Question

Which features of the retinal make it so valuable as a photon absorber in the visual systems of organisms?

Short answer

Retinal is used as a photo absorber due to its conjugation system and the C11-C12 cis bond as shown:

Step-by-step solution

Retinal

Retinal contains the aldehyde bond at the side-chain terminal, which is also called retinal aldehyde. It is bounded to a protein named opsin, which helps in the photon absorption, and it is a derivative of vitamin A, also named vitamin A aldehyde.

Retinal as photon absorber

Retinal is used as a photon absorber and absorbs the peak of 500nm, which is almost equal to sun energy produced by sin; hence when sunlight falls on the eye retina absorbs it, and its cis unsaturated bond between C11 and C12 is d get reduced to a single bond shows the retina absorb light due to cis bond.

Retinal and red bond showing C11-C12 bond