# 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

## 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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