Every living cell needs oxygen and food and must get rid of waste, and these substances move in and out by diffusion. A single-celled organism is so small that it has a large surface area compared with its volume, and substances only have a short distance to travel. Diffusion across its surface is therefore fast enough to supply the whole cell.
The surface area to volume ratio is the surface area divided by the volume. A cube with 1 cm sides has a surface area of 6 cm2 (six faces of 1 cm2) and a volume of 1 cm3, so the ratio is 6 : 1. A cube with 2 cm sides has a surface area of 24 cm2 and a volume of 8 cm3, so the ratio is 3 : 1. With 3 cm sides the figures are 54 cm2 and 27 cm3, a ratio of 2 : 1. As an object gets larger, its volume grows faster than its surface area, so the ratio gets smaller.
A large multicellular organism has a small surface area to volume ratio. Cells deep inside are far from the surface, so the diffusion distance is long and diffusion alone is too slow. The organism needs specialised exchange surfaces, such as the alveoli in the lungs. These have a large surface area, are thin so the diffusion distance is short, and have a good blood supply to keep a steep concentration gradient. It also needs a transport system, the blood, to carry substances between the exchange surfaces and the cells.
