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Rhodopsin

Rhodopsin belongs to the class of G-protein coupled receptors, and consists of two building blocks, an opsin protein called scotopsin and a reversibly covalently bound cofactor; the protein opsin which is linked to 11-cis retinal a prosthetic group while retinal (retinaldehyde) is the light absorbing pigment molecule.

Retinal derives from Vitamin A and is made in the retina. Isomerization of 11-cis-retinal into 11-trans-retinal by light induces a conformational change in the opsin that activates the associated G protein and triggers a second messenger cascade. Rhodopsin opsin is a member of the 7TM receptor family. It is a pigment of the retina that is responsible for the first events in the perception of light.

Rhodopsin of the rods most strongly absorbs green-blue light and therefore appears reddish-purple, which is the reason why it is also known as "visual purple" and is responsible for the Monochromatic vision in the dark. There are several closely related opsins such as the photopsins, which exist that differ only in a few amino acids and in the wavelengths of common light that they absorb most strongly.

All these pigments are found in many different types of the cone cells of the retina and they are the basis of color vision, and humans have three different other opsins beside rhodopsin; the first with absorption maxima for yellowish-green also called photopsin I; the second is the green, named as photopsin II, and then finally the third, bluish-violet light which is photopsin III.

In some, archaea express a proton pump called bacteriorhodopsin to carry out photosynthesis. The same as rhodopsin, bacteriorhodopsin also contains retinal and has seven transmembrane alpha helices, although it is not coupled to a G protein. They say there is an alga known to have an opsin containing its own monolithic light-gated ion channel, channelrhodopsin-2.

For all this Rhodopsin is closely attached to the structure of the eye. In plain English (almost), the entire area within the eye that detects light and color is the called the Retina. The two types of detection cell present, rods and cones, process information coming through the lens and send it down the optic nerve to the brain.

There are around 100 million rod cells in the eyes, they are which detects the degree of lightness entering the eye and their sensitivity is dependent on the amount of Rhodopsin present, which in itself generates within the cells. In this process, rhodopsin is destroyed by bleaching with an exposure to light and therefore rod cells only work in low light as at high illumination the reduced level of this photosensitive pigment leads to a very low sensitivity.

Cone cells are also sensitive to light levels but retain their function up to high illumination via use of the pigment Iodopsin. There are approximately 3 million cone cells in the eyes. The eyes' detection of color is a function of the three types of cone cells present within the retina covering the visible spectrum because each type is sensitive to a different range of wavelengths with maximums corresponding to red, the longer, green, the medium, or blue, the short among them.

The protein rhodopsin contains the protonated retinal-Schiff’s base complex which naturally lies in the inter-membrane pocket and it is formed by the seven trans-membrane a-helical receptors. There are also many flat discs of rhodopsin within the outer segment of a rod cell which, upon different light detection, undergo through photo-isomeric changes from rhodopsin to all-trans retinal. The isomerisation of retinal leads to changes of the shapes of rhodopsin, triggering a sequence of reactions that finally lead to a nerve impulse transmitted to the brain via the optical nerve.

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