Free Essays, Term Papers, Research Papers at Anti Essays

The A vitamins are composed of a group of biologically active molecules called retinoids. There are 3 types of retinoids, retinol, retinal and retinoic acid. A source of retinal in the body is beta-carotene. In the intestine the enzyme beta-carotene oxygenase cleaves the beta-carotene molecule producing 2 molecules of retinal from it. The retinoids have various important roles in the body, such roles are vision, reproduction and the maintaining of the epithelial tissues.
Retinal is an aldehyde formed by the oxidation of retinol. Retinal and retinol can be readily interconverted. The major function of retinal in the human body is in vision. The 11-cis retinal form of retinal attaches to the protein opsin the rod cells to form rhodopsin which is the visual pigment in rod cells. When the rod cells are exposed to light, the rhodopsin experiences photochemical isomerisation and forms all trans retinal and opsin. This process will trigger a nerve impulse which travels through the optic nerve to the brain. The all trans retinal then undergoes isomerisation to form 11-cis retinal which binds to the opsin once again forming rhodopsin. If there is a vitamin A deficiency in the body the first symptoms will be night blindness. The night blindness will lead to a disease known as xerophthalmia which will eventually lead to corneal ulceration and finally blindness. Retinal also plays a role in reproduction.
Both retinol and retinal play a role in reproduction. Retinol and retinal support spermatogenesis in males and prevent foetal resorption in females. However, retinoic acid has no functions in either reproduction or vision. Retinoic acid has a function in maintaining epithelial tissues. If children are only given vitamin A in the form of retinoic acid, they will end up being sterile and blind. This is because for reproduction retinol and retinal are needed and for vision retinal is needed, however, retinal and retinol cannot be obtained from retinoic acid...