Biomolecules

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your JEE Advanced exam.

Biomolecules are organic compounds that form the basis of life. The four major classes are carbohydrates, proteins, lipids, and nucleic acids. Each has a distinct structure and biological function.

Carbohydrates:

Carbohydrates have the general formula $C_n(H_2O)_n$ (literally “hydrates of carbon”). They are classified as:

• Monosaccharides: single sugar units (glucose, fructose, ribose). Simplest carbohydrates — cannot be hydrolysed further.
• Disaccharides: two monosaccharides joined by a glycosidic bond (sucrose = glucose + fructose; maltose = glucose + glucose; lactose = glucose + galactose).
• Polysaccharides: long chains of monosaccharides (starch, cellulose, glycogen).

Glucose ($C_6H_{12}O_6$):

• Open-chain form has an aldehyde group (aldose) and 5 hydroxyl groups
• In solution, it exists mostly (>99%) as the cyclic hemiacetal form: $\alpha$-D-glucopyranose or $\beta$-D-glucopyranose
• The cyclic form has 5-membered ring (furanose) or 6-membered ring (pyranose)
• Most common is $\beta$-D-glucopyranose — all OH groups equatorial (most stable chair conformation)

Key Reactions of Glucose:

1. Molisch test: purple ring with $\alpha$-naphthol + conc. $H_2SO_4$ — general test for carbohydrates
2. Fehling’s test: blue Fehling’s solution $\to$ red $Cu_2O$ precipitate with aldehyde group (reducing sugars)
3. Tollens’ test: ammoniacal $AgNO_3$ $\to$ silver mirror with aldehyde group
4. Osazone formation: with phenylhydrazine — glucose gives yellow crystalline osazone with specific melting point (anomers give same osazone)
5. Glucose does NOT show mutarotation in the cyclic form alone (but open-chain ↔ cyclic interconversion causes overall mutarotation)

⚡ JEE Advanced exam tips:

• Sucrose is NON-reducing (glycosidic bond is between anomers of both units — no free aldehyde or ketone)
• Maltose and lactose are REDUCING sugars (the second glucose/galactose unit has a free anomeric carbon)
• Starch consists of amylose (linear, $\alpha$-1,4 links, 20-30%) and amylopectin (branched, $\alpha$-1,4 main chain + $\alpha$-1,6 branches, 70-80%)
• Cellulose has $\beta$-1,4 links — humans lack the enzyme to hydrolyse it (we cannot digest cellulose = dietary fibre)

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🟡 Standard — Regular Study (2d–2mo)

For JEE Advanced students who want genuine understanding.

Amino Acids and Proteins:

Amino acids have both an amino group ($-NH_2$) and a carboxylic acid group ($-COOH$). The general structure: $H_2N-CH(R)-COOH$.

Classification of Amino Acids:

• Acidic: Asp (D), Glu (E) — second COOH
• Basic: Lys (K), Arg (R), His (H) — second $NH_2$
• Neutral: most others (one $NH_2$, one $COOH$)

At physiological pH (~7.4), amino acids exist as zwitterions: $H_3N^+-CH(R)-COO^-$.

Essential Amino Acids (must be obtained from diet in humans): Valine, Threonine, Tryptophan, Methionine, Histidine, Phenylalanine, Leucine, Isoleucine, Lysine. (VTT TIM HILL — mnemonic.)

Peptide Bond Formation: When two amino acids join: $H_2N-CH(R_1)-COOH + H_2N-CH(R_2)-COOH \to H_2N-CH(R_1)-CONH-CH(R_2)-COOH + H_2O$. The amide bond formed is the peptide bond — it has partial double bond character (C-N bond length ~1.33 Å, between single and double).

Protein Structure:

• Primary: linear sequence of amino acids joined by peptide bonds
• Secondary: local folding (α-helix, β-pleated sheet) stabilised by hydrogen bonds between $C=O$ and $N-H$ groups of peptide bonds
• Tertiary: 3D shape of the entire polypeptide chain (hydrophobic interactions, disulphide bridges, ionic bonds)
• Quaternary: association of multiple polypeptide subunits

Lipids:

Lipids are a diverse group of biological molecules that are hydrophobic or amphipathic. They include:

• Fats and oils (triglycerides): glycerol + 3 fatty acids via ester bonds
• Phospholipids: glycerol + 2 fatty acids + phosphate + amino alcohol (e.g., phosphatidylcholine = lecithin) — form cell membranes
• Steroids: four fused carbon rings (cholesterol, testosterone, oestrogen)

Saturated vs. Unsaturated Fatty Acids:

• Saturated: no C=C bonds (butter, ghee) — higher melting point
• Unsaturated: C=C bonds (vegetable oils) — liquid at room temperature
• Trans fats: artificially hydrogenated vegetable oils — associated with cardiovascular disease

Nucleic Acids (DNA and RNA):

DNA: double helix; sugar = deoxyribose; bases = A, G, C, T (thymine); antiparallel strands; major groove and minor groove.

