Why cells are so small

The Problem of Size. Why are cells so small? Cells are so small that you need a microscope to examine them. To answer this question we have to understand that, in order to survive, cells must constantly interact with their surrounding environment. Gases and food molecules dissolved in water must be absorbed and waste products must be eliminated.

Below that threshold size, the internal components of a cell float freely, buoyed by smaller chemical forces. The typical animal cell has a diameter of about 10 microns 10 millionths of a meter. Larger cells, like the egg cells of the African clawed frog, are up to 1 millimeter in diameter, but examples of such large cells are not frequent. Scientists have attributed this size limit to the difficulty that large-volume cells have obtaining nutrients, an explanation Brangwynne said is not backed by substantial evidence.

Brangwynne and Feric were not thinking about gravity when they began their investigation. Brangwynne wanted to find out why certain types of large particles within cells fuse together upon contact, like water droplets, when floating freely in a cell but not when they are in the cell's nucleus.

By injecting various size plastic beads into the nucleus of the frog egg cells, Branwynne and Feric found evidence for an invisible scaffold that might keep the particles from fusing. Feric next discovered that this matrix could be made up of fibers of the protein actin, which was known to form a cytoskeleton in the parts of cells outside of the nucleus but whose role in the nucleus was not clear.

To test the role of this actin scaffold, the researchers rid the nuclei of the actin polymers, either by treating the nuclei with drugs against the protein, or by making the nucleus pump out the protein. That was genuinely shocking," said Brangwynne. Noting that actin is less abundant and does not appear to form an extensive mesh in smaller cells, Feric's experiments led the researchers to deduce that larger cells have the actin mesh to protect against gravity.

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 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. One reason, therefore, why cells are so small, and there are so many of them, is simple logistics.

But there is another reason and the one given in this story; the tyranny of mathematics. Two mathematical quantities rule the lives of every cell; their surface area and their volume. Each cell is a living globule of cytoplasm in which vast numbers of chemical reactions are taking place millions of times a second. These metabolic furnaces need fuel, everything from food to complex nutrients, and produce waste products that must be eliminated.

Diffusion of these molecules seems to be the main way of getting a chemical compound where it is needed and somewhere else when it is not. Diffusion is very rapid over short distances, but much much slower over longer distances. Lacking a circulatory system like out blood system , cells rely on diffusion to move their molecules around, and thus need to keep distances short.

One of the most important exchanges that takes place is between the inside of the cell and the outside environment.