Chemistry of Vision

Chemistry of Vision
Both rods and cones contain lightsensitive pigments known as rhodopsins. Each rhodopsin molecule consists of a large protein, opsin, which behaves as an enzyme, and a small carotenoid molecule, retinal, a derivative of vitamin A. When a quantum of light strikes the photopigment and is absorbed by the rhodopsin molecule, retinal is isomerized, changing the shape of the molecule. This molecular change triggers the enzymatic activity of opsin, which sets in motion a biochemical sequence of several steps. This complex sequence behaves as an excitatory cascade that vastly amplifies the energy of a single photon to generate a nerve impulse in the rod or cone.

The amount of intact rhodopsin in the retina depends on the intensity of light reaching the eye. A dark-adapted eye contains much rhodopsin and is very sensitive to weak light. Conversely in a light-adapted eye, most of the rhodopsin is split into retinal and opsin. It takes approximately half an hour for a light-adapted eye to accommodate to darkness, while the rhodopsin level gradually increases.