Photosynthetic Pigments Photosynthetic Pigments Pigments are colorful compounds. Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear "colorful". Flowers, corals, and even animal skin contain pigments which give them their colors. More important than their reflection of light is the ability of pigments to absorb certain wavelengths. Because they interact with light to absorb only certain wavelengths, pigments are useful to plants and other autotrophs --organisms which make their own food using photosynthesis. In plants , algae, and cyanobacteria , pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments, each of a different color, to capture more of the sun's energy. There are three basic classes of pigments. Chlorophylls are greenish pigments which contain a porphyrin ring. This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules.

This is the fundamental process by which chlorophyll "captures" the energy of sunlight. There are several kinds of chlorophyll, the most important being chlorophyll "a". This is the molecule which makes photosynthesis possible, by passing its energized electrons on to molecules which will manufacture sugars. All plants, algae, and cyanobacteria which photosynthesize contain chlorophyll "a". A second kind of chlorophyll is chlorophyll "b", which occurs only in "green algae" and in the plants . A third form of chlorophyll which is common is (not surprisingly) called chlorophyll "c", and is found only in the photosynthetic members of the Chromista as well as the dinoflagellates . The differences between the chlorophylls of these major groups was one of the first clues that they were not as closely related as previously thought. Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their color. These compounds are composed of two small six-carbon rings connected by a "chain" of carbon atoms. As a result, they do not dissolve in water, and must be attached to membranes within the cell.

Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments. One very visible accessory pigment is fucoxanthin the brown pigment which colors kelps and other brown algae as well as the diatoms . Phycobilins are water-soluble pigments, and are therefore found in the cytoplasm, or in the stroma of the chloroplast. They occur only in Cyanobacteria and Rhodophyta . The picture at the right shows the two classes of phycobilins which may be extracted from these "algae". The vial on the left contains the bluish pigment phycocyanin, which gives the Cyanobacteria their name. The vial on the right contains the reddish pigment phycoerythrin, which gives the red algae their common name. Phycobilins are not only useful to the organisms which use them for soaking up light energy; they have also found use as research tools. Both pycocyanin and phycoerythrin fluoresce at a particular wavelength.

That is, when they are exposed to strong light, they absorb the light energy, and release it by emitting light of a very narrow range of wavelengths. The light produced by this fluorescence is so distinctive and reliable, that phycobilins may be used as chemical "tags". The pigments are chemically bonded to antibodies, which are then put into a solution of cells. When the solution is sprayed as a stream of fine droplets past a laser and computer sensor, a machine can identify whether the cells in the droplets have been "tagged" by the antibodies. This has found extensive use in cancer research, for "tagging" tumor cells. Biology of Plants: Making Food Photosynthesis is the process by which plants make food from light, water, nutrients, and carbon dioxide. What is chlorophyll? Chlorophyll is the green pigment, or color, found in plants that helps the plant make food. Plants are very important to us. All food people eat comes directly or indirectly from plants. ï¿½ Directly from plants: Indirectly from plants: For example, apples come from an apple tree. The flour used to make bread comes from a wheat plant.

Steak comes from a cow, and we all know that cows are animals, not plants, right? But what does the cow eat? It eats grass and grains—PLANTS! So all the foods we eat come from plants. But what do plants eat? They make their own food! What Do Plants Need to Make Food? Plants need several things to make their own food. They need: chlorophyll, a green pigment found in the leaves of plants (see the layer of chlorophyll in the cross-section of a leaf below) light (either natural sunlight or artificial light, like from a light bulb) carbon dioxide (CO2)(a gas found in the air; one of the gases people and animals breathe out when they exhale) water (which the plant collects through its roots) nutrients and minerals (which the plant collects from the soil through its roots) Plants make food in their leaves. The leaves contain a pigment called chlorophyll, which colors the leaves green. Chlorophyll can make food the plant can use from carbon dioxide, water, nutrients, and energy from sunlight. This process is called photosynthesis. During the process of photosynthesis, plants release oxygen into the air. People and animals need oxygen to breathe.

Chlorophyll and chloroplasts | ASU - Ask A Biologist Ask A Biologist Chlorophyll and chloroplasts show/hide words to know Chlorophyll: the pigment that gives plants their green color and allows them to absorb sunlight... more Chloroplast: a part of a cell found in plants that converts light energy into energy plants can use (sugar). Other living organisms such as algae also have cells that contain chloroplasts. Thylakoid: the disk-shaped parts of a plant cell where light-dependent reactions occur... more The story of chlorophyll and chloroplasts Chloroplasts are tiny factories inside the cells of plants. They are also found in the cells of other organisms that use photosynthesis. Chloroplasts take the energy from the sunlight and use it to make plant food. The food can be used immediately to give cells energy or it can be stored as sugar or starch. If stored, it can be used later when the plant needs to do work, like grow a new branch or make a flower. Chloroplasts up close Inside chloroplasts are special stacks of pancake-shaped structures called thylakoids (Greek thylakos = sack or pouch).

Thylakoids have an outer membrane that surrounds an inner area called the lumen. The light-dependent reactions happen inside the thylakoid. Our cells have mitochondria (Greek mitos = thread, and khondrion = little granule), our energy-producing structures. We don't have any chloroplasts. Plants have both mitochondria and chloroplasts. Both mitochondria and chloroplasts convert one form of energy into another form that cells can use. How did plants get chloroplasts? Chloroplasts were once free-living bacteria! Chloroplasts entered a symbiotic (Greek syn = together, and bios = life) relationship with another cell, which eventually led to the plant cells we have today. Being green Chlorophyll, a green pigment found in chloroplasts, is an important part of the light-dependent reactions. Chlorophyll soaks up the energy from sunlight. It is also the reason why plants are green. You may remember that colors are different wavelengths of light. Chlorophyll captures red and blue wavelengths of light and reflects the green wavelengths. Plants have different types of pigments besides chlorophyll. Some of them also assist in absorbing light energy.

These different pigments are most noticeable during the fall. During that time, plants make less chlorophyll and the other colors are no longer hidden beneath green. But why don't plants have pigments that allow them to capture all wavelengths of light? If you've ever gotten a sunburn you know firsthand that sunlight can be damaging. Plants can also be damaged from excess light energy. Luckily, there are non-chlorophyll pigments in plants that provide a 'sunscreen'. Additional images via Wikimedia Commons. Algae image by Leonardo Ré-Jorge. photosynthesis pigments Pigments: Pigments are "molecules that absorb specific wavelengths (energies) of light and reflect all others." Pigments are colored: the color we see is the net effect of all the light reflecting back at us. Now, what do I mean by absorb? Electrons in molecules can exist at specific energy levels. Normally they exist at the lowest possible energy level they can. However, if enough energy comes along to boost them into the next level, they can "absorb" that energy and occupy that higher level. This is what pigments do. The light they absorb contains' just the right amount' of energy necessary to push them into the next level.

