Fick’s lawSpec 8.5B
In short
Fick’s law states that the rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the thickness of the membrane. Doubling the surface area or the concentration difference doubles the rate, while doubling the membrane thickness halves it. The answer is a relative rate, often given in arbitrary units.
Fick’s law combines the three factors. The rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the thickness of the membrane.
The symbol ∝ means is proportional to. So doubling the surface area doubles the rate, doubling the concentration difference doubles the rate, and doubling the thickness halves the rate.
Using Fick’s law
A membrane has a surface area of 30 units and a thickness of 2 units. The concentration difference across it is 5 units. Calculate the rate of diffusion in arbitrary units.
- Multiply surface area by concentration difference: 30 × 5 = 150.
- Divide by thickness: 150 ÷ 2 = 75.
Answer: 75 arbitrary units
Because the law uses proportionality, the answer is a relative rate and is often given in arbitrary units. Show each step of your working to get method marks.
Quick check
Name the three factors that affect the rate of diffusion.
Show answer
Surface area, concentration gradient and diffusion distance.
What happens to the rate of diffusion if the membrane becomes twice as thick?
Show answer
It halves, if the other factors stay the same.
Write Fick’s law in words.
Show answer
Rate of diffusion is proportional to surface area × concentration difference ÷ thickness of membrane.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026