Biomolecules form the backbone of NEET Chemistry's organic section, accounting for 8-12 marks in the exam. This chapter integrates structure, function, and biological significance—making it essential to understand the why behind molecular arrangements, not just memorize formulas. Master proteins, carbohydrates, and nucleic acids with exam-focused strategies that work.
Carbohydrates are polhydroxy aldehydes or ketones with the general formula Cn(H2O)m. NEET consistently tests classification, structural differences, and functional properties across monosaccharides, disaccharides, and polysaccharides.
Monosaccharides are the fundamental units of carbohydrate chemistry. Glucose and fructose (C6H12O6) are constitutional isomers—glucose is an aldohexose while fructose is a ketohexose. NEET questions frequently ask about Fischer and Haworth projections, the difference between α and β anomers, and the mechanism of mutarotation. The key distinction: glucose exists in both open-chain (aldehydic) and closed-chain (pyranose ring) forms, and the equilibrium between these forms explains its reducing sugar properties.
Know these critical points: Glucose forms a 6-membered pyranose ring; the C1 hydroxyl group determines anomeric configuration (α-glucose has OH below the ring in Haworth, β-glucose has OH above). The C2, C3, C4 configurations determine D/L nomenclature.
Disaccharides are formed by glycosidic bond linkages between two monosaccharides. Sucrose (glucose + fructose via 1,2-glycosidic bond) is a non-reducing sugar because the anomeric carbons of both units participate in bonding. Maltose and lactose have free anomeric carbons, making them reducing sugars. NEET tests this distinction repeatedly—always check which carbons form the glycosidic bond.
For polysaccharides, focus on starch, cellulose, and glycogen. Starch (amylose + amylopectin) contains α-1,4 and α-1,6 linkages; cellulose contains β-1,4 linkages only. This single difference explains why cellulose is indigestible to humans while starch is readily metabolized. Glycogen is the animal storage form with more 1,6 branch points than starch, allowing rapid glucose mobilization.
Proteins are polymers of amino acids linked by peptide bonds. NEET's protein section combines biochemistry with organic chemistry, requiring understanding of amino acid structure, peptide bond formation, and protein hierarchy (primary, secondary, tertiary, quaternary structures). This is high-yield material accounting for 5-8 marks.
All amino acids contain a carboxyl group (−COOH), an amino group (−NH2), a hydrogen, and an R group attached to the same carbon (α-carbon). The R group determines the amino acid's identity and chemical properties. Glycine has H as the R group (simplest amino acid); lysine has a long chain with a terminal amino group (basic); aspartic acid has a carboxyl group (acidic).
The peptide bond forms between the carboxyl carbon of one amino acid and the amino nitrogen of the next, releasing water (condensation reaction). This bond is planar due to partial double-bond character from resonance, restricting rotation around the C−N bond. NEET frequently asks about peptide bond characteristics: its planarity, partial double-bond nature, and the directionality (N-terminus to C-terminus). Know that the peptide bond angle is ~120° (sp2 hybridization of nitrogen).
Primary structure is the amino acid sequence; it's determined by mRNA codons and is unique per protein. Secondary structure involves hydrogen bonding between backbone atoms: α-helices and β-pleated sheets are the main types. α-helix forms when C=O of residue n bonds with N−H of residue n+4; it's stabilized by 3.6 residues per turn and has a pitch of 5.4 Å. β-sheets form when extended chains hydrogen bond laterally.
Tertiary structure is the 3D folding of the entire polypeptide, stabilized by interactions between R groups: hydrogen bonding (polar residues), disulfide bridges (between cysteine residues), ionic bonds (between charged residues), and hydrophobic interactions (nonpolar residues cluster internally). Quaternary structure applies only to multi-subunit proteins like hemoglobin—it describes how individual polypeptide chains associate.
Proteins are classified by function (enzymes, transport, structural, defense), solubility (fibrous vs. globular), and composition (simple proteins containing only amino acids vs. conjugated proteins with non-protein prosthetic groups). Hemoglobin is conjugated (contains heme group with Fe); myoglobin is similar but has one subunit instead of four. Collagen is fibrous and structural; keratin is fibrous. Enzymes are globular and catalytic.
Nucleic acids store and transmit genetic information via sequences of nucleotides. NEET covers DNA and RNA structure, Chargaff's rules, base pairing, and functional differences. This section tests conceptual understanding alongside structural knowledge, accounting for 4-6 exam marks.
Nucleotides consist of three components: a pentose sugar (ribose in RNA, deoxyribose in DNA), a nitrogenous base (purine or pyrimidine), and a phosphate group. The key difference: ribose has a hydroxyl group (−OH) at the 2' carbon; deoxyribose has a hydrogen (−H). This seemingly small difference makes RNA chemically reactive (the 2'−OH participates in hydrolysis) while DNA is more stable.
Bases are either purines (adenine, guanine—double-ring structures) or pyrimidines (cytosine, thymine, uracil—single-ring structures). DNA contains A, G, C, T; RNA contains A, G, C, U. The glycosidic bond connects the nitrogenous base to the 1' carbon of the sugar; the phosphodiester bond links nucleotides via the 3'−OH and 5'−phosphate of adjacent sugars, creating the sugar-phosphate backbone with bases projecting sideways.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose (−H at 2'C) | Ribose (−OH at 2'C) |
| Bases | A, G, C, T | A, G, C, U |
| Structure | Double helix (B-form most common) | Usually single-stranded |
| Stability | Highly stable | Unstable (2'−OH makes it labile) |
| Function | Information storage | Information transfer, catalysis |
DNA forms a double helix stabilized by hydrogen bonds between complementary base pairs: adenine (A) pairs with thymine (T) via 2 hydrogen bonds; guanine (G) pairs with cytosine (C) via 3 hydrogen bonds. The two strands are antiparallel (one runs 5'→3', the other 3'→5'), with the bases stacking inside and the sugar-phosphate backbone on the outside.
Chargaff's rules follow