Any light that does not have enough or has too much energy can not be absorbed and is reflected. The electron in the higher energy level, however, does not 'want' to stay there(i.e. it is unstable). It 'wants' to return to its normal lower energy level. In order to do this it must get rid or release the energy that has put it into the higher energy state to begin with. This can happen several different ways: 1) The extra energy can be converted into molecular motion and lost as heat. 2) Some of the extra energy can be lost as heat energy, while the rest is lost as light. This re-emission of light energy is called florescence. 3)The energy, but not the e- itself, can be passed onto another molecule. This is called resonance. 4)The energy and the e- can be transferred to another molecule. Plant pigments usually utilize the last two of these reactions to convert the sun's energy into their own. When chlorophyll is isolated from the enzymes it is associated with, the second scenario can be seen to happen. What should be the ideal pigment for chloroplasts?

A collection of pigments that would absorb all light and thus appear Black seems a logical choice... but in fact we know this is not true.. plants except for some of the red algae appear green or brown, not black. Why? a number of possible explanation occur... IF plants had pigments that absorbed UV and x-rays this would mean that so much energy could be absorbed in light areas that electrons could be knocked off their orbitals and the molecule destroyed.. IF plants absorbed IR and radio waves, there would not be enough energy for electron transfer, just enough to warm up the molecule Pigments that absorb in the visible region gain just enough energy to boost an electron to the next level... However even in this region, not all visible wavelengths are abosrbed..there is some speculation that in the early competitive wars between photosynthetic bacteria [ and we mean early 3 BYA] , plants specialized so that some would absorb in the red, others green or blue. The survivors in the long run are the one with pigments that absorbed in the red/blue reflecting green.... at least on land. What are these Pigment involved in Photosythesis?: Chlorophyll a: This is the most abundant pigment in plants.

Chlorophyll a absorbs light with wavelengths of 430nm(blue) and 662nm(red). It reflects green light strongly so it appears green to us. It contains a hydrophobic (fat soluble) phytol chain that allow it to be embedded in a lipid membrane. The rest of the structure called a tetrapyrrolic ring rests outside of the membrane . It is this part of the pigment that absorbs the energy from light. The metal at the center of the structure, Mg, can have variable oxidation states . This means that it can accept and donate e- readily depending of the situation. Its flexible, which is very important to the function of the molecule. Chlorophyll b: This molecule has a structure similar to that of chlorophyll a. It absorbs light of 453nm and 642 nm maximally. It is not as abundant as chlorophyll a, and probably evolved later. It helps increase the range of light a plant can use for energy. Carotenoids: This is a class of accessory pigments that occur in all photosynthetic organisms. They are completely hydrophobic (fat soluble) and exist in lipid membranes.

Carotenoids absorb light maximally between 460 nm and 550 nm and appear red, orange, or yellow to us. The most important function of carotenoids seems to be protecting the plant from free radicals formed from ultra violet or other radiation. Free radicals are dangerous because they contain an extra odd e- they don't really want to have. This means that they are constantly trying to get rid of this extra electron. They do this by attacking whatever bonds they can. In animal systems it is speculated that: Vitamin E + radical oxidized ---> radical + Vitamin E oxidized Vitamin E oxidized + carotenoids ----> Vit E + carotenoids oxidized Carotenoids oxidized + Vit C ---> carotenoids + Vit C oxidized In animals, the Vit C can be flushed out of the system since it is soluble in water which none of the other molecules are. Whether this same series of transfers occurs in plants is unknown. In humans, taking Vit E or betacarotenes in themselves is not protection against oxidants unless there is a supply of Vit C to flush it out.. Smokes who maintain low levels of C are not helped with increases of the other 2.

In plants, as the vacuole is a generally safe aqueous repository, the oxidized C may end up here, whereas carotenoids could not .... Xanthophylls are a fourth common class of pigments. They are essentially oxidized Carotenoids and contain oxygen. They are usually red and yellow and do not absorb energy as well as cartenoids. They are also fat soluble. Anthocyanins are a fifth class of pigments. These pigments contain Cu and are stored in the vacuole of a plant because they unlike the other fat soluble pigments are water soluble. Note the structure of the phycocyanins we in the red algae and BG's. How does it differ substantially from the above pigments? Photosynthesis article provided by Encarta Encyclopedia 2000 PHOTOSYNTHESIS INTRODUCTION Photosynthesis, process by which green plants and certain other organisms use the energy of light to convert carbon dioxide and water into the simple sugar glucose. In so doing, photosynthesis provides the basic energy source for virtually all organisms. An extremely important byproduct of photosynthesis is oxygen, on which most organisms depend. Photosynthesis occurs in green plants, seaweeds, algae, and certain bacteria.

These organisms are veritable sugar factories, producing millions of new glucose molecules per second. Plants use much of this glucose, a carbohydrate, as an energy source to build leaves, flowers, fruits, and seeds. They also convert glucose to cellulose, the structural material used in their cell walls. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems, and leaves. The plants can then draw on these reserves for extra energy or building materials. Each year, photosynthesizing organisms produce about 170 billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth. Photosynthesis has far-reaching implications. Like plants, humans and other animals depend on glucose as an energy source, but they are unable to produce it on their own and must rely ultimately on the glucose produced by plants. Moreover, the oxygen humans and other animals breathe is the oxygen released during photosynthesis. Humans are also dependent on ancient products of photosynthesis, known as fossil fuels, for supplying most of our modern industrial energy.

These fossil fuels, including natural gas, coal, and petroleum, are composed of a complex mix of hydrocarbons, the remains of organisms that relied on photosynthesis millions of years ago. Thus, virtually all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known. WHERE PHOTOSYNTHESIS OCCURS Plant photosynthesis occurs in leaves and green stems within specialized cell structures called chloroplasts. One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments. These disklike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes. A stack of thylakoids is called a granum (plural, grana); the grana lie suspended in a fluid known as stroma. Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll, a light-trapping pigment required for photosynthesis.

