Vitamin A as a chromophore

The vitamin A metabolite, 11-cis retinal is an active constituent of the photoreceptor structures involved in the visual process. The eye contains photoreceptors of two kinds, rods and cones. Rods respond to in general (faint) light, whereas the cones respond to coloured light (red, green and blue). Both rods and cones contain 11-cis retinal bound to proteins called opsins. Rods have the opsin 'Rhodopsin' while the cones have the opsin 'Iodopsin'. It is the different opsins that give the receptors different sensitivity for different types of light.