Explain why a large multicellular organism, such as a mammal, requires a specialised exchange surface to absorb oxygen, whereas a single-celled organism such as Amoeba does not.

OCR A-Level Biology A (H420) — 3.1 Exchange surfaces · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Large multicellular organisms have a small surface area to volume ratio, so the body surface alone cannot supply enough oxygen for all cells.
The diffusion distance from the surface to the innermost cells is too great for diffusion alone to meet metabolic demands in time.
A specialised exchange surface (e.g. alveoli) increases the total surface area available for diffusion, maintaining a steeper concentration gradient.
A single‑cell organism such as Amoeba has a large surface area to volume ratio, so oxygen can diffuse directly across the cell membrane quickly enough to meet its needs.

Examiner tips

  • Use the exact terms: surface area to volume ratio, diffusion distance, specialised exchange surface, steeper concentration gradient.
  • Show the logical link between size, diffusion limits and need for specialised structures.
  • Keep each point concise and directly address the question.

Common mistakes

  • Confusing surface area to volume ratio with absolute surface area; students often write “large surface area” instead of “high ratio”. Failing to mention the diffusion distance or the need for a steeper concentration gradient. Using vague phrases like “more oxygen” without explaining how the specialised surface achieves this.

Mark scheme (4 marks)

  1. Large organisms have a small surface area to volume ratio, meaning the body surface alone cannot supply enough oxygen for all cells
  2. In large organisms the diffusion distance from the surface to the innermost cells is too great for diffusion alone to meet metabolic demands in time
  3. A specialised exchange surface increases the total surface area available for diffusion, maintaining a steeper concentration gradient
  4. Amoeba has a large surface area to volume ratio so oxygen can diffuse directly across the cell surface membrane quickly enough to meet the cell's needs

Key terms in this question

specialised exchange surface · single-celled organism

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