Explain why insects, which are small invertebrates, can supply oxygen to their body cells without a heart or blood, but large mammals cannot.

AQA A-Level Biology (7402) — 3.3.1 Surface Area to Volume Ratio · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Insects are small and have a large surface area to volume ratio, so oxygen can diffuse quickly and efficiently to all cells from the body surface. Large mammals have a small surface area to volume ratio, so diffusion alone is too slow and not enough to supply all cells with sufficient oxygen. Therefore large mammals need a specialised transport or circulatory system to deliver oxygen to cells throughout the body.

Examiner tips

  • Use the term ‘surface area to volume ratio’ explicitly; link it to diffusion rate.
  • Show the logical sequence: small size → large ratio → efficient diffusion; large size → small ratio → insufficient diffusion.
  • Mention the need for a circulatory system as the solution for large mammals.

Common mistakes

  • Failing to mention surface area to volume ratio; only saying ‘small insects’ or ‘large mammals’. Not explaining why diffusion is insufficient for large bodies. Using vague terms like ‘transport system’ without specifying circulatory system.

Mark scheme (5 marks)

  1. Insects are small and have a large surface area to volume ratio
  2. So oxygen can diffuse quickly / efficiently to all cells from the body surface
  3. Large mammals have a small surface area to volume ratio
  4. So diffusion alone is too slow / not enough to supply all cells with sufficient oxygen
  5. Large mammals need a specialised transport / circulatory system to deliver oxygen to cells throughout the body

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