Revise: Transport across membranesImportance of diffusion to living organisms
All cells are enclosed by a cell membrane, which is selectively permeable. Molecules can move into or out of cells by diffusion and active transport. Cells can gain or lose water by osmosis.
Diffusion is important to cells because it allows them to gain the useful substances they require to obtain energy and grow, and lets them get rid of waste products.
This table shows examples of substances required by cell and associated waste products.
Substance required by cell
Waste product of cell
Glucose
Carbon dioxide
Oxygen
Urea (made from excess amino acids)
Amino acids
Substance required by cell
Glucose
Waste product of cell
Carbon dioxide
Substance required by cell
Oxygen
Waste product of cell
Urea (made from excess amino acids)
Substance required by cell
Amino acids
Waste product of cell
The diagram below shows the differences in the concentrations of some of these substances between a cell and its environment and the direction that they diffuse.