Explain why a single-celled organism, such as an amoeba, can survive without a specialised gas exchange surface, but a mammal cannot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A single‑cell organism has a very large surface area to volume ratio, so oxygen and glucose can diffuse across the cell membrane quickly enough to meet the cell’s metabolic needs.
A mammal has a much smaller surface area to volume ratio; as body size increases the ratio falls. The body surface area is therefore insufficient for oxygen to diffuse into all cells at the required rate.
Thus a mammal must have a specialised gas‑exchange surface such as lungs or alveoli, which provide a large internal surface area to increase the rate of diffusion and supply enough oxygen.
A mammal has a much smaller surface area to volume ratio; as body size increases the ratio falls. The body surface area is therefore insufficient for oxygen to diffuse into all cells at the required rate.
Thus a mammal must have a specialised gas‑exchange surface such as lungs or alveoli, which provide a large internal surface area to increase the rate of diffusion and supply enough oxygen.
Examiner tips
- Use the term ‘surface area to volume ratio’ and explain its effect on diffusion rates. Mention that larger organisms need specialised structures to increase effective surface area. Show the causal link: small SA:V → insufficient diffusion → need lungs/alveoli.
Common mistakes
- Confusing ‘surface area to volume ratio’ with ‘surface area’ alone. Failing to explain that diffusion is limited by distance and rate. Using vague phrases like ‘more oxygen’ without linking to SA:V.
Mark scheme (5 marks)
- A single-celled organism has a large surface area to volume ratio
- So sufficient oxygen / glucose can diffuse across the cell surface membrane to meet the cell's needs / reach the inside of the cell quickly enough
- A mammal has a small surface area to volume ratio (as body size / volume increases, SA:V ratio decreases)
- The body surface area is not large enough to allow sufficient oxygen to diffuse in to supply all the cells / meet the organism's metabolic demands
- So a mammal needs a specialised gas exchange surface (e.g. lungs / alveoli) with a large surface area to increase the rate of diffusion / supply enough oxygen
Key terms in this question
specialised gas exchange surface · single-celled organism
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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