A light-detecting component (a visual pigment) of rod photoreceptor cells composed of a protein called opsin that is chemically linked to a processed fragment of vitamin A.
Rhodopsin is formed when a protein called opsin is chemically linked to a specialized/processed fragment of vitamin A. Rhodopsin resides almost exclusively in the outer segments of rod photoreceptor cells (near the RPE cells) and can only be formed in the dark. When light strikes, the rhodopsin molecule changes shape, generating the initial signal in the visual process. Ultimately, the vitamin A fragment splits off from the opsin protein to be recycled by the RPE cells and brought back to the photoreceptor outer segments, where it can be reattached to opsin in the dark. This series of reactions in light and dark is called the visual cycle (or phototransduction).