Board exam focus — Biomolecules (CBSE & HBSE)

Living organisms are made of a limited set of chemical elements organised into biomolecules. This chapter analyses the chemical composition of living tissue, distinguishes primary from secondary metabolites, and examines the four major macromolecules — carbohydrates, proteins (with their structural levels), lipids and nucleic acids. It closes with a detailed treatment of enzymes: their properties, mechanism of action, factors affecting activity, classification and cofactors. It bridges chemistry and biology and is high-yield for CBSE, HBSE and NEET.

Chemical Composition, Metabolites, Carbohydrates and Lipids

Analysis of Chemical Composition

If living tissue is ground, filtered through trichloroacetic acid, the filtrate (acid-soluble pool) contains small molecules (amino acids, sugars, nucleotides) while the retentate contains macromolecules (proteins, nucleic acids, polysaccharides, lipids).

• Micromolecules: molecular weight < 1000 Da; in the acid-soluble pool.
• Macromolecules: molecular weight 10,000 Da and above (polymers); acid-insoluble.
• Water is the most abundant chemical in living systems (~70–90%).

Lipids have relatively low molecular weight but appear in the macromolecular (acid-insoluble) fraction because they form aggregates/membranes, not because they are true polymers.

Primary vs Secondary Metabolites

Carbohydrates

Polyhydroxy aldehydes/ketones; general formula often (CH₂O)ₙ.

• Monosaccharides — glucose, fructose, ribose (single sugar unit).
• Disaccharides — sucrose (glucose+fructose), maltose, lactose.
• Polysaccharides — starch (plant storage, gives blue with iodine), glycogen (animal storage), cellulose (structural, β-1,4 glucose, unbranched), chitin (in fungal walls/arthropod exoskeleton).

Lipids

Generally water-insoluble, soluble in organic solvents.

• Fatty acids — long hydrocarbon chains with a –COOH group; saturated (no double bond) or unsaturated (one/more double bonds).
• Glycerides — fatty acid + glycerol (ester bond); fats and oils.
• Phospholipids — contain phosphorus; major component of membranes (e.g., lecithin).

Board point: Cellulose (β-1,4) cannot be digested by humans (no cellulase) but starch/glycogen (α-1,4) can — a structural consequence of glycosidic linkage.

Proteins and Nucleic Acids

Proteins

Polymers of amino acids linked by peptide bonds. There are 20 standard amino acids; an amino acid has an amino group (–NH₂), a carboxyl group (–COOH), an H and a variable R group attached to the α-carbon.

Four levels of protein structure:

Functions: structural (collagen), transport (haemoglobin), catalysis (enzymes), defence (antibodies), hormones (insulin), contraction (actin/myosin).

NEET fact: Collagen is the most abundant protein in the animal world; RuBisCO is the most abundant protein/enzyme in the whole biosphere.

Nucleic Acids

Polymers of nucleotides; carriers of genetic information.

A nucleotide = nitrogenous base + pentose sugar + phosphate.

• Bases: purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA; uracil in RNA).
• Sugar: deoxyribose (DNA) or ribose (RNA).
• Nucleotides join by phosphodiester bonds.

Watson–Crick DNA double helix (key facts):

1. Two anti-parallel strands coiled into a right-handed double helix.
2. Bases pair by H-bonds: A=T (2 bonds), G≡C (3 bonds) — Chargaff's rule (A=T, G=C).
3. Each helical turn ≈ 3.4 nm (10 base pairs); distance between adjacent bases ≈ 0.34 nm.

Trap: DNA has thymine and is double-stranded; RNA has uracil and is usually single-stranded.

Enzymes — Properties, Mechanism, Factors, Classification and Cofactors

Nature and Properties of Enzymes

Enzymes are biocatalysts, almost all proteins (a few RNAs, called ribozymes, are catalytic). They speed up reactions by lowering activation energy without being consumed.

Key properties:

• Highly specific — one enzyme generally acts on one substrate.
• Work under mild conditions (body temperature, near-neutral pH).
• Have an active site that binds the substrate.
• Are regulated and reusable.

Mechanism of Action

1. The substrate (S) binds the enzyme's active site to form an enzyme–substrate (ES) complex.
2. The enzyme strains the substrate, forming a transition state of lower activation energy.
3. Products (P) are formed and released; the enzyme is freed: E + S ⇌ ES → EP → E + P.

'Lock-and-key' (rigid fit) and 'induced-fit' (active site moulds around substrate) models explain specificity.

Factors Affecting Enzyme Activity

Classification (six classes)

1. Oxidoreductases — oxidation–reduction.
2. Transferases — transfer of a group.
3. Hydrolases — hydrolysis of bonds.
4. Lyases — removal of groups forming double bonds.
5. Isomerases — inter-conversion of isomers.
6. Ligases — joining of molecules using ATP.

Cofactors

Many enzymes need a non-protein cofactor; the protein part alone is the apoenzyme, and apoenzyme + cofactor = active holoenzyme.

• Prosthetic groups — tightly bound (e.g., haem in catalase).
• Coenzymes — loosely bound organic molecules, often vitamin-derived (NAD⁺ from niacin, FAD).
• Metal ions — activators (e.g., Zn²⁺, Mg²⁺).

NEET trap: A competitive inhibitor (e.g., malonate vs succinate for succinate dehydrogenase) resembles the substrate and binds the active site; its effect is reversed by increasing substrate concentration.

Frequently asked questions

Are these Biomolecules notes free?

Yes — the Biomolecules notes for Biology (Class 11) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Biomolecules notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Biomolecules chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Biomolecules.