Eye• Starter activity:

• Name the effects in the accumulation of P(fr) in flowering plants

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

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

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

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

Day and night

Rhodopsin

Daylight and darkness• Explain

the difference in how we observe in daylight and darkness.

Reflex arc

• Complete pathway for more/less light

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

Cones Why are there two types of photoreceptor cell? The rods and cones serve two different functions as shown in this table:

Rods   Cones  

Outer segment is rod shaped Outer segment is cone shaped

109 cells per eye, distributed throughout the retina, so used for peripheral vision.

106 cells per eye, found mainly in the fovea, so can only detect images in centre of retina.

Good sensitivity – can detect a single photon of light, so are used for night vision.

Poor sensitivity – need bright light, so only work in the day.

Only 1 type, so only monochromatic vision.3 types (red green and blue), so are responsible for colour vision.

Many rods usually connected to one bipolar cell, so poor acuity (i.e. rods are not good at resolving fine detail).

Each cone usually connected to one bipolar cell, so good acuity (i.e. cones are used for resolving fine detail such as reading).