KA4: communication and signalling: nerve impulse transmission: Initiation of a nerve impulse in response to an environmental stimulus: the vertebrate eye

Question 1

what is the retina?

what does the retina contain?

Answer 1

-the area within the eye that detects light

-two types of photoreceptor cells: rods and cones

Question 2

in animal cells the light sensitive molecule retinal is combined with…

-o_____

-to form the ________ of the eye

Answer 2

-a membrane protein

-opsin

-to form the photoreceptors of the eye

Question 3

describe how rod cells function

Answer 3

in dim light but do not allow colour perception

Question 4

describe how cone cells function

Answer 4

-responsible for colour vision
-only function in bright light.

Question 5

name the retinal-ospin complex found in rod cells

Answer 5

rhodopsin

Question 6

describe the process which triggers nerve impulses in neurons in the retina beginning when retinal absorbs a photon of light

Answer 6

• Retinal absorbs a photon of light
• rhodopsin changes conformation to photoexcited rhodopsin
• A cascade of proteins amplifies the signal
• Photoexcited rhodopsin activates a Gprotein, called transducin, which activates the enzyme phosphodiesterase (PDE)
• A single photoexcited rhodopsin activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE
• PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)
• Each active PDE molecule breaks down thousands of cGMP molecules per second.
• The reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells.
• This results in the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina

Question 7

what does a very high degree of amplification result in?

Answer 7

rod cells being able to respond to low intensities of light

Question 8

describe the importance of different forms of opsin in cone cells

Answer 8

combine with retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths:

red, green, blue or UV