# Science: An Elementary Teacher’s Guide/Cells
## Cell Size

Cells can range in size from the largest, which are about the size of a period at the end of this sentence, to the smallest, which are so tiny they can only be seen with very powerful microscopes. Inside a cell are tiny organs called organelles. The word organelle is a big word that means small organ. These organelles function to provide for the needs of the cell. They work to bring in food supplies, get rid of waste, protect the cell, repair the cell, and help it grow and reproduce.
Why are cells so small?
It all comes down to physical limitations. Cells are small in order to maximize their ratio of surface area to volume. Smaller cells have a higher ratio, which allow more molecules and ions move across the cell membrane. As cells become larger, the surface area to volume ratio decreases.
Why does it matter?
The size of a cell matters because the more edge a cell has, the easier it is for the cell to move around.

## Inner Life of a Cell

Here is a video which demonstrates a small bit of what happens in the inner life of a cell. It begins inside a small blood vessel, with white blood cells crawling along the edges while red blood cells rush past. We zoom in and see proteins on the membrane of the white blood cell interacting with proteins on the membranes of the cells that line the blood vessel. This interaction sets off a cascade of reactions within the white blood cell, so we journey into the cell, seeing its cytoskeleton and organelles, and we watch as messenger RNA gets translated into new proteins, which are packaged by the Golgi and sent to the cell's membrane. The final result is that the white blood cell changes its shape and squeezes between the epithelial cells lining the blood vessel--it is leaving the blood vessel to go fight infection.

## Plant Cell

What is a Plant Cell?
Back to Top Plant cells are eukaryotic cells i.e., the DNA in a plant cell is enclosed within the nucleus. The most important distinctive structure of plant cell is the presence of the cell wall outside the cell membrane. It forms the outer lining of the cell. The cell wall mostly constitutes of cellulose and its main function is providing support and rigidity. Plants cells also contain many membrane bound cellular structures. These organelles carry out specific functions necessary for survival and normal operation of the cells. There are a wide range of operations like producing hormones, enzymes, and all metabolic activities of the cell.
Diagram of Plant Cell Distinctive Features of Plant Cell The features that are distinctive in plant cells are as follows:
* Plant cells contain cell structures like cell wall, plastids, and large vacuoles.
* Cell wall provide plant cells rigidity and structural support and cell to cell interaction.
* Plastids help in storage of plant products.
* Chloroplasts aid in carrying out the process of photosynthesis to produce food for the plants.
* Vacuoles are water-filled, membrane bound organelles which stores useful materials.
* Plants have specialized cells in order to perform certain functions for the survival of plants. Some cells manufacture and store organic molecules, others transport nutrients throughout the plant.
* Some specialized plant cells include: parenchyma cells, collenchyma cells, sclerenchyma cells, water conducting cells and food conducting cells.
-Parts of Plant Cell- Plants cell constitute of membrane bound nucleus and many cellular structures. These organelles carry out functions that are necessary for the proper functioning and survival of the cell. The cell organelles of the plant are enclosed by a cell wall and cell membrane. The constituents of the cell are suspended in the cytoplasm or cytosol.
The parts of the plant cell are as follows:
* Cell wall is the outermost rigid covering of the plant cell. It is a salient feature of plant cell.
* Cell membrane or the plasma membrane is the outer lining of the cell inside the cell wall.
* Cytosol or cytoplasm is the gel-like matrix inside the cell membrane which constitutes all other cell organelles.
* Nucleus is the control center of the cell. It is a membrane bound structure which contains the hereditary material of the cell - the DNA
* Chloroplast is a plastid with green pigment chlorophyll. It traps light energy and converts it to chemical energy by the process of photosynthesis.
* Mitochondria carries out cellular respiration and provides energy to the cells.
* Vacuoles are the temporary storage center of the cell.
* Golgi body is the unit where proteins are sorted and packed.
* Ribosomes are structures that assemble proteins.
* Endoplasmic reticulum are membrane covered organelles that transport materials.