Additional light-trapping pigments, enzymes (organic substances that speed up chemical reactions), and other molecules needed for photosynthesis are also located within the thylakoid membranes. The pigments and enzymes are arranged in two types of units, Photosystem I and Photosystem II. Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules. HOW PHOTOSYNTHESIS WORKS Photosynthesis is a very complex process, and for the sake of convenience and ease of understanding, plant biologists divide it into two stages. In the first stage, the light-dependent reaction, the chloroplast traps light energy and converts it into chemical energy contained in nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), two molecules used in the second stage of photosynthesis. In the second stage, called the light-independent reaction (formerly called the dark reaction), NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and other reactions used to synthesize glucose. These two stages reflect the literal meaning of the term photosynthesis, to build with light.

AThe Light-Dependent Reaction Photosynthesis relies on flows of energy and electrons initiated by light energy. Electrons are minute particles that travel in a specific orbit around the nuclei of atoms and carry a small electrical charge. Light energy causes the electrons in chlorophyll and other light-trapping pigments to boost up and out of their orbit; the electrons instantly fall back into place, releasing resonance energy, or vibrating energy, as they go, all in millionths of a second. Chlorophyll and the other pigments are clustered next to one another in the photosystems, and the vibrating energy passes rapidly from one chlorophyll or pigment molecule to the next, like the transfer of energy in billiard balls. Light contains many colors, each with a defined range of wavelengths measured in nanometers, or billionths of a meter. Certain red and blue wavelengths of light are the most effective in photosynthesis because they have exactly the right amount of energy to energize, or excite, chlorophyll electrons and boost them out of their orbits to a higher energy level. Other pigments, called accessory pigments, enhance the light-absorption capacity of the leaf by capturing a broader spectrum of blue and red wavelengths, along with yellow and orange wavelengths.

None of the photosynthetic pigments absorb green light; as a result, green wavelengths are reflected, which is why plants appear green. Photosynthesis begins when light strikes Photosystem I pigments and excites their electrons. The energy passes rapidly from molecule to molecule until it reaches a special chlorophyll molecule called P700, so named because it absorbs light in the red region of the spectrum at wavelengths of 700 nanometers. Until this point, only energy has moved from molecule to molecule; now electrons themselves transfer between molecules. P700 uses the energy of the excited electrons to boost its own electrons to an energy level that enables an adjoining electron acceptor molecule to capture them. The electrons are then passed down a chain of carrier molecules, called an electron transport chain. The electrons are passed from one carrier molecule to another in a downhill direction, like individuals in a bucket brigade passing water from the top of a hill to the bottom. Each electron carrier is at a lower energy level than the one before it, and the result is that electrons release energy as they move down the chain. At the end of the electron transport chain lies the molecule nicotine adenine dinucleotide (NADP+).

Using the energy released by the flow of electrons, two electrons from the electron transport chain combine with a hydrogen ion and NADP+ to form NADPH. When P700 transfers its electrons to the electron acceptor, it becomes deficient in electrons. Before it can function again, it must be replenished with new electrons. Photosystem II accomplishes this task. As in Photosystem I, light energy activates electrons of the Photosystem II pigments. These pigments transfer the energy of their excited electrons to a special Photosystem II chlorophyll molecule, P680, that absorbs light best in the red region at 680 nanometers. Just as in Photosystem I, energy is transferred among pigment molecules and is then directed to the P680 chlorophyll, where the energy is used to transfer electrons from P680 to its adjoining electron acceptor molecule. From the Photosystem II electron acceptor, the electrons are passed through a different electron transport chain. As they pass along the cascade of electron carrier molecules, the electrons give up some of their energy to fuel the production of ATP, formed by the addition of one phosphorous atom to adenosine diphosphate (ADP).

Eventually, the electron transport carrier molecules deliver the Photosystem II electrons to Photosystem I, which uses them to maintain the flow of electrons to P700, thus restoring its function. P680 in Photosystem II is now electron deficient because it has donated electrons to P700 in Photosystem I. P680 electrons are replenished by the water that has been absorbed by the plant roots and transported to the chloroplasts in the leaves. The movement of electrons in Photosystems I and II and the action of an enzyme split the water into oxygen, hydrogen ions, and electrons. The electrons from water flow to Photosystem II, replacing the electrons lost by P680. Some of the hydrogen ions may be used to produce NADPH at the end of the electron transport chain, and the oxygen from the water diffuses out of the chloroplast and is released into the atmosphere through pores in the leaf. The transfer of electrons in a step-by-step fashion in Photosystems I and II releases energy and heat slowly, thus protecting the chloroplast and cell from a harmful temperature increase. It also provides time for the plant to form NADPH and ATP.

In the words of American biochemist and Nobel laureate Albert Szent-Gyorgyi, "What drives life is thus a little electric current, set up by the sunshine." BThe Light-Independent Reaction The chemical energy required for the light-independent reaction is supplied by the ATP and NADPH molecules produced in the light-dependent reaction. The light-independent reaction is cyclic, that is, it begins with a molecule that must be regenerated at the end of the reaction in order for the process to continue. Termed the Calvin cycle after the American chemist Melvin Calvin who discovered it, the light-independent reactions use the electrons and hydrogen ions associated with NADPH and the phosphorous associated with ATP to produce glucose. These reactions occur in the stroma, the fluid in the chloroplast surrounding the thylakoids, and each step is controlled by a different enzyme. The light-independent reaction requires the presence of carbon dioxide molecules, which enter the plant through pores in the leaf, diffuse through the cell to the chloroplast, and disperse in the stroma. The light-independent reaction begins in the stroma when these carbon dioxide molecules link to sugar molecules called ribulose bisphosphate (RuBP) in a process known as carbon fixation.

With the help of an enzyme, six molecules of carbon dioxide bond to six molecules of RuBP to create six new molecules. Several intermediate steps, which require ATP, NADPH, and additional enzymes, rearrange the position of the carbon, hydrogen, and oxygen atoms in these six molecules, and when the reactions are complete, one new molecule of glucose has been constructed and five molecules of RuBP have been reconstructed. This process occurs repeatedly in each chloroplast as long as carbon dioxide, ATP, and NADPH are available. The thousands of glucose molecules produced in this reaction are processed by the plant to produce energy in the process known as aerobic respiration, used as structural materials, or stored. The regenerated RuBP is used to start the Calvin cycle all over again. PHOTOSYNTHESIS VARIATIONS A majority of plants use these steps in photosynthesis. Plants such as corn and crabgrass that have evolved in hot, dry environments, however, must overcome certain obstacles to photosynthesis. On hot days, they partially close the pores in their leaves to prevent the escape of water. With the pores only slightly open, adequate amounts of carbon dioxide cannot enter the leaf, and the Calvin cycle comes to a halt.

