Explain why a fish uses specialised gill structures to obtain oxygen from water, rather than relying on diffusion across its general body surface.

AQA A-Level Biology (7402) — 3.3.1 Surface Area to Volume Ratio · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A fish is a large, multicellular organism, so its volume is large relative to its surface area. The body surface alone does not provide enough surface area to supply all the cells with sufficient oxygen by diffusion. Gills provide a large surface area for diffusion of oxygen. Gills have a rich blood supply and thin walls, maintaining a steep concentration gradient for diffusion. Diffusion alone across the general body surface would be too slow to meet the metabolic demands of all the cells.

Examiner tips

  • Use the word "large" to describe the fish’s volume and "small" for surface area; mention the steep concentration gradient; link gill structure to oxygen supply.
  • Show the logical chain: large volume → limited surface area → need for specialised structure → gills’ large area and thin walls → efficient diffusion.
  • Use the exact terminology: "gills", "surface area", "diffusion", "concentration gradient".

Common mistakes

  • Confusing the fish’s body surface with the gill surface; writing that the gills are for heat exchange instead of oxygen; omitting the point about the steep concentration gradient.

Mark scheme (5 marks)

  1. A fish is a large/multicellular organism, so its volume is large relative to its surface area
  2. The body surface alone does not provide enough surface area to supply all the cells with sufficient oxygen by diffusion
  3. Gills provide a large surface area for diffusion of oxygen
  4. Gills have a rich blood supply / thin walls, maintaining a steep concentration gradient for diffusion
  5. Diffusion alone across the general body surface would be too slow to meet the metabolic demands of all the cells

Key terms in this question

diffusion

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