Biomolecules — Carbohydrates, Proteins, Lipids, and Nucleotides
🟢 Lite — Quick Review (1h–1d)
Rapid summary for last-minute revision before your exam.
Biomolecules form the structural and functional basis of all living systems. This chapter covers the four major classes — carbohydrates, amino acids/proteins, lipids, and nucleotides/nucleic acids — with emphasis on structures, classifications, and properties relevant to INI CET.
High-Yield Facts for INI CET:
- Monosaccharides: Glucose (aldohexose), fructose (ketohexose); reducing vs non-reducing sugars (glucose, fructose, galactose are reducing; sucrose is non-reducing)
- Peptide bond: Amide bond between amino acids; planar, partial double-bond character; forms the backbone of proteins
- Amino acids: 20 standard; classified as essential (must get from diet: PVTIMM HLL, FVL, K, M, W) and non-essential; acidic (D, E), basic (R, K, H), neutral non-polar (A, G, V, I, L, M, F, Y, W), neutral polar (S, T, N, Q, C)
- Lipid classification: Simple (fatty acids + glycerol = triglycerides), compound (phospholipids, sphingolipids), derived (cholesterol, steroid hormones)
- Nucleotides: Purines (A, G — double ring) and Pyrimidines (C, T, U — single ring); nucleoside = base + sugar; nucleotide = nucleoside + phosphate
⚡ Exam tip: Sucrose (glucose + fructose) is a non-reducing sugar because the glycosidic bond involves the anomeric carbons of both sugars — no free hemiacetal to reduce copper sulfate. Lactose (glucose + galactose) is reducing because the anomeric carbon of galactose is free.
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
Biomolecules — INI CET (AIIMS PG) Study Guide
Carbohydrates — Classification and Structure
Monosaccharides (simple sugars):
- Aldoses: Contain aldehyde group; aldohexoses (glucose, galactose, mannose) most important
- Ketoses: Contain ketone group; most important ketohexose is fructose
- D/L designation: Based on orientation of the chiral carbon farthest from the carbonyl — D if OH is on the right (most body sugars are D)
- Anomers: α (OH down) vs β (OH up) at the anomeric carbon (C1 in aldoses, C2 in ketoses)
Important Monosaccharides:
- Glucose (dextrose): C6H12O6; primary energy source; blood sugar
- Fructose: Fruit sugar; same formula as glucose; found in sucrose; sweeter than glucose
- Galactose: Component of lactose; epimer of glucose at C4
- Ribose: 5-carbon sugar; part of RNA, ATP, NAD, FAD
- Deoxyribose: 2-deoxyribose; missing oxygen at C2; part of DNA
Disaccharides:
| Disaccharide | Composition | Linkage | Reducing? |
|---|---|---|---|
| Maltose | Glucose + Glucose | α1→4 | Yes |
| Sucrose | Glucose + Fructose | α1→β2 | No |
| Lactose | Glucose + Galactose | β1→4 | Yes |
| Trehalose | Glucose + Glucose | α1→α1 | No |
Polysaccharides:
- Starch: Plant storage (amylose = α1→4 linear; amylopectin = α1→4 + α1→6 branch points)
- Glycogen: Animal storage; highly branched (α1→4 + frequent α1→6 branches); liver and muscle
- Cellulose: β1→4 linkages in plants; humans lack cellulase → cannot digest cellulose
- Chitin: β1→4 N-acetylglucosamine; exoskeletons of arthropods, fungal cell walls
Glycemic Index: Measure of how quickly blood glucose rises after consuming carbohydrates; glucose GI = 100; low GI foods (lentils, whole grains) cause slower, more sustained glucose release
Amino Acids and Protein Structure
Classification of Amino Acids:
By side chain (R group) chemistry:
- Non-polar (hydrophobic): Gly, Ala, Val, Leu, Ile, Met, Phe, Tyr, Trp, Pro
- Polar uncharged: Ser, Thr, Asn, Gln, Cys (thiol)
- Polar acidic (negatively charged): Asp, Glu (carboxylate side chains)
- Polar basic (positively charged): Lys, Arg, His (imidazole ring)
By nutritional requirement:
- Essential: Val, Leu, Ile, Phe, Met, Thr, Trp, Lys (VLLIPM TFW — children also need His)
- Non-essential: All others (synthesized in body)
