What is a Cell?
All living things are made up of cells. Each of us has about 50 million million cells - an enormous number which is difficult to imagine. Each cell is a sort of bag made from a sort of skin called a membrane. The inside of a cell is watery and jelly-like. Cells are very small - you can't see them just using your eyes. You need to use a microscope, which makes them look many times bigger that they actually are.
If a cell is cut in half, it will not survive. So a cell can be considered as the smallest part of an organism that can survive on its own. Some organisms have only one cell, while more complicated organisms are made out of lots of cells. All cells have a membrane, which separates them from the outside world. The membrane protects the cell, and allows the cell to be selective about what is allowed in. The membrane is alive too, and can detect and respond to changes in the outside environment.
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*All content on the next five pages came from the Science Net website. I previously used this site, but it is now not accessible. This information comes from a printed copy of the site made before the site went down. I will be happy to relink to this site when and if it becomes available in the future. The original website link address is below.*