To get around this problem, certain hot-weather plants have developed a way to keep carbon dioxide flowing to the stroma without capturing it directly from the air. They open their pores slightly, take in carbon dioxide, and transport it deep within the leaves. Here they stockpile it in a chemical form that releases the carbon dioxide slowly and steadily into the Calvin cycle. With this system, these plants can continue photosynthesis on hot days, even with their pores almost completely closed. A field of corn thus remains green on blistering days when neighboring plants wither, and crabgrass thrives in lawns browned by the summer sun. Bacteria lack chloroplasts, and instead use structures called chromatophores-membranes formed by numerous foldings of the plasma membrane, the membrane surrounding the fluid, or cytoplasm, that fills the cell. The chromatophores house thylakoids similar to plant thylakoids, which in some bacteria contain chlorophyll. For these bacteria, the process of photosynthesis is similar to that of plants, algae, and seaweed.

Many of these chlorophyll-containing bacteria are abundant in oceans, lakes, and rivers, and the oxygen they release dissolves in the water and enables fish and other aquatic organisms to survive. Certain archaebacteria, members of a group of primitive bacteria-like organisms, carry out photosynthesis in a different manner. The mud-dwelling green sulfur and purple sulfur archaebacteria use hydrogen sulfide instead of water in photosynthesis. These archaebacteria release sulfur rather than oxygen, which, along with hydrogen sulfide, imparts the rotten egg smell to mudflats. Halobacteria, archaebacteria found in the salt flats of deserts, rely on the pigment bacteriorhodopsin instead of chlorophyll for photosynthesis. These archaebacteria do not carry out the complete process of photosynthesis; although they produce ATP in a process similar to the light-dependent reaction and use it for energy, they do not produce glucose. Halobacteria are among the most ancient organisms, and may have been the starting point for the evolution of photosynthesis. While it may seem that we understand photosynthesis in detail, decades of experiments have given us only a partial understanding of this important process.

A more thorough understanding of the details of photosynthesis may pave the way for development of crops that are more efficient at using the sun's energy, producing food for increasingly bountiful harvests. Contributed By: Leal G. Dickson "Photosynthesis," Microsoft� Encarta� Encyclopedia 2000. � 1993-1999 Microsoft Corporation. All rights reserved. PHOTOSYNTHESIS PHOTOSYNTHESIS What is Photosynthesis? | Back to Top Photosynthesis is the process by which plants, some bacteria, and some protistans use the energy from sunlight to produce sugar, which cellular respiration converts into ATP , the "fuel" used by all living things. The conversion of unusable sunlight energy into usable chemical energy, is associated with the actions of the green pigment chlorophyll . Most of the time, the photosynthetic process uses water and releases the oxygen that we absolutely must have to stay alive. Oh yes, we need the food as well!

We can write the overall reaction of this process as: 6H2O + 6CO2 ----------> C6H12O6+ 6O2 Most of us don't speak chemicalese, so the above chemical equation translates as: six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen Diagram of a typical plant, showing the inputs and outputs of the photosynthetic process. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Leaves and Leaf Structure | Back to Top Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf. Cross section of a leaf, showing the anatomical features important to the study of photosynthesis: stoma, guard cell, mesophyll cells, and vein.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Water enters the root and is transported up to the leaves through specialized plant cells known as xylem (pronounces zigh-lem). Land plants must guard against drying out (desiccation) and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening (the stoma; plural = stomata; Greek for hole) flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal water is also lost. Cottonwood trees, for example, will lose 100 gallons of water per hour during hot desert days. Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures. Pea Leaf Stoma, Vicea sp. (SEM x3,520).

This image is copyright Dennis Kunkel at , used with permission. The Nature of Light | Back to Top White light is separated into the different colors (=wavelengths) of light by passing it through a prism. Wavelength is defined as the distance from peak to peak (or trough to trough). The energy of is inversely porportional to the wavelength: longer wavelengths have less energy than do shorter ones. Wavelength and other saspects of the wave nature of light. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The order of colors is determined by the wavelength of light. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more red the color. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet. The electromagnetic spectrum. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Light behaves both as a wave and a particle.

Wave properties of light include the bending of the wave path when passing from one material (medium) into another (i.e. the prism, rainbows, pencil in a glass-of-water, etc.). The particle properties are demonstrated by the photoelectric effect. Zinc exposed to ultraviolet light becomes positively charged because light energy forces electrons from the zinc. These electrons can create an electrical current. Sodium, potassium and selenium have critical wavelengths in the visible light range. The critical wavelength is the maximum wavelength of light (visible or invisible) that creates a photoelectric effect. Chlorophyll and Accessory Pigments | Back to Top A pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed). Chlorophyll , the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes. Black pigments absorb all of the wavelengths that strike them. White pigments/lighter colors reflect all or almost all of the energy striking them. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment. Absorption and transmission of different wavelengths of light by a hypothetical pigment.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms (plants, certain protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a . Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths. Molecular model of chlorophyll. The above image is from . Molecular model of carotene. The above image is from . Carotenoids and chlorophyll b absorb some of the energy in the green wavelength. Why not so much in the orange and yellow wavelengths?

Both chlorophylls also absorb in the orange-red end of the spectrum (with longer wavelengths and lower energy). The origins of photosynthetic organisms in the sea may account for this. Shorter wavelengths (with more energy) do not penetrate much below 5 meters deep in sea water. The ability to absorb some energy from the longer (hence more penetrating) wavelengths might have been an advantage to early photosynthetic algae that were not able to be in the upper ( photic) zone of the sea all the time. The molecular structure of chlorophylls. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons. If a pigment absorbs light energy, one of three things will occur. Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. Energy may trigger a chemical reaction, as in photosynthesis.

Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast) or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria. Absorption spectrum of several plant pigments (left) and action spectrum of elodea (right), a common aquarium plant used in lab experiments about photosynthesis. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The structure of the chloroplast and photosynthetic membranes | Back to Top The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana . The areas between grana are referred to as stroma . While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments. Structure of a chloroplast.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Stages of Photosynthesis | Back to Top Photosynthesis is a two stage process. The first process is the Light Dependent Process ( Light Reactions ), requires the direct energy of light to make energy carrier molecules that are used in the second process. The Light Independent Process (or Dark Reactions ) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present. Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts. Overview of the two steps in the photosynthesis process. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

Light Reactions | Back to Top In the Light Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH . Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions). In the Light Independent Process, carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of Hydrogen to form carbohydrates (general formula of carbohydrates is [CH2O]n). The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes. Photosystems are arrangements of chlorophyll and other pigments packed into thylakoids.

