Parts of the Cell

http://www.biology4kids.com/files/cell_main.html

GLE 2; Grade 7

All living organisms on Earth are divided in pieces called cells. There are smaller pieces to cells that include proteins and organelles. There are also larger pieces called tissues and systems. Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth.

There are many types of cells. In biology class, you will usually work with plant-like cells and animal-like cells. We say animal-like because an animal type of cell could be anything from a tiny microorganism to a nerve cell in your brain. Plant cells are easier to identify because they have a protective structure called a cell wall made of cellulose. Plants have the wall; animals do not. Plants also have organelles like the chloroplast (the things that make them green) or large water-filled vacuoles.

Cell MembranesWe have been talking about cells being a unit of organization in biology. Let's look at the cell membrane and see how that membrane keeps all of the pieces inside. When you think about a membrane, imagine it is like a big plastic bag with some tiny holes. That bag holds all of the cell pieces and fluids inside the cell and keeps any nasty things outside the cell. The holes are there to let some things move in and out of the cell

Cell Wall - What's it for?While cell membranes might be around every cell, cell walls made of cellulose are only found around plant cells. Cell walls are made of specialized sugars called cellulose. Cellulose provides a protected framework for a plant cell to survive. It's like taking a water balloon and putting it in a cardboard box. The balloon is protected from the outside world. Cellulose is called a structural carbohydrate (complex sugar) because it is used in protection and support.

Cell walls also help a plant keep its shape. While they do protect the cells, cell walls and cellulose also allow plants to grow to great heights. While you have a skeleton to hold you up, a 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For smaller plants, cell walls are slightly elastic. Wind can push them over and then they bounce back. Big redwoods need strength in high winds and sway very little (except at the top).

Cytoplasm - Filling FluidCytoplasm is the fluid that fills a cell. Scientists used to call the fluid protoplasm. Early on, they didn't know about the many different types of fluids in the cell. There is special fluid in the mitochondria, endoplasmic reticulum, Golgi apparatus, and nucleus. The only two 'plasms' left are cytoplasm (the fluid in the cell also called cytosol) and nucleoplasm (the fluid in the nucleus). Each of those fluids has a very different composition.

The cell organelles are suspended in the cytosol. You will learn that the microfilaments and microtubules set up a "skeleton" of the cell and the cytosol fills the spaces. The cytoplasm has many different molecules dissolved in solution. You'll find enzymes, fatty acids, sugars, and amino acids that are used to keep the cell working. Waste products are also dissolved before they are taken in by vacuoles or sent out of the cell.

Cell Nucleus - Commanding the CellThe cell nucleus acts like the brain of the cell. It helps control eating, movement, and reproduction. If it happens in a cell, chances are the nucleus knows about it. The nucleus is not always in the center of the cell. It will be a big dark spot somewhere in the middle of all of the cytoplasm (cytosol). You probably won't find it near the edge of a cell because that might be a dangerous place for the nucleus to be. If you don't remember, the cytoplasm is the fluid that fills cells.

When the cell is in a resting state there is something called chromatin in the nucleus. Chromatin is made of DNA, RNA, and nuclear proteins. DNA and RNA are the nucleic acids inside of the cell. When the cell is going to divide, the chromatin becomes very compact. It condenses. When the chromatin comes together, you can see the chromosomes. You will also find the nucleolus inside of the nucleus. When you look through a microscope, it looks like a nucleus inside of the nucleus. It is made of RNA and protein. It does not have much DNA at all.

Chloroplasts - Show me the GreenChloroplasts are the food producers of the cell. They are only found in plant cells and some protists. Animal cells do not have chloroplasts. Every green plant you see is working to convert the energy of the sun into sugars. Plants are the basis of all life on Earth. They create sugars, and the byproduct of that process is the oxygen that we breathe. That process happens in the chloroplast. Mitochondria work in the opposite direction and break down the sugars and nutrients that the cell receives.

Chromosomes - Pull up Those GenesChromosomes are the things that make organisms what they are. They carry all of the information used to help a cell grow, thrive, and reproduce. Chromosomes are made up of DNA. Segments of DNA in specific patterns are called genes. Your genes make you who you are. You will find the chromosomes and genetic material in the nucleus of a cell. In prokaryotes, DNA floats in the cytoplasm in an area called the nucleoid.

Endoplasmic Reticulum - Wrapping it UpAnother organelle in the cell is the endoplasmic reticulum (ER). While the function of the nucleus is to act as the cell brain, the ER functions as a packaging system. It does not work alone. The ER works closely with the Golgi apparatus, ribososmes, RNA, mRNA, and tRNA. It creates a network of membranes found through the whole cell. The ER may also look different from cell to cell, depending on the cell's function.

Vacuoles - Storage Bins to the Cells Vacuoles are storage bubbles found in cells. They are found in both animal and plant cells but are much larger in plant cells. Vacuoles might store food or any variety of nutrients a cell might need to survive. They can even store waste products so the rest of the cell is protected from contamination. Eventually, those waste products would be sent out of the cell.

The structure of vacuoles is fairly simple. There is a membrane that surrounds a mass of fluid. In that fluid are nutrients or waste products. Plants may also use vacuoles to store water. Those tiny water bags help to support the plant. They are closely related to objects called vesicles that are found throughout the cell.

In plant cells, the vacuoles are much larger than in animal cells. When a plant cell has stopped growing, there is usually one very large vacuole. Sometimes that vacuole can take up more than half of the cell's volume. The vacuole holds large amounts of water or food. Don't forge that vacuoles can also hold the plant waste products. Those waste products are slowly broken into small pieces that cannot hurt the cell. Vacuoles hold onto things that the cell might need, just like a backpack.

Lysosomes - Little Enzyme Packages You will find organelles called lysosomes in nearly every animal-like eukaryotic cell. Lysosomes hold enzymes that were created by the cell. The purpose of the lysosome is to digest things. They might be used to digest food or break down the cell when it dies. What creates a lysosome? You'll have to visit the Golgi complex for that answer.

A lysosome is basically a specialized vesicle that holds a variety of enzymes. The enzyme proteins are first created in the rough endoplasmic reticulum. Those proteins are packaged in a vesicle and sent to the Golgi apparatus. The Golgi then does its final work to create the digestive enzymes and pinches off a small, very specific vesicle. That vesicle is a lysosome. From there the lysosomes float in the cytoplasm until they are needed. Lysosomes are single-membrane organelles.