Biomolecules

Why This Chapter Matters

Biomolecules is one of the highest-weightage NEET Biology chapters — 8-12 marks. Carbohydrates, proteins, lipids, nucleic acids, and especially ENZYMES are tested every year with very specific questions.

Prerequisites

Core Concepts

1. Chemical Composition of Living Tissue

Inorganic: Water (most abundant, ~70%), minerals

Organic: Proteins, carbohydrates, lipids, nucleic acids (biomolecules)

Water is most abundant chemical in cells.

Proteins are most abundant macromolecule in cells.

2. Carbohydrates

Monosaccharides (simple sugars):

Trioses: glyceraldehyde (3C) — important in respiration

Pentoses: ribose (RNA), deoxyribose (DNA), ribulose (in RuBisCO)

Hexoses: glucose, fructose, galactose (all C₆H₁₂O₆)

Disaccharides (2 monosaccharides + glycosidic bond):

Maltose = Glucose + Glucose (malt sugar, formed in starch digestion)

Sucrose = Glucose + Fructose (table sugar, transport form in plants)

Lactose = Glucose + Galactose (milk sugar)

Polysaccharides:

Starch (α-1,4 and α-1,6 linkages): energy storage in plants; amylose (linear) + amylopectin (branched)

Glycogen (α-1,4 and α-1,6): energy storage in animals and fungi ("animal starch")

Cellulose (β-1,4 linkages): structural in plant cell walls (cannot be digested by humans)

Chitin: structural in fungal cell walls and insect exoskeletons

3. Proteins

Amino acids: 20 types. Each has: NH₂ (amino), COOH (carboxyl), R group (side chain), H — all attached to central α-carbon.

Peptide bond: COOH of one amino acid + NH₂ of next → —CO—NH— + H₂O (condensation)

Protein structure levels:

1° (Primary): Sequence of amino acids (determined by gene)

2° (Secondary): α-helix (H-bonds within chain) or β-pleated sheet (H-bonds between chains)

3° (Tertiary): 3D folding of the polypeptide (held by disulphide bonds, H-bonds, hydrophobic interactions)

4° (Quaternary): Two or more polypeptide chains (e.g., haemoglobin = 4 chains)

Types of proteins:

Structural: collagen, keratin, elastin

Enzymatic: catalysts

Hormonal: insulin, glucagon

Transport: haemoglobin, albumin

Antibodies: immunoglobulins

4. Lipids

Fatty acids: Long hydrocarbon chain + COOH

Saturated: no double bond (solid at RT) — butter, ghee

Unsaturated: 1+ double bonds (liquid at RT) — oils

Triglycerides (Fats): Glycerol + 3 fatty acids (ester bonds)

Phospholipids: Glycerol + 2 fatty acids + phosphate group → major component of cell membrane

Steroids: Cholesterol, cortisol, sex hormones, Vitamin D — ring-based structure

5. Nucleic Acids

Nucleotide = Nitrogenous base + Pentose sugar + Phosphate group

|---|---|---|

Chargaff's rules (for DNA):

A = T, G = C (complementary base pairing)

A+G = T+C (purines = pyrimidines)

6. Enzymes (NEET Favourite!)

Enzymes are biological catalysts — mostly proteins (some RNA = ribozymes)

Properties:

Enzyme terminology:

Substrate: molecule enzyme acts on

Active site: region of enzyme where substrate binds

Product: result of the reaction

Enzyme-substrate complex (ES complex): temporary combination

Cofactors: Non-protein component needed for enzyme activity

Coenzyme: organic cofactor (vitamins like NAD⁺, FAD, CoA)

Prosthetic group: tightly bound cofactor

Activator: metal ions (Mg²⁺, Zn²⁺, Mn²⁺)

Apoenzyme = protein part of enzyme

Holoenzyme = apoenzyme + cofactor

Enzyme inhibition:

Competitive: inhibitor resembles substrate, binds active site → can be overcome by increasing substrate

Non-competitive: inhibitor binds allosteric site (not active site), changes enzyme shape → cannot be overcome

Factors affecting enzyme activity:

Temperature: increases activity up to optimum (~37°C for most human enzymes); then denatures

pH: each enzyme has optimum pH (pepsin = 2, trypsin = 8, salivary amylase = 7)

Substrate concentration: increases activity until saturation (Vmax)

Enzyme concentration: more enzyme → faster reaction (if substrate excess)

Km (Michaelis constant): Substrate concentration at half Vmax. Lower Km = higher enzyme affinity.

Classification of Enzymes (6 classes):

PYQs (NEET)

NEET 2023: Which of the following is an example of a competitive inhibitor?

Malonic acid inhibits succinic dehydrogenase (resembles succinic acid — the substrate).

Answer: Malonic acid (competitive inhibitor of succinic dehydrogenase)

NEET 2022: The sugar present in RNA is:

(A) Deoxyribose (B) Ribose ✓ (C) Ribulose (D) Arabinose

NEET 2021: Enzyme that catalyses the joining of two molecules with the use of ATP:

(A) Hydrolase (B) Lyase (C) Ligase ✓ (D) Isomerase

NEET 2020: Chargaff's rule states that in DNA:

A+G/T+C = 1 (purines = pyrimidines). Also A=T and G=C.

MCQ Practice

Q1. Enzyme active site is identified by:

(A) X-ray crystallography ✓ (B) Enzyme assay (C) Titration (D) Centrifugation

Q2. Lock and key hypothesis was proposed by:

(A) Koshland (induced fit) (B) Emil Fischer ✓ (C) Michaelis (D) Menten

Q3. Which is NOT a macromolecule?

(A) DNA (B) Protein (C) Glycogen (D) Glucose ✓

Q4 (Hard). When competitive inhibitor is added:

(A) Vmax decreases, Km unchanged (B) Vmax unchanged, Km increases ✓ (C) Both decrease (D) Both unchanged

Revision Notes

CARBOHYDRATES:
Monosaccharide: glucose, fructose, ribose, deoxyribose
Disaccharide: maltose(Glu+Glu), sucrose(Glu+Fru), lactose(Glu+Gal)
Polysaccharide: starch(plants), glycogen(animals), cellulose(β-1,4), chitin(fungi/insects)

PROTEIN STRUCTURE LEVELS:
1°= amino acid sequence
2°= α-helix/β-sheet (H-bonds)
3°= 3D folding (disulphide bonds etc.)
4°= multiple polypeptides (haemoglobin)

DNA vs RNA:
DNA: deoxyribose, ATGC, double helix
RNA: ribose, AUGC, usually single-stranded

ENZYMES:
Catalyst = lower activation energy, not consumed
Cofactor = non-protein part (coenzyme=organic, prosthetic=tightly bound)
Competitive inhibition: Km increases, Vmax same
Non-competitive: Vmax decreases, Km same

ENZYME CLASSES: Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase

CHARGAFF'S RULE: A=T, G=C, A+G=T+C (purines=pyrimidines)