In the human eye, the photosensitive compound is composed of

The correct option is opsin and retinal.

To understand why this is the correct answer, let us break down the structure and function of the photosensitive compounds in the human eye step-by-step:
1. Photosensitive Compound (Rhodopsin): In the human eye, the photoreceptor cells (rods and cones) contain light-sensitive pigments. The primary photosensitive pigment in rods is rhodopsin (also known as visual purple).
2. Composition of Rhodopsin: Rhodopsin is a conjugated protein consisting of two main parts:
    • Opsin: This is a protein component (specifically a protein belonging to the G-protein-coupled receptor family).
    • Retinal: This is a non-protein chromophore group, which is an aldehyde of vitamin A (retinol).
3. Mechanism of Action: When light strikes the retina, it induces a conformational change in retinal (specifically, isomerization from 11-cis-retinal to all-trans-retinal). This leads to the dissociation of retinal from opsin, changing the structure of the opsin protein. This structural change initiates a cascade of enzymatic events (via transducin and cyclic GMP phosphodiesterase) that ultimately generates an electrical impulse, which is transmitted to the brain via the optic nerve.
4. Distinction between Retinol and Retinal: While retinol is Vitamin A itself (an alcohol), retinal is the aldehyde derivative that actively binds to opsin to form the visual pigment. Therefore, the combination of opsin and retinol is incorrect, and the correct combination is opsin and retinal.