaviors. Primates have slower rates of development than other mammals their size. They reach maturity later and have longer lifespans. Being dependent on adults for a long maturation period gives young primates plenty of time to learn from their elders. "
primates,T_2097,"Except for humans and a few other species, most modern primates still live in trees at least some of the time. They live primarily in tropical rain forests of Central and South America, Africa, and South Asia. Some primates, such as the gibbon in Figure 14.21, have long arms and curving fingers that allow them to swing from branch to branch high up in trees. This way of traveling is called brachiation. You can watch a gibbon brachiating in this amazing video: MEDIA Click image to the left or use the URL below. URL:  Fruit is the preferred food for almost all primates except humans. However, most primate species are omnivorous and consume a variety of plant and animal foods. For example, they may eat leaves, seeds, bird eggs, insects, and other small animals. Chimpanzees may band together and hunt for animals to kill and eat. They may even sharpen sticks and use them as spears when they hunt. Watch this video to see the incredible teamwork of a group of chimpanzees hunting a monkey:  . MEDIA Click image to the left or use the URL below. URL: "
understanding animal behavior,T_2098,"Why do animals behave in the ways pictured in Figure 15.1? The specific answer depends on what the behavior is. Male flamingoes put on a noisy group show in order to attract females for mating. Frogs call out to attract mates or to warn other frogs to stay away from their territory. Baby ducks follow their mother to stay close to her for protection and survival. Male elephant seals fight to defend their hunting territory from each other. All of these behaviors have the purpose of promoting reproduction or survival. Like the animals pictured above, all animals have behaviors that help them achieve these basic ends. Behaviors that help animals reproduce or survive increase their fitness. Animals with greater fitness have a better chance of passing their genes to the next generation. If genes control behaviors that increase fitness, the behaviors become more common in the species. In other words, they evolve by natural selection. "
understanding animal behavior,T_2099,"All of the animal behaviors pictured in Figure 15.1 are ways that animals act without being taught to act in these ways. Such behaviors are called innate. An innate behavior is any behavior that occurs naturally in all the animals of a given species. An innate behavior is also called an instinct. The first time an animal performs an innate behavior, the animal does it well. The animal doesnt have to practice the behavior in order to get it right or to become better at doing it. Innate behaviors are also predictable. All members of a species perform an innate behavior in the same way. "
understanding animal behavior,T_2100,"There are many other examples of innate behaviors in animals. Even behaviors that seem complex and difficult may be innate. For example, honeybees perform dances in order to communicate about food sources. When a honeybee, like the one in Figure 15.2, finds a food source, it returns to its hive and does a dance, called the waggle dance. The way the bee moves during its dance tells other bees in the hive where to find the food. Honeybees can do the waggle dance without learning it from other bees, so it is an innate behavior. Watch this video to see the waggle dance and find out what it communicates: http://video.nationalgeographic.com/video/weirdest-bees-dance MEDIA Click image to the left or use the URL below. URL:  Three other examples of innate behavior are pictured in Figure 15.3. If an animal were to perform such behaviors incorrectly, it might be less likely to survive or reproduce. Can you explain why each behavior pictured in the figure is important for reproduction or survival? "
understanding animal behavior,T_2101,"Innate behaviors occur in all animals. However, the more intelligent a species is, the fewer innate behaviors it generally has. The human species is the most intelligent animal species, and it has very few innate behaviors. The only innate behaviors in humans are reflex behaviors. A reflex behavior is a simple response that always occurs when a certain stimulus is present. Human reflex behaviors occur mainly in babies. You may have seen a baby exhibit the grasp reflex shown in Figure this way from birth to about 6 months of age. Its easy to see why this might help a baby survive. Grabbing onto something could keep a baby from falling and being injured. "
understanding animal behavior,T_2102,"Other than infant reflexes, human behaviors are mainly learned rather than innate behaviors. Learned behavior is behavior that occurs only after experience or practice. Did you ever teach a dog to sit on command? Thats an example of a learned behavior. The dog wasnt born knowing that it should sit when it hears the word sit. The dog had to learn the behavior. Most animals are capable of learning, but animals that are more intelligent are better at learning and depend more on learned behaviors. The big advantage of learned behaviors over innate behaviors is that learned behaviors are flexible. They can be changed to suit changing conditions. Human beings depend on learned behaviors more than any other species. Think about some of the behaviors you have learned. They might include making a bed, riding a bicycle, using a computer, and playing a sport, to name just a few. You may have learned each of the behaviors in different ways. There are several different ways in which animals learn. They include habituation, observational learning, conditioning, learning through play, and insight learning. "
understanding animal behavior,T_2103,One of the simplest ways of learning that occurs in just about all animals is habituation. Habituation means learning to get used to something after being exposed to it repeatedly. It usually involves getting used to something that is frightening or annoying but not dangerous. Look at the crows in Figure 15.5. They are no longer afraid of the scarecrow. They have gotten used to a human in this location and know that it wont hurt them. Habituation lets animals ignore things that wont harm them. It allows them to avoid wasting time and energy escaping from things that arent really dangerous. 
understanding animal behavior,T_2104,"Do you remember how you learned to tie your shoe laces? You may have watched and copied the behavior of your mom or an older sibling. Learning by watching and copying the behavior of someone else is called observational learning. Human children learn many behaviors this way. Other animals also learn through observational learning. For example, the wolves in Figure 15.6 learned how to hunt in a group by watching and copying the hunting behaviors of older wolves in their pack. "
understanding animal behavior,T_2105,"Conditioning is a way of learning that involves a reward or punishment. If you ever trained a dog to obey a command, you probably gave the dog a tasty treat each time he performed the desired behavior. It may not have been very long before the dog would reliably follow the command in order to get the treat. This is an example of conditioning that involves a reward. Conditioning does not always involve a reward. It can involve a punishment instead. For example, a dog might be scolded each time she jumps up on the sofa. After repeated scolding, she may learn to stay off the sofa. Conditioning occurs in nature as well. Here are just two examples: Bees learn to find nectar in certain types of flowers because they have found nectar in those types of flowers before. In this case, the behavior is learned because it is rewarded with nectar. Many birds learn to avoid eating monarch butterflies, like the one pictured in Figure 15.7. Monarch butterflies taste bad and make birds sick. In this case, the behavior is learned because it is punished with a nasty taste and illness. "
understanding animal behavior,T_2106,"Many animals, especially mammals, spend a lot of time playing when they are young. Although playing is fun, its likely that animals play for other reasons as well. Learning behaviors that will be important in adulthood is one likely outcome of play. Bear cubs, like the two bear cubs in Figure 15.8, frequently play together. They often pretend to be fighting. By play fighting they may be learning skills such as fighting and hunting that they will need as adults. Other young animals may play in different ways. For example, young deer play by running and kicking up their hooves. This may help them learn how to escape from predators. Human children learn by playing as well. For example, playing games and sports may help them learn how to follow rules and work with others. "
understanding animal behavior,T_2107,"Insight learning is learning from past experiences and reasoning. It generally involves coming up with new ways to solve problems. Insight learning generally happens quickly. An animal has a sudden flash of insight. Insight learning requires relatively great intelligence. Human beings use insight learning more than any other species. They have used it to invent the wheel to land astronauts on the moon. Think about problems you have solved. You may have figured out how to solve a new type of math problem or how to get to the next level of a video game. If you relied on your past experiences and reasoning to do it, then you were using insight learning. One type of insight learning is making tools to solve problems. Scientists used to think that humans were the only animals intelligent enough to make tools. In recent decades, however, there have been many observations of other animal species using tools. They range from monkeys and chimpanzees to crows. You can see a monkey using a stone tool in Figure 15.9. She is using the stone to crack open the shells of marine invertebrates such as oysters. Chimpanzees have been observed using sticks to fish for termites in a termite mound. Crows have been seen bending wire to form a hook in order to pull food out of a tube. Behaviors such as these show that other species of animals besides humans can use their experience and reasoning to solve problems. They can learn through insight. "
types of animal behavior,T_2108,"Communication is any way that animals share information. Many animals live in social groups. For these animals, being able to communicate is essential. Communicating increases the ability of group members to cooperate and avoid conflict. Communication may help animals work together to find food and defend themselves from predators. It also helps them find mates and care for their offspring. In addition, communication helps adult animals teach the next generation learned behaviors. Therefore, communication generally improves the chances of animals surviving and reproducing. "
types of animal behavior,T_2109,"Different animal species use a range of senses for communicating. They may communicate using hearing, sight, or smell. Animals that communicate by making and hearing sounds include frogs, birds, and monkeys. Frogs call out to attract mates. Birds may use calls to warn other birds to stay away or to tell them to flock together. Monkeys use warning calls to tell other troop members that a predator is near. Animals may communicate by sight with gestures, body postures, or facial expressions. Look at the cat in Figure 15.11. Theres no mistaking the meaning of its arched back, standing hair, and exposed fangs. Its clearly saying stay away, or else! Bees communicate with a waggle dance. They use it to tell other bees where food is located. A wide range of animals communicate by releasing chemicals they can smell or detect in some other way. They include animals as different as ants and dogs. An ant, for example, releases chemicals to mark the trail to a food source. Other ants in the nest can detect the chemicals with their antennae and find the food. Look at the dog in Figure 15.12. Its marking its territory with a chemical that it releases in urine. It does this to keep other dogs out of its yard. "
types of animal behavior,T_2110,"Humans communicate with each other in a variety of ways. Chiefly, however, we use sound and sight to share information. The most important way that humans communicate is with language. Language is the use of symbols to communi- cate. In human languages, the symbols are words. Words may stand for things, people, actions, feelings, or ideas. By combining words in sentences, language can be used to express very complex thoughts. Another important way that humans communicate is with facial expressions. Look at the facial expressions of the girl in Figure 15.13. You can probably tell what emotion she is trying to convey with each expression. From left to right, she looks happy, sad, and angry. Humans also commonly use gestures and body postures to communicate. You might answer a question by shrugging your shoulders, which means I dont know. You might use a thumbs-up gesture when a friend scores a goal to mean Good job. Can you think of other gestures you commonly use to communicate with others? "
types of animal behavior,T_2111,"Without communication, animals would not be able to live together in groups. Animals that live in groups with other members of their species are called social animals. Social animals include many species of insects, birds, and mammals. Specific examples are ants, bees, crows, wolves, and human beings. "
types of animal behavior,T_2112,"Some species of animals are very social. In these species, members of the group depend completely on one another. Thats because different animals within the group have different jobs. Therefore, group members must work together for the good of all. Most species of bees and ants are highly social animals. Look at the honeybees in Figure 15.14. Honeybees live in colonies that may consist of thousands of individual bees. Generally, there are three types of adult bees in a colony: workers, a queen, and drones. Most of the adult bees in a colony are workers. They cooperate to build the hive, collect food, and care for the young. Each worker has a specific task to perform, depending on its age. Young worker bees clean the hive and feed the offspring. Older worker bees build the waxy honeycomb or guard the hive. The oldest worker bees leave the hive to find food. Each colony usually has one queen. Her only job is to lay eggs. The colony also has a relatively small number of male drones. Their only job is to mate with the queen. "
types of animal behavior,T_2113,"Bees and other social animals must cooperate to live together successfully. Cooperation means working together with others. Members of the group may cooperate by dividing up tasks, defending each other, and sharing food. The ants in Figure 15.15 are sharing food. One ant is transferring food directly from its mouth to the mouth of another colony member. Besides social insects, animals in many other species also cooperate. For example, in meerkat colonies, young female meerkats act as babysitters. They take care of the baby meerkats while their parents are out looking for food. "
types of animal behavior,T_2114,Some of the most important behaviors in animals involve reproduction. They include behaviors to attract mates and behaviors for taking care of the young. 
types of animal behavior,T_2115,"Mating is the pairing of an adult male and an adult female for the purpose of reproduction. In many animal species, females choose the males they will mate with. For their part, males try to show females that they would be better mates than other males. To be chosen as mates, males may perform courtship behaviors. These are special behaviors that help attract a mate. Male courtship behaviors are meant to get the attention of females and show off a males traits. Different species of animals have different courtship behaviors. An example of courtship behavior in birds is shown in Figure 15.16. The bird in the picture is a male sharp-tailed grouse, and hes doing a courtship dance. Each year in the spring, as many as two dozen grouse males gather in a grassy area to perform their courtship dance. Female grouse watch the dance and then mate with the males that put on the best display. You can see a group of male grouse performing their courtship dance in this short video:  . MEDIA Click image to the left or use the URL below. URL: "
types of animal behavior,T_2116,"In most species of birds and mammals, one or both parents care for the young. This may include building a nest or other shelter. It may also include feeding the young and protecting them from predators. Caring for the young increases their chances of surviving. This, in turn, increases the parents fitness, so such behaviors evolve by natural selection. Emperor penguins make great sacrifices to take care of their young. After laying an egg, a penguin mother returns to the sea for two months to feed. Her mate stays behind to keep the egg warm. He balances the egg on top of his feet to keep it warm for the entire time the mother is away. During this time, he goes without food. To survive the cold, he huddles together with other males. If the chick hatches before the mother returns, the father feeds it with a high-protein, high-fat substance he produces just for this purpose. You can see an emperor penguin father feeding his chick in Figure 15.17. "
types of animal behavior,T_2117,"Some species of animals are territorial. This means that they defend an area that typically includes their nest and enough food for themselves and their offspring. Animals generally dont fight to defend their territory. Instead, they are more likely to put on a defensive display. For example, male gorillas may pound on their chest and thump the ground to warn other male gorillas to stay away from their territory. This gets the message across without physical conflict, which would be riskier and take more energy. You can see a male gorilla putting on a defensive display in this video:  . MEDIA Click image to the left or use the URL below. URL: "
types of animal behavior,T_2118,Many animal behaviors occur in repeated cycles. Some cycles of behavior repeat each year. Other cycles of behavior repeat each day. 
types of animal behavior,T_2119,"Examples of behaviors with annual cycles include migration and hibernation. Both are innate behaviors. They are triggered by changes in the environment, such as the days growing shorter in the fall. Migration is the movement of animals from one place to another. Migration is most common in birds, fish, and insects. In the Northern Hemisphere, many species of birds, such as finches and swallows, travel south for the winter. They migrate to areas where it is warmer and where more food is available. They return north in the spring. Migrating animals generally follow the same route each year. They may be guided by the position of the sun, Earths magnetic field, or other clues in the environment. Hibernation is a state in which an animals body processes slow down and its body temperature falls. A hibernating animal uses less energy than usual. This helps it survive during a time of year when food is scarce. Hibernation may last for weeks or even months. Examples of animals that hibernate include some species of bats, squirrels, snakes, and insects (see Figure 15.18). "
types of animal behavior,T_2120,"Many animals go through daily cycles. Daily cycles of behavior are called circadian rhythms. For example, most animals go to sleep when the sun sets down and wake up when the sun rises. These animals are active during the day and called diurnal. Other animals go to sleep when the sun rises and wake up when the sun sets. These animals are active during the night and called nocturnal. Many owls, like the owls in Figure 15.19, are nocturnal. Like some other nocturnal animals, they have large eyes that are specially adapted for seeing when light levels are low. In many species, including the human species, circadian rhythms are controlled by a tiny structure called the biological clock. It is located in the hypothalamus, which is a gland at the base of the brain. The biological clock sends signals to the body. The signals cause regular changes in behavior and body processes. The biological clock, in turn, is controlled by changes in the amount of light entering the eyes. Thats why the biological clock causes changes that repeat every 24 hours. "
choosing healthy foods,T_2164,"MyPlate is a diagram that shows you how to balance foods at each meal. It represents the relative amounts of five food groups that you should put on your plate (and in your cup). You can see MyPlate in Figure 17.6. The five food groups in MyPlate are: 1. 2. 3. 4. 5. Grains, such as whole-grain bread, pasta, and cereal. Vegetables, such as spinach, broccoli, and carrots. Fruits, such as oranges, strawberries, and bananas. Dairy, such as milk, yogurt, and cheese. Protein, such as meat, fish, and beans Follow these guidelines for using MyPlate: Enjoy your food, but eat less. Avoid oversized portions. Make half your plate fruits and vegetables, including both green and yellow or orange vegetables. Make at least half your grains whole grains. Choose fat-free or low-fat milk. Avoid high-sodium foods. Drink water instead of sugary drinks. Youll notice that there is no food group on MyPlate for foods like ice cream, cookies, and potato chips. These foods have little nutritional value. They may also be high in fats, sugars, or salt. They should be eaten only sparingly if at all. "
choosing healthy foods,T_2165,"How do you know which foods contain whole grain and which are low in fat and sodium? Thats where food labels come in. In the U.S., packaged foods must be labeled with nutritional information. A nutrition facts label shows the main nutrients in one serving of the food. Packaged foods must also be labeled with their ingredients. An ingredient is a specific item that a food contains. "
choosing healthy foods,T_2166,"Look at the nutrition facts label in Figure 17.7. Instructions at the right of the label tell you what to look for. At the top of the label, look for the serving size. The serving size tells you how much of the food you should eat to get the nutrients listed on the label. For this food, 1 cup is a serving. The Calories in one serving are listed next. In this food, there are 250 Calories per serving. Next on the nutrition facts label, look for the percent daily values (% DV) of several nutrients. The percent daily value shows what percent of daily needs for a given nutrient that the food provides (based on a 2000- Calorie-per-day diet). A food is low in a nutrient if the %DV is 5% or less. This particular food is low in fiber, vitamin A, vitamin C, and iron. A food is high in a nutrient if the %DV is 20% or more. This food is high in sodium, potassium, and calcium. To learn more about nutrition facts labels and how to use them, watch this video:  MEDIA Click image to the left or use the URL below. URL: "
choosing healthy foods,T_2167,"The food label in Figure 17.8 represents a different food and includes the list of ingredients. The main ingredient is always listed first. The main ingredient is the ingredient that is present in the food in the greatest amount. As you go down the list, the ingredients are present in smaller and smaller amounts. Reading the ingredients lists on food labels can help you choose the healthiest foods. At the top of the list, look for ingredients such as whole grains, vegetables, fruits, and low-fat milk. Ingredients such as these are needed in the greatest amounts for balanced eating. Avoid foods that list fats, oils, sugar, or salt near the top of the list. For good health, you should avoid getting too much of these ingredients. Be aware that ingredients such as corn syrup are sugars. You should also use moderation when eating foods that contain ingredients such as white flour or white rice. These ingredients have been processed, and processing removes nutrients. The word enriched is a clue that an ingredient has been processed. Ingredients are enriched with added nutrients to replace those lost during processing. Even so, they are still likely to have fewer nutrients than unprocessed ingredients. "
choosing healthy foods,T_2168,"Physical activity is an important part of balanced eating. It helps you use up any extra Calories in the foods you eat. You should try to get at least an hour of exercise just about every day (see Figure 17.9). Exercise has many health benefits in addition to balancing the energy in food. For example, it strengthens the bones and muscles and may improve your mood. "
choosing healthy foods,T_2169,"What happens if you dont get enough exercise to balance the food you eat? Any unused energy in the food is stored as fat. If you take in more energy than you use day after day, you will store more and more fat and become overweight. Eventually, you may become obese. Obesity is diagnosed in people who have a high percentage of body fat. A measure called Body Mass Index, or BMI, is often used to diagnose obesity. You can learn more about BMI by watching this video:  MEDIA Click image to the left or use the URL below. URL:  Obesity is associated with many health problems, including high blood pressure and diabetes. People that remain obese during their entire adulthood usually do not live as long as people that stay within a healthy weight range. The current generation of young people in the U.S. is the first generation in our history that may have a shorter life span than their parents because of obesity and the health problems associated with it. "
choosing healthy foods,T_2170,"You can avoid gaining too much weight and becoming obese. Choose healthy foods and balance the energy in food with exercise. To choose healthy foods, use MyPlate and nutrition facts labels. On food labels, pay attention to Calories as well as nutrients. Keep in mind that the average 1113 year old needs about 2000 Calories a day. To balance energy with exercise, aim to get about an hour of physical activity each day. You can use an online calculator like this one to find the number of Calories you use in a wide range of activities: "
communities,T_2375,"Predation is a relationship in which members of one species consume members of another species. The consuming species is called the predator. The species that is consumed is called the prey. In Figure 23.8, the wolves are predators, and the moose is their prey. "
communities,T_2376,"A predator-prey relationship tends to keep the populations of both species in balance. Look at the graph in Figure population also increases. As the number of predators increases, more prey are captured. This causes the prey population to decrease, followed by the predator population decreasing again. "
communities,T_2377,"Some predator species play a special role in their community. They are called keystone species. When the population size of a keystone species changes, the populations of many other species are affected. Prairie dogs, pictured in Figure 23.10, are an example of a keystone species. Their numbers affect most of the other species in their community. Prairie dog actions improve the quality of soil and water for plants, upon which most other species in the community depend. "
communities,T_2378,Both predators and prey have adaptations to predation that evolve through natural selection. Predator adaptations help them capture prey. Prey adaptations help them avoid predators. A common adaptation in both predator and prey species is camouflage. You can see an example in Figure 23.11. You can also see some amazing examples in this video:  MEDIA Click image to the left or use the URL below. URL: 
communities,T_2379,"Competition is a relationship between organisms that depend on the same resources. The resources might be food, water, or space. Competition can occur between organisms of the same species or between organisms of different species. Competition within a species is called intraspecific competition. It leads to natural selection within the species, so the species becomes better adapted to its environment. Competition between different species is called interspecific competition. It might lead to the less well-adapted species going extinct. Or it might lead to one or both species evolving specialized adaptations. For example, competing species might evolve adaptations that allow them to use different food sources. You can see an example in Figure 23.12. "
communities,T_2380,"Symbiosis is a close relationship between two species in which at least one species benefits. For the other species, the relationship may be beneficial, harmful, or neutral. There are three types of symbiosis: mutualism, parasitism, and commensalism. "
communities,T_2381,"Mutualism is a symbiotic relationship in which both species benefit. An example of mutualism is pictured in Figure can inject poison in the anemones prey. The clownfish is protected from the stingers by mucus that covers its body. How do the two species benefit from their close relationship? The anemone provides the clownfish with a safe place to live by keeping away predatory fish. The clownfish also feeds on the remains of the anemones prey. In return, the clownfish helps the anemone catch food by attracting prey with its bright colors. Its feces also provide nutrients to the anemone. "
communities,T_2382,"Parasitism is a symbiotic relationship in which one species benefits and the other species is harmed. The species that benefits is called the parasite. The species that is harmed is called the host. Many species of animals are parasites, at least during some stage of their life cycle. Most animal species are also hosts to one or more parasites. A parasite generally lives in or on its host. An example of a parasite that lives in its host is the hookworm. Figure from their host, which is harmed by the loss of nutrients and blood. Some parasites kill their host, but most do not. Its easy to see why. If a parasite kills its host, the parasite may also die. Instead, parasites usually cause relatively minor damage to their host. "
communities,T_2383,"Commensalism is a symbiotic relationship in which one species benefits while the other species is not affected. An example is the relationship between birds called cattle egrets and cattle (see Figure 23.15). Cattle egrets feed on insects. They follow cattle herds around to take advantage of the insects stirred up by the feet of the cattle. The egrets get ready access to food from the relationship, whereas the cattle are not affected. "
ecosystems,T_2384,"Ecosystems need a constant input of energy to supply the needs of their organisms. Most ecosystems get energy from sunlight. A few ecosystems get energy from chemical compounds. Unlike energy, matter doesnt need to be constantly added to ecosystems. Instead, matter is recycled through ecosystems. Water and elements such as carbon and nitrogen that living things need are used over and over again. "
ecosystems,T_2385,Two important concepts associated with the ecosystem are niche and habitat. 
ecosystems,T_2386,Niche is the role that a particular species plays in its ecosystem. This role includes all the ways that the species interacts with the biotic and abiotic factors in the ecosystem. A major aspect of any niche is how the species obtains energy and matter. Look at Figure 23.16. The grass in the figure obtains energy from sunlight and uses it to convert carbon dioxide and water to sugar by photosynthesis. The deer in the figure gets matter and energy by consuming and digesting the grass. Each species has a different and distinctive niche. 
ecosystems,T_2387,"Another important aspect of a species niche is its habitat. Habitat is the physical environment in which a species lives and to which it has adapted. Features of a habitat depend mainly on abiotic factors, such as temperature and rainfall. These factors influence the traits of the organisms that live there. "
ecosystems,T_2388,"A given habitat may contain many different species. However, each species in the same habitat must have a different niche. Two different species cannot occupy the same niche in the same habitat at the same time. This is called the competitive exclusion principle. What do you think would happen if two species were to occupy the same niche in the same habitat? The two species would compete for everything they needed in the environment. One species might outcompete and replace the other. Or, both species might evolve different specializations so they can fill slightly different niches. "
flow of energy,T_2397,"Living things can be classified based on how they obtain energy. Some use the energy in sunlight or chemical compounds directly to make food. Some get energy indirectly by consuming other organisms, either living or dead. "
flow of energy,T_2398,"Producers are living things that produce food for themselves and other organisms. They use energy and simple inorganic molecules to make organic compounds. Producers are vital to all ecosystems because all organisms need organic compounds for energy. Producers are also called autotrophs. There are two basic types of autotrophs: photoautotrophs and chemoautotrophs. Photoautotrophs use energy in sunlight to make organic compounds by photosynthesis. They include plants, algae, and some bacteria (see Figure 24.1). Chemoautotrophs use energy in chemical compounds to make organic compounds. This process is called chemosynthesis. Chemoautotrophs include certain bacteria and archaea. "
flow of energy,T_2399,"Consumers are organisms that depend on other living things for food. They take in organic compounds by eating or absorbing other living things. Consumers include all animals and fungi. They also include some bacteria and protists. Consumers are also called heterotrophs. There are several different types of heterotrophs depending on exactly what they consume. They may be herbivores, carnivores, or omnivores. Herbivores are heterotrophs that consume producers such as plants or algae. Examples include rabbits and snails. Carnivores are heterotrophs that consume animals. Examples include lions and frogs. Omnivores are heterotrophs that consume both plants and animals. They include crows and human beings. The grizzly bears pictured in Figure 24.2 are also omnivores. "
flow of energy,T_2400,"Decomposers are heterotrophs that break down the wastes of other organisms or the remains of dead organisms. When they do, they release simple inorganic molecules back into the environment. Producers can then use the inorganic molecules to make new organic compounds. For this reason, decomposers are essential to every ecosystem. Imagine what would happen if there were no decomposers. Organic wastes and dead organisms would pile up everywhere, and their nutrients would no longer be recycled. Decomposers are classified by the type of organic matter they break down. They may be scavengers, detritivores, or saprotrophs. Scavengers are decomposers that consume the soft tissues of dead animals. Examples of scavengers include hyenas and cockroaches. Detritivores are decomposers that consume dead leaves, animal feces, and other organic debris that collects on the ground or at the bottom of a body of water. Examples of detritivores include earthworms and catfish. You can see another example in Figure 24.3. Saprotrophs are decomposers that feed on any remaining organic matter that is left after other decomposers do their work. Examples of saprotrophs include fungi and protozoa. "
flow of energy,T_2401,"Energy flows through ecosystems from producers, to consumers, to decomposers. Food chains and food webs are diagrams that model this flow of energy. They represent feeding relationships by showing who eats whom. "
flow of energy,T_2402,A food chain is a diagram that represents a single pathway through which energy flows through an ecosystem. Food chains are generally simpler than what really happens in nature. Thats because most organisms consume and are consumed by more than one species. You can see examples of terrestrial and aquatic food chains in Figure 24.4. See if you can construct a food chain of each type by playing the animation at this link: 
flow of energy,T_2403,A food web is a diagram that represents many pathways through which energy flows through an ecosystem. It includes a number of intersecting food chains. Food webs are generally more similar to what really happens in nature. They show that most organisms consume and are consumed by multiple species. You can see an example of a food web in Figure 24.5. 
flow of energy,T_2404,"Each food chain or food web has organisms at different trophic levels. A trophic level is a feeding position in a food chain or web. The trophic levels are identified in the food web in Figure 24.5. All food chains and webs have at least two or three trophic levels, but they rarely have more than four trophic levels. The trophic levels are: 1. 2. 3. 4. Trophic level 1 = producers that make their own food Trophic level 2 = primary consumers that eat producers Trophic level 3 = secondary consumers that eat primary consumers Trophic level 4 = tertiary consumers that eat secondary consumers Many consumers feed at more than one trophic level. For example, the bivalves in Figure 24.5 eat both producers and primary consumers. Therefore, they feed at trophic levels 2 and 3. "
flow of energy,T_2405,"Energy is passed up a food chain or web from lower to higher trophic levels. However, only about 10 percent of the energy at one level is passed up the next level. This is represented by the ecological pyramid in Figure 24.6. The other 90 percent of energy at each trophic level is used for metabolic processes or given off to the environment as heat. This loss of energy explains why there are rarely more than four trophic levels in a food chain or web. There isnt enough energy left to support additional levels. It also explains why ecosystems need a constant input of energy. You can learn more about ecological pyramids in this video:  . MEDIA Click image to the left or use the URL below. URL: "
flow of energy,T_2406,"Biomass is the total mass of organisms at a trophic level. With less energy at higher trophic levels, there are usually fewer organisms as well. This is also represented in the pyramid in Figure 24.6. Organisms tend to be larger in size at higher trophic levels. However, their smaller numbers result in less biomass. "
ecosystem change,T_2416,"Primary succession occurs in an area that has never before been colonized by living things. Generally, the area is nothing but bare rock. "
ecosystem change,T_2417,Secondary succession occurs in a formerly inhabited area that was disturbed. 
ecosystem change,T_2418,"This type of environment could come about when: a landslide uncovers bare rock a glacier retreats and leaves behind bare rock lava flows from a volcano and hardens into bare rock (see Figure 24.12) Secondary succession could result from a fire, flood, or human action such as farming. For example, a forest fire might kill all the trees and other plants in a forest, leaving behind only charred wood and soil. "
ecosystem change,T_2419,"The first few species to colonize a disturbed area are called pioneer species. In primary succession, pioneer species must be organisms that can live on bare rock. They usually include bacteria and lichens (see Figure 24.12). Along with wind and water, the pioneer species help weather the rock and form soil. Once soil begins to form, plants can move in. The first plants are usually grasses and other small plants that can grow in thin, poor soil. As more plants grow and die, organic matter is added to the soil. This improves the soil and helps it hold water. The improved soil allows shrubs and trees to move into the area. Secondary succession is faster than primary succession. The soil is already in place. After a forest fire, for example, the pioneer species are plants such as grasses and fireweed. You can see a forest in this stage of recovery in Figure area. You can see the amazing real-world story of secondary succession on Mount St. Helens by watching this short video:  . MEDIA Click image to the left or use the URL below. URL: "
ecosystem change,T_2420,"Does a changing ecosystem ever stop changing? Does its community of organisms ever reach some final, stable state? Scientists used to think that ecological succession always ended at a stable state, called a climax community. Now their thinking has changed. Theoretically, a climax community is possible. But continued change is probably more likely for real-world ecosystems. Most ecosystems are disturbed too often to ever develop a climax community. "
biodiversity and extinction,T_2447,"Biodiversity refers to the variety of life and its processes. It includes the variation in living organisms, the genetic differences among them, and the range of communities and ecosystems in which they live. Scientists have identified about 1.9 million species alive today, but they are discovering new species all the time. How many species actually exist in the world? No one knows for sure because only a small percentage of them have already been discovered. Estimates range from 5 to 30 million total species currently in existence. Many of them live on coral reefs and in tropical rainforests (see Figure 25.14). These two ecosystems have some of the greatest biodiversity on the planet. "
biodiversity and extinction,T_2448,"Biodiversity is important to human beings for many reasons. For one thing, biodiversity has direct economic benefits. Here are a few of the economic benefits of biodiversity: Besides food, diverse living things provide us with many different products. Some examples include dyes, rubber, fibers, paper, adhesives, and timber. Living things are an invaluable source of medical drugs. More than half of the most important prescription drugs come from wild species. However, only a fraction of species have yet been studied for their medical potential. Certain species may warn us of toxins in the environment. Amphibians are particularly sensitive to toxins be- cause of their permeable skin. Their current high rates of extinction serve as an early warning of environmental damage and danger to us all. Wild organisms maintain a valuable pool of genetic variation. This is important because most domestic species have been bred to be genetically uniform. This puts domestic crops and animals at great risk of dying out due to disease. Some living things provide inspiration for technology. For example, water strider insects like the one in Figure water quality, among other useful purposes. "
biodiversity and extinction,T_2449,"Biodiversity is important for healthy ecosystems. It generally increases ecosystem productivity and stability. It helps ensure that at least some species will survive environmental change. Biodiversity also provides many other ecosystem services. For example: Plants and algae maintain Earths atmosphere. They add oxygen to the air and remove carbon dioxide when they undertake photosynthesis. Plants help protect the soil. Their roots grip the soil and keep it from washing or blowing away. When plants die, their organic matter improves the soil as it decomposes. Microorganisms purify water in rivers and lakes. They also decompose organic matter and return nutrients to the soil. Certain bacteria fix nitrogen and make it available to plants. Predator species such as birds and spiders control insect pests. They reduce the need for chemical pesticides, which are expensive and may be harmful to human beings and other organisms. Animals, like the bee in Figure below, pollinate flowering plants. Many crop plants depend on pollination by wild animals. "
biodiversity and extinction,T_2450,"Extinction is the complete dying out of a species. Once a species goes extinct, it can never return. More than 99 percent of all the species that ever lived on Earth have gone extinct. Five mass extinctions have occurred in Earths history. They were caused by major geologic and climatic events. The fifth mass extinction wiped out the dinosaurs 65 million years ago. "
biodiversity and extinction,T_2451,"Evidence shows that a sixth mass extinction is happening right now. Species are currently going extinct at the fastest rate since the dinosaurs died out. Dozens of species are going extinct every day. If this rate continues, as many as half of all remaining species could go extinct by 2050. Why are so many species going extinct today? Unlike previous mass extinctions, the sixth mass extinction is due mainly to human actions. "
biodiversity and extinction,T_2452,"The single biggest cause of the sixth mass extinction is habitat loss. A habitat is the area where a species lives and to which it has become adapted. When a habitat is disturbed or destroyed, it threatens all the species that live there with extinction. More than half of Earths land area has been disturbed or destroyed by farming, mining, forestry, or the development of cities, suburbs, and golf courses. Habitats that are rapidly being destroyed include tropical rainforests. They are being cut and burned, mainly to clear the land for farming. Half of Earths mature tropical forests have already been destroyed. At current rates of destruction, they will all be gone by 2090. In the U.S., half of the wetlands and almost all of the tall-grass prairies (see Figure 25.17) have already been destroyed for farming. "
biodiversity and extinction,T_2453,"There are several other causes of the sixth mass extinction. Most of them contribute to habitat destruction. The burning of fossil fuels has increased the greenhouse effect and caused global climate change. Increasing temperatures are changing basic climate factors of habitats, and rising sea levels are covering them with water. These changes threaten many species. Pollution of air, water, and soil makes habitats toxic to many organisms. A well-known example is the near extinction of the peregrine falcon in the mid-1900s due to the pesticide DDT. Humans have over-harvested trees, fish, and other wild species. This threatens not only their survival but the survival of all the other species that depend on them. Humans have introduced exotic species into new habitats. These are species that are not native to the habitat where they are introduced. They may lack predators in the new habitat so they can out-compete native species and drive them extinct. Exotic species may also carry new diseases, prey on native species, and disrupt local food webs. You can read about an example of an exotic species in Figure 25.18. "
biodiversity and extinction,T_2454,"Government policies and laws are needed to protect biodiversity. Such actions have been shown to work in the past. For example, peregrine falcons made an incredible recovery after laws were passed banning the use of DDT. Individuals can also play a role in protecting biodiversity. What can you do? Here are a few suggestions: Start a compost pile to recycle organic wastes. Use the compost to enrich yard and garden soil. It will reduce the need for chemical fertilizers and added water. Make your backyard welcoming to native wildlife. Plant native plants that will provide food and shelter for native animals such as birds and amphibians. Add a water source, such as a fountain or bird bath. Avoid the introduction of exotic species to local habitats. Avoid the use of herbicides and pesticides. In addition to killing garden weeds and pests, they may harm native organisms, such as wildflowers, honey bees, and song birds. Conserve natural resources, including energy resources. Always reduce, reuse, or recycle. Learn more about biodiversity and how to protect it. Then pass on what you learn to others. "
mendels discoveries,T_2547,Mendel was an Austrian Monk who lived in the 1800s. You can see his picture in Figure 6.1. 
mendels discoveries,T_2548,"Mendel didnt call himself a scientist. But he had all the traits of good scientist. He was observant and curious, and he asked a lot of questions. He also tried to find answers to his questions by doing experiments. Working alone in his garden in the mid-1800s, he grew thousands of pea plants over many years. He carefully crossed plants with different traits. Then he observed what traits showed up in their offspring. He repeated each experiment many times. "
mendels discoveries,T_2549,"Pea plants were a good choice to study for several reasons. One reason is that they are easy to grow. They also grow quickly. In addition, peas have many traits that are easy to observe, and each trait exists in two different forms. Figure 6.2 shows the traits that Mendel studied in pea plants. For example, one trait is flower color. Flowers may be either white or violet. Another trait is stem length. Plants may be either tall or short. Pea plants reproduce sexually. The male gametes are released by tiny grains of pollen. The female gametes lie deep within the flowers. Fertilization occurs when pollen from one flower reaches the female gametes in the same or a different flower. This is called pollination. Mendel was able to control which plants pollinated each other. He transferred pollen by hand from flower to flower. "
mendels discoveries,T_2550,"At first, Mendel studied one trait at a time. This was his first set of experiments. These experiments led to his first law, the law of segregation. Then Mendel studied two traits at a time. This was his second set of experiments. These experiments led to his second law, the law of independent assortment. "
mendels discoveries,T_2551,"An example of Mendels first set of experiments is his research on flower color. He transferred pollen from a plant with white flowers to a plant with violet flowers. This is called cross-pollination. Then Mendel observed flower color in their offspring. The offspring formed the first generation (F1). You can see the outcome of this experiment in Figure 6.3. All of the F1 plants had violet flowers. Mendel wondered, ""What happened to the white form of the trait?"" ""Did it disappear?"" If so, the F1 plants should have only violet-flowered offspring. Mendel let the FI plants pollinate themselves. This is called self-pollination. Then he observed flower color in their offspring. These offspring formed the second generation (F2). Surprisingly, the trait of white flowers showed up in the F2 plants. One out of every four F2 plants had white flowers. The other three out of four had violet flowers. In other words, F2 plants with violet flowers and F2 plants with white flowers had a 3:1 ratio. Mendel repeated this experiment with each of the other traits. For each trait, he got the same results. One form of the trait seemed to disappear in the F1 plants. Then it showed up again in the F2 plants. Moreover, the two forms of the trait always showed up in the F2 plants in the same 3:1 ratio. "
mendels discoveries,T_2552,"Based on these results, Mendel concluded that each trait is controlled by two factors. He also concluded that one of the factors for each trait dominates the other. He described the dominating factor as dominant. He used the term recessive to describe the other factor. If both factors are present in an individual, only the dominant factor is expressed. This explains why one form of a trait always seems to disappear in the F1 plants. These plants inherit both factors for the trait, but only the dominant factor shows up. The recessive factor is hidden. When F1 plants reproduce, the two factors separate and go to different gametes. This is Mendels first law, the law of segregation. It explains why both forms of the trait show up again in the F2 plants. One out of four F2 plants inherits two of the recessive factors for the trait. In these plants, the recessive form of the trait is expressed. "
mendels discoveries,T_2553,"Mendel wondered whether different traits are inherited together. For example, are seed form and seed color passed together from parents to offspring? Or do the two traits split up in the offspring? To answer these questions, Mendel studied two traits at a time. For example, he crossed plants that had round, yellow seeds with plants that had wrinkled, green seeds. Then he observed how the two traits showed up in their offspring (F1). You can see the results of this cross in Figure 6.4. All of the F1 plants had round, yellow seeds. The wrinkled and green forms of the traits seemed to disappear in the F1 plants. Then Mendel let the F1 plants self-pollinate. Their offspring, the F2 plants, had all possible combinations of the two traits. You can see this in Figure 6.5. For example there were plants that had round, green seeds, as well as plants that had wrinkled, yellow seeds. In this case the ratios were 9:3:3:1. The ratios are shown across the bottom of Figure 6.5. Mendel repeated this experiment with other combinations of two traits. In each case, he got the same results. One form of each trait seemed to disappear in the F1 plants. Then these forms reappeared in the F2 plants in all possible combinations. Moreover, the different combinations of traits always occurred in the same 9:3:3:1 ratio. "
mendels discoveries,T_2554,The results of Mendels two-trait experiments led to the law of independent assortment. This law states that factors controlling different traits go to gametes independently of each other. This explains why F2 plants have all possible combinations of the two traits. 
mendels discoveries,T_2555,"You might think that Mendels discoveries would have made him an instant science rock star. Hed found the answers to age-old questions about heredity. In fact, Mendels work was largely ignored until 1900. Thats when three other scientists independently arrived at Mendels laws. Only then did people appreciate what a great contribution to science Mendel had made. Mendels discoveries form the basis of the modern science of genetics. Genetics is the science of heredity. For his discoveries, Mendel is now called the ""father of genetics."" Watch this entertaining, upbeat video for an excellent review of Mendels life and work. Its also a good introduction to the next lesson, ""Introduction to Genetics."" MEDIA Click image to the left or use the URL below. URL: "
introduction to genetics,T_2556,"Today we know that the traits of organisms are controlled by genes on chromosomes. A gene can be defined as a section of a chromosome that contains the genetic code for a particular protein. The position of a gene on a chromosome is called its locus. Each gene may have different versions. The different versions are called alleles. Figure 6.6 shows an example in pea plants. It shows the gene for flower color. The gene has two alleles. There is a purple-flower allele and a white-flower allele. Different alleles account for most of the variation in the traits of organisms within a species. In sexually reproducing species, chromosomes are present in cells in pairs. Chromosomes in the same pair are called homologous chromosomes. They have the same genes at the same loci. These may be the same or different alleles. During meiosis, when gametes are produced, homologous chromosomes separate. They go to different gametes. Thus, the alleles for each gene also go to different gametes. "
introduction to genetics,T_2557,"When gametes unite during fertilization, the resulting zygote inherits two alleles for each gene. One allele comes from each parent. "
introduction to genetics,T_2558,The two alleles that an individual inherits make up the individuals genotype. The two alleles may be the same or different. Look at Table 6.1. It shows alleles for the flower-color gene in peas. The alleles are represented by the letters B (purple flowers) and b (white flowers). A plant with two alleles of the same type (BB or bb) is called a homozygote. A plant with two different alleles (Bb) is called a heterozygote. Genotypes BB (homozygote) Bb (heterozygote) bb (homozygote) Phenotypes purple flowers purple flowers white flowers 
introduction to genetics,T_2559,"The expression of an organisms genotype is called its phenotype. The phenotype refers to the organisms traits, such as purple or white flowers. Different genotypes may produce the same phenotype. This will be the case if one allele is dominant to the other. Both BB and Bb genotypes in Table 6.1 have purple flowers. Thats because the B allele is dominant to the b allele, which is recessive. The terms dominant and recessive are the terms Mendel used to describe his ""factors."" Today we use them to describe alleles. In a Bb heterozygote, only the dominant B allele is expressed. The recessive b allele is expressed only in the bb genotype. "
introduction to genetics,T_2560,"Each trait Mendel studied was controlled by one gene with two alleles. In each case, one of the alleles was dominant to the other. This resulted in just two possible phenotypes for each trait. Each trait Mendel studied was also controlled by a gene on a different chromosome. As a result, each trait was inherited independently of the others. With traits like these, its easy to predict which forms of a trait will show up in the offspring of a given set of parents. "
introduction to genetics,T_2561,"Consider a purple-flowered pea plant with the genotype Bb. Half the gametes produced by this parent will have a B allele. The other half will have a b allele. You can see this in Figure 6.7. This is similar to tossing a coin. There is a 50 percent chance of a head and a 50 percent chance of a tail. Like a head or tail, there is a 50 percent chance that any gamete from this parent will have the B allele. There is also a 50 percent chance that any gamete will have the b allele. "
introduction to genetics,T_2562,"Now lets see what happens if two parent pea plants have the Bb genotype. What genotypes are possible for their offspring? And what ratio of genotypes would you expect? The easiest way to find the answer to these questions is with a Punnett square. A Punnett square is a chart that makes it easy to find the possible genotypes in offspring of two parents. Figure of the chart. The gametes produced by the female parent are along the left side of the chart. The different possible combinations of alleles in their offspring can be found by filling in the cells of the chart. If the parents had four offspring, their most likely genotypes would be one BB, two Bb, and one bb. But the genotype ratios of their actual offspring may differ. Thats because which gametes happen to unite is a matter of chance, like a coin toss. The Punnett square just shows the possible genotypes and their most likely ratios. "
introduction to genetics,T_2563,"You know that the B allele is dominant to the b allele. Therefore, you can also use the Punnett square in Figure 6.8 to predict the most likely offspring phonotypes. If the parents had four offspring, their most likely phenotypes would be three with purple flowers (1 BB + 2 Bb) and one with white flowers (1 bb). "
introduction to genetics,T_2564,Inheritance is often more complex than it is for traits like those Mendel studied. Several factors can complicate it. 
introduction to genetics,T_2565,"If a gene has two alleles, one may not be dominant to the other. There are other possibilities. One possibility is called codominance. Another is called incomplete dominance. With codominance, both alleles are expressed equally in heterozygotes. The red and white flower in Figure With incomplete dominance, a dominant allele is not completely dominant. Instead, it is influenced by the recessive allele in heterozygotes. The pink flower in Figure 6.9 is an example. It has an incompletely dominant allele for red petals. It also has a recessive allele for white petals. This results in a flower with pink petals. "
introduction to genetics,T_2566,Many genes have more than two alleles. An example is ABO blood type in people. There are three common alleles for the gene that controls this trait. The allele for type A is codominant with the allele for type B. Both of these alleles are dominant to the allele for type O. The possible genotypes and phenotypes for this trait are shown in Table below Genotype AA AO BB BO AB OO Phenotype Type A Type A Type B Type B Type AB Type O 
introduction to genetics,T_2567,Some traits are controlled by more than one gene. They are called polygenic traits. Each gene for a polygenic trait may have two or more alleles. The genes may be on the same or different chromosomes. Polygenic traits may have many possible phenotypes. Skin color and adult height are examples of polygenic traits in humans. Think about all the variation in the heights of adults you know. Normal adults may range from less than 5 feet tall to more than 7 feet tall. There are people at every gradation of height in between these extremes. 
introduction to genetics,T_2568,"Genes play an important role in determining an organisms traits. However, for many traits, phenotype is influenced by the environment as well. For example, skin color is controlled by genes but also influenced by exposure to sunlight. You can see the effect of sunlight on skin in Figure 6.10. "
introduction to genetics,T_2569,Animals and most plants have two special chromosomes. They are called sex chromosomes. These are chromo- somes that determine the sex of the organism. All of the other chromosomes are called autosomes. Genes on sex chromosomes may be inherited differently than genes on autosomes. 
introduction to genetics,T_2570,"In people, the sex chromosomes are called X and Y chromosomes. Individuals with two X chromosomes are normally females. Individuals with one X and one Y chromosome are normally males. As you can see in Figure sons. "
introduction to genetics,T_2571,"Traits controlled by genes on the sex chromosomes are called sex-linked traits. One gene on the Y chromosome determines male sex. There are very few other genes on the Y chromosome, which is the smallest human chromo- some. There are hundreds of genes on the much larger X chromosome. None is related to sex. Traits controlled by genes on the X chromosome are called X-linked traits. X-linked traits have a different pattern of inheritance than traits controlled by genes on autosomes. With just one X chromosome, males have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. With two X chromosomes, females have two alleles for any X-linked trait, just as they do for autosomal traits. Therefore, a recessive X-linked allele is expressed in females only when they inherit two copies of it. This explains why X-linked recessive traits show up less often in females than males. "
introduction to genetics,T_2572,"An example of a recessive X-linked trait is red-green color blindness. People with this trait cant see red or green colors. This trait is fairly common in males but rare in females. Figure 6.12 is a pedigree for this trait. A pedigree is a chart that shows how a trait is inherited in a family. The mother has one allele for color blindness. She doesnt have color blindness because she also has a dominant normal allele for the gene. Instead, she is called a carrier for the trait. She passes the allele to half of her children. One daughter is a carrier, and one son has the color blindness trait. No matter how many children this couple has, none of the daughters will have color blindness, but half of the sons, on average, will have the trait. Can you explain why? "
advances in genetics,T_2573,A species genome consists of all of its genetic information. The human genome consists of the complete set of genes in the human organism. Its all the DNA of a human being. 
advances in genetics,T_2574,"The Human Genome Project was launched in 1990. It was an international effort to sequence all 3 billion bases in human DNA. Another aim of the project was to identify the more than 20,000 human genes and map their locations on chromosomes. The logo of the Human Genome Project in Figure 6.13 shows that the project brought together experts in many fields. The Human Genome Project was completed in 2003. It was one of the greatest feats of modern science. It provides a complete blueprint for a human being. Its like having a very detailed manual for making a human organism. "
advances in genetics,T_2575,"Knowing the sequence of the human genome is very useful. For example, it helps us understand how humans evolved. Another use is in medicine. It is helping researchers identify and understand genetic disorders. You can learn more about the Human Genome Project and its applications by watching this funny, fast-paced video: http://w MEDIA Click image to the left or use the URL below. URL: "
advances in genetics,T_2576,Sequencing the human genome has increased our knowledge of genetic disorders. Genetic disorders are diseases caused by mutations. Many genetic disorders are caused by mutations in a single gene. Others are caused by abnormal numbers of chromosomes. 
