The eye, rods and pupil responsesEdexcel A-Level Biology A: Subtopic test

1. 1
   An ophthalmologist shines a bright light into one eye of each of two patients and watches the pupil. In the first patient the pupil constricts quickly. In the second patient, the motor neurones that supply the iris muscles have been damaged by a head injury, and the pupil does not constrict.

   (a)

   Which row correctly describes the response of the iris muscles in the first patient?

   [1 mark]

   • ARadial muscles contract and circular muscles relax
   • BCircular muscles contract and radial muscles relax
   • CBoth circular and radial muscles contract together
   • DBoth circular and radial muscles relax

   (b)

   Which sequence shows the pathway of the pupil reflex in the first patient?

   [1 mark]

   • ARetina, motor neurone, central nervous system, sensory neurone, iris muscles
   • BIris muscles, sensory neurone, central nervous system, motor neurone, retina
   • CRetina, relay neurone, iris muscles, central nervous system
   • DRetina, sensory neurone, central nervous system, motor neurone, iris muscles

   (c)

   Explain why the pupil of the second patient does not constrict.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A driver leaves a brightly lit petrol station and pulls onto an unlit country road at night. For the first few minutes she cannot see the road edge clearly, but her vision gradually improves.

   (a)

   How does the driver's pupil change when she moves onto the dark road?

   [1 mark]

   • AThe pupil dilates because the radial muscles contract and the circular muscles relax
   • BThe pupil dilates because the circular muscles contract and the radial muscles relax
   • CThe pupil constricts because the radial muscles contract and the circular muscles relax
   • DThe pupil stays the same size because the iris is under voluntary control

   (b)

   What happens to rhodopsin in the rod cells when the driver is in the bright light of the petrol station?

   [1 mark]

   • AIt is built up from retinal and opsin faster than it is broken down
   • BIt is broken down by acetylcholinesterase into choline and ethanoic acid
   • CIt splits into retinal and opsin
   • DIt stays unchanged but still closes cation channels

   (c)

   Explain why the driver cannot see clearly at first on the dark road.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Vitamin A is needed to make retinal. A man whose diet is deficient in vitamin A has difficulty seeing in dim light, a condition called night blindness. His rod cells contain less rhodopsin than normal.

   (a)

   Explain why the man's lack of vitamin A makes it difficult to detect dim light.

   [3 marks]

   (b)

   In a normal rod cell, light causes cation channels to close. Explain how this leads to action potentials in the optic neurone.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Mammalian eyes must work in light intensities that vary enormously, from moonlight to bright sunshine. Two features that help are the pupil reflex and the rhodopsin in rod cells.

   (a)

   Explain how the pupil reflex adjusts the amount of light entering the eye when a person walks from a dim room into bright sunshine and back again.

   [6 marks]

   (b)

   Evaluate how well the pupil reflex and rhodopsin in the rods together allow vision over a wide range of light intensities.

   [6 marks]

   Total for question 4: 12 marks

End of questions