Rhodopsin

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In vertebrates, rhodopsin, sometimes also known as visual purple, is a photoreceptor protein. It is a pigment of the retina, which is responsible for the first events in the perception of light.

A so-called 7TM G-protein coupled receptor, each scotopsin protein binds as a cofactor a molecule of retinaldehyde, which derives from Vitamin A and is made in the retina. Isomerization of retinaldehyde by light induces a conformational change in rhodopsin and a biochemical cascade by way of the associated G protein. The rhodopsins encoded by different genes differ in the wavelengths of light which they absorb most strongly. This variation causes the cones of the retina to differ in their response and is the basis for color vision. The rhodopsin of the rods most strongly absorbs green-blue light, which is why it is some times called "visual purple" (it appears reddish-purple). Some bacteria express a related protein called bacteriorhodopsin to carry out bacterial photosynthesis.

An excellent summary with pictures can be found here.

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