- Conditionally essential: Arg, Gln, Gly, Pro, Tyr (synthesized but may need supplementation in stress/catabolism)
By metabolic fate:
- Glucogenic: Ala, Cys, Gly, Ser, Val, His, Glu, Gln, Arg, Pro, Thr
- Ketogenic: Leu, Lys (exclusively)
- Both: Ile, Phe, Trp, Tyr, Thr
Protein Structure Levels:
1. Primary (1°): Linear sequence of amino acids; held by peptide bonds; determines all higher structure 2. Secondary (2°): Local regular folding patterns
- α-helix: Right-handed helix; H-bond between C=O of residue n and N-H of residue n+4; 3.6 residues/turn; stabilized by R groups facing outward
- β-sheet: Extended chains lying side-by-side; intra-chain H-bonds; can be parallel (same N→C direction) or antiparallel (opposite); R groups alternate above/below plane
- β-turn: Reversal of peptide chain direction; often contains Pro or Gly; allows folding back 3. Tertiary (3°): 3D shape of entire polypeptide; held by disulfide bonds (Cys-Cys), hydrophobic interactions, ionic bonds, H-bonds, van der Waals forces 4. Quaternary (4°): Association of multiple polypeptide chains (subunits); held by non-covalent interactions; hemoglobin (α2β2) shows cooperative oxygen binding
Denaturation: Loss of 2°, 3°, 4° structure (not 1°); caused by heat, pH, organic solvents, heavy metals; primary structure remains intact
Fibrous vs Globular Proteins:
- Fibrous: Structural (keratin, collagen, elastin); elongated; insoluble; α-helix or triple helix
- Globular: Functional (enzymes, antibodies, hemoglobin); spherical; water-soluble; multiple secondary structures
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Lipids — Structure and Classification
Simple Lipids (Neutral Fats):
- Triglycerides (triacylglycerol): Glycerol + 3 fatty acids; ester bonds; stored in adipose tissue; energy reserve (9 kcal/g vs 4 kcal/g for carbs/protein)
- Waxes: Long-chain fatty acid + long-chain alcohol; waterproof coatings (beeswax, plant leaf waxes)
- Steroids: Cholesterol, cortisol, testosterone, estrogen — four fused carbon rings; not triglycerides
- Cholesterol: 27-carbon sterol; amphipathic (hydroxyl group polar, ring structure non-polar); in cell membranes; precursor to steroid hormones, vitamin D, bile acids
Compound (Conjugated) Lipids:
- Phospholipids: Glycerol backbone + 2 fatty acids + phosphate + polar head group
- Phosphatidylcholine (lecithin): Head = choline; in cell membranes, lung surfactant
- Phosphatidylethanolamine (cephalin): Head = ethanolamine
- Phosphatidylserine: Head = serine; exposed on inner leaflet of plasma membrane
- Phosphatidylinositol: Head = inositol; in signal transduction (PIP2 → IP3 + DAG)
- Cardiolipin: Mitochondrial inner membrane; two phosphatidic acid units; contains linoleic acid
- Sphingolipids: Sphingosine (amino alcohol) backbone + fatty acid + head group
- Ceramides: Sphingosine + fatty acid (N-acylsphingosine) — basic unit of sphingolipids
- Sphingomyelin: Ceramide + phosphorylcholine; in myelin sheaths (nerve insulation)
- Glycosphingolipids (gangliosides): Ceramide + sugar(s); GM1, GM2, GM3 — contain N-acetylneuraminic acid (sialic acid); blood group antigens
Derived Lipids:
- Eicosanoids: Prostaglandins, thromboxanes, leukotrienes — derived from arachidonic acid (ω-6 PUFA); local hormones with diverse actions
- Fat-soluble vitamins: A (retinol — vision), D (calcitriol — Ca²⁺), E (antioxidant), K (γ-carboxylation of clotting factors)
Cell Membrane Structure — Fluid Mosaic Model:
- Bilayer of phospholipids (hydrophobic tails inside)
- Cholesterol (animal cells) — regulates membrane fluidity
- Proteins: Integral (span membrane) vs peripheral (attached to surface)
- Membrane is fluid (lateral diffusion of lipids and proteins)
- Asymmetric (outer leaflet vs inner leaflet differ in lipid composition)
Nucleotides and Nucleic Acids
Nucleotide Structure:
- Base (heterocyclic aromatic ring) + Pentose sugar (ribose for RNA, deoxyribose for DNA) + Phosphate group(s)
