Biomolecules

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Biomolecules — The Chemistry of Life

Living organisms are built from four major classes of organic compounds: carbohydrates, proteins, nucleic acids, and lipids. Each has a distinct structure and function in the cell.

Carbohydrates

• Monosaccharides: glucose ($C_6H_{12}O_6$), fructose, galactose — single sugar units
• Disaccharides: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose)
• Polysaccharides: starch (plant storage), glycogen (animal storage), cellulose (plant structural)
• Key test: Benedict’s test for reducing sugars — brick-red precipitate; Iodine test for starch — blue-black colour

Proteins

• Made of amino acids linked by peptide bonds
• 20 standard amino acids; 9 essential (must be obtained from diet): Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine
• Protein structure levels: Primary (amino acid sequence) → Secondary (alpha-helix, beta-pleated sheet via H-bonds) → Tertiary (3D folding via R-group interactions) → Quaternary (multiple polypeptide subunits)
• ⚡ NEET tip: Denaturation — heat/UV/acids/alkalis break tertiary and quaternary bonds but NOT peptide bonds. Example: boiling an egg (irreversible) vs curdling of milk (reversible)

Lipids

• Fats (solid triglycerides) and oils (liquid triglycerides) — esters of glycerol + 3 fatty acids
• Saturated fats: no C=C bonds → solid at room temperature (e.g., butter)
• Unsaturated fats: ≥1 C=C bond → liquid at room temperature (e.g., groundnut oil)
• Trans fats: artificially hydrogenated → linked to cardiovascular disease
• Phospholipids: glycerol + 2 fatty acids + phosphate group → form cell membrane bilayer

Nucleic Acids

• DNA: double helix, deoxyribose sugar, thymine (T), complementary base pairing (A=T, G≡C)
• RNA: single-stranded, ribose sugar, uracil (U), types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal)
• ATP (adenosine triphosphate): the energy currency of cell — one high-energy phosphate bond yields ~30.5 kJ/mol

⚡ NEET Quick Recall:

• Bond between sugar and phosphate in nucleic acids: phosphodiester bond
• Bond between amino acids in proteins: peptide bond
• Most abundant protein on Earth: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase)
• Glycogen is stored in: liver and muscles

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Biomolecules — Detailed Study Guide

1. Carbohydrates — Structure and Function

Carbohydrates are polyhydroxy aldehydes or ketones. Their empirical formula for many is $C_n(H_2O)_n$.

Classification:

Type	Example	Function
Monosaccharides	Glucose, fructose	Immediate energy source
Disaccharides	Sucrose, lactose, maltose	Transport and storage
Polysaccharides	Starch, cellulose, glycogen	Structural and storage

Key reactions of carbohydrates:

• Hydrolysis of disaccharides: sucrose → glucose + fructose (catalysed by invertase)
• Fehling’s/Benedict’s test: reduces Cu²⁺ to Cu⁺ (brick-red $Cu_2O$ precipitate) — detects aldehydes
• Molisch test: α-naphthol + conc. $H_2SO_4$ → purple ring at interface — general carbohydrate test

Starch vs cellulose: Both polymers of α-D-glucose, but:

• Starch: α(1→4) and α(1→6) linkages → helical structure → storage
• Cellulose: β(1→4) linkages → straight chains forming fibrils → structural (plant cell walls)

2. Amino Acids and Proteins

The 20 standard amino acids differ by their R-group (side chain):

• Non-polar (hydrophobic): Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline, Glycine
• Polar uncharged: Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine
• Positively charged (basic): Lysine, Arginine, Histidine
• Negatively charged (acidic): Aspartic acid, Glutamic acid

Peptide bond formation: Carboxyl group of one amino acid reacts with amino group of another, releasing water (condensation). The bond is planar and rigid.

Protein classification by shape:

• Fibrous proteins: keratin, collagen, myosin — water-insoluble, structural
• Globular proteins: enzymes, hemoglobin, antibodies — water-soluble, functional

Key NEET concepts:

• Insulin: 2 polypeptide chains (A: 21 aa, B: 30 aa) linked by disulphide bridges
• Collagen: triple helix of polypeptide chains — most abundant protein in mammals
• Hemoglobin: quaternary structure — 4 heme groups each containing Fe²⁺

3. Lipids — More Than Just Fats

Fatty acid types:

• Saturated: no double bonds → high melting point
• Unsaturated: mono-unsaturated (1 C=C) e.g., oleic acid; poly-unsaturated (≥2 C=C) e.g., linoleic acid (ω-6), linolenic acid (ω-3)
• Essential fatty acids (must eat): Linoleic acid (ω-6), α-linolenic acid (ω-3)

Compound lipids:

• Phospholipids: main component of cell membrane — hydrophilic head (phosphate), hydrophobic tails (fatty acids)
• Glycolipids: carbohydrates + lipids — cell recognition markers
• Steroids: cholesterol (animal cell membranes), hormones (testosterone, estrogen)

Nucleic acid structure:

• DNA double helix: antiparallel strands, right-handed, 10 bp per turn
• Base pairing: A pairs with T via 2 H-bonds; G pairs with C via 3 H-bonds
• Chargaff’s rule: [A]=[T] and [G]=[C] in double-stranded DNA
• In RNA, uracil replaces thymine; RNA is single-stranded but can form hairpin loops

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for serious preparation.

