Biomolecules

NCERT Line-by-Line Breakdown for NEET 2026

Unit 3: Cell Structure & Function

There is a huge diversity of living organisms in our biosphere. However, elementally, they are all made of the same chemicals. This chapter bridges Biology and Chemistry, explaining the Micromolecules, Macromolecules, and the biological catalysts known as Enzymes.

1. Analysis of Chemical Composition

To analyze the chemical composition of living tissue (vegetable or liver), we grind it in Trichloroacetic acid (Cl₃CCOOH).

🧪 Acid Soluble Pool (Filtrate)

Micromolecules (18-800 Daltons).

Includes: Amino acids, Sugars, Nucleotides, Nitrogen bases.

🛑 Acid Insoluble Pool (Retentate)

Macromolecules (>10,000 Daltons).

Includes: Proteins, Nucleic Acids, Polysaccharides.

Exception: Lipids are micromolecules by weight (< 800 Da) but are found here because they form vesicles and do not filter through.

2. Proteins (Polypeptides)

Heteropolymers of amino acids linked by Peptide bonds. Essential for the structure and function of life.

🏆 NEET Fact:
• Most abundant protein in Animal world: Collagen.
• Most abundant protein in whole Biosphere: RuBisCO.

Structure of Proteins

Level	Description	Bonds involved
Primary	Linear sequence of amino acids (Positional information).	Peptide bonds only.
Secondary	Folding into Helix (α-helix) or Sheet (β-pleated).	Hydrogen bonds.
Tertiary	3D Globular structure. Necessary for biological activity.	H-bonds, Ionic, Disulphide, Hydrophobic.
Quaternary	Assembly of multiple polypeptide chains (e.g., Haemoglobin).	Various non-covalent interactions.

3. Carbohydrates & Nucleic Acids

A. Polysaccharides

Linked by Glycosidic bonds.

Starch: Energy store in plants. Helical secondary structure (Holds I₂ → Blue color).
Cellulose: Structural component (Plant cell wall). No helix (Does not hold I₂). Homopolymer of glucose.
Glycogen: Energy store in animals. Highly branched.
Inulin: Polymer of Fructose.
Chitin: Exoskeleton of arthropods. Complex homopolymer (N-acetyl glucosamine).

B. Nucleic Acids

Polymers of Nucleotides linked by Phosphodiester bonds.

Watson-Crick Model (B-DNA):

Double helix, anti-parallel strands.
Backbone: Sugar-Phosphate. Steps: Nitrogen bases.
Pairing: A=T (2 H-bonds), G≡C (3 H-bonds).
Pitch: 34 Å per turn (10 base pairs). Rise per bp: 3.4 Å.

4. Enzymes (Biocatalysts)

“Almost all enzymes are proteins” (Exception: Ribozymes are Nucleic Acids). They increase the rate of reaction by lowering Activation Energy.

[Image of enzyme activation energy graph]

Factors Affecting Enzyme Activity

Temperature & pH
Bell-shaped curve. Enzymes get denatured at high temp/pH and inactive at low temp.

Substrate Concentration
Velocity increases up to Vmax. Cannot rise further as all enzyme molecules become saturated.

Enzyme Inhibition

Competitive Inhibition: Inhibitor resembles substrate structurally (e.g., Malonate inhibits Succinic dehydrogenase). Used in bacterial pathogen control.

Cofactors

Non-protein part required for enzyme activity. The protein part is called Apoenzyme.

Type	Association	Example
Prosthetic Group	Tightly bound (Organic).	Haem (in Peroxidase/Catalase).
Co-enzyme	Transient/Loose (Organic).	NAD, NADP (contain Vitamins).
Metal Ions	Coordination bonds.	Zinc (in Carboxypeptidase).

📝 Rapid Fire MCQs

Q1. Which of the following is a non-reducing sugar?

A) Glucose
B) Maltose
C) Sucrose
D) Lactose

Click to check Answer

Answer: C) Sucrose (Common table sugar).

Q2. The transition state structure of the substrate formed during an enzymatic reaction is:

A) Permanent and stable
B) Transient and unstable
C) Transient and stable
D) Permanent and unstable