Many Prokaryotes have only one photosystem, Photosystem II (so numbered because, while it was most likely the first to evolve, it was the second one discovered). Eukaryotes have Photosystem II plus Photosystem I. Photosystem I uses chlorophyll a, in the form referred to as P700. Photosystem II uses a form of chlorophyll a known as P680. Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane. Action of a photosystem. This image is from the University of Minnesota page at . Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule. This occurs when the electrons from water are excited by the light in the presence of P680. The energy transfer is similar to the chemiosmotic electron transport occurring in the mitochondria. Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions.

These O-2 ions combine to form the diatomic O2 that is released. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in Photosystem I. Light acts on a molecule of P700 in Photosystem I, causing an electron to be "boosted" to a still higher potential. The electron is attached to a different primary electron acceptor (that is a different molecule from the one associated with Photosystem II). The electron is passed again through a series of redox reactions, eventually being attached to NADP+ and H+ to form NADPH, an energy carrier needed in the Light Independent Reaction. The electron from Photosystem II replaces the excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to NADPH. This energy is used in Carbon Fixation. Cyclic Electron Flow occurs in some eukaryotes and primitive photosynthetic bacteria. No NADPH is produced, only ATP.

This occurs when cells may require additional ATP, or when there is no NADP+ to reduce to NADPH. In Photosystem II, the pumping to H ions into the thylakoid and the conversion of ADP + P into ATP is driven by electron gradients established in the thylakoid membrane. Noncyclic photophosphorylation (top) and cyclic photophosphorylation (bottom). These processes are better known as the light reactions. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The above diagrams present the "old" view of photophosphorylation. We now know where the process occurs in the chloroplast, and can link that to chemiosmotic synthesis of ATP. Chemiosmosis as it operates in photophosphorylation within a chloroplast. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Halobacteria, which grow in extremely salty water, are facultative aerobes, they can grow when oxygen is absent.

Purple pigments, known as retinal (a pigment also found in the human eye) act similar to chlorophyll. The complex of retinal and membrane proteins is known as bacteriorhodopsin, which generates electrons which establish a proton gradient that powers an ADP-ATP pump, generating ATP from sunlight without chlorophyll. This supports the theory that chemiosmotic processes are universal in their ability to generate ATP. Dark Reaction | Back to Top Carbon-Fixing Reactions are also known as the Dark Reactions (or Light Independent Reactions). Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures, diffusing into the cells. Land plants must guard against drying out (desiccation) and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. The Calvin Cycle occurs in the stroma of chloroplasts (where would it occur in a prokaryote?). Carbon dioxide is captured by the chemical ribulose biphosphate (RuBP ). RuBP is a 5-C chemical. Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose. The reactions in this process were worked out by Melvin Calvin (shown below).

The above image is from , Ernest OrlandoLawrence Berkeley National Laboratory. " One of the new areas, cultivated both in Donner and the Old Radiation Laboratory, was the study of organic compounds labeled with carbon-14. Melvin Calvin took charge of this work at the end of the war in order to provide raw materials for John Lawrence's researches and for his own study of photosynthesis. Using carbon-14, available in plenty from Hanford reactors, and the new techniques of ion exchange, paper chromatography, and radioautography, Calvin and his many associates mapped the complete path of carbon in photosynthesis. The accomplishment brought him the Nobel prize in chemistry in 1961. (The preceding information was excerpted from the text of the Fall 1981 issue of LBL Newsmagazine.) Citation Caption: LBL News, Vol.6, No.3, Fall 1981 Melvin Calvin shown with some of the apparatus he used to study the role of carbon in photosynthesis." The first steps in the Calvin ccycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

The first stable product of the Calvin Cycle is phosphoglycerate (PGA) , a 3-C chemical. The energy from ATP and NADPH energy carriers generated by the photosystems is used to attach phosphates to ( phosphorylate ) the PGA. Eventually there are 12 molecules of glyceraldehyde phosphate (also known as phosphoglyceraldehyde or PGAL , a 3-C), two of which are removed from the cycle to make a glucose . The remaining PGAL molecules are converted by ATP energy to reform 6 RuBP molecules, and thus start the cycle again. Remember the complexity of life, each reaction in this process, as in Kreb's Cycle, is catalyzed by a different reaction-specific enzyme . C-4 Pathway | Back to Top Some plants have developed a preliminary step to the Calvin Cycle (which is also referred to as a C-3 pathway), this preamble step is known as C-4.

While most C-fixation begins with RuBP, C-4 begins with a new molecule, phosphoenolpyruvate (PEP), a 3-C chemical that is converted into oxaloacetic acid (OAA, a 4-C chemical) when carbon dioxide is combined with PEP. The OAA is converted to Malic Acid and then transported from the mesophyll cell into the bundle-sheath cell, where OAA is broken down into PEP plus carbon dioxide. The carbon dioxide then enters the Calvin Cycle, with PEP returning to the mesophyll cell. The resulting sugars are now adjacent to the leaf veins and can readily be transported throughout the plant. C-4 photosynthsis involves the separation of carbon fixation and carbohydrate systhesis in space and time. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

The capture of carbon dioxide by PEP is mediated by the enzyme PEP carboxylase, which has a stronger affinity for carbon dioxide than does RuBP carboxylase When carbon dioxide levels decline below the threshold for RuBP carboxylase, RuBP is catalyzed with oxygen instead of carbon dioxide. The product of that reaction forms glycolic acid, a chemical that can be broken down by photorespiration, producing neither NADH nor ATP, in effect dismantling the Calvin Cycle. C-4 plants, which often grow close together, have had to adjust to decreased levels of carbon dioxide by artificially raising the carbon dioxide concentration in certain cells to prevent photorespiration. C-4 plants evolved in the tropics and are adapted to higher temperatures than are the C-3 plants found at higher latitudes. Common C-4 plants include crabgrass, corn, and sugar cane. Note that OAA and Malic Acid also have functions in other processes, thus the chemicals would have been present in all plants, leading scientists to hypothesize that C-4 mechanisms evolved several times independently in response to a similar environmental condition, a type of evolution known as convergent evolution. Photorespiration.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. We can see anatomical differences between C3 and C4 leaves. Leaf anatomy of a C3 (top) and C4 (bottom) plant. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Carbon Cycle | Back to Top Plants may be viewed as carbon sinks , removing carbon dioxide from the atmosphere and oceans by fixing it into organic chemicals. Plants also produce some carbon dioxide by their respiration, but this is quickly used by photosynthesis. Plants also convert energy from light into chemical energy of C-C covalent bonds. Animals are carbon dioxide producers that derive their energy from carbohydrates and other chemicals produced by plants by the process of photosynthesis. The balance between the plant carbon dioxide removal and animal carbon dioxide generation is equalized also by the formation of carbonates in the oceans. This removes excess carbon dioxide from the air and water (both of which are in equilibrium with regard to carbon dioxide).

