What Is Diffusion? GCSE Biology Explained
Learn what diffusion means in GCSE AQA Biology, how concentration affects net movement, examples in living organisms, and common exam mistakes.
Diffusion is the spreading out of particles, resulting in a net movement from an area of higher concentration to an area of lower concentration. It can occur when particles are in a solution or when they are gases.
How diffusion works
Particles move randomly in all directions. If there are more particles in one area than another, more particles will move away from the area of higher concentration than move back towards it. This creates an overall, or net, movement towards the area of lower concentration.
The difference in concentration between two areas is called the concentration gradient. A steeper concentration gradient usually produces a faster rate of diffusion.
Diffusion can occur across cell membranes, allowing substances to move into and out of cells.
Examples of diffusion
During gas exchange in the lungs, oxygen diffuses from the alveoli into the blood because its concentration is higher in the alveoli. Carbon dioxide diffuses in the opposite direction, from the blood into the alveoli.
Urea, a waste product, also diffuses from body cells into the blood plasma so that it can eventually be excreted by the kidneys.
Factors affecting the rate
Diffusion happens faster when:
- the difference in concentration is greater
- the temperature is higher, because particles move more quickly
- the surface area of the membrane is larger
Exchange surfaces such as the alveoli are effective because they have a large surface area and a thin membrane, giving particles a short diffusion distance.
Common exam mistake
Do not say that diffusion is simply particles moving from high to low concentration. Particles move randomly in both directions. It is the net movement that is from higher concentration to lower concentration.
Also, do not confuse diffusion with osmosis. Osmosis describes the movement of water through a partially permeable membrane.
This explanation assumes AQA GCSE Biology 8461 and applies to both Foundation and Higher tiers. Diffusion is part of Cell Biology on Paper 1 and is shared with Combined Science.