RNA: single-stranded (usually); sugar = ribose; bases = A, G, C, U (uracil); several types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).

Chargaff’s Rule: In DNA, number of purine bases = number of pyrimidine bases. A = T, G = C. This was key to determining the double helix structure.

⚡ Common student mistakes:

1. Thinking all carbohydrates are sweet — cellulose is a carbohydrate but not sweet
2. Confusing the direction of alpha helix vs. beta sheet hydrogen bonding
3. Forgetting that both DNA strands are held together by hydrogen bonds (2 between A-T, 3 between G-C)
4. Mixing up the number of ester bonds in a triglyceride (3 ester bonds)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for JEE Advanced mastery of biomolecules.

Mutarotation:

The change in optical rotation when $\alpha$-D-glucose is dissolved in water. $\alpha$-D-glucose has specific rotation $[\alpha]_D = +112°$, $\beta$-D-glucose has $[\alpha]_D = +18.7°$. In water, the solution reaches equilibrium at $[\alpha]_D = +52.7°$. This occurs because the open-chain form interconverts between $\alpha$ and $\beta$ forms through the open-chain aldehyde intermediate.

Enantiomers in Biomolecules:

All amino acids in proteins (except glycine) have the L-configuration (with $NH_2$ on the left in Fischer projection). This is not the same as (+)/(-) optical activity — l-/d- notation is absolute configuration, (+)/(-) is the sign of optical rotation. Most natural amino acids are l- but some D-forms exist in bacterial cell walls and certain antibiotics.

Glucose is D-series (in Fischer projection, the bottom OH is on the right). Fructose from sucrose is D-fructose (but in furanose form, the configuration is different).

DNA Double Helix — The Watson-Crick Model:

The double helix was proposed by Watson and Crick in 1953, based on X-ray diffraction data from Rosalind Franklin.

Key features:

• Two antiparallel strands (one $5’ \to 3’$, the other $3’ \to 5’$)
• Right-handed helix
• Base pairing: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds)
• Helix completes one turn every 10.5 base pairs (~34 Å)
• Distance between adjacent base pairs = 3.4 Å
• Diameter = 20 Å

The stability of DNA double helix comes from:

1. Hydrogen bonds (specificity: A-T, G-C)
2. Base stacking interactions (hydrophobic and van der Waals — this is the major contributor to stability)

Vitamin Cofactors:

Many coenzymes are derived from vitamins:

• NAD⁺ (from niacin/vitamin B3): electron carrier in redox reactions
• FAD/FMN (from riboflavin/vitamin B2): electron carrier
• Coenzyme A (from pantothenic acid/vitamin B5): acetyl group carrier
• Thiamine pyrophosphate (from thiamine/vitamin B1): decarboxylation

Enzyme Kinetics — Michaelis-Menten:

$$v = \frac{V_{max}[S]}{K_M + [S]}$$

Where $V_{max} = k_{cat}[E]{total}$ and $K_M$ is the substrate concentration at which $v = V{max}/2$. $k_{cat}$ (turnover number): number of substrate molecules converted per enzyme per second. For carbonic anhydrase: $k_{cat} = 10^6$ s⁻¹ (one of the fastest known enzymes).

Glycolysis — Key Steps:

Glucose $\to$ 2 pyruvate (in cytoplasm):

1. Hexokinase: glucose $\to$ glucose-6-phosphate (uses 1 ATP)
2. Phosphofructokinase: F6P $\to$ FBP (uses 1 ATP) — rate-limiting step
3. Aldolase: FBP $\to$ G3P + DHAP (reversible)
4. Triose phosphate isomerase: DHAP $\to$ G3P
5. GAP dehydrogenase: G3P $\to$ 1,3-BPG + NADH
6. Phosphoglycerate kinase: 1,3-BPG $\to$ 3PG (produces 1 ATP by substrate-level phosphorylation)
7. Pyruvate kinase: PEP $\to$ pyruvate (produces 1 ATP)

Net: 2 ATP invested, 4 ATP produced, 2 NADH produced. Net gain: 2 ATP, 2 NADH per glucose.

JEE Advanced Previous Year Patterns:

• Carbohydrate classification: common
• Reducing vs. non-reducing sugars: very common
• Amino acid classification and zwitterion: common
• Protein structure levels: common
• DNA/RNA structure: periodic
• Enzyme kinetics: occasionally tested

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