advances in genetics,T_2577,"Table 6.3 lists some genetic disorders caused by mutations in just one gene. It include autosomal and X-linked disorders. It also includes dominant and recessive disorders. Genetic Disorder Marfan syndrome Cystic fibrosis Sickle Cell Anemia Hemophilia A Effect of Mutation Defective protein in tis- sues such as cartilage and bone Defective protein needed to make mucus Defective hemoglobin protein that is needed to transport oxygen in red blood cells Reduced activity of a pro- tein needed for blood to clot Signs of the Disorder Heart and bone defects; unusually long limbs Type of Trait Autosomal dominant Unusually thick mucus that clogs airways in lungs and ducts in other organs Sickle-shaped red blood cells that block blood ves- sels and interrupt blood flow Excessive bleeding that is difficult to control Autosomal recessive Autosomal recessive X-linked recessive Relatively few genetic disorders are caused by dominant alleles. A dominant allele is expressed in everybody who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. They wont pass the allele to the next generation. As a result, the allele may die out of the population. One of the exceptions is Marfan syndrome. It is thought to have affected Abraham Lincoln. Hes pictured in Figure 6.14. His very long limbs are one reason for the suspicion of Marfan syndrome in this former U.S. president. Recessive disorders are more common than dominant ones. Why? A recessive allele is not expressed in heterozy- gotes. These people are called carriers. They dont have the genetic disorder but they carry the recessive allele. They can also pass this allele to their offspring. A recessive allele is more likely than a dominant allele to pass to the next generation rather than die out. "
advances in genetics,T_2578,"In the process of meiosis, paired chromosomes normally separate from each other. They end up in different gametes. Sometimes, however, errors occur. The paired chromosomes fail to separate. When this happens, some gametes get an extra copy of a chromosome. Other gametes are missing a chromosome. If one of these gametes is fertilized and survives, a chromosomal disorder results. You can see examples of such disorders in Table 6.4 Genetic Disorder Down syndrome Genotype Extra copy (complete or partial) of chromosome 21 Turners syndrome One X chromosome and no other sex chromosome (XO) One Y chromosome and two or more X chromosomes (XXY, XXXY) Klinefelters syndrome Phenotypic Effects Developmental delays, distinctive facial appearance, and other abnor- malities Female with short height and inabil- ity to reproduce Male with abnormal sexual devel- opment and reduced level of male sex hormone Most chromosomal disorders involve the sex chromosomes. Can you guess why? The X and Y chromosomes are very different in size. The X is much larger than the Y. This difference in size creates problems. It increases the chances that the two chromosomes will fail to separate properly during meiosis. "
advances in genetics,T_2579,"Treating genetic disorders is one use of biotechnology. Biotechnology is the use of technology to change the genetic makeup of living things for human purposes. Its also called genetic engineering. Besides treating genetic disorders, biotechnology is used to change organisms so they are more useful to people. "
advances in genetics,T_2580,"Biotechnology uses a variety of methods, but some are commonly used in many applications. A common method is the polymerase chain reaction. Another common method is gene cloning. The polymerase chain reaction is a way of making copies of a gene. It uses high temperatures and an enzyme to make new DNA molecules. The process keeps cycling to make many copies of a gene. Gene cloning is another way of making copies of a gene. A gene is inserted into the DNA of a bacterial cell. Figure 6.15 shows how this is done. Bacteria multiply very rapidly by binary fission. Each time a bacterial cell divides, the inserted gene is copied. "
advances in genetics,T_2581,"Biotechnology has many uses. It is especially useful in medicine and agriculture. Biotechnology is used to treat genetic disorders. For example, copies of a normal gene might be inserted into a patient with a defective gene. This is called gene therapy. Ideally, it can cure a genetic disorder. create genetically modified organisms (GMOs). Many GMOs are food crops such as corn. Genes are inserted into plants to give them desirable traits. This might be the ability to get by with little water. Or it might be the ability to resist insect pests. The modified plants are likely to be healthier and produce more food. They may also need less pesticide. produce human proteins. Insulin is one example. This protein is needed to treat diabetes. The human insulin gene is inserted into bacteria. The bacteria reproduce rapidly. They can produce large quantities of the human protein. You can see another example in Figure 6.16. "
advances in genetics,T_2582,"Biotechnology has many benefits. Its pros are obvious. It helps solve human problems. However, biotechnology also raises many concerns. For example, some people worry about eating foods that contain GMOs. They wonder if GMOs might cause health problems. The person in Figure 6.17 favors the labeling of foods that contain GMOs. That way, consumers can know which foods contain them and decide for themselves whether to eat them. Another concern about biotechnology is how it may affect the environment. Negative effects on the environment have already occurred because of some GMOs. For example, corn has been created that has a gene for a pesticide. The corn plants have accidentally cross-pollinated nearby milkweeds. Monarch butterfly larvae depend on milkweeds for food. When they eat milkweeds with the pesticide gene, they are poisoned. This may threaten the survival of the monarch species as well as other species that eat monarchs. Do the benefits of the genetically modified corn outweigh the risks? What do you think? "
archaea,T_2657,"Archaeans are prokaryotes in the Archaea Domain. They were first discovered in extreme environments such as hot springs. For a long time, they were classified as bacteria. As more was learned about them, they were found to be quite different from bacteria. They were finally placed in their own domain in the late 1970s. You can see the incredible story of their discovery in this brief video:  . MEDIA Click image to the left or use the URL below. URL:  The study of archaeans is in its infancy. Scientists still know relatively little about them. New species of archaeans are being discovered all the time. "
archaea,T_2658,"Many archaeans are extremophiles. Extremophiles are organisms that live in extreme conditions. For example, some archaeans live around hydrothermal vents. A hydrothermal vent is a crack on the ocean floor. You can see one in Figure 8.16. Boiling hot, highly acidic water pours out of the vent. These extreme conditions dont deter archaeans. They have evolved adaptations for coping with them. These conditions are like those on ancient Earth. This suggests that archaeans may have evolved very early in Earths history. There are four types of archaean extremophiles. Each type is described below. Extreme conditions pose many challenges to living cells. Archaeans have evolved adaptations that allow them to deal with the challenges. "
archaea,T_2659,"Halophiles are organisms that ""love"" salt. They can survive in very salty water. For example, they have been found in the Great Salt Lake in Utah and the Dead Sea between Israel and Jordan. Both of these bodies of water are much saltier than the ocean. "
archaea,T_2660,"Hyperthermophiles are organisms that ""love"" heat. Some archaeans can survive at very high temperatures. For example, they can grow in hot springs and geysers. One archaean species can even reproduce at 122 C (252 F). This is higher than the boiling point of water. It is the highest recorded temperature for any organism. "
archaea,T_2661,"Acidophiles are organisms that ""love"" acids. They live in very acidic environments, such as acid mine drainage. They are also found near vents of volcanoes. The most acidophilic archaeans can thrive at negative pH values. No other organisms can survive in such acidic conditions. "
archaea,T_2662,"Alkaliphiles are organisms that ""love"" bases. Bases are like the opposite of acids. Basic environments where archaeans are found include Mono Lake in California, pictured in Figure 8.17. Mono Lake is the oldest lake in North America. The water is not only unusually basic. Its also saltier than the ocean. So archaeans that live in the water of Mono Lake must have adaptations to both salty and basic conditions. They are haloalkaliphiles. "
archaea,T_2663,"Not all archaeans live in extreme conditions. In fact, archaeans are now known to live just about everywhere on Earth. They make up as much as 20 percent of Earths total mass of living things. "
archaea,T_2664,"Archaeans have been found in a broad range of habitats. For example, they live in soils, bodies of water, and marshlands. They even live in the human belly button! Archaeans are very common in the ocean. Archaeans in plankton may be some of the most abundant organisms on Earth. "
archaea,T_2665,"Like bacteria, archaeans are important decomposers. For example, archaeans help break down sewage in waste treatment plants. As decomposers, they help recycle carbon and nitrogen. Many archaeans live in close relationships with other organisms. For example, large numbers live inside animals, including humans. Unlike many bacteria, archaeans dont harm their hosts. None of them is known to cause human disease. Archaeans are more likely to help their hosts. For example, archaeans called methanogens live inside the gut of cows (see Figure 8.18). They help cows digest tough plant fibers made of cellulose. They produce methane gas as a waste product. "
alligators and crocodiles,T_2694,"Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles belonging to Crocodilia are the closest living relatives of birds. Reptiles and birds are the only known living descendants of the dinosaurs. Some would day that alligators and crocodiles actually look like small dinosaurs. Dinosaurs that evolved wings are the ancestors of birds. The basic crocodilian body plan ( Figure 1.1) is a very successful one and has changed little over time. Modern species actually look very similar to their Cretaceous ancestors of 84 million years ago. All species of crocodilians have similar body structures, including an elongated snout, powerful jaws, muscular tail, large protective scales, streamlined body, and eyes and nostrils that are positioned on top of the head. "
alligators and crocodiles,T_2695,"Crocodilians have a flexible, semi-erect posture. They can walk either in a low, sprawled belly walk, or hold their legs more directly underneath them to perform the high walk. Most other reptiles can only walk in a sprawled position. All crocodilians have, like humans, teeth set in bony sockets. But unlike mammals, they replace their teeth through- out life. Crocodiles and gharials (large crocodilians with longer jaws) have salivary glands on their tongue, which are used to remove salt from their bodies. This helps with life in a saltwater environment. Crocodilians are often seen lying with their mouths open, a behavior called gaping. One of its functions is probably to cool them down. The crocodilian digestive system is highly adapted to their lifestyle. Crocodilians are known to swallow stones, known as gastroliths, which help digest their prey. The crocodilian stomach is divided into two chambers. The first is powerful and muscular. The other stomach is the most acidic digestive system of any animal. It can digest mostly everything from their prey, including bones, feathers, and horns! All crocodilians are carnivores. They feed on live animals such as birds, small mammals and fish. Crocodilians use several methods of attack when pursuing live prey. One approach is that of the ambush. The crocodilian lies motionless beneath the waters surface with only their nostrils above the water line. This keeps them concealed while they watch for prey that approaches the waters edge. The crocodilian then lunges out of the water, taking their prey by surprise and dragging it from the shoreline into deep water where the prey is killed. The sex of developing crocodilians is determined by the temperature of the eggs during incubation, when eggs are kept warm before they hatch. This means that the sex of crocodilians is not determined genetically. If the eggs are kept at a cold or a hot temperature, then their offspring may be all male or all female. To get both male and female offspring, the temperature must be kept within a narrow range. Female crocodilians care for the young after they hatch, providing them with protection until they grow large enough to defend themselves. In many species of crocodilians, the female carries her tiny offspring in her mouth. "
alligators and crocodiles,T_2696,"Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more advanced than those of other reptiles. Because of their aquatic habitat, the eyes, ears, and nostrils are all located on the same ""face"" in a line one after the other. The crocodiles have advanced sensory organs. They see well during the day and may even have color vision, and they also have excellent night vision. A third transparent eyelid, the nictitating membrane, protects their eyes underwater. The eardrums are located behind the eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when they dive. This prevents water from entering their external head openings. Their jaws are covered with sensory pits, which hold bundles of nerve fibers that respond to the slightest disturbance in surface water. Crocodiles can detect vibrations and small pressure changes in water. This makes it possible for them to sense prey and danger even in total darkness, and becomes very useful when the animal is submerged in the water. Like mammals and birds, and unlike other reptiles, crocodiles have a four-chambered heart. But, unlike mammals, blood with and without oxygen can be mixed. See Supersize Crocs at "
amphibians,T_2697,"What group of animals begins its life in the water, but then spends most of its life on land? Amphibians! Amphibians are a group of vertebrates that has adapted to live in both water and on land. Amphibian larvae are born and live in water, and they breathe using gills. The adults live on land for part of the time and breathe both through their skin and with their lungs as their lungs are not sufficient to provide the necessary amount of oxygen. There are approximately 6,000 species of amphibians. They have many different body types, physiologies, and habitats, ranging from tropical to subarctic regions. Frogs, toads, salamanders ( Figure 1.1), newts, and caecilians are all types of amphibians. "
amphibians,T_2698,"Transition to life on land meant significant changes to both external and internal features. In order to live on land, amphibians replaced gills with another respiratory organ, the lungs. Other adaptations include: One of the many species of amphibian is this dusky salamander. Skin that prevents loss of water. Eyelids that allow them to adapt to vision outside of the water. An eardrum developed to separate the external ear from the middle ear. A tail that disappears in adulthood (in frogs and toads). "
amphibians,T_2699,"Like fish, amphibians are ectothermic vertebrates. They belong to the class Amphibia. There are three orders: 1. Urodela, containing salamanders and newts. 2. Anura, containing frogs and toads. 3. Apoda, containing caecilians. "
amphibians,T_2700,"Most amphibians live in fresh water, not salt water. Their habitats can include areas close to springs, streams, rivers, lakes, swamps and ponds. They can be found in moist areas in forests, meadows and marshes. Amphibians can be found almost anywhere there is a source of fresh water. Although there are no true saltwater amphibians, a few can live in brackish (slightly salty) water. Some species do not need any water at all, and several species have also adapted to live in drier environments. Most amphibians still need water to lay their eggs. "
amphibians,T_2701,"Amphibians reproduce sexually. The life cycle of amphibians happens in the following stages: 1. Egg Stage: Amphibian eggs are fertilized in a number of ways. External fertilization, employed by most frogs and toads, involves a male gripping a female across her back, almost as if he is squeezing the eggs out of her. The male releases sperm over the females eggs as they are laid. Another method is used by salamanders, whereby the male deposits a packet of sperm onto the ground. The female then pulls it into her cloaca, a single opening for her internal organ systems. Therefore, fertilization occurs internally. By contrast, caecilians and tailed frogs use internal fertilization, just like reptiles, birds, and mammals. The male deposits sperm directly into the females cloaca. 2. Larval stage: When the egg hatches, the organism is legless, lives in water, and breathes with gills, resembling their evolutionary ancestors (fish). 3. During the larval stage, the amphibian slowly transforms into an adult by losing its gills and growing four legs. Once development is complete, it can live on land. "
angiosperms,T_2702,"Angiosperms, in the phylum Anthophyta, are the most successful phylum of plants. This category also contains the largest number of individual plants ( Figure 1.1). Angiosperms evolved the structure of the flower, so they are also called the flowering plants. Angiosperms live in a variety of different environments. A water lily, an oak tree, and a barrel cactus, although different, are all angiosperms. "
angiosperms,T_2703,"Even though flowers may look very different from each other, they do have some structures in common. The structures are explained in the picture below ( Figure 1.2). The green outside of a flower that often looks like a leaf is called the sepal ( Figure 1.3). All of the sepals together are called the calyx, which is usually green and protects the flower before it opens. All of the petals ( Figure 1.3) together are called the corolla. They are bright and colorful to attract a particular pollinator, an animal that carries pollen from one flower to another. Examples of pollinators include birds and insects. Angiosperms are the flowering plants. A complete flower has sepals, petals, sta- mens, and one or more carpels. The next structure is the stamen, consisting of the stalk-like filament that holds up the anther, or pollen sac. The pollen, which is found at the top of the stamen, is the male gametophyte. At the very center is the carpel, which is divided into three different parts: (1) the sticky stigma, where the pollen lands, (2) the tube of the style, and (3) the large, bottom part, known as the ovary. The ovary holds the ovules, the female gametophytes. When the ovules are fertilized, the ovule becomes the seed and the ovary becomes the fruit. The following table summarizes the parts of the flower ( Table 1.1). Part sepals calyx corolla stamens filament anther carpel Definition The green outside of the flower. All of the sepals together, or the outside of the flower. The petals of a flower collectively. The part of the flower that produces pollen. Stalk that holds up the anther. The structure that contains pollen in a flower. Female part of the flower; includes the stigma, style, and ovary. style ovary ovules This image shows the difference between a petal and a sepal. "
angiosperms,T_2704,"Flowering plants can reproduce two different ways: 1. Self-pollination: Pollen falls on the stigma of the same flower. This way, a seed will be produced that can turn into a genetically identical plant. 2. Cross-fertilization: Pollen from one flower travels to a stigma of a flower on another plant. Pollen travels from flower to flower by wind or by animals. Flowers that are pollinated by animals such as birds, butterflies, or bees are often colorful and provide nectar, a sugary reward, for their animal pollinators. "
angiosperms,T_2705,"Angiosperms are important to humans in many ways, but the most significant role of angiosperms is as food. Wheat, rye, corn, and other grains are all harvested from flowering plants. Starchy foods, such as potatoes, and legumes, such as beans, are also angiosperms. And, as mentioned previously, fruits are a product of angiosperms that increase seed dispersal and are nutritious. There are also many non-food uses of angiosperms that are important to society. For example, cotton and other plants are used to make cloth, and hardwood trees are used for lumber. "
animal behaviors,T_2706,"Barking, purring, and playing are just some of the ways in which dogs and cats behave. These are examples of animal behaviors. Animal behavior is any way that animals act, either alone or with other animals. "
animal behaviors,T_2707,"Can you think of examples of animal behaviors? What about insects and birds? How do they behave? Pictured below are just some of the ways in which these, and other animals act ( Figure 1.1). Look at the pictures and read about the behaviors. Think about why the animal is behaving that way. These pictures show examples of animal behaviors. Why do the animals behave these ways? "
animal behaviors,T_2708,"Why do animals behave the way they do? The answer to this question depends on what the behavior is. A cat chases a mouse to catch it. A mother dog nurses her puppies to feed them. All of these behaviors have the same purpose: getting or providing food. All animals need food for energy. They need energy to move around. In fact, they need energy just to stay alive. Energy allows all the processes inside cells to occur. Baby animals also need energy to grow and develop. Birds and wasps build nests to have a safe place to store their eggs and raise their young. Many other animals build nests for the same reason. Animals protect their young in other ways, as well. For example, a mother dog not only nurses her puppies. She also washes them with her tongue and protects them from strange people or other animals. All of these behaviors help the young survive and grow up to be adults. Rabbits run away from foxes and other predators to stay alive. Their speed is their best defense. Lizards sun themselves on rocks to get warm because they cannot produce their own body heat. When they are warmer, they can move faster and be more alert. This helps them escape from predators and also find food. All of these animal behaviors are important. They help the animals get food for energy, make sure their young survive, or ensure that they, themselves, survive. Behaviors that help animals or their young survive, increase the animals fitness. Animals with higher fitness have a better chance of passing their genes on to the next generation. If genes control behaviors that increase fitness, the behaviors become more common in the species. This occurs through the process of evolution by natural selection. "
animal communication,T_2709,"What does the word ""communication"" make you think of? Talking on a cell phone? Texting? Writing? Those are just a few of the ways in which human beings communicate. Most other animals also communicate. Communication is any way in which animals share information, and they do this in many different ways. Do all animals talk to each other? Probably not, but many do communicate. Like human beings, many other animals live together in groups. Some insects, including ants and bees, are well known for living in groups. In order for animals to live together in groups, they must be able to communicate with each other. Animal communication, like most other animal behaviors, increases the ability to survive and have offspring. This is known as fitness. Communication increases fitness by helping animals find food, defend themselves from predators, mate, and care for offspring. "
animal communication,T_2710,"Some animals communicate with sound. Most birds communicate this way. Birds use different calls to warn other birds of danger, or to tell them to flock together. Many other animals also use sound to communicate. For example, monkeys use warning cries to tell other monkeys in their troop that a predator is near. Frogs croak to attract female frogs as mates. Gibbons use calls to tell other gibbons to stay away from their area. "
animal communication,T_2711,"Another way some animals communicate is with sight. By moving in certain ways or by making faces, they show other animals what they mean. Most primates communicate in this way. For example, a male chimpanzee may raise his arms and stare at another male chimpanzee. This warns the other chimpanzee to keep his distance. The chimpanzee pictured below may look like he is smiling, but he is really showing fear ( Figure 1.1). He is communicating to other chimpanzees that he will not challenge them. Look at the peacock pictured below ( Figure 1.2). Why is he raising his beautiful tail feathers? He is also communicating. He is showing females of his species that he would be a good mate. This peacock is using his tail feathers to communicate. What is he ""saying""? "
animal communication,T_2712,"Some animals communicate with scent. They release chemicals that other animals of their species can smell or detect in some other way. Ants release many different chemicals. Other ants detect the chemicals with their antennae. This explains how ants are able to work together. The different chemicals that ants produce have different meanings. Some of the chemicals signal to all of the ants in a group to come together. Other chemicals warn of danger. Still other chemicals mark trails to food sources. When an ant finds food, it marks the trail back to the nest by leaving behind a chemical on the ground. Other ants follow the chemical trail to the food. Many other animals also use chemicals to communicate. You have probably seen male dogs raise their leg to urinate on a fire hydrant or other object. Did you know that the dogs were communicating? They mark their area with a chemical in their urine. Other dogs can smell the chemical. The scent of the chemical tells other dogs to stay away. "
animal communication,T_2713,"Like other animals, humans communicate with one another. They mainly use sound and sight to share information. The most important way in which humans communicate is with language. Language is the use of symbols to communicate. In human languages, the symbols are words. They stand for many different things. Words stand for things, people, actions, feelings, or ideas. Think of several common words. What does each word stand for? Another important way in which humans communicate is with facial expressions. Look at the face of the young child pictured below ( Figure 1.3). Can you tell from her face how she is feeling? Humans also use gestures to communicate. What are people communicating when they shrug their shoulders? When they shake their head? These are just a few examples of the ways in which humans share information without using words. What does this girls face say about how she is feeling? "
animal like protists,T_2714,"Animal-like protists are called protozoa. Protozoa are single-celled eukaryotes that share some traits with animals. Like animals, they can move, and they are heterotrophs. That means they eat things outside of themselves instead of producing their own food. Animal-like protists are very small, measuring only about 0.010.5mm. Animal-like protists include the flagellates, ciliates, and the sporozoans. "
animal like protists,T_2715,"There are many different types of animal-like protists. They are different because they move in different ways. Flagellates have long flagella, or tails. Flagella rotate in a propeller-like fashion, pushing the protist through its environment ( Figure 1.1). An example of a flagellate is Trypanosoma, which causes African sleeping sickness. Other protists have what are called transient pseudopodia, which are like temporary feet. The cell surface extends out to form feet-like structures that propel the cell forward. An example of a protist with pseudopodia is the amoeba. The ciliates are protists that move by using cilia. Cilia are thin, very small tail-like projections that extend outward from the cell body. Cilia beat back and forth, moving the protist along. Paramecium has cilia that propel it. The sporozoans are protists that produce spores, such as the toxoplasma. These protists do not move at all. The spores develop into new protists. These flagellates all cause diseases in humans. A video of the animal-like amoeba can be viewed at: http://commons.wikimedia.org/wiki/File:Amoeba_engulfing_ "
arachnids,T_2721,"Arachnids are a class of joint-legged invertebrates in the subphylum Chelicerata. They live mainly on land but are also found in fresh water and in all marine environments, except for the open ocean. There are over 100,000 named species, including many species of spiders, scorpions, daddy-long-legs, ticks, and mites ( Figure 1.1). There may be up to 600,000 species in total, including unknown ones. "
arachnids,T_2722,"Arachnids have the following characteristics: 1. Four pairs of legs (eight total). You can tell the difference between an arachnid and an insect because insects have three pairs of legs (six total). 2. Arachnids also have two additional pairs of appendages. The first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps, help the organisms feed, move, and reproduce. (left) A daddy-long-legs spider. (right) Various diseases are caused by bacteria that are spread to humans by arachnids, like the tick shown here. 3. 4. 5. 6. 7. 8. 9. Arachnids do not have antennae or wings. The arachnid body is organized into the cephalothorax, a fusion of the head and thorax, and the abdomen. To adapt to living on land, arachnids have internal breathing systems, like a trachea or a book lung. Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Several groups are venomous. They release the venom from specialized glands to kill prey or enemies. Several mites are parasitic, and some of those are carriers of disease. Arachnids usually lay eggs, which hatch into immature arachnids that are similar to adults. Scorpions, however, give birth to live young. "
arachnids,T_2723,"The arachnids are divided into eleven subgroups. Below are the four most familiar subgroups, with a description of each ( Table 1.1). Subgroup Representative Organisms Approximate Number of Species Characteristics Subgroup Araneae Representative Organisms Spiders Approximate Number of Species 40,000 Characteristics Found all over the world, ranging from tropics to the Arctic, some in extreme environments. All produce silk, which is used for trapping insects in webs, aiding in climbing, producing egg sacs, and wrapping prey. Nearly all spiders inject venom to protect themselves or to kill prey. Only about 200 species have bites that can be harmful to humans. Subgroup Opiliones Representative Organisms Daddy-long-legs Approximate Number of Species 6,300 Characteristics Known for extremely long walking legs. No silk nor poison glands. Many are omnivores, eating small insects, plant material, and fungi. Some are scavengers, eating decaying animal and other matter. Mostly nocturnal (come out at night) and colored in hues of brown. A number of diurnal (come out during the day) species have vivid patterns of yellow, green, and black. Subgroup Scorpiones Representative Organisms Scorpions Approximate Number of Species 2,000 Characteristics Characterized by a tail with six segments, the last bearing a pair of venom glands and a venom-injecting barb. Predators of small arthropods and insects. They use pincers to catch prey. Then they either crush it or inject it with a fast-acting venom, which is used to kill or paralyze the prey. Only a few species are harmful to humans. Nocturnal; during the day, they find shelter in holes or under rocks. Unlike the majority of arachnids, scorpions produce live young. The young are carried about on the mothers back until they have molted at least once. They reach an age of between four to 25 years. Subgroup Acarina Representative Organisms Mites and ticks Approximate Number of Species 30,000 Characteristics Most are small (no more than 1.0 mm in length), but some ticks, and one species of mite, may grow to be 10-20 mm in length. Live in nearly ev- ery habitat, includ- ing aquatic and ter- restrial. Many are parasitic, affecting both in- vertebrates and ver- tebrates. They may transmit diseases to humans and other mammals. Those that feed on plants may damage crops. "
arthropods,T_2727,"How often do you think you see an arthropod? Well, have you ever looked up close at an ant? A spider? A fly? A moth? With over a million described species (and many more yet to be described) in the phylum containing arthro- pods, chances are, you encounter one of these organisms every day, without even leaving your house. Arthropods are a very diverse group of animals. In fact, they are the biggest group of animals on the planet, with upwards of 5 million distinct species. "
arthropods,T_2728,"Arthropods belong to the phylum Arthropoda, which means jointed feet, and includes four living subphyla. Chelicerata, which includes spiders ( Figure 1.1), mites, and scorpions. In these animals, the first pair of appendages are often modified as fangs or pincers, and are used to manipulate food. Spiders have eight legs. Myriapoda, which includes centipedes and millipedes. All of these animals live on land, and can have anywhere from ten to nearly 200 pairs of appendages. Hexapoda, which includes the insects. These animals dominate the land. All hexapods have three pairs (six appendages) of walking appendages. Crustacea, which includes lobsters, crabs, barnacles, crayfish, and shrimp. These animals dominate the ocean, and usually have a set of anterior appendages that are modified as mandibles, which function in grasping, biting, and chewing food. Spiders are one type of arthropod. "
arthropods,T_2729,"Characteristics of arthropods include: 1. A segmented body ( Figure 1.2) with a head, a thorax, and abdomen segments. 2. Appendages on at least one segment. They can be used for feeding, sensory reception, defense, and locomo- tion. In addition to legs, antennas and mouth parts are considered modified appendages. 3. A nervous system. 4. A hard exoskeleton made of chitin, which gives them physical protection and resistance to drying out. In order to grow, arthropods shed this outer covering in a process called molting. 5. An open circulatory system with hemolymph, a blood-like fluid. A series of hearts move the hemolymph into the body cavity where it comes in direct contact with the tissues. Hemolymph is involved with oxygen distribution. 6. A complete digestive system with a mouth and an anus. 7. Aquatic arthropods use gills to exchange gases. These gills have a large surface area in contact with the water, so they can absorb more oxygen. 8. Land-living arthropods have internal surfaces that help exchange gasses. Insects and most other terrestrial species have a tracheal system, where air sacs lead into the body from pores in the exoskeleton. These pores cover a large part of their external body surface. Others use book lungs, gills modified for breathing air, as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen absorbing skin. Land crabs sometimes have two different structures: one used for breathing underwater, and another used to absorb oxygen from the air. "
basic and applied science,T_2759,"Science can be ""basic"" or ""applied."" The goal of basic science is to understand how things workwhether it is a single cell, an organism made of trillions of cells, or a whole ecosystem. Scientists working on basic science questions are simply looking to increase human knowledge of nature and the world around us. The knowledge obtained through the study of the subspecialties of the life sciences is mostly basic science. Basic science is the source of most scientific theories. For example, a scientist that tries to figure out how the body makes cholesterol, or what causes a particular disease, is performing basic science. This is also known as basic research. Additional examples of basic research would be investigating how glucose is turned into cellular energy or determining how elevated blood glucose levels can be harmful. The study of the cell (cell biology), the study of inheritance (genetics), the study of molecules (molecular biology), the study of microorganisms and viruses (microbiology and virology), the study of tissues and organs (physiology) are all types of basic research, and have all generated lots of information that is applied to humans and human health. Applied science is using scientific discoveries, such as those from basic research, to solve practical problems. For example, medicine, and all that is known about how to treat patients, is applied science based on basic research ( Figure 1.1). A doctor administering a drug to lower a persons cholesterol is an example of applied science. Applied science also creates new technologies based on basic science. For example, designing windmills to capture wind energy is applied science ( Figure 1.2). This technology relies, however, on basic science. Studies of wind patterns and bird migration routes help determine the best placement for the windmills. Surgeons operating on a person, an example of applied science. Windmills capturing energy, an example of applied science. "
biotechnology in agriculture,T_2760,"Energy must constantly flow through an ecosystem for the system to remain stable. What exactly does this mean? Essentially, it means that organisms must eat other organisms. Food chains ( Figure 1.1) show the eating patterns in an ecosystem. Food energy flows from one organism to another. Arrows are used to show the feeding relationship between the animals. The arrow points from the organism being eaten to the organism that eats it. For example, an arrow from a plant to a grasshopper shows that the grasshopper eats the leaves. Energy and nutrients are moving from the plant to the grasshopper. Next, a bird might prey on the grasshopper, a snake may eat the bird, and then an owl might eat the snake. The food chain would be: plant  grasshopper  bird  snake  owl. A food chain cannot continue to go on and on. For example the food chain could not be: plant  grasshopper  spider  frog  lizard  fox  hawk. Food chains only have 4 or 5 total levels. Therefore, a chain has only 3 or 4 levels for energy transfer. This food chain includes producers and consumers. How could you add decom- posers to the food chain? In an ocean ecosystem, one possible food chain might look like this: phytoplankton  krill  fish  shark. The producers are always at the beginning of the food chain, bringing energy into the ecosystem. Through photosynthesis, the producers create their own food in the form of glucose, but also create the food for the other organisms in the ecosystem. The herbivores come next, then the carnivores. When these consumers eat other organisms, they use the glucose in those organisms for energy. In this example, phytoplankton are eaten by krill, which are tiny, shrimp-like animals. The krill are eaten by fish, which are then eaten by sharks. Could decomposers be added to a food chain? Each organism can eat and be eaten by many different types of organisms, so simple food chains are rare in nature. There are also many different species of fish and sharks. So a food chain cannot end with a shark; it must end with a distinct species of shark. A food chain does not contain the general category of ""fish,"" it will contain specific species of fish. In ecosystems, there are many food chains. Since feeding relationships are so complicated, we can combine food chains together to create a more accurate flow of energy within an ecosystem. A food web ( Figure 1.2) shows the feeding relationships between many organisms in an ecosystem. If you expand our original example of a food chain, you could add deer that eat clover and foxes that hunt chipmunks. A food web shows many more arrows, but still shows the flow of energy. A complete food web may show hundreds of different feeding relationships. "
bird reproduction,T_2761,"How do birds reproduce? We know that chickens lay eggs. But how do they do that? It all starts with behavior aimed at attracting a mate. In birds, this will involve a type of display, usually performed by the male. Some displays are very elaborate and may include dancing, aerial flights, or wing or tail drumming. Most male birds also sing a type of song to attract females. If they are successful at attracting a female, it will lead to breeding. Birds reproduce by internal fertilization, during which the egg is fertilized inside the female. Like reptiles, birds have cloaca, or a single exit and entrance for sperm, eggs, and waste. The male brings his sperm to the female cloaca. The sperm fertilizes the egg. Then the hard-shelled egg develops within the female. The hard-shelled eggs have a fluid-filled amnion, a thin membrane forming a closed sac around the embryo. Eggs are usually laid in a nest. "
bird reproduction,T_2762,"Why do you think eggs come in so many different colors? Birds that make nests in the open have camouflaged eggs ( Figure 1.1). This gives the eggs protection against predation. Some species, like ground-nesting nightjars, have pale eggs, but the birds camouflage the eggs with their feathers. To protect their young, different species of birds make different nests. Birds of all types, from hummingbirds to ostriches, make nests. Many can be elaborate, shaped like cups, domes, plates, mounds, or burrows. However, some birds, like the common guillemot, do not use nests. Instead, they lay their eggs on bare cliffs. Emperor penguins do not have a nest at all; they sit on eggs to keep them warm before they hatch, a process called incubation. How else might a bird help protect its young from predators? Most species locate their nests in areas that are hidden, in order to avoid predators. Large birds, or those that nest in groups, may build nests in the open, since they are more capable of defending their young. Nest and eggs of the common moorhen, showing camouflaged eggs. "
bird reproduction,T_2763,"In birds, 90% to 95% of species are monogamous, meaning the male and female remain together for breeding for a few years or until one mate dies. Birds of all types, from parrots to eagles and falcons, are monogamous. Usually, the parents take turns incubating the eggs. Birds usually incubate their eggs after the last one has been laid. In polygamous species, where there is more than one mate, one parent does all of the incubating. The wild turkey is an example of a polygamous bird. The length and type of parental care varies widely amongst different species of birds. At one extreme, in a group of birds called the magapodes (which are chicken-like birds), parental care ends at hatching. In this case, the newly- hatched chick digs itself out of the nest mound without parental help and can take care of itself right away. These birds are called precocial. Other precocial birds include the domestic chicken and many species of ducks and geese. At the other extreme, many seabirds care for their young for extended periods of time. For example, the chicks of the Great Frigatebird receive intensive parental care for six months, or until they are ready to fly, and then take an additional 14 months of being fed by the parents ( Figure 1.2). These birds are the opposite of precocial birds and are called altricial. In most animals, male parental care is rare. But it is very common in birds. Often both parents share tasks such as defense of territory and nest site, incubation, and the feeding of chicks. Since birds often take great care of their young, some birds have evolved a behavior called brood parasitism. This happens when a bird leaves her eggs in another birds nest. The host bird often accepts and raises the parasite birds eggs. Great Frigatebird adults are known to care for their young for up to 20 months after hatching, the longest in a bird species. Here, a young bird is begging for food. "
birds,T_2764,"How many different types of birds can you think of? Robins, ostriches, hummingbirds, chickens, and eagles. All of these are birds, but they are very different from one another. There is an amazingly wide variety of birds. Like amphibians, reptiles, mammals, and fish, birds are vertebrates. What does that mean? It means they have a backbone. Almost all birds have forelimbs modified as wings, but not all birds can fly. In some birds, the wings have evolved into other structures. Birds are in the class Aves. All birds have the following key features: they are endothermic (warm-blooded), have two legs, and lay eggs. Birds range in size from the tiny two-inch bee hummingbird to the nine-foot ostrich ( Figure 1.1). With approxi- mately 10,000 living species, birds are the most numerous vertebrates with four limbs. They live in diverse habitats around the globe, from the Arctic to the Antarctic. The ostrich can reach a height of nine feet! Pictured here is an ostrich with her young in the Negev Desert, southern Israel. "
birds,T_2765,"The digestive system of birds is unique, with a gizzard that contains swallowed stones for grinding food. Birds do not have teeth. What do you think the stones do? They help them digest their food. Defining characteristics of modern birds also include: Feathers. High metabolism. A four-chambered heart. A beak with no teeth. A lightweight but strong skeleton. Production of hard-shelled eggs. Which of the above traits do you think might be of importance to flight? "
birds,T_2766,"In comparing birds with other vertebrates, what do you think distinguishes them the most? In most birds, flight is the obvious difference. Birds have adapted their body plan for flight: Their skeleton is especially lightweight, with large, air-filled spaces connecting to their respiratory system. Their neck bones are flexible. Birds that fly have a bony ridge along the breastbone that the flight muscles attach to ( Figure 1.2). This allows them to remain stable in the air as they fly. Birds also have wings that function as an aerofoil. The surface of the aerofoil is curved to help the bird control and use the air currents to fly. Aerofoils are also found on the wings of airplanes. A bony ridge along the breastbone (green) allows birds to remain stable as they fly. What other traits do you think might be important for flight? Feathers help because theyre more lightweight than scales or fur. A birds wing shape and size will determine how a species flies. For example, many birds have powered flight at certain times, requiring the flapping of their wings, while at other times they soar, using up less energy ( Figure 1.3). One birds flight. A flightless cormorant can no longer fly, but it uses its wings for swimming. "
blood pressure,T_2772,"The health of your whole body depends on the good health of your cardiovascular system. One measure of the health of your cardiovascular system is blood pressure. Blood pressure occurs when circulating blood puts pressure on the walls of blood vessels. Since blood pressure is primarily caused by the beating of your heart, the walls of the arteries move in a rhythmic fashion. Blood in arteries is under the greatest amount of pressure. The pressure of the circulating blood slowly decreases as blood moves from the arteries and into the smaller blood vessels. Blood in veins is not under much pressure. "
blood pressure,T_2773,"Blood pressure is read as two numbers. The first number is the systolic pressure. The systolic pressure is the pressure on the blood vessels when the heart beats. This is the time when there is the highest pressure in the arteries. The diastolic pressure, which is the second number, is when your blood pressure is lowest, when the heart is resting between beats. Healthy ranges for blood pressure are: Systolic: less than 120 Diastolic: less than 80 Blood pressure is written as systolic/diastolic. For example, a reading of 120/80 is said as ""one twenty over eighty."" These measures of blood pressure can change with each heartbeat and over the course of the day. Pressure varies with exercise, emotions, sleep, stress, nutrition, drugs, or disease. Studies have shown that people whose systolic pressure is around 115, rather than 120, have fewer health problems. Clinical trials have shown that people who have blood pressures at the low end of these ranges have much better long term cardiovascular health. Blood pressure is measured with a sphygmomanometer ( Figure 1.1). A digital sphygmomanometer is made of an inflatable cuff and a pressure meter to measure blood pressure. This reading shows a blood pressure of 126/70. Hypertension, which is also called ""high blood pressure,"" occurs when a persons blood pressure is always high. Hypertension is said to be present when a persons systolic blood pressure is always 140 or higher, and/or if the persons diastolic blood pressure is always 90 or higher. Having hypertension increases a persons chance for developing heart disease, having a stroke, or suffering from other serious cardiovascular diseases. Hypertension often does not have any symptoms, so a person may not know that he or she has high blood pressure. For this reason, hypertension is often called the ""silent killer."" Treatments for hypertension include diet changes, exercise, and medication. Foods thought to lower blood pressure include skim milk, spinach, beans, bananas and dark chocolate. Some health conditions, as well as a persons lifestyle and genetic background, can put someone at a higher risk for developing high blood pressure. As a person cannot alter their genetic background, lifestyle changes may be necessary to reduce the chance of developing high blood pressure. These changes include getting enough exercise, limiting the amount of sodium (salt) in the diet, not being overweight, not drinking alcohol to excess, and not smoking. Low blood pressure is not usually a concern, as long as there are no problems associated with the low pressure. Symptoms associated with low blood pressure include dizziness or lightheadedness, fainting, dehydration and unusual thirst, lack of concentration, blurred vision, nausea, and fatigue. "
centipedes and millipedes,T_2819,"Centipedes and millipedes belong to the subphylum Myriapoda, which contains 13,000 species. They all live on land, which makes sense as all those legs are more adapted to a terrestrial lifestyle, as opposed to an aquatic lifestyle. The Myriapoda are divided among four classes: (1) Chilopoda (centipedes), (2) Diplopoda (millipedes), (3) Sym- phyla (symphylans), and (4) Pauropoda (pauropods). They range from having over 750 legs to having fewer than ten legs. They have a single pair of antennae and simple eyes. "
centipedes and millipedes,T_2820,"Myriapoda are mostly found in moist forests, where they help to break down decaying plant material. A few live in grasslands, semi-arid habitats, or even deserts. Most myriapods are decomposers, with the majority herbivores breaking down decaying plant material, but centipedes are nighttime predators. Centipedes roam around looking for small animals to bite and eat; their prey includes insects, spiders, and other small invertebrates. If the centipede is large enough, it will even attack small vertebrates, like lizards. Although not generally considered dangerous to humans, many from this group can cause temporary blistering and discoloration of the skin. "
centipedes and millipedes,T_2821,"Centipedes (""hundred feet"") ( Figure 1.1) are fast, predatory carnivores, and venomous. There are around 3,300 described species, ranging from one tiny species (less than half an inch in length) to one giant species (the Peruvian giant yellow-leg centipede or Amazonian giant centipede), which may grow larger than 12 inches. This giant centipede has been known to attack, kill and eat much larger animals, including tarantulas. Centipedes have one pair of legs per body segment, with the first pair of legs behind the head modified into a pair of fangs containing a poison gland. Many centipedes also guard their eggs and young by curling around them. Centipede. "
centipedes and millipedes,T_2822,"Most millipedes are slower than centipedes and feed on leaf litter and loose organic material. They can be distin- guished from centipedes by looking at the number of legs per body segment. Millipedes have two pairs of legs per body segment, while centipedes have a single pair of legs per body segment. Millipedes protect their eggs from predators by using a nest of hard soil. Millipedes are not poisonous. They lack the pair of fangs containing a poison gland that centipedes have. "
centipedes and millipedes,T_2823,"The third class, Symphyla, contains 200 species. Symphylans resemble centipedes but are smaller and translucent. These arthropods have an elongated body, with three pairs of thoracic legs and about nine pairs of abdominal legs, giving this class 12 pairs total. Many spend their lives in soil feeding on plant roots, but some do live in trees. "
centipedes and millipedes,T_2824,"The pauropods are typically 0.5-2.0 mm long and live on all continents except Antarctica. They are usually found in soil, leaf litter, or other moist places. They feed on fungi and decaying organic matter, and are essentially harmless. Adult pauropods have 11 or 12 body segments and 9-11 pairs of legs. They also possess unique forked antennae and a distinctive pattern of movement characterized by rapid burst of movement and frequent abrupt changes in direction. Over 700 species have been described, and they are believed to be closely related to millipedes. "
choosing healthy foods,T_2838,"Foods such as whole grain breads, fresh fruits, and fish provide nutrients you need for good health. But different foods give you different types of nutrients. You also need different amounts of each nutrient. How can you choose the right mix of foods to get the proper balance of nutrients? Three tools can help you choose foods wisely: MyPyramid, MyPlate, and food labels. "
choosing healthy foods,T_2839,"MyPyramid ( Figure 1.1) is a diagram that shows how much you should eat each day of foods from six different food groups. It recommends the amount of nutrients you need based on your age, your gender, and your level of activity. The six food groups in MyPyramid are: Grains, such as bread, rice, pasta, and cereal. Vegetables, such as spinach, broccoli, carrots, and sweet potatoes. Fruits, such as oranges, apples, bananas, and strawberries. Oils, such as vegetable oil, canola oil, olive oil, and peanut oil. Dairy, such as milk, yogurt, cottage cheese, and other cheeses. Meat and beans, such as chicken, fish, soybeans, and kidney beans. MyPyramid can help you choose foods wisely for good health. Each colored band represents a different food group. The key shows which food group each color represents. Which colored band of MyPyramid is widest? Which food group does it represent? In MyPyramid, each food group is represented by a band of a different color. For example, grains are represented by an orange band, and vegetables are represented by a green band. The wider the band, the more foods you should choose from that food group each day. The orange band in MyPyramid is the widest band. This means that you should choose more foods from the grain group than from any other single food group. The green, blue, and red bands are also relatively wide. Therefore, you should choose plenty of foods from the vegetable, dairy, and fruit groups as well. You should choose the fewest foods from the food group with the narrowest band. Which band is narrowest? Which food group does it represent? Are you wondering where foods like ice cream, cookies, and potato chips fit into MyPyramid? The white tip of MyPyramid represents foods such as these. These are foods that should be eaten only in very small amounts and not very often. Such foods contain very few nutrients and are called nutrient-poor. Instead, they are high in fats, sugars, and sodium, which are nutrients that you should limit in a healthy eating plan. Ice cream, cookies, and potato chips are also high in calories. Eating too much of them may lead to unhealthy weight gain. "
choosing healthy foods,T_2840,"Make at least half your daily grain choices whole grains. Examples of whole grains are whole wheat bread, whole wheat pasta, and brown rice. Choose a variety of different vegetables each day. Be sure to include both dark green vegetables, such as spinach and broccoli, and orange vegetables, such as carrots and sweet potatoes. Choose a variety of different fruits each day. Select mainly fresh fruits rather than canned fruits, and whole fruits instead of fruit juices. When choosing oils, choose unsaturated oils, such as olive oil, canola oil, or vegetable oil. Choose low-fat or fat-free milk and other dairy products. For example, select fat-free yogurt and low-fat cheese. For meats, choose fish, chicken, and lean cuts of beef. Also, be sure to include beans, nuts, and seeds. "
choosing healthy foods,T_2841,"In June 2011, the United States Department of Agriculture replaced My Pyramid with MyPlate. MyPlate depicts the relative daily portions of various food groups ( Figure 1.2). See  for more information. MyPlate is a visual guideline for balanced eating, replacing MyPyramid in 2011. The following guidelines accompany MyPlate: 1. Balancing Calories Enjoy your food, but eat less. Avoid oversized portions. 2. Foods to Increase Make half your plate fruits and vegetables. Make at least half your grains whole grains. Switch to fat-free or low-fat (1%) milk. 3. Foods to Reduce "
choosing healthy foods,T_2842,"In the United States and other nations, packaged foods are required by law to have nutrition facts labels. A nutrition facts label ( Figure 1.3) shows the nutrients in a food. Packaged foods are also required to list their ingredients. The information listed at the right of the label tells you what to look for. At the top of the label, look for the serving size. The serving size tells you how much of the food you should eat to get the nutrients listed on the label. A cup of food from the label pictured below is a serving. The calories in one serving are listed next. In this food, there are 250 calories per serving. Reading nutrition facts labels can help you choose healthy foods. Look at the nutrition facts label shown here. Do you think this food is a good choice for a healthy eating plan? Why or why not? Next on the nutrition facts label, look for the percent daily values (% DV) of nutrients. Remember the following tips when reading a food label: A food is low in a nutrient if the percent daily value of the nutrient is 5% or less. The healthiest foods are low in nutrients such as fats and sodium. A food is high in a nutrient if the percent daily value of the nutrient is 20% or more. The healthiest foods are high in nutrients such as fiber and proteins. "
choosing healthy foods,T_2843,"Look at MyPyramid ( Figure 1.1). Note the person walking up the side of the pyramid. This shows that exercise is important for balanced eating. Exercise helps you use any extra energy in the foods you eat. The more active you are, the more energy you use. You should try to get at least an hour of physical activity just about every day. Pictured below are some activities that can help you use extra energy ( Figure 1.4). "
choosing healthy foods,T_2844,"Any unused energy in food is stored in the body as fat. This is true whether the extra energy comes from carbohy- drates, proteins, or lipids. What happens if you take in more energy than you use, day after day? You will store more and more fat and become overweight. Eventually, you may become obese. Obesity is having a very high percentage of body fat. Obese people are at least 20 percent heavier than their healthy weight range. The excess body fat of obesity is linked to many diseases. Obese people often have serious health problems, such as diabetes, high blood pressure, and high cholesterol. They are also more likely to develop arthritis and some types of cancer. People who remain obese during their entire adulthood usually do not live as long as people who stay within a healthy weight range. The current generation of children and teens is the first generation in our history that may have a shorter life than their parents. The reason is their high rate of obesity and the health problems associated with obesity. You can avoid gaining weight and becoming obese. The choice is yours. Choose healthy foods by using MyPyramid and reading food labels. Then get plenty of exercise to balance the energy in the foods you eat. "
chordates,T_2845,"Did you know that fish, amphibians, reptiles, birds, and mammals are all related? They are all chordates. Chordates are a group of animals that includes vertebrates, as well as several closely related invertebrates. Chordates (phylum Chordata) are named after a feature they all share, a notochord. A notochord is a hollow nerve cord along the back. "
chordates,T_2846,"Chordates are defined by a set of four characteristics that are shared by these animals at some point during their development. In some chordates, all four traits are present in the adult animal and serve important functions. However, in many chordates, including humans, some traits are present only during the embryonic stage. After that, these traits may disappear. All chordates have four main traits ( Figure 1.1): 1. Post-anal tail: The tail is opposite the head and extends past the anus. 2. Dorsal hollow nerve cord: ""Dorsal"" means that the nerve cord runs along the top of the animal. In some animals, the nerve cord develops into the brain and spinal cord. 3. Notochord: The notochord lies below the nerve cord. It is a rigid structure where muscles attach. 4. Pharyngeal slits: Pharyngeal slits are used to filter out food from water by some simple chordates. In most chordates, however, they are only present during the embryonic stages and serve no apparent purpose. Body Plan of a Typical Chordate. The body plan of a chordate includes a post- anal tail, notochord, dorsal hollow nerve cord, and pharyngeal slits. "
chordates,T_2847,"The chordates are divided into nine classes. Five of the classes are the fish, amphibians, reptiles, birds, and mammals. There are actually five classes of marine chordates (for example, sharks are cartilaginous fish which are distinct from bony fish), and these will be discussed in additional concepts. The chordate phylum is broken down into three subphyla: 1. Urochordata: The tunicates, pictured in the introduction, make up this group. The urochordates are sessile (non-moving) marine animals with sack-like bodies and tubes for water movement. Urochordates have a notochord and nerve cord only during the larval stage. 2. Cephalochordata: Cephalochordates include the lancelets ( Figure 1.2), fish-like marine animals often found half-buried in the sand. Cephalochordates have a notochord and nerve cord but no backbone. 3. Vertebrata: Humans and other mammals, along with fish, amphibians, reptiles, and birds, fall in this category. In vertebrates, the notochord is typically smaller and surrounded by a backbone. The lancelet, an example of a chordate, is found in shallow ocean waters. "
cnidarians,T_2856,"Cnidarians, in the phylum Cnidaria, include organisms such as the jellyfish, corals, and sea anemones. These animals are found in shallow ocean water. You might know that these animals can give you a painful sting if you step on them. Thats because cnidarians have stinging cells known as nematocysts. Cnidarians use nematocysts to catch their food. When touched, the nematocysts release a thread of poison that can be used to paralyze prey. Cnidarians are among the simplest of the so-called ""higher"" organisms, but are also among the most beautiful. "
cnidarians,T_2857,"The body plan of cnidarians is unique because these organisms show radial symmetry, making these animals very different from those that evolved before them. Radial symmetry means that they have a circular body plan, and any cut through the center of the animal leaves two equal halves. The cnidarians have two basic body forms: 1. Polyp: The polyp is a cup-shaped body with the mouth facing upward, such as a sea anemone and coral. 2. Medusa: The medusa is a bell-shaped body with the mouth and tentacles facing downward, such as a jellyfish. Unlike the sponges which evolved prior to cnidarians, the cnidarians are made up of true tissues. The inside of a cnidarian is called the gastrovascular cavity, a large space that helps the organism digest and move nutrients around the body. The cnidarians also have nerve tissue organized into a net-like structure, known as a nerve-net, with connected nerve cells dispersed throughout the body. Cnidarians do not have true organs, however. Reproduction is by asexual budding (polyps) or sexual formation of gametes (medusae, some polyps). The result of sexual reproduction is a larva, which can swim on its own. "
cnidarians,T_2858,"Some types of cnidarians are also known to form colonies. Two examples are described below. 1. The Portuguese Man o War ( Figure 1.1) looks like a single organism but is actually a colony of polyps. One polyp is filled with air to help the colony float, while several feeding polyps hang below with tentacles. The tentacles are full of nematocysts. The Portuguese Man o War is known to cause extremely painful stings to swimmers and surfers who accidentally brush up against it in the water. 2. Coral reefs ( Figure below) look like big rocks, but they are actually alive. They are built from cnidarians called corals. The corals are sessile (non-moving) polyps that can use their tentacles to feed on ocean creatures that pass by. Their skeletons are made up of calcium carbonate, which is also known as limestone. Over long periods of time, their skeletons build on each other to produce large structures known as coral reefs. Coral reefs are important habitats for many different types of ocean life. Corals are colonial cnidarians. "
competition,T_2859,"Recall that ecology is the study of how living organisms interact with each other and with their environment. But how do organisms interact with each other? Organisms interact with each other through various mechanisms, one of which is competition. Competition occurs when organisms strive for limited resources. Competition can be for food, water, light, or space. This interaction can be between organisms of the same species (intraspecific) or between organisms of different species (interspecific). Intraspecific competition happens when members of the same species compete for the same resources. For example, two trees may grow close together and compete for light. One may out-compete the other by growing taller to get more available light. As members of the same species are usually genetically different, they have different characteristics, and in this example, one tree grows taller than the other. The organism that is better adapted to that environment is better able to survive. The other organism may not survive. In this example, it is the taller tree that is better adapted to the environment. Interspecific competition happens when individuals of different species strive for a limited resource in the same area. Since any two species have different traits, one species will be able to out-compete the other. One species will be better adapted to its environment, and essentially ""win"" the competition. The other species will have lower reproductive success and lower population growth, resulting in a lower survival rate. For example, cheetahs and lions feed on similar prey. If prey is limited, then lions may catch more prey than cheetahs. This will force the cheetahs to either leave the area or suffer a decrease in population. Looking at different types of competition, ecologists developed the competitive exclusion principle. The principle states that species less suited to compete for resources will either adapt, move from the area, or die out. In order for two species within the same area to coexist, they may adapt by developing different specializations. This is known as character displacement. An example of character displacement is when different birds adapt to eating different types of food. They can develop different types of bills, like Darwins Finches ( Figure 1.1). Therefore, competition for resources within and between species plays an important role in evolution through natural selection. An example of character displacement, showing different types of bill for eating different types of foods, in Darwins or Galapagos Finches. "
consumers and decomposers,T_2866,"Recall that producers make their own food through photosynthesis. But many organisms are not producers and cannot make their own food. So how do these organisms obtain their energy? They must get their energy from other organisms. They must eat other organisms, or obtain their energy from these organisms some other way. The organisms that obtain their energy from other organisms are called consumers. All animals are consumers, and they eat other organisms. Fungi and many protists and bacteria are also consumers. But, whereas animals eat other organisms, fungi, protists, and bacteria ""consume"" organisms through different methods. The consumers can be placed into different groups, depending on what they consume. Herbivores are animals that eat producers to get energy. For example, rabbits and deer are herbivores that eat plants. The caterpillar pictured below ( Figure 1.1) is a herbivore. Animals that eat phytoplankton in aquatic environments are also herbivores. Carnivores feed on animals, either herbivores or other carnivores. Snakes that eat mice are carnivores. Hawks that eat snakes are also carnivores ( Figure 1.1). Omnivores eat both producers and consumers. Most people are omnivores, since they eat fruits, vegetables, and grains from plants, and also meat and dairy products from animals. Dogs, bears, and raccoons are also omnivores. Examples of consumers are caterpillars (herbivores) and hawks (carnivore). "
consumers and decomposers,T_2867,"Decomposers ( Figure 1.2) get nutrients and energy by breaking down dead organisms and animal wastes. Through this process, decomposers release nutrients, such as carbon and nitrogen, back into the environment. These nutrients are recycled back into the ecosystem so that the producers can use them. They are passed to other organisms when they are eaten or consumed. Many of these nutrients are recycled back into the soil, so they can be taken up by the roots of plants. The stability of an ecosystem depends on the actions of the decomposers. Examples of decomposers include mushrooms on a decaying log. Bacteria in the soil are also decomposers. Imagine what would happen if there were no decomposers. Wastes and the remains of dead organisms would pile up and the nutrients within the waste and dead organisms would not be released back into the ecosystem. Producers would not have enough nutrients. The carbon and nitrogen necessary to build organic compounds, and then cells, allowing an organism to grow, would be insufficient. Other nutrients necessary for an organism to function properly would also not be sufficient. Essentially, many organisms could not exist. Examples of decomposers are (a) bacte- ria and (b) fungi. "
control of insects,T_2868,"Though insects can be very important, some are also considered pests. Common insect pests include: 1. 2. 3. 4. Parasitic insects, such as mosquitoes, lice, and bed bugs. Insects that transmit diseases, including mosquitoes and flies. Insects that damage structures, such as termites ( Figure 1.1). Insects that destroy crops, including locusts and weevils. Many scientists who study insects are involved in various forms of pest control. Most utilize insect-killing chemicals, but more and more rely on other methods. Ways to control insect pests are described below. "
control of insects,T_2869,"Biological control of pests in farming is a method of controlling pests by using other insects (or other natural predators of the pests). Biological control of insects relies on predation and parasitism. Insect predators, such as ladybugs and lacewings, consume a large number of other insects during their lifetime. If you add ladybugs to your farm or garden, they will help keep insect pests, such as aphids, under control. Aphids are among the most destructive insect pests on cultivated plants in temperate regions, so any control of these pests is beneficial. Ladybugs also consume mites, scale insects and small caterpillars. The larvae of many hoverfly species also feed upon aphids, with one larva consuming up to fifty aphids a day, which is about 1,000 in its lifetime. They also eat fruit tree spider mites and small caterpillars. Dragonflies are important predators of mosquitoes, and can be used to control this pest. Parasitic insects include insects such as wasps and flies that lay their eggs on or in the body of an insect host, which is then used as a food for developing larvae. The host is ultimately killed. Caterpillars also tend to be one likely target of parasitic wasps. "
control of insects,T_2870,"Chemical control of pests involves the use of insecticides. Insecticides, which are also known as pesticides, are most often used to kill insects. Insecticides are chemicals that kill insects. The U.S. spends $9 billion each year on pesticides. Disadvantages to using pesticides include human, fish, and honeybee poisonings, and the contamination of meat and dairy products. When choosing to use an insecticide, there are numerous points to consider. Negative effects of the pesticide should try to be minimized. Important questions to consider include the following. What is the chemicals success against the target pest? Will the insecticide provide the desired level of control of the pest? If the answer is no, other methods of control should be considered. Does the chemical have an impact on natural enemies of the pest? In large scale efforts to rid areas of mosquitoes, the insecticide used also killed the dragonfly. This effort removed a natural predator of the mosquito. This may be an unacceptable negative effect of using the insecticide. How susceptible is the crop to insect damage? If the crop is not heavily damaged, only minor pest control may be needed. This may affect the amount or type of insecticide used. How toxic is the chemical to the environment and humans? Some older insecticides are extremely poisonous. Keep in mind that users of these poisons have a community responsibility to minimize the contamination of the surrounding environment, as well as keeping animals, surrounding crops and humans safe. Does using the pesticide result in the development of resistance? If so, this can make additional use of the pesticide less effective. As the resistance will be passed to future generations of the insect (which is natural selection in action), this could be considered a negative side-effect of pesticide use. "
crustaceans,T_2871,"Crustaceans are a large group of arthropods, consisting of almost 52,000 species. The majority of crustaceans are aquatic. Some live in the ocean, while others live in fresh water. A few groups have adapted to living on land, such as land crabs, hermit crabs, and woodlice ( Figure 1.1). Crustaceans are among the most successful animals, and can be considered the dominant aquatic animals. Though small, crustaceans are numerous enough to be the main source of energy for large ocean mammals. They are found as much in the oceans as insects are on land. "
crustaceans,T_2872,"Six classes of crustaceans are generally recognized ( Table 1.1). Class Branchiopoda Characteristics Mostly small, freshwater animals that feed on plankton and detritus. Examples Brine shrimp Class Remipedia Cephalocarida Maxillopoda Ostracoda Malacostraca Characteristics A small class of blind organisms found in deep caves connected to salt water. Small crustaceans, with an eye- less head covered by a horseshoe- shaped shield; has two pairs of an- tennae and two pairs of jaws. Mostly small, with a small abdomen, and generally no appendages. Small animals with bivalve shells. The largest class, with the largest and most familiar animals. This class has the greatest diversity of body forms. Examples Nectiopoda Horseshoe shrimp Barnacles, copepods Seed shrimp Crabs, lobsters, woodlice shrimp, krill, A terrestrial arthropod, a species of woodlice. "
crustaceans,T_2873,"Remember that crustaceans are an arthropod subphylum, and that arthropod means ""jointed feet."" As expected, the majority of crustaceans can move. A few groups are parasitic and live attached to their hosts. Adult barnacles ( Figure 1.2) cannot move, so they attach themselves headfirst to a rock or log. "
crustaceans,T_2874,"Characteristics of crustaceans include: 1. An exoskeleton that may be bound together, such as in the carapace, the thick back shield seen in many crustaceans that often forms a protective space for the gills. 2. A main body cavity with an expanded circulatory system. Blood is pumped by a heart located near the back. 3. A digestive system consisting of a straight tube that has a gastric mill for grinding food and a pair of digestive glands that absorb food. 4. Structures that function like kidneys to remove wastes. These are located near the antennae. 5. A brain that exists in the form of ganglia, or connections between nerve cells. 6. Crustaceans periodically shed the outer skeleton, grow rapidly for a short time, and then form another hard skeleton. They cannot grow underneath their outer exoskeleton. They are very vulnerable during this time, as they lack their hard shell. "
crustaceans,T_2875,"Most crustaceans have separate sexes, so they reproduce sexually using eggs and sperm. Many land crustaceans, such as the Christmas Island red crab, mate every season and return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land when the environment is damp. In some crustaceans, the females keep the eggs until they hatch into free-swimming larvae. "
cyclic behavior of animals,T_2876,Many animal behaviors change in a regular way. They go through cycles. Some cycles of behavior repeat each year. Other cycles of behavior repeat every day. 
