Cambridge IGCSE Biology: Sense Organs (Topic 14.2)

Overview

• Sense Organs: Groups of receptor cells that respond to environmental changes (stimuli) such as light, sound, touch, temperature, and chemicals.
• Focus of Lesson: The eye, its structure, function, and related physiological processes.

Structure and Function of the Eye

Main Structures:

1. Cornea

   • Tough, transparent, curved layer at the front of the eye.
   • Protects internal structures and refracts light to help focus.

2. Iris

   • Colored ring of muscles behind the cornea.
   • Controls the size of the pupil.

3. Pupil

   • Circular opening in the center of the iris.
   • Allows light to enter the eye.

4. Lens

   • Clear, flexible structure behind the iris.
   • Refracts light to focus it onto the retina.
   • Held by ligaments and changes shape for focusing on objects at different distances.

5. Retina

   • Lining inside the back of the eye.
   • Contains light-sensitive receptor cells.
   • Sends electrical impulses via the optic nerve to the brain resulting in sight.

6. Blind Spot

   • Area on the retina in front of the optic nerve.
   • Contains no light-sensitive cells, creating a blind spot.

Pupil Reflex

• Function: The diameter of the pupil adjusts based on light intensity to protect the retina.
• Bright Light: Iris muscles constrict, reducing pupil size to protect retina.
• Dim Light: Iris muscles dilate, increasing pupil size to improve vision.
• Reflex is involuntary and cannot be consciously controlled.

Extended Content

Pupil Reflex Mechanism

• Antagonistic Muscles: Circular and radial muscles in the iris.
  • Bright Light: Circular muscles contract, radial muscles relax, constricting the pupil.
  • Dim Light: Radial muscles contract, circular muscles relax, dilating the pupil.

Accommodation (Focusing on Near and Distant Objects)

• Distant Objects

  • Ciliary muscle relaxes, widens, pulling on suspensory ligaments.
  • Lens flattens, reducing refraction angle.

• Near Objects

  • Ciliary muscle contracts, lens returns to convex shape.
  • Increases refraction angle, focusing on the retina.

Light-Sensitive Cells in the Retina

1. Rods

   • Sensitive to low light intensities.
   • Important for night vision.
   • Images formed in shades of gray.

2. Cones

   • Enable color distinction in bright light.
   • Three types: respond to red, green, blue light.
   • Concentrated at the fovea.

Fovea

• Description: Small depression in the retina center.
• Function: Responsible for central vision and visual acuity.
• Concentration: High concentration of cone cells for detailed vision.

Peripheral Vision

• Less defined images formed on other retina parts.

Conclusion

• Comprehensive understanding of sense organs, specifically the eye, is essential for the IGCSE Biology exam.
• Next topic: Hormones (14.3).

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