A student compares gas exchange in a single-celled organism, such as an amoeba, with gas exchange in a large multicellular organism, such as a mammal. Explain why a large multicellular organism needs a specialised gas exchange surface, whereas a single-celled organism does not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Single-celled organisms exchange gases directly across their outer cell membrane. Large multicellular organisms, such as mammals, have specialised gas exchange surfaces such as alveoli in the lungs.
Model answer (5 marks)
A single‑cell organism has a high surface area to volume ratio, so oxygen and carbon dioxide can diffuse directly across the cell membrane fast enough to meet its metabolic needs.
A large multicellular organism has a much smaller surface area to volume ratio; the outer body surface alone cannot supply all cells with sufficient oxygen by diffusion.
Therefore it develops specialised gas‑exchange surfaces such as alveoli, which greatly increase the total surface area available for diffusion.
These surfaces are very thin and have a short diffusion pathway, allowing gases to move rapidly down a concentration gradient.
A dense capillary network supplies blood to the alveoli, maintaining a low oxygen concentration in the blood and a high concentration in the alveolar air, so the gradient is kept steep and diffusion remains efficient.
A large multicellular organism has a much smaller surface area to volume ratio; the outer body surface alone cannot supply all cells with sufficient oxygen by diffusion.
Therefore it develops specialised gas‑exchange surfaces such as alveoli, which greatly increase the total surface area available for diffusion.
These surfaces are very thin and have a short diffusion pathway, allowing gases to move rapidly down a concentration gradient.
A dense capillary network supplies blood to the alveoli, maintaining a low oxygen concentration in the blood and a high concentration in the alveolar air, so the gradient is kept steep and diffusion remains efficient.
Examiner tips
- Use the key terms: surface area to volume ratio, specialised gas‑exchange surface, alveoli, thin membrane, capillary network.
- Explain the sequence: ratio → need for surface → alveoli → thinness → blood supply.
- Show the cause–effect link between size and diffusion limitation.
- Mention the concentration gradient explicitly.
Common mistakes
- Failing to mention the surface area to volume ratio difference.
- Confusing the role of blood supply with the surface area alone.
- Using vague phrases like "more efficient" without explaining how the gradient is maintained.
Mark scheme (5 marks)
- A single-celled organism has a large surface area to volume ratio, so gases can diffuse across the cell membrane quickly enough to meet the cell's needs.
- A large multicellular organism has a small surface area to volume ratio, so its outer surface is insufficient to supply all its cells with enough oxygen by diffusion.
- Specialised gas exchange surfaces, such as alveoli, provide a large surface area to increase the rate of diffusion.
- Gas exchange surfaces are thin / have a short diffusion pathway, so gases diffuse across quickly down a concentration gradient.
- A good blood supply / capillary network maintains the concentration gradient at the gas exchange surface, ensuring efficient diffusion continues.
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
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- Decode the mark scheme abbreviations →
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