Diffusion is the random motion of molecules, with net movement from areas of higher to lower concentration (Chapter 5). The distance through which molecules can diffuse quickly depends in part on how great the concentration difference is. Because diffusion supplies key molecules for metabolism, it exerts a strong constraint on the size, shape, and function of cells. Diffusion is effective only over small distances and so it strictly limits the size and shape of bacterial cells, as discussed in Chapter 26. Diffusion also constrains how eukaryotic organisms function.

Oxygen provides a good example. Most eukaryotes require oxygen for respiration. If a cell or tissue must rely on diffusion for its oxygen supply, its thickness is limited by the concentration difference in oxygen between the cell or tissue and its environment. The concentration difference itself depends on the amount of oxygen in the environment and the rate at which oxygen is used for respiration inside the organism. In shallow water that is in direct contact with the atmosphere, diffusion permits animals to reach thicknesses of about 1 mm to 1 cm, depending on their metabolic rate. Of course, a quick swim along the seacoast will reveal many animals much larger than this. And, obviously, you are larger than this. How do you and other large animals get enough oxygen to all of your cells to enable you to survive?