## Animal Cell

Animal Cell Definition Animal cells are eukaryotic. Animal cells are have outer boundary known as the plasma membrane. The nucleus and the organelles of the cell are bound by a membrane. The genetic material (DNA) in animal cells is within the nucleus that is bound by a double membrane. The cell organelles have a vast range of functions to perform like hormone and enzyme production to providing energy for the cells.
The components of animal cells are centrioles, cilia and flagella, endoplasmic reticulum, golgi apparatus, lysosomes, microfilaments, microtubules, mitochondria, nucleus, peroxisomes, plasma membrane and ribosomes. Parts of Animal CellBack to Top Animal cell contains membrane bound nucleus, it also contains other membrane bound cellular organelles. These cellular organelles carry out specific functions that are necessary for the normal functioning of the cell. Animal cells lack cell wall, a large vacuole and plastids. Due to the lack of the cell wall, the shape and size of the animal cells are mostly irregular. The constituents of animal cells are structures like centrioles, cilia and flagella and lysosomes.
Parts of the animal cell are as follows:
Cell membrane - forms the outer covering of the cell, and is semi-permeable.
Cytoplasm - is a gel-like matrix where all the other cell organelles are suspended inside the cell.
Nucleus - contains the hereditary material DNA and directs the activities of the cell.
Centrioles - organize the microtubules assembly during cell division.
Endoplasmic Reticulum - are a network of membranes composed of rough and smooth endoplasmic reticulum.
Golgi complex - is responsible for storing, packaging of cellular products.
Lysosomes - are enzyme sacs, that digest cellular wastes.
Microtubules - are hollow rods, function primarily as support and shape to the cell.
Mitochondria - is the site for cellular respiration and producers of energy.
Ribosomes - are made of RNA and proteins, and are sites for protein synthesis.
Nucleolus - is the structure within the nucleus and helps in synthesis of ribosomes.
Nucleopore - is the tiny hole in the nuclear membrane, allows the movement of nucleic acids and proteins in/out of the cell.
Animal Cell Structure Animal cells are eukaryotic cells, the nucleus and other organelles of the cell are bound by membrane.
Animal Cell Structure
Cell membrane
* It is a semi-permeable barrier, allowing only a few molecules to move across it.
* Electron microscopic studies of cell membrane shows the lipid bi-layer model of the plasma membrane, it also known as the fluid mosaic model.
* The cell membrane is made up of phospholipids which has polar(hydrophillic) heads and non-polar (hydrophobic) tails.
* The cell membrane surrounds, protects and insulates every single cell of every organ or tissue in our body.
Cytoplasm
* The fluid matrix that fills the cell is the cytoplasm.
* The cellular organelles are suspended in this matrix of the cytoplasm.
* This matrix maintains the pressure of the cell, ensures the cell doesn't shrink or burst.
* The cytoplasm is highly structured: it is not some kind of soup, even though it is made out of 75–80% water.
Nucleus
* Nucleus is the house for most of the cells genetic material- the DNA and RNA.
* The nucleus is surrounded by a porous membrane known as the nuclear membrane.
* The RNA moves in/out of the nucleus through these pores.
* Proteins needed by the nucleus enter through the nuclear pores.
* The RNA helps in protein synthesis through transcription process.
* The nucleus controls the activity of the cell and is known as the control center.
* The nucleolus is the dark spot in the nucleus, and it is the location for ribosome formation.
Ribosomes
* Ribosomes is the site for protein synthesis where the translation of the RNA takes place.
* As protein synthesis is very important to the cell, ribosomes are found in large number in all cells.
* Ribosomes are found freely suspended in the cytoplasm and also are attached to the endoplasmic reticulum.
Endoplasmic reticulum
* ER is the transport system of the cell. It transports molecules that need certain changes and also molecules to their destination.
* ER is of two types, rough and smooth.
* ER bound to the ribosomes appear rough and is the rough endoplasmic reticulum; while the smooth ER do not have the ribosomes.
Lysosomes
* It is the digestive system of the cell.
* They have digestive enzymes helps in breakdown the waste molecules and also help in detoxification of the cell.
* If the lysosomes were not membrane bound the cell could not have used the destructive enzymes.
Centrosomes
* It is located near the nucleus of the cell and is known as the 'microtubule organizing center' of the cell.
* Microtubules are made in the centrosome.
* During mitosis the centrosome aids in dividing of the cell and moving of the chromosome to the opposite sides of the cell.
Vacuoles
* They are bound by single membrane and small organelles.
* In many organisms vacuoles are storage organelles.
* Vesicles are smaller vacuoles which function for transport in/out of the cell.
* The single large vacuole of the cell is surrounded by a membrane, called the tonoplast, and filled with a solution of water, dissolved ions , sugars, amino acids , and other materials.
Golgi bodies
* Golgi bodies are the packaging center of the cell.
* The Golgi bodies modify the molecules from the rough ER by dividing them into smaller units with membrane known as vesicles.
* They are flattened stacks of membrane-bound sacs.
Mitochondria
* Mitochondria is the main energy source of the cell.
* They are called the power house of the cell because energy(ATP) is created here.
* Mitochondria consists of inner and outer membrane.
* It is spherical or rod shaped organelle.
* It is an organelle which is independent as it has its own hereditary material.
Peroxisomes
* Peroxisomes are single membrane bound organelle that contain oxidative enzymes that are digestive in function.
* They help in digesting long chains of fatty acids and amino acids and help in synthesis of cholesterol.
Cytoskeleton
* It is the network of microtubules and microfilament fibres.
* They give structural support and maintain the shape of the cell.
Cilia and Flagella
* Cilia and flagella are structurally identical structures.
* They are different based on the function they perform and their length.
* Cilia are short and are in large number per cell while flagella are longer and are fewer in number.
* They are organelles of movement.
* The flagellar motion is undulating and wave-like whereas the ciliary movement is power stroke and recovery stroke.