cyclic behavior of animals,T_2877,"An example of a behavior with a yearly cycle is hibernation. Hibernation is a state in which an animals body processes are slower than usual, and its body temperature falls. An animal uses less energy than usual during hibernation. This helps the animal survive during a time of year when food is scarce. Hibernation may last for weeks or months. Animals that hibernate include species of bats, squirrels, and snakes. Most people think that bears hibernate. In fact, bears do not go into true hibernation. In the winter, they go into a deep sleep. However, their body processes do not slow down very much. Their body temperature also remains about the same as usual. Bears can be awakened easily from their winter sleep. Another example of a behavior with a yearly cycle is migration. Migration is the movement of animals from one place to another. Migration is an innate behavior that is triggered by changes in the environment. For example, animals may migrate when the days get shorter in the fall. Migration is most common in birds, fish, and insects. In the Northern Hemisphere, many species of birds, including robins and geese, travel south for the winter. They migrate to areas where it is warmer and where there is more food. They return north in the spring. A flock of migrating geese is pictured below ( Figure 1.1). These geese are flying south for the win- ter. Flocks of geese migrate in V-shaped formations. Some animals migrate very long distances. The map shown below shows the migration route of a species of hawk called Swainsons hawk ( Figure 1.2). About how many miles do the hawks travel from start to finish? Are you surprised that birds migrate that far? Some species of birds migrate even farther. Whales also are known to migrate thousands of miles each year to take advantage of warmer waters in the winter months. The great migration of millions of zebra, wildebeest and other antelope in East Africa also occurs yearly. Each year around 1.5 million wildebeest and 300,000 zebra (along with other antelope) go in search of food and water, traveling a distance of around 1800 miles. Birds and other migrating animals follow the same routes each year. How do they know where to go? It depends on the species. Some animals follow landmarks, such as rivers or coastlines. Other animals are guided by the position of the sun, the usual direction of the wind, or other clues in the environment. "
cyclic behavior of animals,T_2878,"Many animal behaviors change at certain times of day, day after day. For example, most animals go to sleep when the sun sets and wake up when the sun rises. Animals that are active during the daytime are called diurnal. Some animals do the opposite. They sleep all day and are active during the night. These animals are called nocturnal. Examples of nocturnal animals include bats, foxes, possums, skunks and coyotes. Many mammals (including humans), insects, reptiles and birds are diurnal. Animals may eat and drink at certain times of day as well. Humans have daily cycles of behavior, too. Most people start to get sleepy after dark and have a hard time sleeping when it is light outside. Daily cycles of behavior are called circadian rhythms. In many species, including humans, circadian rhythms are controlled by a tiny structure called the biological clock. This structure is located in a gland at the base of the brain. The biological clock sends signals to the body. The signals cause regular changes in behavior and body processes. The amount of light entering the eyes helps control the biological clock. The clock causes changes that repeat every 24 hours. The migration route of Swainsons hawk starts in North America and ends in South America. Scientists learned their mi- gration route by attaching tiny tracking devices to the birds. The birds were then tracked by satellite. On the migra- tion south, the hawks travel almost 5,000 miles from start to finish. "
diversity of birds,T_2897,"Turkey, hummingbird, penguin, parrot, owl and eagle. These are just some of the many different types of birds. If you just think about the birds in this list, the differences are striking. About 10,000 bird species belong to 29 different orders within the class Aves. They live and breed on all seven continents. The tropics are home to the greatest biodiversity of birds. The diversity among birds is striking. Birds can vary greatly in size and color. Some fly, some swim, some just walk or run. Some are savage carnivores, others are gentle herbivores. Some are low on the food chain, others are at the top. Birds live in a variety of different habitats. Birds that live in different habitats will encounter different foods and different predators. Birds can be carnivores (feeding on other animals), herbivores (feeding on plants), or generalists (feeding on a variety of foods). The lifestyle of the bird can affect what it looks like. For example, can you think of some examples of beaks that are adapted to the type of food a bird eats? Carnivorous birds include hawks, falcons, eagles, osprey, vultures and owls. Herbivorous birds include the goose, cockatoo and parrot. The American Crow is an example of a generalist. In addition, a specialist is a bird (or other animal) that is specially adapted to eat a certain food. An example of a specialist is a hummingbird, whose long, thin beak is excellent for reaching into flowers for nectar, but not very good for eating other foods. Waterfowl are birds that live on the water. These include ducks, geese, swans, and pelicans, to name a few. Landfowl are ground-feeding birds such as chickens and turkeys. Penguins are a group of flightless birds adapted for life in the water with flippers. Diurnal raptors are birds of prey that hunt during the day. These include falcons, eagles and hawks. Nocturnal raptors hunt during the night. These include various types of owls. Parrots are brightly colored and very intelligent. They are found in the tropics and include cockatoos, parrots, and parakeets. "
diversity of birds,T_2898,"The size and shape of the beak is related to the food the bird eats and can vary greatly among different birds. Parrots have down-curved, hooked bills, which are well-adapted for cracking seeds and nuts ( Figure 1.1). Hummingbirds, on the other hand, have long, thin, pointed bills, which are adapted for getting the nectar out of flowers ( Figure 1.1). Hawks, eagles, falcons and owls have a sharp, hooked beak. (left) The down-curved, hooked bill of a scarlet macaw, a large colorful parrot. (right) A long, thin and pointed bill of the hummingbird. "
diversity of birds,T_2899,"Bird feet can also vary greatly among different birds. Some birds, such as gulls and terns and other waterfowl, have webbed feet used for swimming or floating ( Figure 1.2). Other birds, such as herons, gallinules, and rails, have four long spreading toes, which are adapted for walking delicately in the wetlands ( Figure 1.2). You can predict how the beaks and feet of birds will look depending on where they live and what type of food they eat. Flightless birds also have long legs that are adapted for running. Flightless birds include the ostrich and kiwi. Raptors have clawed feet. They also have strong legs. Hawks, eagles and falcons also have excellent vision and they hunt by sight. Owls, with excellent hearing, can hunt by that sense alone. See Wild African Vulture Birds Scavage Bones of Dead Animals at  (left) The webbed feet of a great black- backed gull. (right) The long spreading toes of an American purple gallinule. Click image to the left or use the URL below. URL: "
ecosystems,T_2912,"Ecology is the study of ecosystems. That is, ecology is the study of how living organisms interact with each other and with the nonliving part of their environment. An ecosystem consists of all the nonliving factors and living organisms interacting in the same habitat. Recall that living organisms are biotic factors. The biotic factors of an ecosystem include all the populations in a habitat, such as all the species of plants, animals, and fungi, as well as all the micro-organisms. Also recall that the nonliving factors are called abiotic factors. Abiotic factors include temperature, water, soil, and air. You can find an ecosystem in a large body of fresh water or in a small aquarium. You can find an ecosystem in a large thriving forest or in a small piece of dead wood. Examples of ecosystems are as diverse as the rainforest, the savanna, the tundra, or the desert ( Figure 1.1). The differences in the abiotic factors, such as differences in temperature, rainfall, and soil quality, found in these areas greatly contribute to the differences seen in these ecosystems. Ecosystems can include well known sites, such as the Great Barrier Reef off the coast of Australia and the Greater Yellowstone Ecosystem of Yellowstone National Park, which actually includes a few different ecosystems, some with geothermal features, such as Old Faithful geyser. Desert Botanical Gardens in Phoenix, Ari- zona. Ecosystems need energy. Many ecosystems get their energy in the form of sunlight, which enters the ecosystem through photosynthesis. This energy then flows through the ecosystem, passed from producers to consumers. Plants are producers in many ecosystems. Energy flows from plants to the herbivores that eat the plants, and then to carnivores that eat the herbivores. The flow of energy depicts interactions of organisms within an ecosystem. Matter is also recycled in ecosystems. Biogeochemical cycles recycle nutrients, like carbon and nitrogen, so they are always available. These nutrients are used over and over again by organisms. Water is also continuously recycled. The flow of energy and the recycling of nutrients and water are examples of the interactions between organisms and the interactions between the biotic and abiotic factors of an ecosystem. "
fields in the life sciences,T_2934,"The life sciences are the study of living organisms. They deal with every aspect of living organisms, from the biology of cells, to the biology of individual organisms, to how these organisms interact with other organisms and their environment. The life sciences are so complex that most scientists focus on just one or two subspecialties. If you want to study insects, what would you be called? An entomologist. If you want to study the tiny things that give us the flu, then you need to enter the field of virology, the study of viruses. If you want to study the nervous system, which life science field is right for you ( Table 1.1, Table 1.2, and Table 1.3)? Field Botany Zoology Marine biology Focus Plants Animals Organisms living in oceans Field Freshwater biology Microbiology Bacteriology Virology Entomology Taxonomy Focus Organisms living in and around freshwater lakes, streams, rivers, ponds, etc. Microorganisms Bacteria Viruses Insects The classification of organisms "
fields in the life sciences,T_2935,"Field Cell biology Anatomy Morphology Physiology Immunology Neuroscience Developmental biology and embryology Genetics Biochemistry Molecular biology Epidemiology Evolution Focus Cells and their structures/functions Structures of animals Form and structure of living organisms Physical and chemical functions of tissues and organs Mechanisms inside organisms that protect them from disease and infection The nervous system Growth and development of plants and animals Genetic makeup of living organisms and heredity Chemistry of living organisms Nucleic acids and proteins How diseases arise and spread The changing of species over time Field Ecology Biogeography Population biology Focus How various organisms interact with their environ- ments Distribution of living organisms The biodiversity, evolution, and environmental biology of populations of organisms During the study of the life sciences, you will study cell biology, genetics, molecular biology, botany, microbi- ology, zoology, evolution, ecology, and physiology. Cell biology is the study of cellular structure and function ( Figure 1.1). Genetics is the study of heredity, which is the passing of traits (and genes) from one generation to the next. Molecular biology is the study of molecules, such as DNA and proteins. Ecologists study ecosystems, which are made of both living and nonliving parts of the environment. A botanist may work in a botanical garden, where plant life can be studied. What will you study with the other subspecialties? This illustration shows a virus among red blood cells. Which fields study red blood cells and viruses? (Keep in mind that viruses are actually much smaller than cells.) Other life science subspecialties include biogeography, which is the study of where organisms live and at what abundance. "
food webs,T_2946,"Energy must constantly flow through an ecosystem for the system to remain stable. What exactly does this mean? Essentially, it means that organisms must eat other organisms. Food chains ( Figure 1.1) show the eating patterns in an ecosystem. Food energy flows from one organism to another. Arrows are used to show the feeding relationship between the animals. The arrow points from the organism being eaten to the organism that eats it. For example, an arrow from a plant to a grasshopper shows that the grasshopper eats the leaves. Energy and nutrients are moving from the plant to the grasshopper. Next, a bird might prey on the grasshopper, a snake may eat the bird, and then an owl might eat the snake. The food chain would be: plant  grasshopper  bird  snake  owl. A food chain cannot continue to go on and on. For example the food chain could not be: plant  grasshopper  spider  frog  lizard  fox  hawk. Food chains only have 4 or 5 total levels. Therefore, a chain has only 3 or 4 levels for energy transfer. This food chain includes producers and consumers. How could you add decom- posers to the food chain? In an ocean ecosystem, one possible food chain might look like this: phytoplankton  krill  fish  shark. The producers are always at the beginning of the food chain, bringing energy into the ecosystem. Through photosynthesis, the producers create their own food in the form of glucose, but also create the food for the other organisms in the ecosystem. The herbivores come next, then the carnivores. When these consumers eat other organisms, they use the glucose in those organisms for energy. In this example, phytoplankton are eaten by krill, which are tiny, shrimp-like animals. The krill are eaten by fish, which are then eaten by sharks. Could decomposers be added to a food chain? Each organism can eat and be eaten by many different types of organisms, so simple food chains are rare in nature. There are also many different species of fish and sharks. So a food chain cannot end with a shark; it must end with a distinct species of shark. A food chain does not contain the general category of ""fish,"" it will contain specific species of fish. In ecosystems, there are many food chains. Since feeding relationships are so complicated, we can combine food chains together to create a more accurate flow of energy within an ecosystem. A food web ( Figure 1.2) shows the feeding relationships between many organisms in an ecosystem. If you expand our original example of a food chain, you could add deer that eat clover and foxes that hunt chipmunks. A food web shows many more arrows, but still shows the flow of energy. A complete food web may show hundreds of different feeding relationships. For more information on food chains, see A Million Sharks at  . "
frogs and toads,T_2949,"Frogs and toads are amphibians in the order Anura. In terms of classification, there is actually not a big difference between frogs and toads. Frogs often have long legs that are good for hopping, skin that is smooth and moist, and special pads on their toes that help them climb. Toads are more heavyset with shorter legs, and usually have drier skin, often with warty-looking bumps. Frogs are more likely to live in or near water than toads. Frogs are found in many areas of the world, from the tropics to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species (about 88% of amphibian species are frogs), they are among the most diverse groups of vertebrates. Frogs range in size from less than 0.5 inches in species in Brazil and Cuba to the over 1-foot (33 cm) long goliath frog of Cameroon, which can weigh up to 7 pounds. That is 1-foot from the nose to the back of the body, not including the length of the legs. Even the largest frogs are significantly smaller than common reptiles. "
frogs and toads,T_2950,"Adult frogs are characterized by long hind legs, a short body, webbed finger-like parts, and the lack of a tail. They also have a three-chambered heart, as do all tetrapods except birds and mammals. Most frogs live part of the time in water and part of the time on land. They move easily on land by jumping or climbing. To become great jumpers, frogs evolved long hind legs and long ankle bones. They also have a short backbone with only ten vertebrae. Frog and toad skin hangs loosely on the body, and skin texture can be smooth, warty, or folded. Frogs and toads dont have fur, feathers, or scales on their skin. Instead, they have a moist and permeable skin layer covered with mucous glands. Their special skin allows them to breathe through their skin in addition to using their lungs. They are vulnerable to water loss through the skin in dry conditions, which is why they need to live near water or in moist environments. The thin layer of mucous keeps the skin moist. In order to live on land and in water, frogs have three eyelid membranes: one is see-through to protect the eyes underwater, and the two other ones let them see on land. Frogs also have a tympanum, which acts like a simple ear. They are found on each side of the head. In some species, the tympanum is covered by skin. A tree frog. Notice the powerful muscles in the limbs and the coverings around the eyes. "
frogs and toads,T_2951,"Frogs typically lay their eggs in puddles, ponds, or lakes. Their larvae, or tadpoles, have gills, a tail, but no legs, and need to live in water. If fact, they are quite similar to a fish. Tadpoles develop into adult frogs in water ( Figure You may hear males ""ribbiting,"" producing a mating call used to attract females to the bodies of water best for mating and breeding. Frog calls can occur during the day or night. Each frog species has a different call that is used to attract mates and warn off rivals. When a female picks a male whose call she likes, the male grabs her and squeezes across her back and around her abdomen. This causes the female to release her eggs. The male then fertilizes the eggs and, in some species, also guards them. "
frogs and toads,T_2952,"Adult frogs are meat-eaters and eat mostly insects, spiders, slugs and worms. Larger species will eat mice, birds, and even other small reptiles and amphibians. Frogs do not have teeth on their lower jaw, so they usually swallow their food whole. Some frogs have teeth on the upper jaw that are used to hold the prey in place. Frogs and toads are responsible for keeping a large part of the worlds insect population under control. They catch these insects using their long tongue. The frog tongue is about a third the length of the frogs body, though they can Frogs develop from tadpoles, which de- velop from eggs. Notice the formation of the two powerful back legs used for jumping. grow even longer. They can easily reach 12 inches long in an adult frog. Frogs tongues are attached to the front of their mouths rather than at the back like humans. They release a sticky substance at the precise moment of impact with their food. When a frog catches an insect it throws its sticky tongue out of its mouth and wraps it around its prey. The frogs tongue then snaps back and throws the food down its throat. This happens about as fast as a blink of your eyes. "
fungi,T_2953,"Ever notice blue-green mold growing on a loaf of bread? Do you like your pizza with mushrooms? Has a physician ever prescribed an antibiotic for you? If so, then you have encountered fungi. Fungi are organisms that belong to the Kingdom Fungi ( Figure 1.1). Our environment needs fungi. Fungi help decompose matter to release nutrients and make nutritious food for other organisms. Fungi are all around us and are useful in many ways. These many different kinds of organisms demonstrate the huge diversity within the Kingdom Fungi. "
fungi,T_2954,"If you had to guess, would you say a fungus is a plant or an animal? Scientists used to debate about which kingdom to place fungi in. Finally they decided that fungi were plants. But they were wrong. Now, scientists know that fungi are not plants at all. Fungi are very different from plants. The main difference between plants and fungi is how they obtain energy. Plants are autotrophs, meaning that they make their own ""food"" using the energy from sunlight. Fungi are heterotrophs, which means that they obtain their ""food"" from outside of themselves. In other words, they must ""eat"" their food like animals do. But they dont really eat. Instead, they absorb their nutrients. Yeasts, molds, and mushrooms are all different kinds of fungi. There may be as many as 1.5 million species of fungi ( Figure 1.2). You can easily see bread mold and mushrooms without a microscope, but most fungi you cannot see. Fungi are either too small to be seen without a microscope, or they live where you cannot see them easilydeep in the soil, under decaying logs, or inside plants or animals. Some fungi even live in, or on top of, other fungi. "
fungi,T_2955,"Fungi can grow fast because they are such good eaters. Fungi have lots of surface area, and this large surface area eats or absorbs. Surface area is how much exposed area an organism has, compared to their overall volume. Most of a mushrooms surface area is actually underground. If you see a mushroom in your yard, that is just a small part of a larger fungus growing underground. These are the steps involved in fungi ""eating"": 1. Fungi squirt special enzymes into their environment. 2. The enzymes help digest large organic molecules, similar to cutting up your food before you eat. 3. Cells of the fungi then absorb the broken-down nutrients. "
fungi classification,T_2956,"Scientists used to think that fungi were members of the plant kingdom. They thought this because fungi had several similarities to plants. For example: Fungi and plants have similar structures. Plants and fungi live in the same kinds of habitats, such as growing in soil. Plants and fungi cells both have a cell wall, which animals do not have. "
fungi classification,T_2957,"However, there are a number of characteristics that make fungi different from plants: 1. Fungi cannot make their own food like plants can, since they do not have chloroplasts and cannot carry out photosynthesis. Fungi are more like animals because they are heterotrophs, as opposed to autotrophs, like plants, that make their own food. Fungi have to obtain their food, nutrients and glucose, from outside sources. 2. The cell walls in many species of fungi contain chitin. Chitin is tough carbohydrate found in the shells of animals such as beetles and lobsters. The cell wall of a plant is made of cellulose, not chitin. 3. Unlike many plants, most fungi do not have structures, such as xylem and phloem, that transfer water and nutrients. "
fungi classification,T_2958,"The Kingdom Fungi can be broken down into several phyla. Each phyla has some unique traits. And even within the same phyla there are many differences among the fungi. Various types of fungi are pictured below ( Table 1.1). Notice how different each of these organisms are from one another. Type of Fungi Molds Examples Penicillium Mushrooms Morels, shiitake, cremini, oyster Single-celled yeasts Bakers yeast "
fungi reproduction,T_2959,"Different fungi reproduce in different ways. Many fungi reproduce both sexually and asexually. However, some reproduce only sexually and some only asexually. Asexual reproduction involves just one parent and sexual repro- duction involves two parents. "
fungi reproduction,T_2960,"Through asexual reproduction, new organisms are produced that are genetically identical to the parent. That is, they have exactly the same DNA. Fungi reproduce asexually through three methods: 1. Spores: Spores are formed by the fungi and released to create new fungi. This is the powdery substance released by puffballs. Spores are haploid reproductive cells found in some bacteria, plants, algae, fungi, and protozoa. Theoretically, spores can reproduce asexually to produce countless offspring. Obviously this does not happen. If it did, the world would be covered by genetically identical fungi. 2. Budding: The fungus grows a new part of its body, which eventually breaks off. The broken-off piece becomes a new organism ( Figure 1.1). 3. Fragmentation: In this method, a piece of the mycelium, the body of the fungus, splits off. The resulting fragment can eventually produce a new colony of fungi. Asexual reproduction is faster and produces more fungi than sexual reproduction. This form of reproduction is controlled by many different factors. Outside conditions, such as the availability of food, determine when a fungus undergoes asexual reproduction. "
fungi reproduction,T_2961,"Almost all fungi can reproduce sexually. But why reproduce sexually when asexual reproduction is much quicker? Sexual reproduction brings together traits from the two parents. This increases the genetic diversity of the species. In plants and animals, sexual reproduction occurs when sperm and egg from two parents join to make a new individual. In fungi, however, two haploid hyphae meet together and their nuclei fuse. Instead of calling a hyphae male or female, they have different mating types, such as (+) and (-) ( Figure 1.2). The common mushroom, a fruiting body, results after sexual reproduction when two hyphae, one (+) and one (-), mate, forming a mycelium with sporangia. No- tice, when the sporangia burst, the spores are released from the fruiting body. "
gymnosperms,T_2971,"Gymnosperms have seeds, but they do not produce fruit. Instead, the seeds of gymnosperms are usually found in cones. There are four phyla of gymnosperms: 1. Conifers 2. Cycads 3. Ginkgoes 4. Gnetophytes "
gymnosperms,T_2972,"Conifers, members of the phylum Coniferophyta, are probably the gymnosperms that are most familiar to you. Conifers include trees such as pines, firs, spruces, cedars, and the coastal redwood trees in California, which are the tallest living vascular plants. Conifers have their reproductive structures in cones, but they are not the only plants to have that trait ( Figure 1.1). Conifer pollen cones are usually very small, while the seed cones are larger. Pollen contains gametophytes that produce the male gamete of seed plants. The pollen, which is a powder-like substance, is carried by the wind to fertilize the seed cones that contain the female gamete ( Figure 1.2). Conifers have many uses. They are important sources of lumber and are also used to make paper. Resins, the sticky substance you might see oozing out of a wound on a pine tree, are collected from conifers to make a variety of products, such as the solvent turpentine and the rosin used by musicians and baseball players. The sticky rosin improves the pitchers hold on the ball or increases the friction between the bow and the strings to help create music from a violin or other stringed instrument. "
gymnosperms,T_2973,"Cycads, in the phylum Cycadophyta, are also gymnosperms. They have large, finely-divided leaves and grow as short shrubs and trees in tropical regions. Like conifers, they produce cones, but the seed cones and pollen cones are always on separate plants ( Figure 1.3). One type of cycad, the Sago Palm, is a popular landscape plant. During the Age of the Dinosaurs (about 65 to 200 million years ago), cycads were the dominant plants. So you can imagine dinosaurs grazing on cycad seeds and roaming through cycad forests. The end of a pine tree branch bears the male cones that produce the pollen. Cycads bear their pollen and seeds in cones on separate plants. "
gymnosperms,T_2974,"Ginkgoes, in the phylum Ginkgophyta, are unique because they are the only species left in the phylum. Many other species in the fossil record have gone extinct ( Figure 1.4). The ginkgo tree is sometimes called a ""living fossil"" because it is the last species from its phylum. One reason the ginkgo tree may have survived is because it was often grown around Buddhist temples, especially in China. The ginkgo tree is also a popular landscape tree today in American cities because it can live in polluted areas better than most plants. Ginkgoes, like cycads, has separate female and male plants. The male trees are usually preferred for landscaping because the seeds produced by the female plants smell terrible when they ripen. Ginkgo trees are gymnosperms with broad leaves. "
gymnosperms,T_2975,"Gnetophytes, in the phylum Gnetophyta, are a very small and unusual group of plants. Ephedra is an important member of this group, since this desert shrub produces the ephedrine used to treat asthma and other conditions. Welwitschia produces extremely long leaves and is found in the deserts of southwestern Africa ( Figure 1.5). Overall, there are about 70 different species in this diverse phylum. One type of gnetophyte is Welwitschia. "
habitat and niche,T_2977,"Each organism plays a particular role in its ecosystem. A niche is the role a species plays in the ecosystem. In other words, a niche is how an organism makes a living. A niche will include the organisms role in the flow of energy through the ecosystem. This involves how the organism gets its energy, which usually has to do with what an organism eats, and how the organism passes that energy through the ecosystem, which has to do with what eats the organism. An organisms niche also includes how the organism interacts with other organisms, and its role in recycling nutrients. Once a niche is left vacant, other organisms can fill that position. For example when the Tarpan, a small wild horse found mainly in southern Russia, became extinct in the early 1900s, its niche was filled by a small horse breed, the Konik ( Figure 1.1). Often this occurs as a new species evolves to occupy the vacant niche. A species niche must be specific to that species; no two species can fill the same niche. They can have very similar niches, which can overlap, but there must be distinct differences between any two niches. If two species do fill the same niche, they will compete for all necessary resources. One species will out compete the other, forcing the other species to adapt or risk extinction. This is known as competitive exclusion. When plants and animals are introduced, either intentionally or by accident, into a new environment, they can occupy the existing niches of native organisms. Sometimes new species out-compete native species, and the native species may go extinct. They can then become a serious pest. For example, kudzu, a Japanese vine, was planted in the southeastern United States in the 1870s to help control soil loss. Kudzu had no natural predators, so it was able to out-compete native species of vine and take over their niches ( Figure 1.2). "
habitat and niche,T_2978,"The habitat is the physical area where a species lives. Many factors are used to describe a habitat. The average amount of sunlight received each day, the range of annual temperatures, and average yearly rainfall can all describe a habitat. These and other abiotic factors will affect the kind of traits an organism must have in order to survive there. The temperature, the amount of rainfall, the type of soil and other abiotic factors all have a significant role in determining the plants that invade an area. The plants then determine the animals that come to eat the plants, and so on. A habitat should not be confused with an ecosystem: the habitat is the actual place of the ecosystem, whereas the ecosystem includes both the biotic and abiotic factors in the habitat. Habitat destruction means what it sounds likean organisms habitat is destroyed. Habitat destruction can cause a population to decrease. If bad enough, it can also cause species to go extinct. Clearing large areas of land for housing developments or businesses can cause habitat destruction. Poor fire management, pest and weed invasion, and storm damage can also destroy habitats. National parks, nature reserves, and other protected areas all preserve habitats. Santa Cruz Island off the California coast has diverse habitats including a coastline with steep cliffs, coves, gigantic caves, and sandy beaches. "
habitat destruction,T_2979,"From a human point of view, a habitat is where you live, go to school, and go to have fun. Your habitat can be altered, and you can easily adapt. Most people live in a few different places and go to a number of different schools throughout their life. But a plant or animal may not be able to adapt to a changed habitat. A habitat is the natural home or environment of an organism. Humans often destroy the habitats of other organisms. Habitat destruction can cause the extinction of species. Extinction is the complete disappearance of a species. Once a species is extinct, it can never recover. Some ways humans cause habitat destruction are by clearing land and by introducing non-native species of plants and animals. "
habitat destruction,T_2980,"Clearing land for agriculture and development is a major cause of habitat destruction. Within the past 100 years, the amount of total land used for agriculture has almost doubled. Land used for grazing cattle has more than doubled. Agriculture alone has cost the United States half of its wetlands ( Figure 1.1) and almost all of its tallgrass prairies. Native prairie ecosystems, with their thick fertile soils, deep-rooted grasses, diversity of colorful flowers, burrowing prairie dogs, and herds of bison and other animals, have virtually disappeared ( Figure 1.3). Wetlands such as this one in Cape May, New Jersey, filter water and protect coastal lands from storms and floods. The Flint Hills contain some of the largest remnants of tallgrass prairie habitat remaining in North America. "
habitat destruction,T_2981,"Other habitats that are being rapidly destroyed are forests, especially tropical rainforests. The largest cause of deforestation today is slash-and-burn agriculture (shown in the opening image). This means that when people want to turn a forest into a farm, they cut down all of the trees and then burn the remainder of the forest. This technique is used by over 200 million people in tropical forests throughout the world. As a consequence of slash-and-burn agriculture, nutrients are quickly lost from the soil. This often results in people abandoning the land within a few years. Then the top soil erodes and desertification can follow. Desertification Herds of bison also made up part of the tallgrass prairie community. turns forest into a desert, where it is difficult for plants to grow. Half of the Earths mature tropical forests are gone. At current rates of deforestation, all tropical forests will be gone by the year 2090. "
habitat destruction,T_2982,"One of the main causes of extinction is introduction of exotic species into an environment. These exotic and new species can also be called invasive species or non-native species. These non-native species, being new to an area, may not have natural predators in the new habitat, which allows their populations to easily adapt and grow. Invasive species out-compete the native species for resources. Sometimes invasive species are so successful at living in a certain habitat that the native species go extinct ( Figure 1.4). Recently, cargo ships have transported zebra mussels, spiny waterfleas, and ruffe (a freshwater fish) into the Great Lakes ( Figure 1.5). These invasive species are better at hunting for food. They have caused some of the native species to go extinct. Invasive species can disrupt food chains, carry disease, prey on native species directly, and out-compete native species for limited resources, like food. All of these effects can lead to extinction of the native species. "
habitat destruction,T_2983,"Other causes of habitat destruction include poor fire management, overfishing, mining ( Figure 1.6), pollution, and storm damage. All of these can cause irreversible changes to a habitat and ecosystem. "
habitat destruction,T_2984,"A habitat that is quickly being destroyed is the wetland. By the 1980s, over 80% of all wetlands in parts of the U.S. were destroyed. In Europe, many wetland species have gone extinct. For example, many bogs in Scotland have been lost because of human development. Another example of species loss due to habitat destruction happened on Madagascars central highland plateau. From 1970 to 2000, slash-and-burn agriculture destroyed about 10% of the countrys total native plants. The area turned into a wasteland. Soil from erosion entered the waterways. Much of the river ecosystems of several large rivers were also destroyed. Several fish species are almost extinct. Also, some coral reef formations in the Indian Ocean are completely lost. "
human uses of fungi,T_3036,"Fungi are extremely important to the ecosystem because they are one of the major decomposers of organic material. Decomposing organic material is how fungi acquire energy. But fungi have other roles in addition to being decom- posers. How do fungi help people? They are used to help prepare food and beverages, and they have many other uses. "
human uses of fungi,T_3037,"Yeasts are crucial for the fermentation process that makes beer, wine, and bread. Fermentation occurs in the absence of oxygen and allows the first step of cellular respiration, glycolysis, to continue. Some fungi are used in the production of soy sauce and tempeh, a source of protein used in Southeast Asia. Fungi can produce antibiotics, such as penicillin. Antibiotics are important medicines that kill bacteria, and penicillin was the first identified cure against many deadly bacterial species. Antibiotics only treat bacterial diseases; they can not be used to treat viral or fungal diseases. Mushrooms are fungi that are eaten by people all over the globe. The video Bread Mold Kills Bacteria at  explains Alexander Flemings famous discovery that lead to the discovery of the antibiotic penicillin. Antibiotics are used to kill Saccharomyces cerevisiae, a single-celled fungus called brewers or bakers yeast, is used in the baking of bread and in making wine and beer through fermentation. harmful bacteria. See Alexander Fleming 1881 - 1955 at  Click image to the left or use the URL below. URL: "
human uses of fungi,T_3038,"Some of the best known types of fungi are mushrooms, which can be edible or poisonous ( Figure 1.2). Many species are grown commercially, but others are harvested from the wild. When you order a pizza with mushrooms or add them to your salad, you are most likely eating Agaricus bisporus, known as white or button mushrooms, the most commonly eaten species. Other mushroom species are gathered from the wild for people to eat or for commercial sale. Many mushroom species are poisonous to humans. Some mushrooms will simply give you a stomachache, while others may kill you. Some mushrooms you can eat when they are cooked but are poisonous when raw. So if you find mushrooms in the wild, dont eat them until you are certain they are safe! Have you ever eaten blue cheese? Do you know what makes it blue? You guessed it. A fungus. For certain types of cheeses, producers add fungal spores to milk curds to promote the growth of mold, which makes the cheese blue. Molds used in cheese production are safe for humans to eat. Some fungi are poisonous and must be avoided. "
human vision,T_3039,"Think about all the ways that students use their sense of sight during a typical school day. As soon as they open their eyes in the morning, they might look at the clock to see what time it is. Then, they might look out the window to see what the weather is like. They probably look in a mirror to comb their hair. In school, they use their eyes to read the board, their textbooks, and the expressions on their friends faces. After school, they might keep their eye on the ball while playing basketball ( Figure 1.1). Then, they might read their homework assignment and text messages from their friends. Sight, or vision, is the ability to see light. It depends on the eyes detecting light and forming images. It also depends on the brain making sense of the images, so that we know what we are seeing. Human beings and other primates depend on vision more than many other animals. Its not surprising, then, that we have a better sense of vision than many other animals. Not only can we normally see both distant and close-up objects clearly, but we can also see in three dimensions and in color. But humans do not have the best vision. You may think you see things fairly clearly. Imagine if you could see even better. How about 8 times better? Raptors, or birds of prey, including the eagles, hawks and falcons can see up to 8 times more clearly than the sharpest human eye. A golden eagle for example can see a rabbit from a mile away. Why do you think they have such a good sense of vision? Other animals also have much better night vision then us. "
human vision,T_3040,"Did you ever use 3-D glasses to watch a movie, like the boy pictured below ( Figure 1.2)? If you did, then you know that the glasses make people and objects in the movie appear to jump out of the screen. They make images on the flat movie screen seem more realistic because they give them depth. Thats the difference between seeing things in two dimensions and three dimensions. We are able to see in three dimensions because we have two eyes facing the same direction but a few inches apart. As a result, we see objects and people with both eyes at the same time but from slightly different angles. Hold up a finger a few inches away from your face, and look at it, first with one eye and then with the other. Youll notice that your finger appears to move. Now hold up your finger at arms length, and look at it with one eye and then the other. Your finger seems to move less than it did when it was closer. Although you arent aware of it, your brain constantly uses such differences to determine the distance of objects. "
human vision,T_3041,"For animals like us that see in color, it may be hard to imagine a world that appears to be mainly shades of gray. You can get an idea of how many other animals see the world by looking at a black-and-white picture of colorful objects. For example, look at the apple on the tree pictured below ( Figure 1.3). In the top picture, they appear in color, the way you would normally see them. In the bottom picture they appear without color, in shades of gray ( Figure 1.4). Humans with color vision see the apple on this tree; the bright red color of the apple stands out clearly from the green background of leaves. This black-and-white picture gives an idea of how many animals see the world. Dogs and cats would see the green and red colors as shades of gray; they are able to see blue, but red and green appear the same to them. Many animals see just one or two colors. Some see colors that we cannot see. Apes and chimps see the same colors as us. But whereas many animals cannot see colors, some animals see colors that we cannot. The range of color vision of bees and butterflies for example, extends beyond the visible spectrum of light we can see. The leaves of the flowers they pollinate have special ultraviolet patterns which guide the insects deep into the flower. "
human vision,T_3042,"Why do you think primates, including humans, evolved the ability to see in three dimensions and in color? To answer that question, you need to know a little about primate evolution. Millions of years ago, primate ancestors lived in trees. To move about in the trees, they needed to be able to judge how far away the next branch was. Otherwise, they might have a dangerous fall. Being able see in depth was important. It was an adaptation that would help tree-living primates survive. Primate ancestors also mainly ate fruit. They needed to be able to spot colored fruits in the leafy background of the trees ( Figure 1.5). They also had to be able to judge which fruits were ripe and which were still green. Ripe fruits are usually red, orange, yellow, or purple. Being able to see in color was important for finding food. It was an adaptation that would help fruit-eating primates survive. "
humans and primates,T_3043,"The great apes are the members of the biological family Hominidae, which includes four living genera: chimpanzees, gorillas, orangutans and humans. Among these four genera are just seven species, two of each except humans, which has only one species, Homo sapiens. "
humans and primates,T_3044,"The Great Apes are large, tailless primates, ranging in size from the pygmy chimpanzee, at 66-88 pounds in weight, to the gorilla, at 300-400 pounds ( Figure 1.1). In all species, the males are, on average, larger and stronger than the females. A Western Lowland gorilla, member of the great apes. The gorilla is the largest of the hominids, weighing up to 309-397 lbs. Most living primate species are four-footed, but all are able to use their hands for gathering food or nesting materials. In some cases, hands are used as tools, such as when gorillas use sticks to measure the depth of water ( Figure 1.2). Chimpanzees sharpen sticks to use as spears in hunting; they also use sticks to gather food and to fish for termites. Most primate species eat both plants and meat ( omnivorous), but fruit is the preferred food among all but humans. In contrast, humans eat a large amount of highly processed, low fiber foods, and unusual proportions of grains and vertebrate meat. As a result of our diets, human teeth and jaws are markedly smaller for our size than those of other apes. Humans may have been eating cooked food for a million years or more, so perhaps our teeth adapted to eating cooked food. Gorillas and chimpanzees live in family groups of approximately five to ten individuals, although larger groups are sometimes observed. The groups include at least one dominant male, and females leave the group when they can mate. Orangutans, however, generally live alone. "
humans and primates,T_3045,"Gorillas, chimpanzees, and humans have more than 97% of their DNA sequence in common. This means that a similar percent of the amino acid sequences of the proteins will be the same, resulting in many proteins with similar or identical functions. All organisms in the Hominidae communicate with some kind of language. They can also create simple cultures beyond the family or group of animals. Having a culture means that knowledge and behaviors can be passed on from generation to generation. "
humans and primates,T_3046,"Specialized features of Homo sapiens include the following: small front teeth (canines and incisors) and very large molars relative to other primate species, a fully upright posture resulting in bipedalism (walking on two limbs instead of four), shortening of the arms relative to the legs, increased usefulness (dexterity) of the hands, increase in brain size, especially in the frontal lobes and a decrease in bone mass of the skull and face. See Communication - the Jane Goodall Institute at  , Com- paring the Human and Chimpanzee Genomes at http://wrl.it/show/197403/12898478 , and Discovering Gibbons at Click image to the left or use the URL below. URL: "
importance of arthropods,T_3048,"Have you ever been startled by a bee landing on a flower? Or surprised by a swarm of pill bugs when you overturned a rock? These arthropods might seem a little scary to you, but they are actually performing important roles in the environment. Arthropods are important to the ecosystem and to humans in many ways. "
importance of arthropods,T_3049,"Many species of crustaceans, especially crabs, lobsters ( Figure 1.1), shrimp, prawns, and crayfish, are consumed by humans, and are now farmed on a large commercial scale. Nearly 10,000,000 tons of arthropods as food were produced in 2005. Over 70% by weight of all crustaceans caught for consumption are shrimp and prawns. Over 80% is produced in Asia, with China producing nearly half the worlds total. Insects and their grubs are at least as nutritious as meat, and are eaten both raw and cooked in many cultures. Beetles, locusts, butterflies, ants, and stinkbugs (which have an apple flavor) are insects that are regularly eaten by people in dozens of countries. In fact, there are more than 1,900 edible insect species on Earth, hundreds of which are already part of the diet of about two billion people worldwide. This is just under one of every three people worldwide, and this number should continue to grow in the future. The intentional cultivation of arthropods and other small animals for human food, referred to as minilivestock, is now emerging in animal husbandry as an ecologically sound concept. However, the greatest contribution of arthropods to human food supply is by pollination. Three-fourths of the worlds flowering plants and about 35% of the worlds food crops depend on animal pollinators to reproduce and increase crop yields. More than 3,500 species of native bees pollinate crops. Some scientists estimate that one out of every three bites of food we eat exists because of animal pollinators, including birds and bats and arthropods like bees, butterflies and moths, and beetles and other insects. Lobsters are one kind of arthropod food source. "
importance of arthropods,T_3050,"Humans use mites to prey on unwanted arthropods on farms or in homes. Other arthropods are used to control weed growth. Populations of whip scorpions added to an environment can limit the populations of cockroaches and crickets. Millipedes also control the harmful growth of destructive fungi and bacteria. When the numbers of millipedes is low, the imbalance between predator and prey can cause harmful microorganisms to flourish, and it can became difficult to manage plagues and diseases through natural processes. Cockroaches, spiders, mites, ticks and all other insects considered as carnivorous, prey on smaller species to maintain ecological balance. Thus, communities that have a good balance of these arthropods tend to have better pest control. "
importance of arthropods,T_3051,"Many arthropods have extremely important roles in ecosystems. Arthropods are of ecological importance because of their sheer numbers and extreme diversity. As mentioned above, bees, wasps, ants, butterflies, moths, flies and beetles are invaluable agents of pollination. Pollens and grains became accidentally attached to their chests and legs and are transferred to other agricultural crops as these animals move about, either by walking or flying. Most plants actually produce scents to send signals to insects that food (in the form of nectar) is available. Mites, ticks, centipedes, and millipedes are decomposers, meaning they break down dead plants and animals and turn them into soil nutrients. This is an important role because it supplies the plants with the minerals and nutrients necessary for life. It also keeps dead material from accumulating in the environment. Plants then pass along those minerals and nutrients to the animals that eat the plants. "
importance of arthropods,T_3052,"Arthropods are also invaluable to humans, as they are used in many different human-made products. Examples are: Bees produce honey and their honeycombs contain beeswax, widely used for making candles, furniture wax and polishes, waxed papers, antiseptics, and fillings for surgical uses. The pollens stored in honeycombs were discovered to have a rich mixture of vitamins, enzymes, and amino acids that could provide medical benefits. They were used as ingredients for supplements and medications that could provide relief for colds, asthma, and hay fever. Silk produced by arthropods, like those produced by caterpillars to protect their cocoons, is strong enough to use and be woven into fabrics, a discovery first used in ancient Chinas silk industry. The spiders web was discovered as an additional material that could provide strength, and has became essential raw materials for Kevlar vests, fishing nets, surgical sutures, and adhesives, as they contained natural antiseptics. "
importance of biodiversity,T_3053,"Biodiversity is a measurement of the amount of variation of the species in a given area. More specifically, biodi- versity can be defined as the variety of life and its processes, including the variety of living organisms, the genetic differences among them, and the communities and ecosystems in which they occur. A place such as a coral reef has many different species of plants and animals. That means the coral reef is a ecosystem with high biodiversity ( Figure 1.1). Because of its biodiversity, the rainforest shown above is an ecosystem with extreme importance. Why is biodiversity so important? In addition to maintaining the health and stability of the ecosystem, the diversity of life provides us with many benefits. Extinction is a threat to biodiversity. Does it matter if we are losing thousands of species each year? The answer is yes. It matters even if we consider not only the direct benefits to humans, but also the benefits to the ecosystems. The health and survival of ecosystems is related to that ecosystems biodiversity. Coral reefs are one of the biomes with the highest biodiversity on Earth. "
importance of biodiversity,T_3054,"Economically, there are many direct benefits of biodiversity. As many as 40,000 species of fungi, plants, and animals provide us with many varied types of clothing, shelter, medicines and other products. These include poisons, timber, fibers, fragrances, papers, silks, dyes, adhesives, rubber, resins, skins, furs, and more. According to one survey, 57% of the most important prescription drugs come from nature. Specifically, they come from bacteria, fungi, plants, and animals ( Figure 1.2). But only a small amount of species with the ability to give us medicines have been explored. The loss of any species may mean the loss of new medicines, which will have a direct effect on human health. Aspirin originally came from the bark of the white willow tree, pictured here. "
importance of biodiversity,T_3055,"Nature has inspired many technologies in use today. Bionics, also known as biomimetics or biomimicry, uses organisms to inspire technology or engineering projects. By studying animals and their traits, we are able to gain valuable information that we can put to use to help us. For example, rattlesnake heat-sensing pits helped inspire the development of infrared sensors. Zimbabwes Eastgate Centre ( Figure 1.3) was inspired by the air-conditioning efficiency of a termite mound ( Figure 1.4). Design of the Eastgate Centre (brown building), in Zimbabwe, which requires just 10% of the energy needed for a con- ventional building of the same size, was inspired by a biological design. "
importance of biodiversity,T_3056,"Biodiversity also has many benefits to ecosystems. High biodiversity makes ecosystems more stable. What can happen to an ecosystem if just one species goes extinct? What if that one species was a producer or decomposer? Would the loss of a producer have an effect on all the organisms that relied on that producer? If a decomposer vanishes, are there other decomposers to fill the void? Maybe the resulting species will adapt. Other species may fill in the niche left by the extinct species. But the extinction of one species could have a ""domino"" effect, resulting in the extinction of other species. This could greatly effect the stability of the whole ecosystem. The air-conditioning efficiency of this ter- mite mound was the inspiration for the Eastgate Centre. One important role of biodiversity is that it helps keep the nutrients, such as nitrogen, in the soil. For example, a diversity of organisms in the soil allows nitrogen fixation and nutrient recycling to happen. Biodiversity also allows plants to be pollinated by different types of insects. And of course, different species of fungi are necessary to recycle wastes from dead plants and animals. These are just a few of the many examples of how biodiversity is important for ecosystems. "
importance of birds,T_3057,"You are probably familiar with birds as food. People have always hunted birds for food. People eventually discovered that certain wild fowl (ducks, chickens, turkeys) could be tamed. This discovery led to the development of poultry, which is domesticated fowl that farmers raise for meat and eggs. Chickens are probably the oldest kinds of poultry. Chickens were domesticated in Asia at least 3,000 years ago. Since then, farmers have developed other poultry, including ducks, geese, guineafowl, pheasants, and turkeys. Around the world, people consume all these birds, and even more exotic birds, like ostriches. Today, chickens rank as the most widely raised poultry by far. Farmers throughout the world produce hundreds of millions of chickens annually for meat and eggs. Ducks and turkeys rank second and third in production worldwide. Ducks are raised for both meat and eggs. Turkeys are raised mainly for meat. Can you think of other ways that birds are important? "
importance of birds,T_3058,"1. In agriculture, humans harvest bird droppings for use as fertilizer. These droppings have a high content of nitrogen, phosphate, and potassium, three nutrients essential for plant growth. 2. Chickens are also used as an early warning system of human diseases, such as West Nile virus. Mosquitoes carry the West Nile virus, bite young chickens and other birds, and infect them with the virus. When chickens or other birds become infected, humans may also become infected in the near future. 3. Birds have important cultural relationships with humans. Birds are common pets in the Western world. Common bird pets include canaries, parrots, finches, and parakeets. Sometimes, people act cooperatively with birds. For example, the Borana people in Africa use birds to guide them to honey that they use in food. 4. Birds also play prominent and diverse roles in folklore, religion, and popular culture. They have been featured in art since prehistoric times, when they appeared in early cave paintings. Many young child know of Big Bird, a very large canary of Sesame Street fame. 5. Feathers are also used all over the world to stuff pillows, mattresses, sleeping bags, coats, and quilting. Goose feathers are preferred because they are soft. Manufacturers often mix goose feathers with down feathers to provide extra softness. "
importance of birds,T_3059,"Birds are obviously important members of many ecosystems. They are integral parts of food chains and food webs. In a woodland ecosystem for example, some birds get their food mainly from plants. Others chiefly eat small animals, such as insects or earthworms. Birds and bird eggs, in turn, serve as food for such animals as foxes, raccoons, and snakes. The feeding relationships among all the animals in an ecosystem help prevent any one species from becoming too numerous. Birds play a vital role in keeping this balance of nature. In addition to being important parts of food webs, birds play other roles within ecosystems. 1. Birds eat insects. They are a natural way to control pests in gardens, on farms, and other places. A group of birds gliding through the air can easily eat hundreds of insects each day. Insect eating birds include warblers, bluebirds and woodpeckers. 2. Nectar-feeding birds are important pollinators, meaning they move the pollen from flower to flower to help fertilize the sex cells and create new plants. Hummingbirds, sunbirds, and the honey-eaters are common pollinators. 3. Many fruit-eating birds help disperse seeds. After eating fruit, they carry the seeds in their intestines and deposit them in new places. Fruit-eating birds include mockingbirds, orioles, finches and robins. 4. Birds are often important to island ecology. In New Zealand, the kereru and kokako are important browsers, or animals that eat or nibble on leaves, tender young shoots, or other vegetation ( Figure 1.1). Seabirds add nutrients to soil and to water with their production of guano, their dung. The kereru (left) and the kokako (right) are important browser species in New Zealand "
importance of echinoderms,T_3060,Echinoderms are important for the ecosystem. They are also a source of food and medicine for humans. 
