Factors affecting the rate of diffusionSpec 8.4B
In short
The rate of diffusion across an exchange surface depends on three factors: surface area, concentration gradient and diffusion distance. A larger surface area, a steeper concentration gradient and a shorter diffusion distance, meaning a thinner membrane, all make diffusion faster. In the lungs, millions of alveoli, constant blood flow and thin walls make diffusion fast.
Three factors affect how fast particles diffuse across an exchange surface.
| Factor | Effect on rate of diffusion | Example in the body |
|---|---|---|
| Surface area | Larger surface area means a faster rate, because more particles can cross at once | Millions of alveoli in the lungs |
| Concentration gradient | A steeper gradient (bigger difference in concentration) means a faster rate | Blood flow and breathing keep the oxygen concentration lower in the blood than in the alveoli |
| Diffusion distance (thickness of the membrane) | A shorter distance means a faster rate | Alveolus and capillary walls are one cell thick |
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. The bigger the difference, the faster the net movement.
A short diffusion distance means a thin membrane. Do not say that a thin membrane gives a larger surface area, because these are two separate factors.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026