On this page
Vitamins take up two pages in the Class 12 Biomolecules chapter, so they are easy to skip. They should not be. The syllabus asks for their classification and functions, and the questions are quick marks if you know a short list well and costly if you mix up two entries. This post covers what a vitamin is, why some dissolve in fat and others in water, the NCERT deficiency table and a way to revise it in one sitting.
What do the syllabi ask about vitamins?
Both the JEE (Main) 2026 syllabus and the NEET (UG)-2026 syllabus list "VITAMINS – Classification and functions" in the Biomolecules unit. The CBSE Chemistry curriculum for 2026-27 includes vitamins in Unit 10. The Biomolecules section of the JEE (Advanced) 2026 brochure covers carbohydrates, proteins and nucleic acids and does not name vitamins. Everything below comes from section 10.4 of the NCERT Class 12 chapter Biomolecules. Check the syllabus for your own exam year.
What exactly is a vitamin?
NCERT defines vitamins as organic compounds needed in the diet in small amounts to perform specific biological functions, whose absence causes specific diseases. They belong to different chemical classes, so they cannot be defined by structure. Most cannot be made in our bodies, though plants can make almost all of them and gut bacteria make some.
The name has a history examiners like. NCERT explains that the word "vitamine" came from "vital amine", because the first compounds found had amino groups. Most later ones did not, so the final "e" was dropped. The early work was recognised with the 1929 Nobel Prize in Physiology or Medicine, shared by Christiaan Eijkman "for his discovery of the antineuritic vitamin" and Frederick Gowland Hopkins "for his discovery of the growth-stimulating vitamins". The chemistry followed: the 1937 Nobel Prize in Chemistry went to Walter Haworth for work on carbohydrates and vitamin C, and to Paul Karrer for work on carotenoids, flavins and vitamins A and B2.
How are vitamins classified?
NCERT uses solubility.
Fat-soluble: A, D, E and K. They dissolve in fats and oils but not in water, and are stored in the liver and adipose (fat-storing) tissue.
Water-soluble: the B group and C. They are readily excreted in urine and cannot be stored, so they must be supplied regularly in the diet. The exception is vitamin B12, which can be stored.
A memory hook: the fat-soluble ones spell "DEKA" (D, E, K, A). Everything else is water-soluble.
Why does structure decide solubility?
Water dissolves molecules that can hydrogen-bond with it, so molecules with many –OH groups and a high share of oxygen tend to be water-soluble. Long hydrocarbon chains dissolve in fats instead. You can show the difference with a quick calculation.
Worked example: oxygen by mass. Use C = 12.011, H = 1.008 and O = 15.999.
Vitamin C (ascorbic acid) is C6H8O6. Its molar mass is 6(12.011) + 8(1.008) + 6(15.999) = 72.066 + 8.064 + 95.994 = 176.12 g/mol, which matches PubChem's entry for ascorbic acid.
Oxygen share: 95.994 / 176.12 = 0.545, or 54.5%.
Vitamin A as retinol is C20H30O, with a molar mass of 286.5 g/mol according to PubChem's entry for retinol.
Oxygen share: 15.999 / 286.5 = 0.056, or 5.6%.
Retinol is a 20-carbon molecule with a single –OH group, so it behaves like a fat. Vitamin C is small and dense with oxygen, so it dissolves in water. Composition does not prove solubility, but it is a good first check.
Which deficiency goes with which vitamin?
These are NCERT's Table 10.3 entries:
A: xerophthalmia (hardening of the cornea) and night blindness. Sources: fish liver oil, carrots, butter, milk.
B1 (thiamine): beri beri. Sources: yeast, milk, green vegetables, cereals.
B2 (riboflavin): cheilosis (fissuring at the corners of the mouth and lips), digestive disorders and burning sensation of the skin.
B6 (pyridoxine): convulsions.
B12: pernicious anaemia. Sources: meat, fish, egg, curd.
C (ascorbic acid): scurvy (bleeding gums). Sources: citrus fruits, amla, green leafy vegetables.
D: rickets in children and osteomalacia in adults. Sources: sunlight, fish, egg yolk.
E: increased fragility of red blood cells and muscular weakness. Sources: vegetable oils such as wheat germ and sunflower oil.
K: increased blood clotting time. Source: green leafy vegetables.
This is exam chemistry, not health advice. NCERT itself notes that excess vitamins are also harmful and that vitamin pills should not be taken without a doctor's advice.
Which mistakes cost marks?
Forgetting the B12 exception. It is water-soluble but can be stored.
Reversing vitamin K. A deficiency increases clotting time, so blood clots more slowly.
Mixing up B1 and B2. Beri beri is B1; cheilosis is B2.
Calling every vitamin an amine. The name is historical; most have no amino group.
Writing "vitamin D comes only from food". NCERT lists exposure to sunlight first.
For the rest of the chapter, read our post on sugars, proteins and DNA in Biomolecules, and for proteins in more depth, protein structure from primary to quaternary.
A one-sitting revision routine
Write "DEKA" and list the fat-soluble vitamins. Write the rest as water-soluble and mark B12.
Cover NCERT's table and write each vitamin's deficiency disease from memory. Check, and circle the ones you missed.
Write one source for each vitamin without looking.
Redo the oxygen calculation for any formula your teacher gives you and predict its solubility.
The next day, repeat step 2 only for the circled entries.
If Biomolecules feels like a list without logic, it helps to see the chemistry behind it. The Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can connect hydrogen bonding and polarity to this chapter.
Start now: cover the list above and write all nine deficiency diseases from memory.
