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Cambridge IGCSE Biology · 0610

Chapter 14: Coordination and response (Part 2)

Sense organs

Definition
Groups of receptor cells that respond to specific stimuli: light, sound, touch, temperature, and chemicals.

Structure and function of the eye

  1. Cornea: Transparent layer that refracts light into the eye.
  2. Iris: Coloured part that controls how much light enters by adjusting pupil diameter.
  3. Pupil: The opening that allows light into the eye.
  4. Lens: Changes shape to focus light onto the retina.
  5. Retina: Contains light receptors (photoreceptors) sensitive to different colours.
  6. Optic nerve: Carries impulses from the photoreceptors to the brain.
  7. Blind spot: The point where the optic nerve leaves the eye; it has no receptors.
Transverse section of the human eye with labels for cornea, iris, pupil, lens, retina, optic nerve, blind spot, and fovea
Diagram 1: Structure of the human eye. A transverse section with clear labels for all parts listed above, including the position of the fovea.

Exam Traps

  • Do not say the cornea changes shape to focus — the lens accommodates by changing shape.
  • Avoid placing photoreceptors in the cornea or optic nerve — they are in the retina.

Pupil reflex

Purpose
To protect the retina from damage in bright light and allow better vision in low light.
Mechanism
Controlled by antagonistic muscles in the iris:
  • Bright light: Circular muscles contract, radial muscles relax → pupil constricts.
  • Dim light: Circular muscles relax, radial muscles contract → pupil dilates.

Exam Traps

  • Do not say both iris muscle sets contract in bright light — circular contract, radial relax.
  • Avoid confusing pupil reflex with accommodation — pupil size controls light entry, not focus distance.

Accommodation and photoreceptors

Accommodation
Focusing on objects at different distances by changing lens shape:
  • Near objects: Ciliary muscles contract, suspensory ligaments relax → lens becomes fatter/more curved.
  • Distant objects: Ciliary muscles relax, suspensory ligaments contract → lens becomes thinner/less curved.
Rods and cones
  • Rods: Greater sensitivity for night vision; distributed at the periphery of the retina.
  • Cones: Three types for colour vision in bright light; concentrated at the fovea.
  • Fovea: A section in the middle of the retina providing the clearest image.

Exam Traps

  • Do not say ciliary muscles relax for near vision — they contract to make the lens fatter.

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