There was a question about “retinal”.

Can you please tell me which chapter was the question from?

it must be from either biology or from biochemistry topic.
Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A (the number of which varies from species to species). Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision. Bound to proteins called type 1 rhodopsins, retinal allows certain microorganisms to convert light into metabolic energy.

Vertebrate animals ingest retinal directly from meat, or produce retinal from one of two carotenes (α-carotene, β-carotene), and also from another type of carotenoid known as β-cryptoxanthin, a type of xanthophyll, these must be obtained from plants or other photosynthetic organisms. No other carotenoids can be converted by animals to retinal, and some carnivores cannot convert any carotenoids at all. The other main forms of vitamin A, retinol, and a partially active form, retinoic acid, may both be produced from retinal.