importance of echinoderms,T_3061,"Echinoderms play numerous ecological roles. Sand dollars and sea cucumbers burrow into the sand, providing more oxygen at greater depths of the sea floor. This allows more organisms to live there. In addition, starfish prevent the growth of algae on coral reefs. This allows the coral to filter-feed more easily. And many sea cucumbers provide a habitat for parasites such as crabs, worms, and snails. Echinoderms are also an important step in the ocean food chain. Echinoderms are the staple diet of many animals, including the sea otter. On the other hand, echinoderms eat seaweed and keep its growth in check. Recall that the sea urchin is a grazer, mainly feeding on algae on the coral and rocks. Recently, some marine ecosystems have been overrun by seaweed. Excess seaweed can destroy entire reefs. Scientists believe that the extinction of large quantities of echinoderms has caused this destruction ( Figure 1.1). A large die-off of the sea urchin, Diadema antillarum, in the Caribbean Sea coin- cided with increases in algal growth in some areas but not others. "
importance of echinoderms,T_3062,"In some countries, echinoderms are considered delicacies. Around 50,000 tons of sea urchins are captured each year for food. They are consumed mostly in Japan, Peru, Spain and France. Both male and female gonads of sea urchins are also consumed. The taste is described as soft and melting, like a mixture of seafood and fruit. Sea cucumbers are considered a delicacy in some southeastern Asian countries. In China they are used as a basis for gelatinous soups and stews. "
importance of echinoderms,T_3063,"Echinoderms are also used as medicine and in scientific research. For example, some sea cucumber toxins slow down the growth rate of tumor cells, so there is an interest in using these in cancer research. Sea urchins are also model organisms used in developmental biology research. Sea urchins have been used to study the mechanisms of fertilization and egg activation, physiological processes that occur during early development, and the regulation of differentiation in the early embryo. In addition, the molecular basis of early development was studied in sea urchins. Gametes can be obtained easily, sterility is not required, and the eggs and early embryos of many commonly used species are beautifully transparent. In addition, the early development of sea urchin embryos is a highly conserved process. When a batch of eggs is fertilized, all of the resulting embryos typically develop at the same time. This makes biochemical and molecular studies of early embryos possible in the sea urchin, and has led to a number of major discoveries. "
importance of echinoderms,T_3064,"The hard skeleton of echinoderms is used as a source of lime by farmers in some areas where limestone is unavailable. Lime is added to the soil to allow plants to take up more nutrients. About 4,000 tons of the animals are used each year for this purpose. "
importance of insects,T_3065,"Many insects are considered to be pests by humans. However, insects are also very important for numerous reasons. "
importance of insects,T_3066,"Insects can be found in every environment on Earth. While a select few insects, such as the Arctic Wooly Bear Moth, live in the harsh Arctic climate, the majority of insects are found in the warm and moist tropics. Insects have adapted to a broad range of habitats, successfully finding their own niche, because they will eat almost any substance that has nutritional value. Insects are crucial components of many ecosystems, where they perform many important functions. They aerate the soil, pollinate blossoms, and control insect and plant pests. Many insects, especially beetles, are scavengers, feeding on dead animals and fallen trees, thereby recycling nutrients back into the soil. As decomposers, insects help create top soil, the nutrient-rich layer of soil that helps plants grow. Burrowing bugs, such as ants and beetles, dig tunnels that provide channels for water, benefiting plants. Bees, wasps, butterflies, and ants pollinate flowering plants ( Figure 1.1). Gardeners love the big-eyed bug and praying mantis because they control the size of certain insect populations, such as aphids and caterpillars, which feed on new plant growth. Finally, all insects fertilize the soil with the nutrients from their droppings. Bees are important pollinators of flower- ing plants. "
importance of insects,T_3067,"Insects have tremendous economic importance. Some insects produce useful substances, such as honey, wax, lacquer, and silk. Honeybees have been raised by humans for thousands of years for honey. The silkworm greatly affected human history. When the Chinese used worms to develop silk, the silk trade connected China to the rest of the world. Adult insects, such as crickets, as well as insect larvae, are also commonly used as fishing bait. "
importance of insects,T_3068,"Insects, of course, are not just eaten by people. Insects are the sole food source for many amphibians, reptiles, birds, and mammals, making their roles in food chains and food webs extremely important. It is possible that food webs could collapse if insect populations decline. In some parts of the world, insects are used for food by humans. Insects are a rich source of protein, vitamins, and minerals, and are prized as delicacies in many third-world countries. In fact, it is difficult to find an insect that is not eaten in one form or another by people. Among the most popular are cicadas, locusts, mantises, grubs, caterpillars, crickets, ants, and wasps. Many people support this idea to provide a source of protein in human nutrition. From South America to Japan, people eat roasted insects, like grasshoppers or beetles. "
importance of insects,T_3069,"Insects have also been used in medicine. In the past, fly larvae ( maggots) were used to treat wounds to prevent or stop gangrene. Gangrene is caused by infection of dead flesh. Maggots only eat dead flesh, so when they are placed on the dead flesh of humans, they actually clean the wound and can prevent infection. Some hospitals still use this type of treatment. "
importance of mammals,T_3070,"Mammals play many important roles in ecosystems, and they also benefit people. "
importance of mammals,T_3071,"Mammals have important roles in the food webs of practically every ecosystem. Mammals are important members of food chains and food webs, as grazers and predators. Mammals can feed at various levels of food chains, as herbivores, insectivores, carnivores and omnivores. Mammals also interact with other species in many symbiotic relationships. For example, bats have established mutually beneficial relationships with plants. Nectar-feeding bats receive a tasty treat from each flower, and, in return, they pollinate the flowers. That means they transfer pollen from one flower to another, allowing the plant to reproduce. Non-flying mammalian pollinators include marsupials, primates, and rodents. In most cases, these animals visit flowers to eat their nectar, and end up with pollen stuck to their bodies. When the animal visits another flower to eat the nectar, the pollen is transferred to that flower. Fruit-eating bats ( Figure 1.1) also receive food from plants. In return, they help these plants spread their seeds. When bats consume fruit, they also consume the seeds within the fruit. Then they carry the seeds in their guts to far-away locations. Zebras have been known to befriend ostriches. In this symbiotic relationship, both species benefit. The ostrich, with its terrible senses of smell and hearing and the zebra with its poor eyesight, are both able to warn the other when danger is near. The zebra can smell or hear certain dangers approaching, while the ostrich can see other dangers. Both are prepared to warn one another at a moments notice so they can each flee when necessary. Baboons and impala have a similar relationship. Impala are one of the most common prey species for all predators and need to be constantly alert. Impala have good hearing and eyesight, raising an alarm when danger is near. Baboons use trees to check for danger and bark an alarm when danger is sensed. What do the baboons receive? Male baboons sometimes prey on young impala soon after birth. So, though both alert others to dangers, sometimes this is not the best of relationships for young impala. Zebra and wildebeest are found together on the African savanna grazing different parts of the same grass. The zebra grazes the tougher parts of the plant, saving the softer parts for the wildebeest. A zebra will move into an area of tall grass before other herbivores and graze the grass down to the area that the wildebeest prefers. Bats, like this Egyptian fruit bat, play an important role in seed dispersal. "
importance of mammals,T_3072,"We see examples of mammals (other than people!) serving our needs everywhere. We have pets that are mammals, such as dogs and cats. Mammals are also used around the world for transport. For example, horses, donkeys, mules, or camels ( Figure 1.2) may be the primary means of transport in some parts of the world. Mammals also do work for us. Service dogs can be trained to help the disabled. These include guide dogs, which are assistance dogs trained to lead blind and visually impaired people around obstacles. Horses and elephants can carry heavy loads. Humans also use some mammals for food. For example, cows and goats are commonly raised for their milk and meat. Mammals more highly developed brains have made them ideal for use by scientists in studying such things as learning, as seen in maze studies of mice and rats. "
importance of mammals,T_3073,"Mammals have also played a significant role in different cultures folklore and religion. For example, the grace and power of the cougar have been admired in the cultures of the native peoples of the Americas. The Inca city of Cuzco is designed in the shape of a cougar, and the thunder god of the Inca, Viracocha, has been associated with the animal. In North America, mythological descriptions of the cougar have appeared in the stories of several American Indian tribes. Important mammals include Dolly the sheep, Lassie the dog, and flipper the dolphin. Dolly was the first mammal to be cloned from an adult somatic (body) cell, using the process of nuclear transfer. Lassie was a collie dog who appeared in seven full length feature films in the 1940s and 1950s, starting with Lassie Come Home in 1943. Additional Lassie movies were made as recently as 2005. Between 1954 and 1973, the Lassie television series aired, with plenty of additional productions as recently as 2007. Flipper was a bottle nose dolphin that starred in a television series between 1964 and 1967. The most famous mammal may be King Kong, the giant gorilla that terrorized New York City in 1933 in the movie of the same name. "
importance of mollusks,T_3074,"Mollusks are important in a variety of ways; they are used as food, for decoration, in jewelry, and in scientific studies. They are even used as roadbed material and in vitamin supplements. "
importance of mollusks,T_3075,"Edible species of mollusks include numerous species of clams, mussels, oysters, scallops, marine and land snails, squid, and octopuses. Many species of mollusks, such as oysters, are farmed in order to produce more than could be found in the wild ( Figure 1.1). Today, fisheries in Europe, Japan, and the US alone produce over 1 billion pounds of oyster meat each year. Abalone (a marine gastropod mollusk), a great delicacy, can fetch up to three hundred dollars per pound. Eating mollusks is associated with a risk of food poisoning from toxins that accumulate in molluscs under certain conditions, and many countries have regulations to reduce this risk. At certain times of the year, (usually the warmer months) many species of saltwater mollusks become very poisonous due to an algal bloom known as ""red tide."" The mollusks filter feed on the tiny creatures (called dinoflagellates in the bloom) that produce the toxins. Eating shellfish during a red tide can cause serious illness and even death to humans. Tastes in molluscan food vary tremendously from one person to the next and from culture to culture; however, when it comes to a question of survival, most mollusks are edible. Some are considered delicacies such as oysters and escargot (a snail that lives in trees), while others such as the clams and mussels of freshwater ponds and streams are less likely to be consumed due to taste, but none-the-less are very edible. Land-based mollusks are also eaten. France alone consumes 5 million pounds of escargot every year. Of course, some people are allergic to mollusks and need to be careful about consuming any kind of shelled animals. "
importance of mollusks,T_3076,"Two natural products of mollusks used for decorations and jewelry are pearls and nacre. A pearl is the hard, round object produced within the mantle of a living shelled mollusk. Pearls are produced by many bivalves when a tiny particle of sand or grit is trapped between the mantle and the shell. Its as if the mollusk has a splinter. The mollusk forms a protective covering around the irritant. Most pearls used as jewelry are made by pearl oysters and freshwater mussels; most of the ones sold are cultured and not wild. Natural pearls have been highly valued as gemstones and objects of beauty for many centuries. The most desirable pearls are produced by oysters and river mussels. The substance used to form the pearl covering is made from the mother of pearl material that lines the interior of the shell. Mother of pearl is also known as nacre. Nacre is the iridescent inner shell layer. It can be found in buttons, watch faces, knives, guns, and jewelry. It is also used to decorate various musical instruments. "
importance of mollusks,T_3077,"Several mollusks are ideal subjects for scientific investigation of the nervous system. The giant squid has a sophis- ticated nervous system and a complex brain for study. The California sea slug, also called the California sea hare, is used in studies of learning and memory because it has a simple nervous system, consisting of just a few thousand large, easily identified neurons. These neurons are responsible for a variety of learning tasks. Some slug brain studies have even allowed scientists to better understand human brains. Some octopuses and squid are incredibly smart. They are capable of learning to solve problems and do mazes. "
importance of protists,T_3078,"Humans could not live on Earth if it were not for protists. Why? Plant-like protists produce almost one-half of the oxygen on the planet through photosynthesis. Other protists decompose and recycle nutrients that humans need to live. All protists make up a huge part of the food chain. Humans use protists for many other reasons: Many protists are also commonly used in medical research. For example, medicines made from protists are used in treatment of high blood pressure, digestion problems, ulcers, and arthritis. Other protists are used in scientific studies. For example, slime molds (including D. discoideum, a soil-living protist) are used to analyze the chemical signals in cells. Protists are also valuable in industry. Look on the back of a milk carton. You will most likely see carrageenan, which is extracted from red algae. This is used to make puddings and ice cream solid ( Figure 1.1). Chemicals from other kinds of algae are used to produce many kinds of plastics. "
importance of reptiles,T_3079,"Reptiles play an important role in the life of humans. In addition to playing an important role in many food chains, which keep the populations of small animals under control, reptiles serve as food, pets, and have played roles in art and culture for thousands of years. "
importance of reptiles,T_3080,"Reptiles are important as food sources for people: Green iguanas, a type of large lizard, are eaten in Central America. The tribals of Irulas from Andhra Pradesh and Tamil Nadu in India are known to eat some of the snakes they catch. Cantonese snake soup is consumed by local people in the fall to prevent colds. The soup is believed to warm up their body of those who eat it. Cooked rattlesnake meat is commonly consumed in parts of the Midwestern United States. You can eat rattlesnake meat without worry of the poisonous venom. Other snake meat is consumed throughout the world. Turtle soup is consumed throughout the world. "
importance of reptiles,T_3081,"Reptiles also make good pets. In the Western world, some snakes, especially less aggressive species, like the ball python or corn snake, are kept as pets. Turtles, particularly small land-dwelling and freshwater turtles, are also common pets. Among the most popular are Russian tortoises, Greek spur-thighed tortoises, and terrapins. Large constrictor snakes like pythons, boa constrictors, and anacondas are powerful wild animals capable of killing an adult human, and they are commonly kept as pets. Many people dont think this is a wise idea, as these reptiles pose dangerous threats to people, especially children. Reptiles are capable of recognizing people by voice, sight and smell; many are capable of learning. Some species actually benefit from interaction with humans. When cared for properly, all live as long or longer than mammalian pets of similar size. Having a reptile as a pet, you get to learn about everything from adaptation, behavior and the environment, to nutrition, camouflage and reproductive strategies. Learning about the natural history and proper captive care of these animals just might change your world outlook and get you thinking more about the environment as a whole. Keep in mind that if you want to have a snake as a pet, that there are no herbivorous snakes, and you must be willing to feed it a proper diet. Be prepared to feed your snake, or other reptile, mice, rats, birds eggs, insects, or fish. And these need to be served raw. Of course, the herbivorous reptiles, such as the green iguanas and some tortoises, are much easier to feed. They eat foods such as chopped collard greens, romaine lettuce, chopped squash and bananas. "
importance of reptiles,T_3082,"Finally, reptiles play a significant role in folklore, religion, and popular culture. The Moche people of ancient Peru worshipped reptiles and often put lizards in their art. Snakes or serpents are connected to healing and to the Devil. Since snakes shed and then heal again, they are a symbol of healing and medicine, as shown in the Rod of Asclepius ( Figure 1.1). In Egyptian history, the Nile cobra is found on the crown of the pharaoh. This snake was worshiped as one of the gods. Reptiles have also played roles in more recent popular culture. Unforgettable reptiles include Leonardo, Donatello, Michaelangelo, and Raphael, otherwise known as the Teenage Mutant Ninja Turtles, and Godzilla, one of the most famous movie reptiles who has been terrorizing Japanese cities for years. Dino, from The Flintstones is one of the more lovable television reptiles. On the other hand is Nagini from the Harry Potter series. This tremendously long snake (roughly 12 feet) is difficult to forget as she was very important to Lord Voldemort. Though her appearances are far and few between, her unwavering loyalty to the Dark Lord makes her one of the more infamous reptiles. "
importance of seedless plants,T_3083,"Seedless plants have been tremendously useful to humans. Without these plants evolving millions of years ago, life as we know it would be very different. "
importance of seedless plants,T_3084,"The greatest influence seedless plants have had on human society is in the formation of coal millions of years ago. When the seedless plants died, became buried deep in the Earth, and were exposed to heat and pressure, coal formed. Coal is essentially made of the fossilized carbon from these plants. Now coal is burned to provide energy, such as electricity. "
importance of seedless plants,T_3085,"But some seedless plants still have uses in society today. Peat moss is commonly used by gardeners to improve soils, since it is really good at absorbing and holding water ( Figure 1.1). Depending on the location, ferns have several different uses worldwide. Ferns are found in many gardens as ornaments, and are used as indoor plants. In tropical regions, the fern is used as a food source by many locals. The fronds can also be used to weave hats and baskets. The fiddleheads of certain species of ferns are used in gourmet food. Some species of ferns, such as the maidenhair fern, are used as medicines. In Southeast Asia, the fern is used in rice fields as a biological fertilizer. Much of the worlds fossil fuels consist of remains of ferns and their relatives. The horsetails reedy exterior and silica content made it popular as a metal polisher and abrasive cleanser. Herbalists still use horsetail to treat a variety of kidney/bladder problems, including inflammation, infection, and kidney stones, and it is used as a remedy for brittle nails. Club moss is also used to treat kidney ailments and digestive problems. Club moss spores can be dusted onto the skin and provide relief from itching and irritation, and provide the skin with protection. Extinct forests of club moss have fossilized and developed into huge beds of coal. Sphagnum, or peat moss, is commonly added to soil to help absorb water, and keep it in the soil. "
innate behavior of animals,T_3097,"Many animal behaviors are ways that animals act, naturally. They dont have to learn how to behave in these ways. Cats are natural-born hunters. They dont need to learn how to hunt. Spiders spin their complex webs without learning how to do it from other spiders. Birds and wasps know how to build nests without being taught. These behaviors are called innate. An innate behavior is any behavior that occurs naturally in all animals of a given species. An innate behavior is also called an instinct. The first time an animal performs an innate behavior, the animal does it well. The animal does not have to practice the behavior in order to get it right or become better at it. Innate behaviors are also predictable. All members of a species perform an innate behavior in the same way. From the examples described above, you can probably tell that innate behaviors usually involve important actions, like eating and caring for the young. There are many other examples of innate behaviors. For example, did you know that honeybees dance? The honeybee pictured below has found a source of food ( Figure 1.1). When the bee returns to its hive, it will do a dance. This dance is called the waggle dance. The way the bee moves during its dance tells other bees in the hive where to find the food. Honeybees can do the waggle dance without learning it from other bees, so it is an innate behavior. Besides building nests, birds have other innate behaviors. One example occurs in gulls, which are pictured below ( Figure 1.2); one of the chicks is pecking at a red spot on the mothers beak. This innate behavior causes the mother Left: This mother gull will feed her chick after it pecks at a red spot on her beak. Both pecking and feeding behaviors are innate. Right: When these baby birds open their mouths wide, their mother in- stinctively feeds them. This innate behav- ior is called gaping. to feed the chick. In many other species of birds, the chicks open their mouths wide whenever the mother returns to the nest ( Figure 1.2). This innate behavior, called gaping, causes the mother to feed them. Another example of innate behavior in birds is egg rolling. It happens in some species of water birds, like the graylag goose ( Figure 1.3). Graylag geese make nests on the ground. If an egg rolls out of the nest, a mother goose uses her bill to push it back into the nest. Returning the egg to the nest helps ensure that the egg will hatch. "
innate behavior of animals,T_3098,"All animals have innate behaviors, even human beings. Can you think of human behaviors that do not have to be learned? Chances are, you will have a hard time thinking of any. The only truly innate behaviors in humans are called reflex behaviors. They occur mainly in babies. Like innate behaviors in other animals, reflex behaviors in human babies may help them survive. An example of a reflex behavior in babies is the sucking reflex. Newborns instinctively suck on a nipple that is placed in their mouth. It is easy to see how this behavior evolved. It increases the chances of a baby feeding and surviving. Another example of a reflex behavior in babies is the grasp reflex ( Figure 1.4). Babies instinctively grasp an object placed in the palm of their hand. Their grip may be surprisingly strong. How do you think this behavior might increase a babys chances of surviving? This female graylag goose is a ground- nesting water bird. Before her chicks hatch, the mother protects the eggs. She will use her bill to push eggs back into the nest if they roll out. This is an example of an innate behavior. How could this behavior increase the gooses fitness? One of the few innate behaviors in human beings is the grasp reflex. It occurs only in babies. "
insect food,T_3099,"What do insets eat? Practically anything they want. There are so many different insects, that among all of them, no potential food is safe. Lots of insects eat plants, some insects eat other insects, and some even drink blood. Many insects eat nectar from plants. And some insects will eat whatever scraps of food you leave lying around. A few insects, such as mayflies and some moths, never eat. Thats because their lives are over in just a few hours or days. Once these insects become adults, they lay eggs, and then die. On the other hand, some insects are very healthy eaters. A silkworm eats enough leaves to increase its weight more than 4,000 times in just 56 days, as the silkworm increases in size about 10,000 times since birth. A locust eats its own weight in plants every day. Just imagine eating your own weight in food every day. You probably couldnt. You would most likely get very sick even if you tried. "
insect food,T_3100,"Insects eat in many different ways and they eat a huge range of foods. Around half are plant-eaters, feeding on leaves, roots, seeds, nectar, or wood. Aphids and leafhoppers suck up the sap from plants. Praying mantises are predators, hunting other small creatures, including insects like moths, caterpillars, flies, beetles, and spiders. Insects like mosquitoes and aphids have special mouthparts that help them pierce and suck. Others, like assassin bugs ( Figure 1.1) and certain species of female mosquitoes, eat other insects. Fleas and lice are parasites, eating the flesh or blood of larger animals without killing them. Insects have different types of appendages (arms and legs) adapted for capturing and feeding on prey. They also have special senses that help them detect prey. Furthermore, insects have a wide range of mouthparts used for feeding. An assassin bug feasts on a beetle. Examples of chewing insects include dragonflies, grasshoppers, and beetles. These insects use one pair of jaws to bite off bits of food and grind them down. Another pair of jaws helps to push the food down the throat. Some larvae also have chewing mouthparts, as in the caterpillar stages of moths and butterflies ( Figure 1.2). Caterpillar feeding on a host plant. Some insects use siphoning, as if sucking through a straw, like moths and butterflies. This long mouth-tube that they use to suck up the nectar of the flower is called a proboscis. Some moths, however, have no mouthparts at all. Some insects obtain food by sponging, like the housefly. Sponging means that the mouthpart can absorb liquid food and send it to the esophagus. The housefly is able to eat solid food by releasing saliva and dabbing it over the food. As the saliva dissolves the food, the sponging mouthpart absorbs the liquid food. Sponging Chewing Siphoning Used to suck liquids "
insect reproduction and life cycle,T_3101,"Most insects can reproduce very quickly within a short period of time. With a short generation time, they evolve faster and can quickly adjust to environmental changes. Most insects reproduce by sexual reproduction. The female produces eggs, which are fertilized by the male, and then the eggs are usually placed near the required food. In some insects, there is asexual reproduction during which the offspring come from a single parent. In this type of reproduction, the offspring are almost identical to the mother. This is most often seen in aphids and scale insects. With a few exceptions, all insect life begins as an egg. After leaving the egg, insects must grow and transform until reaching adulthood. Only the adult insect can mate and reproduce. The physical transformation of an insect from one stage of its life cycle to another is known as metamorphosis. "
insect reproduction and life cycle,T_3102,"An insect can have one of three types of metamorphosis and life cycles ( Table 1.1). Metamorphosis describes how insects transform from an immature or young insect into an adult insect in at least two stages. Insects may undergo gradual metamorphosis (incomplete), where transformation is subtle, or complete metamorphosis, where each stage of the life cycle appears quite different from the others. In some insects, there may be no true metamorphosis at all. Type of Metamorphosis None Characteristics Examples Silverfish, firebrats, springtails Only difference between adult and larvae (young or non-adult insects) is size. Occurs in the most primitive insects. Newborn insect looks like a tiny version of the adult. Incomplete Three stages: egg, nymph, and adult. Young, called nymphs, usu- ally similar to adult. Growth occurs during the nymph stage. Wings then appear as buds on nymphs or early forms. When last molt is completed, wings expand to full adult size. Dragonflies, grasshoppers, mantids, cockroaches, termites Type of Metamorphosis Complete Characteristics Most insects undergo this type. Each stage of the life cy- cleegg, larva, pupa, and adultlooks different from the others. Immature and adult stages have different forms, have different behaviors, and live in different habitats. Immature form is called lar- vae and remains similar in form but increases in size. Larvae usually have chew- ing mouthparts even if adult mouthparts are sucking ones. At last larval stage of de- velopment, insect forms into pupa ( Figure 1.1) and does not eat or move. During pupa stage, wing development begins, after which the adult emerges. Examples Butterflies, moths, flies, ants, bees, beetles The chrysalis (pupal stage) of a monarch butterfly. "
insects,T_3103,"Insects, with over a million described species, are the most diverse group of animals on Earth. They may be found in nearly all environments on the planet. No matter where you travel, you will see organisms from this group. Adult insects range in size from a minuscule fairy fly to a 21.9-inch-long stick insect ( Figure 1.1). "
insects,T_3104,"Characteristics of Insects include: Segmented bodies with an exoskeleton. The outer layer of the exoskeleton is called the cuticle. It is made up of two layers. The outer layer, or exocuticle, is thin, waxy, and water-resistant. The inner layer is much thicker. The exocuticle is extremely thin in many soft-bodied insects, such as caterpillars. The segments of the body are organized into three distinct but joined units: a head, a thorax, and an abdomen ( Figure 1.2 and Table 1.1). A diagram of a human and an insect, com- paring the three main body parts: head, thorax, and abdomen. Structure Head Thorax Abdomen Description A pair of antennae, a pair of compound eyes, and three sets of appendages that form the mouthparts. Six segmented legs and two or four wings. Contains most of the digestive, respiratory, excretory, and reproductive structures. A nervous system that is divided into a brain and a ventral nerve cord. Respiration that occurs without lungs. Insects have a system of internal tubes and sacs that oxygen travels through to reach body tissues. Air is taken in through the spiracles, openings on the sides of the abdomen. A closed digestive system, with one long enclosed coiled tube which runs lengthwise through the body, from the mouth to the anus. A circulatory system that is simple and consists of only a single tube with openings. The tube pulses and circulates blood-like fluids inside the body cavity. Various types of movement. Insect movement can include flight, walking, and swimming. Insects were the Fireflies Reproduction and predation: Some species produce flashes to attract mates; other species to attract prey. Sound Production By moving appendages Cicadas Ultrasound clicks Moths Loudest sounds among insects; have special muscles to produce sounds. Predation: Produced mostly by moths to warn bats. Chemical Wide range of insects have evolved chemical communication; chemi- cals are used to attract, repel, or provide other kinds of information; use of scents is especially well de- veloped in social insects. Dance Language Moths Honey bees Antennae of males ( Figure 1.4) can detect pheromones (chemicals released by animals that influence the behavior of others within the same species) of female moths over distances of many miles. Honey bees are the only inverte- brates to have evolved this type of communication; length of dance represents distance to be flown. "
insects,T_3105,"Social insects, such as termites, ants, and many bees and wasps ( Figure 1.5), are the most familiar social species. They live together in large, well-organized colonies. Only those insects which live in nests or colonies can home. Homing means that an insect can return to a single hole among many other apparently identical holes, even after a long trip or after a long time. A few insects migrate in groups. For example, the monarch butterfly flies between Mexico and North America each spring and fall ( Figure 1.5). (left) Damage to this nest brings the work- ers and soldiers of this social insect, the termite, to repair it. (center ) A wasp build- ing its nest. (right) Monarch butterflies in an overwintering cluster. "
insects,T_3106,"Insects are divided into two major groups: 1. Wingless: Consists of two orders, the bristle tails and the silverfish. 2. Winged insects: All other orders of insects. They are named below. Mayflies; dragonflies and damselflies; stoneflies; webspinners; angel insects; earwigs; grasshoppers, crickets, and katydids; stick insects; ice-crawlers and gladiators; cockroaches and termites; mantids; lice; thrips; true bugs, aphids, and cicadas; wasps, bees, and ants; beetles; twisted-winged parasites; snakeflies; alderflies and dobsonflies; lacewings and antlions; hangingflies (including fleas); true flies; caddisflies; and butterflies, moths, and skippers. "
introduction to ecology,T_3107,"Life Science can be studied at many different levels. You can study small things like cells. Or you can study big things like a group of animals. You can also study the biosphere, which is any area in which organisms live. The study of the biosphere is part of ecology, the study of how living organisms interact with each other and with their environment. "
introduction to ecology,T_3108,"Ecology involves many different fields, including geology, soil science, geography, meteorology, genetics, chemistry, and physics. You can also divide ecology into the study of different organisms, such as animal ecology, plant ecology, insect ecology, and so on. Ecologists also study biomes. A biome is a large community of plants and animals that live in the same place. For example, ecologists can study the biomes as diverse as the Arctic, the tropics, or the desert ( Figure 1.1). They may want to know why different species live in different biomes. They may want to know what would make a particular biome or ecosystem stable. Can you think of other aspects of a biome or ecosystem that ecologists could study? Ecologists do two types of research: An example of a biome, the Atacama Desert, in Chile. 1. Field studies. 2. Laboratory studies. Field studies involve collecting data outside in the natural world. An ecologist who completes a field study may travel to a tropical rainforest to study, count, and classify all of the insects that live in a certain area. Laboratory studies involve working inside, usually in a controlled environment. Sometimes, ecologists collect data from the field, and then they analyze that data in the lab. Also, they use computer programs to predict what will happen to organisms that live in a specific area. For example, they may make predictions about what happens to insects in the rainforest after a fire. "
introduction to ecology,T_3109,"All organisms have the ability to grow and reproduce. To grow and reproduce, organisms must get materials and energy from the environment. Plants obtain their energy from the sun through photosynthesis, whereas animals obtain their energy from other organisms. Either way, these plants and animals, as well as the bacteria and fungi, are constantly interacting with other species as well as the non-living parts of their ecosystem. An organisms environment includes two types of factors: 1. Abiotic factors are the parts of the environment that are not living, such as sunlight, climate, soil, water, and air. 2. Biotic factors are the parts of the environment that are alive, or were alive and then died, such as plants, animals, and their remains. Biotic factors also include bacteria, fungi and protists. Ecology studies the interactions between biotic factors, such as organisms like plants and animals, and abiotic factors. For example, all animals (biotic factors) breathe in oxygen (abiotic factor). All plants (biotic factor) absorb carbon dioxide (abiotic factor) and need water (abiotic factor) to survive. Can you think of another way that abiotic and biotic factors interact with each other? "
invertebrates,T_3110,"Animals are often identified as being either invertebrates or vertebrates. These are terms based on the skeletons of the animals. Vertebrates have a backbone made of bone or cartilage ( cartilage is a flexible supportive tissue. You have cartilage in your ear lobes.). Invertebrates, on the other hand, have no backbone ( Figure 1.1). Invertebrates live just about anywhere. There are so many invertebrates on this planet that it is impossible to count them all. There are probably billions of billions of invertebrates. They come in many shapes and sizes, live practically anywhere and provide many services that are vital for the survival of other organisms, including us. They have been observed in the upper reaches of the atmosphere, in the driest of the deserts and in the canopies of the wettest rainforests. They can even be found in the frozen Antarctic or on the deepest parts of the ocean floor. Snails are an example of invertebrates, animals without a backbone. All vertebrate organisms are in the phylum Chordata. Invertebrates, which make up about 95% (or more) of the animal kingdom, are divided into over 30 different phyla, some of which are listed below ( Table 1.1). Numerous invertebrate phyla have just a few species; some have only one described species, yet these are classified into separate phyla because of their unique characteristics. For example, sponges, with pores throughout their body, are from the phylum Porifera. Crabs and lobsters, with jointed appendages, are from the phylum Arthropoda. Phylum Porifera Cnidaria Platyhelminthes Nematoda Mollusca Annelida Arthropoda Echinodermata Meaning Pore bearer Stinging nettle Flat worms Thread like Soft Little ring Jointed foot Spiny skin Examples Sponges Jellyfish, corals Flatworms, tapeworms Nematodes, heartworm Snails, clams Earthworms, leeches Insects, crabs Sea stars, sea urchins "
learned behavior of animals,T_3128,"Just about all human behaviors are learned. Learned behavior is behavior that occurs only after experience or practice. Learned behavior has an advantage over innate behavior: it is more flexible. Learned behavior can be changed if conditions change. For example, you probably know the route from your house to your school. Assume that you moved to a new house in a different place, so you had to take a different route to school. What if following the old route was an innate behavior? You would not be able to adapt. Fortunately, it is a learned behavior. You can learn the new route just as you learned the old one. Although most animals can learn, animals with greater intelligence are better at learning and have more learned behaviors. Humans are the most intelligent animals. They depend on learned behaviors more than any other species. Other highly intelligent species include apes, our closest relatives in the animal kingdom. They include chimpanzees and gorillas. Both are also very good at learning behaviors. You may have heard of a gorilla named Koko. The psychologist, Dr. Francine Patterson, raised Koko. Dr. Patterson wanted to find out if gorillas could learn human language. Starting when Koko was just one year old, Dr. Patterson taught her to use sign language. Koko learned to use and understand more than 1,000 signs. Koko showed how much gorillas can learn. See A Conversation with Koko at  . Think about some of the behaviors you have learned. They might include riding a bicycle, using a computer, and playing a musical instrument or sport. You probably did not learn all of these behaviors in the same way. Perhaps you learned some behaviors on your own, just by practicing. Other behaviors you may have learned from other people. Humans and other animals can learn behaviors in several different ways. The following methods of learning will be explored below: 1. 2. 3. 4. 5. Habituation (forming a habit) Observational learning Conditioning Play Insight learning "
learned behavior of animals,T_3129,"Habituation is learning to get used to something after being exposed to it for a while. Habituation usually involves getting used to something that is annoying or frightening, but not dangerous. Habituation is one of the simplest ways of learning. It occurs in just about every species of animal. You have probably learned through habituation many times. For example, maybe you were reading a book when someone turned on a television in the same room. At first, the sound of the television may have been annoying. After a while, you may no longer have noticed it. If so, you had become habituated to the sound. Another example of habituation is shown below ( Figure 1.1). Crows and most other birds are usually afraid of people. They avoid coming close to people, or they fly away when people come near them. The crows landing on this scarecrow have become used to a human in this place. They have learned that the scarecrow poses no danger. They are no longer afraid to come close. They have become habituated to the scarecrow. Can you see why habituation is useful? It lets animals ignore things that will not harm them. Without habituation, animals might waste time and energy trying to escape from things that are not really dangerous. "
learned behavior of animals,T_3130,"Observational learning is learning by watching and copying the behavior of someone else. Human children learn many behaviors this way. When you were a young child, you may have learned how to tie your shoes by watching your dad tie his shoes. More recently, you may have learned how to dance by watching a pop star dancing on TV. Most likely, you have learned how to do math problems by watching your teachers do problems on the board at school. Can you think of other behaviors you have learned by watching and copying other people? Other animals also learn through observational learning. For example, young wolves learn to be better hunters by watching and copying the skills of older wolves in their pack. Another example of observational learning is how some monkeys have learned to wash their food. They learned by watching and copying the behavior of other monkeys. "
learned behavior of animals,T_3131,"Conditioning is a way of learning that involves a reward or punishment. Did you ever train a dog to fetch a ball or stick by rewarding it with treats? If you did, you were using conditioning. Another example of conditioning is shown in the video below; the rats have been taught to play basketball by being rewarded with food pellets. What do you think would happen if the rats were no longer rewarded for this behavior? Click image to the left or use the URL below. URL:  Conditioning also occurs in wild animals. For example, bees learn to find nectar in certain types of flowers because they have found nectar in those flowers before. Humans learn behaviors through conditioning, as well. A young child might learn to put away his toys by being rewarded with a bedtime story. An older child might learn to study for tests in school by being rewarded with better grades. Can you think of behaviors you have learned by being rewarded for them? Conditioning does not always involve a reward. It can involve a punishment, instead. A toddler might be punished with a time-out each time he grabs a toy from his baby brother. After several time-outs, he may learn to stop taking his brothers toys. A dog might be scolded each time she jumps up on the sofa. After repeated scolding, she may learn to stay off the sofa. A bird might become ill after eating a poisonous insect. The bird may learn from this punishment to avoid eating the same kind of insect in the future. "
learned behavior of animals,T_3132,"Most young mammals, including humans, like to play. Play is one ways they learn the skills that they will need as adults. Think about how kittens play. They pounce on toys and chase each other. This helps them learn how to be better predators when they are older. Big cats also play. The lion cubs pictured below are playing and practicing their hunting skills at the same time ( Figure 1.2). The dogs are playing tug-of-war with a toy ( Figure 1.2). What do you think they are learning by playing together this way? Other young animals play in different ways. For example, young deer play by running and kicking up their hooves. This helps them learn how to escape from predators. Left: These two lion cubs are playing. They are not only having fun, but they are also learning how to be better hunters. Right: These dogs are really playing. This play fighting can help them learn how to be better predators. Human children learn by playing as well. For example, playing games and sports can help them learn to follow rules and work with others. The toddlers pictured below are playing in the sand ( Figure 1.3). They are learning about the world through play. What do you think they might be learning? Playing in a sandbox is fun for young children. It can also help them learn about the world. "
learned behavior of animals,T_3133,"Insight learning is learning from past experiences and reasoning. It usually involves coming up with new ways to solve problems. Insight learning generally happens quickly. An animal has a sudden flash of insight. Insight learning requires relatively great intelligence. Human beings use insight learning more than any other species. They have used their intelligence to solve problems ranging from inventing the wheel to flying rockets into space. Think about problems you have solved. Maybe you figured out how to solve a new type of math problem or how to get to the next level of a video game. If you relied on your past experiences and reasoning to do it, then you were using insight learning. One type of insight learning is making tools to solve problems. Scientists used to think that humans were the only animals intelligent enough to make tools. In fact, tool-making was believed to set humans apart from all other animals. In 1960, primate expert Jane Goodall discovered that chimpanzees also make tools. She saw a chimpanzee strip leaves from a twig. Then he poked the twig into a hole in a termite mound. After termites climbed onto the twig, he pulled the twig out of the hole and ate the insects clinging to it. The chimpanzee had made a tool to fish for termites. He had used insight to solve a problem. Since then, chimpanzees have been seen making several different types of tools. For example, they sharpen sticks and use them as spears for hunting. They use stones as hammers to crack open nuts. Scientists have also observed other species of animals making tools to solve problems. A crow was seen bending a piece of wire into a hook. Then the crow used the hook to pull food out of a tube. An example of a gorilla using a walking stick is shown below ( Figure 1.4). Behaviors such as these show that other species of animals can use their experience and reasoning to solve problems. They can learn through insight. This gorilla is using a branch as a tool. She is leaning on it to keep her balance while she reaches down into swampy water to catch a fish. "
levels of ecological organization,T_3134,"Ecosystems can be studied at small levels or at large levels. The levels of organization are described below from the smallest to the largest: A species is a group of individuals that are genetically related and can breed to produce fertile young. Individuals are not members of the same species if their members cannot produce offspring that can also have children. The second word in the two word name given to every organism is the species name. For example, in Homo sapiens, sapiens is the species name. A population is a group of organisms belonging to the same species that live in the same area and interact with one another. A community is all of the populations of different species that live in the same area and interact with one another. A community is composed of all of the biotic factors of an area. An ecosystem includes the living organisms (all the populations) in an area and the non-living aspects of the environment ( Figure 1.1). An ecosystem is made of the biotic and abiotic factors in an area. Satellite image of Australias Great Barrier Reef, an example of a marine ecosys- tem. The biosphere is the part of the planet with living organisms ( Figure 1.2). The biosphere includes most of Earth, including part of the oceans and the atmosphere. Ecologists study ecosystems at every level, from the individual organism to the whole ecosystem and biosphere. They can ask different types of questions at each level. Examples of these questions are given in Table 1.1, using the zebra (Equus zebra) as an example. Ecosystem Level Individual Population Community Ecosystem Question How do zebras keep water in their bodies? What causes the growth of a zebra populations? How does a disturbance, like a fire or predator, affect the number of mammal species in African grasslands? How does fire affect the amount of food available in grassland ecosystems? "
lizards and snakes,T_3145,"Lizards and snakes belong to the largest order of reptiles, Squamata. Lizards are a large group of reptiles, with nearly 5,000 species, living on every continent except Antarctica. Some places are just too cold for lizards. "
lizards and snakes,T_3146,"Lizards and snakes are distinguished by scales or shields and movable quadrate bones, which make it possible to open the upper jaw very wide. Quadrate bones are especially visible in snakes, because they are able to open their mouths very wide to eat large prey ( Figure 1.1). Without this ability, the snake diet would be very limited. "
lizards and snakes,T_3147,"Key features of lizards include: Four limbs. External ears. Movable eyelids. A short neck. A long tail, which they can shed in order to escape from predators. They eat insects. Vision, including color vision, is well-developed in lizards. You may have seen a lizard camouflaged to blend in with its surroundings. Since they have great vision, lizards communicate by changing the color of their bodies. They also communicate with chemical signals called pheromones. Adult lizards range from one inch in length, like some Caribbean geckos, to the nearly 10-foot-long Komodo dragon ( Figure 1.2). A Komodo dragon, the largest of the lizards, attaining a length of ten feet. Ko- modo dragons will eat just about anything and they often attack deer, goats, pigs, dogs and, occasionally, humans. With 40 lizard families, there is an extremely wide range of color, appearance, and size of lizards. Many lizards are capable of regenerating lost limbs or tails. Almost all lizards are carnivorous, meaning they eat animals, although most are so small that insects are their primary prey. However, some have reached sizes where they can prey on birds and mammals. On the other hand, a few species of lizards exclusively eat plants. "
lizards and snakes,T_3148,"Have you ever tried catching a lizard? Many lizards are good climbers or fast sprinters. Some can run on two feet, such as the collared lizard. Some, like the basilisk, can even run across the surface of water to escape danger. Many lizards can change color in response to their environments or in times of stress ( Figure 1.3). The most familiar example is the chameleon, but more subtle color changes can occur in other lizard species. A species of lizard, showing general body form and camouflage against back- ground. "
lizards and snakes,T_3149,"Some lizard species, including the glass lizard and flap-footed lizards, have evolved to lose their legs, or their legs are so small that they no longer work. This provides these species an evolutionary advantage in their way of life. Legless lizards almost look like snakes, though structures leftover from earlier stages of evolution remain. For example, flap-footed lizards can be distinguished from snakes by their external ears. "
lizards and snakes,T_3150,"Snakes are different from legless lizards because they do not have eyelids, limbs, external ears, or forelimbs. The more than 2,700 species of snake can be found on every continent except Antarctica and range in size from the tiny, 4-inch-long thread snake to pythons, to the over 17-foot-long anaconda ( Figure 1.4). In order to fit inside of snakes narrow bodies, paired organs, such as kidneys, appear one in front of the other instead of side by side. Snakes eyelids are transparent spectacle scales which remain permanently closed. Most snakes are not venomous, but some have venom capable of causing painful injury or death to humans. However, snake venom is primarily used for killing prey rather than for self-defense. Snakes that are kept as pets can have their venom removed without affecting the health of the snake. Most snakes use specialized belly scales, which grip surfaces to move ( Figure 1.5). In the shedding of scales, known as molting, the complete outer layer of skin is shed in one layer ( Figure 1.6). Molting replaces old and worn skin, allows the snake to grow, and helps it get rid of parasites such as mites and ticks. Although different snake species reproduce in different ways, all snakes use internal fertilization, where fertiliza- tion of the egg takes place inside the female. The male uses sex organs stored in its tail to transfer sperm to the female. Most species of snakes lay eggs, and most species abandon these eggs shortly after laying them. A species of anaconda, one of the largest snakes, which can be as long as 17 feet. A close-up of scales on a scarlet kingsnake, showing a tricolored pattern of red, black, and white bands. Notice the distinction between the belly scales and the rest of the snakes scales. "
lizards and snakes,T_3151,"All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails, or insects. Because snakes cannot bite or tear their food to pieces, prey must be swallowed whole. Therefore, the body size of a snake has a major influence on its eating habits. A snake can usually estimate in advance if a prey is too large. The snakes jaw is unique in the animal kingdom. Snakes have a very flexible lower jaw, the two halves of which are not rigidly attached. They also have many other joints in their skull, allowing them to open their mouths wide A Centralian carpet python shedding its skin. enough to swallow their prey whole. Some snakes have a venomous bite, which they use to kill their prey before eating it. Other snakes kill their prey by strangling them, and still others swallow their prey whole and alive. After eating, snakes enter a resting stage, while the process of digestion takes place. The process is highly efficient, with the snakes digestive enzymes dissolving and absorbing everything but the preys hair and claws! "
mammal characteristics,T_3158,"What is a mammal? These animals range from bats, cats, and rats to dogs, monkeys, elephants, and whales. They walk, run, swim, and fly. They live in the ocean, fly in the sky, walk on the prairies, and run in the savanna. There is a tremendous amount of diversity within the group in terms of reproduction, habitat, and adaptation for living in those different habitats. What allows them to live in such diverse environments? They have evolved specialized traits, unlike those of any other group of animal. Mammals (class Mammalia) are endothermic (warm-blooded) vertebrate animals with a number of unique characteristics. In most mammals, these include: The presence of hair or fur. Sweat glands. Glands specialized to produce milk, known as mammary glands. Three middle ear bones. A neocortex region in the brain, which specializes in seeing and hearing. Specialized teeth. A four-chambered heart. There are approximately 5,400 mammalian species, ranging in size from the tiny 1-2 inch bumblebee bat to the 108-foot blue whale. These are distributed in 29 orders, 153 families, and about 1,200 genera. There are three types of mammals, characterized by their method of reproduction. All mammals, except for a few, are viviparous, meaning they produce live young instead of laying eggs. The monotremes, however, have birdlike and reptilian characteristics, such as laying eggs and a cloaca. An example of a monotreme is the platypus with its birdlike beak and egg-laying characteristics. The echidnas are the only other monotreme mammals. A second type of mammal, the marsupial mammal, includes kangaroos, wallabies, koalas and possums. These mammals give birth to underdeveloped embryos, which then climb from the birth canal into a pouch on the front of the mothers body, where it feeds and continues to grow. The remainder of mammals, which is the majority of mammals, are placental mammals. These mammals develop in the mothers uterus, receiving nutrients across the placenta. Placental mammals include humans, rabbits, squirrels, whales, elephants, shrews, and armadillos. Dogs and cats, and sheep, cattle and horses are also placental mammals. Mammals are also the only animal group that evolved to live on land and then back to live in the ocean. Whales, dolphins, and porpoises have all adapted from land-dwelling creatures to a life of swimming and reproducing in the water ( Figure 1.1). Whales have evolved into the largest mammals. See Mammals- San Diego Kids at http://kids.sandiegozoo.org/animals/mammals and The Cheetah Orphans at material. Listen to They Might Be Giants - Mammal at  for a description of numerous mammal traits. "
mammal classification,T_3159,"Traditionally, mammals were divided into groups based on their characteristics. Scientists took into consideration their anatomy (body structure), their habitats, and their feeding habits. Mammals are divided into three subclasses and about 26 orders. Some of the groups of mammals include: 1. Lagomorphs include hares and rabbits. Rabbits and hares characteristically have long ears, a short tail, and strong hind limbs that provide for a bouncing method of locomotion. They are all are small to medium-sized terrestrial herbivores. 2. Rodents include rats, mice, and other small gnawing mammals. They have a single pair of continuously growing incisors (teeth) in each of the upper and lower jaws that must be kept short by gnawing. 3. Carnivores include cats and lions and tigers, dogs and wolves, polar bears, and other meat eaters. 4. Insectivores include moles and shrews ( Figure 1.1). These mammals eat primarily insects, other arthropods, and earthworms. 5. Bats include the vampire bat. These mammals have forelimbs that form webbed wings, making bats the only mammals naturally capable of true and sustained flight. One of the subgroups of mammals is the insectivores, including this shrew. 6. Primates include monkeys, apes and humans. These mammals are characterized by detailed development of the hands and feet, a shortened snout, and a large brain. 7. Ungulates include hoofed animals, such as deer, sheep, goats, pigs, buffalo, elephants and giraffes ( Figure a thick nail rolled around the tip of the toe. The ungulates (hoofed animals), like the giraffe here, is one of the subgroups of mammals. Mammals can also be grouped according to the adaptations they form to live in a certain habitat. For example, terrestrial mammals with leaping kinds of movement, as in some marsupials and lagomorphs, typically live in open "
mammal reproduction,T_3160,"You probably realize that cats, dogs, people, and other mammals dont typically lay eggs. There are exceptions, however. Egg-laying is possible among the monotremes, mammals with birdlike and reptilian characteristics. Recall that mammals can be classified into three general groups, based on their reproductive strategy: the monotremes, the marsupials and the placental mammals. The egg-laying monotremes, such as echidnas ( Figure 1.1) and platypuses ( Figure 1.1), use one opening, the cloaca, to urinate, release waste, and reproduce, just like birds. They lay leathery eggs, similar to those of lizards, turtles, and crocodilians. Monotremes feed their young by sweating milk from patches on their bellies, as they lack the nipples present on other mammals. All other mammals give birth to live young and belong to one of two different categories, the marsupials and the placental mammals. A marsupial is an animal in which the embryo, which is often called a joey, is born at an immature stage. Development must be completed outside the mothers body. Most female marsupials have an abdominal pouch or skin fold where there are mammary glands. The pouch is a place for completing the development of the baby. Although blind, without fur, and with only partially formed hind legs, the tiny newborns have well developed forelimbs with claws that enable them to climb their way into their mothers pouch where they drink their mothers milk and continue their development. Marsupials include kangaroos, koalas, and opossums. Other marsupials are the wallaby and the Tasmanian Devil. Most marsupials live in Australia and nearby areas. ( Figure The echidna (right) is a member of the monotremes, the most primitive order of mammals. Another monotreme, the platy- pus (left), like other mammals in this or- der, lays eggs and has a single opening for the urinary, genital, and digestive or- gans. The majority of mammals are placental mammals. These are mammals in which the developing baby is fed through the mothers placenta. Female placental mammals develop a placenta after fertilization. A placenta is a spongy structure that passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta allows the fetus to grow for a long time within the mother. A marsupial mammal, this eastern gray kangaroo has a joey (young kangaroo) in its abdominal pouch. Some mammals are alone until a female can become pregnant. Others form social groups with big differences between sexes, such as size differences, a trait called sexual dimorphism. Dominant males are those that are the largest or best-armed. These males usually have an advantage in mating. They may also keep other males from mating with females within a group. This is seen in elephant seals ( Figure 1.3), and also with elk, lions and non- human primates, including the orangutans and gorillas. Male elk grow antlers, while female elk do not have antlers. Adult male lions are not only larger than females, they have a mane of long hair on the side of the face and top of the head. "
mass extinctions,T_3161,"An organism goes extinct when all of the members of a species die out and no more members remain. Extinctions are part of natural selection. Species often go extinct when their environment changes, and they do not have the traits they need to survive. Only those individuals with the traits needed to live in a changed environment survive (Survival of the Fittest) ( Figure 1.1). Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes such as volcanic eruptions and earthquakes ( Figure 1.2). Since life began on Earth, there have been several major mass extinctions. If you look closely at the geological time scale, you will find that at least five major mass extinctions have occurred in the past 540 million years. In each mass extinction, over 50% of animal species died. Though species go extinct frequently, a mass extinction in which such a high percentage of species go extinct is rare. The total number of mass extinctions could be as high as 20. It is probable that we are currently in the midst of another mass extinction. Two of the largest extinctions are described below: The fossil of Tarbosaurus, one of the land dinosaurs that went extinct during one of the mass extinctions. At the end of the Permian Period, it is estimated that about 99.5% of individual organisms went extinct! Up to 95% of marine species perished, compared to only 70% of land species. Some scientists theorize that the extinction was caused by the formation of Pangaea, or one large continent made out of many smaller ones. One large continent has a smaller shoreline than many small ones, so reducing the shoreline space may have The supercontinent Pangaea encompassed all of todays continents in a single land mass. This configuration limited shallow coastal areas which harbor marine species. This may have contributed to the dramatic event which ended the Permianthe most massive extinction ever recorded. At the end of the Cretaceous Period, or 65 million years ago, all dinosaurs (except those which led to birds) went extinct. Some scientists believe a possible cause is a collision between the Earth and a comet or asteroid. The collision could have caused tidal waves, changed the climate, increased atmospheric dust and clouds, and reduced sunlight by 10-20%. A decrease in photosynthesis would have resulted in less plant food, leading to the extinction of the dinosaurs. Evidence for the extinction of dinosaurs by asteroid includes an iridium-rich layer in the Earth, dated at 65.5 million years ago. Iridium is rare in the Earths crust but common in comets and asteroids. Maybe the asteroid that hit the Earth left the iridium behind. After each mass extinction, new species evolve to fill the habitats where old species lived. This is well documented in the fossil record. "
mendels laws and genetics,T_3167,Do you remember what happened when Mendel crossed purple flowered-plants and white flowered-plants? All the offspring had purple flowers. There was no blending of traits in any of Mendels experiments. Mendel had to come up with a theory of inheritance to explain his results. He developed a theory called the law of segregation. 
