Why cells are small: surface area to volume ratio

A cell takes in nutrients and oxygen and gets rid of wastes through its surface, the cell membrane, but it uses those materials throughout its volume. As a cell grows, its volume increases faster than its surface area. For a cube-shaped cell, doubling the length of each side multiplies the surface area by 4 but the volume by 8, so each bit of membrane must serve twice as much of the cell's inside. A cube 1 unit on a side has a surface-area-to-volume ratio of 6 to 1; a cube 2 units on a side has 24 to 8, or 3 to 1. If a cell grew too large, it could not move materials across its membrane fast enough to supply its interior, which is one main reason most cells stay small and a growing cell divides instead of growing forever. Many cells and body structures increase their surface area to speed up exchange: the folds of the small intestine and its fingerlike villi, the millions of tiny air sacs (alveoli) in the lungs, and the thin, flat shapes of some cells all provide more surface for materials to pass through. That is also why large organisms are made of many small cells, not a few giant ones.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report