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Why are cells so small?

As Mendel describes in this story, cells are so small they cannot normally be seen with the naked eye. There are also a lot of cells in the average organism, a fact that astonished their discoverer, Robert Hooke, who calculated from his observations of cork cells that there must be over a billion cells in one "cubick inch", a fact most remarkable. The average adult human has a lot more than this, about 60 trillion according to one calculation - a fact even more remarkable. A pint of blood has over 5 billion specialized cells floating in it, and you scrape off your skin or slough off from your intestines close to 700 billion cells a day!

Why?

Cells have a finite life span which, like any kind of complex machinery, occasionally breaks down. They can be repaired for a time, but sooner or later it is easier and more efficient to discard the older cell and recycle it constituents into a new cell with fewer problems. This renewal goes on constantly throughout the life time of a multicellular organism. Cells arise by division, specialize, function and carry out their roles, then age and eventually die or are lost. The total organism remains the same throughout this process, and (usually) has a longer time on earth than any one of its cells.

For the body of an animal or plant, small, specialized cells are easier to replace and turnover without disruption than would be the case if an organism was made up of just a few very large cells. Imagine what it would be like if each of your eyes was a single cell. When it came time for the eye cells to be replaced, you would be either blind or have an extra eye growing in your face until the change over could take place. As it is, you have about 125 million cells in your eye which are responsible for capturing light rays, and as a few of them are replaced every day, you never notice the change.




Science@a Distance
© 2002, Professor John Blamire