Biology IGCSE Section II - (4) Movement in and out of cells
4.1 Diffusion Diffusion is the net movement of molecules from a region of high concentration to a region of lower concentration, down a concentration gradient as a result of their random movement.
4.2 Active transport Active transport is the movement of ions in or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration against a concentration gradient, using energy released during respiration. The cell membrane contains carrier proteins, which combine with the ion and change shape to carry the ion or molecule across the cell membrane. To change shape carrier proteins must use energy.
4.3 Osmosis Osmosis is the diffusion of water molecules from a region of their higher concentration (dilute solution) to a region of their lower concentration (concentrated solution) thought a partially permeable membrane.