Explain how the structure of the alveolar wall in the human lung is adapted to maximise the rate of gas exchange by diffusion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The alveolar wall is only one cell thick – a single layer of squamous epithelial cells – which minimises the diffusion distance for O₂ and CO₂.
The epithelial cells are flattened, further reducing the distance across the gas‑exchange surface.
The wall is covered by a thin film of moisture (and surfactant), allowing gases to dissolve and diffuse in solution.
The large number of alveoli gives a very large total surface area, increasing the overall rate of diffusion in accordance with Fick’s law (rate ∝ surface area).
The epithelial cells are flattened, further reducing the distance across the gas‑exchange surface.
The wall is covered by a thin film of moisture (and surfactant), allowing gases to dissolve and diffuse in solution.
The large number of alveoli gives a very large total surface area, increasing the overall rate of diffusion in accordance with Fick’s law (rate ∝ surface area).
Examiner tips
- Use the word ‘minimises’ and ‘reduces’ to show cause–effect. Mention the single‑cell thickness and squamous shape. Include the moisture film and surfactant. State the link to Fick’s law and surface area.
- common_mistakes
- :
- Omitting the single‑cell thickness or the squamous shape. Failing to mention the moisture film or surfactant. Not linking the large surface area to Fick’s law.
Mark scheme (4 marks)
- The alveolar wall is only one cell thick (a single layer of squamous epithelial cells), which minimises the diffusion distance for oxygen and carbon dioxide.
- The alveolar epithelial cells are squamous (flattened), which further reduces the diffusion distance across the gas-exchange surface.
- The alveolar surface is covered by a thin film of moisture / surfactant, allowing gases (oxygen and carbon dioxide) to dissolve and diffuse across the membrane in solution.
- The large number of alveoli provides a very large total surface area, increasing the overall rate of diffusion according to Fick's law (rate ∝ surface area).
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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