Explain how the structure of the gills of a bony fish allows efficient ventilation with water, ensuring a continuous flow of water over the gill surfaces.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Bony fish (Osteichthyes) use a paired opercular pumping mechanism to drive water continuously and unidirectionally over their gills, even when the fish is stationary.
Model answer (4 marks)
1. The buccal cavity and opercular cavity act as pressure pumps, creating alternating suction and positive pressure that keeps water moving.
2. The operculum opens and closes in sync with the mouth, producing a pressure differential that forces water across the gills.
3. Water flows unidirectionally over the gill lamellae, giving the fish a long diffusion path and maintaining the O₂/CO₂ gradient.
4. The gill arch supports many filaments, providing a large surface area for gas exchange as water passes through.
2. The operculum opens and closes in sync with the mouth, producing a pressure differential that forces water across the gills.
3. Water flows unidirectionally over the gill lamellae, giving the fish a long diffusion path and maintaining the O₂/CO₂ gradient.
4. The gill arch supports many filaments, providing a large surface area for gas exchange as water passes through.
Examiner tips
- Use the phrase "pressure pump/suction pump" to show understanding of the mechanism. Mention the coordinated opening/closing of mouth and operculum. Explain unidirectional flow and its effect on diffusion. Highlight the large surface area of the gill arch and filaments.
Common mistakes
- Confusing the buccal cavity with the opercular cavity. Saying water flows in both directions. Forgetting to mention the pressure differential. Not linking the large surface area to increased gas exchange.
Mark scheme (4 marks)
- The buccal cavity and opercular cavity act as pressure pumps/suction pumps working in alternating phases to maintain near-continuous water flow.
- The operculum (gill cover) opens and closes in a coordinated sequence with the mouth, maintaining a positive pressure differential that drives water across the gills.
- Water flows unidirectionally over the gill lamellae, which maximises the time available for diffusion and maintains the concentration gradient for oxygen uptake / carbon dioxide removal.
- The gill arch supports multiple gill filaments, which are arranged to offer a large surface area through which water must pass, maximising contact between water and the respiratory surface.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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