Fossil fuels, such as petroleum and coal, as well as more recent fuels such as peat and wood generate carbon dioxide when burned. Fossil fuels are formed ultimately by organic processes, and represent also a tremendous carbon sink. Human activity has greatly increased the concentration of carbon dioxide in air. This increase has led to global warming, an increase in temperatures around the world, the Greenhouse Effect. The increase in carbon dioxide and other pollutants in the air has also led to acid rain , where water falls through polluted air and chemically combines with carbon dioxide, nitrous oxides, and sulfur oxides, producing rainfall with pH as low as 4. This results in fish kills and changes in soil pH which can alter the natural vegetation and uses of the land. The Global Warming problem can lead to melting of the ice caps in Greenland and Antarctica, raising sea-level as much as 120 meters. Changes in sea-level and temperature would affect climate changes, altering belts of grain production and rainfall patterns. Learning Objectives | Back to Top After completing this chapter you should be able to: Study the general equation for photosynthesis and be able to indicate in which process each reactant is used and each product is produced.

List the two major processes of photosynthesis and state what occurs in those sets of reactions. Distinguish between organisms known as autotrophs and those known as heterotrophs as pertains to their modes of nutrition. Explain the significance of the ATP/ADP cycle. Describe the nature of light and how it is associated with the release of electrons from a photosystem. Describe how the pigments found on thylakoid membranes are organized into photosystems and how they relate to photon light energy. Describe the role that chlorophylls and the other pigments found in chloroplasts play to initiate the light-dependent reactions. Describe the function of electron transport systems in the thylakoid membrane. Explain the role of the two energy-carrying molecules produced in the light-dependent reactions (ATP and NADPH) in the light-independent reactions. Describe the Calvin-Benson cycle in terms of its reactants and products. Explain how C-4 photosynthesis provides an advantage for plants in certain environments. Describe the phenomenon of acid rain, and how photosynthesis relates to acid rain and the carbon cycle.. Review Questions | Back to Top 1.

The organic molecule produced directly by photosynthesis is: a) lipids; b) sugar; c) amino acids; d) DNA 2. The photosynthetic process removes ___ from the environment. a) water; b) sugar; c) oxygen; d) chlorophyll; e) carbon dioxide 3. The process of splitting water to release hydrogens and electrons occurs during the _____ process. a) light dependent; b) light independent; c) carbon fixation; d) carbon photophosphorylation; e) glycolysis 4. The process of fixing carbon dioxide into carbohydrates occurs in the ____ process. a) light dependent; b) light independent; c) ATP synthesis; d) carbon photophosphorylation; e) glycolysis 5. Carbon dioxide enters the leaf through ____. a) chloroplasts; b) stomata: c) cuticle; d) mesophyll cells; e) leaf veins 6. The cellular transport process by which carbon dioxide enters a leaf (and by which water vapor and oxygen exit) is ___. a) osmosis; b) active transport; c. co- transport; d) diffusion; e) bulk flow 7.

Which of the following creatures would not be an autotroph? a) cactus; b) cyanobacteria; c) fish; d) palm tree; e) phytoplankton 8. The process by which most of the world's autotrophs make their food is known as ____. a) glycolysis; b) photosynthesis; c) chemosynthesis; d) herbivory; e) C-4 cycle 9. The process of ___ is how ADP + P are converted into ATP during the Light dependent process. a) glycolysis; b) Calvin Cycle; c) chemiosmosis; d) substrate-level phosphorylation; e) Kreb's Cycle 10. Once ATP is converted into ADP + P, it must be ____. a) disassembled into components (sugar, base, phosphates) and then ressembled; b) recharged by chemiosmosis; c) converted into NADPH; d) processed by the glycolysis process; e) converted from matter into energy. 11. Generally speaking, the longer the wavelenght of light, the ___ the available energy of that light.

a) smaller; b) greater; c) same 12. The section of the electromagnetic spectrum used for photosynthesis is ___. a) infrared; b) ultraviolet; c) x-ray; d) visible light; e) none of the above 13. The colors of light in the visible range (from longest wavelength to shortest) is ___. a) ROYGBIV; b) VIBGYOR; c) GRBIYV; d) ROYROGERS; e) EBGDF 14. The photosynthetic pigment that is essential for the process to occur is ___. a) chlorophyll a; b) chlorophyll b; c) beta carotene; d) xanthocyanin; e) fucoxanthin 15. When a pigment reflects red light, _____. a) all colors of light are absorbed; b) all col;ors of light are reflected; c) green light is reflected, all others are absorbed; d) red light is reflected, all others are absorbed; e) red light is absorbed after it is reflected into the internal pigment molecules. 16. Chlorophyll a absorbs light energy in the ____color range.

a) yellow-green; b) red-organge; c) blue violet; d) a and b; e) b and c. 17. A photosystem is ___. a) a collection of hydrogen-pumping proteins; b) a collection of photosynthetic pigments arranged in a thylakjoid membrane; c) a series of electron-accepting proteins arranged in the thylakoid membrane; d. found only in prokaryotic organisms; e) multiple copies of chlorophyll a located in the stroma of the chloroplast. 18. The individual flattened stacks of membrane material inside the chloroplast are known as ___. a) grana; b) stroma; c) thylakoids; d) cristae; e) matrix 19. The fluid-filled area of the chloroplast is the ___. a) grana; b) stroma; c) thylakoids; d) cristae; e) matrix 20. The chloroplast contains all of these except ___. a) grana; b) stroma; c) DNA; d) membranes; e) endoplasmic reticulum 21.

The chloroplasts of plants are most close in size to __. a) unfertilized human eggs; b) human cheek cells; c) human nerve cells; d) bacteria in the human mouth; e) viruses 22. Which of these photosynthetic organisms does not have a chloroplast? a) plants; b) red algae; c) cyanobacteria; d) diatoms; e) dinoflagellates 23. The photoelectric effect refers to ____. a) emission of electrons from a metal when energy of a critical wavelength strikes the metal; b) absorbtion of electrons from the surrounding environment when energy of a critical wavelength is nearby; c) emission of electrons from a metal when struck by any wavelength of light; d) emission of electrons stored in the daytime when stomata are open at night; e) release of NADPH and ATP energy during the Calvin Cycvle when light iof a specific wavelength strikes the cell. 24. Light of the green wavelengths is commonly absorbed by which accessory pigment? a) chlorophyll a; b) chlorophyll b; c) phycocyanin; d) beta carotene 25.

