"Cis-Trans Isomerism in Vision
The retina, the light-detector layer in the back of our eyes, contains colored compounds called visual pigments. They are insoluble in water and can be extracted from the retina with aqueous detergents. In the dark these pigments are reddish (their name, rhodopsin, comes from a Greek word meaning rose-colored), but the color fades upon exposure to light.
Rhodopsin molecules contain a protein called opsin plus a derivative of vitamin A called 11-cis-retinal. In the dark, 11-cis-retinal fits nicely into the folds of the surrounding opsin. When light hits the rhodopsin, the 11-cis-retinal becomes all-trans-retinal and no longer fits into the cavity of opsin. The opsin and the all-trans-retinal separate. The change in rhodopsin conformation is eventually transmitted to the nerve cells in the eye and then the brain. The stereoisomerism of retinal is thus an important part of the vision process. Note that only one of the five double bonds is affected in this transformation, but when this one changes from cis to trans, the shape of the entire molecule changes. An enzyme later catalyzes the change of all-trans-retinal back to 11-cis-retinal so that it can once again bind opsin and wait for the next exposure to light.
The retinas of vertebrates have two kinds of cells that contain rhodopsin. These cells are distinguished by their shapes: rods and cones. The cones, which function in bright light and are used in color vision, are concentrated in the central portion of the retina, called the macula, and are responsible for the greatest visual acuity. The remaining area of the retina consists mostly of rods, which are used for peripheral and night vision. 11-cis-retinal is present in both rods and cones. However, the opsin is somewhat different in the two kinds of cells, and the cones have three different opsins, one kind each for perception of blue, green, and red colors."
