A fish obtains oxygen from water by passing water over its gills. Explain how the gills of a fish are adapted to allow efficient gas exchange.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Fish live in an aquatic environment where oxygen is dissolved in water. Water is taken in through the mouth and passed over the gill surfaces before leaving through the gill covers. Oxygen must move from the water into the blood inside the gills, whilst carbon dioxide moves in the opposite direction.
Model answer (5 marks)
1. Gills have a large surface area, which increases the rate of diffusion of oxygen into the blood and carbon dioxide out.
2. The gill surfaces are very thin, essentially one cell thick, allowing gases to diffuse rapidly.
3. A concentration gradient is maintained between the water and the blood, driving diffusion.
4. The gills are richly supplied with capillaries, so oxygen is removed quickly from the blood.
5. The combination of large area, thinness, gradient and rich blood supply makes gas exchange efficient.
2. The gill surfaces are very thin, essentially one cell thick, allowing gases to diffuse rapidly.
3. A concentration gradient is maintained between the water and the blood, driving diffusion.
4. The gills are richly supplied with capillaries, so oxygen is removed quickly from the blood.
5. The combination of large area, thinness, gradient and rich blood supply makes gas exchange efficient.
Examiner tips
- Use the exact terms: large surface area, thin surface, concentration gradient, rich blood supply. Show the causal link: each adaptation improves diffusion rate. Keep answer concise, one sentence per point.
Common mistakes
- Failing to mention the thinness of the gill epithelium. Using vague phrases like "good for gas exchange" instead of specific adaptations. Omitting the role of the rich capillary network.
Mark scheme (5 marks)
- Gills have a large surface area
- Large surface area increases the rate of diffusion of oxygen into the blood (and carbon dioxide out)
- Gill surfaces are thin / one cell thick
- A concentration gradient is maintained between the water and the blood
- Gills have a rich blood supply / are well supplied with capillaries to remove oxygen quickly
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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