# A large multicellular animal, such as a whale, cannot rely on diffusion alone to supply oxygen to its cells. Explain why diffusion alone is insufficient for a large multicellular organism and describe two features that allow efficient exchange of substances to occur.

> OCR GCSE Biology A: Gateway Science (J247) — B2.2 The challenges of size · Explain · 4 marks

> A whale is one of the largest animals on Earth. Every cell in its body requires a continuous supply of oxygen and nutrients, and must remove waste products such as carbon dioxide.

## Mark scheme (4 marks)

1. The diffusion distance from the surface to the inner cells is too large / too far for diffusion to be fast enough
2. A large surface area (of an exchange surface / specialised organ) increases the rate of exchange of substances
3. A thin exchange surface / short diffusion pathway maintains a steep concentration gradient, speeding up exchange
4. A transport system / good blood supply maintains a steep concentration gradient by continuously removing or delivering substances

## Key terms

- [diffusion](https://www.gradenine.co.uk/glossary/diffusion)
- [multicellular organism](https://www.gradenine.co.uk/glossary/multicellular-organism)

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](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/a-large-multicellular-animal-such-as-b79624fd) · Published by Druglandscape Ltd.