Describe the distribution of rod cells and cone cells in the retina, and explain how this distribution affects a person's ability to see in dim light and to detect colour.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Rod cells are found mainly at the periphery of the retina and are absent from the fovea, whereas cone cells are concentrated in the fovea at the centre of the retina.
Because rod cells are highly sensitive to low light intensities, they enable vision in dim light, but they only provide monochrome (black‑and‑white) vision.
Cone cells, which are sensitive to bright light, detect colour. There are three types of cone cells, each sensitive to a different wavelength (red, green or blue). Colour vision therefore requires bright light.
Because rod cells are highly sensitive to low light intensities, they enable vision in dim light, but they only provide monochrome (black‑and‑white) vision.
Cone cells, which are sensitive to bright light, detect colour. There are three types of cone cells, each sensitive to a different wavelength (red, green or blue). Colour vision therefore requires bright light.
Examiner tips
- State the peripheral distribution of rods and central concentration of cones. Explain rods give low‑light (monochrome) vision and cones give colour vision in bright light. Use the terms "fovea", "low light", "colour vision" and "red, green, blue".
Common mistakes
- Saying rods are in the centre of the retina. Confusing rods with cones for colour vision. Omitting the fovea or the three cone types.
Mark scheme (4 marks)
- Rod cells are distributed at the periphery of the retina / evenly distributed around the retina (and are absent at the fovea)
- Cone cells are concentrated at the fovea (in the centre of the retina)
- Rod cells are sensitive to low light intensities, so they allow vision in dim light (monochromatic / black and white vision)
- Cone cells are sensitive to bright light and detect different colours / there are three types of cone cell each sensitive to a different colour (red, green, blue), so colour vision requires bright light
Key terms in this question
rod cells · cone cells · retina · dim light
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →