Eye

A2 biology edexcel

1. Eye
•Starter activity:
•Name the effects in the accumulation of P(fr) in
flowering plants

2. Learning objective
•To be able to distinguish between ‘rods’ and
‘cones’
•To be able to name and describe the retinal
pigments
•To explain how pigments affect our sight

3. Retina
•Briefly describe the 3 layers of the retina
•1) Outer layer (photoreceptor) contains rods
and cones inside pigmented epithelial cells of
the choroid
•2) Middle layer contain bipolar neurones with
the synapses of the rods and cones
•3) Inner layer contains ganglion cells with
axons of the optic nerve

5. Rods
•Using text book answer the following questions
•Q) Name the region where rods are not found?
•A) fovea region
•Q) Name the region and role of the mitochondria?
•A) To generate ATP for the resynthesis of rhodopsin
•Q) The photosensitive pigment rhodopsin is found in what region of the rod?
•A) Outer segment
•Q) The inner segment contains large numbers of?
•A) Mitochondria, polysomes (and a nucleus)
•Q) The vesicles in the outer segment are called?
•A) Lamellae
•Q) What is the role for the synthesis of proteins in the polysomes?
•A) Production of visual pigments and the formation of vesicles
•Q) How is the outer segment is connected to the inner segment?
•A) Cytoplasm and a pair of cilia
•Q) What are rods sensitive to and involved in which process?
•A) Light intensities and vision in the dark
•Q) Name the photosensitive pigment found in rods?
•A) Rhodopsin
•Q) What is the base of the inner segment connected to?
•A) Synaptic region
•Q) What do the cells in the synaptic region form?
•A) Form a synapse with a bipolar neurone

6. Rhodopsin
•Rhodopsin is formed when the protein (opsin)
is combined the derivative vitamin A (retinal).
•Retinal exists in 2 forms cis/trans isomer
•When light is exposed to retinal (cis-isomer)
this changes into (trans-isomer) and vice versa
•When rhodopsin is in the cis-isomer and is
exposed to photons of light it quickly changes
into trans-isomer causing a change in the
shape of the (retinal/opsin) known as
‘bleaching’
•The reverse action requires enzymes so it
takes more time

7. Day and night

10. Rhodopsin

11. Daylight and darkness
•Explain
the
difference
in how we
observe in
daylight
and
darkness.

12. Reflex arc
•Complete pathway for more/less light

13. Cones
•They contain the photosensitive pigment iodopsin
•They are sensitive to the wavelength of light
•There are 3 types responding to (red, blue, green)
•This requires 3 different types of iodopsin for each
colour
•The perception of colour depends upon how much
stimulation has occurred on the cone from the light
reflected from the object
•The cones synaptic region has a monsynaptic bipolar
neurone allowing more acuity (clear) vision
•Explain the difference between (rod and cone) in the
amount of connections to the bipolar cell

14. Cones
Why are there two types of photoreceptor cell? The rods and cones serve two different
functions as shown in this table:
Each cone usually connected to one
bipolar cell, so good acuity (i.e. cones are
used for resolving fine detail such as
reading).
Many rods usually connected to one
bipolar cell, so poor acuity (i.e. rods are
not good at resolving fine detail).
3 types (red green and blue), so are
responsible for colour vision.
Only 1 type, so only monochromatic vision.
Poor sensitivity – need bright light, so only
work in the day.
Good sensitivity – can detect a single
photon of light, so are used for night vision.
10
6
cells per eye, found mainly in the
fovea, so can only detect images in centre
of retina.
10
9
cells per eye, distributed throughout the
retina, so used for peripheral vision.
Outer segment is cone shapedOuter segment is rod shaped
Cones
Rods