Explain how the structure of an alveolus is adapted to facilitate efficient gas exchange in the human lung.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Alveoli have a very large total surface area, increasing the rate of diffusion of gases
Alveolar walls are one cell thick / very thin epithelium, so the diffusion distance is short, increasing the rate of diffusion
Alveolar walls are moist / lined with a thin film of water, allowing gases to dissolve and diffuse across the epithelium
Alveoli are surrounded by an extensive network of capillaries / rich blood supply, maintaining a steep concentration gradient for both oxygen and carbon dioxide
Alveolar walls are one cell thick / very thin epithelium, so the diffusion distance is short, increasing the rate of diffusion
Alveolar walls are moist / lined with a thin film of water, allowing gases to dissolve and diffuse across the epithelium
Alveoli are surrounded by an extensive network of capillaries / rich blood supply, maintaining a steep concentration gradient for both oxygen and carbon dioxide
Examiner tips
- Use the exact phrases from the mark scheme
- Show each point clearly and separately
- Keep the answer concise and to the point
Common mistakes
- Failing to mention the large surface area
- Confusing the thickness of the alveolar wall with the thickness of capillary walls
- Omitting the moist lining or capillary network
Mark scheme (4 marks)
- Alveoli have a very large total surface area, increasing the rate of diffusion of gases
- Alveolar walls are one cell thick / very thin epithelium, so the diffusion distance is short, increasing the rate of diffusion
- Alveolar walls are moist / lined with a thin film of water, allowing gases to dissolve and diffuse across the epithelium
- Alveoli are surrounded by an extensive network of capillaries / rich blood supply, maintaining a steep concentration gradient for both oxygen and carbon dioxide
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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