Explain how the structure of the alveoli is adapted to allow efficient gas exchange in the lungs.

AQA A-Level Biology (7402) — 3.3.2 Gas Exchange · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Alveoli have a very large surface area, which allows many gas molecules to diffuse across at once and therefore speeds up the rate of diffusion.
The walls of the alveoli are only one cell thick and are moist, giving a very short diffusion distance.
Each alveolus is surrounded by a dense network of capillaries, providing a good blood supply.
This arrangement maintains a steep concentration gradient for oxygen and carbon dioxide, ensuring efficient gas exchange.

Examiner tips

Mention all five features in a logical order; link each structure to its functional benefit.
Use the phrase "steep concentration gradient" to show understanding of diffusion principles.
Show that you know the alveoli are moist and one‑cell‑thick to reduce diffusion distance.
Keep the answer concise – 5 marks, so one sentence per point is enough.

Common mistakes

Failing to state that the alveolar walls are only one cell thick.
Omitting the moist nature of the alveolar wall or the capillary network.
Using vague terms like "large area" without explaining the diffusion benefit.

Mark scheme (5 marks)

Alveoli have a large surface area
Large surface area allows more gas molecules to diffuse across at once / speeds up rate of diffusion
Alveoli have a thin (one cell thick / moist) wall / short diffusion distance
Alveoli have a good blood supply / are surrounded by capillaries
This maintains a steep concentration gradient (for oxygen and/or carbon dioxide)

Key terms in this question

alveoli