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Photoreception in rod cellsEdexcel International A Level Biology: Revision notes

Section 1

Rod cells and rhodopsin

Rod cells are photoreceptors in the retina that work in dim light. Each contains the pigment rhodopsin in its outer segment. Rhodopsin is made of:

  • Opsin, a protein
  • Retinal, a light-absorbing molecule derived from vitamin A

When light is absorbed, retinal changes shape and separates from opsin. The rhodopsin is broken down (bleached).

Key termsretinarhodopsinopsinretinal

Section 2

Rod cells in the dark

In the dark, cation channels in the rod cell membrane are open, so sodium ions diffuse into the outer segment. The rod is kept depolarised.

A depolarised rod continuously releases an inhibitory neurotransmitter onto the bipolar neurone. The bipolar neurone is therefore not depolarised and no action potentials are generated in the optic neurone.

Key termscation channelbipolar neurone
Common mistake

A rod cell hyperpolarises, not depolarises, when it absorbs light.

Section 3

Rod cells in the light

  1. Light is absorbed by rhodopsin; retinal changes shape and separates from opsin.
  2. The cation channels close, so sodium ions stop entering.
  3. Sodium ions are still removed from the cell, so the membrane becomes more negative: hyperpolarisation.
  4. The rod releases less inhibitory neurotransmitter.
  5. The bipolar neurone is no longer inhibited, so it is depolarised.
  6. This stimulates the optic neurone, which generates action potentials that pass along the optic nerve to the brain.
Key termshyperpolarisationoptic neurone
Exam tip

Remember the double reversal: light causes less inhibitory neurotransmitter, and removing inhibition is what stimulates the bipolar neurone.

Section 4

Dark adaptation

In bright light most rhodopsin is bleached and the rods stay hyperpolarised, so they cannot respond to dim light.

In the dark, rhodopsin is resynthesised from opsin and retinal. This takes several minutes and uses ATP, which is why it takes time for vision to improve when moving from bright light to dim light.

Key termsbleachingdark adaptation

Must know

  • Rhodopsin is opsin plus retinal
  • In the dark, cation channels are open, sodium ions enter and the rod is depolarised
  • A depolarised rod releases inhibitory neurotransmitter onto the bipolar neurone
  • Light changes retinal, closes the cation channels and hyperpolarises the rod
  • Less inhibitory neurotransmitter is released, so the bipolar neurone depolarises and the optic neurone produces action potentials

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Photoreception in rod cells

  1. A vision scientist is recording the membrane potential of a single rod cell taken from the retina of a dark-adapted animal. She records first in complete darkness and then when a brief flash of light is delivered.
    Describe what happens to rhodopsin when a rod cell absorbs light.2 marks
  2. In a further experiment the scientist exposes a dark-adapted rod cell to a dim flash of light and measures changes in the rod cell and in the bipolar neurone that it synapses with.
    Explain why absorption of light causes the rod cell membrane to become hyperpolarised.2 marks
  3. A driver leaves a brightly lit road tunnel and enters a poorly lit country lane at night. For several minutes she can see almost nothing, but her vision then slowly improves.
    Describe how absorption of light by rod cells in the driver's retina leads to an action potential in an optic neurone.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).