Fish gills are adapted for efficient gas exchange between water and the blood. Describe the features of fish gills that make them well-adapted for gas exchange.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Fish extract dissolved oxygen from water as it flows over their gills. The gills contain thin, plate-like structures called lamellae, which are richly supplied with blood capillaries. Water flows over the lamellae in the opposite direction to the blood flowing through the capillaries.
Model answer (5 marks)
Large surface area – the lamellae are thin, plate‑like structures that give a huge area for diffusion.
Thin walls – the gill membrane is only a few micrometres thick, giving a short diffusion distance for O₂ and CO₂.
Rich capillary supply – capillaries run through the lamellae, carrying O₂ away and keeping the concentration gradient high.
Counter‑current flow – water moves opposite to the blood, so the gradient is maintained along the whole lamella.
Moist surface – the gills are wet, allowing gases to dissolve in the water before diffusing across the membrane.
Thin walls – the gill membrane is only a few micrometres thick, giving a short diffusion distance for O₂ and CO₂.
Rich capillary supply – capillaries run through the lamellae, carrying O₂ away and keeping the concentration gradient high.
Counter‑current flow – water moves opposite to the blood, so the gradient is maintained along the whole lamella.
Moist surface – the gills are wet, allowing gases to dissolve in the water before diffusing across the membrane.
Examiner tips
- Use the exact terms from the mark scheme – e.g. ‘large surface area’, ‘thin walls’, ‘rich capillary supply’, ‘counter‑current flow’, ‘moist surface’.
- Show the five points clearly, each as a separate sentence or bullet, to match the 5 marks.
Common mistakes
- Mixing up the direction of flow – students say water and blood flow in the same direction.
- Omitting the counter‑current point or describing it as ‘parallel flow’.
- Using vague phrases like ‘good for gas exchange’ instead of the specific features listed.
Mark scheme (5 marks)
- Large surface area (provided by the lamellae) allows more molecules to diffuse across at once
- Thin walls / short diffusion distance so oxygen and carbon dioxide can diffuse quickly across the membrane
- Rich blood capillary supply carries oxygen away from the gill surface, maintaining a large concentration gradient
- Water flows in the opposite direction to blood (countercurrent flow) so a concentration gradient is maintained along the entire length of the lamella
- Moist surface allows gases to dissolve before diffusing across the membrane
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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