Explain why multicellular organisms need both an exchange surface and a mass transport system, whereas single-celled organisms do not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Single‑cell organisms have a large surface area to volume ratio, so diffusion alone can supply all cells with oxygen and nutrients and remove waste.
Multicellular organisms have a small surface area to volume ratio, so diffusion alone is too slow to supply all cells with the required oxygen and nutrients and to remove waste.
Therefore, multicellular organisms need a specialised exchange surface (e.g. alveoli) and a mass transport system (e.g. the circulatory system) to transport substances to and from all cells.
Multicellular organisms have a small surface area to volume ratio, so diffusion alone is too slow to supply all cells with the required oxygen and nutrients and to remove waste.
Therefore, multicellular organisms need a specialised exchange surface (e.g. alveoli) and a mass transport system (e.g. the circulatory system) to transport substances to and from all cells.
Examiner tips
- Use the exact terms ‘surface area to volume ratio’, ‘diffusion’, ‘exchange surface’, ‘mass transport system’.
- Show the logical progression: ratio → diffusion sufficiency → insufficiency → need for specialised systems.
- Keep answer concise – 5 marks, so one sentence per point.
Common mistakes
- Confusing surface area to volume ratio with surface area alone.
- Failing to mention both exchange surface and mass transport system.
- Using vague terms like ‘blood vessels’ without specifying the need for a transport system.
Mark scheme (5 marks)
- Single-celled organisms have a large surface area to volume ratio
- So diffusion alone is sufficient to supply all cells with oxygen/nutrients and remove waste products
- Multicellular organisms have a small surface area to volume ratio
- So diffusion alone is too slow / insufficient to supply all cells with the required oxygen and nutrients and to remove all waste products
- Therefore multicellular organisms require a specialised exchange surface (e.g. alveoli) AND a mass transport system (e.g. the circulatory system) to transport substances to and from all cells
Key terms in this question
exchange surface · mass transport system · multicellular organism · single-celled organism
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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