<prompt> Which of the following is a provitamin -AscorbicacidVitamin-Eb-caroteneVitamin-K  <response>: Ans. is c i.e., b-carotene o Two group of compounds have vitamin A activity -Vitamin A: - Vitamin A refers to three biologically active Vitamers i.e.. isomers of vitamin: - RetinolQ an alcohol, retinalQ an aldehyde, and retinoic acidQ an acid. All of these are found only in animai They are polyisoprenoid compounds comprising two distinct components : i a cyclohexenyl ring and ii A side chain of several isoprene units attached to cyclohexenyl ring. The term Retinoids is used to define these three compounds retinol, retinal, and retinoic acid and other synthetic compounds with vitamin A activity.Carotenoids : - These are provitamins of vitamin A which are present in plants. These are precursors of vitamin A which are cleaved to yield retinal retinaldehyde and then retinol and retinoic acid. The most important carotenoid is b-carotene, which consists of two molecules of vitamin A retinal joined end to end. 