Organisms need exchange

Living organisms need to exchange materials like oxygen, glucose, excretory products like urea, and heat with their environment.

This exchange occurs across plasma membranes.

Surface area to volume ratio affects exchange

The rate at which substances diffuse across an organism's outer surface depends on its surface area to volume ratio (SA:V).

How SA:V affects rate of diffusion:

1. High SA:V - These organisms have a large surface area relative to their volume so the diffusion of substances is fast.
2. Low SA:V - These organisms have a small surface area relative to their volume so the diffusion of substances is slower.

Generally, smaller organisms have a higher SA:V while larger organisms have a lower SA:V.

Calculating the surface area to volume ratio of a cube

To calculate the surface area (SA) of a cube:

SA = length x width x 6

To calculate the volume (V) of a cube:

V = length x width x depth