Why Do Cells Stay Small?

Substances are moving in and out of the cell continuously. For the cell to thrive, these materials must be able to move back and forth quickly and efficiently.  While the cell membrane provides an important barrier that can regulate which materials move in and out, the process of simple diffusion is also super important.   Water, carbon dioxide, and oxygen, are three small molecules that move by diffusion into the cell.

Cells are limited in size because the outside (the plasma membrane) must transport oxygen, carbon dioxide and other molecules into and out of the cell. As a cell gets bigger, the outside (surface area) is unable to keep up with the inside (volume), because the 3-D inside grows at a faster rate than the 2-D outside. So if a cell grows larger instead of dividing, diffusion will be too slow and the cell will not be able to obtain oxygen and get rid of wastes efficiently, which ultimately would kill the cell.

In this exploration, students use models of a cell made with gelatin to observe how long it takes for nutrients (vinegar) to move into a small cell and a large cell. The experiment is colorful and exciting.