Vision Has Great Chemistry
The primary chemical reaction in vision, the one responsible for generating an impulse in the optic nerve, involves cis–trans isomerization around one of the double bonds in the retinal portion of rhodopsin. When rhodopsin is active (that is, when it can respond to visible light), the double bond between carbon atoms 11 and 12 of the retinal (11-cis-retinal) has the cis orientation. Under the influence of light, an isomerizationreaction occurs at this double bond, producing all-trans-retinal. Because the all-trans form of retinal cannot bind to opsin, all-trans-retinal and free opsin are released. As a result of this reaction, an electrical impulse is generated in the optic nerve and transmitted to the brain to be processed as a visual event. The active form of rhodopsin is regenerated by enzymatic isomerization of the all-trans-retinal back to the 11-cis form and subsequent re-formationof the rhodopsin.
Vitamin A deficiency can have drastic consequences, as would be predicted from its importance in vision. Night blindness-and even total blindness-can result, especially in children. On the other hand, an excess of vitamin A can have harmful effects, such as bone fragility. Lipid-soluble compounds are not excreted as readily as water-soluble substances, and excessive amounts of lipid-soluble vitamins can accumulate in adipose tissue.