Home / iGCSE Biology (0610)-14.2 Sense organs – iGCSE Style Questions Paper 4

iGCSE Biology (0610)-14.2 Sense organs – iGCSE Style Questions Paper 4

IGCSE Biology (0610) Practice Questions Paper 4 - All Chapters

Question

(a) The human retina contains receptor cells called rods and cones.
Fig. 2.1 shows the distribution of receptor cells in a human retina.

Using the information in Fig. 2.1, describe the differences in the distribution of rods and cones.

(b) Nocturnal animals are active at night.
Suggest how the number of receptor cells in the retina of a nocturnal animal differs from those in the retina of an animal that is active in the day.
Explain your suggestion.
(c) In humans, the size of the iris increases in bright light conditions.
During this response one effector in the iris contracts and one effector relaxes.
(i) State the name of this response.
(ii) State the name of the effector that contracts in this response. 
(iii) State the name of the type of action shown by the paired effectors during this response.

(d) The optic nerve contains many neurones.
Fig. 2.2 shows a synapse between two neurones.

(i) State the names of the parts labelled A, B and C in Fig. 2.2.

(ii) Explain how part C in Fig. 2.2 moves across the synaptic gap.

▶️ Answer/Explanation
Detailed solution

(a)
Based on Fig. 2.1, the differences are:

  • Overall, there are significantly more rods than cones across the retina.
  • The number of cones peaks at the fovea (at \(0\) arbitrary units on the x-axis).
  • There are no rods present at the fovea.
  • The number of rods peaks on either side of the fovea (around \(20\) arbitrary units) and generally increases as you move towards the fovea before dropping to zero.
  • Cones are present in low, relatively constant numbers outside the fovea.

(b)
A nocturnal animal would have a greater number (or proportion) of rod cells compared to a diurnal animal.
Explanation: Rod cells are highly sensitive to low light intensities, making them essential for night vision. Cones, which require brighter light to function and provide colour vision, would be less important; thus, nocturnal animals may have fewer cones.

(c)
(i) Pupil reflex.
(ii) Circular muscle (of the iris).
(iii) Antagonistic action.
Explanation: In bright light, the pupil constricts to limit light entry. This is achieved by the contraction of the circular muscles and the relaxation of the radial muscles. Since these two muscle sets work in opposition, their action is described as antagonistic.

(d)
(i)
A: Vesicle
B: Receptor (protein)
C: Neurotransmitters

(ii) Part C (neurotransmitters) moves by diffusion. This occurs due to the random movement of particles from an area of higher concentration (released from the vesicle) to an area of lower concentration (across the synaptic gap towards the postsynaptic membrane).

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