CIE IGCSE Biology

Revision Notes

14.1.3 Sense Organs: The Eye

Sense Organs

  • Receptors are groups of specialised cells
  • They detect a change in the environment and stimulate electrical impulses in response
  • Sense organs contain groups of receptors that respond to specific stimuli

Sense Organs table, IGCSE & GCSE Biology revision notes

  • Once the receptor cell in the sense organ has been stimulated, it generates an electrical impulse
  • This is passed on to a sensory neurone which carries the impulse to the central nervous system
  • Here a response will be decided on and the impulse will be passed to a motor neurone (via a relay neurone)
  • The motor neurone carries the impulse to the effector (muscle or gland)
  • The effector carries out the response

Structure of the Eye & Functions

  • The eye is a sense organ containing receptor cells that are sensitive to light (rod cells) and colour (cone cells)

The eye, IGCSE & GCSE Biology revision notesThe eye

 

Function of the parts of the eye:

Function of the parts of the eye table, IGCSE & GCSE Biology revision notes

The Pupil Reflex

  • This is a reflex action carried out to protect the retina from damage in bright light and protect us from not seeing objects in dim light
  • In dim light the pupil dilates (widens) in order to allow as much light into the eye as possible
  • In bright light the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina

The pupil reflex, IGCSE & GCSE Biology revision notesThe pupil reflex

  • In dim light, the pupil dilates (becomes larger) to allow more light to enter the eye to improve vision. 
  • In bright light, the pupil constricts (gets smaller) to allow less light to enter the eye to protect the retina from damage.
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How does the Pupil Reflex Work?

DIM LIGHT table, IGCSE & GCSE Biology revision notes

BRIGHT LIGHT table, IGCSE & GCSE Biology revision notes

Eye reflex comparison table, IGCSE & GCSE Biology revision notes

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Accomodation Viewing Near & Distant Objects

  • The way the lens brings about fine focusing is called accommodation
  • The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose
  • The changes are brought about by the contraction or relaxation of the ciliary muscles

Diagram showing the eye when an object is close up, IGCSE & GCSE Biology revision notesDiagram showing the eye when an object is close up

  • Ciliary muscles contract
  • Suspensory ligaments slacken
  • This allows lens to become fatter
  • Light is refracted more

Eye when an object is far away, IGCSE & GCSE Biology revision notesDiagram showing the eye when an object is far away

The eye when an object is far away table, IGCSE & GCSE Biology revision notes

Exam Tip

This is something you can work out in an exam if you have forgotten – staring at your hand right in front of your eye will make your eyes feel tight after a few seconds. This is because the ciliary muscles are contracted. Staring at an object far away feels relaxing and comfortable because the ciliary muscles are relaxed.

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Rods & Cones

  • There are two types of receptor cells in the retina:
    • Rods which are sensitive to dim light
    • Cones which distinguish between different colours in bright light
  • There are 3 types of cone cells which are sensitive to different colours of light (red, blue and green)
  • The fovea is an area on the retina where almost all of the cone cells are found
  • Rod cells are found all over the retina, other than the area where the optic nerve attaches to the retina – there are no light-sensitive cells at all in this area, and so it is known as the blind spot

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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