Biomolecules are the organic molecules that form the chemical basis of all living organisms. For NEET 2026, the four major classes — carbohydrates, proteins, vitamins, and nucleic acids — are tested with 2–3 direct questions every year. Mastery of this chapter means memorising classifications, structural features, and disease associations with precision.

Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones with the general formula $(CH_2O)_n$. They are classified by the number of monomer units:

• Monosaccharides — single sugar units that cannot be hydrolysed further. Glucose ($C_6H_{12}O_6$, SMILES: OC[C@@H](O)[C@H](O)[C@@H](O)[C@@H](O)C=O) is an aldohexose (aldehyde group + 6 carbons). Fructose ($C_6H_{12}O_6$) is a ketohexose (ketone group + 6 carbons). Both are isomers but structurally distinct.
• Disaccharides — formed by joining two monosaccharides via a glycosidic bond with loss of water. Sucrose (glucose + fructose, $\alpha$-1,2 linkage) is the only common non-reducing disaccharide because the glycosidic bond involves the anomeric carbons of both units. Lactose (glucose + galactose, $\beta$-1,4) and maltose (glucose + glucose, $\alpha$-1,4) are reducing because one anomeric carbon remains free.
• Polysaccharides — many monosaccharide units linked together. Starch consists of amylose (linear, $\alpha$-1,4 linkages) and amylopectin (branched: $\alpha$-1,4 main chain + $\alpha$-1,6 at branch points). Cellulose has $\beta$-1,4 linkages — humans cannot digest it because we lack cellulase. Glycogen (animal starch) is like amylopectin but more highly branched.

The cyclic forms of glucose (pyranose, 6-membered ring) exist as two anomers: $\alpha$-D-glucose (C1-OH axial/pointing down in Haworth) and $\beta$-D-glucose (C1-OH equatorial/pointing up). Interconversion between the two via the open-chain form is called mutarotation.

Proteins

Amino acids are the building blocks of proteins. Their general structure is H$_2$N–CHR–COOH (SMILES for glycine: NCC(=O)O). At the isoelectric point (pI), the amino acid exists as a zwitterion: $^+H_3N$–CHR–COO$^-$, with no net charge and no migration in an electric field. Glycine is the only amino acid without a chiral centre.

The peptide bond (–CO–NH–) is a covalent amide bond formed by condensation between the –COOH of one amino acid and the –NH$_2$ of another, releasing water. It has partial double-bond character (restricted rotation). Proteins are organised into four structural levels:

1. Primary — the linear sequence of amino acids held by peptide bonds (covalent).
2. Secondary — local folding patterns. The $\alpha$-helix is stabilised by intramolecular hydrogen bonds between the C=O of residue $n$ and the N–H of residue $n+4$. The $\beta$-pleated sheet is stabilised by intermolecular hydrogen bonds between adjacent polypeptide chains.
3. Tertiary — the overall 3D shape of a single polypeptide chain, stabilised by disulfide bonds (–S–S–), hydrophobic interactions, ionic bonds, and hydrogen bonds. Example: myoglobin.
4. Quaternary — the assembly of two or more polypeptide subunits. Example: haemoglobin (4 subunits: 2$\alpha$ + 2$\beta$).

Denaturation disrupts secondary, tertiary, and quaternary structures by breaking hydrogen bonds, disulfide bonds, etc., but it does not break peptide bonds. The primary structure (amino acid sequence) is preserved.

Vitamins

Vitamins are organic micronutrients required in small quantities. The NEET-critical divide is water-soluble vs fat-soluble:

• Fat-soluble (ADEK): A (retinol — night blindness, xerophthalmia), D (calciferol — rickets in children, osteomalacia in adults), E (tocopherol — infertility), K (phylloquinone — delayed blood clotting).
• Water-soluble (B-complex + C): B1/thiamine (beriberi), B2/riboflavin (cheilosis), B3/niacin (pellagra), B6/pyridoxine (convulsions), B9/folic acid (megaloblastic anaemia), B12/cyanocobalamin (pernicious anaemia), C/ascorbic acid (scurvy).

Nucleic Acids

DNA is a right-handed double helix with complementary antiparallel strands. Base pairing: A–T (2 hydrogen bonds) and G–C (3 hydrogen bonds). Higher G–C content → higher melting temperature ($T_m$). DNA uses deoxyribose sugar; RNA uses ribose and replaces thymine with uracil. Three types of RNA: mRNA (carries genetic code), tRNA (cloverleaf, delivers amino acids), rRNA (ribosome structure).

The single most-tested NEET facts from this chapter: (1) sucrose is non-reducing because both anomeric carbons are locked in the glycosidic bond; (2) G–C has 3 hydrogen bonds; (3) denaturation does not break peptide bonds.