9.7 detection of light by mammals

Question 1

what are the two types of photoreceptors in the retina?

Answer 1

rods and cones

Question 2

what is the fovea?

Answer 2

area of the retina containing only cones, region of highest visual activity

Question 3

which photoreceptor has their own sensory neurones?

Answer 3

cones

Question 4

what is the visual pigment in rods?

Answer 4

rhodopsin

Question 5

what is rhodopsin made up of?

Answer 5

opsin and retinal

Question 6

what happens when a photon of light hits a rhodopsin molecule?

Answer 6

rhodopsin is bleached and breaks down into opsin and retinal

Question 7

what happenes to rods in dim light?

Answer 7

1- rhodpsin breaks down to retinal+opsin
2- opsin, acting as an enzyme, closes the Na+ channels in the outer segment so there is no facilitated diffusion
3- Na+ is actively pumped out making the inside more negative
4- neurone is now hyperpolarised
5- causing the bipolar neurone to become depolarised
6- bipolar neurones release neurotransmitter to the ganglion cells and an action potential is initiated

Question 8

what happened to rod cells in the dark?

Answer 8

1- retinol+opsin reform rhodopsin
2- Na+ channels are open so Na+ can diffuse back into the outer segment
3- Na+ actively pumped out of the inner segment using ATP which maintains the concentration gradient between the outer and inner segments
4- inner segment slightly depolarises
5- neurotransmitter glutamate is released binding to the bipolar cell
6- bipolar cell is not depolarised and therefore there is no action potential initiated in ganglion cells

Question 9

what happens when light hits cones?

Answer 9

1- iodopsin is broken down but needs more light than rhodopsin so colours are not easily distinguished in dim light
2- each type of cone has it’s own iodopsin