VISUAL PIGMENT

any of a number of related substances that function in light reception by animals by transforming light energy into electrical (nerve) potentials. It is believed that all animals employ the same basic pigment structure, consisting of a coloured molecule, or chromophore (the carotenoid retinal, sometimes called retinene), and a protein, or opsin, of moderate size. Retinal1 is derived from vitamin A1; retinal2 is derived from vitamin A2. Many vertebrate animals have two or more visual pigments. Scotopsin pigments are associated with vision in dim light and, in vertebrates, are found in the rod cells of the retina; the retinal1 forms are called rhodopsins, and the retinal2 forms porphyropsins. Photopsin pigments operate in brighter light than scotopsins and occur in the vertebrate cone cells; they differ from the scotopsins only in the characteristics of the opsin fraction. The retinal1 forms are called iodopsins; the retinal2 forms cyanopsins.