Diffusion is the passive movement of substances down a concentration gradient. Surface area, difference in concentration and the thickness of the exchange surface affect the rate of diffusion.

Substances will move from an area of high concentration to an area of low concentration (diffusion.) This often happens when two different environments are connected by a membrane e.g. inside a cell and outside a cell.

A bigger surface area increases the rate of diffusion because there are more opportunities for substances to move across a membrane.

The bigger the difference in concentration the faster the diffusion.

Thickness of the exchange surface affects how far substances have to travel to diffuse into a space: the thicker the surface the longer it takes.