## Cell wall

Found only in plant cells, the cell wall provides the cell with additional strength. Cell walls are thick walls built around the cell. These walls are made from cellulose. The cell walls location is outside the cell membrane.
* Why do plants have cell walls and not animals?
Think about how a plant grows. Plants grow tall, towards the Sun's light. In order to provide plants with the strength necessary to support their weight, the cells within the plant have this hard cell covering. If a tree were soft and mushy like an animal, do you think they could stand strong and tall? What would happen if the cells in your body had cell walls? If an animal's body were made of plant cells, the animal would be very stiff and unable to move easily. Instead of cell walls, animals use other creative solutions to give them strength. For example, many animal's bodies are built on a structure of bones. These bones allow the animal to have the strength to stand up, but the flexibility to move quickly.
The function of the Cell Wall is to:
- By giving the cell a definite shape and structure.
- Prevent water loss.
- The protection against infection and mechanical stress.
- It separates the interior of the cell to the outer environment.
- The help in osmotic-regulation.

## Cell membrane

Found in both plant and animal cells, the cell membrane is the outside wall of a cell. In plant cells, it is a second wall, and is found just inside the main cell wall. The cell membranes found in animal cells contain a chemical called cholesterol. This chemical makes the membrane harder. Plant cells do not need cholesterol because they have a cell wall, as a result, their cell membranes are softer.
In order for a cell to remain healthy, the cell needs to be able to bring in food, and get rid of waste. Look closely at this picture. You will notice that the cell membrane has small openings, or doorways. These openings allow the cells to move materials in and out of the cell.

## What Are Cellular Membranes Made Of?

With few exceptions, cellular membranes — including plasma membranes and internal membranes — are made of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains. Glycerol is a three-carbon molecule that functions as the backbone of these membrane lipids. Within an individual glycerophospholipid, fatty acids are attached to the first and second carbons, and the phosphate group is attached to the third carbon of the glycerol backbone. Variable head groups are attached to the phosphate. Space-filling models of these molecules reveal their cylindrical shape, a geometry that allows glycerophospholipids to align side-by-side to form broad sheets.

## What Do Membranes Do?

Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly. Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly. On the other hand, cell membranes restrict diffusion of highly charged molecules, such as ions, and large molecules, such as sugars and amino acids. The passage of these molecules relies on specific transport proteins embedded in the membrane.

## How Diverse Are Cell Membranes?

In contrast to prokaryotes, eukaryotic cells have not only a plasma membrane that encases the entire cell, but also intracellular membranes that surround various organelles. In such cells, the plasma membrane is part of an extensive endomembrane system that includes the endoplasmic reticulum (ER), the nuclear membrane, the Golgi apparatus, and lysosomes. Membrane components are exchanged throughout the endomembrane system in an organized fashion. For instance, the membranes of the ER and the Golgi apparatus have different compositions, and the proteins that are found in these membranes contain sorting signals, which are like molecular zip codes that specify their final destination.
Mitochondria and chloroplasts are also surrounded by membranes, but they have unusual membrane structures — specifically, each of these organelles has two surrounding membranes instead of just one. The outer membrane of mitochondria and chloroplasts has pores that allow small molecules to pass easily. The inner membrane is loaded with the proteins that make up the electron transport chain and help generate energy for the cell. The double membrane enclosures of mitochondria and chloroplasts are similar to certain modern-day prokaryotes and are thought to reflect these organelles' evolutionary origins.

