Vision 2 - transduction

Question 1

what is visible light

Answer 1

an electromagnetic wave

Question 2

what is the definition of phototransduction

Answer 2

the conversion of light energy to an electrochemical response by photoreceptors

Question 3

what are the photoreceptors in the eye

Answer 3

rods and cones on the retina

Question 4

how is an action potential generated in the eye

Answer 4

the phototransduced rods and cones activate optic nerve neurons - fire an action potential

Question 5

what is the structure of a rod

Answer 5

outer - contains stacks of discs/lamellar which have photoreceptors

inner - nucleus, axon then synaptic terminal

Question 6

what is the structure of a cone

Answer 6

outer - contains stacks of discs/lamellar which have photoreceptors

inner - cell organelles

Question 7

what is the structure of the lamellae

Answer 7

made from a cell membrane with integrated visual pigments

• in rods = rhodopsin
• in cones = cone opsins S, M and L

Question 8

what visual pigments are in rods

Answer 8

rhodopsin

Question 9

what visual pigments are in cones

Answer 9

cone opsins S, M and L

Question 10

what comprises the visual pigment rhodopsin

Answer 10

opsin and 11-cis-retinal

Question 11

what is a chromophore

Answer 11

an atom or group whose presence is responsible for the colour of a compound - contains 11-cis-retinal - formed from dietary vitamin A

Question 12

what is 11-cis-retinal

Answer 12

the light sensitive component of visual pigments

Question 13

how does bleaching of the visual pigment occur

Answer 13

chromophore rests in opsin

• light falls on it
• 11-cis-retinal is isomerised to all-trans-retinal
• all-trans-retinal cannot fit into opsin
• rhodopsin splits
• results in bleaching of visual pigment

Question 14

how does bleaching of the visual pigment result in phototransduction

Answer 14

the phototransduction cascade

Question 15

what role does vitamin A play in the visual pigment

Answer 15

visual pigment regeneration

Question 16

what is different about photoreceptor cells in relation to other cells in the body

Answer 16

when at rest (in the dark) kept in a depolarised state by open Na+/Ca+ channels

Question 17

what occurs in the phototransduction cascade

Answer 17

photon of light hits rhodopsin on rods - activated rhodopsin causes transducer GDP to covert to transducer GTP - causes Phosphodiesterase (PDE) to convert to activated PDE - causes cGMP to convert to GMP - CLOSES Na+ CHANNELS - relative hyper polarisation of photoreceptor cell - hyperpolarisation transmitted by a flux of Ca+ ions to the synapse - Stimulates retinal cell

Question 18

how does visual pigment regeneration occur

Answer 18

rhodopsin splits to form all-trans-retinal (AT-Rol) - needs to be regenerated

IN PIGMENT EPITHELIAL CELL:
- AT-Rol (with dietary it A from liver - converted to 11-cis-rol - converted to 11-cis-ral (coversions occur when retinal esters synthesised)

IN PHOTORECEPTOR:
- 11-cis-ral is now called 11-cis-retinal - joins with opsin to form more rhodopsin

Question 19

what are the consequences of vitamin A deficiency

Answer 19

vision affected - e.g. night blindness, abnormal conjunctive and corneal epithelium

Question 20

when can vitamin A deficiency occur

Answer 20

any condition that might affect vit A absorption, malnutrition, malabsorption syndromes e.g. coeliac disease

Question 21

what are signs of vitamin A deficiency

Answer 21

bitots spots in conjunctiva - can be first indication

corneal ulceration

corneal melting - can lead to future corneal opacification