How We See 2 - Transduction

Question 1

What is visible light an example of?

Answer 1

Electromagnetic wave

Question 2

What is phototransduction?

Answer 2

Conversion of light energy to an electrochemical response by photoreceptors (rods and cones)

Question 3

Where are photoreceptors?

Answer 3

On the retina

Question 4

What do the phototransduced rods and cones activate and how?

Answer 4

Optic nerve cells by generating an action potential

Question 5

What is each lamellae made up of?

Answer 5

Cell membrane which has the visual pigment rhodopsin in rods and coneopsins S, M and L in cones

Question 6

What is the cell in the retina called where rods and cones exist?

Answer 6

Muller cell

Question 7

What is integrated into the cell membrane of lamellae in rods?

Answer 7

Rhodopsin

Question 8

What is integrated into the cell membrane of lamellae in cones?

Answer 8

Coneopsins S, M and L

Question 9

What do the molecules rhodopsin, conopsins S, M and L differ in?

Answer 9

Spectral sensitivity

Question 10

Where on the Muller cells in the retina are photoreceptors found?

Answer 10

On stacks of lamellae in the outer segment

Question 11

What wavelength does the blue cone best absorb?

Answer 11

437nm

Question 12

What wavelength do rods best absorb?

Answer 12

498nm

Question 13

What wavelength do green cones best absorb?

Answer 13

533nm

Question 14

What wavelength do red cones best absorb?

Answer 14

564nm

Question 15

What is the structure of rhodopsin?

Answer 15

Integral transmembrane helical protein

Question 16

What is chromosphore?

Answer 16

Part of a molecule responsible for changing its colour

Question 17

What is opsin?

Answer 17

Protein which forms part of the visual pigment rhodopsin and is released by the action of light

Question 18

Where is chomosphore found in rhodopsin?

Answer 18

Nesting in the opsin

Question 19

Why is vitamin A important for sight?

Answer 19

It is used to form opsin within rhodopsin

Question 20

What are the two importnat parts of rhodopsin?

Answer 20

Opsin and retinal

Question 21

What happens to 11-cis retinal when light falls on it?

Answer 21

Isomerises to all trans retinal

Question 22

Why does rhodosin split from all trans retinal

Answer 22

All-trans retinal cnnot fir into the opsin, resulting in bleaching of the visual pigment purple

Question 23

Explain the process of rhodopsin causing the phototransduction cascade?

Answer 23

1) Chomophore nests in the protein opsin which is part of retinal
2) When light falls on 11-cis retinal it isomerises to all-trans retinal
3) All-trans retinal cannot fit into the opsin so rhodopsin splits
4) Results in bleaching of the visual purple which causes photoconduction due to the phototransduction cascade

Question 24

What vitamin is responsible for visual pigment regeneratoin?

Question 25

When are photoreceptor cells at rest, and in what polar state are they?

Answer 25

At rest in th edark in a depolarised state by open Na/Ca channels

Question 26

What maintains at rest photoreceptor cells in a depolarised state?

Answer 26

Open Na/Ca channels

Question 27

Explain the phototransduction cascade?

Answer 27

1) Photo of ligh hits rhodopsin
2) Which becomes activated rhodopsin
3) Becomes transducin-GTP, then phosphodiesterase (acitvated) then GMP

4) Na channel closes

5) Relative hyperpolarisation of photoreceptor cell
6) hyperpolarisation transmitted by a flux of Ca ions to the synapse with bipolar cell
7) Stimulates ultimately the retinal cell

Question 28

Explain visual pigment regeneration?

Answer 28

Question 29

What does vitamin A deficiency lead to in relation to the eyes?

Answer 29

Since vitamin A is supplied through the diet any condition that affects vitamin A absorption will affect vision, causing blindness

Also essential for healthy epithelium so conjunctiva and corneal epithleium are also abnormal

Question 30

What are examples of conditions where vitamin A deficiency can occur?

Answer 30

Malnutrition

Malabsorption conditions such as coeliac disease or sprue

Question 31

What are signs of vitamin A deficiency?

Answer 31

Bitot’s spot in conjunctiva

Corneal ulceration

Corneal melting which leads to opacification of the cornea