Chapter 9: Biomolecules – Class 11 Biology

Biomolecules are the backbone of life processes and form the foundation of biochemistry and molecular biology. Chapter 9 of Class 11 NCERT Biology explores the types of organic and inorganic compounds found in living organisms. These molecules include carbohydrates, proteins, lipids, nucleic acids, enzymes, and various metabolites. Understanding these molecules is vital for NEET aspirants, as many biology questions test concepts from this chapter.

This chapter builds on your understanding of the cell and its structure from Chapter 8: Cell – The Unit of Life and transitions into understanding what makes up a living cell at the molecular level. A deep grasp of biomolecular interactions will also help in studying digestion, respiration, photosynthesis, and molecular genetics.

What Are Biomolecules?

Biomolecules are organic and inorganic substances produced by living organisms. They can be broadly categorized into:

• Macromolecules: Large molecules such as proteins, nucleic acids, and polysaccharides.
• Micromolecules: Small and simple molecules including water, minerals, vitamins, and small metabolites.

Major Classes of Biomolecules

Carbohydrates

• Monosaccharides: Simple sugars like glucose, fructose. They serve as instant energy sources.
• Disaccharides: Composed of two monosaccharide units (e.g., sucrose = glucose + fructose). Linked by glycosidic bonds.
• Polysaccharides: Long chains of monosaccharide units (e.g., starch, glycogen, cellulose). They function as energy reserves and structural components.

Molecular Structure:

• Glucose (C₆H₁₂O₆) shows a cyclic structure in its pyranose form.

Table: Examples of Carbohydrates

Proteins

• Polymers of amino acids linked by peptide bonds.
• Each amino acid has an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen, and a variable R-group.
• 20 amino acids are known to be involved in protein synthesis.

Protein Structure:

1. Primary: Linear sequence of amino acids
2. Secondary: Hydrogen bonding forms α-helices or β-pleated sheets
3. Tertiary: 3D folding due to side-chain interactions
4. Quaternary: Association of multiple polypeptides

Table: Types of Proteins

Lipids

• Not polymers, but large biomolecules composed of fatty acids and glycerol.
• Include fats, phospholipids, and steroids.

Key Concepts:

• Saturated fats: No double bonds (e.g., palmitic acid)
• Unsaturated fats: One or more double bonds
• Phospholipids: Form biological membranes
• Steroids: Include hormones like testosterone and cholesterol

Table: Types of Lipids

Nucleic Acids

• Long chains of nucleotides
• Two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)

Nucleotide Structure:

• Composed of nitrogen base (A, G, C, T/U), pentose sugar (ribose or deoxyribose), and a phosphate group.

Table: Comparison of DNA and RNA

Enzymes

• Enzymes are biological catalysts made of proteins. They accelerate chemical reactions without undergoing permanent changes.

Properties and Action

• Catalytic Efficiency: Enzymes increase the rate of reaction by lowering activation energy.
• Substrate Specificity: Enzymes act on specific substrates via the active site.
• Enzyme-Substrate Complex: Intermediate formed during the reaction.

Mechanism of Action:

1. Substrate (S) binds to enzyme (E) forming ES complex.
2. Transition state achieved with lower activation energy.
3. Products (P) are released and enzymes remain unchanged.

Factors Affecting Enzyme Activity:

• Temperature and pH: Each enzyme has an optimum value. Deviation leads to denaturation or reduced activity.
• Substrate concentration: Increased concentration increases velocity to a point (Vmax).
• Inhibitors:
  • Competitive inhibitors resemble substrates.
  • Non-competitive inhibitors bind elsewhere on the enzyme.

Enzyme Classification (IUBMB):

Co-factors

• Maybe metal ions, co-enzymes, or prosthetic groups.
• Zinc and Mg²⁺ are common metal cofactors.
• Vitamins like NAD, NADP function as coenzymes.

NEET Illustrative Questions

1. Which of the following is a disaccharide?
   a) Glucose
   b) Fructose
   c) Maltose
   d) Ribose
   Answer: c) Maltose
2. What is the bond between two amino acids in a protein?
   a) Glycosidic bond
   b) Hydrogen bond
   c) Peptide bond
   d) Disulfide bond
   Answer: c) Peptide bond
3. Which of the following enzymes helps in the conversion of starch to sugar during germination?
   a) Lipase
   b) Amylase
   c) Protease
   d) Ligase
   Answer: b) Amylase
4. Which of these is not a component of a nucleotide?
   a) Nitrogenous base
   b) Phosphate group
   c) Fatty acid
   d) Pentose sugar
   Answer: c) Fatty acid
5. The R-group in amino acids decides:
   a) The solubility of DNA
   b) Type of lipid formed
   c) Specific properties of amino acids
   d) Nature of carbohydrate
   Answer: c) Specific properties of amino acids

FAQs

1. What are biomolecules?

Biomolecules are organic and inorganic substances produced by living organisms that are essential for metabolism and cell function.

2. What is the difference between monosaccharides and disaccharides?

Monosaccharides are single sugar molecules, whereas disaccharides consist of two sugar units joined by glycosidic bonds.

3. How are proteins structured?

Proteins have four levels of structural organization—primary, secondary, tertiary, and quaternary—each contributing to function.

4. What makes enzymes efficient catalysts?

Enzymes lower activation energy and have high substrate specificity, which makes biochemical reactions highly efficient.

5. Why is DNA important?

DNA stores genetic information required for heredity, development, and metabolic functions.

6. What is a nucleotide composed of?

Each nucleotide contains a nitrogenous base, a pentose sugar, and a phosphate group.

7. What are enzyme cofactors?

Cofactors are non-protein chemical compounds (like metal ions or coenzymes) that assist in enzyme activity.

8. How does temperature affect enzyme activity?

Enzyme activity increases with temperature up to an optimum, after which the enzyme denatures and activity drops.

Conclusion

Chapter 9 – Biomolecules is a key part of NEET preparation. A solid understanding of carbohydrates, lipids, proteins, nucleic acids, and enzymes is essential for excelling in biochemistry and other interconnected chapters in Class 12. Diagrams, molecular structures, and classification tables are frequently tested. Regular revision of these core concepts along with solving NEET-based MCQs will ensure higher retention and better scores.