1) Protein 
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction c...
(from the article `coloration`) Vision itself depends on a biochrome that consists of a protein, opsin, attached to a chromophore. The chromophore may be either retinal (vitamin ... Rhodopsin is made up of a protein called opsin and a photosensitive chemical derived from vitamin A, 11-cis-retinaldehyde. Photons of light entering ... ...
<protein> General term for the apoproteins of the rhodopsin family. ... (18 Nov 1997) ... 
General term for the apoproteins of the rhodopsin family.
[n] - retinal protein formed by the action of light on rhodopsin
noun retinal protein formed by the action of light on rhodopsin
any of several compounds that form the protein component of the light-sensitive retina pigment, rhodopsin.