Cell

It is estimated that an adult human body contains about 60 trillion cells. Most of these cells, with some exceptions, are so small that a microscope is necessary to see them. The small size of cells fulfills a distinct purpose in the functioning of the body. If cells were larger, many of the processes that cells perform could not occur efficiently. To visualize this concept, think about the intestinal epithelial cells discussed earlier. What if the intestinal epithelium were composed of one, large cell instead of thousands of small cells? A large cell has a large volume, or contents. The surface area, or membrane, of this large cell is the site through which nutrients enter the small intestine for delivery to the bloodstream. Because the volume of this large cell is so large, the surface area, by comparison, is relatively small. Large cells, therefore, have a small surface area to volume ratio. Only so many nutrients can pass through the limited membrane area of this large cell. With a small surface area to volume ratio, the amount of substances passing into and out of the cell is severely restricted.

However, if the intestinal epithelium is divided into thousands of smaller cells, the volume stays the same, but the surface area—the number of cell membranes—greatly increases. Many more nutrients can pass through the intestinal epithelium cells. Small cells, therefore, have a large surface area to volume ratio. The large surface area to volume ratio of small cells makes the transport of substances into and out of cells extremely efficient.

Another reason for the small size of cells is that control of cellular processes is easier in a small cell than in a large cell. Cells are dynamic, living things. Cells transport substances from one place to another, reproduce themselves, and produce various enzymes and chemicals for export to the extracellular environment. All of these activities are accomplished under the direction of the nucleus, the control center of the cell. If the nucleus had to control a large cell, then this direction might break down. Substances transported from one place to another would have to traverse great distances to reach their destinations; reproduction of a large cell would be an extremely complicated endeavor; and products for export would not be as efficiently produced. Smaller cells, because of their more manageable size, are much more efficiently controlled than larger cells.