"Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both 
the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision in low-light conditions.[1] Exposed to light, the pigment immediately photobleaches, and it takes about 30 minutes[2] to regenerate fully in humans."
"Rhodopsin consists of the protein moiety opsin and a reversibly covalently bound cofactor, retinal. Opsin, a bundle of seven transmembrane helices connected to each other by protein loops, binds retinal (a photoreactive chromophore), which is located in a central pocket on the seventh helix at a lysine residue. Retinal lies horizontally with relation to the membrane. Each outer segment disc contains thousands of visual pigment molecules. About half the opsin is within the lipid bilayer. Retinal is produced in the retina from Vitamin A, from dietary beta-carotene. Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change (bleaching) in opsin continuing with metarhodopsin II, which activates the associated G protein transducin and triggers a second messenger cascade."