The function of the electron transport proteins in the thyakoid membranes is ___. a) production of ADP by chemiosmosis; b) production of NADPH by substrate-level phosphorylation; c) pumping of hydrogens into the thylakoid space for later generation of ATP by chemiosmosis; d) pumping of hydrogens into the inner cristae space for later generation of ATP by chemiosmosis; e) preparation of water for eventual incorporation into glucose 26. ATP is known as the energy currency of the cell because ____. a) ATP is the most readily usable form of energy for cells; b) ATP passes energy along in an electron transport chain; c) ATP energy is passed to NADPH; d) ATP traps more energy than is produced in its formation; e) only eukaryotic cells use this energy currency. 27. Both cyclic and noncyclic photophosphorylation produce ATP. We can infer that the purpose of ATP in photosynthesis is to ____.

a) supply hydrogen to the carbohydrate; b) supply carbon to the carbohydrate; c) supply energy that can be used to form a carbohydrate; d) transfer oxygens from the third phosphate group to the carbohydrate molecule; e) convert RuBP into PGA 28. The role of NADPH in oxygen-producing photosynthesis is to ____. a) supply hydrogen to the carbohydrate; b) supply carbon to the carbohydrate; c) supply energy that can be used to form a carbohydrate; d) transfer oxygens from the third phosphate group to the carbohydrate molecule; e) convert RuBP into PGA. 29. The dark reactions require all of these chemicals to proceed except ___. a) ATP; b) NADPH; c) carbon dioxide; d) RUBP; e) oxygen 30. The first stable chemical formed by the Calvin Cycle is _____. a) RUBP; b) RU/18; c) PGA; d) PGAL; e) Rubisco 31.

The hydrogen in the carbohydrate produced by the Calvin Cycle comes from ___ a.) ATP; b) NADPH; c) the environment if the pH is very acidic; d) a and b; e) a and c 32. The carbon incorporated into the carbohydrate comes from ___. a) ATP; b) NADPH; c) carbon dioxide; d) glucose; e) organic molecules 33. C-4 photosynthesis is so named because _____. a) it produces a three carbon compound as the first stable product of photosynthesis; b) it produces a four carbon compound as the first stable produc of photosynthesis; c) it produces four ATP and four NADPH molecules for carbon fixation.; d) there are only four steps in this form of carbon fixation into carbohydrate. Photosynthesis, Chloroplast | Learn Science at Scitable Figure Detail What Is Photosynthesis? Why Is it Important? Most living things depend on photosynthetic cells to manufacture the complex organic molecules they require as a source of energy. Photosynthetic cells are quite diverse and include cells found in green plants, phytoplankton, and cyanobacteria.

During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. These sugar molecules are the basis for more complex molecules made by the photosynthetic cell, such as glucose. Then, via respiration processes, cells use oxygen and glucose to synthesize energy-rich carrier molecules, such as ATP, and carbon dioxide is produced as a waste product. Therefore, the synthesis of glucose and its breakdown by cells are opposing processes. Figure Detail The building and breaking of carbon-based material — from carbon dioxide to complex organic molecules (photosynthesis) then back to carbon dioxide (respiration) — is part of what is commonly called the global carbon cycle. Indeed, the fossil fuels we use to power our world today are the ancient remains of once-living organisms, and they provide a dramatic example of this cycle at work. The carbon cycle would not be possible without photosynthesis, because this process accounts for the "building" portion of the cycle (Figure 2). However, photosynthesis doesn't just drive the carbon cycle — it also creates the oxygen necessary for respiring organisms.

Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth. What Cells and Organelles Are Involved in Photosynthesis? Figure 3: Structure of a chloroplast © 2010 Nature Education All rights reserved. Photosynthetic cells contain special pigments that absorb light energy. Different pigments respond to different wavelengths of visible light. Chlorophyll, the primary pigment used in photosynthesis, reflects green light and absorbs red and blue light most strongly. In plants, photosynthesis takes place in chloroplasts, which contain the chlorophyll. Chloroplasts are surrounded by a double membrane and contain a third inner membrane, called the thylakoid membrane, that forms long folds within the organelle. In electron micrographs, thylakoid membranes look like stacks of coins, although the compartments they form are connected like a maze of chambers. The green pigment chlorophyll is located within the thylakoid membrane, and the space between the thylakoid and the chloroplast membranes is called the stroma (Figure 3, Figure 4).

Chlorophyll A is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. For example, the photosynthetic protists called dinoflagellates, which are responsible for the "red tides" that often prompt warnings against eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and the red pigments responsible for their dramatic coloration. Figure 4: Diagram of a chloroplast inside a cell, showing thylakoid stacks Shown here is a chloroplast inside a cell, with the outer membrane (OE) and inner membrane (IE) labeled. Other features of the cell include the nucleus (N), mitochondrion (M), and plasma membrane (PM). At right and below are microscopic images of thylakoid stacks called grana. Note the relationship between the granal and stromal membranes. © 2004 Nature Publishing Group Soll, J. & Schleiff, E. Protein import into chloroplasts.

Nature Reviews Molecular Cell Biology 5, 198-208 (2004) doi:10.1038/nrm1333. All rights reserved. Figure Detail What Are the Steps of Photosynthesis? Photosynthesis consists of both light-dependent reactions and light-independent reactions. In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5). Figure 5: The light and dark reactions in the chloroplast The chloroplast is involved in both stages of photosynthesis. The light reactions take place in the thylakoid. There, water (H2O) is oxidized, and oxygen (O2) is released.

The electrons that freed from the water are transferred to ATP and NADPH. The dark reactions then occur outside the thylakoid. In these reactions, the energy from ATP and NADPH is used to fix carbon dioxide (CO2). The products of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism. Note that the dark reaction takes place in the stroma (the aqueous fluid surrounding the stacks of thylakoids) and in the cytoplasm. Chemistry for Biologists: Photosynthesis Leaves and leaf structure | The structure of the chloroplast and photosynthetic membranes | Stages of photosynthesis | Non-cyclic phosphorylation (the Z scheme) | Chemiosmosis and ATP synthesis | Cyclic phosphorylation | The light-independent reactions | Summary of stages of photosynthesis | Factors affecting the rate of photosynthesis | Test your knowledge Photosynthesis Photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed.

