Explain why a whale requires a specialised circulatory system to deliver oxygen to its cells, but a flatworm of similar body length does not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A blue whale can reach 30 metres in length. A bootlace worm can also reach 30 metres in length but has a ribbon-like body only a few millimetres thick throughout.
Model answer (5 marks)
A whale has a very large volume but a relatively small surface area, giving a low surface area to volume ratio. Consequently the distance that oxygen must diffuse from the body surface to the deepest cells is great, and diffusion alone would be too slow to meet the metabolic demand. Therefore a specialised circulatory system is required to transport oxygen rapidly over long distances to all cells.
A flatworm of the same length is very thin and ribbon‑like, giving a high surface area to volume ratio. The diffusion distance from the surface to any cell is short, so oxygen can reach all cells quickly by simple diffusion and a circulatory system is unnecessary.
A flatworm of the same length is very thin and ribbon‑like, giving a high surface area to volume ratio. The diffusion distance from the surface to any cell is short, so oxygen can reach all cells quickly by simple diffusion and a circulatory system is unnecessary.
Examiner tips
- Use the terms ‘surface area to volume ratio’, ‘diffusion distance’ and ‘circulatory system’ as they are key words in the mark scheme.
- Show the contrast: low ratio in whale → long diffusion path, high ratio in flatworm → short diffusion path.
- Mention that the whale’s cells are deep inside the body, while the flatworm’s cells are close to the surface.
- Keep the answer concise – 5 marks only.
Common mistakes
- Confusing surface area to volume ratio with absolute size; students may say ‘whale is big’ instead of ‘low ratio’. Forgetting to explain why diffusion is too slow in the whale (e.g. not mentioning long diffusion distance). Using vague phrases like ‘big body’ or ‘small body’ without linking to surface area or diffusion.
Mark scheme (5 marks)
- The whale has a large volume relative to its surface area (low surface area to volume ratio)
- The flatworm / ribbon-like organism has a large surface area relative to its volume (high surface area to volume ratio) because it is very thin / flat
- Oxygen can diffuse from the flatworm's surface to every cell quickly / the diffusion distance to all cells is short
- In the whale, most cells are deep inside the body / far from the surface, so diffusion alone is too slow to supply enough oxygen
- A specialised circulatory system transports oxygen quickly / over long distances to all cells in the whale
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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