Table 1 shows results from an experiment using a model of an alveolus. A thin membrane separated air on one side from a solution on the other. The amount of...
Question 1 Report
Table 1 shows results from an experiment using a model of an alveolus. A thin membrane separated air on one side from a solution on the other. The amount of oxygen that crossed the membrane in one minute was measured.
| Membrane thickness / mm | Oxygen transferred in 1 min / cm3 |
|---|---|
| 0.02 | 18.4 |
| 0.05 | 11.2 |
| 0.10 | 5.9 |
| 0.20 | 2.8 |
(a) Describe the effect of membrane thickness on oxygen transfer. [2]
(b) Explain how this result relates to the structure of alveoli in human lungs. [3]
(c) Calculate the decrease in oxygen transferred when membrane thickness increases from 0.02 mm to 0.10 mm. [1]
(d) State two other features of alveoli that increase the rate of oxygen uptake. [2]
(e) Explain why a good blood supply maintains rapid diffusion into the blood. [2]
Answer Details
(a) Oxygen transfer decreases as membrane thickness increases. For example, it falls from 18.4 cm3 per min at 0.02 mm to 2.8 cm3 per min at 0.20 mm. [2]
(b) Alveolar walls are very thin, only one cell thick. This gives oxygen a short diffusion distance, so oxygen can diffuse rapidly from alveolar air into the blood. [3]
(c)
\[18.4\ \text{cm}^3\text{ min}^{-1}-5.9\ \text{cm}^3\text{ min}^{-1}=12.5\ \text{cm}^3\text{ min}^{-1}\]
The decrease is 12.5 cm3 per min. [1]
(d) Two other alveolar adaptations are a large surface area, produced by many alveoli, and a moist lining. A good capillary supply and ventilation maintaining high oxygen concentration in alveolar air are also acceptable. [2]
(e) A good blood supply carries oxygen away from the alveoli. This keeps the oxygen concentration in the blood lower than in alveolar air, maintaining a steep concentration gradient and rapid diffusion. [2]
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