Photosynthesis may be summarised by the word equation: carbon dioxide + water glucose + oxygen The conversion of usable sunlight energy into chemical energy is associated with the action of the green pigment chlorophyll. Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll a absorbs its energy from the violet-blue and reddish orange-red wavelengths, and little from the intermediate (green-yellow-orange) wavelengths. Chlorophyll - click on image to open All chlorophylls have: a lipid-soluble hydrocarbon tail (C20H39 -) a flat hydrophilic head with a magnesium ion at its centre; different chlorophylls have different side-groups on the head The tail and head are linked by an ester bond.

Leaves and leaf structure Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf. Water enters the root and is transported up to the leaves through specialized plant cells known as xylem vessels. Land plants must guard against drying out and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through the stoma (the singular of stomata), flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal of water is also lost. Cottonwood trees, for example, will lose 100 gallons (about 450 dm3) of water per hour during hot desert days.

The structure of the chloroplast and photosynthetic membranes The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments. Structure of a chloroplast Stages of photosynthesis When chlorophyll a absorbs light energy, an electron gains energy and is 'excited'. The excited electron is transferred to another molecule (called a primary electron acceptor). The chlorophyll molecule is oxidized (loss of electron) and has a positive charge. Photoactivation of chlorophyll a results in the splitting of water molecules and the transfer of energy to ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP).

The chemical reactions involved include: condensation reactions - responsible for water molecules splitting out, including phosphorylation (the addition of a phosphate group to an organic compound) oxidation/reduction (redox) reactions involving electron transfer Photosynthesis is a two stage process.

The Light dependent reactions, a light-dependent series of reactions which occur in the grana, and require the direct energy of light to make energy-carrier molecules that are used in the second process: light energy is trapped by chlorophyll to make ATP (photophosphorylation) at the same time water is split into oxygen, hydrogen ions and free electrons: 2H2O 4H+ + O2 + 4e- (photolysis) the electrons then react with a carrier molecule nicotinamide adenine dinucleotide phosphate (NADP), changing it from its oxidised state (NADP+) to its reduced state (NADPH): NADP+ + 2e- + 2H+ NADPH + H+ The light-independent reactions, a light-independent series of reactions which occur in the stroma of the chloroplasts, when the products of the light reaction, ATP and NADPH, are used to make carbohydrates from carbon dioxide (reduction); initially glyceraldehyde 3-phosphate (a 3-carbon atom molecule) is formed.

The light-dependent reactions When light energy is absorbed by a chlorophyll molecule its electrons gain energy and move to higher energy levels in the molecule (photoexcitation). Sufficient energy ionises the molecule, with the electron being 'freed' leaving a positively charged chlorophyll ion. This is called photoionisation. In whole chloroplasts each chlorophyll molecule is associated with an electron acceptor and an electron donor. These three molecules make up the core of a photosystem. Two electrons from a photoionised chlorophyll molecule are transferred to the electron acceptor. The positively charged chlorophyll ion then takes a pair of electrons from a neighbouring electron donor such as water. An electron transfer system (a series of chemical reactions) carries the two electrons to and fro across the thylakoid membrane. The energy to drive these processes comes from two photosystems: Photosystem II (PSII) (P680) Photosystem I (PSI) (P700) It may seem confusing, but PSII occurs before PSI. It is named because it was the second to be discovered and hence named second. The energy changes accompanying the two sets of changes make a Z shape when drawn out.

This is why the electron transfer process is sometimes called the Z scheme. Key to the scheme is that sufficient energy is released during electron transfer to enable ATP to be made from ADP and phosphate. Chemiosmosis and ATP synthesis The components of non-cyclic phosphorylation are found in the thylakoid membranes of the chloroplast. Electrons passing through the transport chain provide energy to pump H+ ions from the stroma, across the thylakoid membrane into the thylakoid compartment. H+ ions are more concentrated in the thylakoid compartment than in the stroma. We say there is an electrochemical gradient. H+ ions diffuse from the high to the low regions of concentration. This drives the production of ATP. Cyclic phosphorylation The net effect of non-cyclic phosphorylation is to pass electrons from water to NADP. Energy released enables the production of ATP. But much more ATP is needed to drive the light-independent reactions. This extra energy is obtained from cyclic phosphorylation. This involves only Photosystem I which generates excited electrons. These are transferred to the electron transport chain between PSII and PSI, rather than to NADP+ and so no NADPH is formed.

The cycle is completed by electrons being transported back to PSI by the electron transport system. The light-independent reactions In the Light-Independent Process (the Dark reaction) carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of hydrogen to form carbohydrates. The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes. Carbon dioxide combines with a five-carbon sugar, ribulose 1,5-biphosphate (RuBP). A six-carbon sugar forms but is unstable. Each molecule breaks down to form two glycerate 3-phosphate (GP) molecules. These glycerate 3-phosphate (GP) molecules are phosphorylated by ATP into glycerate diphosphate molecules. These are reduced by NADPH to two molecules of glyceraldehyde 3-phosphate (GALP).

Of each pair of GALP molecules produced: one molecule is the initial end product of photosynthesis; it is quickly converted to glucose and other carbohydrates, lipids or amino acids one molecule forms RuBP through a series of chemical reactions The first steps in the Calvin cycle The first stable product of the Calvin Cycle is phosphoglycerate (PGA), a 3-C chemical. The energy from ATP and NADPH energy carriers generated by the photosystems is used to phosphorylate the PGA. Eventually there are 12 molecules of glyceraldehyde phosphate (also known as phosphoglyceraldehyde or PGAL, a 3-C), two of which are removed from the cycle to make a glucose. The remaining PGAL molecules are converted by ATP energy to reform six RuBP molecules, and thus start the cycle again. Factors affecting the rate of photosynthesis The main factors are light intensity, carbon dioxide concentration and temperature, known as limiting factors. As light intensity increases, the rate of the light-dependent reaction, and therefore photosynthesis generally, increases proportionately. As light intensity is increased however, the rate of photosynthesis is eventually limited by some other factor. Chlorophyll a is used in both photosystems. The wavelength of light is also important.

PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a high proportion of energy concentrated in these wavelengths will produce a high rate of photosynthesis. An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor. Photosynthesis is dependent on temperature. It is a reaction catalysed by enzymes. As the enzymes approach their optimum temperatures the overall rate increases. Above the optimum temperature the rate begins to decrease until it stops.

Question: What pigment found in plant leaves uses energy from sunlight to provide food?
Answer: Chlorophyl