Explain the principle of the surface area to volume ratio and how it applies to gas diffusion in different organisms.

The surface area to volume ratio of an object will decrease as the size of the object increases. This means that a small object with a small volume will have a bigger surface area, as a proportion or fraction of the volume. This can be demonstrated by cubes of different sizes: a 1cm3 cube has a surface area of 6cm2: a SA:V ratio of 6:1, but a 8cm3 cube has a surface area of 24cm2: a SA:V ratio of 3:1.This principle can be applied to organisms to explain the different diffusion systems they have: a bacterium with a very small size does not need a specially designed diffusion system as gases can diffuse over its body surface due to its high SA:V ratio - lots of surface area over which gases can move into the organism. In comparison, large organisms like fish or humans need fins or lungs respectively to increase their surface area for gas diffusion as their SA:V ratio is too small for efficient diffusion.