# 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

## 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

- [iris](https://www.gradenine.co.uk/glossary/iris)
- [retina](https://www.gradenine.co.uk/glossary/retina)

## Related

- [Revision notes for Pearson Edexcel International GCSE Biology (4BI1)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/describe-the-roles-of-the-iris-1789bdb1) · Published by Druglandscape Ltd.