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 and living systems

Diffusion

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:

• particles move faster, eg when they're warmer
• concentration difference is greater, eg by replacing air in the lungs with fresh air rich in oxygen
• distance for diffusion is kept to a minimum, eg by having a thin layer of cellslining the lungs
• the surface area for diffusion is increased eg by folding

Materials like oxygen, carbon dioxide and glucose enter and leave cells by diffusion. When a cell is respiring the concentration of oxygen and glucose inside the cell is lower than the surrounding blood so these substances move into the cell.

As carbon dioxide is produced the concentration inside the cell increases to a level higher than the surrounding blood, so carbon dioxide diffuses out of the cell. Cell membranes are very thin to allow materials to diffuse through them easily.

lung oxygen air space red blood cells

alveoli (moist lining, good blood supply)

lung carbon dioxide blood plasma air space alveoli (moist lining, good blood supply) digestive system food molecules eg glucose stomach, intestine blood villi, microvilli, good blood supply uterus food molecules and oxygen mother's blood supply foetus's blood supply placenta with villi, thin walls and good blood supply leaf oxygen leaf cell air space

thin leaves, with air spaces and spongy layer