Explain how the structure of alveoli makes them well adapted for efficient gaseous exchange in the lungs.

OCR GCSE Biology A: Gateway Science (J247) — B2.1 Supplying the cell · Explain · 4 marks · View as Markdown

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

The lungs are the site of gaseous exchange in mammals. Oxygen must pass from the air into the blood, and carbon dioxide must pass from the blood into the air. This exchange happens across the walls of the alveoli.

Model answer (4 marks)

Alveoli have a very large surface area, which increases the area over which gas can diffuse.

The large surface area speeds up the rate of diffusion of oxygen and carbon dioxide.

Alveolar walls are extremely thin, only one cell thick, allowing gases to cross quickly.

Because blood flow continuously removes oxygen and carries away carbon dioxide, a steep concentration gradient is maintained across the alveolar wall, further driving efficient diffusion.

Examiner tips

Use the exact terms: large surface area, thin walls, steep concentration gradient.
Show the causal link: large area → faster diffusion; thin walls → less resistance; gradient → driving force.
Keep each point short and to the point to fit the 4‑mark structure.

Common mistakes

Failing to mention the thinness of the alveolar wall or the concentration gradient.
Using vague phrases like “good for gas exchange” without the specific reasons.
Excessive explanation or irrelevant details that take up marks.

Mark scheme (4 marks)

Alveoli provide a large surface area
A large surface area increases / speeds up the rate of diffusion
Alveoli walls are very thin / one cell thick
A steep concentration gradient is maintained (e.g. by the blood supply / continuous blood flow removing oxygen)

Key terms in this question

alveoli · gaseous exchange