Editing Retinol (section)

==Chemistry==
Many different geometric isomers of retinol, retinal and retinoic acid are possible as a result of either a ''[[trans isomer|trans]]'' or ''[[Cis-trans isomerism|cis]]'' configuration of four of the five [[double bond]]s found in the [[polyene]] chain. The ''cis'' isomers are less stable and can readily convert to the all-''trans'' configuration (as seen in the structure of all-''trans''-retinol shown at the top of this page). Nevertheless, some ''cis'' isomers are found naturally and carry out essential functions. For example, the 11-''cis''-retinal isomer is the [[chromophore]] of [[rhodopsin]], the [[vertebrate]] [[Photoreceptor protein|photoreceptor]] molecule. Rhodopsin is composed of the 11-cis-retinal covalently linked via a [[Schiff base]] to the [[opsin]] protein (either rod opsin or blue, red, or green cone opsins). The process of vision relies on the light-induced isomerisation of the chromophore from 11-''cis'' to all-''trans'' resulting in a change of the conformation and activation of the photoreceptor molecule.<ref name=bs/>

Many of the non-visual functions of vitamin A are mediated by retinoic acid, which regulates gene expression by activating nuclear [[retinoic acid receptor]]s.<ref name="Duester_2008">{{cite journal | vauthors = Duester G | title = Retinoic acid synthesis and signaling during early organogenesis | journal = Cell | volume = 134 | issue = 6 | pages = 921–931 | date = September 2008 | pmid = 18805086 | pmc = 2632951 | doi = 10.1016/j.cell.2008.09.002 }}</ref> The non-visual functions of vitamin A are essential in the immunological function, reproduction, and embryonic development of vertebrates as evidenced by the impaired growth, susceptibility to infection, and birth defects observed in populations receiving suboptimal vitamin A in their diet.