Opsins

The family of proteins that bind to retinene are known collectively as opsins and in human beings there are several that are responsible for varying aspects of color and black and white vision.

retinene=retinal + opsin = rhodopsin

The rhodopsin then is a complex of an opsin protein and the retinene molecule. It sits in the cell membrane as a receptor, with regions both within and without the cell. The retinene attaches at a point within the cell membrane.

Proteins are the key to living systems and it is the shape of proteins that give them their distinct functions. Proteins are made of smaller units, amino acids, each of which has it's own unique electrochemical properties. These amino acids are linked in sequences, long chains which are unique to each protein. The shape and electrical charges associated with these amino acid units cause these chains to fold, twist, and bend to give the protein it's unique shape and it's unique properties. Adding a new molecule, with other electrochemical properties to a protein, can change the shape, and the properties of the protein.

The opsins contain a region on the molecule where the retinene molecule can fit and bind. But only the cis (bent) configuration of retinene fits this way. This is a bit like forcing spring into a bent position, like a mousetrap. When a photon of light strikes the rhodopsin, the cis-retinene straightens out to the trans form. In some rhodopsins, the retinene comes away completely, in others the trans-retinene is retained in a different formation called a meta-rhodopsin. In both cases, the removal of the cis-retinene cause electrochemical changes in the protein which provide a mechanism for creating an electrical neural signal. This has been shown to occur on a time scale of about one hundred millionths of a second.*