# Explain how the ultrastructure of a retinal rod cell is adapted for the detection of low-intensity light.

> IB DP Biology Higher Level (2023 syllabus) — B2.3 Cell specialisation · Explain · 4 marks

> Rod cells are photoreceptor neurons located in the peripheral retina. They are responsible for monochromatic vision in dim-light conditions and contain a specialised photopigment called rhodopsin.

## Mark scheme (4 marks)

1. The outer segment contains a large number of flattened membranous discs, which provides an extensive surface area for the high density of rhodopsin molecules, increasing the probability of photon capture.
2. The inner segment contains a high density of mitochondria, supplying the large amounts of ATP required for continuous active transport (e.g. Na⁺/K⁺ pumps) needed to restore the dark current after each signal.
3. Rhodopsin is a highly sensitive photopigment; a single photon can trigger a conformational change in retinal (the chromophore), allowing rod cells to function in very dim light — unlike cone cells which require brighter conditions.
4. Many rod cells converge onto a single bipolar / ganglion cell (retinal convergence), so subthreshold signals from multiple rods are summated, increasing sensitivity to dim light at the cost of spatial resolution.

## Key terms

- [rod cell](https://www.gradenine.co.uk/glossary/rod-cell)

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