1.4 - photoreception in animals

Question 1

What is the retina?

Answer 1

the area within the eye that detects light and contains two types of photoreceptor cells.

Question 2

Name the two types of photoreceptor cells…

Answer 2

Rod and cone cells.

Question 3

Describe rod cells…

Answer 3

contains one type of light-sensitive pigment; function in dim light, do not allow colour perception; sensitive to light intensity changes; nocturnal animals have greater proportion in retina, better night vision.

Question 4

Describe cone cells…

Answer 4

colour vision and only function in bright light; sensitive to specific wavelengths of light; cone cells allow animals to have colour vision; colour blind people lack a particular cone cell type.

Question 5

What is retinal?

Answer 5

a light absorbing molecule

Question 6

What is opsin?

Answer 6

a membrane protein

Question 7

What is the retinal-opsin complex?

Answer 7

rhodopsin, eyes photoreceptors.

Question 8

What happens when retinal absorbs a photon of light?

Answer 8

rhodopsin changes conformation to photoexcited rhodopsin.

Question 9

Describe rhodopsin activation (summarised)…

Answer 9

1. light stimulation of rhodopsin lead to activation of a G-protein
2. activated G-protein activates cGMP phosphodiesterase (PDE)
3. PDE hydrolyses cGMP, reducing its concentration
4. leads to closure of Na+ channels

Question 10

How do rod cells respond to low light intesity?

Answer 10

High degree of amplification

Question 11

How do cone cells work?

Answer 11

In cone cells, different forms of opsin combine with retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: red, blue, green and UV.

Question 12

How is a nerve impulse generated?

Answer 12

1. retinal absorbs a photon of light and rhodopsin changes to photoexcited rhodopsin
2. activates G-protein called transducin (1 photoexcited rhodopsin = hundreds of transducin molecules)
3. transducin activates phosphodieasterase (PDE) (1 transducin = 1PDE)
4. PDE catalyses hydrolysis of cyclic GMP (cGMP)
5. the reduction of cGMP concentration as result of hydrolysis causes closure of ion channels in rod cells membrane.
6. the inwards leakage of + ions (Na+ and Ca+) is halted so the membrane potential increases; hyperpolarisation (more negative charge) triggers nerve impulses in neurons in the retina.