Biomolecules — Deep Dive

1. Enzymes — Proteins That Catalyse Reactions

Enzymes are globular proteins that lower activation energy. They follow the lock-and-key model (Fischer, 1894) or induced-fit model (Koshland, 1958).

Cofactors:

• Metal ions: $Fe^{2+/3+}$ (catalase), $Zn^{2+}$ (carbonic anhydrase), $Mg^{2+}$ (ATPases)
• Coenzymes: non-protein organic helpers — NAD⁺, NADP⁺, Coenzyme A, FAD, biotin
• Prosthetic groups: tightly bound cofactors (e.g., heme in hemoglobin)

Enzyme kinetics (Michaelis-Menten): $$v = \frac{V_{max}[S]}{K_m + [S]}$$

• $K_m$: substrate concentration at which velocity is half of $V_{max}$
• Competitive inhibition: inhibitor resembles substrate, increases $K_m$, $V_{max}$ unchanged
• Non-competitive inhibition: inhibitor binds elsewhere, decreases $V_{max}$, $K_m$ unchanged

2. Vitamins — Essential Organic Compounds

Vitamins are classified as:

Vitamin	Type	Source	Deficiency disease
Vitamin A	Fat-soluble	Carrots, fish oil	Night blindness, Xerophthalmia
Vitamin B₁	Water-soluble	Whole grains, pulses	Beriberi
Vitamin C	Water-soluble	Citrus fruits, amla	Scurvy (bleeding gums, poor wound healing)
Vitamin D	Fat-soluble	Sunlight, fish oil	Rickets (children), Osteomalacia (adults)
Vitamin K	Fat-soluble	Green leafy vegetables	Bleeding disorders

3. Mineral Ions — Inorganic Essential Nutrients

• Macro-nutrients needed in >1g/day: N, P, K, Ca, Mg, S
• Micro-nutrients needed in <1mg/day: Fe, Mn, Zn, Cu, B, Mo, Cl
• ⚡ NEET fact: Iron is a component of hemoglobin and cytochrome; Magnesium is central atom in chlorophyll ($C_{55}H_{70}O_5N_4Mg$)

4. Secondary Metabolites

Plants produce compounds not directly involved in growth/reproduction:

• Alkaloids: nicotine (Nicotiana), morphine (Papaver somniferum), caffeine (Coffee arabica)
• Terpenoids: rubber, essential oils (eucalyptus, menthol)
• Phenolics: tannins, flavonoids (quercetin, anthocyanins — give colour to fruits)

5. Water and Its Properties

Water constitutes 70–90% of living organisms. Its unique properties:

• High specific heat → temperature stability
• High latent heat of vaporisation → cooling mechanism
• Cohesion and adhesion → capillary action in plants
• Ice floats → aquatic life insulation
• Polarity → excellent solvent (“universal solvent”)

Hydrogen bonds in biomolecules:

• Between base pairs in DNA (2 bonds A-T, 3 bonds G-C)
• Between polypeptide chains in protein secondary structure
• Between water molecules → high boiling point and surface tension

6. Previous Year NEET Questions on Biomolecules

• 2023 Qn: “Which amino acid is not encoded by the genetic code?” → Answer: Selenocysteine (21st amino acid, UGA codon in presence of SECIS element)
• 2022 Qn: “The enzyme that catalyzes the formation of DNA strands…” → DNA polymerase
• 2021 Qn: “Which of the following is a non-reducing sugar?” → Sucrose (no free anomeric carbon after glycosidic bond formation)

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📊 NEET UG Exam Essentials

Detail	Value
Questions	200 (180 mandatory + 10 optional)
Time	3h 20min
Marks	720
Section	Physics (50), Chemistry (50), Biology (100)
Negative	−1 for wrong answer
Qualifying	50th percentile (general category)

🎯 High-Yield Topics for NEET UG

• Human Physiology — 18 marks
• Genetics & Evolution — 16 marks
• Ecology & Environment — 12 marks
• Organic Chemistry (Reactions) — 15 marks
• Electrodynamics (Physics) — 18 marks
• Chemical Equilibrium — 10 marks

📝 Previous Year Question Patterns

• Q: “A particle moves in a circle…” [2024 Physics — 2 marks]
• Q: “Identify the incorrect statement about DNA…” [2024 Biology — 4 marks]
• Q: “The major product ofFriedel-Crafts acylation is…” [2024 Chemistry — 3 marks]

💡 Pro Tips

• NCERT Biology is the single most important resource — 80%+ questions are from NCERT lines
• Focus on Human Physiology, Genetics, and Ecology — together they make ~40% of Biology
• In Physics, master Electrostatics + Current Electricity + Magnetism (combined ~20%)
• Organic Chemistry: learn named reactions with mechanisms — they repeat across years

🔗 Official Resources

• NTA Official
• NEET Syllabus PDF

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