1.15 Fick's law
6 exam questions worth 15 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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According to Fick's law, the rate of diffusion across a membrane is greater when
Choose one answer.
Correct Not quite: the answer is the surface area is larger
Mark scheme
C (the surface area is larger)
Model answer
C: the surface area is larger. A larger surface area lets more particles cross at once, so diffusion is faster.
The rate of diffusion across a membrane is proportional to:
(a) A membrane has a surface area of 30 units, a concentration difference of 4 units and a thickness of 2 units. Calculate the relative rate of diffusion. (2 marks)
(b) The thickness of the membrane doubles. All the other values stay the same. State the new relative rate. (1 mark)
Mark scheme
(a)
- (30 × 4) ÷ 2 (1)
- = 60 (1)
(b) 30 (1)
Model answer
(a) rate = (30 × 4) ÷ 2 = 120 ÷ 2 = 60.
(b) The thickness is now 4, so the rate is (30 × 4) ÷ 4 = 30. Doubling the thickness halves the rate.
Use Fick's law to explain how the structure of the alveoli makes gas exchange efficient.
Mark scheme
- millions of alveoli give a large surface area, so the rate of diffusion is greater (1)
- the wall is one cell thick / thin, so the diffusion distance is short (1)
- blood flow and breathing keep a high concentration difference / steep concentration gradient (1)
Model answer
The alveoli have a very large surface area, which increases the rate of diffusion. Their walls are one cell thick, so the particles have a short distance to travel. Blood flow carries oxygen away and breathing brings fresh air, which keeps the concentration difference high.
In emphysema, the walls of the alveoli break down.
Explain why a person with emphysema may find it difficult to breathe.
Mark scheme
- the surface area of the alveoli is reduced (1)
- so less oxygen diffuses into the blood / the rate of diffusion falls (1)
Model answer
The surface area of the alveoli is reduced, so less oxygen can diffuse into the blood each second, which makes the person short of breath.
Describe how blood flow helps to keep a high concentration difference for diffusion in the small intestine.
Mark scheme
- the blood carries away the absorbed food / glucose (1)
- so the concentration of glucose in the blood stays low / a steep concentration gradient is kept (1)
Model answer
The blood constantly carries absorbed glucose away from the small intestine, so the concentration in the blood stays low and the gradient from the gut to the blood stays steep.
Explain how the structure of a fish gill is adapted for gas exchange, using ideas from Fick's law.
Mark scheme
Any four from:
- many filaments / lamellae give a large surface area (1) so a faster rate of diffusion (1)
- the lamellae are very thin (1) so a short diffusion distance / faster diffusion (1)
- water flowing over the gills and blood flowing through them (1) keep a high concentration difference / steep gradient (1)
Model answer
A gill has many filaments and lamellae, giving a large surface area, which increases the rate of diffusion. The lamellae are very thin, so oxygen has a short distance to diffuse. Water flowing over the gills and blood flowing through them keep the concentration difference high, so diffusion stays fast.
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