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 · View as Markdown

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

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.

Model answer (4 marks)

1. The outer segment is packed with many flattened membranous discs, giving a huge surface area for rhodopsin molecules, so more photons can be captured.
2. The inner segment is rich in mitochondria, providing the ATP needed for the Na⁺/K⁺ pumps that restore the dark current after each photon‑induced signal.
3. Rhodopsin is extremely sensitive; a single photon can change the retinal chromophore, allowing rods to respond to very low light levels, unlike cones.
4. Rods converge onto one bipolar/ganglion cell, summating sub‑threshold signals from several rods, which boosts sensitivity to dim light but reduces spatial resolution.

Examiner tips

  • Use the exact terms (outer/inner segment, flattened discs, rhodopsin, convergence).
  • Show the link between structure and function for each point.

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 in this question

rod cell

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