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 they can rely on diffusion alone to exchange all substances – oxygen, nutrients and waste.
Multicellular organisms have a small surface area to volume ratio; diffusion alone is too slow to supply all cells with oxygen and nutrients and to remove waste.
Therefore a specialised exchange surface (e.g. lungs, gills) increases the surface area and the rate of diffusion, and a mass transport system (e.g. a circulatory system) rapidly moves substances to and from all cells.
Multicellular organisms have a small surface area to volume ratio; diffusion alone is too slow to supply all cells with oxygen and nutrients and to remove waste.
Therefore a specialised exchange surface (e.g. lungs, gills) increases the surface area and the rate of diffusion, and a mass transport system (e.g. a circulatory system) rapidly moves substances to and from all cells.
Examiner tips
- Mention surface area/volume ratio for both types of organisms. Explain how diffusion limits multicellular life. Show that specialised surfaces and transport systems solve this limitation.
Common mistakes
- Confusing the need for transport systems in single‑cell organisms. Omitting the role of diffusion in single‑cell organisms. Using vague terms like "more cells" without linking to surface area/volume ratio.
Mark scheme (5 marks)
- Single-celled organisms have a large surface area to volume ratio
- So single-celled organisms can rely on diffusion alone to exchange all substances / oxygen, nutrients and waste
- Multicellular organisms have a small surface area to volume ratio
- So diffusion alone is insufficient / too slow to supply all cells with oxygen and nutrients and remove waste products
- A specialised exchange surface increases the surface area / rate of diffusion, and a mass transport system (e.g. circulatory system) rapidly moves substances to and from all cells
Key terms in this question
exchange surface · mass transport system · multicellular organism · single-celled organism
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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