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Physiology

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Vision

Question
7 out of 11
 

During the dark phase of visual cycle, which form of vitamin A combines with opsin to make Rhodopsin



A All trans-Retinaldehyde

B all trans-Retinol

C 11-cis-Retinaldehyde

D 11-cis-Retinol

Ans. C 11-cis-Retinaldehyde

First know about the forms of Vitamin A

a. Retinol : A primary alcohol containing a bionone ring with an unsaturated side chain.

b. Retinal: It is an aldehyde which is derived from the oxidation of alcohol. It can exist in two forms

i. 11-cis retinal

ii. 11-trans retinal

c. Retinoic acid à The acid derived from oxidation of retinal

(Oxidation) (Oxidation)

Retinol ---------------à Retinal ------------------------à Retinoic acid

(Retinaldehyde)

Visual cycle .

a. Retinal is a component of the visual pigment of rods and cone cells.

b. Rhodopsin is the visual pigment present in rod cells of the retina during the dark phase. Q Rhodopsin consists of 11-cis retinal specificany bound to the protein opsin.

c. When rhodopsin is exposed to light a series of photochemical isomerization occurs resulting in the bleaching of the visual pigment and release of all Trans retinal and opsin.

d. This process triggers a nerve impulse that is transmitted by the optic nerve to the brain. Regeneration of rhodopsin requires isomerization of all transretinal back to 11-cis retinal. Trans retinal after being released from rhodopsin is isomerized to 11-cis retinal which spontaneously combines with opsin to form rhodopsin thus completing the cycle.

e. Similar reactions are responsible for color vision in the cone cells. Q

All trans retinol

All trans retinal

11 cis retinal

ß Opsin

Rhodopsin

Light à Opsin

All trans retinal

f. So, the crux is

i. In the dark phase à 11 cis retinal exists with opsinQ

ii. In the light phase à 11 trans retinal exists with opsinQ

Vision Flashcard List

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