Scheme 5 describes the chemistry of vision. In retinol, the cone cells are responsible for vision in bright light, while the rod cells are responsible for the vision in the dim light. In rod cells, the vitamin A is oxidized to aldehyde that undergoes isomerization of trans double to cis double bond at C-11 carbon. The latter reacts with protein opsin to form an imine with (C-11 Z)-retinal, giving rhodopsin. When rhodopin absorbs light, the cis double bond isomerizes to a trans double bond. This geometry change causes an electrical signal to be sent to the brain, where it is perceived as a visual image. The trans isomer of rhodospin is not stable, thus, it undergoes hydrolysis to give (11E)-retinal and opsin. (C-11 E)-Retinal is then transformed back to (C-11 Z)-retinal to complete the vision cycle.

Scheme 5. The Chemistry of Vision

Scheme 5