Detection of light by mammalsEdexcel A-Level Biology B: Mind map

Exam questions on Detection of light by mammals

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.
   Explain why a person cannot detect light that falls on the point where the optic nerve leaves the retina.2 marks
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.
   Explain how the breakdown of rhodopsin leads to an action potential in a ganglion cell.2 marks
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.
   Explain why the faint light is seen better when she looks slightly to one side of it.3 marks