mendels laws and genetics,T_3168,"Mendel proposed that each pea plant had two hereditary factors for each trait. There were two possibilities for each hereditary factor, such as a purple factor or white factor. One factor is dominant to the other. The other trait that is masked is called the recessive factor, meaning that when both factors are present, only the effects of the dominant factor are noticeable ( Figure 1.1). Although you have two hereditary factors for each trait, each parent can only pass on one of these factors to the offspring. When the sex cells, or gametes (sperm or egg), form, the heredity factors must separate, so there is only one factor per gamete. In other words, the factors are ""segregated"" in each gamete. Mendels law of segregation states that the two hereditary factors separate when gametes are formed. When fertilization occurs, the offspring receive one hereditary factor from each gamete, so the resulting offspring have two factors. The law of segregation predates our understanding or meiosis. Mendel developed his theories without an under- standing of DNA, or even the knowledge that DNA existed. Quite a remarkable feat! In peas, purple flowers are dominant to white. If one of these purple flowers is crossed with a white flower, all the offspring will have purple flowers. "
mendels laws and genetics,T_3169,"This law explains what Mendel had seen in the F1 generation when a tall plant was crossed with a short plant. The two heredity factors in this case were the short and tall factors. Each individual in the F1 would have one of each factor, and as the tall factor is dominant to the short factor (the recessive factor), all the plants appeared tall. In describing genetic crosses, letters are used. The dominant factor is represented with a capital letter (T for tall) while the recessive factor is represented by a lowercase letter (t). For the T and t factors, three combinations are possible: TT, Tt, and tt. TT plants will be tall, while plants with tt will be short. Since T is dominant to t, plants that are Tt will be tall because the dominant factor masks the recessive factor. In this example, we are crossing a TT tall plant with a tt short plant. As each parent gives one factor to the F1 generation, all of the F1 generation will be Tt tall plants. When the F1 generation (Tt) is allowed to self-pollinate, each parent will give one factor (T or t) to the F2 generation. So the F2 offspring will have four possible combinations of factors: TT, Tt, tT, or tt. According to the laws of probability, 25% of the offspring would be tt, so they would appear short. And 75% would have at least one T factor and would be tall. "
mendels pea plants,T_3170,"What does the word ""inherit"" mean? You may have inherited something of value from a grandparent or another family member. To inherit is to receive something from someone who came before you. You can inherit objects, but you can also inherit traits. For example, you can inherit a parents eye color, hair color, or even the shape of your nose and ears! Genetics is the study of inheritance. The field of genetics seeks to explain how traits are passed on from one generation to the next. In the late 1850s, an Austrian monk named Gregor Mendel ( Figure 1.1) performed the first genetics experiments. To study genetics, Mendel chose to work with pea plants because they have easily identifiable traits ( Figure 1.2). For example, pea plants are either tall or short, which is an easy trait to observe. Furthermore, pea plants grow quickly, so he could complete many experiments in a short period of time. Mendel also used pea plants because they can either self-pollinate or be cross-pollinated. Self-pollination means that only one flower is involved; the flowers own pollen lands on the female sex organs. Cross pollination is done by hand by moving pollen from one flower to the stigma of another (just like bees do naturally). As a result, one plants sex cells combine with another plants sex cells. This is called a ""cross."" These crosses produce offspring Gregor Mendel, the ""father"" of genetics. Characteristics of pea plants. (or ""children""), just like when male and female animals mate. Since Mendel could move pollen between plants, he could carefully control and then observe the results of crosses between two different types of plants. He studied the inheritance patterns for many different traits in peas, including round seeds versus wrinkled seeds, white flowers versus purple flowers, and tall plants versus short plants. Because of his work, Mendel is considered the ""Father of Genetics."" "
mendels pea plants,T_3171,"In one of Mendels early experiments, he crossed a short plant and a tall plant. What do you predict the offspring of these plants were? Medium-sized plants? Most people during Mendels time would have said medium-sized. But an unexpected result occurred. Mendel observed that the offspring of this cross (called the F1 generation) were all tall plants! Next, Mendel let the F1 generation self-pollinate. That means the tall plant offspring were crossed with each other. He found that 75% of their offspring (the F2 generation) were tall, while 25% were short. Shortness skipped a generation. But why? In all, Mendel studied seven characteristics, with almost 20,000 F2 plants analyzed. All of his results were similar to the first experimentabout three out of every four plants had one trait, while just one out of every four plants had the other. For example, he crossed purple flowered-plants and white flowered-plants. Do you think the colors blended? No, they did not. Just like the previous experiment, all offspring in this cross (the F1 generation) were one color: purple. In the F2 generation, 75% of plants had purple flowers and 25% had white flowers ( Figure 1.3). There was no blending of traits in any of Mendels experiments. The results of Mendels experiment with purple flowered and white flowered-plants numerically matched the results of his experiments with other pea plant traits. "
microevolution and macroevolution,T_3173,"Does evolution only happen gradually through small changes? Or is it possible that drastic environmental changes can cause new species to evolve? Or can both small and large changes occur? Evolutionary changes can be both big and small. Some evolutionary changes do not create new species, but result in changes at the population level. A population is a group of organisms of the same species that live in the same area ( Figure 1.1). But what exactly is the definition of a species? A species is a group of organisms that have similar characteristics (they are genetically similar) and can mate with one another to produce fertile offspring. This school of fish are considered mem- bers of the same species because they are able to mate with one another. They are also considered a population because they live in the same part of the ocean. "
microevolution and macroevolution,T_3174,"You already know that evolution is the change in species over time. Most evolutionary changes are small and do not lead to the creation of a new species. When populations change in small ways over time, the process is called microevolution. Microevolution results in changes within a species. An example of microevolution is the evolution of mosquitoes that cannot be killed by pesticides, called pesticide- resistant mosquitoes. Imagine that you have a pesticide that kills most of the mosquitoes in your state. Through a random mutation, some of the mosquitoes have resistance to the pesticide. As a result of the widespread use of this pesticide, most of the remaining mosquitoes are the pesticide-resistant mosquitoes. When these mosquitoes repro- duce the next year, they produce more mosquitoes with the pesticide-resistant trait. Soon, most of the mosquitoes in your state are resistant to the pesticide. This is an example of microevolution because the number of mosquitoes with this trait changed. However, this evolutionary change did not create a new species of mosquito because the pesticide-resistant mosquitoes can still reproduce with other non-pesticide-resistant mosquitoes. "
microevolution and macroevolution,T_3175,"Macroevolution refers to much bigger evolutionary changes that result in new species. Macroevolution may happen: 1. When microevolution occurs repeatedly over a long period of time and leads to the creation of a new species. 2. As a result of a major environmental change, such as a volcanic eruption, earthquake, or an asteroid hitting Earth, which changes the environment so much that natural selection leads to large changes in the traits of a species. After thousands of years of isolation from each other, Darwins finch populations have experienced both microevo- lution and macroevolution. These finch populations cannot breed with other finch populations when they are brought together. Since they do not breed together, they are classified as separate species. "
modern genetics,T_3184,"Mendel laid the foundation for modern genetics, but there were still a lot of questions he left unanswered. What exactly are the dominant and recessive factors that determine how all organisms look? And how do these factors work? Since Mendels time, scientists have discovered the answers to these questions. Genetic material is made out of DNA. It is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge of DNA and chromosomes, we can explain Mendels findings and build on them. In this concept, we will explore the connections between Mendels work and modern genetics. "
modern genetics,T_3185,"Recall that our DNA is wound into chromosomes. Each of our chromosomes contains a long chain of DNA that encodes hundreds, if not thousands, of genes. Each of these genes can have slightly different versions from individual to individual. These variants of genes are called alleles. Each parent only donates one allele for each gene to an offspring. For example, remember that for the height gene in pea plants there are two possible factors. These factors are alleles. There is a dominant allele for tallness (T) and a recessive allele for shortness (t). "
modern genetics,T_3186,"Genotype is a way to describe the combination of alleles that an individual has for a certain gene ( Table 1.1). For each gene, an organism has two alleles, one on each chromosome of a homologous pair of chromosomes (think of it as one allele from Mom, one allele from Dad). The genotype is represented by letter combinations, such as TT, Tt, and tt. When an organism has two of the same alleles for a specific gene, it is homozygous (homo means ""same"") for that gene. An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different alleles (Tt) for a certain gene, it is known as heterozygous (hetero means different). Genotype Homozygous Heterozygous Homozygous dominant Homozygous recessive Definition Two of the same allele One dominant allele and one reces- sive allele Two dominant alleles Two recessive alleles Example TT or tt Tt TT tt Phenotype is a way to describe the traits you can see. The genotype is like a recipe for a cake, while the phenotype is like the cake made from the recipe. The genotype expresses the phenotype. For example, the phenotypes of Mendels pea plants were either tall or short, or they were purple-flowered or white-flowered. Can organisms with different genotypes have the same phenotypes? Lets see. What is the phenotype of a pea plant that is homozygous dominant (TT) for the tall trait? Tall. What is the phenotype of a pea plant that is heterozygous (Tt)? It is also tall. The answer is yes, two different genotypes can result in the same phenotype. Remember, the recessive phenotype will be expressed only when the dominant allele is absent, or when an individual is homozygous recessive (tt) ( Figure 1.1). Different genotypes (AA, Aa, aa or TT, Tt, tt) will lead to different phenotypes, or different appearances of the organism. "
molecular evidence for evolution,T_3187,"Arguably, some of the best evidence of evolution comes from examining the molecules and DNA found in all living things. Beginning in the 1940s, scientists studying molecules and DNA have confirmed conclusions about evolution drawn from other forms of evidence. Molecular clocks are used to determine how closely two species are related by calculating the number of differences between the species DNA sequences or amino acid sequences. These clocks are sometimes called gene clocks or evolutionary clocks. The fewer the differences, the less time since the species split from each other and began to evolve into different species ( Figure 1.1). A chicken and a gorilla will have more differences between their DNA and amino acid sequences than a gorilla and an orangutan. That means the chicken and gorilla had a common ancestor a very long time ago, while the gorilla and orangutan shared a more recent common ancestor. This provides additional evidence that the gorilla and orangutan are more closely related than the gorilla and the chicken. Which pair of organisms would have more molecular differences, a mammal and a bird, a mammal and a frog, or a mammal and a fish? On the other hand, animals may look similar but can have very different DNA sequences and evolutionary ancestry. Which would have more DNA sequences in common, a whale and a horse, or a whale and a shark? Almost all organisms are made from DNA with the same building blocks. The genomes (all of the genes in an organism) of all mammals are almost identical. The genomes, or all the DNA sequences of all the genes of an organism, have been determined for many different organisms. The comparison of genomes provides new information about the relationships among species and how evolution occurs ( Figure 1.2). Molecular evidence for evolution also includes: 1. The same biochemical building blocks, such as amino acids and nucleotides, are found in all organisms, from bacteria to plants and animals. Recall that amino acids are the building blocks of proteins, and nucleotides are the building blocks of DNA and RNA. 2. DNA and RNA determine the development of all organisms. 3. The similarities and differences between the genomes confirm patterns of evolution. "
natural selection,T_3204,"The theory of evolution by natural selection means that the inherited traits of a population change over time. Inherited traits are features that are passed from one generation to the next. For example, your eye color is an inherited trait. You inherited your eye color from your parents. Inherited traits are different from acquired traits, or traits that organisms develop over a lifetime, such as strong muscles from working out ( Figure 1.1). Natural selection explains how organisms in a population develop traits that allow them to survive and reproduce. Natural selection means that traits that offer an advantage will most likely be passed on to offspring; individuals with those traits have a better chance of surviving. Evolution occurs by natural selection. Take the giant tortoises on the Galpagos Islands as an example. If a short-necked tortoise lives on an island with fruit located at a high level, will the short-necked tortoise survive? No, it will not, because it will not be able to reach the food it needs to survive. If all of the short necked tortoises die, and the long-necked tortoises survive, then, over time, only the long-necked trait will be passed down to offspring. All of the tortoises with long-necks will be Human earlobes may be attached or free. You inherited the particular shape of your earlobes from your parents. Inherited traits are influenced by genes, which are passed on to offspring and future genera- tions. Things not influenced by genes are not passed on to your offspring. Natural selection only operates on traits like ear- lobe shape that have a genetic basis, not on traits that are acquired, like a summer tan. ""naturally selected"" to survive. Organisms that are not well-adapted, for whatever reason, to their environment, will naturally have less of a chance of surviving and reproducing. Every plant and animal depends on its traits to survive. Survival may include getting food, building homes, and attracting mates. Traits that allow a plant, animal, or other organism to survive and reproduce in its environment are called adaptations. Natural selection occurs when: 1. There is some variation in the inherited traits of organisms within a species. Without this variation, natural selection would not be possible. 2. Some of these traits will give individuals an advantage over others in surviving and reproducing. 3. These individuals will be likely to have more offspring. Imagine how in the Arctic, dark fur makes a rabbit easy for foxes to spot and catch in the snow. Therefore, white fur is a beneficial trait that improves the chance that a rabbit will survive, reproduce, and pass the trait of white fur on to its offspring ( Figure 1.2). Through this process of natural selection, dark fur rabbits will become uncommon over time. Rabbits will adapt to have white fur. In essence, the selection of rabbits with white fur - the beneficial trait - is a natural process. "
natural selection,T_3205,"Scientists estimate that there are between 5 million and 30 million species on the planet. But why are there so many? Different species are well-adapted to live and survive in many different types of environments. As environments change over time, organisms must constantly adapt to those environments. Diversity of species increases the chance that at least some organisms adapt and survive any major changes in the environment. For example, if a natural disaster kills all of the large organisms on the planet, then the small organisms will continue to survive. "
nonvascular plants,T_3218,"Nonvascular seedless plants, as their name implies, lack vascular tissue. Vascular tissue is specialized tissue that transports water, nutrients, and food in plants. As they lack vascular tissue, they also do not have true roots, stems, or leaves. Nonvascular plants do often have a leafy appearance, though, and they can have stem-like and root-like structures. These plants are very short because they cannot move nutrients and water up a stem. Nonvascular seedless plants, also known as bryophytes, are classified into three phyla: 1. Mosses 2. Hornworts 3. Liverworts "
nonvascular plants,T_3219,"Mosses are most often recognized as the green fuzz on damp rocks and trees in a forest. If you look closely, you will see that most mosses have tiny stem-like and leaf-like structures. This is the gametophyte stage. Remember that a gametophyte is haploid, having only one set of chromosomes. The gametophyte produces the gametes that, after fertilization, develop into the diploid sporophyte with two sets of chromosomes. The sporophyte forms a capsule, called the sporangium, which releases spores ( Figure 1.1). Sporophytes sprout up on stalks from this bed of moss gametophytes. Notice that both the sporophytes and gametophytes exist at the same time. "
nonvascular plants,T_3220,"Hornworts are named for their appearance. The ""horn"" part of the name comes from their hornlike sporophytes, and wort comes from the Anglo-Saxon word for herb. The hornlike sporophytes grow from a base of flattened lobes, which are the gametophytes ( Figure 1.2). They usually grow in moist and humid areas. In hornworts, the horns are the sporo- phytes that rise up from the leaflike ga- metophyte. "
nonvascular plants,T_3221,"Liverworts have two distinct appearances: they can either be leafy like mosses or flattened and ribbon-like. Liver- worts get their name from the type with the flattened bodies, which can resemble a liver ( Figure 1.3). Liverworts can often be found along stream beds. Liverworts with a flattened, ribbon-like body are called thallose liverworts. "
organization of living things,T_3228,"When you see an organism that you have never seen before, you probably put it into a group without even thinking. If it is green and leafy, you probably call it a plant. If it is long and slithers, you probably call it as a snake. How do you make these decisions? You look at the physical features of the organism and think about what it has in common with other organisms. Scientists do the same thing when they classify, or put into categories, living things. Scientists classify organisms not only by their physical features, but also by how closely related they are. Lions and tigers look like each other more than they look like bears, but are lions and tigers related? Evolutionarily speaking, yes. Evolution is the change in a species over time. Lions and tigers both evolved from a common ancestor. So it turns out that the two cats are actually more closely related to each other than to bears. How an organism looks and how it is related to other organisms determines how it is classified. "
organization of living things,T_3229,"People have been concerned with classifying organisms for thousands of years. Over 2,000 years ago, the Greek philosopher Aristotle developed a classification system that divided living things into several groups that we still use today, including mammals, insects, and reptiles. Carolus (Carl) Linnaeus (1707-1778) ( Figure 1.1) built on Aristotles work to create his own classification system. He invented the way we name organisms today, with each organism having a two word name. Linnaeus is considered the inventor of modern taxonomy, the science of naming and grouping organisms. In the 18th century, Carl Linnaeus invented the two-name system of naming organisms (genus and species) and introduced the most complete classification system then known. Linnaeus developed binomial nomenclature, a way to give a scientific name to every organism. In this system, each organism receives a two-part name in which the first word is the genus (a group of species), and the second word refers to one species in that genus. For example, a coyotes species name is Canis latrans. Latrans is the species and Canis is the genus, a larger group that includes dogs, wolves, and other dog-like animals. Here is another example: the red maple, Acer rubra, and the sugar maple, Acer saccharum, are both in the same genus and they look similar ( Figure 1.2). Notice that the genus is capitalized and the species is not, and that the whole scientific name is in italics. Tigers (Panthera tigris) and lions (Panthera leo) have the same genus name, but are obviously different species. The names may seem strange, but the names are written in a language called Latin. These leaves are from two different species of trees in the Acer, or maple, genus. The green leaf (far left) is from the sugar maple, and the red leaf (center ) are from the red maple. One of the character- istics of the maple genus is winged seeds (far right). "
organization of living things,T_3230,"Modern taxonomists have reordered many groups of organisms since Linnaeus. The main categories that biologists use are listed here from the most specific to the least specific category ( Figure 1.3). All organisms can be classified into one of three domains, the least specific grouping. The three domains are Bacteria, Archaea, and Eukarya. The Kingdom is the next category after the Domain. All life is divided among six kingdoms: Kingdom Bacteria, Kingdom Archaea, Kingdom Protista, Kingdom Plantae, Kingdom Fungi, and Kingdom Animalia. This diagram illustrates the classification categories for organisms, with the broad- est category (kingdom) at the bottom, and the most specific category (species) at the top. We are Homo sapiens. Homo is the genus of great apes that includes modern humans and closely related species, and sapiens is the only living species of the genus. "
organization of living things,T_3231,"Even though naming species is straightforward, deciding if two organisms are the same species can sometimes be difficult. Linnaeus defined each species by the distinctive physical characteristics shared by these organisms. But two members of the same species may look quite different. For example, people from different parts of the world sometimes look very different, but we are all the same species ( Figure 1.4). So how is a species defined? A species is defined as a group of similar individuals that can interbreed with one another and produce fertile offspring. A species does not produce fertile offspring with other species. "
origin of species,T_3236,"The creation of a new species is called speciation. Most new species develop naturally. But humans have also artificially created new breeds and species for thousands of years. New species develop naturally through the process of natural selection. Due to natural selection, organisms with traits that better enable them to adapt to their environment will tend to survive and reproduce in greater numbers. Natural selection causes beneficial heritable traits to become more common in a population and unfavorable heritable traits to become less common. For example, a giraffes neck is beneficial because it allows the giraffe to reach leaves high in trees. Natural selection caused this beneficial trait to become more common than short necks. As new changes in the DNA sequence are constantly being generated in a populations gene pool (changing the populations allele frequencies), some of these changes will be beneficial and result in traits that allow adaptation and survival. Natural selection causes evolution of a species as these beneficial traits become more common within a population. Evolution can occur within a species without completely resulting in a new species. Therefore, evolution and speciation are not the same. "
origin of species,T_3237,"Artificial selection occurs when humans select which plants or animals to breed in order to pass on specific traits to the next generation. For example, a farmer may choose to breed only cows that produce the best milk. Farmers would also avoid breeding cows that produce less milk. In this way, selective breeding of the cows would increase milk quality and quantity. Humans have also artificially bred dogs to create new breeds ( Figure 1.1). Artificial Selection: Humans used artificial selection to create these different breeds. Both dog breeds are descended from the same wolves, and their genes are almost identical. "
origin of species,T_3238,"There are two main ways that speciation happens naturally. Both processes create new species by reproductively isolating populations of the same species from each other. Organisms can be geographically isolated or isolated by a behavior. Either way, they will no longer be able to mate. Over a long period of time, usually thousands of years, each of the isolated populations evolves in a different direction, forming distinct species. How do you think scientists test whether two populations are separate species? They bring species from two populations back together again. If the two populations do not mate and produce fertile offspring, they are separate species. "
origin of species,T_3239,"Allopatric speciation occurs when groups from the same species are geographically isolated for long periods. Imagine all the ways that plants or animals could be isolated from each other: Emergence of a mountain range. Formation of a canyon. New rivers or streams. Here are two examples of allopatric speciation: Darwin observed thirteen distinct finch species on the Galpagos Islands that had evolved from the same ancestor. Different finch populations lived on separate islands with different environments. They evolved to best adapt to those particular environments. Later, scientists were able to determine which finches had evolved into distinct species by bringing members of each population together. The birds that could not mate were a separate species. When the Grand Canyon in Arizona formed, two populations of one squirrel species were separated by the giant canyon. After thousands of years of isolation from each other, the squirrel populations on the northern wall of the canyon looked and behaved differently from those on the southern wall ( Figure 1.2). North rim squirrels have white tails and black bellies. Squirrels on the south rim have white bellies and dark tails. They cannot mate with each other, so they are different species. Abert squirrel (left) on the southern rim of the Grand Canyon. Kaibab squirrel (right) found on northern rim of the Grand Canyon. "
origin of species,T_3240,"Sympatric speciation occurs when groups from the same species stop mating because of something other than physical or geographic separation. The behavior of two groups that live in the same region is an example of such separation. The separation may be caused by different mating seasons, for example. Sympatric speciation is more difficult to identify. Here are two examples of sympatric speciation: Some scientists suspect that two groups of orcas (killer whales) live in the same part of the Pacific Ocean part of the year but do not mate. The two groups hunt different prey species, eat different foods, sing different songs, and have different social interactions ( Figure 1.3). Two groups of Galpagos Island finch species lived in the same space, but each had his or her own distinct mating signals. Members of each group selected mates according to different beak structures and bird calls. The behavioral differences kept the groups separated until they formed different species. "
plant characteristics,T_3265,"Plants have adapted to a variety of environments, from the desert to the tropical rain forest to lakes and oceans. In each environment, plants have become crucial to supporting animal life. Plants are the food that animals eat. Plants also provide places for animals, such as insects and birds, to live; many birds build nests in plants. From tiny mosses to gorgeous rose bushes to extremely large redwood trees ( Figure 1.1), the organisms in this kingdom, Kingdom Plantae, have three main features. They are all: 1. Eukaryotic. 2. Photosynthetic. 3. Multicellular. Recall that eukaryotic organisms also include animals, protists, and fungi. Eukaryotes have cells with nuclei that contain DNA, and membrane-bound organelles, such as mitochondria. Photosynthesis is the process by which plants capture the energy of sunlight and use carbon dioxide from the air (and water) to make their own food, the carbohydrate glucose. Plants have chloroplasts, the organelle of photosynthesis, and are known as producers and autotrophs. Other organisms are heterotrophic consumers, meaning they must obtain their nutrients from another organism, as these organisms lack chloroplasts. Lastly, plants must be multicellular, composed of more than one cell. There are no single-celled plants. Recall that some protists, such as algae, are eukaryotic and photosynthetic but are not considered plants. Unlike plants, algae is mostly unicellular. "
plant classification,T_3266,"Plants are formally divided into 12 phyla (plural for phylum), and these phyla are gathered into four groups ( Figure 1. Nonvascular plants evolved first. They are distinct from the algae because they keep the embryo inside of the reproductive structure after fertilization. These plants do not have vascular tissue, xylem or phloem, to transport nutrients, water, and food. Examples include mosses, liverworts, and hornworts. Without vascular tissue, these plants do not grow very tall. 2. Seedless vascular plants evolved to have vascular tissue after the nonvascular plants but do not have seeds. Examples include the ferns, whisk ferns, club mosses, and horsetails. Vascular tissue allowed these plants to grow taller. 3. Gymnosperms evolved to have seeds but do not have flowers. Examples of gymnosperms include the Redwood, Fir, and Cypress trees. Gymnos means ""naked"" in Greek; the seeds of gymnosperms are naked, not protected by flowers. 4. Flowering plants, or angiosperms, evolved to have vascular tissue, seeds, and flowers. Examples of an- giosperms include magnolia trees, roses, tulips, and tomatoes. The plant kingdom contains a diversity of organisms. "
plant hormones,T_3267,"Plants may not move, but that does not mean they dont respond to their environment. Plants can sense gravity, light, touch, and seasonal changes. For example, you might have noticed how a house plant bends toward a bright window. Plants can sense and then grow toward the source of light. Scientists say that plants are able to respond to ""stimuli,"" or somethingusually in the environmentthat results in a response. For instance, light is the stimulus, and the plant moving toward the light is the ""response."" Hormones are special chemical messengers molecules that help organisms, including plants, respond to stimuli in their environment. In order for plants to respond to the environment, their cells must be able to communicate with other cells. Hormones send messages between the cells. Animals, like humans, also have hormones, such as testosterone or estrogen, to carry messages from cell to cell. In both plants and animals, hormones travel from cell to cell in response to a stimulus; they also activate a specific response. "
plant hormones,T_3268,"Five different types of plant hormones are involved in the main responses of plants, and they each have different functions ( Table 1.1). Hormone Ethylene Gibberellins Cytokinins Abscisic Acid Auxins Function Fruit ripening and abscission Break the dormancy of seeds and buds; promote growth Promote cell division; prevent senescence Close the stomata; maintain dormancy Involved in tropisms and apical dominance "
plant hormones,T_3269,"The hormone ethylene has two functions. It (1) helps ripen fruit and (2) is involved in the process of abscission, the dropping of leaves, fruits, and flowers. When a flower is done blooming or a fruit is ripe and ready to be eaten, ethylene causes the petals or fruit to fall from a plant ( Figure 1.1 and Figure 1.2). Ethylene is an unusual plant hormone because it is a gas. That means it can move through the air, and a ripening apple can cause another apple to ripen, or even over-ripen. Thats why one rotten apple spoils the whole barrel! Some farmers spray their green peppers with ethylene gas to cause them to ripen faster and become red peppers. You can try to see how ethylene works by putting a ripe apple or banana with another unripe fruit in a closed container or paper bag. What do you think will happen to the unripe fruit? "
plant hormones,T_3270,"Gibberellins are hormones that cause the plant to grow. When gibberellins are applied to plants by scientists, the stems grow longer. Some gardeners or horticulture scientists add gibberellins to increase the growth of plants. The hormone ethylene causes flower petals to fall from a plant, a process known as abscission. Dwarf plants (small plants), on the other hand, have low levels of gibberellins ( Figure 1.3). Another function of gibberellins is to stop dormancy (resting time) of seeds and buds. Gibberellins signal that its time for a seed to germinate (sprout) or for a bud to open. Dwarf plants like this bonsai tree often have unusually low concentrations of gib- berellins. "
plant hormones,T_3271,"Cytokinins are hormones that cause plant cells to divide. Cytokinins were discovered from attempts to grow plant tissue in artificial environments ( Figure 1.4). Cytokinins prevent the process of aging (senescence). So florists sometimes apply cytokinins to cut flowers, so they do not get old and die. Cytokinins promote cell division and are necessary for growing plants in tissue cul- ture. A small piece of a plant is placed in sterile conditions to regenerate a new plant. "
plant hormones,T_3272,"Abscisic acid is misnamed because it was once believed to play a role in abscission (the dropping of leaves, fruits, and flowers), but we now know abscission is caused by ethylene. The actual role of abscisic acid is to close the stomata, the tiny openings in leaves that allow substances to enter and leave, and to maintain dormancy. When a plant is stressed due to lack of water, abscisic acid tells the stomata to close. This prevents water loss through the stomata. When the environment is not good for a seed to germinate, abscisic acid signals for the dormancy period of the seed to continue. Abscisic acid also tells the buds of plants to stay in the dormancy stage. When conditions improve, the levels of abscisic acid drop and the levels of gibberellins increase, signaling that is time to break dormancy ( Figure "
plant hormones,T_3273,"Auxins are hormones that play a role in plant growth. Auxins produced at the tip of the plant are involved in apical dominance, when the main central stem grows more strongly than other stems and branches. When the tip of the plant is removed, the auxins are no longer present, and the side branches begin to grow. This is why pruning a plant by cutting off the main branches helps produce a fuller plant with more branches. You actually need to cut branches off of a plant for it to grow more branches! Auxins are also involved in tropisms, responses to stimuli in the environment "
plant like protists,T_3274,"Plant-like protists are known as algae ( Figure 1.1). They are a large and diverse group. Plant-like protists are autotrophs. This means that they produce their own food. They perform photosynthesis to produce sugar by using carbon dioxide and water, and the energy from sunlight, just like plants. Unlike plants, however, plant-like protists do not have true stems, roots, or leaves. Most plant-like protists live in oceans, ponds, or lakes. Protists can be unicellular (single-celled) or multicellular (many-celled). Seaweed and kelp are examples of multicellular, plant-like protists. Kelp can be as large as trees and form a ""forest"" in the ocean ( Figure 1.2). Plant-like protists are essential to the ecosystem. They are the base of the marine food chain, and they produce oxygen through photosynthesis for animals to breathe. They are classified into a number of basic groups ( Table Red algae are a very large group of protists making up about 5,0006,000 species. They are mostly multicellular and live in the ocean. Many red algae are seaweeds and help create coral reefs. Macrocystis pyrifera (giant kelp) is a type of multicellular, plant-like protist. Phylum Description Chlorophyta Green algae (related to higher plants) Red algae Brown algae Diatoms, golden-brown algae, yellow-green algae Dinoflagellates Euglenoids Rhodophyta Phaeophyta Chrysophyta Pyrrophyta Euglenophyta Approximate Number of Species 7,500 5,000 1,500 12,000 Chlamydomnas, Volvox Porphyra Macrocystis Cyclotella 4,000 1,000 Gonyaulax Euglena "
plants adaptations for life on land,T_3276,"The first photosynthetic organisms were bacteria that lived in the water. So, where did plants come from? Evidence shows that plants evolved from freshwater green algae, a protist ( Figure 1.1). The similarities between green algae and plants is one piece of evidence. They both have cellulose in their cell walls, and they share many of the same chemicals that give them color. So what separates green algae from green plants? There are four main ways that plants adapted to life on land and, as a result, became different from algae: The ancestor of plants is green algae. This picture shows a close up of algae on the beach. 1. In plants, the embryo develops inside of the female plant after fertilization. Algae do not keep the embryo inside of themselves but release it into water. This was the first feature to evolve that separated plants from green algae. This is also the only adaptation shared by all plants. 2. Over time, plants had to evolve from living in water to living on land. In early plants, a waxy layer called a cuticle evolved to help seal water in the plant and prevent water loss. However, the cuticle also prevents gases from entering and leaving the plant easily. Recall that the exchange of gassestaking in carbon dioxide and releasing oxygenoccurs during photosynthesis. 3. To allow the plant to retain water and exchange gases, small pores (holes) in the leaves called stomata also evolved ( Figure 1.2). The stomata can open and close depending on weather conditions. When its hot and dry, the stomata close to keep water inside of the plant. When the weather cools down, the stomata can open again to let carbon dioxide in and oxygen out. 4. A later adaption for life on land was the evolution of vascular tissue. Vascular tissue is specialized tissue that transports water, nutrients, and food in plants. In algae, vascular tissue is not necessary since the entire body is in contact with the water, and the water simply enters the algae. But on land, water may only be found deep in the ground. Vascular tissues take water and nutrients from the ground up into the plant, while also taking food down from the leaves into the rest of the plant. The two vascular tissues are xylem and phloem. Xylem is responsible for the transport of water and nutrients from the roots to the rest of the plant. Phloem carries the sugars made in the leaves to the parts of the plant where they are needed. "
predation,T_3282,"Predation is another mechanism in which species interact with each other. Predation is when a predator organism feeds on another living organism or organisms, known as prey. The predator always lowers the preys fitness. It does this by keeping the prey from surviving, reproducing, or both. Predator-prey relationships are essential to maintaining the balance of organisms in an ecosystem. Examples of predator-prey relationships include the lion and zebra, the bear and fish, and the fox and rabbit. There are different types of predation, including: true predation. grazing. parasitism. True predation is when a predator kills and eats its prey. Some predators of this type, such as jaguars, kill large prey. They tear it apart and chew it before eating it. Others, like bottlenose dolphins or snakes, may eat their prey whole. In some cases, the prey dies in the mouth or the digestive system of the predator. Baleen whales, for example, This lion is an example of a predator on the hunt. eat millions of plankton at once. The prey is digested afterward. True predators may hunt actively for prey, or they may sit and wait for prey to get within striking distance. Certain traits enable organisms to be effective hunters. These include camouflage, speed, and heightened senses. These traits also enable certain prey to avoid predators. In grazing, the predator eats part of the prey but does not usually kill it. You may have seen cows grazing on grass. The grass they eat grows back, so there is no real effect on the population. In the ocean, kelp (a type of seaweed) can regrow after being eaten by fish. Predators play an important role in an ecosystem. For example, if they did not exist, then a single species could become dominant over others. Grazers on a grassland keep grass from growing out of control. Predators can be keystone species. These are species that can have a large effect on the balance of organisms in an ecosystem. For example, if all of the wolves are removed from a population, then the population of deer or rabbits may increase. If there are too many deer, then they may decrease the amount of plants or grasses in the ecosystem. Decreased levels of producers may then have a detrimental effect on the whole ecosystem. In this example, the wolves would be a keystone species. Prey also have adaptations for avoiding predators. Prey sometimes avoid detection by using camouflage ( Figure background. Mimicry is a related adaptation in which a species uses appearance to copy or mimic another species. For example, a non-poisonous dart frog may evolve to look like a poisonous dart frog. Why do you think this is an adaptation for the non-poisonous dart frog? Mimicry can be used by both predators and prey ( Figure 1.3). Parasitism is a type of symbiotic relationship and will be described in the Symbiosis concept. Camouflage by the dead leaf mantis makes it less visible to both its predators and prey. If alarmed, it lies motionless on the rainforest floor of Madagascar, Africa, camouflaged among the actual dead leaves. It eats other animals up to the size of small lizards. An example of mimicry, where the Viceroy butterfly (right) mimics the unpleasant Monarch butterfly (left). Both butterfly species are avoided by predators to a greater degree than either one would be without mimicry. "
primates,T_3294,"If primates are mammals, what makes them seem so different from most mammals? Primates, including humans, have several unique features. Some adaptations give primates advantages that allow them to live in certain habitats, such as in trees. Other features have allowed them to adapt to complex social and cultural situations. Primates are mostly omnivorous, meaning many primate species eat both plant and animal material. The order contains all of the species commonly related to lemurs, monkeys, and apes. The order also includes humans ( Figure 1.1). Key features of primates include: Five fingers, known as pentadactyl. Several types of teeth. Certain eye orbit characteristics, such as a postorbital bar, or a bone that runs around the eye socket. An opposable thumb, a finger that allows a grip that can hold objects. (top left) Ring-tailed lemurs. Lemurs be- long to the prosimian group of primates. (top right) One of the New World mon- keys, a squirrel monkey. (bottom left) Chimpanzees belong to the great apes, one of the groups of primates. (bottom right) Reconstruction of a Neanderthal man, belonging to an extinct subspecies of Homo sapiens. This subspecies of humans lived in Europe and western and central Asia from about 100,000 40,000 BCE. Whats the difference between monkeys and apes? The easiest way to distinguish monkeys from the other primates is to look for a tail. Most monkey species have tails, but no apes or humans do. Monkeys are much more like other mammals than apes and humans are. "
primates,T_3295,"In intelligent mammals, such as primates, the cerebrum is larger compared to the rest of the brain. A larger cerebrum allows primates to develop higher levels of intelligence. Primates have the ability to learn new behaviors. They also engage in complex social interactions, such as fighting and play. "
primates,T_3296,"Old World species, such as apes and some monkeys ( Figure 1.1 and Figure 1.2), tend to have significant size differences between the sexes. This is known as sexual dimorphism. Males tend to be slightly more than twice as heavy as females. This dimorphism may have evolved when one male had to defend many females. Old World generally refers to monkeys of Africa and Asia. New World refers to monkeys of the Americas. New World species, including tamarins (squirrel-sized monkeys) and marmosets (very small primitive monkeys) ( Figure 1.2), form pair bonds, which is a partnership between a mating pair that lasts at least one season. The pair cooperatively raise the young and generally do not show a significant size difference between the sexes. Old World monkeys do not tend to form monogamous relationships. (left) An Old World monkey, a species of macaque, in Japan. (center ) A New World species of monkey, a tamarin. (right) Another New World species of monkey, the pygmy marmoset. "
primates,T_3297,"Non-human primates live mostly in Central and South America, Africa, and South Asia. Since primates evolved from animals living in trees, many modern species still live mostly in trees. Other species live on land most of the time, such as baboons ( Figure 1.3) and the Patas monkey. Only a few species live on land all of the time, such as the gelada and humans. Primates live in a diverse number of forested habitats, including rain forests, mangrove forests and mountain forests to altitudes of over 9,800 feet. The combination of opposable thumbs, short fingernails, and long, inward-closing fingers has allowed some species to develop the ability to move by swinging their arms from one branch to another ( Figure 1.4). Another feature for climbing are expanded finger-like parts, such as those in tarsiers, which improve grasping ( Figure 1.4). A few species, such as the proboscis monkey, De Brazzas monkey, and Allens swamp monkey, evolved webbed fingers so they can swim and live in swamps and aquatic habitats. Some species, such as the rhesus macaque and the Hanuman langur, can even live in cities by eating human garbage. (left) A gibbon shows how its limbs are modified for hanging from trees. (right) A species of tarsier, with expanded digits used for grasping branches. "
protist characteristics,T_3312,"Protists are eukaryotes, which means their cells have a nucleus and other membrane-bound organelles. Most, but not all, protists are single-celled. Other than these features, they have very little in common. You can think about protists as all eukaryotic organisms that are neither animals, nor plants, nor fungi. Although Ernst Haeckel set up the Kingdom Protista in 1866, this kingdom was not accepted by the scientific world until the 1960s. These unique organisms can be so different from each other that sometimes Protista is called the junk drawer"" kingdom. Just like a junk drawer, which contains items that dont fit into any other category, this kingdom contains the eukaryotes that cannot be put into any other kingdom. Therefore, protists can seem very different from one another. "
protist characteristics,T_3313,"Most protists are so small that they can be seen only with a microscope. Protists are mostly unicellular (one-celled) eukaryotes. A few protists are multicellular (many-celled) and surprisingly large. For example, kelp is a multicellular protist that can grow to be over 100-meters long ( Figure 1.1). Multicellular protists, however, do not show cellular specialization or differentiation into tissues. That means their cells all look the same and, for the most part, function the same. On the other hand, your cells often are much different from each other and have special jobs. Kelp is an example of a muticellular pro- tist. "
protist characteristics,T_3314,"A few characteristics are common between protists. 1. 2. 3. 4. They are eukaryotic, which means they have a nucleus. Most have mitochondria. They can be parasites. They all prefer aquatic or moist environments. "
protist characteristics,T_3315,"For classification, the protists are divided into three groups: 1. Animal-like protists, which are heterotrophs and have the ability to move. 2. Plant-like protists, which are autotrophs that photosynthesize. 3. Fungi-like protists, which are heterotrophs, and they have cells with cell walls and reproduce by forming spores. But remember, protists are not animals, nor plants, nor fungi ( Figure 1.2). "
protists nutrition,T_3316,"The cells of protists need to perform all of the functions that other cells do, such as grow and reproduce, maintain homeostasis, and obtain energy. They also need to obtain ""food"" to provide the energy to perform these functions. Recall that protists can be plant-like, fungi-like, or animal-like. That means that protists can obtain food like plants, fungi, or animals do. There are many plant-like protists, such as algae, that get their energy from sunlight through photosynthesis. Some of the fungus-like protists, such as the slime molds ( Figure 1.1), decompose decaying matter. The animal-like protists must ""eat"" or ingest food. Some animal-like protists use their ""tails"" to eat. These protists are called filter-feeders. They acquire nutrients by constantly whipping their tails, called flagellum, back and forth. The whipping of the flagellum creates a current that brings food into the protist. Other animal-like protists must ""swallow"" their food through a process called endocytosis. Endocytosis happens when a cell takes in substances through its membrane. The process is described below: 1. The protist wraps around its prey, which is usually bacteria. 2. It creates a food vacuole, a sort of ""food storage compartment,"" around the bacteria. 3. The protist produces toxins which paralyze its prey. 4. Once digested, the food material moves through the vacuole and into the cytoplasm of the protist. Also, some of the animal-like and fungi-like protists are parasitic, harming their hosts as they obtain nutrients. Fungi-like protists absorb nutrients meant for their host, harming the host in the process. Slime molds live on decaying plant life and in the soil. "
punnett squares,T_3319,"A Punnett square is a special tool derived from the laws of probability. It is used to predict the possible offspring from a cross, or mating between two parents. An example of a Punnett square ( Figure 1.1) shows the results of a cross between two purple flowers that each have one dominant factor and one recessive factor (Bb). The Punnett square of a cross between two purple flowers (Bb). A Punnett square can be used to calculate what percentage of offspring will have a certain trait. To create a Punnett square, perform the following steps: 1. 2. 3. 4. Take the factors from the first parent and place them at the top of the square (B and b). Take the factors from the second parent and line them up on the left side of the square (B and b). Pull the factors from the top into the boxes below. Pull the factors from the side into the boxes next to them. The possible offspring are represented by the letters in the boxes, with one factor coming from each parent. Results: Top left box: BB, or purple flowers Top right box: Bb, or purple flowers Lower left box: Bb, or purple flowers Lower right box: bb, or white flowers Only one of the plants out of the four, or 25% of the plants, has white flowers (bb). The other 75% have purple flowers (BB, Bb), because the purple factor (B) is the dominant factor. This shows that the color purple is the dominant trait in pea plants. Now imagine you cross one of the white flowers (bb) with a purple flower that has both a dominant and recessive factor (Bb). The only possible gamete in the white flower is recessive (b), while the purple flower can have gametes with either dominant (B) or recessive (b). Practice using a Punnett square with this cross (see Table 1.1). b Bb bb B b b Bb bb Did you find that 50% of the offspring will be purple, and 50% of the offspring will be white? "
reproduction in seedless plants,T_3329,"Seedless plants can reproduce asexually or sexually. Some seedless plants, like hornworts and liverworts, can reproduce asexually through fragmentation. When a small fragment of the plant is broken off, it can form a new plant. "
reproduction in seedless plants,T_3330,"Like all plants, nonvascular plants have an alternation of generations life cycle. That means they alternate between diploid cell stages (having two sets of chromosomes) and haploid cell stages (having one set of chromosomes) during their life cycle. Recall the haploid stage is called the gametophyte, and the diploid stage is called the sporophyte. In the life cycle of the nonvascular seedless plants, the gametophyte stage is the longest part of the cycle. The gametophyte is the green photosynthetic carpet that you would recognize as a moss. The life cycle of nonvascular seedless plants can be described as follows: 1. The male gametophyte produces flagellated sperm that must swim to the egg formed by the female game- tophyte. For this reason, sexual reproduction must happen in the presence of water. Therefore, nonvascular plants tend to live in moist environments. Though the life of a nonvascular seedless plant is a cycle, this can be considered the initial step in the life cycle. 2. Following fertilization, the sporophyte forms. The sporophyte is connected to, and dependent on, the gameto- phyte. 3. The sporophyte produces spores that will develop into gametophytes and start the cycle over again. "
reproduction in seedless plants,T_3331,"For the seedless vascular plants, the sporophyte stage is the longest part of the cycle, but the cycle is similar to nonvascular plants. For example, in ferns, the gametophyte is a tiny heart-shaped structure, while the leafy plant we recognize as a fern is the sporophyte. The ferns sporangia, where spores are produced, are often on the underside of the fronds ( Figure 1.1). Like nonvascular plants, ferns also have flagellated sperm that must swim to the egg. Unlike nonvascular plants, once fertilization takes place, the gametophyte will die, and the sporophyte will live independently. This fern is producing spores underneath its fronds. "
reproductive behavior of animals,T_3332,"Some of the most important animal behaviors involve mating. Mating is the pairing of an adult male and female to produce young. Adults that are most successful at attracting a mate are most likely to have offspring. Traits that help animals attract a mate and have offspring increase their fitness. As the genes that encode these traits are passed to the next generation, the traits will become more common in the population. "
reproductive behavior of animals,T_3333,"In many species, females choose the male they will mate with. For their part, males try to be chosen as mates. They show females that they would be a better mate than the other males. To be chosen as a mate, males may perform courtship behaviors. These are special behaviors that help attract a mate. Male courtship behaviors get the attention of females and show off a males traits. These behaviors are often observed as direct competition between males. Different species have different courtship behaviors. One example is a peacock raising his tail feathers. The colorful peacock is trying to impress females of his species with his beautiful feathers. Another example of courtship behavior in birds is the blue-footed booby. He is doing a dance to attract a female for mating. During the dance, he spreads out his wings and stamps his feet on the ground. You can watch the following video of a blue-footed booby doing his courtship dance at:  . Click image to the left or use the URL below. URL:  Courtship behaviors occur in many other species. For example, males in some species of whales have special mating songs to attract females as mates. Frogs croak for the same reason. Male deer clash antlers to court females. Male jumping spiders jump from side to side to attract mates. Courtship behaviors are one type of display behavior. A display behavior is a fixed set of actions that carries a specific message. Although many display behaviors are used to attract mates, some display behaviors have other purposes. For example, display behaviors may be used to warn other animals to stay away, as you will read below. "
