# Explain how the structure of a gill lamella in a bony fish is adapted to maximise the efficiency of gas exchange with the surrounding water.

> IB DP Biology Higher Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks

## Mark scheme (4 marks)

1. Gill lamellae have a large surface area, increasing the area available for diffusion of gases.
2. The epithelium of the lamella is very thin / one cell thick, reducing the diffusion distance so gases can diffuse rapidly across.
3. A rich capillary network / dense blood supply within the lamella maintains a steep concentration gradient by continuously removing oxygen and delivering carbon dioxide.
4. Countercurrent flow of blood and water across the lamella means blood always encounters water with a higher oxygen concentration, maintaining a concentration gradient along the entire length and maximising oxygen uptake.

## Key terms

- [gill lamella](https://www.gradenine.co.uk/glossary/gill-lamella)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-69a314e9) · Published by Druglandscape Ltd.