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2.1.2Topic 2 · Genes and Health

Fick’s Law of Diffusion — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 35 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

2.1.2 Fick's Law of Diffusion — Edexcel A Level Biology A (SNAB) 9BI0 notes: State Fick's law and use it to predict how changes in surface area, concentration difference and membrane thickness affect the rate of diffusion.Fick's law describes the rate of diffusion across an exchange surface.

  • rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness of membrane
  • pulmonary fibrosis
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Diffusion distance

Part 1 of 7Statement of the law

Fick's law describes the rate of diffusion across an exchange surface. It states that the rate of diffusion is directly proportional to the surface area and to the difference in concentration across the surface, and inversely proportional to the thickness of the exchange surface (the diffusion distance).

Written as a relationship:

rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness of membrane

This is a proportionality rather than an equation with fixed constants at A Level, but it lets you reason quantitatively. Doubling the surface area doubles the rate; halving the diffusion distance doubles the rate; doubling the concentration difference doubles the rate.

Part 2 of 7Applying each variable

Surface area. Biological exchange surfaces are folded or branched to increase area: microvilli on epithelial cells, alveoli in lungs, lamellae on gill filaments, and root hair cells that extend the absorbing surface of the root.

Concentration difference. This is kept as large as possible by constantly removing the diffusing substance on one side and supplying it on the other. In lungs, ventilation keeps alveolar oxygen high and carbon dioxide low, while circulation removes oxygen and brings carbon dioxide. In fish, counter-current flow keeps the gradient favourable along the whole length of the lamella.

Thickness. Short diffusion distances are achieved by flattened, single-layer cells. The alveolar wall and the capillary wall are each one cell thick, and the two are closely pressed together.

Part 3 of 7Comparison table

FactorChangeEffect on rateBiological example
Surface areaIncreaseIncreasesAlveoli, villi, root hairs
Concentration differenceIncreaseIncreasesVentilation; blood flow
Membrane thicknessDecreaseIncreasesSquamous epithelium
Surface areaDecreaseDecreasesEmphysema destroys alveolar walls
ThicknessIncreaseDecreasesFibrosis thickens alveolar wall; fluid in lungs

Part 4 of 7Using the law with disease

Lung diseases make Fick's law visible.

Pulmonary fibrosis
In pulmonary fibrosis scar tissue thickens the alveolar wall, increasing the diffusion distance, so oxygen uptake falls.
Emphysema
In emphysema the walls between alveoli break down, forming fewer, larger air spaces: surface area drops.
In pneumonia or oedema, fluid collects in the alveoli, which lengthens the pathway and reduces the gradient.
All lead to breathlessness and low blood oxygen saturation.

Exercise has the opposite effect on the gradient: faster, deeper breathing and a higher cardiac output remove oxygen from the alveoli into the blood more quickly and renew it from the air, so the concentration difference is larger and gas exchange speeds up.

Part 5 of 7Interpreting data

If a question gives rates of diffusion for models of different sizes, look for the pattern in relation to the SA:V ratio.

Interpreting data
Agar blocks of different sizes placed in dilute acid, for example, show that smaller blocks (larger SA:V) change colour throughout faster.
State the trend, quote paired figures, and explain it by Fick's law rather than simply describing the numbers.

When asked to evaluate an adaptation, always link it to one of the three variables in the law and say whether it raises or lowers the rate.

Part 6 of 7Worked reasoning with Fick's law

Questions often present a scenario and ask you to predict a change in rate.

1
A good method is to take the three variables in turn and decide whether each rises, falls or stays constant.
2
For example, a swimmer at the start of a sprint breathes faster and the heart beats faster.
3
Alveolar oxygen concentration stays high and blood arriving at the lungs has a lower oxygen concentration, so the difference is larger.
4
The surface area and diffusion distance are unchanged, so the rate rises because of the gradient alone.

Now consider a cell membrane in which the cell grows to twice its diameter.

1
Surface area increases four times but volume eight times.
2
The area available per unit volume has halved, so the supply of oxygen per unit volume falls even though the absolute rate of diffusion is greater.
3
This is why Fick's law is combined with SA:V to explain size limits.
4
Finally, remember that Fick's law applies to any diffusing substance: it explains nutrient absorption by villi and the uptake of mineral ions by root hairs equally well as it does gas exchange.

Part 7 of 7Exam-style questions

  1. State Fick's law of diffusion. [2]2 marks

  2. A patient has fibrosis of the lungs. Use Fick's law to explain why the patient has low blood oxygen levels. [3]3 marks

  3. Cell A has a surface area of 40 µm2 and a membrane 8 nm thick. Cell B has a surface area of 80 µm2 and a membrane 8 nm thick. The concentration difference is the same. Compare the rate of diffusion. [2]2 marks

Key terms

TermMeaning
Fick's lawThe rate of diffusion is proportional to surface area and concentration difference, and inversely proportional to the thickness of the exchange surface.
Diffusion distanceThe thickness of the barrier or distance molecules must cross.
Counter-current flowFlow of two fluids in opposite directions, keeping a concentration gradient along the whole exchange surface.
FibrosisThickening and scarring of lung tissue which lengthens the diffusion pathway.
EmphysemaLung disease in which alveolar walls break down, reducing surface area.
MicrovilliTiny folds of the cell surface membrane that increase surface area.

Avoid these mistakes

1
Stating that rate is proportional to thickness. It is inversely proportional to thickness.
2
Forgetting that all three variables must be considered, not just surface area.
3
Describing a concentration gradient without saying how it is maintained (ventilation, blood flow, counter-current).
4
Writing that diffusion is active. Diffusion needs no ATP; it relies on the kinetic energy of molecules.

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