## Cytoplasm

As you travel through the cell membrane and enter the cell, you will find yourself floating in a kind of jelly. This jelly that fills the inside of a cell is called Cytoplasm. Cytoplasm helps to hold the cell's organelles (small organs) in place.
Cytoplasm also gives the cell structure. Think of a balloon. An empty balloon does not have much structure. However, if we fill it with something, like water, it begins to take shape. Cytoplasm helps give cells structure. Cytoplasm also helps the cell move proteins, chromosomes and other materials, including the cells organelles, around the cell.

## Endoplasmic reticulum

A cell also has a system of tiny roads. These roads are actually tubes called the endoplasmic reticulum. These clear tubes travel throughout all parts of the cell. Some go from the nuclear membrane to the outside cellular membrane. Others travel to different organelles. Throughout the cell, the endoplasmic reticulum carries materials where they need to go. The endoplasmic reticulum helps things move around a cell. Whenever you hear the word endoplasmic reticulum, just think “cellular highway.” So climb aboard the endoplasmic reticulum as we prepare to travel to, and explore the rest of the cell.

## Ribosomes

As you travel along the endoplasmic reticulum, you will notice that stuck to the sides of this tube are several small balls called ribosomes. Ribosomes are like small factories, that use available materials to build proteins. These proteins can then be used by the cell for other purposes, such as to build new structures, repair damage, and direct chemical reactions. Why do you think ribosomes are found on the walls of the endoplasmic reticulum? When a ribosome is done building a protein, it can release it directly into the endoplasmic reticulum where it can then be transported to wherever it is needed. Not all ribosomes end up attached to the inside of the endoplasmic reticulum. Many simply float around in the cytoplasm. Ribosomes are created in the nucleolus, which is found inside the cell's nucleus.

## Lysosomes

As you travel through the endoplasmic reticulum, look out into the cytoplasm. You will see small round objects called Lysosomes. Lysosomes are filled with enzymes that are used to break up and partially digest food. The food particles are broken up into smaller pieces, which can then be passed on to another organelle called mitochondria. We will learn about the mitochondria shortly. Another important job that the lysosomes perform is to break down and digest older parts of a cell. As a cell ages, parts of it need to be replaced. The lysosomes break down the old parts so that the materials can be reused to build new parts.

## Mitochondria

The organelle responsible for energy production inside a cell is called a Mitochondrion. These pill shaped organelles take food and break it apart into water and carbon dioxide. As food is broken down, an enormous amount of energy is created. AN example of mitochondria: Think of the power generators that produce electricity for our cities. As these power generators break up fuel, they create electricity which is captured and transmitted throughout the city. Within living things, cells which are more active need more energy. As a result, these cells have more mitochondria. In a similar way, bigger cities need more power, and as a result, these cities have more power plants.

## Golgi Apparatus

The Golgi Apparatus is responsible for taking the proteins which were created by the ribosomes and making them bigger and better. Think of an assembly line where cars are made. The first worker creates a car frame. The next worker adds an engine, or seats, or other parts. When the golgi apparatus is done, it releases the new proteins into the cell, where they can be used to strengthen and build up the cell.
Golgi apparatus

## Vacuoles & Chloroplasts

Vacuoles are small sacs are filled with food and water. They are used by cells as storage tanks. All plant cells have vacuoles, but not all animal cells do. The primary place where plants store water is within its vacuoles. When a plant's vacuoles are filled with water they become plump, giving the plant strength. What happens when you do not water a plant? It begins to wilt, becoming softer. This is because the vacuoles found inside the plants cells are running out of water.
A chloroplast is a small pill-shaped organelle found only in plants. Chloroplasts are green because they are filled with a green pigment, or chemical called chlorophyll. Chlorophyll is used by a plant to capture energy from the sun, which can later be used to create food.

## Quiz

Try this quick quiz and test what you have learned by reading this chapter!
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