Diffusion — AQA GCSE Biology
Diffusion is one of the ways substances move in and out of cells. It is a passive process — no energy is needed.
What is diffusion?
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration (down a concentration gradient). It happens because particles move randomly and spread out.
It is passive — it does not require energy from respiration.
Examples in living organisms
- Oxygen and carbon dioxide moving in gas exchange (e.g. in the lungs and in leaves).
- Digested food molecules moving from the gut into the blood.
- Urea moving from cells into the blood for excretion.
Factors affecting the rate of diffusion
The rate of diffusion increases with:
- a steeper concentration gradient (bigger difference in concentration),
- a higher temperature (particles move faster),
- a larger surface area of the membrane.
Surface area to volume ratio
Single-celled organisms have a large surface area to volume ratio, so diffusion alone supplies their needs. Larger organisms have a smaller ratio and need specialised exchange surfaces (e.g. alveoli, villi) and transport systems. Good exchange surfaces have a large surface area, thin walls and a good blood/air supply to keep the gradient steep.
Exam tips
- Define diffusion precisely: net movement, high to low concentration, down the gradient.
- Remember diffusion is passive (no energy), unlike active transport.
- Learn the three factors that increase the rate.
- Link the surface area to volume ratio to why large organisms need exchange surfaces.