1
Triple only
[1 mark]Which change would decrease the rate of diffusion across an exchange surface?
- Increasing the surface area
- Increasing the concentration difference
- Increasing the thickness of the membrane
- Increasing the surface area and the concentration difference
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Answer: C C (increasing the thickness of the membrane)
2
Triple only
[2 marks]In some lung diseases the walls of the alveoli become thicker. Explain how this affects the rate of diffusion of oxygen into the blood.
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- diffusion distance increases (1)
- so the rate of diffusion decreases / less oxygen enters the blood (1)
3
Triple only
[3 marks]A membrane has a surface area of 50 units and a thickness of 4 units. The concentration difference across it is 8 units. Use Fick’s law to calculate the rate of diffusion in arbitrary units.
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- surface area × concentration difference = 50 × 8 = 400 (1)
- ÷ thickness = 400 ÷ 4 (1)
- 100 (arbitrary units) (1)
4
Triple only
[3 marks]A student says that the rate of diffusion across a membrane will double if both the surface area and the thickness of the membrane are doubled. Use Fick’s law to evaluate the student’s idea.
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- rate of diffusion is proportional to surface area (1)
- and inversely proportional to thickness, so doubling both cancels out (1)
- rate stays the same, so the student is not correct (1)