Biomolecules for JEE Main 2026: Carbs, Proteins & DNA

Biomolecules is a two-to-three question chapter in JEE Main Chemistry that rewards memorisation of well-organised facts. It tests carbohydrate classification and reactions, amino acid properties, protein structure levels, enzyme characteristics, and nucleic acid base pairing. The questions are predictable and fact-based — no calculation is required. A student who organises the key facts into a compact reference can guarantee full marks here in under a week of focused revision.

Carbohydrates: Classification and Key Reactions

Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that hydrolyse to give them). Monosaccharides (glucose, fructose, galactose) cannot be hydrolysed further. Disaccharides hydrolyse to give two monosaccharides: maltose → glucose + glucose; sucrose → glucose + fructose; lactose → glucose + galactose. Polysaccharides: starch = amylose (linear, α-glucose) + amylopectin (branched, α-glucose); cellulose = linear, β-glucose (the β-linkage makes it indigestible for humans); glycogen = animal starch, highly branched.

Glucose reactions JEE tests: with HNO₃ → saccharic acid (confirms CHO and COOH groups); with HCN → cyanohydrin; with hydroxylamine → oxime; with Tollens' or Fehling's reagent → positive result only for reducing sugars (glucose, fructose, maltose, lactose are all reducing; sucrose is non-reducing). The distinction between reducing and non-reducing sugars is tested directly. Take a free biomolecules mock to check your classification speed.

Amino Acids and Proteins

Amino acids have a central carbon bearing an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group. At physiological pH, amino acids exist as zwitterions (both –NH₃⁺ and –COO⁻ present). Essential amino acids cannot be synthesised by the body and must come from diet — valine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan (mnemonic: "Very Little Is Liked, Most Prefer Them"). Glycine is the simplest amino acid with no chiral centre; all others (except glycine) are L-form in proteins.

Protein structural levels: primary (sequence of amino acids linked by peptide bonds); secondary (α-helix or β-pleated sheet from hydrogen bonding); tertiary (3D folding from disulphide bonds, hydrophobic interactions, hydrogen bonds, ionic interactions); quaternary (association of two or more polypeptide chains — haemoglobin is a classic example). Denaturation disrupts secondary and tertiary structure without breaking primary structure (peptide bonds remain intact). Enzymes are biological catalysts with high specificity, optimal pH, and temperature — denaturation causes irreversible loss of activity.

Nucleic Acids and Vitamins

DNA has the double helix structure with base pairs: Adenine–Thymine (2 H-bonds) and Guanine–Cytosine (3 H-bonds). RNA has Uracil instead of Thymine. The sugar in DNA is deoxyribose; in RNA it is ribose. The backbone consists of alternating sugar-phosphate units with bases pointing inward. Three types of RNA: mRNA (carries genetic information), tRNA (brings amino acids to ribosomes), rRNA (structural component of ribosomes).

Vitamins: water-soluble (B complex, C) and fat-soluble (A, D, E, K). The mnemonic ADEK covers fat-soluble vitamins. Deficiency diseases JEE tests: Vitamin A → night blindness; Vitamin B₁ → beriberi; Vitamin B₁₂ → pernicious anaemia; Vitamin C → scurvy; Vitamin D → rickets; Vitamin K → blood clotting disorder. JEE typically asks one deficiency-disease question per session. For the complete organic chemistry context, see our polymers guide and our chemistry in everyday life guide.