Factors affecting the rate of diffusion — Edexcel GCSE Biology
How surface area, concentration gradient and diffusion distance change the rate of diffusion, and how to use Fick’s law.
Revision notes
2 short notes, in the order of the specification. Each one 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.
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.
Exam questions
4 exam-style questions (9 marks), each with its mark scheme.
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7 cards: flip them, mark what you knew, and practise the rest.
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The whole of exchange and transport in animals on one page, so you can see where this subtopic fits.
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Quick check questions
Name the three factors that affect the rate of diffusion.
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Surface area, concentration gradient and diffusion distance.
What happens to the rate of diffusion if the membrane becomes twice as thick?
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It halves, if the other factors stay the same.
Write Fick’s law in words.
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Rate of diffusion is proportional to surface area × concentration difference ÷ thickness of membrane.
Frequently asked questions
What affects the rate of diffusion?
The rate of diffusion is affected by surface area, concentration gradient and diffusion distance. A larger surface area lets more particles cross at once, a steeper concentration gradient gives faster net movement, and a shorter distance, meaning a thinner membrane, speeds it up. In the lungs, millions of thin-walled alveoli make diffusion fast.
How does the concentration gradient affect diffusion?
A steeper concentration gradient makes diffusion faster. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, so the bigger the difference, the faster the net movement. In the lungs, blood flow and breathing keep the oxygen concentration lower in the blood than in the alveoli.
What is Fick’s law?
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, and doubling the thickness halves it. Answers are usually relative rates in arbitrary units, so show each step of working.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026