Q.1. Define vitamins.
Ans. Vitamins have been defined as organic compounds occurring in natural foods either as such or as utilizable “precursors”, which are required in minute amounts for normal growth, maintenance and reproduction, for normal nutrition and health.
Q.2. How the name vitamin derived?
Ans. The first vitamin isolated from rice polishing was found to be an “amine” and as it was found to be vital for growth, it was called as “vitamine”, Later on “e” was omitted.
Q.3. How will you classify vitamins?
Ans. Vitamins are classified according to its solubility into two major groups:
- Fat soluble vitamins, and
- Water soluble vitamins.
Q.4. Name the fat soluble vitamins.
Ans. Fat soluble vitamins are:
- Vitamin A, Vitamin D, Vitamin E, and Vitamin K.
Q.5. What are retinoids?
Ans. Retinoids are three forms of vitamin A:
- Vitamin A alcohol (retinol)
- Vitamin A aldehyde (retinal or retinene)
- Vitamin A acid (retinoic acid)
Q.6. What is the structural characteristics of retinoids?
Ans.
- All the three compounds contain as common structural unit:
- A trimethyl cyclohexenyl ring (β-ionone ring), and
- An all “trans” configurated polyene chain (isoprenoid chain) with 4 double bonds.
Q.7. What are provitamins A?
Ans. Provitamins A : are the carotenoid pigments which are hydrocarbon polyene pigments, yellow or red, widely distributed in nature.
They are:
- α-carotene, β-carotene, γ-carotene, cryptoxanthine and lycopene.
Q.8. Why β-carotene gives 2 mols of vitamin A whereas other carotenoids give only one molecule of vitamin A?
Ans.
- Carotenoids undergo symmetrical oxidative scission to yield vitamin A.
- β-carotene contains two β-ionone rings at both ends connected by 18 carbon hydrocarbon chain. Hence oxidative scission in the middle of the molecule yields 2 mols of vitamin A
- Other carotenoids viz., α, γ and cryptoxanthine contains only one β-ionone ring, so they yield only 1 mole of vitamin A.
Q.9. Where in the body the carotenoids are converted to vitamin A?
Ans. In man, liver is the only organ where carotenes undergo oxidative scission and converted to vitamin A.
Q.10. How retinal and retinoic acid are carried in the blood?
Ans.
- Retinal is transported in the blood bound to a specific retinol binding protein (RBP), and
- Retinoic acid is carried in the blood bound to another specific retinoic binding protein (RBP).
Q.11. What is rhodopsin (visual purple)?
Ans.
- Rhodopsin (visual purple) is a photosensitive pigment present in rod cells of retina responsible for visual acuity in dim light (night vision).
- Chemically it is a conjugated protein with mol wt 40,000. It contains opsin as its apo-protein and retinene (retinal or retinaldehyde) as 11-cis retinal as prosthetic group.
Q.12. What is Wald’s visual cycle” (or rhodopsin cycle)?
Ans.
- When light falls on rhodopsin it is split to form “opsin” + all “trans”-retinal after a series of changes viz. bathorhodopsin → Lumirhodopsin → metarhodopsin I and II, and finally to all transretinal