# Explain why a whale requires a specialised circulatory system to deliver oxygen to its cells, but a flatworm of similar body length does not.

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

> 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.

## Mark scheme (5 marks)

1. The whale has a large volume relative to its surface area (low surface area to volume ratio)
2. 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
3. Oxygen can diffuse from the flatworm's surface to every cell quickly / the diffusion distance to all cells is short
4. In the whale, most cells are deep inside the body / far from the surface, so diffusion alone is too slow to supply enough oxygen
5. A specialised circulatory system transports oxygen quickly / over long distances to all cells in the whale

## Key terms

- [circulatory system](https://www.gradenine.co.uk/glossary/circulatory-system)

## Related

- [Revision notes for AQA A-Level Biology (7402)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-whale-requires-a-6e310a28) · Published by Druglandscape Ltd.