reproductive behavior of animals,T_3334,"In most species of birds and mammals, one or both parents care for their offspring. Caring for the young may include making a nest or other shelter. It may also include feeding the young and protecting them from predators. Caring for offspring increases their chances of surviving. Birds called killdeers have an interesting way of protecting their chicks. When a predator gets too close to her nest, a mother killdeer pretends to have a broken wing. The mother walks away from the nest holding her wing as though it were injured ( Figure 1.1). The predator thinks she is injured and will be easy prey. The mother leads the predator away from the nest and then flies away. In most species of mammals, parents also teach their offspring important skills. For example, meerkat parents teach their pups how to eat scorpions without being stung. A scorpion sting can be deadly, so this is a very important skill. "
reproductive behavior of animals,T_3335,"Some species of animals are territorial. This means that they defend their area. The area they defend usually contains their nest and enough food for themselves and their offspring. A species is more likely to be territorial if there is not very much food in their area. Having a larger territory could mean more prey or food. Animals generally do not defend their territory by fighting. Instead, they are more likely to use display behavior. The behavior tells other animals to stay away. It gets the message across without the need for fighting. Display behavior is generally safer and uses less energy than fighting. Male gorillas use display behavior to defend their territory. They pound on their chests and thump the ground with their hands to warn other male gorillas to keep away from their area. The robin displays his red breast to warn other robins to stay away ( Figure 1.2). The red breast of this male robin is easy to see. The robin displays his bright red chest to defend his territory. It warns other robins to keep out of his area. Some animals deposit chemicals to mark the boundary of their territory. This is why dogs urinate on fire hydrants and other objects. Cats may also mark their territory by depositing chemicals. They have scent glands in their face. They deposit chemicals by rubbing their face against objects. "
reptiles,T_3337,"What reptiles can you name? Snakes, alligators, and crocodiles are all reptiles. Modern reptiles live on every continent except Antarctica. They range in size from the newly-discovered Jaragua Sphaero (a dwarf gecko), at 0.6 inches, to the saltwater crocodile, at up to 23 feet. There are four living orders of reptiles: 1. 2. 3. 4. Squamata, which includes lizards, snakes, and amphisbaenids (or worm-lizards). Crocodilia, which includes crocodiles, gharials ( Figure 1.1), caimans, and alligators. Testudines, which includes turtles and tortoises. Sphenodontia, which includes tuatara ( Figure 1.1). A gharial crocodile (left). A tuatara (right). "
reptiles,T_3338,"Reptiles are tetrapods (four-legged) and ectothermic, meaning their internal temperature depends on the temperature of their environment. This is why you may see reptiles sunbathing as they use the energy from the sun to warm their bodies. Usually the sense organs of reptiles, like ears, are well developed, though snakes do not have external ears. All reptiles have advanced eyesight. Reptiles also have a sense of smell. Crocodilians, turtles, and tortoises smell like most other land vertebrates. But, some lizards, and all snakes, smell with their tongues, which is flicked out of the mouth to pick up scent molecules from the air. Reptiles also have several adaptations for living on land. They have a skin covered in scales to protect them from drying out. All reptiles have lungs to breathe air. Reptiles are also amniotes, which means their embryos are surrounded by a thin membrane. This membrane protects the embryo from the harsh conditions of living on land. Reptile eggs are also surrounded by a protective shell, which may be either flexible or inflexible. "
reptiles,T_3339,"Most reptiles reproduce sexually, meaning there are two parents involved. In some families of lizards and one snake family, however, asexual reproduction is possible. This is when only one parent is involved in creating new life. For example, the gecko females can make tiny clones of themselves without the aid of a male. All reptiles have a cloaca, a single exit and entrance for sperm, eggs, and waste, located at the base of the tail. Most reptiles lay amniotic eggs covered with leathery or hard shell. These eggs can be placed anywhere as they dont have to be in a moist environment, like the eggs of amphibians. However, not all species lay eggs, as certain species of squamates can give birth to live young. Unlike the amphibians, there are no larval stages of development. The young reptiles look like miniature versions of the adult. The young reptiles are generally left to fend for themselves. However, some reptiles provide care for their young. For example, crocodiles and alligators may defend their young from predators. "
role of amphibians,T_3358,"Humans have used amphibians for a number of purposes for thousands of years, if not longer. Amphibians play significant roles in many food webs and are thus an important part of many ecosystems. For example, frogs keep insect populations stable. Extinction of frogs, or just significant decreases in the frog population, would probably have serious consequences for agricultural crops. Humans have also consumed amphibians, especially frogs, probably since they first ate meat. More recently, amphibians have been tremendously useful in research. "
role of amphibians,T_3359,"Amphibians play important roles in many ecosystems, especially as middle players in many food chains and food webs. In addition to consuming many worms and insects and other arthropods, and even some small reptiles and mammals and fish, they are prey for turtles and snakes, as well as some fish and birds. Tadpoles keep waterways clean by feeding on algae. Frogs are raised as a food source for humans. Frog legs are a delicacy in China, France, the Philippines, northern Greece, and the American south, especially the Frensh-speaking parts of Louisiana. Only the upper joint of the hind leg is served, which has a single bone similar to the upper joint of a chicken or turkey wing. They are commonly prepared by grilling or deep frying, sometimes breaded, though they can also be served with garlic, or turned into a soup or stew. Some estimates have well over a billion frogs harvested a year as food. Thats about one frog harvested for every seven people on the planet. "
role of amphibians,T_3360,"Amphibians have long been used in scientific research, especially developmental and physiological processes, largely due to their unique ability to undergo metamorphosis, and in some species, to regenerate limbs. Amphibians are also used in cloning research. Cloning involves making identical copies of a parent organism, and the large amphibian egg helps in this process. They are also used to study embryos because their eggs lack shells, so it is easy to watch their development. The African clawed frog, Xenopus laevis, is a species that is studied to understand aspects of developmental biology. It is a good model organism because it is easy to raise in a lab and has a large embryo, which is easy to study ( Figure 1.1). Many Xenopus genes have been identified and cloned, especially those involved in development. Developing Xenopus embryos can be easily observed and studied with a basic microscope, though the eggs are large enough to see without a microscope. Because of their size, the exact developmental stage after fertilization can be easily determined. This allows proteins that are used at a specific developmental time to be collected and analyzed. Identification of Xenopus genes and proteins has allowed the identification of corresponding genes and proteins from humans. Many environmental scientists believe that amphibians, including frogs, indicate when an environment is damaged. When species of frogs begin to decline, it often indicates that there is a bigger problem within the ecosystem. This could have dramatic effects on food webs and ecosystems. Frog embryos are often studied to better understand how development works. "
role of amphibians,T_3361,"Amphibians can be found in folklore, fairy tales, and popular culture. Numerous legends have developed over the centuries around the mystical properties of the salamander. Its name originates from the Persian words for fire and within,"" so many of these legends are related to fire. This connection likely originates from the tendency of many salamanders to live inside rotting logs. When placed into the fire, salamanders would escape from the logs, lending to the belief that the salamander was created from flames. Unforgettable amphibians Kermit the Frog ( Figure 1.2) and his popular saying Its not easy being green. Frogger, from the video game of the same name, has been teaching children about the dangers of the road and alligator-filled moats for years. And all it takes is a kiss from a princess to turn a frog into a prince, as told in The Frog Prince story. Kermit the Frog balloon is flown at the Annual Macys Thanksgiving Day Parade. "
safety in the life sciences,T_3365,"There can be some very serious safety risks in scientific research. If researchers are not careful, they could poison themselves or contract a deadly illness. The kinds of risks that scientists face depend on the kind of research they perform. For example, a scientist working with bacteria in a laboratory faces different risks than a scientist studying the behavior of lions in Africa, but both scientists must still follow safety guidelines. Safety practices must be followed when working with the hazardous things such as parasites, radiation and radioactive materials, toxins, and wild animals. Also, carcinogens, which are chemical that cause cancer, pathogens, which are disease-causing virus, bacteria or fungi, and teratogens, which are chemical that cause deformities in developing embryos, are extremely hazardous, and extreme care must be used when working with these items as well. For example, scientists studying dangerous organisms such as Yersinia pestis, the cause of bubonic plague, use special equipment that helps keep the organism from escaping the lab. A biohazard is any biological material that could make someone sick, including disease-causing organisms. There- fore, a used needle is a biohazard because it could harbor blood contaminated with a disease-causing organism. Bacteria grown in a laboratory are also biohazards if they could potentially cause disease. Science laboratory safety and chemical hazard signs. "
safety in the life sciences,T_3366,"If you perform an experiment in your classroom, your teacher will explain how to be safe. Professional scientists follow safety rules as well, especially for the study of dangerous organisms like the bacteria that cause bubonic plague ( Figure 1.2). Sharp objects, chemicals, heat, and electricity are all used at times in laboratories. Below is a list of safety guidelines that you should follow when in the laboratory: Be sure to obey all safety guidelines given in lab instructions and by your teacher. Follow directions carefully. Tie back long hair. Wear closed toe shoes with flat heels and shirts with no hanging sleeves, hoods, or drawstrings. Use gloves, goggles, or safety aprons when instructed to do so. Broken glass should only be cleaned up with a dust pan and broom. Never touch broken glass with your bare hands. Never eat or drink anything in the science lab. Table tops and counters could have dangerous substances on them. Be sure to completely clean materials like test tubes and beakers. Leftover substances could interact with other substances in future experiments. If you are using flames or heat plates, be careful when you reach. Be sure your arms and hair are kept far away from heat. Alert your teacher immediately if anything out of the ordinary occurs. An accident report may be required if someone is hurt. Also, the teacher must know if any materials are damaged or discarded. Scientists studying dangerous organisms such as Yersinia pestis, the cause of bubonic plague, use special equipment that helps keep the organism from escap- ing the lab. "
safety in the life sciences,T_3367,"A field scientist studies an organism in a natural setting, which is not usually an indoor laboratory. Scientists who work outdoors are also required to follow safety regulations. These safety regulations are designed to prevent harm to themselves, other humans, animals, and the environment. If scientists work outside the country, they are required to learn about and follow the laws and restrictions of the country in which they are doing research. For example, entomologists following monarch butterfly ( Figure 1.3) migrations between the United States and Mexico must follow regulations in both countries. Before biologists can study protected wildlife or plant species, they must apply for permission to do so, usually from the government. This is important to protect these fragile species. For example, if scientists collect rare butterflies, they must first get a permit. They must also be careful to not disturb the habitat. "
salamanders,T_3368,"Salamanders are characterized by slender bodies, short legs, and long tails. They are most closely related to the caecilians, little-known legless amphibians ( Figure 1.1). Most of the animals in the salamander order look like a cross between a lizard and a frog. They have moist, smooth skin like frogs and long tails like lizards. Salamanders are found in most moist or arid habitats in the Northern Hemisphere, but can also be found south of the equator. They live on all continents except Antarctica and Australia. Salamanders live in or near water or on moist ground, often in a swamp. Some species live in water most of their life, some live their entire adult life on land, and some live in both habitats. Some salamanders live in caves. These salamanders have pale skin and reduced eyes as they have adapted to living in complete darkness in underground pools of water. The reduced eyes are similar to other organisms that live in caves or underground. Salamanders are carnivorous, eating only other animals, not plants. They will eat almost any smaller animal, such as worms, centipedes, crickets, spiders, and slugs. Some will even eat small invertebrates. Finally, salamanders have the ability to grow back lost limbs, as well as other body parts. This process is known as regeneration. Salamanders have developed ways not to be eaten. Most salamanders have brightly colored, poisonous skin. The bold color tells predators not to eat the salamander. Many salamanders have glands on the back of the neck or on the tail that give off a poisonous or bad-tasting liquid. Some species can even shed their tail during an attack and grow a new one later. Some salamanders stand high on its legs and waves its tail to scare away danger. One particular salamander, the ribbed newt, has needle-like rib tips. It can squeeze its muscles to make the rib tips pierce through its skin and into its enemy, telling the predator to stay away, a feature unique among the animal kingdom. The marbled salamander (left) shows the typical salamander body plan: slender body, short legs, long tail, and moist skin. Caecilian (right) are a type of legless am- phibian most closely related to salaman- ders. "
salamanders,T_3369,"Different salamanders breathe in different ways. In those that have gills, breathing occurs through the gills as water passes over the gill slits. Sirens keep their gills all their lives, which allows them to breathe underwater. Species that live on land lose their gills as they grow older. These salamanders develop lungs that are used in breathing, much like breathing in mammals. Other land-living salamanders do not have lungs or gills. These are called lungless salamanders. Instead, they ""breathe,"" or exchange gases, through their skin. This requires blood vessels that exchange gases to be spread throughout the skin. "
salamanders,T_3370,"Salamanders are generally small. However, some can reach a foot or more, as in the mudpuppy of North America. In Japan and China, the giant salamander reaches 6 feet and weighs up to 66 pounds ( Figure 1.2). "
salamanders,T_3371,"Salamanders belong to a group of approximately 500 species of amphibians. The order Urodela, containing sala- manders and newts, is divided into three suborders: 1. Giant salamanders, including the hellbender and Asiatic salamanders. 2. Advanced salamanders, including lungless salamanders, mudpuppies, and newts. Newts are salamanders that spend most of each year living on land. 3. Sirens. Sirens are salamanders that have lungs as well as gills and never develop beyond the larval stage. Sirens have only two legs, but the other salamander species develop four legs as adults, with fleshy toes at the end of each foot. The legs on four-legged salamanders are so short that the salamander belly drags on the ground as the animal walks. Sirens have long, strong tails that are flat to help sirens swim like a fish, with the tail swinging from side to side. The Pacific giant salamander can reach up to 6 feet in length and weigh up to 66 pounds. "
scientific investigation,T_3372,"The scientific method is a process used to investigate the unknown ( Figure 1.1). It is the general process of a scientific investigation. This process uses evidence and testing. Scientists use the scientific method so they can find information. A common method allows all scientists to answer questions in a similar way. Scientists who use this method can reproduce another scientists experiments. Almost all versions of the scientific method include the following steps, although some scientists do use slight variations. 1. 2. 3. 4. 5. 6. 7. Make observations. Identify a question you would like to answer based on the observation. Find out what is already known about your observation (research). Form a hypothesis. Test the hypothesis. Analyze your results and draw conclusions. Communicate your results. "
scientific investigation,T_3373,"Imagine that you are a scientist. While collecting water samples at a local pond, you notice a frog with five legs instead of four ( Figure 1.2). As you start to look around, you discover that many of the frogs have extra limbs, Steps of a Scientific Investigation. A scientific investigation typically has these steps. extra eyes, or no eyes. One frog even has limbs coming out of its mouth. These are your observations, or things you notice about an environment using your five senses. A frog with an extra leg. "
scientific investigation,T_3374,"The next step is to ask a question about the frogs. You may ask, ""Why are so many frogs deformed?"" Or, ""Is there something in their environment causing these defects, like water pollution?"" Yet, you do not know if this large number of deformities is ""normal"" for frogs. What if many of the frogs found in ponds and lakes all over the world have similar deformities? Before you look for causes, you need to find out if the number and kind of deformities is unusual. So besides finding out why the frogs are deformed, you should also ask: ""Is the percentage of deformed frogs in this pond greater than the percentage of deformed frogs in other places?"" "
scientific investigation,T_3375,"No matter what you observe, you need to find out what is already known about your questions. For example, is anyone else doing research on deformed frogs? If yes, what did they find out? Do you think that you should repeat their research to see if it can be duplicated? During your research, you might learn something that convinces you to change or refine your question. From this, you will construct your hypothesis. A pond with frogs. "
scientific investigation,T_3376,"A hypothesis is a proposed explanation that tries to explain an observation. A good hypothesis allows you to make more predictions. For example, you might hypothesize that a pesticide from a nearby farm is running into the pond and causing frogs to have extra legs. If thats true, then you can predict that the water in a pond of non-deformed frogs will have lower levels of that pesticide. Thats a prediction you can test by measuring pesticide levels in two sets of ponds, those with deformed frogs and those with nothing but healthy frogs. Every hypothesis needs to be written in a way that it can: 1. 2. 3. 4. Be tested using experiments to collect evidence. Be proven wrong. Provide measurable results. Provide yes or no answers. For example, do you think the following hypothesis meets the four criteria above? Lets see. Hypothesis: ""The number of deformed frogs in five ponds that are polluted with chemical X is higher than the number of deformed frogs in five ponds without chemical X."" Of course, next you will have to test your hypothesis. "
scientific investigation,T_3377,"To test the hypothesis, an experiment will be done. You would count the healthy and deformed frogs and measure the amount of chemical X in all of the ponds. The hypothesis will be either true or false. Doing an experiment will test most hypotheses. The experiment may generate evidence in support of the hypothesis. The experiment may also generate evidence proving the hypothesis false. Once you collect your data, it will need to be analyzed. "
scientific investigation,T_3378,"If a hypothesis and experiment are well designed, the experiment will produce results that you can measure, collect, and analyze. The analysis should tell you if the hypothesis is true or false. Refer to the table for the experimental results ( Table 1.1). Polluted Pond 1 2 3 4 5 Average: Number of Deformed Frogs 20 23 25 26 21 23 Non-Polluted Pond 1 2 3 4 5 Average: Number of Deformed Frogs 23 25 30 16 20 22.8 Your results show that pesticide levels in the two sets of ponds are different, but the average number of deformed frogs is almost the same. Your results demonstrate that your hypothesis is false. The situation may be more complicated than you thought. This gives you new information that will help you decide what to do next. Even if the results supported your hypothesis, you would probably ask a new question to try to better understand what is happening to the frogs and why. "
scientific investigation,T_3379,"If a hypothesis and experiment are well designed, the results will indicate whether your hypothesis is true or false. If a hypothesis is true, scientists will often continue testing the hypothesis in new ways to learn more. If a hypothesis is false, the results may be used to come up with and test a new hypothesis. A scientist will then communicate the results to the scientific community. This will allow others to review the information and extend the studies. The scientific community can also use the information for related studies. Scientists communicate their results in a number of ways. For example, they may talk to small groups of scientists and give talks at large scientific meetings. They will also write articles for scientific journals. Their findings may also be communicated to journalists. If you conclude that frogs are deformed due to a pesticide not previously measured, you would publish an article and give talks about your research. Your conclusion could eventually help find solutions to this problem. "
scientific investigation,T_3380,"A summery video of the scientific method, using the identification of DNA structure as an example, is shown in this video by MIT students:  . "
scientific theories,T_3381,"One goal of a scientist is to find answers to scientific questions. To do this, scientists first develop a hypothesis, which is a proposed explanation that tries to explain an observation. To collect evidence to support (or disprove) their hypothesis, scientists must do experiments. Evidence is: 1. A direct, physical observation of something or a process over time. 2. Usually something measurable or ""quantifiable."" 3. The data resulting from an experiment. For example, an apple falling to the ground is evidence in support of the law of gravity. A bear skeleton in the woods would be evidence of the presence of bears. Looking at the image below might be confusing at first because this evidence seems to defy the law of gravity ( Figure 1.1). Of course water cannot be poured out of bottle and flow upward. The law of gravity is a scientific law, which is a statement describing what always happens under certain conditions in nature. Scientific laws are developed from lots of collected information. If many experiments are performed, and lots of evidence is collected in support of a general hypothesis, a scientific theory can be developed. Scientific theories are well established explanations of evidence, usually tested and confirmed by many different people. Scientific theories usually have a lot of evidence in support of the theory, and no evidence disproving the theory. Scientific theories produce information that helps us understand our world. For example, the idea that matter is made up of atoms is a scientific theory. Scientists accept this theory as a fundamental principle of basic science. A scientific theory must stand up to all scientific testing. Thus, when scientists find new evidence, they can change their theories. In addition to the germ theory of disease, other scientific theories are the cell theory and the theory of evolution. "
scientific ways of thinking,T_3382,"Modern science is a way of understanding the physical world, based on observable evidence, reasoning, and repeated testing. That means scientists explain the world based on their own observations. If they develop new ideas about the way the world works, they set up a way to test these new ideas. "
scientific ways of thinking,T_3383,"A scientist is always trying to find the truth and discover new truths. How can you think like a scientist? Thinking like a scientist is based on asking and answering questions. Though you may not know it, you do this all day long. Scientists ask questions, and then make detailed observations to try to ask more specific questions and develop a hypothesis. They may design and perform an experiment to try to answer their question and test their hypothesis. From the results of their experiment, scientists draw conclusions. A conclusion describes what the evidence tells the scientist. Scientists ask questions: The key to being a great scientist is to ask questions. Imagine you are a scientist in the African Congo. While in the field, you observe one group of healthy chimpanzees on the north side of the jungle. On the other side of the jungle, you find a group of chimpanzees that are mysteriously dying. What questions might you ask? A good scientist might ask the following two questions: 1. ""What differs between the two environments where the chimpanzees live?"" 2. ""Are there differences in behavior between the two groups of chimpanzees?"" Scientists make detailed observations: To observe means to watch and study attentively. A person untrained in the sciences may only observe, ""The chimps on one side of the jungle are dying, while chimps on the other side of the jungle are healthy."" A scientist, however, will make more detailed observations. Can you think of ways to make this observation more detailed? What about the number of chimps? Are they male or female? Young or old? What do they eat? A good scientist may observe, ""While all seven adult females and three adult males on the north side of the jungle are healthy and show normal behavior, four female and five male chimps under the age of five on the south side have died."" Detailed observations can ultimately help scientists design their experiments and answer their questions. From these observations, a scientist will develop a hypothesis to explain the observations. A hypothesis is the scientists proposed explanation for his observations. The scientists hypothesis may be that ""Young chimps on the south side die due to a lack of nutrients in their diet."" An adult and infant chimpanzee (Pan troglodytes). Scientists find answers using tests: When scientists want to answer a question, they search for evidence using experiments. An experiment is a test to see if their explanation is right or wrong. Evidence is made up of the observations a scientist makes during an experiment. To study the cause of death in the chimpanzees, scientists may give the chimps nutrients in the form of nuts, berries, and vitamins to see if they are dying from a lack of food. This test is the experiment. If fewer chimps die, then the experiment shows that the chimps may have died from not having enough food. This is the evidence. Scientists question the answers: Good scientists are skeptical. Scientists never use only one piece of evidence to form a conclusion. For example, the chimpanzees in the experiment may have died from a lack of food, but can you think of another explanation for their death? They may have died from a virus, or from another less obvious cause. More experiments need to be completed before scientists can be sure. Science is about finding the truth, no matter what. So good scientists constantly question their own conclusions. They also find other scientists to confirm or disagree with their evidence. "
seasonal changes in plants,T_3384,"Have you seen the leaves of plants change colors? During what time of year does this happen? What causes it to happen? Plants can sense changes in the seasons. Leaves change color and drop each autumn in some climates ( Figure 1.1). Certain flowers, like poinsettias, only bloom during the winter. And, in the spring, the winter buds on the trees break open, and the leaves start to grow. How do plants detect time of year? Although you might detect seasonal changes by the change in temperature, this is not the way in which plants know the seasons are changing. Plants determine the time of year by the length of daylight, known as the photoperiod. Because of the tilt of the Earth, during winter days, there are less hours of light than during summer days. Thats why, in the winter, it starts getting dark very early in the evening, and then stays dark while youre getting ready for school the next morning. But in the summer it will be bright early in the morning, and the sun will not set until late that night. With a light-sensitive chemical, plants can sense the differences in day length. For example, in the fall, when the days start to get shorter, the trees sense that there is less sunlight. The plant is stimulated, and it sends messages telling the leaves to change colors and fall. This is an example of photoperiodism, the reaction of organisms, such as plants, to the length of day or night. Photoperiodism is also the reaction of plants to the length of light and dark periods. Many flowering plants sense the length of night, a dark period, as a signal to flower. Each plant has a different photoperiod, or night length. When the plant senses the appropriate length of darkness, resulting in an appropriate length of daylight, it flowers. Flowering plants are classified as long-day plants or short-day plants. Long-day plants flower when the length of daylight exceeds the necessary photoperiod, and short-day plants flower when the day length is shorter than the necessary photoperiod. Long-day plants include carnations, clover, lettuce, wheat, and turnips. Short-day plants include cotton, rice, and sugar cane. "
seeds and seed dispersal,T_3385,Plants seem to grow wherever they can. How? Plants cant move on their own. So how does a plant start growing in a new area? 
seeds and seed dispersal,T_3386,"If youve ever seen a plant grow from a tiny seed, then you might realize that seeds are amazing structures. A seed is a plant ovule containing an embryo. The seed allows a plant embryo to survive droughts, harsh winters, and other conditions that would kill an adult plant. The tiny plant embryo can simply stay dormant, in a resting state, and wait for the perfect environment to begin to grow. In fact, some seeds can stay dormant for hundreds of years! Another impressive feature of the seed is that it stores food for the young plant after it sprouts. This greatly increases the chances that the tiny plant will survive. So being able to produce a seed is a beneficial adaptation, and, as a result, seed plants have been very successful. Although the seedless plants were here on Earth first, today there are many more seed plants than seedless plants. Learn more about seeds in the Seeds Massachusetts Institute of Technology video at "
seeds and seed dispersal,T_3387,"For a seed plant species to be successful, the seeds must be dispersed, or scattered around in various directions. If the seeds are spread out in many different areas, there is a better chance that some of the seeds will find the right conditions to grow. But how do seeds travel to places they have never been before? To aid with seed dispersal, some plants have evolved special features that help their seeds travel over long distances. One such strategy is to allow the wind to carry the seeds. With special adaptations in the seeds, the seeds can be carried long distances by the wind. For example, you might have noticed how the ""fluff"" of a dandelion moves in the wind. Each piece of fluff carries a seed to a new location. If you look under the scales of pine cone, you will see tiny seeds with ""wings"" that allow these seeds to be carried away by the wind. Maple trees also have specialized fruits with wing-like parts that help seed dispersal ( Figure 1.1). Maple trees have fruits with wings that help the wind disperse the seeds. Some flowering plants grow fleshy fruit that helps disperse their seeds. When animals eat the fruit, the seeds pass through an animals digestive tract unharmed. The seeds germinate after they are passed out with the animals feces. Berries, citrus fruits, cherries, apples, and a variety of other types of fruits are all adapted to be attractive to animals, so the animals will eat them and disperse the seed ( Figure 1.2). Some non-fleshy fruits are specially adapted for animals to carry them on their fur. You might have returned from a walk in the woods to find burrs stuck to your socks. These burrs are actually specialized fruits designed to carry seeds to a new location. "
social behavior of animals,T_3404,"Why is animal communication important? Without it, animals would not be able to live together in groups. Animals that live in groups with other members of their species are called social animals. Social animals include many species of insects, birds, and mammals. Specific examples of social animals are ants, bees, crows, wolves, lions, and humans. To live together with one another, these animals must be able to share information. "
social behavior of animals,T_3405,"Some species of animals are very social. In these species, members of the group depend completely on one another. Different animals within the group have different jobs. Therefore, group members must work together for the good of all. Most species of ants and bees are highly social animals. Ants live together in large groups called colonies ( Figure 1.1). A colony may have millions of ants, making communication among the ants very important. All of the ants in the colony work together as a single unit. Each ant has a specific job, and most of the ants are workers. Their job is to build and repair the colonys nest. Worker ants also leave the nest to find food for themselves and other colony members. The workers care for the young as well. Other ants in the colony are soldiers. They defend the colony against predators. Each colony also has a queen. Her only job is to lay eggs. She may lay millions of eggs each month. A few ants in the colony are called drones. They are the only male ants in the colony. Their job is to mate with the queen. Honeybees and bumblebees also live in colonies ( Figure 1.2). Each bee in the colony has a particular job. Most of the bees are workers. Young worker bees clean the colonys hive and feed the young. Older worker bees build the waxy honeycomb or guard the hive. The oldest workers leave the hive to find food. Each colony usually has one queen that lays eggs. The colony also has a small number of male drones. They mate with the queen. "
social behavior of animals,T_3406,"Ants, bees, and other social animals must cooperate. Cooperation means working together with others. Members of the group may cooperate by sharing food. They may also cooperate by defending each other. Look at the ants pictured below ( Figure 1.3). They show very clearly why cooperation is important. A single ant would not be able to carry this large bee back to the nest to feed the other ants. With cooperation, the job is easy. Animals in many other species cooperate. For example, lions live in groups called prides ( Figure 1.4). All the lions in the pride cooperate, though there is still serious competition among the males. Male lions work together to defend the other lions in the pride. Female lions work together to hunt. Then, they share the meat with other pride members. Another example of cooperation is seen with meerkats. Meerkats are small mammals that live in Africa. They also live in groups and cooperate with one another. For example, young female meerkats act as babysitters. They take care of the baby meerkats while their parents are away looking for food. Members of this lion pride work together. Males cooperate by defending the pride. Females cooperate by hunting and shar- ing the food. "
structural evidence for evolution,T_3409,"Even though two different species may not look similar, they may have similar internal structures that suggest they have a common ancestor. That means both evolved from the same ancestor organism a long time ago. Common ancestry can also be determined by looking at the structure of the organism as it first develops. "
structural evidence for evolution,T_3410,"Some of the most interesting kinds of evidence for evolution are body parts that have lost their use through evolution ( Figure 1.1). For example, most birds need their wings to fly. But the wings of an ostrich have lost their original use. Structures that have lost their use through evolution are called vestigial structures. They provide evidence for evolution because they suggest that an organism changed from using the structure to not using the structure, or using it for a different purpose. Penguins do not use their wings, known as flippers, to fly in the air. However, they do use them to move in the water. The theory of evolution suggests that penguins evolved to use their wings for a different purpose. A whales pelvic bones, which were once attached to legs, are also vestigial structures. Whales are descended from land-dwelling ancestors that had legs. Homologous structures are structures that have a common function and suggest common ancestry. For example, homologous structures include the limbs of mammals, such as bats, lions, whales, and humans, which all have a common ancestor. Different mammals may use their limbs for walking, running, swimming or flying. The method the mammal uses to move is considered a common function. "
structural evidence for evolution,T_3411,"Some of the oldest evidence of evolution comes from embryology, the study of how organisms develop. An embryo is an animal or plant in its earliest stages of development. This means looking at a plant or animal before it is born or hatched. Centuries ago, people recognized that the embryos of many different species have similar appearances. The embryos of some species are even difficult to tell apart. Many of these animals do not differ much in appearance until they develop further. Some unexpected traits can appear in animal embryos. For example, human embryos have gill slits just like fish! In fish they develop into gills, but in humans they disappear before birth. The presence of the gill slits suggests that a long time ago humans and fish shared a common ancestor. The similarities between embryos suggests that these animals are related and have common ancestors. For example, humans did not evolve from chimpanzees. But the similarities between the embryos of both species suggest that we have an ancestor in common with chimpanzees. As our common ancestor evolved, humans and chimpanzees went down different evolutionary paths and developed different traits. "
succession,T_3412,"When you see an older forest, its easy to picture that the forest has been there forever. This is not the case. Ecosystems are ""dynamic."" This means that ecosystems change over time. That forest may lie on land that was once covered by an ocean millions of years ago. Lightning may have sparked a fire in a forest, destroying much of the plant life there. Or the forest may have been cut down at one point for agricultural use, then abandoned and allowed to re-grow over time. During the ice ages, glaciers once covered areas that are tropical rainforests today. Both natural forces and human actions cause ecosystems to change. If there is a big ecosystem change caused by natural forces or human actions, the plants and animals that live there may be destroyed. Or they may be forced to leave. Over time, a new community will develop, and then that community may be replaced by another. You may see several changes in the plant and animal composition of the community over time. Ecological succession is the constant replacement of one community by another. It happens after a big change in the ecosystem. And, of course, succession occurs on brand new land. "
succession,T_3413,"Primary succession is the type of ecological succession that happens on new landslands where life has not yet existed. Primary succession can take place after lava flow cools and hardens into new land, or a glacier recedes exposing new land. Since the land that results from these processes is completely new land, soil must first be produced. How is soil produced? Primary succession always starts with a pioneer species. This is the species that first lives in the habitat. If life is to begin on barren rock, which is typical of new land, the pioneer species would be an organism such as a lichen ( Figure 1.1). A lichen is actually an organism formed from two species. It results from a symbiotic relationship between a fungus and an algae or cyanobacteria. The lichen is able to thrive as both the fungus and the algae or bacteria contribute to the relationship. The fungus is able to absorb minerals and nutrients from the rock, while algae supplies the fungus with sugars through photosynthesis. Since lichens can photosynthesize and do not rely on soil, they can live in environments where other organisms cannot. As a lichen grows, it breaks down the rock, which is the first step of soil formation. Primary succession on a rock often be- gins with the growth of lichens. What do lichens help create? The pioneer species is soon replaced by other populations. Abiotic factors such as soil quality, water, and climate will determine the species that continue the process of succession. Mosses and grasses will be able to grow in the newly created soil. During early succession, plant species like grasses that grow and reproduce quickly will take over the landscape. Over time, these plants improve the soil and a few shrubs can begin to grow. Slowly, the shrubs are replaced by small trees. Small trees then are succeeded by larger trees. Since trees are more successful at competing for resources than shrubs and grasses, a forest may be the end result of primary succession. "
succession,T_3414,"Sometimes ecological succession occurs in areas where life has already existed. These areas already have soil full of nutrients. Secondary succession is the type of succession that happens after something destroys the habitat, such as a flood or other natural disaster. Abandoning a field that was once used for agriculture can also lead to secondary succession ( Figure 1.2). In this case, the pioneer species would be the grasses that first appear. Lichen would not be necessary as there is already nutrient-rich soil. Slowly, the field would return to its natural state. A forest fire can alter a habitat such that secondary succession occurs ( Figure 1.3 and Figure 1.4). Although the area will look devastated at first, the seeds of new plants are underground. They are waiting for their chance to grow. This land was once used for growing crops. Now that the field is abandoned, secondary succession has begun. Pio- neer species, such as grasses, appear first, and then shrubs begin to grow. Just like primary succession, the burned forest will go through a series of communities, starting with small grasses, then shrubs, and finally bigger trees. "
succession,T_3415,"A climax community ( Figure 1.5) is the end result of ecological succession. The climax community is a stable balance of all organisms in an ecosystem, and will remain stable unless a disaster strikes. After the disaster, succession will start all over again. Depending on the climate of the area, the climax community will look different. In the tropics, the climax community might be a tropical rainforest. At the other extreme, in northern parts of the world, the climax community might be a coniferous forest. Though climax communities are stable, are they truly the final community of the habitat? Or is it likely that sometime in the future, maybe a long time in the future, the community of populations will change, and another stable, climax community will thrive? "
symbiosis,T_3416,"Symbiosis describes a close and long-term relationship between different species. At least one species will benefit in a symbiotic relationship. These relationships are often necessary for the survival of one or both organisms. There are three types of symbiotic relationships: mutualism, communalism, and parasitism. Mutualism is a symbiotic relationship in which both species benefit. Commensalism is a symbiotic relationship in which one species benefits while the other is not affected. Parasitism is a symbiotic relationship in which the parasitic species benefits while the host species is harmed. An example of a mutualistic relationship is between herbivores (plant-eaters) and the bacteria that live in their intestines. The bacteria get a place to live. Meanwhile, the bacteria help the herbivore digest food. Both species benefit, so this is a mutualistic relationship. The clownfish and the sea anemones also have a mutualistic relationship. The clownfish protects the anemone from anemone-eating fish, and the stinging tentacles of the anemone protect the clownfish from predators ( Figure 1.1). Another example of this type of symbiotic relationship is the relationship between the plover bird and the African crocodile. The tiny blackbird acts as a toothpick for the fierce crocodile, and helps by removing tiny morsels of food that are stuck between the crocodiles teeth. These food remains are the source of food for the bird. Another example is between the ostrich and the zebra. The ostrich always moves with the herd of zebras since it has a poor sense of hearing and smell, whereas the zebra has very sharp senses. The ostrich has a keen sense of sight, which the zebra lacks. Hence, these two species depend on each other to warn one another of any nearby imposing dangers. Commensal relationships may involve an organism using another for transportation or housing. For example, spiders build their webs on trees. The spider gets to live in the tree, but the tree is unaffected. Other commensal relationships exist between cattle egrets and livestock. Cattle egrets are mostly found in meadows and grasslands are always seen near cattle, horses and other livestock. These birds feed on the insects that come out of the field due to the movement of the animals. They even eat ticks, fleas, and other insects off the back of animals. The relationship between tigers and golden jackals is also commensalism. The jackal alerts the tiger to a kill and feeds on the remains of the prey left by the tiger. This is not a mutualistic relationship as the tiger does not provide anything to the jackal. Parasites may live either inside or on the surface of their host. An example of a parasite is a hookworm. Hookworms are roundworms that affect the small intestine and lungs of a host organism. They live inside of humans and cause them pain. However, the hookworms must live inside of a host in order to survive. Parasites may even kill the host they live on, but then they also kill their host organism, so this is rare. Parasites are found in animals, plants, and fungi. Hookworms are common in the moist tropic and subtropic regions. There is very little risk of getting a parasite in industrialized nations. Clownfish in a sea anemone. "
symbiotic relationships of fungi,T_3417,"Fungi dont live in isolation. They often interact with other species. In fact, fungi can be dependent on another or- ganism for survival. When two species live close together and form a relationship, it is called symbiosis. Symbiosis can be beneficial to one or both organisms, or sometimes one organism hurts the other. Some of the partners in these relationships include plants, algae, insects and other animals, and even humans. "
symbiotic relationships of fungi,T_3418,"If it were not for fungi, many plants would go hungry. In the soil, fungi grow closely around the roots of plants, and they begin to help each other. The plant roots together with the special root-dwelling fungi are called mycorrhizae ( Figure 1.1). As plants and fungi form a close relationship, the plant and the fungus feed one another. The plant provides sugars to the fungus that the plant makes through photosynthesis, which the fungus cannot do. The fungus then provides minerals and water to the roots of the plant. Since the plant and the fungus are helping each other out, this is a mutualistic relationship, a type of symbiosis known as mutualism. In a mutualistic relationship, both organisms benefit. These roots (brown) and the mycorrhizae (white) help to feed one another. "
symbiotic relationships of fungi,T_3419,"Have you ever seen an organism called a lichen? Lichens are crusty, hard growths that you might find on trees, logs, walls, and rocks ( Figure 1.2). Although lichens may not be the prettiest organisms in nature, they are unique. A lichen is really two organisms, sometimes referred to as a composite organism, that live very closely together: a fungus and a bacterium or an alga. The cells from the alga or bacterium live inside the fungus. Besides providing a home, the fungus also provides nutrients. In turn, the bacterium or the alga provides energy to the fungus by performing photosynthesis, obtaining energy directly from the sun. A lichen is also an example of a mutualistic relationship. Because lichens can grow on rocks, these organisms are some of the earliest life forms in new ecosystems. "
symbiotic relationships of fungi,T_3420,"Many insects have a symbiotic relationship with certain types of fungi: Ants and termites grow fungi in underground fungus gardens that they create. When the ants or termites have eaten a big meal of wood or leaves, they also eat some fungi from their gardens. The fungi help them digest the wood or leaves. The fungi secrete certain enzymes that the ants or termites cannot produce on their own. Ambrosia beetles live in the bark of trees. Like ants and termites, they grow fungi inside the bark of trees and use it to help digest their food. "
symbiotic relationships of fungi,T_3421,"Although lots of symbiotic relationships help both organisms, sometimes one of the organisms is harmed. When that happens, the organism that benefits, and is not harmed, is called a parasite. This type of relationship is known as parasitism. Examples of parasitic fungi include the following: Beginning in 1950, Dutch Elm trees in the United States began to die. Since then, most of these trees have been eliminated. The disease was caused by a fungus that acted as a parasite. The fungus that killed the trees was carried by beetles to the trees. Some parasitic fungi cause human diseases such as athletes foot and ringworm. These fungi feed on the outer layer of warm, moist skin. Though its name may suggest otherwise, ringworm is not caused by a worm, but by a fungus. "
terrestrial biomes,T_3425,"A terrestrial biome is an area of land with a similar climate that includes similar communities of plants and animals. Different terrestrial biomes are usually defined in terms of their plants, such as trees, shrubs, and grasses. Factors such as latitude, humidity, and elevation affect biome type: Latitude means how far a biome is from the equator. Moving from the poles to the equator, you will find (in order) Arctic, boreal, temperate, subtropical, and tropical biomes. Humidity is the amount of water in the air. Air with a high concentration of water will be called humid. Moving away from the most humid climate, biomes will be called semi-humid, semi-arid, or arid (the driest). Elevation measures how high land is above sea level. It gets colder as you go higher above sea level, which is why you see snow-capped mountains. Terrestrial biomes include grasslands, forests, deserts, and tundra. Grasslands are characterized as lands dominated by grasses rather than large shrubs or trees and include the savanna and temperate grasslands. Forests are dominated by trees and other woody vegetation and are classified based on their latitude. Forests include tropical, temperate, and boreal forests (taiga). Deserts cover about one fifth of the Earths surface and occur where rainfall is less than 50 cm (about 20 inches) each year. Tundra is the coldest of all the biomes. The tundra is characterized for its frost-molded landscapes, extremely low temperatures, little precipitation, poor nutrients, and short growing seasons. There are two main types of tundra, Arctic and Alpine tundras. Terrestrial biomes ( Figure 1.1) lying within the Arctic and Antarctic Circles do not have very much plant or animal life. Biomes with the highest amount of biodiversity, that is the most variation in plant and animal life, are near the equator ( Figure 1.2). One of the terrestrial biomes, taiga, is an evergreen forest of the subarctic, covering extensive areas of northern North Amer- ica and Eurasia. This taiga is along the Denali Highway in Alaska. "
the biosphere,T_3426,"The highest level of ecological organization is the biosphere. It is the part of Earth, including the air, land, surface rocks, and water, where life is found. Parts of the lithosphere, hydrosphere, and atmosphere make up the biosphere. The lithosphere is the outermost layer of the Earths crust; essentially land is part of the lithosphere. The hydrosphere is composed of all the areas that contain water, which can be found on, under, and over the surface of Earth. The atmosphere is the layer of gas that surrounds the planet. The biosphere includes the area from about 11,000 meters below sea level to 15,000 meters above sea level. It overlaps with the lithosphere, hydrosphere, and atmosphere. Land plants and animals are found on the lithosphere, freshwater and marine plants and animals are found in the hydrosphere, and birds and other flying animals are found in the atmosphere. Of course, there are countless bacteria, protists, and fungi that are also found in the biosphere. "
the biosphere,T_3427,"The Gaia hypothesis states that the biosphere is its own living organism. The hypothesis suggests that the Earth is self-regulating and tends to achieve a stable state, known as homeostasis. For example the composition of our atmosphere stays fairly consistent, providing the ideal conditions for life. When carbon dioxide levels increase in the atmosphere, plants grow more quickly. As their growth continues, they remove more carbon dioxide from the atmosphere. In this way, the amount of carbon dioxide stays fairly constant without human intervention. For a better understanding of how the biosphere works and various dysfunctions related to human activity, scientists have simulated the biosphere in small-scale models. Biosphere 2 ( Figure 1.1) is a laboratory in Arizona that contains 3.15 acres of closed ecosystems. Ecosystems of Biosphere 2 are an ocean ecosystem with a coral reef, mangrove wetlands, a tropical rainforest, a savannah grassland and a fog desert. See  for additional information. Additional biosphere projects include BIOS-3, a closed ecosystem in Siberia, and Biosphere J, located in Japan. "
tracing evolution,T_3441,How fast is evolution? Can you actually see evolution happening within your lifetime? Usually evolution takes a long time. So how can we visualize how it has happened? 
