Vitamin A is a terpene and an alcohol, of which the corresponding
aldehyde is retinal, the trigger for light reception in
our eyes (right). Before a photon of light strikes it, retinal,
which has a cis conformation about the double bond at Position 11,
is attached to the protein opsin to form a molecule of rhodopsin.
When light hits the visual photoreceptors, the energy of the photon
flips the cis double bond in retinal over to the trans form. This
is the trigger which, in a way that is not entirely clear, initiates
a nerve impulse from eye to brain.
Rhodopsin with trans-retinal is unstable and falls apart
into opsin and retinal. Trans-retinal is reconverted biochemically
to the cis isomer, reconnected to a molecule of opsin, and the trap
is set for another photon of light.