Cells need to take in useful substances and remove other substances, such as waste, in order to function effectively. Exchange of materials occurs between the cell and its surrounding environment across the cell membrane. Depending on the circumstances, this is achieved via three transport processes - diffusion, osmosis and active transport. To make exchange as efficient as possible, larger organisms have evolved specialised exchange surfaces.
Diffusion is the process by which particles of a substance spread out from each other, moving from a region where they are in high concentration to a region oflow concentration. In the same way as a ball will roll from a high point to a low point down a gradient, particles…