tracing evolution,T_3442,"How long did it take for the giraffe to develop a long neck? How long did it take for the Galpagos finches to evolve? How long did it take for whales to evolve from land mammals? These, and other questions about the rate of evolution, are difficult to answer. The rate of evolution depends on how many of an organisms genes have changed over a period of time. Evolution is usually so gradual that we do not see the change for many, many generations. The rate of evolution also depends on the generation time of a particular species. Not all organisms evolve at the same rate. Humans took millions of years to evolve from a mammal that is now extinct. It is very difficult to observe evolution in humans. However, there are organisms that are evolving so fast that you can observe evolution! A human takes about 22 years to go through one generation. But some bacteria go through over a thousand generations in less than two months. Some bacteria go through many generations in a few days. And sometimes a bacterial generation is as fast as 20 minutes! We can actually trace their evolution as it is happening. "
tracing evolution,T_3443,"If evolution can take a very long time, how can we visualize how it happens? Charles Darwin came up with the idea of an evolutionary tree to represent the relationships between different species and their common ancestors ( Figure 1.1). The base of the tree represents the ancient ancestors of all life. The separation into large branches shows where these original species evolved into new species. The branches keep splitting into smaller and smaller branches as species continue to evolve into more and more species. Some species are represented by short twigs spurting out of the tree, then stopping. These are species that went extinct before evolving into new species. Other Trees of Life have been created by other scientists ( Figure would that be? Animal, plant, fungi protist, or none of those? Darwin drew this version of the Tree of Life on the left to represent how species evolve and diverge into separate direc- tions. Each point on the tree where one branch splits off from another represents the common ancestor of the species on the separate branches. Scientists have drawn many different versions of the Tree of Life to show different features of evo- lution. The Tree of Life on the right was made by Ernst Haeckel in 1879. "
tropisms,T_3446,"Plants may not be able to move to another location, but they are able to change how they grow in response to their environment. Growth toward or away from a stimulus is known as a tropism ( Table 1.1). Auxins, a class of plant hormones, allow plants to curve in specific directions as they grow. The auxin moves to one side of the stem, where it starts a chain of events that cause rapid cell growth on just that one side of the stem. With one side of the stem growing faster than the other, the plant begins to bend. Name Phototropism Gravitropism Thigmotropism Stimulus Light Gravity Touch "
tropisms,T_3447,"You might have noticed that plants bend toward the light. This is an example of a tropism where light is the stimulus, known as phototropism ( Figure 1.1). To obtain more light for photosynthesis, leaves and stems grow toward the light. On the other hand, roots grow away from light. This is beneficial for the roots, because they need to obtain water and nutrients from deep within the ground. "
tropisms,T_3448,"So, how do the roots of seeds underground know to grow downward? How do the roots deep in the soil know which way is up? Gravitropism is a growth toward or away from the pull of gravity ( Figure 1.2). Shoots, the new growth of a plant, also show a gravitropism, but in the opposite direction. If you place a plant on its side, the stem and new leaves will curve upward. "
tropisms,T_3449,"Plants also have a touch response called thigmotropism. If you have ever seen a morning glory or the tendrils of a pea plant twist around a pole, then you know that plants must be able to sense the pole. Thigmotropism works much like the other tropisms. The plant grows straight until it comes in contact with the pole. Then, the side of the stem that is in contact with the pole grows slower than the opposite side of the stem. This causes the stem to bend around the pole. "
turtles,T_3450,"Turtles are reptiles in the order Testudines. If you have seen turtles before, what is the most noticeable thing about them? Their shells. Most turtle bodies are covered by a special shell developed from their ribs. Their shells can be bony or cartilaginous, made from a more flexible supportive tissue. About 300 species are alive today, and some are highly endangered. Like other reptiles, turtles cannot regulate their body temperature, except with behavioral means, such as burrowing underground. The major difference between turtles and tortoises is that the land dwelling ones are called tortoises and water dwelling are called turtles. Turtles are broken down into two groups, based on how they bring their neck back into their shell: 1. Cryptodira, which can draw their neck inside and under their spine. 2. Pleurodira, which fold their necks to one side. "
turtles,T_3451,"Although many turtles spend large amounts of their lives underwater, they can also spend much of their lives on dry land and breathe air. Turtles cannot breathe in water, but can hold their breath for long periods of time. Turtles must surface at regular intervals to refill their lungs. The position of a turtles eyes can give a clue to their natural habitat. Most turtles that spend most of their lives on land have their eyes looking down at objects in front of them. Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species of turtles can hide from predators in shallow water, where they lie entirely submerged in water except for their eyes and nostrils. Sea turtles ( Figure 1.1) have glands near their eyes that produce salty tears, which remove excess salt taken in from the water they drink. A species of sea turtle, showing place- ment of eyes, shell shape, and flippers. Turtles have exceptional night vision due to the unusually large number of cells that sense light in their eyes. This allows them to be active at any time of the day. Turtles also have color vision. Turtles dont lay eggs underwater. Turtles lay slightly soft and leathery eggs, like other reptiles. The eggs of the largest species are spherical, while the eggs of the rest are longer in shape. After internal fertilization, a female is ready to lay her eggs, she places a large numbers of eggs in holes dug into mud or sand. They are then covered and left to grow and develop by themselves. When the turtles hatch, they squirm their way to the surface and head toward the water. They need to get to the water as fast as possible before they are fed upon by animals such as seabirds, crabs, and raccoons. "
turtles,T_3452,"Turtles may appear slow and harmless when they are out of the water, but in the water is another story. Turtles can be either herbivores or carnivores, with most sea turtles carnivorous. Turtles have a rigid beak and use their jaws to cut and chew food. Instead of teeth, the upper and lower jaws of the turtle are covered by horny ridges. Carnivorous, or animal-eating turtles usually have knife-sharp ridges for slicing through their prey. But as the turtle is not a very fast animal, and it cannot quickly turn its head to snap at prey, it does have some limitations. Sea turtles typically feed on jellyfish, sponges and other soft-bodied organisms. Some species of sea turtle with stronger jaws eat shellfish, while other species, such as the green sea turtle, do not eat any meat at all. Herbivorous turtles have serrated ridges that help them cut through tough plants. "
turtles,T_3453,"The largest turtle is the great leatherback sea turtle ( Figure 1.2), which can have a shell length of seven feet and can weigh more than 2,000 pounds. The only surviving giant tortoises are on the Seychelles and Galpagos Islands and can grow to over four feet in length and weigh about 670 pounds ( Figure 1.3). The smallest turtle is the speckled padloper tortoise of South Africa, measuring no more than three inches in length, and weighing about five ounces. The largest ever turtle was the know extinct Archelon genus, a Late Cretaceous sea turtle known to have been up to 15 ft long, and 16 ft wide from flipper to flipper. The closest living relative of this genus is the leatherback sea turtle. It was the giant Galpagos tortoises that Charles Darwin studied during his voyage on the Beagle, providing significant evidence that he used to support his theory of evolution. A giant tortoise can grow to over feet ft in length and weigh about 670 lb. These animals can easily live over 100 years, spending their days grazing on grass, leaves, and cactus, basking in the sun, and napping nearly 16 hours each day. "
types of archaea,T_3454,"The first archaea described could survive in extremely harsh environments in which no other organisms could survive. As a result, archaea are often distinguished by the environment in which they live. "
types of archaea,T_3455,"The halophiles, which means ""salt-loving,"" live in environments with high levels of salt ( Figure 1.1). They have been identified in the Great Salt Lake in Utah and in the Dead Sea between Israel and Jordan, which have salt concentrations several times that of the oceans. "
types of archaea,T_3456,"The thermophiles live in extremely hot environments. For example, they can grow in hot springs, geysers, and near volcanoes. Unlike other organisms, they can thrive in temperatures near 100C, the boiling point of water! "
types of archaea,T_3457,"Methanogens can also live in some strange places, such as swamps and inside the guts of cows and termites. They help these animals break down cellulose, a tough carbohydrate made by plants ( Figure 1.2). This is an example of a mutualistic relationship. Methanogens are named for their waste product, a gas called methane. Cows are able to digest grass with the help of the methanogens in their gut. "
types of archaea,T_3458,"Although archaea are known for living in unusual environments, such as the Dead Sea, inside hot springs, and in the guts of cows, they also live in more common environments. For example, new research shows that archaea are abundant in the soil. They also live among the plankton in the ocean ( Figure 1.3). Therefore, scientists are just beginning to discover some of the important roles that archaea have in the environment. Thermococcus gammatolerans are another type of archaea. "
types of mollusks,T_3463,"There are approximately 160,000 living species and probably 70,000 extinct species of mollusks. They are typically divided into ten classes, of which two are extinct. The major classes of living mollusks include gastropods, bivalves, and cephalopods ( Figure 1.1). "
types of mollusks,T_3464,"Gastropods include snails and slugs. They use their foot to crawl. They have a well-developed head. There are many thousands of species of sea snails and sea slugs, as well as freshwater snails, freshwater limpets, land snails and land slugs. Gastropods live in many diverse habitats, from gardens to deserts and mountains. They also live in rivers, lakes and the ocean. Most shelled gastropods have a one-piece shell that is typically coiled or spiraled, but not all gastropods have shells. Gastropods have no sense of hearing, but they can see and have a keen sense of smell. In land-based gastropods, the olfactory organs (for smell) are the most important. These are located on the tentacles. "
types of mollusks,T_3465,"Bivalves include clams, scallops, oysters, and mussels. As their name implies, they have two parts of their shell, which can open and close. Bivalves live in both marine and freshwater habitats. Most bivalves have a pair of large gills that enable them to extract oxygen from the water (to breathe) and to capture food. Water is drawn into the bivalve and washes over the gills. Mucus on the gills helps capture food and cilia transfer the food particles to the mouth. Once in the mouth, food passes into the stomach to be digested. Bivalves have a mouth, heart, intestine, gills, and stomach, but no head. Bivalves have a muscular foot, which in many species such as clams, is used to anchor their body to a surface or dig down into the sand. "
types of mollusks,T_3466,"Cephalopods include the octopus and squid. They have a prominent head and a well-developed brain. Typically the foot has been modified into a set of arms or tentacles. Members of this class can change color. They can also change texture and body shape, and, and if those camouflage techniques dont work, they can still ""disappear"" in a cloud of ink. Cephalopods have three hearts that pump blue blood, theyre jet powered by their muscular foot, and theyre found in all oceans of the world. Cephalopods are thought to be the most intelligent of invertebrates. They have eyes and other senses that rival those of humans. Many cephalopods are active and efficient predators. What features do you think allows for this? (left) An example of a gastropod species, the ostrich foot. (right) A Caribbean reef squid, an example of a cephalopod. "
vascular seedless plants,T_3476,"For these plants, the name says it all. Vascular seedless plants have vascular tissue but do not have seeds. Remember that vascular tissue is specialized tissue that transports water and nutrients throughout the plant. The development of vascular tissue allowed these plants to grow much taller than nonvascular plants, forming ancient swamp forests. Most of these large vascular seedless plants are now extinct, but their smaller relatives still remain. Vascular tissue includes xylem, which transports water from the roots to the rest of the plant; and phloem, which transports sugars and nutrients from the leaves throughout the plant. Seedless vascular plants include: 1. 2. 3. 4. Clubmosses. Ferns. Horsetails. Whisk ferns. "
vascular seedless plants,T_3477,"Clubmosses are so named because they can look similar to mosses ( Figure 1.1). Clubmosses are not true mosses, though, because they have vascular tissue. The club part of the name comes from club-like clusters of sporangia found on the plants. One type of clubmoss is called the ""resurrection plant"" because it shrivels and turns brown when it dries out but then quickly turns green when watered again. Clubmosses can resemble mosses; how- ever, clubmosses have vascular tissue, while mosses do not. "
vascular seedless plants,T_3478,"Ferns are the most common seedless vascular plants ( Figure 1.2). They usually have large divided leaves called fronds. In most ferns, fronds develop from a curled-up formation called a fiddlehead ( Figure 1.3). The fiddlehead looks like the curled decoration on the end of a stringed instrument, such as a fiddle. Leaves unroll as the fiddleheads grow and expand. Ferns grow in a variety of habitats, ranging in size from tiny aquatic species to giant tropical plants. "
vascular seedless plants,T_3479,"Horsetails have hollow, ribbed stems and are often found in marshes ( Figure 1.4). Whorls of tiny leaves around the stem make the plant look like a horses tail, but these soon fall off and leave a hollow stem that can perform photosynthesis. This is unusual since photosynthesis most often occurs in leaves. The stems are rigid and rough to the touch because they are coated with a scratchy mineral. Because of their scratchy texture, these plants were once used as scouring pads for cleaning dishes. "
vascular seedless plants,T_3480,"Whisk ferns have green branching stems with no leaves, so they resemble a whisk broom ( Figure 1.5). Another striking feature of the whisk ferns is its spherical yellow sporangia. Ferns are common in the understory of the tropical rainforest. The first leaves of most ferns appear curled up into fiddleheads. "
vertebrate characteristics,T_3481,"Vertebrates are animals with backbones. These include fish, amphibians, reptiles, birds, and mammals. "
vertebrate characteristics,T_3482,"The primary feature shared by all vertebrates is the vertebral column, or backbone. The vertebral column protects the spinal cord. Other typical vertebrate traits include: The cranium (skull) to protect the brain. The brain is attached to the spinal cord. An internal skeleton. The internal skeleton supports the animal, protects internal organs, and allows for movement. A defined head region with a brain. The head region has an accumulation of sense organs. Living vertebrates range in size from a carp species, as little as 0.3 inches, to the blue whale, which can be as large as 110 feet ( Figure 1.1). A species of carp and an image of the blue whale (a mammal), the largest liv- ing vertebrate, reaching up to 110 feet long. Shown below it is the smallest whale species, Hectors dolphin (about 5 feet in length), and beside it is a human. These images are not to scale. The carp is greatly exaggerated in size and is even smaller than depicted when compared to the blue whale. "
vertebrate characteristics,T_3483,"Vertebrates, or subphylum Vertebrata, are all members of the phylum Chordata. Although there is some disagreement on how to classify animals, the traditional system divides the vertebrates into seven classes ( Table 1.1). Class Agnatha Chondrichthyes Common Name Jawless fishes Cartilaginous fishes Osteichthyes Amphibia Bony fishes Amphibians Reptilia Reptiles Aves Birds Characteristics No jaws or scales. Skeletons consisting of hard, rubber-like carti- lage. Skeletons made of bone. Spend part of their lives under water and part on land Have lungs to breathe on land, skin that does not need to be kept wet, and produces a watertight (amniotic) egg. Produces watertight eggs and protects eggs from predators. Examples Lampreys, hagfish Sharks, rays Tuna, bass, salmon, trout Frogs, toads, salamanders Turtles, snakes, lizards, alligators Ostriches, penguins, flamingos, parrots Class Mammalia Common Name Mammals Characteristics Nourish young with milk through mammary glands. Examples Dogs, cats, bears, mon- keys, humans "
what are biomes,T_3494,"Tropical rainforests and deserts are two familiar types of biomes. A biome is an area with similar populations of organisms. This can easily be seen with a community of plants and animals. Remember that a community is all of the populations of different species that live in the same area and interact with one another. Different biomes, such as a forest ( Figure 1.1) or a desert, obviously have different communities of plants and animals. How are the plants and animals different in the rainforest than those in the desert? Why do you think they are so different? The differences in the biomes are due to differences in the abiotic factors, especially climate. Climate is the typical weather in an area over a long period of time. The climate includes the amount of rainfall and the average temperature in the region. Obviously, the climate in the desert is much different than the climate in the rainforest. As a result, different types of plants and animals live in each biome. There are into two major groups of biomes: 1. Terrestrial biomes, which are land-based, such as deserts and forests. 2. Aquatic biomes, which are water-based, such as ponds and lakes. The abiotic factors, such as the amount of rainfall and the temperature, are going to influence other abiotic factors, such as the quality of the soil. This, in turn, is going to influence the plants that migrate into the ecosystem and thrive Tropical rainforest landscape in Hawaii. Notice how the plants are different from those in the desert. in that biome. Recall that migration is the movement of an organism into or out of a population. It can also refer to a whole new species moving into a habitat. The type of plants that live in a biome are going to attract a certain type of animal to that habitat. It is the interaction of the abiotic and biotic factors that describe a biome and ecosystem. In aquatic biomes, abiotic factors such as salt, sunlight and temperature play significant roles. For example, a hot dry biome is going to be completely different from a moderate wet biome. The soil quality will be different. Together, these will result in different plants being able to occupy each biome. Different plants will attract different animals (herbivores) to eat these plants. These animals, in turn, will attract different (carnivores) animals to eat the herbivores. So it is the abiotic factors that determine the biotic factors of an ecosystem, and together these define the biome. "
what is science,T_3495,"Are you like the teen in Figure 1.1? Do you ever wonder why things happen? Do you like to find out how things work? If so, then you are already thinking like a scientist. Scientists also wonder how and why things happen. They are curious about the world. To answer their questions, they make many observations. Then they use logic to draw general conclusions. "
what is science,T_3496,"Drawing general conclusions from many individual observations is called induction. It is a hallmark of scientific thinking. To understand how induction works, think about this simple example. Assume you know nothing about gravity. In fact, pretend youve never even heard of gravity. Perhaps you notice that whenever you let go of an object it falls to the ground. For example, you drop a book, and it crashes to the floor. Your pencil rolls to the edge of the desk and down it goes. You throw a ball into the air, and it falls back down. Based on many such observations (Figure 1.2), you conclude that all objects fall to the ground. Now assume that someone gives you your first-ever helium balloon. You discover that it rises up into the air if you dont hold on to it. Based on this new observation, do you throw out your first idea about falling objects? No; you decide to observe more helium balloons and try to find other objects that fall up instead of down. Eventually, you come to a better understanding based on all your observations. You conclude that objects heavier than air fall to the ground but objects lighter than air do not. Your new conclusion is better because it applies to a wider range of observations. You can learn more about induction, including its limits, by watching the video at this link: http://w MEDIA Click image to the left or use the URL below. URL: "
what is science,T_3497,"The above example shows how science generally advances. New evidence is usually used to improve earlier ideas rather than entirely replace them. In this way, scientists gradually refine their ideas and increase our understanding of the world. On the other hand, sometimes science advances in big leaps. This has happened when a scientist came up with a completely new way of looking at things. For example, Albert Einstein came up with a new view of gravity. He said it was really just a dent in the fabric of space and time. Different conclusions can be drawn from the same observations, and its not possible to tell which one is correct. For example, based on observations of the sun moving across the sky, people in the past couldnt tell whether the sun orbits Earth or Earth orbits the sun. Both models of the solar system are pictured in Figure 1.3. It wasnt until strong telescopes were invented that people could make observations that let them choose the correct idea. Not sure which idea is correct? You can learn more by watching the student-created video at this link: "
what is science,T_3498,"Some ideas in science gain the status of theories. Scientists use the term ""theory"" differently than it is used in everyday language. You might say, ""I think the dog ate my homework, but its just a theory."" In other words, its just one of many possible explanations for the missing work. However, in science, a theory is much more than that. "
what is science,T_3499,"A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence. An example is the kinetic theory of matter. According to this theory, all matter consists of tiny particles that are in constant motion. Particles move at different speeds in matter in different states. You can see this in Figure 1.4 and at the following URL: http://preparatorychemistry.com/Bishop_KMT_frames.htm . Particles in solids move the least; particles in gases move the most. These differences in particle motion explain why solids, liquids, and gases look and act differently. Think about how ice and water differ, or how water vapor differs from liquid water. The kinetic theory of matter explains the differences. You can learn more about this theory in the chapter States of Matter. "
what is science,T_3500,"Scientific laws are often confused with scientific theories, but they are not the same thing. A scientific law is a statement describing what always happens under certain conditions in nature. It answers ""how"" questions but not ""why"" questions. An example of a scientific law is Newtons law of gravity. It describes how all objects attract each other. It states that the force of attraction is greater for objects that are closer together or have more mass. However, the law of gravity doesnt explain why objects attract each other in this way. Einsteins theory of general relativity explains why. You can learn more about Newtons law of gravity and Einsteins theory in the chapter Forces, and at the following link:  . "
what is science,T_3501,"People have wondered about the natural world for as long as there have been people. So its no surprise that modern science has roots that go back thousands of years. The Table 1.1 describes just a few milestones in the history of science. A much more detailed timeline is available at the link below. Often, new ideas were not accepted at first because they conflicted with accepted views of the world. A good example is Copernicus idea that the sun is the center of the solar system. This idea was rejected at first because people firmly believed that Earth was the center of the solar system and the sun moved around it. Date Scientific Discovery Date 3500 BC Mesopotamian calendar 600 BC Thales 350 BC Aristotle 400 AD to 1000 AD Early Chinese Seismograph Scientific Discovery Several ancient civilizations studied astronomy. They recorded their observations of the movements of stars, the sun, and the moon. We still use the calendar developed by the Mesopotamians about 5500 years ago. It is based on cycles of the moon. The ancient Greek philosopher Thales proposed that natural events, such as lightning and earthquakes, have natural causes. Up until then, people blamed such events on gods or other supernatural causes. Thales has been called the ""father of science"" for his ideas about the natural world. The Greek philosopher Aristotle argued that truth about the natural world can be discovered through observa- tion and induction. This idea is called empiricism. Aristotles empiricism laid the foundation for the meth- ods of modern science. When Europe went through the Dark Ages, European science withered. However, in other places, science still flourished. For example: In North Africa, the scientist Alhazen studied light. He used experiments to test competing theories about light. In China, scientists invented compasses. They also invented seismographs to measure earth- quakes. They studied astronomy as well. Date Mid-1500s to late 1600s Scientific Discovery The Scientific Revolution occurred in Europe. This was the beginning of modern Western science. Many scientific advances were made during this time. Copernicus proposed that the sun, not Earth, is the center of the solar system. Galileo improved the telescope and made im- portant discoveries in astronomy. He discovered evidence that supported Copernicus theory. Newton proposed the law of gravity. Galileo 2001 Many scientists around the world worked together to complete the genetic sequence of human chromosomes. This amazing feat will help scientists understand, and perhaps someday cure, genetic diseases. Human Chromosomes "
what is science,T_3502,"Throughout history, women and people of color have rarely had the same chances as white males for education and careers in science. But they have still made important contributions to science. The Table 1.2 gives just a few examples of their contributions to physical science. More contributions are described at these links: Contributor Marie Curie (1867-1934) Description Marie Curie was the first woman to win a Nobel Prize. She won the 1903 Nobel Prize in physics for the discovery of radiation. She won the 1911 Nobel Prize in chemistry for discovering the elements radium and polonium. Contributor Lise Meitner (1878-1968) Description Lise Meitner was one of the scientists who discovered nuclear fission. This is the process that creates enor- mous amounts of energy in nuclear power plants. Irene Joliot-Curie (18971956) Irene Joliot-Curie, daughter of Marie Curie, won the 1935 Nobel prize in chemistry, along with her husband, for the synthesis of new radioactive elements. Maria Goeppert-Mayer (19061972) Maria Goeppert-Mayer was a co-winner of the 1963 Nobel prize in physics for discoveries about the struc- ture of the nucleus of the atom. Ada E. Yonath (1939present) Ada E. Yonath was a co-winner of the 2009 Nobel prize in chemistry. She made important discoveries about ribosomes, the structures in living cells where proteins are made. Contributor Shirley Ann Jackson (1946-present) Description Shirley Ann Jackson earned a doctoral degree in physics. She became the chair of the US Nuclear Regulatory Commission. Ellen Ochoa (1958-present) Ellen Ochoa is an inventor, research scientist, and NASA astronaut. She has flown several space missions. "
the scope of physical science,T_3503,"Physical science can be defined as the study of matter and energy. Matter refers to all the ""stuff"" that exists in the universe. It includes everything you can see and many things that you cannot see, including the air around you. Energy is what gives matter the ability to move and change. Energy can take many forms, such as electricity, heat, and light. Physical science can be divided into chemistry and physics. Chemistry focuses on matter and energy at the scale of atoms and molecules. Physics focuses on matter and energy at all scales, from atoms to outer space. "
the scope of physical science,T_3504,"Chemistry is the study of the structure, properties, and interactions of matter. Important concepts in chemistry include physical changes, such as water freezing, and chemical reactions, such as fireworks exploding. Chemistry concepts can answer all the questions on the left page of the notebook in Figure 1.5. Do you know the answers? "
the scope of physical science,T_3505,"Physics is the study of energy and how it interacts with matter. Important concepts in physics include motion, forces such as magnetism and gravity, and different forms of energy. Physics concepts can answer all the questions on the right page of the notebook in Figure 1.5. "
the scope of physical science,T_3506,"Physical science explains much of what you observe and do in your daily life. In fact, you depend on physical science for almost everything that makes modern life possible. You couldnt drive a car, text message, or send a tweet without decades of advances in chemistry and physics. You wouldnt even be able to turn on a light. Figure ""hows"" and ""whys"" about them as you read the rest of this book. "
the scope of physical science,T_3507,People with training in physical science are employed in a variety of places. There are many career options. Just four are described in Figure 1.7. Many more are described at the URL below. Do any of these careers interest you? http://diplomaguide.com/article_directory/sh/page/Physical%20Science/sh/Job_Titles_and_Careers_List.html . 
pressure of fluids,T_3612,"All fluids exert pressure like the air inside a tire. The particles of fluids are constantly moving in all directions at random. As the particles move, they keep bumping into each other and into anything else in their path. These collisions cause pressure, and the pressure is exerted equally in all directions. When particles are crowded together in one part of their container, they quickly spread out to fill their container. They always move from an area of higher pressure to an area of lower pressure. Thats why air entering a tire quickly spreads throughout the tire. "
pressure of fluids,T_3613,"Pressure is the result of force acting on a given area. It can be represented by the equation: Pressure = Force Area Pressure shows how concentrated the force is on a given area. The smaller the area to which force is applied, the greater the pressure is. Think about pressing a pushpin, like the one in Figure 15.2, into a bulletin board. You apply force with your thumb to the broad head of the pushpin. However, the force that the pushpin applies to the bulletin board acts only over the tiny point of the pin. This is a much smaller area, so the pressure the point applies to the bulletin board is much greater than the pressure you apply with you thumb. As a result, the pin penetrates the bulletin board with ease. "
pressure of fluids,T_3614,"In the equation for pressure, force is expressed in newtons (N) and area is expressed in square meters (m2 ). Therefore, pressure is expressed in N/m2 , which is the SI unit for pressure. This unit is also called the pascal (Pa). It is named for the scientist Blaise Pascal, whose discovery about pressure in fluids is described later in this lesson. Pressure may also be expressed in the kilopascal (kPa), which equals 1000 pascals. For example, the correct air pressure inside a mountain bike tire is usually about 200 kPa. "
pressure of fluids,T_3615,"When you know how much force is acting on a given area, you can calculate the pressure that is being applied to the area using the equation for pressure given above. For example, assume that a big rock weighs 500 newtons and is resting on the ground on an area of 0.5 m2 . The pressure exerted on the ground by the rock is: Pressure = 500 N = 1000 N/m2 = 1000 Pa, or 1 kPa 0.5 m2 Sometimes pressure but not force is known. To calculate force, the equation for pressure can be rewritten as: Force = Pressure  Area For example, suppose another rock exerts 2 kPa of pressure over an area of 0.3 m2 . How much does the rock weigh? Change 2 kPa to 2000 N/m2 and substitute it for pressure in the force equation: Force (Weight) = 2000 N/m2  0.3 m2 = 600 N Problem Solving Problem: A break dancer has a weight of 450 N. She is balancing on the ground on one hand. The palm of her hand has an area of 0.02 m2 . How much pressure does her hand exert on the ground? Solution: Use the equation Pressure = Force Area . Pressure = 450 N = 22500 Pa, or 22.5 kPa 0.02 m2 You Try It! Problem: If the break dancer lies down on the ground on her back, her weight is spread over an area of 0.75 m2 . How much pressure does she exert on the ground in this position? "
pressure of fluids,T_3616,"Both the water in the ocean and the air in the atmosphere exert pressure because of their moving particles. The ocean and atmosphere also illustrate two factors that affect pressure in fluids: depth and density. A fluid exerts more pressure at greater depths. Deeper in a fluid, all of the fluid above results in more weight pressing down. This causes greater pressure. Denser fluids such as water exert more pressure than less dense fluids such as air. The particles of denser fluids are closer together, so there are more collisions in a given area. This is illustrated in Figure 15.3 for water and air. "
pressure of fluids,T_3617,"As you go deeper in the ocean, the pressure exerted by the water increases steadily. The diagram in Figure 15.4 shows how pressure changes with depth. For every additional meter below the surface, pressure increases by 10 kPa. At 30 meters below the surface, the pressure is double the pressure at the surface. At a depth greater than 500 meters, the pressure is too great for humans to withstand without special equipment to protect them. Around 9000 meters below the surface, in the deepest part of the ocean, the pressure is tremendous. You can see a video demonstration of changes in water pressure with depth at this URL:  (0:42). MEDIA Click image to the left or use the URL below. URL:  Because of the pressure of the water, divers who go deeper than about 40 meters below the surface must return to the surface slowly and stop for several minutes at one or more points in their ascent. Thats what the divers in Figure water as they swim closer to the surface. If they were to rise to the surface too quickly, the gases dissolved in their blood would form bubbles and cause serious health problems. "
pressure of fluids,T_3618,"Like water in the ocean, air in the atmosphere exerts pressure that increases with depth. Most gas molecules in the atmosphere are pulled close to Earths surface by gravity. As a result, air pressure decreases quickly at lower altitudes and then more slowly at higher altitudes. This is illustrated in Figure 15.6. Air pressure is greatest at sea level, where the depth of the atmosphere is greatest. At higher altitudes, the pressure is less because the depth of the atmosphere is less. For example, on top of Mount Everest, the tallest mountain on Earth, air pressure is only about one-third of the pressure at sea level. At such high altitudes, low air pressure makes it hard to breathe and is dangerous to human health. The pressure of air in the atmosphere allows you to do many things, from sipping through a straw to simply breathing (see Figure 15.7). When you first suck on a straw, you remove air from the straw, so the air pressure in the straw is lower than For more examples of how we use air pressure, watch the video at this URL:  MEDIA Click image to the left or use the URL below. URL: "
pressure of fluids,T_3619,"Some of the earliest scientific research on fluids was conducted by a French mathematician and physicist named Blaise Pascal (16231662). Pascal was a brilliant thinker. While still a teen, he derived an important theorem in mathematics and also invented a mechanical calculator. One of Pascals contributions to our understanding of fluids is known as Pascals law. "
pressure of fluids,T_3620,"Pascals law states that a change in pressure at any point in an enclosed fluid is transmitted equally throughout the fluid. A simple example may help you understand Pascals law. Assume you have a small packet of ketchup, like the one in Figure 15.8. If you open one end of the packet and then apply pressure to the other end, what will happen? Ketchup will squirt out the open end. The pressure you exert on the packet is transmitted throughout the ketchup. When the pressure reaches the open end, it forces ketchup out of the packet. To see a video about Pascals law, go to this URL:  (2:59). MEDIA Click image to the left or use the URL below. URL:  The ability of fluids to transmit pressure in this way can be very useful besides providing ketchup for your French fries! For example, the hydraulic car lift in Figure 15.9 contains fluid that transmits pressure and raises a car so a mechanic can work on it from below. The fluid used, usually a type of oil, cant be compressed. Force is placed on the fluid in a narrow cylinder, and the fluid transmits the pressure throughout the hydraulic system. When the pressure reaches the fluid in the wide cylinder, it forces the cylinder upward, along with the car. The force applied to the car is much greater than the force applied to the fluid in the narrow cylinder. Why? When pressure acts over a wider area, it creates a larger force. Thats because force equals pressure multiplied by the area over which it acts, as you saw above in the equation Force = Pressure  Area. Besides hydraulic car lifts, other equipment that uses hydraulic fluid to increase force ranges from brakes to bull- dozers. Even the controls in airplanes use hydraulics. Because of the force-multiplying effect, a flick of a switch can raise or lower heavy wing flaps or landing gear. You can see animations of hydraulic systems at these URLs: http://science.howstuffworks.com/transport/engines-equipment/hydraulic1.htm http://home.wxs.nl/~brink494/hydr_e.htm "
pressure of fluids,T_3621,"Another important law about pressure in fluids was described by Daniel Bernoulli, a Swiss mathematician who lived during the 1700s. Bernoulli used mathematics to arrive at his law. Bernoullis law states that pressure in a moving fluid is less when the fluid is moving faster. For an animation of this law, go to the URL below. Bernoullis law explains how the wings of both airplanes and birds create lift that allows flight (see Figure 15.10). The shape of the wings causes air to flow more quickly and air pressure to be lower above the wings than below them. This allows the wings to lift the plane or bird above the ground against the pull of gravity. A spoiler on a race car, like the one in Figure 15.10, works in the opposite way. Its shape causes air to flow more slowly and air pressure to be greater above the spoiler than below it. As a result, air pressure pushes the car downward, giving its wheels better traction on the track. "
pressure of fluids,T_3622,"Northern California has a storied, 500-year history of sailing. But despite this rich heritage, scientists and boat designers continue to learn more each day about what makes a sail boat move. Contrary to what you might expect, the physics of sailing still present some mysteries to modern sailors. For more information on the physics of sailing, see http://science.kqed.org/quest/video/the-physics-of-sailing/ . MEDIA Click image to the left or use the URL below. URL: "
scientific investigation,T_3737,"Scientists investigate the world in many ways. In different fields of science, researchers may use different methods and be guided by different theories and hypotheses. However, most scientists, including physical scientists, usually follow the general approach shown in Figure 2.1. This approach typically includes the following steps: Identify a research question or problem. Form a hypothesis. Gather evidence, or data, to test the hypothesis. Analyze the evidence. Decide whether the evidence supports the hypothesis Draw conclusions. Communicate the results. Scientists may follow these steps in a different sequence. Or they may skip or repeat some of the steps. Which steps are repeated in Figure 2.1? "
scientific investigation,T_3738,"A scientific investigation begins with a question or problem. Often, the question arises because a scientist is curious about something she has observed. An observation is any information that is gathered with the senses. People often have questions about things they see, hear, or observe in other ways. For example, a teen named Tara has a bracelet with a magnetic clasp, like the one shown in Figure 2.2. Tara has noticed that the two magnets in the clasp feel harder to pull apart on cold days than on warm days. She wonders whether temperature affects the strength of a magnet. "
scientific investigation,T_3739,"Tara is curious. She decides to investigate. She begins by forming a hypothesis. A hypothesis is a potential answer to a question that can be tested by gathering information. If it isnt possible to gather evidence to test an answer, then it cannot be used as a scientific hypothesis. In fact, the question it addresses may not even be answerable by science. For example, in the childrens television show Sesame Street, there was a large Snuffalufagus (kind of like an elephant). But Snuffy would disappear whenever people came around. So if someone said ""Is there a Snuffy on Sesame Street?,"" that question would be unanswerable by science, since there isnt any test that can be performed because Snuffy would disappear as soon as a scientist showed up. Can you think of other examples of questions outside the realm of science? This important distinction, that evidence taken in by observation is experimented on by a scientist, is what separates legitimate science from other things which may pretend to be science. Fields which claim to be scientific but dont use the scientific method are called ""pseudoscience."" If a person cant gather data through some sort of instrument or sense information, they cant form a scientific conclusion. If there is no way to prove the hypothesis false, there is no scientific claim either. For example, if a friend told you that Snuffy visited him every day, but he was invisible whenever anyone walked into the room, this claim is not scientific, since there is no way to prove him false. Developing a hypothesis may require creativity as well as reason. However, in Taras case, the hypothesis is simple. She hypothesizes that a magnet is stronger at lower temperatures. Based on her hypothesis, Tara makes a prediction. If she cools a magnet, then it will pick up more metal objects, such as paper clips. Predictions are often phrased as ""if-then"" statements like this one. Is Taras prediction correct? She decides to do an experiment. "
scientific investigation,T_3740,"An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. There must be at least two variables in an experiment. They are called the manipulated variable and the responding variable. The manipulated variable (also called the ""independent variable"") is a factor that is changed by the re- searcher. For example, Tara will change the temperature of a magnet. Temperature is the manipulated variable in her experiment. The responding variable (also called the ""dependent variable"") is a factor that the researcher predicts will change if the manipulated variable changes. Tara predicts the number of paper clips attracted by the magnet will be greater at lower temperatures. Number of paper clips is the responding variable in her experiment. Tara wonders what other variables might affect the strength of a magnet. She thinks that the size and shape of a magnet might affect its strength. These are variables that must be controlled. A control is a variable that is held constant so it wont influence the outcome of an experiment. By using the same magnet at different temperatures, Tara is controlling for any magnet variables that might affect the results. What other variables should Tara control? (Hint: What about the paper clips?) "
scientific investigation,T_3741,"Not everything in physical science is as easy to study as magnets and paper clips. Sometimes its not possible or desirable to do experiments. There are some things with which a person simply cannot experiment. A distant star is a good example. Scientists study stars by making observations with telescopes and other devices. Often, its important to investigate a problem in the real world instead of in a lab. Scientists do field studies to gather real-world evidence. You can see an example of a field study in Figure 2.3. "
scientific investigation,T_3742,"Researchers should always communicate their results. By sharing their results, they may be able to get helpful feedback from other scientists. Reporting on research also lets other scientists repeat the investigation to see whether they get the same results. Getting the same results when an experiment is repeated is called replication. If results can be replicated, it means they are more likely to be correct. Replication of investigations is one way that a hypothesis may eventually become a theory. Scientists can share their results in various ways. For example, they can write articles for peer-reviewed science journals. Peer review means that the work is analyzed by peers, in this case other scientists. This is the best way to ensure that the results are accurate and reported honestly. Another way to share results with other scientists is with presentations at scientific meetings (see Figure 2.4). Creating websites and writing articles for newspapers and magazines are ways to share research with the public. Why might this be important? "
scientific investigation,T_3743,"Ethics refers to rules for deciding between right and wrong. Ethics is an important issue in science. Scientific research must be guided by ethical rules, including those listed below. The rules help ensure that the research is done safely and the results are reliable. Following the rules furthers both science and society. You can learn more about the role of ethics in science by following the links at this URL:  Ethical Rules for Scientific Research Scientific research must be reported honestly. It is wrong and misleading to make up or change research results. Scientific researchers must try to see things as they really are. They should avoid being biased by the results they expect or want to get. Researchers must be careful. They should take pains to avoid errors in their data. Researchers studying human subjects must tell their subjects about any potential risks of the research. Subjects also must be told that they can refuse to participate in the research. Researchers must inform coworkers, students, and members of the community about any risks of the research. They should proceed with the research only if they have the consent of these groups. Researchers studying living animals must treat them humanely. They should provide for their needs and do what they can to avoid harming them (see Figure 2.5). Sometimes, science can help people make ethical decisions in their own lives, although science is unlikely to be the only factor involved. For example, scientific evidence shows that human actions are affecting Earths climate. Actions such as driving cars are causing Earth to get warmer. Does this mean that it is unethical to drive a car to work or school? What if driving is the only way to get there? As this example shows, ethical decisions are likely to be influenced by many factors, not just science. Can you think of other factors that might affect ethical decisions such as this one? "
science skills,T_3744,One of the most important aspects of measuring is the system of units used for measurement. Remember the Mars Climate Orbiter that opened this chapter? It shows clearly why a single system of measurement units is needed in science. 
science skills,T_3745,"The measurement system used by most scientists is the International System of Units, or SI. Table 2.2 lists common units in this system. SI is easy to use because everything is based on the number 10. Basic units are multiplied or divided by powers of ten to arrive at bigger or smaller units. Prefixes are added to the names of the basic units to indicate the powers of ten. For example, the meter is the basic unit of length. The prefix kilo- means 1000, so a kilometer is 1000 meters. Can you infer what the other prefixes in the table mean? If not, you can find out at this URL: http://physics.nist.gov/cuu/Units/prefixes.html . Variable Length Volume Mass Basic SI Unit (English Equivalent) meter (m) (1 m = 39.37 in) cubic meter (m3 ) (1 m3 = 1.3 yd3 ) gram (g) (1 g = 0.04 oz) Related SI Units Equivalent Units kilometer (km) decimeter (dm) centimeter (cm) millimeter (mm) micrometer (m) nanometer (nm) liter (L) milliliter (mL) kilogram (kg) milligram (mg) = 1000 m = 0.1 m = 0.01 m = 0.001 m = 0.000001 m = 0.000000001 m = 1 dm3 = 1 cm3 = 1000 g = 0.001 g The SI system has units for other variables in addition to the three shown here in Table 2.2. Some of these other units are introduced in later chapters. Problem Solving Problem: Use information in Table 2.2 to convert 3 meters to inches. Solution: 3 m = 3  39.37 in = 118.11 in You Try It! Problem: Rod needs to buy 1 m of wire for a science experiment. The wire is sold by the yard, not the meter. If he buys 1 yd of wire, will he have enough? (Hint: How many inches are there in 1 yd? In 1 m?) "
science skills,T_3746,"The SI scale for measuring temperature is the Kelvin scale. However, some scientists use the Celsius scale instead. If you live in the U.S., you are probably more familiar with the Fahrenheit scale. Table 2.3 compares all three temperature scales. What is the difference between the boiling and freezing points of water on each of these scales? Scale Kelvin Celsius Fahrenheit Freezing Point of Water 273 K 0C 32F Boiling Point of Water 373 K 100C 212F Each 1-degree change on the Kelvin scale is equal to a 1-degree change on the Celsius scale. This makes it easy to convert measurements between Kelvin and Celsius. For example, to go from Celsius to Kelvin, just add 273. How would you convert a temperature from Kelvin to Celsius? Converting between Celsius and Fahrenheit is more complicated. The following conversion factors are used: Celsius ! Fahrenheit : ( C  1.8) + 32 = F Fahrenheit ! Celsius : ( F 32)  1.8 = C Problem Solving Problem: Convert 10C to Fahrenheit. Solution: (10C  1.8) + 32 = 50F You Try It! Problem: The weather forecaster predicts a high temperature today of 86F. What will the temperature be in Celsius? "
science skills,T_3747,"Measuring devices must be used correctly to get accurate measurements. Figure 2.6 shows the correct way to use a graduated cylinder to measure the volume of a liquid. Follow these steps when using a graduated cylinder to measure liquids: Place the cylinder on a level surface before adding liquid. Move so your eyes are at the same level as the top of the liquid in the cylinder. Read the mark on the glass that is at the lowest point of the curved surface of the liquid. This is called the meniscus. At the URLs below, you can see the correct way to use a metric ruler to measure length and a beam balance to measure mass.  (beam balance) (5:14) "
science skills,T_3748,"Measurements should be both accurate and precise. Accuracy is how close a measurement is to the true value. For example, 66 mL is a fairly accurate measure- ment of the liquid in Figure 2.6. Precision is how exact a measurement is. A measurement of 65.5 mL is more precise than a measurement of 66 mL. But in Figure 2.6, it is not as accurate. You can think of accuracy and precision in terms of a game like darts. If you are aiming for the bulls-eye and get all of the darts close to it, you are being both accurate and precise. If you get the darts all close to each other somewhere else on the board, you are precise, but not accurate. And finally, if you get the darts spread out all over the board, you are neither accurate nor precise. "
science skills,T_3749,"Record keeping is very important in scientific investigations. Follow the tips below to keep good science records. Use a bound laboratory notebook so pages will not be lost. Write in ink for a permanent record. Record the steps of all procedures. Record all measurements and observations. Use drawings as needed. Date all entries, including drawings. "
science skills,T_3750,Doing science often requires calculations. Converting units is just one example. Calculations are also needed to find derived quantities. 
science skills,T_3751,"Derived quantities are quantities that are calculated from two or more different measurements. Examples include area and volume. Its easy to calculate these quantities for a simple shape. For a rectangular solid, like the one in Figure 2.7, the formulas are: Area (of each side) = length  width (l  w) Volume = length  width  height (l  w  h) Helpful Hints When calculating area and volume, make sure that: all the measurements have the same units. answers have the correct units. Area should be in squared units, such as cm2 ; volume should be in cubed units, such as cm3 . Can you explain why? Naturally, not all derived quantities will have the same types of units. In the examples above, the only fundamental unit used was meters for the length of one of the sides of the box. However, if you had a quantity like speed (a derived quantity), it would be equal to distance traveled (which is meters) divided by the amount of time you spent traveling that distance (which is in seconds). Therefore your speed would be measured in meters per second. "
science skills,T_3752,"Assume you are finding the area of a rectangle with a length of 6.8 m and a width of 6.9 m. When you multiply the length by the width on your calculator, the answer you get is 46.92 m2 . Is this the correct answer? No; the correct answer is 46.9 m2 . The correct answer must be rounded down so there is just one digit to the right of the decimal point. Thats because the answer cannot have more digits to the right of the decimal point than any of the original measurements. Using extra digits implies a greater degree of precision than actually exists. The correct number of digits is called the number of significant figures. To learn more about significant figures and rounding, you can watch the videos at the URLs below.  (3:20)  (8:30) "
science skills,T_3753,"Quantities in science may be very large or very small. This usually requires many zeroes to the left or right of the decimal point. Such numbers can be hard to read and write accurately. Thats where scientific notation comes in. Scientific notation is a way of writing very large or small numbers that uses exponents. Numbers are written in this format: a  10b The letter a stands for a decimal number. The letter b stands for an exponent, or power, of 10. For example, the number 300 is written as 3.0  102 . The number 0.03 is written as 3.0  10 2 . Figure 2.8 explains how to convert numbers to and from scientific notation. For a review of exponents, watch:  You Try It! Problem: Write the number 46,000,000 in scientific notation. "
science skills,T_3754,"In a scientific investigation, a researcher may make and record many measurements. These may be compiled in spreadsheets or data tables. In this form, it may be hard to see patterns or trends in the data. Descriptive statistics and graphs can help organize the data so patterns and trends are easier to spot. Example: A vehicle checkpoint was set up on a busy street. The number of vehicles of each type that passed by the checkpoint in one hour was counted and recorded in Table 2.4. These are the only types of vehicles that passed the checkpoint during this period. Type of Vehicle 4-door cars 2-door cars SUVs Number 150 50 80 Type of Vehicle vans pick-up trucks Number 50 70 "
science skills,T_3755,"A descriptive statistic sums up a set of data in a single number. Examples include the mean and range. The mean is the average value. It gives you an idea of the typical measurement. The mean is calculated by summing the individual measurements and dividing the total by the number of measurements. For the data in Table 2.4, the mean number of vehicles by type is: (150 + 50 + 80 + 50 + 70)  5 = 80. (There are two other words people can sometimes use when they use the word ""average."" They might be referring to a quantity called the ""median"" or the ""mode."" Youll see these quantities in later courses, but for now, well just say the average is the same thing as the mean.) The range is the total spread of values. It gives you an idea of the variation in the measurements. The range is calculated by subtracting the smallest value from the largest value. For the data in Table 2.4, the range in numbers of vehicles by type is: 150 - 50 = 100. "
science skills,T_3756,"Graphs can help you visualize a set of data. Three commonly used types of graphs are bar graphs, circle graphs, and line graphs. Figure 2.9 shows an example of each type of graph. The bar and circle graphs are based on the data in Table 2.4, while the line graph is based on unrelated data. You can see more examples at this URL:  Bar graphs are especially useful for comparing values for different types of things. The bar graph in Figure Circle graphs are especially useful for showing percents of a whole. The circle graph in Figure 2.9 shows the percent of all vehicles counted that were of each type. Line graphs are especially useful for showing changes over time. The line graph in Figure 2.9 shows how distance from school changed over time when some students went on a class trip. Helpful Hints Circle graphs show percents of a whole. What are percents? Percents are fractions in which the denominator is 100. Example: 30% = 30/100 Percents can also be expressed as decimal numbers. Example: 30% = 0.30 You Try It! Problem: Show how to calculate the percents in the circle graph in Figure 2.9. Need a refresher on percents, fractions, and decimals? Go to this URL: "
science skills,T_3757,"Did you ever read a road map, sketch an object, or play with toy trucks or dolls? No doubt, the answer is yes. What do all these activities have in common? They all involve models. A model is a representation of an object, system, or process. For example, a road map is a representation of an actual system of roads on the ground. Models are very useful in science. They provide a way to investigate things that are too small, large, complex, or distant to investigate directly. Figure 2.10 shows an example of a model in chemistry. To be useful, a model must closely represent the real thing in important ways, but it must be simpler and easier to manipulate than the real thing. Do you think the model in Figure 2.10 meets these criteria? "
science skills,T_3758,"Research in physical science can be exciting, but it also has potential dangers. Whether in the lab or in the field, knowing how to stay safe is important. "
science skills,T_3759,"Lab procedures and equipment may be labeled with safety symbols. These symbols warn of specific hazards, such as flames or broken glass. Learn the symbols so you will recognize the dangers. A list of common safety symbols is shown in Figure 2.11. Do you know how to avoid each hazard? You can learn more at this URL: "
science skills,T_3760,"Following basic safety rules is the best way to stay safe in science. Safe practices help prevent accidents. Several lab safety rules are listed below. Different rules may apply when you work in the field. But in all cases, you should always follow your teachers instructions. Lab Safety Rules Wear safety gear, including goggles, an apron, and gloves. Wear a long-sleeved shirt and shoes that completely cover your feet. Tie back your hair if it is long. Do not eat or drink in the lab. Never work alone. Never perform unauthorized experiments. Never point the open end of a test tube at yourself or another person. Always add acid to water never water to acid and add the acid slowly. To smell a substance, use your hand to fan vapors toward your nose rather than smell it directly. This is demonstrated in Figure 2.12. When disposing of liquids in the sink, flush them down the drain with lots of water. Wash glassware and counters when you finish your lab work. Thoroughly wash your hands with soap and water before leaving the lab. Even when you follow the rules, accidents can happen. Immediately alert your teacher if an accident occurs. Report all accidents, even if you dont think they are serious. "
technology,T_3761,"Technology is the application of knowledge to real-world problems. It includes methods and processes as well as devices like computers and cars. An example is the Bessemer process. It is a cheap method of making steel that was invented in the 1850s. It is just one of many technological advances that have occurred in manufacturing. Technology is also responsible for most of the major advances in agriculture, transportation, communications, and medicine. Clearly, technology has had a huge impact on people and society. It is hard to imagine what life would be like without it. Professionals in technology are generally called engineers. Most engineers have a strong background in physical science. There are many different careers in engineering. You can learn about some of them at the URLs below. "
technology,T_3762,The development of new technology is called technological design. It is similar to scientific investigation. Both processes use evidence and logic to solve problems. 
