Explain why a large multicellular organism, such as a mammal, requires a specialised exchange surface, 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 outer body surface alone cannot supply the metabolic demands of all cells by diffusion alone. Therefore they develop specialised exchange surfaces (e.g. alveoli, villi) that increase the surface area available for diffusion or have features that increase the rate of diffusion.
Single‑cell organisms such as amoeba have a high surface area to volume ratio; their cell membrane provides sufficient area for exchange by diffusion to meet their needs.

Examiner tips

  • Use the term ‘surface area to volume ratio’ early. Explain why a small ratio limits diffusion. Mention that specialised surfaces increase area or diffusion rate. Contrast with single‑cell high ratio.

Common mistakes

  • Failing to mention surface area to volume ratio. Confusing ‘volume’ with ‘mass’. Saying specialised surfaces ‘reduce’ diffusion instead of ‘increase’ it.

Mark scheme (4 marks)

  1. Large organisms have a small surface area to volume ratio
  2. The outer body surface alone is insufficient to supply the metabolic demands of all cells by diffusion alone
  3. Specialised exchange surfaces increase the surface area available for diffusion / have features that increase the rate of diffusion
  4. Single-celled organisms have a high surface area to volume ratio so the cell surface membrane provides sufficient area for exchange by diffusion to meet the cell's needs

Key terms in this question

exchange surface · single-celled organism · multicellular organism

Related

More Exchange surfaces questions

▶ Try answering this question with AI marking (free) →