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) (i) Frogs are classified as amphibians.
State two features of amphibians that distinguish them from all other vertebrates.
(ii) Fig. 3.1 is a photograph of a cross-section of a frog’s eye

Identify the structures labelled G and H in Fig. 3.1.

(i) Identify cell structures K, L and M in Fig. 3.2.

(ii) State the names of the two parts of the mammalian central nervous system

(c) Fig. 3.3 is a graph showing the distribution of rod cells and cone cells across the retina in a human eye.

(i) On Fig. 3.3 draw:
• the letter P to show the location of the fovea
• the letter Q to show the location of the blind spot.
(ii) Describe and explain the distribution of rod cells and cone cells shown in Fig. 3.3.

(iii) Some mammals are nocturnal which means they are active at night and sleep during the day.
Suggest how the number and distribution of rod cells and cone cells across the retina of a nocturnal mammal would differ from a human retina.

▶️ Answer/Explanation
Solution

(a)(i) Any two from:
• Eggs laid in water / larvae live in water, adults live on land.
• Gas exchange occurs through skin (and lungs).
• Moist skin (no scales).

Explanation: Amphibians are unique due to their dual life stages (aquatic larvae, terrestrial adults) and permeable skin for respiration.

(a)(ii) G: cornea ; H: lens.

Explanation: The cornea (G) is the transparent outer layer, while the lens (H) focuses light onto the retina.

(b)(i) K: cell membrane ; L: cytoplasm ; M: nucleus.

Explanation: Basic cell structures: the membrane (K) encloses the cell, cytoplasm (L) is the gel-like interior, and the nucleus (M) contains genetic material.

(b)(ii) Brain and spinal cord.

Explanation: The central nervous system (CNS) consists of the brain (processing center) and spinal cord (signal pathway).

(c)(i) • P (fovea): Highest cone cell density (center of the graph).
• Q (blind spot): No rods/cones (dip in the graph).

Explanation: The fovea has maximal cones for sharp vision, while the blind spot lacks photoreceptors (optic nerve exit).

(c)(ii) Description:
1. More rods than cones overall.
2. Cone peak at the fovea; no rods there.
3. Rods dominate peripherally.
Explanation:
• Rods detect dim light (night vision).
• Cones detect color/bright light (high acuity at fovea).

(c)(iii) Nocturnal mammals have more rods (for night vision) and fewer cones (reduced color detection). Rods may extend into the fovea.

Explanation: Adaptations for low-light vision prioritize rod cells over cones.

Question

(a) Define the term sense organ.

(b) Fig. 2.1 shows the structure of the eye. It also shows the pathway taken by nerve impulses which help bring about changes in the eye in order to bring light to a focus.

(i) Draw a label line and a letter X on Fig. 2.1 to identify a motor neurone.

(ii) Describe how the events that occur at a synapse generate an impulse in the next neurone.

(iii) Describe and explain the process of accommodation in the eye to view a near object. Use the letters in Fig. 2.1 in your answer.

(c) Describe the distribution and function of rods and cones in the eye.

(d) Red‑green colour blindness is a sex‑linked characteristic. It is controlled by a gene on the X chromosome. There are two alternative versions of this gene:

  • no colour blindness \(X^A\)
  • colour blindness \(X^a\)

(i) State the term used to describe an alternative version of a gene.

(ii) State the genotype of a male with colour blindness.

▶️ Answer/Explanation
Solution

(a) Ans: A sense organ is a group of receptor cells or tissues that detect specific stimuli (e.g., light, sound) and transmit impulses to sensory neurones.

(b)(i) Ans: Label line and letter X should mark the neurone from the brain to the eye (motor neurone).

(b)(ii) Ans: At a synapse, neurotransmitters are released from vesicles in the presynaptic neurone, diffuse across the synaptic cleft, and bind to receptors on the postsynaptic neurone, generating a new impulse.

(b)(iii) Ans: For near vision:

  • Ciliary muscles (E) contract.
  • Suspensory ligaments (K) slacken.
  • Lens (F) becomes more convex, increasing refraction to focus light onto the retina (A/B).

(c) Ans:

  • Distribution: Rods and cones are in the retina (A). Cones are concentrated in the fovea (B); blind spot (C) lacks both.
  • Function: Rods detect dim light (grayscale); cones detect bright light and colour (different wavelengths).

(d)(i) Ans: Allele.

(d)(ii) Ans: \(X^aY\) (males have one X chromosome; \(X^a\) denotes colour blindness).

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