NEET Chem — Biomolecules

Chapter Overview & Weightage

Biomolecules is a high-volume memorisation chapter that delivers 2-3 NEET questions per paper. The marks are easy if you’ve drilled carbohydrate classifications, amino acid types, vitamin deficiencies, and DNA/RNA structure.

Key Concepts You Must Know

Carbohydrates

• Monosaccharides: glucose (aldohexose), fructose (ketohexose), ribose (aldopentose), 2-deoxyribose.
• Disaccharides: sucrose (glucose + fructose, non-reducing), maltose (glucose + glucose, reducing), lactose (glucose + galactose, reducing).
• Polysaccharides: starch (amylose + amylopectin, plants), glycogen (animals), cellulose (plant cell wall, β-1,4 linkage).
• Reducing vs non-reducing: free anomeric carbon → reducing. Sucrose has both anomeric carbons in the glycosidic bond → non-reducing.

Amino Acids and Proteins

• 20 standard amino acids, all α-amino acids in proteins. Essential: 9 (must come from diet) — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine.
• Zwitterion: at isoelectric pH, amino acid exists as $\text{NH}_3^+\text{-CH(R)-COO}^-$.
• Peptide bond: amide linkage between $-\text{COOH}$ and $-\text{NH}_2$ of adjacent amino acids.
• Protein structure: primary (sequence), secondary (α-helix, β-sheet via H-bonds), tertiary (3D folding via S-S, H-bonds, ionic, hydrophobic), quaternary (multi-subunit, e.g., haemoglobin).
• Denaturation: loss of secondary, tertiary, quaternary structure (heat, pH, urea). Primary intact.

Nucleic Acids

• DNA: 2-deoxyribose, bases A-T-G-C, double helix.
• RNA: ribose, bases A-U-G-C, single strand (mostly).
• Chargaff’s rule (DNA): A = T, G = C, A + G = T + C (purine = pyrimidine).
• Watson-Crick: B-form DNA, right-handed, 10 base pairs per turn, 3.4 Å per base pair, 34 Å per turn, diameter 20 Å.
• mRNA, tRNA, rRNA — three RNA types in protein synthesis.

Vitamins

• Fat-soluble: A (retinol, night blindness), D (calciferol, rickets), E (tocopherol, sterility), K (clotting).
• Water-soluble: B-complex (B₁ thiamine, B₂ riboflavin, B₃ niacin, B₅, B₆ pyridoxine, B₇ biotin, B₉ folic acid, B₁₂ cobalamin), C (ascorbic acid, scurvy).

Enzymes

• Active site: small region where substrate binds.
• Specificity: lock-and-key vs induced-fit models.
• Inhibitors: competitive (compete with substrate) vs non-competitive.

Important Frameworks

Solved Previous Year Questions

PYQ 1 (NEET 2024)

Which of the following is a non-reducing sugar? (a) Maltose (b) Lactose (c) Sucrose (d) Glucose

Solution: (c) Sucrose. Both anomeric carbons (one from glucose, one from fructose) are involved in the glycosidic bond, leaving no free hemiacetal/hemiketal. So no reducing activity.

PYQ 2 (NEET 2023)

The vitamin associated with the deficiency disease scurvy is:

Solution: Vitamin C (ascorbic acid). Memorise the disease-vitamin pairs:

• Vitamin A → night blindness
• Vitamin B₁ → beri-beri
• Vitamin B₃ → pellagra
• Vitamin C → scurvy
• Vitamin D → rickets (children), osteomalacia (adults)
• Vitamin K → clotting issues

PYQ 3 (NEET 2022)

In which structure of protein is the α-helix found?

Solution: Secondary structure. α-helix and β-pleated sheet are the two main secondary structure motifs, stabilised by H-bonds along the peptide backbone.

Difficulty Distribution

• Easy (60%): Direct identification (which is reducing/non-reducing, which is essential amino acid, which vitamin causes which disease).
• Medium (30%): Distinguishing α vs β anomers, structure-function questions on proteins/DNA.
• Hard (10%): Multi-concept questions linking biomolecules to enzyme mechanism or genetic code.

Expert Strategy

Common Traps