Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
8.5B Calculate the rate of diffusion using Fick’s law: surface area…
Full specification wording
Topic 8 – Exchange and transport in animals: 8.5B Calculate the rate of diffusion using Fick’s law: surface area concentration difference diffusion thickness of membrane rate of × α
Fick’s law links the rate of diffusion to three measurable features of a surface.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 5 of 10
Study notes
From the checked teaching notes for this specification point.
The idea
Fick’s law links the rate of diffusion to three measurable features of a surface. The rate is proportional to the surface area available and to the concentration difference across the surface, and inversely proportional to the thickness of the membrane. In symbols, rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of membrane. To calculate, multiply surface area by concentration difference, then divide by thickness, keeping units consistent. For example, a surface of 0.5 m² with a concentration difference of 4 mol/m³ across a membrane 0.01 m thick gives (0.5 × 4) ÷ 0.01 = 200 in the matching units. Doubling the surface area doubles the rate; doubling the thickness halves it.
What earns the marks
- States that rate of diffusion is directly proportional to surface area.
- States that rate of diffusion is directly proportional to concentration difference.
- States that rate of diffusion is inversely proportional to thickness of membrane.
- Correctly combines the three variables as (surface area × concentration difference) ÷ thickness.
- Applies the relationship to a numerical case, multiplying area by concentration difference before dividing by thickness.
- Recognises that increasing thickness decreases the rate while increasing area or concentration difference increases it.
Common mistakes and how to put them right
Multiplying by thickness instead of dividing.
Put it right: The learner should divide by thickness because a thicker membrane slows diffusion.
Adding surface area and concentration difference rather than multiplying.
Put it right: The learner should multiply the two proportional quantities.
Ignoring units or mixing them.
Put it right: The learner should keep area, concentration and thickness in consistent units before calculating.
Exam tips
- Write the relationship first, then substitute values, so the structure of the calculation is visible.
- Check the direction of each change: larger area and larger concentration difference raise the rate, larger thickness lowers it.
- Keep units consistent and state the unit of the answer where the question supplies units.
Try the lesson
The start of the Topic 8 – Exchange and transport in animals: 8.5B Calculate the rate of diffusion using Fick’s law: surface area concentration difference diffusion thickness of membrane rate of × α lesson, made from checked teaching notes for this specification point.
Have a go first
A membrane of area 50 cm² and thickness 5 mm has a concentration difference of 6 units. Using Fick's law, what is the rate of diffusion in arbitrary units?
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