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THE CELL
In every living thing on Earth there are cells. A very small, complex unit of PROTOPLASM, usually with a nucleus, cytoplasm, and enclosing membrane is termed “the cell”. Cells keep living things organized. One of the main purposes of cells is to organize. You have brain cells, stomach cells, bone cells, and many other types of cells. Each is different from the others.
In the same way there are different kinds of cells inside you, different organisms have different types of cells. Trees have different cells that you and so do monkeys. Each of those cells is different in some way. It might be the DNA, or the number of MITOCHONDRIA. Every cell has a specific job to do.
There are even some creatures on Earth that don’t have all of the basic parts of a cell, or even a cell. Bacteria are missing nuclear membranes to hold their DNA inside. Viruses don’t have cells; they are just a bit of DNA or RNA and a protein coat. Animal cells do not have chloroplasts or cell walls. There is so much variety in the world, but here we are going to talk about the basic structures you might find in cells. Just remember, not every creature has all of the pieces.
All cells have to survive and live, so they need energy. But what do you use the energy for? Nerve cells conduct electrical impulses through the body. There are cells in your intestine that absorb nutrients from the food you eat. Stomach cells secrete an acid to digest your food. No matter what they do, they convert food into energy.
Where do you get the food? Some plant cells have an organelle called a CHLOROPLAST that takes energy from the sun and converts it into sugars. There are some cells that digest their food outside of the cell and then absorb it.
The NUCLEUS is like the brain of the cell. If you take the nucleus out of a cell it dies. It is the thing that tells every part of the cell what to do. It even tells the cell when to divide. Cells that have nuclei are called EUKARYOTES.
Inside the nucleus is all of the stuff used in cell reproduction. Scientists call that “stuff” DNA. DNA is short for DESOXYNUCLEIC ACID (isn’t DNA much easier to say?). The cell’s DNA has the instructions that tell the cell what it should do, and what it should become. DNA tells the cell what kind of proteins to make and the best things the cell should do to survive.
Around the nucleus is another membrane (different from the cell membrane). The nuclear membrane holds the nucleus together. Scientists call the membrane the NUCLEAR ENVELOPE. Just like in the other membranes and cell wall, this one has tiny holes. Pieces of protein and RNA can pass through these holes.
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When the cell is just sitting around, there is something called CHROMATIN in the nucleus. We just talked about DNA. Chromatin is made up of DNA, RNA and proteins. When the cell is going to divide, the chromatin becomes very compact, it condenses. When it comes together, you can see the things that scientists call chromosomes.
If you see a picture of a nucleus, you might see a small dark area inside the nucleus, almost like a tiny nucleus inside the nucleus. This dark area is called the NUCLEUS. The nucleus is made up of protein and RNA with very little DNA.
Дайте відповіді на запитання.
1.Who has cells?
2.What is usually termed as “the cell”?
3.What keep living things organized?
4.What is the main purpose of cells?
5.What kinds of cells does a man usually have?
6.What kind of job does every cell have to do?
7.Name creatures that have no all of the basic parts of a cell?
8.What do all cells need in order to survive and live?
9.What do cells in man’s intestine absorb from the food?
10.What is the nucleus of the cell like?
11.What happens if you take the nucleus out of a cell?
12.How do we usually call the cells that have nuclei?
13.In what way do usually scientists call DNA?
14.What kinds of instructions does DNA have?
15.What is there around the nucleus?
16.How do scientists call the membrane?
17.What are the basic parts of chromatin?
18.What one might see if he looks at the picture of a nucleus?
19.What are the basic parts of the nucleus?
Вправи
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