Explain about the Vitamin A?

Vitamin A, one of the fat soluble vitamins, refers to a sub-group of retinoid that possess the biological activity of all-trans-retinol. The term 'retinoid s' includes both naturally-occurring forms of vitamin A (rctinol and retinyl esters) and its synthetic analogues which possess the biological activity of the most active geometric isomer, all-trans-retinol. Look at Figure, which illustrates the different forms of vitamin A.

Retinol (an alcohol) can only be found in animal sources, Retinol is referred to as pre-formed vitamin A, as it is present in foods already in the active form and does not require any conversion. Retinol is oxidized reversibly to retinal, which exhibits all the biological activities of retinol, or further oxidized to retinoic acid, which is active in animal growth, but not in vision or reproduction. The form of vitamin A involved in vision is Il-cis-retinal, whereas the primary storage forms are retinyl esters the most common of which is retinyl palmitate.