Diffusion is the process by which useful materials and waste products move in and out of cells. Substances diffuse down a concentration gradient - from an area of high concentration to an area of low concentration.
Diffusion occurs in many living systems, for example the movement of carbon dioxide and oxygen in the lungs and in leaves.
Diffusion is a model that explains how material spreads in gases and liquids. It's an important idea in biology as it can explain how useful materials and waste products can move in and out of cells.
Diffusion occurs when molecules collide, causing them to spread out. This gives the effect that the substance is moving from an area of high concentration to an area of low concentration (or ‘down a concentration gradient’). When the particles are evenly spread out diffusion appears to stop.
Diffusion can be speeded up when: