Biomolecules is one of the most scoring chapters of Class 12 Chemistry. Questions come every year in the CBSE board exam, and the same chapter is asked in NEET. These Biomolecules Class 12 notes cover carbohydrates, proteins, enzymes, vitamins, nucleic acids and hormones in simple language, with the points examiners actually ask.
What are biomolecules?
Biomolecules are the chemical compounds made by living cells to run life processes. The four main groups you must know are carbohydrates, proteins, nucleic acids and lipids. Vitamins and hormones are also studied in this chapter because the body needs them in small amounts to work properly.
Carbohydrates
Carbohydrates are compounds of carbon, hydrogen and oxygen, mostly with the formula Cx(H2O)y. They are classified by how many sugar units they give on hydrolysis.
- Monosaccharides: single sugar units that cannot be broken further, like glucose and fructose.
- Oligosaccharides: 2 to 10 units. Sucrose (glucose + fructose), maltose (glucose + glucose) and lactose (glucose + galactose) are the exam favourites.
- Polysaccharides: long chains of many units, like starch, cellulose and glycogen.
Reducing vs non-reducing sugars: a sugar with a free aldehyde or ketone group reduces Fehling and Tollens reagents. Glucose, maltose and lactose are reducing; sucrose is non-reducing because its reactive groups are locked in the bond between glucose and fructose. Also remember: starch is the plant storage carbohydrate, glycogen is the animal storage carbohydrate, and cellulose builds plant cell walls.
Proteins
Proteins are polymers of alpha-amino acids joined by peptide bonds (the -CO-NH- link formed when the acid group of one amino acid reacts with the amine group of the next, releasing water). In water, an amino acid exists as a zwitter ion — a single molecule carrying both a positive and a negative charge.
Protein structure has four levels: primary (the sequence of amino acids), secondary (folding into an alpha-helix or beta-pleated sheet held by hydrogen bonds), tertiary (the full 3-D folding) and quaternary (two or more chains packed together). Denaturation means the higher structures break on heating or pH change while the primary sequence stays — like boiling an egg or curdling of milk. Fibrous proteins (keratin in hair) are thread-like and water-insoluble; globular proteins (albumin, insulin) are round and usually soluble.
Enzymes
Enzymes are biological catalysts, and almost all of them are globular proteins. Two points matter for exams: they are highly specific (one enzyme usually speeds up one reaction) and they work best at body temperature and a particular pH. Example: maltase converts maltose into glucose.
Vitamins
Vitamins are needed in tiny amounts, and their absence causes deficiency diseases. Learn this table by heart.
| Vitamin | Type | Deficiency disease |
|---|---|---|
| A | Fat soluble | Night blindness |
| B1 | Water soluble | Beri-beri |
| B12 | Water soluble | Pernicious anaemia |
| C | Water soluble | Scurvy (bleeding gums) |
| D | Fat soluble | Rickets |
| E | Fat soluble | Muscular weakness, fertility problems |
| K | Fat soluble | Poor blood clotting |
Fat-soluble vitamins (A, D, E, K) are stored in the body; water-soluble ones (B group and C) are not stored, so we need them regularly in food.
Nucleic acids: DNA and RNA
Nucleic acids are polymers of nucleotides. Each nucleotide = a nitrogen base + a pentose sugar + a phosphate group. DNA has deoxyribose sugar and the bases A, G, C, T; it is double-stranded (the famous double helix) and stores hereditary information. RNA has ribose sugar and the bases A, G, C, U; it is usually single-stranded and helps in protein synthesis. In DNA, A pairs with T and G pairs with C through hydrogen bonds.
Hormones
Hormones are chemical messengers made by glands and carried by blood. Examples to quote: insulin (controls blood sugar; its lack causes diabetes), thyroxine (needs iodine; its lack causes goitre) and adrenaline (the emergency hormone).
Quick revision points
- Sucrose = glucose + fructose; it is a non-reducing sugar.
- Peptide bond = -CO-NH-; amino acids exist as zwitter ions.
- Denaturation destroys secondary and tertiary structure, not primary.
- A-T and G-C pairing in DNA; RNA has U in place of T.
- Scurvy = vitamin C, rickets = vitamin D, night blindness = vitamin A.
FAQs on Biomolecules Class 12
Is Biomolecules important for boards and NEET?
Yes. CBSE asks direct questions worth 3 to 5 marks from it, and NEET asks 1 to 2 questions almost every year, mostly from vitamins, sugars and DNA vs RNA.
What is the difference between DNA and RNA?
DNA has deoxyribose sugar, bases A G C T, and is double-stranded. RNA has ribose sugar, bases A G C U, and is usually single-stranded.
Why is sucrose called a non-reducing sugar?
Because the aldehyde group of glucose and the ketone group of fructose are both used up in the bond joining them, so no free reducing group is left.
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