More Cell Information
The human body is made of billions upon billions of cells. That tells you something about cells - they are tiny. Most cells are so small that they can be seen only with a microscope. In fact, the existence of cells became known only after the microscope was invented and a guy named Hooke saw some cork cells. He names them cells after the rooms that monks lived in, which are called cells. It's no surprise that plant cells were discovered first, because most plant cells are larger than animal cells.
An animal cell of average size is about 1/1000 of an inch long. The smallest cells are probably the bacteria. These one-celled forms of life can barely be seen even with a microscope. Nerve cells are probably the largest cells (some of them are more than 3 feet long), although some argue that the ostrich egg may be the largest cell.
There are a great many kinds of cells. Each kind, as a rule, specializes in one particular activity. For example, in animals some cells expand and contract; these are muscle cells. Other cells are sensitive to light; these cells form the retina of the eye.
Modern microscopes show that cells are far from simple. They contain many complicated structures, or organelles, and they are constantly active. Cells convert good substances into energy that in turn is used to do various kinds of work. The cell also makes new cell material - more living matter.
With a few exceptions, a single cell does not live very long. But if it divides, the two new cells start life afresh. That is, instead of aging and dying, the old cell becomes two new cells in a process called cell division.
Because of its constant activity, the cell is much more than a structural unit in a living organism. It is itself the unit of life. Life is an activity, or a process, forever changing, even though the appearance may remain the same. This is one of the most important ideas in biology.
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