Question

In the chemistry of vision in mammals, the photosensitive substance is called
A. Rhodopsin
B. Melanin
C. Sclerotin
D. Retinol

Answer 1

Hint: Rhodopsin is also known as visual purple which consists of a large protein called opsin and retinal. Opsins are a group of light-sensitive proteins found in photoreceptor cells of the retina. Opsin is attached to a molecule of the carotenoid retinal. It is a G-protein coupled receptor (GPCR). It is classified as a chromoprotein.

Complete answer:
• Rhodopsin is the pigment present in the rod of the retina that is photosensitive. So, here the answer would be rhodopsin.
• Retinal (an aldehyde of vitamin A) is common for both rods and cones but the protein opsin differs because amino acid differs in them. Retinal itself is light orange, but when attached to opsin, the deep purplish-red color of Rhodopsin is produced.
• Process of generation of nerve impulse-
• When light is absorbed by Rhodopsin, the retinal is split from the protein, and the rod becomes partially bleached. At the same time, a nerve impulse is generated. More the light strikes the rods, more will be bleaching of Rhodopsin.
• This process of bleaching is reversible. Some Rhodopsin may be synthesized directly from its breakdown products, retinal and opsin. Also, fresh retinal continuously gets manufactured in the eye by the oxidation of vitamin A.
• Process of bleaching-
Rhodopsin $\to$ Retinene Scotopsin $+$ Energy $+$ nerve impulse

Hence, the correct answer is option (A).

Note: Rods are chiefly for vision in dim light that is why twilight (scotopic) vision is the function of the rods. For the light to be absorbed there must be a light-absorbing substance, a pigment. This light-absorbing pigment is called the photo pigment. Photo pigments are conjugated proteins.