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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Diffusion is too slow over the long distances required in a large animal – the distance from the body surface to the inner cells is too great for oxygen and nutrients to reach cells quickly enough.
Large surface area of specialised organs (e.g. lungs, gills) increases the rate of exchange.
Thin exchange surfaces or short diffusion pathways keep a steep concentration gradient, speeding up diffusion.
A transport system such as a good blood supply continuously removes waste and delivers oxygen, maintaining the gradient.
Large surface area of specialised organs (e.g. lungs, gills) increases the rate of exchange.
Thin exchange surfaces or short diffusion pathways keep a steep concentration gradient, speeding up diffusion.
A transport system such as a good blood supply continuously removes waste and delivers oxygen, maintaining the gradient.
Examiner tips
- Use the word ‘diffusion’ and explain the distance problem; mention surface area, thin surfaces and blood supply as the two features; keep each point concise and use exam‑style terminology.
Common mistakes
- Saying diffusion is ‘slow’ without linking it to distance; confusing surface area with volume; forgetting to mention the transport system or the steep concentration gradient.
Mark scheme (4 marks)
- The diffusion distance from the surface to the inner cells is too large / too far for diffusion to be fast enough
- A large surface area (of an exchange surface / specialised organ) increases the rate of exchange of substances
- A thin exchange surface / short diffusion pathway maintains a steep concentration gradient, speeding up exchange
- A transport system / good blood supply maintains a steep concentration gradient by continuously removing or delivering substances
Key terms in this question
diffusion · multicellular organism
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