Explain why large multicellular organisms need a mass transport system, but 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 substances can diffuse directly in and out of the cell across the whole surface efficiently. In contrast, large multicellular organisms have a small surface area to volume ratio; diffusion alone would be too slow to supply all cells with sufficient oxygen, glucose, etc., or to remove waste. Therefore they require a mass transport system such as a circulatory system to carry substances rapidly over long distances throughout the body.
Examiner tips
- Use the key terms surface area to volume ratio, diffusion, and mass transport system.
- Show the contrast between single‑cell and multicellular organisms.
- Mention that diffusion is sufficient for small cells but not for large bodies.
- Include an example of a transport system (e.g. circulatory system).
Common mistakes
- Failing to mention surface area to volume ratio.
- Confusing diffusion with active transport.
- Not explaining why diffusion is insufficient for large organisms.
Mark scheme (5 marks)
- Single-celled organisms have a large surface area to volume ratio
- So substances can diffuse directly in/out of the cell quickly / efficiently over the whole surface
- Large multicellular organisms have a small surface area to volume ratio
- Diffusion alone would be too slow / insufficient to supply all cells with enough substances (e.g. oxygen/glucose) or remove waste
- A mass transport system (e.g. the circulatory system/blood) carries substances rapidly to all cells / over long distances throughout the body
Key terms in this question
mass transport system · multicellular
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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