Explain how the pupil of the human eye changes size in bright light and describe the role of the antagonistic muscles involved in this response.

Pearson Edexcel International GCSE Biology (4BI1) — 2.15 Coordination and response: humans · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

In bright light the pupil constricts, becoming smaller.
1. The circular (constrictor) muscles of the iris contract.
2. The radial (dilator) muscles relax.
3. This reduces the diameter of the pupil.
4. Consequently less light enters the eye, protecting the retina from excess light.

Examiner tips

  • Use the word 'constricts' and mention both muscle types; link muscle action to pupil size and light reduction.
  • Show the cause–effect chain: bright light → muscle action → pupil size change → light reduction.
  • Include the protective effect on the retina for full marks.

Common mistakes

  • Confusing the direction of muscle action (e.g., saying dilator muscles contract).
  • Omitting the role of the circular muscles or the protective effect on the retina.

Mark scheme (4 marks)

  1. In bright light, the pupil constricts / becomes smaller / decreases in diameter
  2. The circular (constrictor) muscles of the iris contract
  3. The radial (dilator) muscles of the iris relax
  4. This reduces the amount of light entering the eye / reaching the retina, preventing damage to the retina / photoreceptors

Key terms in this question

pupil · antagonistic

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