Biomolecules

Biomolecules

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Biomolecules are organic compounds synthesised by living cells, built primarily from C, H, O, N, S, and P, and grouped into carbohydrates, proteins, nucleic acids, lipids, vitamins, and hormones. The monosaccharide glucose has molecular formula C₆H₁₂O₆; sucrose (a disaccharide) is C₁₂H₂₂O₁₁; starch and cellulose share the empirical unit (C₆H₁₀O₅)ₙ. Proteins are polymers of α-amino acids joined by peptide (C–N) bonds formed via condensation, with twenty standard residues. Nucleic acids are polymers of nucleotides linked by 3′–5′ phosphodiester bonds, each nucleotide carrying a nitrogenous base, pentose sugar, and phosphate. Enzymes are globular protein catalysts obeying Michaelis–Menten kinetics: v = V_max [S] / (K_m + [S]). For CUET UG, focus on the four biomolecule families, their linkages, and identifying reducing vs non-reducing sugars.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Carbohydrates

Polyhydroxy aldehydes or ketones classified as monosaccharides (e.g., glucose, fructose), disaccharides (sucrose, maltose, lactose), and polysaccharides (starch, glycogen, cellulose). A glycosidic bond forms between the anomeric carbon (C1 in aldoses, C2 in ketoses) of one sugar and a hydroxyl of another. Reducing sugars (glucose, maltose, lactose) carry a free anomeric carbon and reduce Fehling’s/Tollens’ reagents; sucrose is non-reducing because both anomeric carbons are locked in the bond. Starch = α-1,4-linked amylose (linear) + α-1,4/α-1,6-branched amylopectin; glycogen is more highly branched; cellulose uses β-1,4 linkages that human α-amylase cannot hydrolyse.

Amino Acids and Proteins

The 20 standard α-amino acids share the backbone H₂N–CHR–COOH. A peptide bond forms by condensation between –COOH of one residue and –NH₂ of the next. Structural hierarchy:

Level	Stabilised by	Description
Primary	Covalent peptide bonds	Linear residue sequence
Secondary	Hydrogen bonds	α-helix, β-pleated sheet
Tertiary	Disulfide bridges, hydrophobic, ionic, H-bonds	3D folding of one chain
Quaternary	Subunit interactions	Assembly of multiple chains

Denaturation disrupts 2°, 3°, and 4° structures — heat, pH change, or heavy metals unfold the protein without hydrolysing peptide bonds; the process is often reversible (e.g., renaturation of ribonuclease).

Nucleic Acids

A nucleotide = nitrogenous base + pentose sugar (ribose in RNA, deoxyribose in DNA) + phosphate. Purines (adenine, guanine) have a fused double ring; pyrimidines (cytosine, thymine, uracil) have one ring. A nucleoside lacks the phosphate. Polymerisation yields 3′–5′ phosphodiester bonds. Chargaff’s rule states A = T and G ≡ C in double-stranded DNA; RNA replaces thymine with uracil.

Lipids and Vitamins

Fatty acids are saturated (no C=C) or unsaturated (one or more C=C). Cis-unsaturated fatty acids have kinks that lower melting points. Triglycerides = glycerol esterified with three fatty acids. Phospholipids are amphipathic and self-assemble into bilayers. Vitamins divide into fat-soluble (A, D, E, K) and water-soluble (B-complex, C); NAD⁺, FAD, and CoA are coenzymes derived from B vitamins.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Enzyme Kinetics and Inhibition

The Michaelis–Menten equation v = V_max [S] / (K_m + [S]) describes a hyperbolic rate–substrate curve. K_m equals the substrate concentration at half-maximal velocity and inversely reflects enzyme–substrate affinity. A Lineweaver–Burk plot (1/v vs 1/[S]) linearises the data, giving slope K_m/V_max and y-intercept 1/V_max. Competitive inhibitors raise apparent K_m without changing V_max; non-competitive inhibitors lower V_max while leaving K_m unchanged. Cofactors are inorganic (e.g., Zn²⁺ in carbonic anhydrase), while coenzymes are organic derivatives such as NAD⁺ (from niacin) carrying hydride equivalents.

Common Mistakes and Traps

• Marking sucrose as a reducing sugar — both anomeric carbons (C1 of glucose, C2 of fructose) are engaged in the glycosidic linkage.
• Assuming humans digest cellulose — we lack the β-1,4-glucosidase (cellulase) that hydrolyses β-linkages; only α-linkages in starch and glycogen are attacked by salivary/pancreatic amylase.
• Claiming denaturation breaks peptide bonds — it only disrupts non-covalent interactions and disulfide bridges; primary structure survives.
• Forgetting that essential amino acids must come from the diet because the human body cannot synthesise their carbon skeletons (e.g., lysine, tryptophan, methionine, threonine, phenylalanine, valine, leucine, isoleucine, histidine — eight to ten in total).

Worked Example

A solution contains an enzyme at 25 °C. When [S] = 2.0 × 10⁻³ M, v = 47 µmol·min⁻¹; when [S] = 2.0 × 10⁻² M, v = 148 µmol·min⁻¹. Applying the Michaelis–Menten equation and solving the simultaneous pair yields V_max ≈ 200 µmol·min⁻¹ and K_m ≈ 6.5 × 10⁻³ M. The Beer–Lambert law A = ε c l is routinely tested alongside enzymes for calculating product concentration from absorbance.

Practice Prompts

1. Compare the structural consequences of replacing α-1,4 glycosidic linkages in starch with β-1,4 linkages as in cellulose.
2. Using a Lineweaver–Burk sketch, predict and justify the curve shift when a competitive inhibitor is added at constant [S].

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