A student compares a small unicellular organism living in pond water with a large multicellular organism such as an earthworm. Describe how the surface area to volume ratio differs between the two organisms and explain how this difference affects the way each organism obtains oxygen.

OCR GCSE Biology A: Gateway Science (J247) — B2.2 The challenges of size · Describe · 4 marks · View as Markdown

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

Unicellular organisms such as Amoeba live in pond water and obtain all the oxygen they need directly through their outer surface. Earthworms are much larger multicellular organisms that also live in damp environments.

Model answer (4 marks)

The unicellular organism has a very large surface area to volume ratio, so a large proportion of its mass is exposed to the surrounding water. This allows it to obtain sufficient oxygen by simple diffusion directly through its outer surface (cell membrane). In contrast, the earthworm has a much lower surface area to volume ratio; most of its cells are far from the outer surface. Diffusion alone cannot supply all cells with oxygen, so the earthworm has evolved a transport system (blood vessels and specialised respiratory surfaces) to distribute oxygen throughout its body.

Examiner tips

  • Use the terms "surface area to volume ratio" and "diffusion" for the unicellular organism; mention "transport system" or "specialised respiratory surface" for the earthworm.
  • Show the contrast clearly: large ratio → diffusion; small ratio → transport system.
  • Include the word "oxygen" in both parts of the answer.

Common mistakes

  • Confusing the direction of oxygen movement (students may say oxygen moves into the earthworm but not specify diffusion vs transport).
  • Using vague phrases like "more oxygen" instead of explaining the mechanism.
  • Omitting the term "surface area to volume ratio" or describing it incorrectly.

Mark scheme (4 marks)

  1. The unicellular organism has a large/high surface area to volume ratio (compared to the multicellular organism / earthworm)
  2. The unicellular organism can obtain sufficient oxygen by diffusion directly through its outer surface / cell membrane
  3. The earthworm / large multicellular organism has a low surface area to volume ratio, so diffusion through the outer surface alone is not sufficient to supply all cells with oxygen
  4. The earthworm / large multicellular organism therefore requires a transport system (and/or a specialised exchange surface) to distribute oxygen to all cells

Key terms in this question

surface area to volume ratio · multicellular organism

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