Photoreception in rod cellsEdexcel International A Level Biology: Subtopic test

1. 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.

   (a)

   Which two components make up the light-absorbing pigment rhodopsin?

   [1 mark]

   • AOpsin and iodopsin
   • BOpsin and retinal
   • CRetinal and melanin
   • DRetinal and haemoglobin

   (b)

   What is happening to sodium ions in the rod cell in complete darkness?

   [1 mark]

   • AThey are leaving the outer segment through open cation channels
   • BThey are entering only through voltage-gated channels in the axon
   • CThey are prevented from entering because the cation channels are closed
   • DThey are entering the outer segment through open cation channels, keeping the membrane depolarised

   (c)

   Describe what happens to rhodopsin when a rod cell absorbs light.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   What happens to the membrane potential of the rod cell when it absorbs the light?

   [1 mark]

   • AIt depolarises as more sodium ions enter
   • BIt stays unchanged because rods only respond to bright light
   • CIt hyperpolarises and becomes more negative
   • DIt reverses to a positive potential as an action potential is generated

   (b)

   What happens to the release of inhibitory neurotransmitter from the rod onto the bipolar neurone when the rod is illuminated?

   [1 mark]

   • AIt decreases, so the bipolar neurone is no longer inhibited
   • BIt increases, so the bipolar neurone is inhibited more strongly
   • CIt stays the same because the rod is not affected by light
   • DIt stops permanently once any light has been absorbed

   (c)

   Explain why absorption of light causes the rod cell membrane to become hyperpolarised.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Describe how absorption of light by rod cells in the driver's retina leads to an action potential in an optic neurone.

   [3 marks]

   (b)

   Explain why it takes the driver several minutes before she can see in the dim light.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Some people inherit a mutation in the gene for opsin. The altered opsin cannot bind to retinal, so no functional rhodopsin is made in their rod cells. These people struggle to see in dim light.

   (a)

   Describe the sequence of events in a normal rod cell, from the dark state to an action potential in the optic neurone, when light is absorbed.

   [6 marks]

   (b)

   Predict and explain the effect of the mutation on the membrane potential of a rod cell, on neurotransmitter release and on action potentials in the optic neurone when the person is in dim light.

   [6 marks]

   Total for question 4: 12 marks

End of questions