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14.4 Receptors
AQA Biology
Cells of the retina
Specification reference
Learning outcome
After completing this worksheet you should be able to:
explain the differences in sensitivity to light, sensitivity to colour, and visual
acuity between rod and cone cells.
The retina is the light-sensitive layer at the rear of the eye. Retinas possess two
types of photoreceptor: rod cells and cone cells. Here you will learn how the
molecular biology and the optic nerve connections of rods and cones determine the
light-sensing properties of the eye.
The table below outlines how the biology of rod and cone cells affects the function of
the eye:
Rod cells
Cone cells
Sensitivity to light
Contains the photosensitive
pigment rhodopsin, which is
more sensitive to light than
Contains the photosensitive
pigment iodopsin, which is less
sensitive to light than rhodopsin.
Sensitivity to colour
Only one form of rhodopsin
exists; therefore rod cells are not
sensitive to differences in the
wavelengths of light (i.e. colour).
Three forms of iodopsin exist in
humans, each of which is
sensitive to a different wavelength
of light. Each of the three types of
cone cell responds to a different
colour. The overall colour we
perceive depends on the relative
numbers of the three cone cells
that are stimulated.
Visual acuity (i.e.
sharpness or resolution
of vision)
Poor acuity because several rods
share a single neural pathway
(via a bipolar neurone) to the
Good acuity because each cone
cell has a single neural pathway
(via a bipolar neurone) to the
© Oxford University Press 2017
This resource sheet may have been changed from the original
AQA Biology
14.4 Receptors
Explain why rod cells rather than cone cells enable vision at night.
(2 marks)
Red cloths are traditionally used by bullfighters to encourage bulls to charge.
However, bulls cannot see the colour red. Suggest why.
(1 mark)
Birds can see ultraviolet wavelengths of light. Suggest how birds are able to see
ultraviolet light.
(2 marks)
A person’s peripheral vision relies principally on rod cells. Suggest how rod cells
are distributed on the retina.
(1 mark)
A person looked at several small white dots clustered close together on a sheet
of black paper. Explain why cone cells would be able to distinguish the separate
dots, but rod cells may not be able to distinguish them.
(3 marks)
Rod cells respond slower to light than cone cells. The stimulation received by rod
cells over 100 ms is combined to produce a single neural signal to the brain.
Suggest how this difference in response affects:
a the relative sensitivity of rod and cone cells to light
(2 marks)
b the ability of rod and cone cells to detect movement.
(2 marks)
© Oxford University Press 2017
This resource sheet may have been changed from the original