Describe the roles of the iris and the retina in allowing a person to see clearly in dim light.

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

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

Model answer (4 marks)

The iris controls the amount of light entering the eye – in dim light the pupil dilates.
The radial muscles of the iris contract (and the circular muscles relax) to enlarge the pupil.
The retina contains rod cells that are highly sensitive to low light intensities.
Rod cells transmit nerve impulses to the brain via the optic nerve.

Examiner tips

  • Use the exact terms ‘iris’, ‘pupil’, ‘radial muscles’, ‘circular muscles’, ‘rod cells’, ‘optic nerve’.
  • Show the cause–effect sequence: dim light → pupil dilates → more light reaches retina → rods detect light → signals sent to brain.

Common mistakes

  • Confusing the iris with the cornea or lens.
  • Mentioning cone cells instead of rod cells for dim light.
  • Omitting the role of the radial and circular muscles in pupil dilation.

Mark scheme (4 marks)

  1. The iris controls the amount of light entering the eye / pupil dilates in dim light
  2. The radial muscles of the iris contract (and circular muscles relax) to dilate the pupil
  3. The retina contains receptor cells / rod cells that are sensitive to low light intensities
  4. Rod cells / receptor cells send nerve impulses to the brain via the optic nerve

Key terms in this question

iris · retina

Related

More Coordination and response: humans questions

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