technology,T_3763,"Figure 2.13 shows the steps of the technological design process. Consider the problem of developing a solar- powered car. Many questions would have to be researched in the design process. For example, what is the best shape for gathering the suns rays? How will the energy from the sun be stored? Will a back-up energy source be needed? After researching the answers, possible designs are developed. This takes imagination as well as reason. Then a model is made of the best design, and the model is tested. This allows any problems with the design to be worked out before a final design is selected. "
technology,T_3764,"Technological design always has constraints. Constraints are limits on the design. Common constraints include: laws of nature, such as the law of gravity. properties of the materials used. cost of producing a technology. Ethical concerns are also constraints on many technological designs. Like scientists, engineers must follow ethical rules. For example, the technologies they design must be as safe as possible for people and the environment. Engineers must weigh the benefits and risks of new technologies, and the benefits should outweigh the risks. "
technology,T_3765,"Technology advances as new materials and processes are invented. Computers are a good example. Table 2.5 and the videos below show some of the milestones in their evolution. The evolution of modern computers began in the 1930s. Computers are still evolving today. How have computers changed during your lifetime?  (4:11) MEDIA Click image to the left or use the URL below. URL:  (5:36) MEDIA Click image to the left or use the URL below. URL:  Computer (Year) ENIAC (1946) US Army Photo ERMA (1955) Description Like other early computers, the huge ENIAC computer used vacuum tubes for electrical signals. This made it very large and expensive. It could do just one task at a time. It had to be rewired to change programs. Thats what the women in this photo are doing. The ERMA computer represented a new computer technology. It used transistors instead of vacuum tubes. This allowed computers to be smaller, cheaper, and more energy efficient. Computer (Year) PDP-8 (1968) Description By the late 1960s, tiny transistors on silicon chips were invented. They increased the speed and efficiency of computers. They also allowed computers to be much smaller. The PDP-8 computer pictured here was the first ""mini"" computer. Macintosh 128K (1984) The next major advance in computers was the develop- ment of microprocessors. A microprocessor consisted of thousands of integrated circuits placed on a tiny sili- con chip. This allowed computers to be more powerful and even smaller. The computer pictured here is the first Macintosh personal computer. MacBook Air (2010) The computers of the 21st century are tiny compared with the lumbering giants of the mid-1900s. Their problem-solving abilities are also immense compared with early computers. The diversity of software pro- grams available today allows users to undertake an immense variety of tasks and no rewiring is needed! "
technology,T_3766,"Technology is sometimes referred to as applied science, but it has a different goal than science. The goal of science is to increase knowledge. The goal of technology is to use knowledge for practical purposes. Although they have different goals, technology and science work hand in hand. Each helps the other advance. Scientific knowledge is needed to create new technologies. New technologies are used to further science. The microscope is a good example. Scientific knowledge of light allowed 17th century lens makers to make the first microscopes. This new technology let scientists view a world of tiny objects they had never before seen. Figure "
technology,T_3767,"Whats 100,000 times thinner than a strand of hair? A nanometer. Discover the nanotech boom in Berkeley, where researchers are working to unlock the potential of nanoscience to battle global warming and disease. For more information on nanotechnology, see http://science.kqed.org/quest/video/nanotechnology-takes-off/ . MEDIA Click image to the left or use the URL below. URL: "
technology,T_3768,"The goal of technology is to solve peoples problems. Therefore, the problems of society generally set the direction that technology takes. Technology, in turn, affects society. It may make peoples lives easier or healthier. Two examples are described in Figure 2.15. You can read about other examples at these URLs: http://mezocore.wordpress.com/ "
technology,T_3769,"Everyday, women living in the refugee camps of Darfur, Sudan must walk for up to seven hours outside the safety of the camps to collect firewood for cooking, putting them at risk for violent attacks. Now, researchers at Lawrence Berkeley National Laboratory have engineered a more efficient wood-burning stove, which is greatly reducing both the womens need for firewood and the threats against them. For more information on these stoves, see http://scien MEDIA Click image to the left or use the URL below. URL: "
behavior of gases,T_3945,"Pressure is defined as the amount of force pushing against a given area. How much pressure a gas exerts depends on the amount of gas. The more gas particles there are, the greater the pressure. You usually cannot feel it, but air has pressure. The gases in Earths atmosphere exert pressure against everything they contact. The atmosphere rises high above Earths surface. It contains a huge number of individual gas particles. As a result, the pressure of the tower of air above a given spot on Earths surface is substantial. If you were standing at sea level, the amount of force would be equal to 10.14 newtons per square centimeter (14.7 pounds per square inch). This is illustrated in Figure 4.11. "
behavior of gases,T_3946,"For a given amount of gas, scientists have discovered that the pressure, volume, and temperature of a gas are related in certain ways. Because these relationships always hold in nature, they are called laws. The laws are named for the scientists that discovered them. "
behavior of gases,T_3947,"Boyles law was discovered in the 1600s by an Irish chemist named Robert Boyle. According to Boyles law, if the temperature of a gas is held constant, increasing the volume of the gas decreases its pressure. Why is this the case? As the volume of a gas increases, its particles have more room to spread out. This means that there are fewer particles bumping into any given area. This decreases the pressure of the gas. The graph in Figure 4.12 shows this relationship between volume and pressure. Because pressure and volume change in opposite directions, their relationship is called an inverse relationship. You can see an animation of the relationship at this URL:  A scuba diver, like the one in Figure 4.13, releases air bubbles when he breathes under water. As he gets closer to the surface of the water, the air bubbles get bigger. Boyles law explains why. The pressure of the water decreases as the diver gets closer to the surface. Because the bubbles are under less pressure, they increase in volume even though the amount of gas in the bubbles remains the same. "
behavior of gases,T_3948,"Charless law was discovered in the 1700s by a French physicist named Jacques Charles. According to Charless law, if the pressure of a gas is held constant, increasing the temperature of the gas increases its volume. What happens when a gas is heated? Its particles gain energy. With more energy, the particles have a greater speed. Therefore, they can move more and spread out farther. The volume of the gas increases as it expands and takes up more space. The graph in Figure 4.14 shows this relationship between the temperature and volume of a gas. You can see an animation of the relationship at this URL:  . Roger had a latex balloon full of air inside his air-conditioned house. When he took the balloon outside in the hot sun, it got bigger and bigger until it popped. Charless law explains why. As the gas in the balloon warmed in the sun, its volume increased. It stretched and expanded the latex of the balloon until the balloon burst. "
behavior of gases,T_3949,"Amontons law was discovered in the late 1600s by a French physicist named Guillaume Amonton. According to Amontons law, if the volume of a gas is held constant, increasing the temperature of the gas increases its pressure. Why is this the case? A heated gas has more energy. Its particles move more and have more collisions, so the pressure of the gas increases. The graph in Figure 4.15 shows this relationship. A woman checked the air pressure in her tires before driving her car on a cold day (see Figure 4.16). The tire pressure gauge registered 29 pounds of pressure per square inch. After driving the car several miles on the highway, the woman stopped and checked the tire pressure again. This time the gauge registered 32 pounds per square inch. Amontons law explains what happened. As the tires rolled over the road, they got warmer. The air inside the tires also warmed. As it did, its pressure increased. "
air pressure and altitude,T_4113,"Because gas particles in the airlike particles of all fluidsare constantly moving and bumping into things, they exert pressure. The pressure exerted by the air in the atmosphere is greater close to Earths surface and decreases as you go higher above the surface. You can see this in the Figure 1.1. Q: Denver, Colorado, is called the mile-high city because it is located 1 mile (1.6 km) above sea level. What is the average atmospheric pressure that high above sea level? A: From the Figure 1.1, the average atmospheric pressure 1.6 km above sea level is about 85 kPa. "
air pressure and altitude,T_4114,"There are two reasons why air pressure decreases as altitude increases: density and depth of the atmosphere. Most gas molecules in the atmosphere are pulled close to Earths surface by gravity, so gas particles are denser near the surface. With more gas particles in a given volume, there are more collisions of particles and therefore greater pressure. The depth (distance from top to bottom) of the atmosphere is greatest at sea level and decreases at higher altitudes. With greater depth of the atmosphere, more air is pressing down from above. Therefore, air pressure is greatest at sea level and falls with increasing altitude. On top of Mount Everest, which is the tallest mountain on Earth, air pressure is only about one-third of the pressure at sea level. "
air pressure and altitude,T_4115,"The pressure of air in the atmosphere allows us to do many things, from sipping through a straw to simply breathing. You can see in the Figures 1.2 and 1.3 how we use air pressure in both of these ways. When you first suck on a straw, you remove air from the straw, so the air pressure in the straw is lower that the air pressure on the surface of the drink. A fluid always flows from an area of higher pressure to an area of lower pressure, so the drink moves up the straw and into your mouth. "
amontonss law,T_4125,"There was no additional air in the tire the second time Lawrence checked the air pressure, but something did change between the two measurements. The tires had rolled over 10 miles of road on the trip to school. Any time one surface moves over another, it causes friction. Friction is a force that opposes the motion of two surfaces that are touching, and friction between two surfaces always generates heat. Quickly rub your hands together and youll feel the heat generated by the friction between them. As the tires moved over the road, friction between the tires and road generated heat. In short, the tires got warmer and so did the air inside them. "
amontonss law,T_4126,"The space inside a car tire is more-or-less fixed, so it has a constant volume. What happens when the volume of air is constant and its temperature increases? Lawrence found out the answer to that question when he measured the air pressure in his tire. Increasing the temperature of a gas such as air, while holding its volume constant, increases the pressure of the gas. This relationship between temperature and pressure of a gas is called Amontons law. It was first proposed by a French scientist named Guillaume Amontons in the late 1600s. Amontons gas law is just one of three commonly known gas laws. The other two are Boyles law and Charles law. Q: How does Amontons gas law explain the difference in air pressure in Lawrences tire? A: The tireand the air inside itgot warmer because of friction with the road. The volume of air inside the tire was more-or-less constant, so the pressure of the air increased when it got warmer. "
amontonss law,T_4127,"Why does the pressure of a gas increase as it gets warmer? Particles of a gas are constantly moving and bumping into things. The force of the collisions is measured by pressure. Pressure is the amount of force exerted on a given area, such as pounds of force per square inch. When gas particles heat up and gain energy, they move faster. This causes more collisions and greater pressure. Therefore, heating particles of gas in a closed space causes the pressure of the gas to increase. "
bernoullis law,T_4156,"Bernoullis law states that the pressure of a moving fluid such as air is less when the fluid is moving faster. Pressure is the amount of force applied per given area. The law is named for Daniel Bernoulli, a Swiss mathematician who discovered it during the 1700s. Bernoulli used mathematics to arrive at his law. "
bernoullis law,T_4157,"Did you ever wonder how the wings of airplanes or birds allow them to fly? Bernoullis law provides the answer. Look at the wings of the plane and hawk in the Figure 1.1. The shape of the wings causes air to flow more slowly below them than above them. This causes air pressure to be greater below the wings than above them. The difference in air pressure lifts the plane or bird above the ground. Q: How does a spoiler on a racecar use Bernoullis law? A: A spoiler on a racecar is like an upside-down wing. Its shape causes air to flow more slowlyand air pressure to be greaterabove the spoiler than below it. As a result, air pressure pushes the car downward, helping it to stay on the track. "
boyles law,T_4178,"What does popping bubble wrap have to do with science? Actually, it demonstrates an important scientific law, called Boyles law. Like other laws in science, this law describes what always happens under certain conditions. Boyles law is one of three well-known gas laws, which state the relationships among temperature, volume, and pressure of gases. (The other two gas laws are Charles law and Amontons law.) According to Boyles law, if the temperature of a gas is held constant, then decreasing the volume of the gas increases its pressureand vice versa. Thats what happens when you squeeze the bubbles of bubble wrap. You decrease the bubbles volume, so the air pressure inside the bubbles increases until they pop. "
boyles law,T_4179,"Boyles law is named for Robert Boyle, the English scientist who discovered this relationship between gas volume and pressure. Boyle based the law on his own controlled experiments. He published his results, along with detailed descriptions of his procedures and observations, in the 1660s. These steps were unheard of in his day. Mainly because of his careful research and the details he provided about it, Boyle has been called the father of modern chemistry. "
boyles law,T_4180,"Imagine a container of gas molecules like the one in the Figure 1.1. The container in the sketch has a lid that can be pushed down to shrink the volume of the gas inside. Notice what happens as the lid is lowered. The gas molecules crowd closer together because there is less space for them to occupy and they have nowhere else to go. Gas molecules have a lot of energy. They are always moving and bouncing off each other and anything else in their path. When gas molecules bump into things, it creates pressure. Pressure is greater when gas molecules occupy a smaller space, because the greater crowding results in more collisions. In other words, decreasing the volume of a gas increases its pressure. "
boyles law,T_4181,"As the volume of gas in the container pictured in the Figure 1.1 gets smaller, the pressure of the gas molecules becomes greater. When two variables change in opposite directions like this, the variables have an inverse, or upside-down, relationship. Q: How could you show an inverse relationship with a graph? Sketch a graph to show what the relationship between gas volume and pressure might look like. Let the x-axis represent volume (V) and the y-axis represent pressure (P). A: Did you sketch a graph like the one in the Figure 1.2? Lets see why this graph is correct. Find the point on the line where volume is smallest. Thats were pressure is highest. Then find the point where volume is largest. Thats where pressure is lowest. Whenever you see a graph with this shape, it usually represents variables that have an inverse relationship, like gas volume and pressure. "
charless law,T_4217,"The popularity of hot air balloons got scientists thinking about gases and what happens to them when they heat up. In the early 1800s, two French scientistsJacques Charles and Joseph Gay-Lussacdecided to investigate how changes in the temperature of a gas affect the amount of space it takes up, or its volume. They heated air and measured how its volume changed. The two scientists already knew that the pressure of a gas affects it volume. This had been demonstrated back in the 1660s by the English scientist Robert Boyle. So Charles and Gay-Lussac controlled the effects of pressure by keeping it constant in their experiments. Based on the results of the research, Charles developed a scientific law about gases. It is one of three well-known gas laws, the others being Boyles law and Amontons law. According to Charless law, when the pressure of a gas is held constant, increasing its temperature increases its volume. The opposite is also true: decreasing the temperature of a gas decreases it volume. "
communication in science,T_4256,"The last step of most scientific investigations is reporting the results. When scientists communicate their findings, they add to the body of scientific knowledge, and thats how science advances. Science generally builds on previous knowledge, sometimes advancing in giant steps but more often in baby steps. The brick building analogy in the Figure 1.1 may help you better understand why communication is important in science. When scientists communicate about their research, they may also get useful feedback from other scientists. For example, comments from other scientists might help them improve their research design or interpret their findings in a different way. Other scientists can also repeat the research to see if they get the same results. Q: Why might it be important for other scientists to repeat an investigation? A: If an investigation is repeated and different results are obtained, then it throws doubt on the original research. On the other hand, if the same results are obtained, scientists can place more confidence in them. "
communication in science,T_4257,"The posters shown in the opening image are just one of several ways that scientists may communicate about their research. Some of the most common ways scientists communicate are listed below. You can think of scientific knowledge as a brick building, and the work of a single sci- entist as an individual brick. Considered by itself, the work of a single scientist may not seem that important, yet it may be an important piece of the overall structure. But unless a scientist communicates re- search results, that single brick may never be added to the building. Scientists may present papers about their research at scientific conferences. This is a good way to quickly reach an audience of other scientists who are most interested in the research topic. Scientists may publish articles about their research in peer-reviewed science journals. Peer review means that the work is analyzed by peers, in other words, by other scientists. The articles are published only if the other scientists are convinced that the research is accurate and honest. Scientists may testify about their research before congress if their findings relate to matters of public policy, such as environmental pollution. Scientists may communicate about their research to the general public. For example, they might create a Web site about their research, blog about it, or write articles for newspapers or magazines. Q: Why might it be important for scientists to communicate about their research to the general public? Give an example. A: Communicating to the general public might be important if the research is directly related to peoples lives. For example, assume that a scientist has investigated how driving habits are related to car crashes. She might write a newspaper article to share the research results with the public so they can adopt driving habits that reduce the risk of crashes. "
ethics in science,T_4429,"Ethics refers to deciding whats right and whats wrong. Making ethical decisions involves weighing right and wrong in order to make the best choice. The ethical problem of the Pacific yew has both right and wrong aspects. Its right to save lives with the cancer drug that comes from the tree bark, but its wrong to endanger the tree and risk its extinction. Q: What do you think is the most ethical decision about the Pacific yew? Should the bark be used to make the drug and possibly save human lives? Or should this be prohibited in order to protect the tree from possible extinction? A: This is tough ethical dilemma, and there is no right or wrong answer. Ethical dilemmas such as this often spur scientists to come up with new solutions to problems. Thats what happened in the case of the Pacific yew. Scientists tackled and solved the problem of determining the chemical structure of the anti-cancer drug so it could be synthesized in labs. This is a win-win solution to the problem. The synthetic drug is now available to save lives, and the trees are no longer endangered by being stripped of their bark. "
ethics in science,T_4430,"Ethics is an important consideration in science. Scientific investigations must be guided by what is right and what is wrong. Thats where ethical rules come in. They help ensure that science is done safely and that scientific knowledge is reliable. Here are some of the ethical rules that scientists must follow: Scientific research must be reported honestly. It is wrong and misleading to make up or change research results. Scientific researchers must try to see things as they really are. They should avoid being biased by the results they expect or hope to get. Researchers must be careful. They should do whatever they can to avoid errors in their data. Researchers must inform coworkers and members of the community about any risks of their research. They should do the research only if they have the consent of these groups. Researchers studying living animals must treat them humanely. They should provide for their needs and take pains to avoid harming them. Researchers studying human subjects must tell their subjects that they have the right to refuse to participate in the research. Human subjects also must be fully informed about their role in the research, including any potential risks. You can read about a terrible violation of this ethical rule in the Figure 1.1. "
ethics in science,T_4431,"Sometimes, science can help people make ethical decisions in their own lives. For example, scientific evidence shows that certain human actionssuch as driving cars that burn gasolineare contributing to changes in Earths climate. This, in turn, is causing more severe weather and the extinction of many species. A number of ethical decisions might be influenced by this scientific knowledge. Q: For example, should people avoid driving cars to work or school because it contributes to climate change and the serious problems associated with it? What if driving is the only way to get there? Can you think of an ethical solution? A: This example shows that ethical decisions may not be all or nothing. For example, rather than driving alone, people might carpool with others. This would reduce their impact on climate change. They could also try to reduce their impact in other ways. For example, they might turn down their thermostat in cold weather so their furnace burns less fuel. "
field study,T_4443,"Although experiments are the gold standard for scientific investigations, sometimes its not possible or desirable to do experiments. Often its important to investigate a problem in the real world instead of in a lab. An investigation that gathers evidence in the real worldas the environmental chemist above is doingis called a field study. Q: Why are field studies important for environmental scientists? A: To learn about the environment, scientists need to take measurements and make observations in the real world. This means gathering evidence in field studies; collecting samples of water from a river is one example of this method. "
field study,T_4444,"The environmental scientist above will gather samples of river water in several different locations. Then he will take the samples back to a lab to analyze them. He will do tests to identify any pollutants in the samples. Taking samples from different locations may help him identify the source of any pollution he finds. Pollution can enter a river from a single source, such as a waste water pipe from a factory. This is called point-source pollution. Or pollution can enter a river in runoff rainwater that picks up pollutants as it runs over the ground. This type of pollution enters the river everywhere. This is called nonpoint-source pollution. Q: Assume that the river is polluted only by nonpoint-source pollution. Describe how the samples of river water would compare in terms of the pollutants they contain. A: All of the samples would contain about the same amount and types of pollutants. Q: How might point-source pollution be identified? A: Just one sample might be polluted. This would be the sample taken at, or just downstream from, the single source of pollution. "
gases,T_4469,"A gas is one of four well-known states of matter. (The other three are solid, liquid, and plasma). The particles of a gas can pull apart from each other and spread out. As a result, a gas does not have a fixed shape or a fixed volume. In fact, a gas always spreads out to take up whatever space is available to it. If a gas is enclosed in a container, it spreads out until it has the same volume as the container. Q: The sketches in the Figure 1.1 represent two identical sealed boxes that contain only air particles (represented by dots). There are more air particles in box B than box A. Which box contains a greater volume of air? A: This is a trick question! The air inside each box expands to fill the available space, which is identical for both boxes. There are more air particles in box B, but the volume of air is exactly the same in both boxes. "
gases,T_4470,"Particles of gas are constantly moving in all directions at random. As a result, they are always bumping into each other and other things. This is modeled in the Figure 1.2. The force of the particles against things they bump into creates pressure. Pressure is defined in physics as the amount of force pushing against a given area. How much pressure a gas exerts depends on the number of gas particles in a given space and how fast they are moving. The more gas particles there are and the faster they are moving, the greater the pressure they create. The arrows show that particles of a gas move randomly in all directions. Q: Look at box A and box B in the previous question. Is air pressure the same in both boxes? Why or why not? A: Air pressure is greater in box B. Thats because there are more air particles in box B to bump into each other and into the sides of the container. Therefore, the particles in box B exert more force on a given area. "
gases,T_4471,"We live in a sea of air called the atmosphere. Can you feel the air in the atmosphere pressing against you? Not usually, but air actually exerts a lot of pressure because theres so much of it. The atmosphere rises high above Earths surface, so it contains a huge number of gas particles. Most of them are concentrated close to Earths surface because of gravity and the weight of all the air in the atmosphere above them. As a result, air pressure is greatest at sea level and drops rapidly as you go higher in altitude. The Figure 1.3 shows how air pressure falls from sea level to the top of the atmosphere. In the graph, air pressure is measured in a unit called the millibar (mb). The SI unit of pressure is newton per square centimeter (N/cm2 ). Q: The top of Mount Everest is almost 9 km above sea level. What is the pressure of the atmosphere at this altitude? A: Air pressure at the top of Mount Everest is about 260 mb. This is only about 25 percent of air pressure at sea level, which is 1013.2 mb. No wonder its hard for climbers to breathe when they get close to Mount Everests summit! "
history of science,T_4503,"People have probably wondered about the natural world for as long as there have been people. So its no surprise that science has roots that go back thousands of years. Some of the earliest contributions to science were made by Greek philosophers more than two thousand years ago. It wasnt until many centuries later, however, that the scientific method and experimentation were introduced. The dawn of modern science occurred even more recently. It is generally traced back to the scientific revolution, which took place in Europe starting in the 1500s. "
history of science,T_4504,"A Greek philosopher named Thales, who lived around 600 BCE, has been called the father of science for his ideas about the natural world. He proposed that natural events such as lightning and earthquakes have natural causes. Up until then, people understood such events to be the acts of gods or other supernatural forces. Q: Why was Thales idea about natural causes such an important contribution to science? A: Natural causes can be investigated and understood, whereas gods or other supernatural causes are above nature and not suitable for investigation. Just a few hundred years after Thales, the Greek philosopher Aristotle made a very important contribution to science. You can see what Aristotle looked like in the Figure 1.1. Prior to Aristotle, other philosophers believed that they could find the truth about the natural world by inward reflectionin other words, just by thinking about it. Aristotle, in contrast, thought that truth about the natural world could come only from observations of nature and inductive reasoning. He argued that knowledge of nature must be based on evidence and logic. This idea is called empiricism, and it is the basis of science today. Aristotle introduced the idea of empiricism around 350 BCE. It is a hallmark of modern science. "
history of science,T_4505,"In the first 1000 years CE, Europe went through a period called the Dark Ages. Science and learning in general were all but abandoned. However, in other parts of the world science flourished. During this period, some of the most important contributions to science were made by Persian scholars. For example, during the 700s CE, a Persian scientist named Geber introduced the scientific method and experimentation in chemistry. His ideas and methods were later adopted by European chemists. Today, Geber is known as the father of chemistry. "
history of science,T_4506,"Starting in the mid-1500s, a scientific revolution occurred in Europe. This was the beginning of modern Western science. Many scientific advances were made during a period of just a couple of hundred years. The revolution in science began when Copernicus made the first convincing arguments that the sunnot Earthis the center of what we now call the solar system. (You can see both models of the solar system in the Figure 1.2.) This was a drastic shift in thinking about Earths place in the cosmos. Around 1600, the Italian scientist Galileo greatly improved the telescope, which had just been invented, and made many important discoveries in the field of astronomy. Some of Galileos observations provided additional evidence for Copernicus sun-centered solar system. Q: Copernicus ideas about the solar system were so influential that the scientific revolution is sometimes called the Copernican revolution. Why do you think Copernicus ideas led to a revolution in science? A: Copernicus ideas about the solar system are considered to be the starting point of modern astronomy. They changed how all future scientists interpreted observations in astronomy. They also led to a flurry of new scientific investigation. Other contributions to science that occurred during the scientific revolution include: Keplers laws of planetary motion The model on the left shows what people believed about the solar system before Copernicus introduced the model on the right. Newtons law of universal gravitation Newtons three laws of motion "
history of science,T_4507,"Another major shift in science occurred with the work of Albert Einstein (the rock star scientist pictured in the opening image). In 1916, Einstein published his general theory of relativity. This theory relates matter and energy. It also explains gravity as a property of space and time (rather than a property of matter as Newton thought). Einsteins theory has been supported by all evidence and observations to date, whereas Newtons law of gravity does not apply to all cases. Einsteins theory is still the accepted explanation for gravity today. Q: How might Einsteins theory have influenced the course of science? A: Einsteins theory suggested new areas of investigation. Many predictions based on the theory were later found to be true. For example, black holes in the universe were predicted by Einsteins theory and later confirmed by scientific evidence. "
hypothesis,T_4520,"The word hypothesis can be defined as an ""educated guess."" For example, it might be an educated guess about why a natural event occurs. But not all hypotheseseven those about the natural worldare scientific hypotheses. What makes a statement a scientific hypothesis rather than just an educated guess? A scientific hypothesis must meet two criteria: A scientific hypothesis must be testable. A scientific hypothesis must be falsifiable. "
hypothesis,T_4521,"For a hypothesis to be testable means that it is possible to make observations that agree or disagree with it. If a hypothesis cannot be tested by making observations, it is not scientific. Consider this statement: ""There are invisible creatures all around us that we can never observe in any way."" This statement may or may not be true, but it is not a scientific hypothesis. Thats because it cant be tested. Given the nature of the hypothesis, there are no observations a scientist could make to test whether or not it is false. "
hypothesis,T_4522,"A hypothesis may be testable, but even that isnt enough for it to be a scientific hypothesis. In addition, it must be possible to show that the hypothesis is false if it really is false. Consider this statement: There are other planets in the universe where life exists. This statement is testable. If it is true, it is at least theoretically possible to find evidence showing that its true. For example, a spacecraft could be sent from Earth to explore the universe and report back if it discovers an inhabited planet. If such a planet were found, it would prove the statement is true. However, the statement isnt a scientific hypothesis. Why? If it is false, its not possible to show that its false. The spacecraft may never find an inhabited planet, but that doesnt necessarily mean there isnt one. Given the vastness of the universe, we would never be able to check every planet for life! "
hypothesis,T_4523,"Lets consider one last example, which is illustrated in the Figure 1.1: ""Any two objects dropped at the same time from the same height will reach the ground at the same time (assuming the absence of air resistance)."" Is this statement testable? Yes. You could drop two objects at the same time from the same height and observe when they reach the ground. Of course, you would have to drop the objects in the absence of air to prevent air resistance, but at least such a test is theoretically possible. Is the statement falsifiable if it really is false? Again, the answer is yes. You can easily test many combinations of two objects and if any two objects do not reach the ground at the same time, then the hypothesis is false. If a hypothesis really is false, it should be relatively easy to disprove it. "
hypothesis,T_4524,"If the hypothesis above about falling objects really were false, it is likely that this would be discovered sooner or later after enough objects had been dropped. It takes just one exception to disprove a hypothesis. But what if the hypothesis really is true? Can this be demonstrated as well? No; it would require testing all possible combinations of objects to show that they always reach the ground at the same time. This is impossible. New objects are being made all the time that would have to be tested. Its always possible an exception would be found in the future to disprove the hypothesis. Although you cant prove conclusively that a hypothesis is true, the more evidence you gather in support of it, the more likely it is to be true. "
nature of science,T_4644,"Science is more about gaining knowledge than it is about simply having knowledge. Science is a way of learning about the natural world that is based on evidence and logic. In other words, science is a process, not just a body of facts. Through the process of science, our knowledge of the world advances. "
nature of science,T_4645,"Scientists may focus on very different aspects of the natural world. For example, some scientists focus on the world of tiny objects, such as atoms and molecules. Other scientists devote their attention to huge objects, such as the sun and other stars. But all scientists have at least one thing in common. They want to understand how and why things happen. Achieving this understanding is the goal of science. Have you ever experienced the thrill of an exciting fireworks show like the one pictured in the Figure 1.1? Fireworks show how the goal of science leads to discovery. Fireworks were invented at least 2000 years ago in China, but explaining how and why they work didnt happen until much later. It wasnt until scientists had learned about elements and chemical reactions that they could explain what caused fireworks to create brilliant bursts of light and deep rumbling booms. Fireworks were invented long before sci- entists could actually explain how and why they explode. "
nature of science,T_4646,"Sometimes learning about science is frustrating because scientific knowledge is always changing. But thats also what makes science exciting. Occasionally, science moves forward in giant steps. More commonly, however, science advances in baby steps. Giant steps in science may occur if a scientist introduces a major new idea. For example, in 1666, Isaac Newton introduced the idea that gravity is universal. People had long known that things fall to the ground because they are attracted by Earth. But Newton proposed that everything in the universe exerts a force of attraction on everything else. This idea is known as Newtons law of universal gravitation. Q: How do you think Newtons law of universal gravitation might have influenced the advancement of science? A: Newtons law allowed scientists to understand many different phenomena. It explains not only why things always fall down toward the ground or roll downhill. It also explains the motion of many other objects. For example, it explains why planets orbit the sun. The idea of universal gravity even helped scientists discover the planets Neptune and Pluto. The caption and diagram in the Figure 1.2 explain how. Baby steps in science occur as small bits of evidence gradually accumulate. The accumulating evidence lets scientists refine and expand on earlier ideas. For example, the scientific idea of the atom was introduced in the early 1800s. But scientists came to understand the structure of the atom only as evidence accumulated over the next two centuries. Their understanding of atomic structure continues to expand today. The advancement of science is sometimes a very bumpy road. New knowledge and ideas arent always accepted at first, and scientists may be mocked for their ideas. The idea that Earths continents drift on the planets surface is a good example. This idea was first proposed by a scientist named Alfred Wegener in the early 1900s. Wegener also proposed that all of the present continents had once formed one supercontinent, which he named Pangaea. You can see a sketch of Pangaea in Figure 1.3. Other scientists not only rejected Wegeners ideas, but ridiculed Wegener for even suggesting them. It wasnt until the 1950s that enough evidence had accumulated for scientists to realize that Wegener had been right. Unfortunately, Wegener did not live long enough to see his ideas accepted. A: Several types of evidence support Wegeners ideas. For example, similar fossils and rock formations have been found on continents that are now separated by oceans. It is also now known that Earths crust consists of rigid plates that slide over molten rock below them. This explains how continents can drift. Even the shapes of todays continents show how they once fit together, like pieces of a giant jigsaw puzzle. "
observation,T_4684,"An observation is any information that is gathered with the senses. Our senses include vision, hearing, touch, smell, and taste. We see with our eyes, hear with our ears, touch with our hands, smell with our nose, and taste with our tongue. We can also extend our senses and our ability to make observations by using instruments such as microscopes, telescopes, and thermometers. Q: How do these instruments extend human senses and our ability to make observations? A: Microscopes and telescopes extend the sense of vision. They allow us to observe objects that are too small (microscopes) or too distant (telescopes) for the unaided eye to see. Thermometers extend the sense of touch. Using our sense of touch, we can only feel how warm or cold something is relative to our own temperature or the temperature of something else. Thermometers allow us to measure precisely how warm or cold something is. "
observation,T_4685,"Besides raising questions for investigation, observations play another role in scientific investigations. They help scientists gather evidence. For example, to investigate whether a chemical change has occurred, a scientist might observe whether certain telltale signs are present. In some chemical changes, for example, a substance turns from one color to another. You can see an example of this in the Figure 1.1. In other chemical changes, an odor is produced or gas bubbles are released. All of these changes can be observed with the senses. Some of these pennies are shiny and copper colored. Thats how pennies look when they are new. The older pennies are dull and brown. Copper at the surface of these pennies has combined with air to become a different substance with different properties. The change in color shows that a chemical change has occurred. Q: Some chemical changes release heat. How could this change be observed? A: The sense of touchor a thermometercould be used to observe an increase in temperature. "
oceanic pressure,T_4686,"Pressure is the amount of force acting on a given area. As you go deeper in the ocean, the pressure exerted by the water increases steadily. Thats because there is more and more water pressing down on you from above. The Figure 1.1 shows how pressure changes with depth. For each additional meter below the surface, pressure increases by 10 kPa. At 30 meters below the surface, the pressure is double the pressure at the surface. At a depth greater than 500 meters, the pressure is too great for humans to withstand without special equipment to protect them. At nearly 11,000 meters below the surface, the pressure is tremendous. "
oceanic pressure,T_4687,"Scuba divers can dive without special vehicles because they dont go very deep below the surface of the water. Nonetheless, because of the pressure of the water, scuba divers who go deeper than about 40 meters must return to the surface slowly. They must stop for several minutes at one or more points in their ascent. Thats what the divers in the Figure 1.2 are doing. The stops are needed to let the pressure inside their body adjust to the decreasing pressure of the water as they swim closer to the surface. If they were to rise to the surface too quickly, the gases dissolved in their blood would form bubbles and cause serious health problems. Q: Why would dissolved gases form bubbles as pressure decreases? A: Less gas can dissolve in a fluid at lower pressure. Therefore, as pressure decreases, gases come out of solution and form bubbles. "
pascals law,T_4699,Pressure is the amount of force acting on a given area. It is represented by the equation: Pressure = Force Area The pressure exerted by a fluid increases if more force is applied or if the same force is applied over a smaller area. The equation for pressure can be rewritten as: Force = Pressure  Area This equation shows that the same pressure applied to a greater area increases the force. 
pascals law,T_4700,"Some of the earliest scientific research on pressure in fluids was conducted by a French mathematician and physicist named Blaise Pascal (1623-1662). The SI unit of pressure, the Pascal (Pa), is named for him because of his important research. One of Pascals major contributions is known as Pascals law. This law states that a change in pressure at any point in an enclosed fluid is transmitted equally throughout the fluid. "
pascals law,T_4701,"A simple example may help you understand Pascals law. Toothpaste is a fluid that is enclosed in a tube with a small opening at one end. Look at the toothpaste tube in the Figure 1.1. When any part of the tube is squeezed, toothpaste squirts out the open end. The pressure applied to the tube is transmitted equally throughout the toothpaste. When the pressure reaches the open end, it forces toothpaste out through the opening. "
pascals law,T_4702,"The ability of fluids to transmit pressure in this way can be very usefulbesides getting toothpaste out of a tube! For example, hydraulic brakes in a car use fluid to transmit pressure, and when they do, they also increase force. You can see how hydraulic brakes work in the Figure 1.2. A: The arrows representing the force applied by the break cylinder are larger than the arrow representing the force applied by the brake pedal mechanism. A larger arrow indicates greater force. Q: How do hydraulic brakes increase the force that is applied to the brake shoes? A: The pressure exerted by the fluid on the brake shoes is applied over a larger area. When pressure acts over a larger area, it increases the force (Force = Pressure  Area). Hydraulic car lifts also use fluid to transmit pressure and increase force. The lifts are used to raise cars, which are very heavy, so mechanics can work on them from underneath. Controls in airplanes use fluids to transmit pressure and increase force so a flick of a switch can raise or lower heavy landing gear. "
physical science careers,T_4713,"Physical science is the study of matter and energy. It includes the sciences of chemistry and physics. Most careers in physical science require a 4-year college degree in one of these fields. Some careers require more advanced education as well. For example, an astronaut might have a masters degree or even a doctoral degree. Q: Besides becoming an astronaut, a degree in physical science can prepare you for many other careers. What careers do you think might be available to people with degrees in physical science? A: People with degrees in physical science might become pharmacists, forensic technicians, or research scientists, to name just three possible careers. Four additional careers in physical science are described below. "
physical science careers,T_4714,Training in the physical science field of chemistry or physics is needed for the careers described in the Figure 1.1. Do any of these careers interest you? 
pressure in fluids,T_4736,"All fluids exert pressure like the air inside a tire. The particles of fluids are constantly moving in all directions at random. As the particles move, they keep bumping into each other and into anything else in their path. These collisions cause pressure, and the pressure is exerted equally in all directions. When particles are crowded together in one part of an enclosed space, such as the air particles first entering a tire, they quickly spread out to fill all the available space. Thats because particles of fluids always move from an area of higher pressure to an area of lower pressure. This explains why air entering a tire through a tiny opening quickly fills the entire tire. "
pressure in fluids,T_4737,"Pressure is defined as the amount of force acting on a given area. Therefore, pressure can be represented by this equation: Pressure = Force Area Pressure shows how concentrated the force is on a given area. The smaller the area to which force is applied, the greater the pressure is. Think about pressing a pushpin, like the one in the Figure 1.1, into a bulletin board. You apply force with your thumb to the broad head of the pushpin. However, the force that the pushpin applies to the bulletin board acts only over the tiny point of the pin. This is a much smaller area, so the pressure the point applies to the bulletin board is much greater than the pressure you apply with your thumb. As a result, the pin penetrates the bulletin board with ease. "
pressure in fluids,T_4738,"In the above equation for pressure, force is expressed in Newtons (N) and area is expressed in square meters (m2 ). Therefore, pressure is expressed in N/m2 , which is the SI unit for pressure. This unit is also called the Pascal (Pa). It is named for the scientist Blaise Pascal whose discoveries about pressure in fluids led to a law of the same name. Pressure may also be expressed in the kilopascal (kPa), which equals 1000 Pascals. For example, the correct air pressure inside a mountain bike tire is usually about 200 kPa. "
pressure in fluids,T_4739,"When you know how much force is acting on a given area, you can calculate the pressure that is being applied to the area using the equation for pressure given above. For example, assume that a rock weighs 5000 N and is resting on the ground on an area of 0.5 m2 . The pressure exerted on the ground by the rock is: N = 10000 N/m2 = 10000 Pa, or 10 kPa Pressure = 5000 0.5 m2 Sometimes pressure but not force is known. To calculate force, the equation for pressure can be rewritten as: Force = Pressure  Area For example, suppose another rock exerts 10 kPa of pressure over an area of 0.4 m2 . How much does the rock weigh? Change 10 kPa (10,000 Pa) to 10,000 N/m2 . Then substitute this value for pressure in the force equation as follows: Force (Weight) = 10,000 N/m2  0.4 m2 = 4,000 N Q: The break-dancer in the Figure 1.2 has a weight of 800 N. He is balancing on the palm of one hand. If the palm of his hand has an area of 0.02 m2 , how much pressure is he exerting on the ground? A: Use the equation for pressure: 800 N Pressure = 0.02 m2 = 40000 Pa, or 40 kPa Q: If the break-dancer lies down on the ground on his back, his weight is spread over an area of 0.75 m2 . How much pressure does he exert on the ground in this position? A: On his back, the pressure he exerts is: Pressure = 800 N 0.75 m2 = 1100 Pa, or 1.1 kPa "
pressure in fluids,T_4740,"Two factors influence the pressure of fluids. They are the depth of the fluid and its density. A fluid exerts more pressure at greater depths. Deeper in a fluid, all of the fluid above it results in more weight pressing down. This causes greater pressure the deeper you go. Denser fluids such as water exert more pressure than less dense fluids such as air. The particles of denser fluids are closer together, so there are more collisions of particles in a given area. The difference in density of water and air is illustrated in the Figure 1.3. "
replication in science,T_4797,"Scientists also have to check their work. The results of an investigation are not likely to be well accepted unless the investigation is repeatedusually many timesand the same result is always obtained. Getting the same result when an experiment is repeated is called replication. If research results can be replicated, it means they are more likely to be correct. Repeated replication of investigations may turn a hypothesis into a theory. On the other hand, if results cannot be replicated they are likely to be incorrect. "
replication in science,T_4798,"The following example shows why replication is important in science. In 1998, a British researcher published an article in a medical journal reporting that he had found a link between a common childhood vaccine and autism (see Figure 1.1). According to the article, children in his study developed autism soon after receiving the vaccine. Following publication of the article, many parents refused to have their children vaccinated. Several epidemics occurred as a result, and some children died of the diseases. Soon after the original study was published, other researchers tried to replicate the research. However, it could not be replicated. No other studies could find a link between the vaccine and autism. As a result, scientists became convinced that the original results were incorrect. Eventually, investigators determined that the original study was a fraud. They learned that its author had received a large amount of money to find evidence that the vaccine causes autism, so he faked his research results. If other scientists had not tried to replicate the research, the truth might never have come out. "
safety in science,T_4803,"Lab procedures and equipment may be labeled with safety symbols. These symbols warn of specific hazards, such as flames or broken glass. Learn the symbols so you will recognize the dangers. Then learn how to avoid them. Many common safety symbols are shown below. Q: Do you know how you can avoid these hazards? A: Wearing protective gear is one way to avoid many hazards in science. For example, to avoid being burned by hot objects, use hot mitts to protect your hands. To avoid eye hazards, such as harsh liquids splashed into the eyes, wear safety goggles. "
safety in science,T_4804,"Following basic safety rules is another important way to stay safe in science. Safe practices help prevent accidents. Several lab safety rules are listed below. Different rules may apply when you work in the field. But in all cases, you should always follow your teachers instructions. Lab Safety Rules Wear long sleeves and shoes that completely cover your feet. If your hair is long, tie it back or cover it with a hair net. Protect your eyes, skin, and clothing by wearing safety goggles, an apron, and gloves. Use hot mitts to handle hot objects. Never work in the lab alone. Never engage in horseplay in the lab. Never eat or drink in the lab. Never do experiments without your teachers approval. Always add acid to water, never the other way around, and add the acid slowly to avoid splashing. Take care to avoid knocking over Bunsen burners, and keep them away from flammable materials such as paper. Use your hand to fan vapors toward your nose rather than smelling substances directly. Never point the open end of a test tube toward anyoneincluding yourself! Clean up any spills immediately. Dispose of lab wastes according to your teachers instructions. Wash glassware and counters when you finish your work. Wash your hands with soap and water before leaving the lab. "
safety in science,T_4805,"Even when you follow the rules, accidents can happen. Immediately alert your teacher if an accident occurs. Report all accidents, whether or not you think they are serious. "
scientific experiments,T_4811,"An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. For example, the speed of an object down a ramp might be one variable, and the steepness of the ramp might be another. "
scientific experiments,T_4812,"There must be at least two variables in any experiment: a manipulated variable and a responding variable. A manipulated variable is a variable that is changed by the researcher. A manipulated variable is also called an independent variable. A responding variable is a variable that the researcher predicts will change if the manipulated variable changes. A responding variable is also called a dependent variable. You can learn how to identify manipulated and responding variables in an experiment by watching this video about bouncing balls: Click image to the left or use the URL below. URL:  Q: If you were to do an experiment to find out what influences the speed of an object down a ramp, what would be the responding variable? How could you measure it? A: The responding variable would be the speed of the object. You could measure it indirectly with a stopwatch. You could clock the time it takes the object to travel from the top to the bottom of the ramp. The less time it takes, the faster the average speed down the ramp. Q: What variables might affect the speed of an object down a ramp? A: Variables might include factors relating to the ramp or to the object. An example of a variable relating to the ramp is its steepness. An example of a variable relating to the object is the way it movesit might roll or slide down the ramp. Either of these variables could be manipulated by the researcher, so you could choose one of them for your manipulated variable. "
scientific experiments,T_4813,"Assume you are sliding wooden blocks down a ramp in your experiment. You choose steepness of the ramp for your manipulated variable. You want to measure how changes in steepness affect the time it takes a block to reach the bottom of the ramp. You decide to test two blocks on two ramps, one steeper than the other, and see which block reaches the bottom first. You use a shiny piece of varnished wood for one ramp and a rough board for the other ramp. You raise the rough board higher so it has a steeper slope (see sketch below). You let go of both blocks at the same time and observe that the block on the ramp with the gentler slope reaches the bottom sooner. Youre surprised, because you expected the block on the steeper ramp to go faster and get to the bottom first. Q: What explains your result? A: The block on the steeper ramp would have reached the bottom sooner if all else was equal. The problem is that all else was not equal. The ramps varied not only in steepness but also in smoothness. The block on the smoother ramp went faster than the block on the rougher ramp, even though the rougher ramp was steeper. This example illustrates another important aspect of experiments: experimental controls. A control is a variable that must be held constant so it wont influence the outcome of an experiment. The control can be used as a standard for comparison between experiments. In the case of your ramp experiment, smoothness of the ramps should have been controlled by making each ramp out of the same material. For other examples of controls in an experiment, watch the video below. It is Part II of the above video on bouncing balls. Click image to the left or use the URL below. URL:  Q: What other variables do you think might influence the outcome of your ramp experiment? How could these other variables be controlled? A: Other variables might include variables relating to the block. For example, a smoother block would be expected to go down a ramp faster than a rougher block. You could control variables relating to the block by using two identical blocks. "
scientific induction,T_4819,"Inductive reasoning is the process of drawing general conclusions based on many clues, or pieces of evidence. Many crimes are solved using inductive reasoning. It is also the hallmark of science and the basis of the scientific method. Q: How might the police detective pictured above use inductive reasoning to solve the crime? A: The detective might gather clues that provide evidence about the identity of the person who committed the crime. For example, he might find fingerprints or other evidence left behind by the perpetrator. The detective might eventually find enough clues to be able to conclude the identity of the most likely suspect. "
scientific induction,T_4820,"A simple example will help you understand how inductive reasoning works in science. Suppose you grew up on a planet named Quim, where there is no gravity. In fact, assume youve never even heard of gravity. You travel to Earth (on a student exchange program) and immediately notice things are very different here than on your home planet. For one thing, when you step out of your spacecraft, you fall directly to the ground. Then, when you let go of your communications device, it falls to the ground as well. On Quim, nothing ever falls to the ground. For example, if you had let go of your communications device back home, it would have just stayed in place by your upper appendage. You notice that everything you let go of falls to the ground. Using inductive reasoning, you conclude that all objects fall to the ground on Earth. Then, you make the observation pictured (Figure 1.2). You see round objects rising up into the sky, rather than falling toward the ground as you expect. Clearly, your first conclusionalthough based on many pieces of evidenceis incorrect. You need to gather more evidence to come to a conclusion that explains all of your observations. Evidence that not everything falls to the ground on Earth. Q: What conclusion might you draw based on the additional evidence of the balloons rising instead of falling? A: With this and other evidence, you might conclude that objects heavier than air fall to the ground but objects lighter than air do not. "
scientific induction,T_4821,"Inductive reasoning cant solve a crime or arrive at the correct scientific conclusion with 100 percent certainty. Its always possible that some piece of evidence remains to be found that would disprove the conclusion. Thats why jurors in a trial are told to decide whether the defendant is guilty without a reasonable doubtnot without a shred of doubt. Similarly, a scientific theory is never really proven conclusively to be true. However, it can be supported by so much evidence that it is accepted without a reasonable doubt. Click image to the left or use the URL below. URL: "
scientific law,T_4822,"It may seem like common sense that bumper cars change their motion when they collide. Thats because all objects behave this way - its the law! A scientific law, called Newtons third law of motion, states that for every action there is an equal and opposite reaction. Thus, when one bumper car acts by ramming another, one or both cars react by pushing apart. Q: What are some other examples of Newtons third law of motion? What actions are always followed by reactions? A: Other examples of actions and reactions include hitting a ball with a bat and the ball bouncing back; and pushing a swing and the swing moving away. "
scientific law,T_4823,Newtons third law of motion is just one of many scientific laws. A scientific law is a statement describing what always happens under certain conditions. Other examples of laws in physical science include: Newtons first law of motion Newtons second law of motion Newtons law of universal gravitation Law of conservation of mass Law of conservation of energy Law of conservation of momentum 
scientific law,T_4824,"Scientific laws state what always happen. This can be very useful. It can let you let you predict what will happen under certain circumstances. For example, Newtons third law tells you that the harder you hit a softball with a bat, the faster and farther the ball will travel away from the bat. However, scientific laws have a basic limitation. They dont explain why things happen. Why questions are answered by scientific theories, not scientific laws. Q: You know that the sun always sets in the west. This could be expressed as a scientific law. Think of something else that always happens in nature. How could you express it as a scientific law? A: Something else that always happens in nature is water flowing downhill rather than uphill. This could be expressed as the law, When water flows over a hill, it always flows from a higher to a lower elevation. "
scientific process,T_4830,"Investigations are at the heart of science. They are how scientists add to scientific knowledge and gain a better understanding of the world. Scientific investigations produce evidence that helps answer questions. Even if the evidence cannot provide answers, it may still be useful. It may lead to new questions for investigation. As more knowledge is discovered, science advances. "
scientific process,T_4831,"Scientists investigate the world in many ways. In different fields of science, researchers may use different methods and be guided by different theories and questions. However, most scientists follow the general steps outlined in the Figure 1.1. This approach is sometimes called the scientific method. Keep in mind that the scientific method is a general approach and not a strict sequence of steps. For example, scientists may follow the steps in a different order. Or they may skip or repeat some of the steps. "
scientific process,T_4832,"A simple example will help you understand how the scientific method works. While Cody eats a bowl of cereal (Figure 1.2), he reads the ingredients list on the cereal box. He notices that the cereal contains iron. Cody is The general steps followed in the scientific method. studying magnets in school and knows that magnets attract objects that contain iron. He wonders whether there is enough iron in a flake of the cereal for it to be attracted by a strong magnet. He thinks that the iron content is probably too low for this to happen, even if he uses a strong magnet. Cody makes an observation that raises a question. Curiosity about observations is how most scientific investigations begin. Q: If Cody were doing a scientific investigation, what would be his question and hypothesis? A: Codys question would be, Is there enough iron in a flake of cereal for it to be attracted by a strong magnet? His hypothesis would be, The iron content of a flake of cereal is too low for it to be attracted by a strong magnet. Q: Based on this evidence, what should Cody conclude? A: Cody should conclude that his hypothesis is incorrect. There is enough iron in a flake of cereal for it to be attracted by a strong magnet. Q: If Cody were a scientist doing an actual scientific investigation, what should he do next? A: He should report his results to other scientists. "
scientific theory,T_4833,"The term theory is used differently in science than it is used in everyday language. A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence. Because it is so well supported, a scientific theory has a very good chance of being a correct explanation for events in nature. Because it is a broad explanation, it can explain many observations and pieces of evidence. In other words, it can help connect and make sense of many phenomena in the natural world. "
scientific theory,T_4834,"A number of theories in science were first proposed many decades or even centuries ago, but they have withstood the test of time. An example of a physical science theory that has mainly withstood the test of time is Daltons atomic theory. John Dalton was a British chemist who lived in the late 1700s and early 1800s. Around 1800, he published his atomic theory, which is one of the most important theories in science. According to Daltons atomic theory, all substances consist of tiny particles called atoms. Furthermore, all the atoms of a given element are identical, whereas the atoms of different elements are always different. These parts of Daltons atomic theory are still accepted today, although some other details of his theory have since been disproven. Dalton based his theory on many pieces of evidence. For example, he studied many substances called compounds. These are substances that consist of two or more different elements. Dalton determined that a given compound always consists of the same elements in exactly the same proportions, no matter how small the sample of the compound. This idea is illustrated for the compound water in the Figure 1.1. Dalton concluded from this evidence that elements must be made up of tiny particles in order to always combine in the same specific proportions in any given compound. Water is a compound that consists of the elements hydrogen (H) and oxygen (O). Like other compounds, the smallest particles of water are called molecules. Each molecule of water (H2 O) contains two atoms of hydrogen and one atom of oxygen. Q: Dalton thought that atoms are the smallest particles of matter. Scientists now know that atoms are composed of even smaller particles. Does this mean that the rest of Daltons atomic theory should be thrown out? A: The discovery of particles smaller than atoms doesnt mean that we should scrap the entire theory. Atoms are still known to be the smallest particles of elements that have the properties of the elements. Also, it is atomsnot particles of atomsthat combine in fixed proportions in compounds. Instead of throwing out Daltons theory, scientists have refined and expanded on it. There are many other important physical science theories. Here are three more examples: Einsteins theory of gravity Kinetic theory of matter Wave-particle theory of light "
scientific theory,T_4835,"The formation of scientific theories is generally guided by the law of parsimony. The word parsimony means thriftiness. The law of parsimony states that, when choosing between competing theories, you should select the theory that makes the fewest assumptions. In other words, the simpler theory is more likely to be correct. For example, you probably know that Earth and the other planets of our solar system orbit around the sun. But several centuries ago, it was believed that Earth is at the center of the solar system and the other planets orbit around Earth. While it is possible to explain the movement of planets according to this theory, the explanation is unnecessarily complex. Q: Why do you think parsimony is an important characteristic of scientific theories? A: The more assumptions that must be made to form a scientific theory, the more chances there are for the theory to be incorrect. If one assumption is wrong, so is the theory. Conversely, the theory that makes the fewest assumptions, assuming it is well supported by evidence, is most likely to be correct. "
scope of physical science,T_4838,"Physical science is the study of matter and energy. That covers a lot of territory because matter refers to all the stuff that exists in the universe. It includes everything you can see and many things that you cannot see, including the air around you. Energy is also universal. Its what gives matter the ability to move and change. Electricity, heat, and light are some of the forms that energy can take. "
scope of physical science,T_4839,"Physical science, in turn, can be divided into chemistry and physics. Chemistry is the study of matter and energy at the scale of atoms and molecules. For example, the synthetic fibers in the swimmers suit were created in labs by chemists. Physics is the study of matter and energy at all scalesfrom the tiniest particles of matter to the entire universe. Knowledge of several important physics conceptssuch as motion and forcescontributed to the design of the swimmers suit. Q: Its not just athletes that depend on physical science. We all do. What might be some ways that physical science influences our lives? A: We depend on physical science for just about everything that makes modern life possible. You couldnt turn on a light, make a phone call, or use a computer without centuries of discoveries in chemistry and physics. The Figure matter makes each action possible. Each of these pictures represents a way that physical science influences our lives. "
technological design process,T_4908,"The process in which tri-ATHLETE was created and perfected is called technological design. This is the process in which most new technologies are developed. Technological design is similar to scientific investigation. Both processes rely on evidence and reason, and follow a logical sequence of steps to solve problems or answer questions. The process of designing a new technology includes much more than just coming up with a good idea. Possible limitations, or constraints, on the design must be taken into account. These might include factors such as the cost or safety of the new product or process. Making and testing a model of the design are also important. These steps ensure that the design actually works to solve the problem. This process also gives the designer a chance to find problems and modify the design if necessary. No solution is perfect, but testing and refining a design assures that the technology will provide a workable solution to the problem it is intended to solve. "
technological design process,T_4909,"The technological design process can be broken down into the series of steps shown in the flowchart in Figure 1.1. Typically, some of the steps have to be repeated, and the steps may not always be done in the sequence shown. This flowchart illustrates the steps of the technological design process. Consider the problem of developing a solar-powered car. Many questions would have to be researched in the design process. For example, what is the best shape for gathering the suns rays? How will sunlight be converted to useable energy to run the car? Will a back-up energy source be needed? After researching the answers, possible designs are developed. This generally takes imagination as well as sound reasoning. Then a model must be designed and tested. This allows any problems with the design to be worked out before a final design is selected and produced. Q: Assume you want to design a product that lets a person in a wheelchair carry around small personal items so they are easy to access. What questions might you research first? A: You might research questions such as: What personal items are people likely to need to carry with them? What types of carriers or holders are there that might be modified for use by people in a wheelchair? Where might a carrier be attached to a wheelchair or person in a wheelchair without interfering with the operation of the chair or hindering the person? Q: Suppose you have come up with a possible solution to the problem described in the previous question. How might you make a model of your idea? How could you test your model? A: First, you might make a sketch of your idea. Then you could make an inexpensive model using simple materials such as cardboard, newspaper, tape, or string. You could test your model by trying to carry several personal items in it while maneuvering around a room in a wheelchair. You would also want to make sure that you could do things like open doors, turn on light switches, and get in and out of the chair without the carrier getting in the way. "
technology and science,T_4910,"The Hubble space telescope shows that technology and science are closely related. Technology uses science to solve problems, and science uses technology to make new discoveries. However, technology and science have different goals. The goal of science is to answer questions and increase knowledge. The goal of technology is to find solutions to practical problems. Although they have different goals, science and technology work hand in hand, and each helps the other advance. Scientific knowledge is used to create new technologies such as the space telescope. New technologies often allow scientists to explore nature in new ways. "
technology and science,T_4911,"The Hubble telescope was put into orbit around Earth in the 1990s, but scientists have been using telescopes to make discoveries for hundreds of years. The first telescope was invented in the early 1600s. The inventor was probably a Dutch lens maker named Hans Lippershey. He and his telescope are pictured in Figure 1.1. Lippershey used scientific knowledge of the properties of light and lenses to design his telescope. Lippersheys new technology quickly spread all over Europe. Almost immediately, the Italian scientist and inventor Galileo started working to improve Lippersheys design. In just two years, Galileo had made a more powerful telescope. It could make very distant objects visible to the human eye. The Figure 1.2 shows Galileo demonstrating his powerful telescope. It appears to be focused on the moon. Galileo started using his telescope to explore the night sky. He soon made some remarkable discoveries. He observed hills and valleys on the moon and spots on the sun. He discovered that Jupiter has moons and that the sun rotates on its axis. With his discoveries, Galileo was able to prove that the sun, not Earth, is at the center of the solar system. This discovery played an important role in the history of science. It led to a scientific revolution that gave birth to modern Western science. And it all began with technology! Galileo is shown here presenting his tele- scope to government leaders. They must have been impressed. They gave him a life-long job as a university professor and doubled his salary. "
technology and science,T_4912,"There are many other examples that show how technology and science work together. Two are pictured in Figure Like the invention of the telescope, the invention of the microscope also depended on scientific knowledge of light and lenses. Q: How do you think the invention of the microscope helped science advance? A: The microscope let scientists view a world of tiny objects they had never seen before. It led to many important scientific discoveries, including the discovery of cells, which are the basic building blocks of all living things. Seismometers and spectrometers are both technological devices that led to im- portant scientific discoveries. "
technology careers,T_4915,"Companies that design and build roller coasters employ a range of technology professionals. Technology is the application of science to real-world problems. Professionals in technology are generally called engineers. Engineers are creative problem solvers. They use math and science to design and develop just about everythingfrom roller coasters to video games. Q: Whether engineers are designing and developing roller coasters or video games, they need many of the same skills and the same basic knowledge. What skills and knowledge do you think all engineers might need? A: All engineers need basic knowledge of math and science, particularly physical science. For example, to design and build a roller coaster, they would need to know about geometry, as well as forces and motion, which are important topics in physical science. In addition, all engineers must have skills such as logical thinking and creativity. The ability to envision three-dimensional structures from two-dimensional drawings is also helpful for most engineers. "
technology careers,T_4916,"Different types of engineers, such as electrical and mechanical engineers, must work together to build roller coasters and most other engineering projects. You can learn about these and other technology careers at the URLs listed here and in the Figure 1.1. These are just a few of many possible careers in technology. Do any of these careers interest you? "
women and people of color in science,T_5012,"Dr. Ochoa is one of just a few dozen female astronauts in the U.S. She is also the first Hispanic woman in the world to go into space. Although females make up more than half of the U.S. population, fewer than 25 percent of U.S. astronauts are women. Women are also under-represented in science, especially physical sciences such as chemistry and physics. What explains this? Throughout history, womenand also people of color of both gendershave rarely had the same chances as white males for education and careers in science. Cultural, social, and economic biases have made it far harder for them than for white males to excel in this area. This explains why there have been fewer scientists among their ranks. "
women and people of color in science,T_5013,"Despite their relative lack of opportunities, women and people of color have made many important contributions to science. Several have won Nobel prizes for their discoveries. Just a few of their contributions to physical science are presented in Table 1.1. Scientist Marie Curie (1867-1934) Contribution Marie Curie was the first woman to win a Nobel prizeand she won two of them! She won the 1903 Nobel prize for physics for her discovery of radiation. She won the 1911 Nobel prize for chemistry for her discovery of the elements radium and polonium. C. V. Raman (1888-1970) C. V. Raman was an Indian scientist who won the 1930 Nobel prize for physics. He made important discoveries about how light travels through transparent materials. He was also made a knight of the British Empire for his work. Maria Goeppert-Mayer (1906-1972) Maria Goeppert-Mayer was a German-born American scientist who won the 1963 Nobel prize for physics. She helped to develop a new model of the nucleus of the atom. She was just the second woman to win a Nobel prize for physics, after Marie Curie. Ada E. Yonath (1939-present) Ada E. Yonath was a co-winner of the 2009 Nobel prize in chemistry. She made important discoveries about ribosomes, the structures in living cells where proteins are made. Scientist Mario Molina (1943-present) Contribution Mario Molina is a Mexican-born scientist who won the 1995 Nobel prize for chemistry. He helped to discover how the ozone layer in the atmosphere is being destroyed by pollution. He has received more than 18 honorary degrees for his contributions and even has an asteroid named after him. "