Fish gills are adapted for efficient gas exchange. Describe the adaptations of fish gills that allow efficient gas exchange between the water and the blood.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Fish extract dissolved oxygen from water as it passes over their gills. The gills contain many gill filaments, each covered in smaller projections called lamellae. A dense network of capillaries runs through each lamella.
Model answer (5 marks)
1. Large surface area – many gill filaments and lamellae maximise the area available for diffusion.
2. Short diffusion distance – lamellae and capillary walls are very thin, so O₂ and CO₂ travel a short distance.
3. Moist surfaces – the gill surface is wet, allowing gases to dissolve in the water before diffusing.
4. Large concentration gradient – capillaries continuously remove O₂ from the lamellae, keeping blood O₂ low and maintaining a steep gradient.
5. Dense capillary network – capillaries run close to the exchange surface, supplying blood to all parts of the gill.
2. Short diffusion distance – lamellae and capillary walls are very thin, so O₂ and CO₂ travel a short distance.
3. Moist surfaces – the gill surface is wet, allowing gases to dissolve in the water before diffusing.
4. Large concentration gradient – capillaries continuously remove O₂ from the lamellae, keeping blood O₂ low and maintaining a steep gradient.
5. Dense capillary network – capillaries run close to the exchange surface, supplying blood to all parts of the gill.
Examiner tips
- Use the exact terms: surface area, diffusion distance, moist surfaces, concentration gradient, capillary network.
- Show each point clearly, matching the five mark‑scheme items.
- Keep each point brief but complete – no extra explanation needed.
Common mistakes
- Mixing up the direction of diffusion or saying gases move from blood to water.
- Forgetting to mention the thinness of lamellae or capillary walls.
- Using vague phrases like "lots of surface" instead of "large surface area".
Mark scheme (5 marks)
- Large surface area (provided by the many gill filaments and lamellae) allows more molecules to diffuse across at once
- Short diffusion distance — the walls of the lamellae / capillaries are very thin so oxygen and carbon dioxide do not have to travel far
- Moist surfaces allow gases to dissolve before diffusing across the membrane
- Large concentration gradient maintained — capillaries continuously carry oxygen away from the gill surface, keeping the concentration of oxygen in the blood low
- Dense / extensive capillary network means blood flows close to the exchange surface, supplying blood to all parts of the gill
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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