- Nucleoside: Base + sugar (no phosphate)
- Nucleotide: Base + sugar + phosphate (1-3 phosphates)
Purines (Adenine, Guanine): Double-ring structure (pyrimidine + imidazole); A pairs with T/U; G pairs with C Pyrimidines (Cytosine, Thymine, Uracil): Single six-membered ring; C pairs with G; T pairs with A; U pairs with A (in RNA); T has methyl group at C5 (distinguishes from U)
Nucleic Acid Structure:
- DNA: Deoxyribose sugar (no OH at C2); double helix (antiparallel strands; B-form: 10.5 bp/turn; major and minor grooves); bases: A, T, G, C; inherited genetic information
- RNA: Ribose sugar (OH at C2); single-stranded (most); A, U, G, C; types: mRNA (template), tRNA (adapter), rRNA (ribosomal), miRNA/siRNA (gene regulation)
- Backbone: Sugar-phosphate bonds (phosphodiester bonds link 3’→5’)
- Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds); A=U (in RNA); G-U wobble pairs
Nucleoside Diphosphates and Triphosphates:
- ATP: The energy currency of the cell; adenine + ribose + 3 phosphates
- GTP: Used in protein synthesis (translation), signal transduction (G-proteins)
- cAMP: Second messenger (formed from ATP by adenylyl cyclase; degraded by phosphodiesterase)
- cGMP: Second messenger (formed from GTP; involved in vasodilation — nitric oxide pathway)
Important Nucleotide-Dependent Enzymes and Coenzymes:
- NAD⁺/NADH: Electron carrier in redox reactions (dehydrogenases); niacin (B3) is precursor
- NADP⁺/NADPH: Reducing power for biosynthesis; niacin (B3) is precursor
- FAD/FADH₂: Electron carrier; riboflavin (B2) is precursor
- CoA (Coenzyme A): Acyl group carrier; pantothenic acid (B5) is precursor
- TPP (thiamine pyrophosphate): Aldehyde transfers; thiamine (B1) is precursor
- PLP (pyridoxal phosphate): Transamination; pyridoxine (B6) is precursor
Enzyme Classification and Kinetics
The 6 Enzyme Classes:
- Oxidoreductases: Transfer electrons (dehydrogenases, oxidases, reductases)
- Transferases: Transfer chemical groups (methyl, acetyl, phosphate, amino)
- Hydrolases: Cleave bonds by adding water (esterases, proteases, glycosidases)
- Lyases: Non-hydrolytic cleavage (adds/ removes double bonds, groups)
- Isomerases: Rearrange atoms (epimerases, mutases)
- Ligases: Join two molecules (synthetases, carboxylases)
Enzyme Kinetics (Michaelis-Menten):
- v = Vmax[S]/(Km + [S])
- Km: Substrate concentration at half-maximal velocity; reflects enzyme-substrate affinity (lower Km = higher affinity)
- Vmax: Maximum velocity when all enzyme is saturated with substrate
Inhibition:
- Competitive: Inhibitor resembles substrate; competes for active site; increases Km, unchanged Vmax
- Non-competitive: Inhibitor binds site other than active site; decreases Vmax, unchanged Km
- Uncompetitive: Inhibitor binds only to ES complex; decreases both Vmax and Km
- Lineweaver-Burk: 1/v vs 1/[S] plot; competitive shifts intersection up (same Y-intercept); non-competitive shifts left (same X-intercept)
Allosteric Enzymes: Exhibit sigmoid kinetics (S-shaped); multiple subunits with cooperative binding; F6P on PFK-1 (glycolysis); O₂ on hemoglobin (not an enzyme but classic allosteric protein)
⚡ INI CET High-Yield: The glycosidic bond in sucrose connects the anomeric carbons of both glucose and fructose — this eliminates the free hemiacetal needed for reducing properties. Lactose is reducing because only glucose’s anomeric carbon is involved in the glycosidic bond; galactose’s anomeric carbon remains free. In protein structure, remember that disulfide bonds (Cys-Cys) stabilize tertiary structure; hydrogen bonds between peptide groups stabilize secondary structure. Cholesterol in membranes decreases fluidity at high temperature and increases fluidity at low temperature.
Content adapted based on your selected roadmap duration. Switch tiers using the selector above.