Detection of light by mammalsEdexcel A-Level Biology B: Subtopic test

1. 1
   A student studies a section of a human retina. Light entering the eye passes through a layer of ganglion cells and a layer of bipolar neurones before reaching the photoreceptor cells at the back of the retina. The axons of the ganglion cells leave the eye together at one point as the optic nerve, and no photoreceptor cells are present at that point.

   (a)

   What is the name of the region where the optic nerve leaves the retina and there are no photoreceptors?

   [1 mark]

   • ABlind spot
   • BFovea
   • CChoroid
   • DPupil

   (b)

   In which order do signals pass through the retina towards the brain?

   [1 mark]

   • AGanglion cell, bipolar neurone, photoreceptor
   • BPhotoreceptor, bipolar neurone, ganglion cell
   • CBipolar neurone, photoreceptor, ganglion cell
   • DPhotoreceptor, ganglion cell, bipolar neurone

   (c)

   Explain why a person cannot detect light that falls on the point where the optic nerve leaves the retina.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Rod cells contain the pigment rhodopsin in the membranes of the discs in their outer segments. In darkness, Na⁺ channels in the membrane of a rod are open, so the rod is partly depolarised and continuously releases an inhibitory transmitter onto a bipolar neurone. When rhodopsin absorbs light it breaks down (bleaches), the Na⁺ channels close and the rod becomes hyperpolarised.

   (a)

   What is the effect of light on a rod cell?

   [1 mark]

   • AIt depolarises the rod, so more transmitter is released
   • BIt opens Na⁺ channels, so Na⁺ ions enter the rod
   • CIt hyperpolarises the rod, so less transmitter is released
   • DIt stops the rod producing rhodopsin permanently

   (b)

   Rhodopsin is made of which two components?

   [1 mark]

   • AActin and myosin
   • BHaemoglobin and iron
   • COpsin and chlorophyll
   • DOpsin and retinal

   (c)

   Explain how the breakdown of rhodopsin leads to an action potential in a ganglion cell.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A person walks from bright sunlight into a dim room. After a while she notices that she can see a faint light on the wall better if she looks slightly to one side of it than if she looks straight at it. In the fovea of the human retina there are about 150 000 cones per mm² and no rods. At 20° from the fovea there are about 4 000 cones per mm² and 150 000 rods per mm².

   (a)

   Explain why the faint light is seen better when she looks slightly to one side of it.

   [3 marks]

   (b)

   Calculate the percentage of photoreceptors that are rods at 20° from the fovea. Explain why the fovea gives clearer detail than the retina at 20°.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An optometrist sees two patients. Patient 1 has an inherited condition in which rod cells gradually die throughout the retina. Patient 2 has a condition in which cone cells die only in the fovea.

   (a)

   Explain how the features of rod and cone cells and their distribution in the retina allow vision in both dim and bright light.

   [6 marks]

   (b)

   Explain the symptoms that you would expect each patient to report in bright light and in dim light.

   [6 marks]

   Total for question 4: 12 marks

End of questions