Credit: Department of Ophthalmology, University of Utah.
Rhodopsin, also known as visual purple, is a widely distributed pigment concerned with light sensitivity. It consists of a protein called opsin with a prosthetic group called retinal of which vitamin A is an essential precursor. Rhodopsin occurs in the rod cells of vertebrate retinas. It is chemically changed by the light, the change being the basis of how rods work as light-sensitive receptors. Rhodopsin increases in amount in the dark, raising the sensitivity of rods to faint light (the dark adaptation).