CBSE Class 11 Biology Study Module: Biomolecules (NCERT-Based Revision Notes)

Course and Examination Framework

Course: CBSE Class 11 Biology

Board: Central Board of Secondary Education

Unit III: Cell Structure and Function

Chapter 9: Biomolecules

Assessment Focus: Conceptual Clarity, Chemical Structures

Examination Level: CBSE Class 11 Annual & School Examinations

Syllabus Base: Strictly as per NCERT

CBSE Class 11 Biology Chapter 9 – Biomolecules | NCERT Study Module & Revision Notes

Introduction to Biomolecules

Living organisms are composed of both inorganic and organic compounds, collectively known as biomolecules. These molecules form the chemical basis of life and are essential for cellular structure, metabolism, growth, and reproduction. Biomolecules include carbohydrates, proteins, lipids, nucleic acids, and enzymes. Understanding their chemical nature, structure, and biological roles is crucial for mastering cellular processes and metabolism.

This chapter explains how simple chemical units assemble into complex macromolecules and how these molecules interact to sustain life.

Section I: Chemical Composition of Living Cells

Cells contain a variety of elements, mainly:

• Carbon (C)
• Hydrogen (H)
• Oxygen (O)
• Nitrogen (N)
• Phosphorus (P)
• Sulphur (S)

These elements combine to form organic molecules with carbon backbones, which are characteristic of living systems.

Types of Biomolecules

1. Micromolecules – Low molecular weight compounds
   • Amino acids
   • Sugars
   • Nucleotides
   • Fatty acids
2. Macromolecules – High molecular weight compounds
   • Polysaccharides
   • Proteins
   • Nucleic acids

Section II: Carbohydrates

Definition

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, generally following the formula (CH₂O)n.

Classification of Carbohydrates

1. Monosaccharides

• Simplest sugars
• Cannot be hydrolysed further
• Examples: Glucose, fructose, ribose

Functions:

• Primary source of energy
• Structural components of nucleic acids

2. Oligosaccharides

• Formed by condensation of 2–10 monosaccharides
• Example: Sucrose (glucose + fructose)

3. Polysaccharides

• Long chains of monosaccharides
• Examples:
  • Starch – Storage carbohydrate in plants
  • Glycogen – Storage carbohydrate in animals
  • Cellulose – Structural component of plant cell wall

Functions:

• Energy storage
• Structural support

Structure–Function Relationship

• Glucose rings allow rapid energy release
• Cellulose has β-glycosidic bonds, making it strong and insoluble

Section III: Proteins

Definition

Proteins are polymers of amino acids linked by peptide bonds and contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulphur.

Amino Acids

• Basic units of proteins
• General structure:
  NH₂–CH(R)–COOH
• 20 standard amino acids are present in proteins

Levels of Protein Structure

1. Primary Structure
   • Linear sequence of amino acids
2. Secondary Structure
   • α-helix and β-pleated sheet
   • Stabilized by hydrogen bonds
3. Tertiary Structure
   • Three-dimensional folding
   • Determines protein function
4. Quaternary Structure
   • Association of multiple polypeptide chains

Functions of Proteins

• Structural (keratin, collagen)
• Enzymatic (enzymes)
• Transport (haemoglobin)
• Regulatory (hormones)
• Defensive (antibodies)

Denaturation of Proteins

• Loss of biological activity due to heat, pH, or chemicals
• Primary structure remains intact

Section IV: Lipids

Definition

Lipids are hydrophobic organic compounds insoluble in water but soluble in organic solvents.

Types of Lipids

1. Simple Lipids

• Fats and oils
• Composed of fatty acids and glycerol

2. Complex Lipids

• Phospholipids
• Glycolipids
• Lipoproteins

3. Derived Lipids

• Steroids
• Cholesterol

Functions of Lipids

• Energy storage
• Structural component of cell membranes
• Hormone synthesis
• Thermal insulation

Unique Feature

Lipids are not true polymers, unlike carbohydrates and proteins.

Section V: Nucleic Acids

Definition

Nucleic acids are polymers of nucleotides responsible for storage and transmission of genetic information.

Types of Nucleic Acids

1. DNA (Deoxyribonucleic Acid)

• Genetic material in most organisms
• Double-stranded helix

2. RNA (Ribonucleic Acid)

• Single-stranded
• Types: mRNA, tRNA, rRNA

Structure of Nucleotide

Each nucleotide consists of:

• Nitrogenous base
• Pentose sugar
• Phosphate group

Functions

• DNA stores genetic information
• RNA helps in protein synthesis

Section VI: Enzymes

Definition

Enzymes are biological catalysts that speed up biochemical reactions without being consumed.

Characteristics of Enzymes

• Proteinaceous in nature
• Highly specific
• Reusable
• Effective at optimum pH and temperature

Mechanism of Enzyme Action

Lock and Key Model

• Substrate fits into enzyme’s active site

Induced Fit Model

• Enzyme changes shape to bind substrate

Factors Affecting Enzyme Activity

• Temperature
• pH
• Substrate concentration
• Enzyme concentration

Section VII: Metabolism

Definition

Metabolism refers to all biochemical reactions occurring within a cell or organism.

Types of Metabolic Reactions

1. Anabolism

• Constructive reactions
• Example: Photosynthesis
• Energy-consuming

2. Catabolism

• Destructive reactions
• Example: Respiration
• Energy-releasing

Metabolic Pathways

• Linear pathways
• Cyclic pathways
• Spiral pathways

Role of Enzymes in Metabolism

• Control rate of reactions
• Maintain cellular efficiency
• Regulate metabolic balance

Section VIII: Concept of Living State

Living systems are characterized by:

• Continuous metabolic activity
• Self-regulation
• Ability to respond to stimuli

Metabolism is a defining property of life.

Revision Notes: Exam-Focused Key Points

• Biomolecules form the chemical foundation of life
• Carbohydrates are primary energy sources
• Proteins have four structural levels
• Lipids are hydrophobic and non-polymeric
• DNA is the genetic material
• Enzymes lower activation energy
• Metabolism includes anabolic and catabolic reactions
• Chemical structures are frequently tested in exams

Conclusion

The chapter Biomolecules explains how life is governed by chemical substances and reactions. Understanding the structure, classification, and functions of carbohydrates, proteins, lipids, nucleic acids, enzymes, and metabolism provides a strong conceptual base for advanced topics such as genetics, physiology, and biotechnology. Thorough revision of this chapter is essential for scoring well in CBSE Class 11 Biology examinations.

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