Explain why, as an organism increases in size, it requires specialised exchange surfaces and a transport system to survive.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
As an organism grows, its volume increases faster than its surface area, so the surface area to volume ratio decreases. A smaller ratio means the outer surface cannot supply oxygen, nutrients or remove waste quickly enough for all cells. Diffusion distances from the surface to cells deep inside become too long, limiting the rate of mass transfer. Therefore specialised exchange surfaces (e.g. gills, alveoli, villi) are required to increase surface area relative to volume, enhancing the rate of exchange. Additionally, a transport system (circulatory or similar) is needed to carry substances rapidly from these exchange surfaces to body cells and to remove waste products.
Examiner tips
- Use the phrase "surface area to volume ratio" and explain its change with size. Mention diffusion limits and the need for larger surface area. Show that a transport system is required to overcome diffusion distances. Keep the answer concise and use exam‑specific terminology.
Common mistakes
- Failing to mention the decrease in surface area to volume ratio. Confusing surface area with volume. Omitting the role of a transport system or the concept of diffusion limits.
Mark scheme (5 marks)
- As an organism gets larger, its volume increases faster than its surface area
- The smaller surface area to volume ratio means the outer surface is not large enough to supply all the cells with oxygen/nutrients/remove waste quickly enough
- Diffusion distances from the outer surface to cells deep inside the organism become too large/long
- Specialised exchange surfaces provide a large surface area relative to the organism's volume, increasing the rate of exchange
- A transport system is needed to carry substances quickly from the exchange surfaces to the body cells (and remove waste)
Key terms in this question
exchange surface · transport system
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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