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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Gill lamellae are thin, one‑cell‑thick sheets of epithelium that give a very large surface area for diffusion. The lamellae are packed close together, so the total area available for gas exchange is maximised. The epithelium is only one cell thick, which minimises the diffusion distance for O₂ and CO₂. Inside each lamella there is a dense network of capillaries that continuously remove O₂ from the water and deliver CO₂ to the blood, keeping a steep concentration gradient. Blood and water flow in opposite directions (counter‑current flow), so the blood always meets water that still has a higher O₂ concentration, maintaining the gradient along the whole length of the lamella and allowing the greatest possible uptake of oxygen.

Examiner tips

  • Mention large surface area, thin epithelium, rich capillary network, and counter‑current flow in separate points.
  • Use the exact terms ‘large surface area’, ‘one‑cell‑thick epithelium’, ‘dense capillary network’, ‘counter‑current flow’.
  • Show the sequence: surface area → thinness → capillaries → counter‑current to maximise gradient.
  • Keep each point concise and directly linked to the efficiency of gas exchange.

Common mistakes

  • Failing to state that the epithelium is one cell thick.
  • Not mentioning counter‑current flow or confusing it with parallel flow.
  • Using vague terms like ‘thin’ or ‘many blood vessels’ instead of the specific descriptors required.

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 in this question

gill lamella

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