CBSE Class 11 Biology Study Module: Cell – The Unit of Life (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 8: Cell – The Unit of Life

Assessment Focus: Definitions, Diagrams, Comparisons

Examination Level: CBSE Class 11 Annual & School Examinations

Syllabus Base: Strictly as per NCERT

CBSE Class 11 Biology Chapter 8 – Cell: The Unit of Life | NCERT Study Module & Revision Notes

Introduction: Cell – The Structural and Functional Unit of Life

All living organisms are composed of cells. The cell is the smallest unit capable of performing all vital life functions such as metabolism, growth, and reproduction. The study of cells, known as cytology, forms the foundation of modern biology. This chapter explains the origin of cell theory, types of cells, and detailed structure and function of cell organelles, helping students understand how cellular organization supports life processes.

Section I: Discovery of Cell and Cell Theory

Discovery of Cell

• In 1665, Robert Hooke observed cork under a microscope and coined the term cell.
• Antonie van Leeuwenhoek later observed living cells using improved microscopes.

Cell Theory

The cell theory was proposed by Matthias Schleiden and Theodor Schwann.

Postulates of Cell Theory

1. All living organisms are composed of cells and cell products.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (added later by Rudolf Virchow).

Significance of Cell Theory

• Explains continuity of life
• Establishes unity among living organisms
• Forms the basis of cell biology and genetics

Section II: Types of Cells

Cells are broadly classified based on the presence or absence of a true nucleus.

Prokaryotic Cells

General Characteristics

• No true nucleus
• Genetic material is not enclosed by nuclear membrane
• Membrane-bound organelles absent
• Size ranges from 1–10 µm
• Example: Bacteria, Cyanobacteria

Structure of Prokaryotic Cell

Cell Envelope

• Glycocalyx: Protective outer layer
• Cell wall: Made of peptidoglycan
• Plasma membrane: Selectively permeable

Cytoplasm

• Contains ribosomes (70S)
• No membrane-bound organelles

Nucleoid

• Single circular DNA molecule
• Not associated with histone proteins

Appendages

• Flagella: Locomotion
• Pili/Fimbriae: Attachment and conjugation

Eukaryotic Cells

General Characteristics

• True nucleus present
• Membrane-bound organelles present
• Linear chromosomes with histones
• Larger in size (10–100 µm)
• Examples: Plant and animal cells

Differences Between Prokaryotic and Eukaryotic Cells

(Highly important comparison for exams)

Section III: Cell Membrane (Plasma Membrane)

Structure

• Composed of phospholipid bilayer
• Proteins embedded within bilayer
• Described by Fluid Mosaic Model (Singer and Nicolson)

Functions

• Selective permeability
• Transport of substances
• Cell recognition and communication
• Maintains internal environment

Transport Mechanisms

• Diffusion
• Facilitated diffusion
• Active transport
• Endocytosis and exocytosis

Section IV: Cell Wall

Occurrence

• Present in plant cells, fungi, and bacteria
• Absent in animal cells

Composition

• Plant cell wall mainly made of cellulose

Functions

• Provides rigidity and shape
• Protects cell from mechanical injury
• Prevents osmotic bursting

Section V: Nucleus

Structure

• Nuclear membrane (double-layered)
• Nuclear pores
• Nucleoplasm
• Nucleolus
• Chromatin

Functions

• Controls cellular activities
• Stores genetic material
• Regulates cell division and protein synthesis

Section VI: Endoplasmic Reticulum (ER)

Types

Rough Endoplasmic Reticulum (RER)

• Ribosomes attached
• Protein synthesis

Smooth Endoplasmic Reticulum (SER)

• Lipid synthesis
• Detoxification
• Calcium storage

Section VII: Golgi Apparatus

Structure

• Stacks of flattened sacs (cisternae)
• Cis face (forming) and trans face (maturing)

Functions

• Modification, packaging, and transport of proteins
• Formation of secretory vesicles
• Cell surface renewal

Section VIII: Lysosomes

Characteristics

• Membrane-bound sacs
• Contain hydrolytic enzymes

Functions

• Intracellular digestion
• Removal of worn-out organelles
• Known as suicidal bags of the cell

Section IX: Mitochondria

Structure

• Double membrane
• Outer membrane smooth
• Inner membrane folded into cristae
• Inner space called matrix

Functions

• Site of aerobic respiration
• ATP synthesis
• Known as powerhouse of the cell

Special Feature

• Contains its own DNA and ribosomes

Section X: Plastids (Plant Cells Only)

Types of Plastids

1. Chloroplast
   • Photosynthesis
   • Contains chlorophyll
   • Thylakoids and grana present
2. Chromoplast
   • Imparts colour to fruits and flowers
3. Leucoplast
   • Storage of food materials
   • Amyloplast, Elaioplast, Proteinoplast

Section XI: Ribosomes

Characteristics

• Non-membranous
• Made of rRNA and proteins

Functions

• Protein synthesis

Types

• 70S (Prokaryotes, mitochondria)
• 80S (Eukaryotic cytoplasm)

Section XII: Cytoskeleton

Components

• Microtubules
• Microfilaments
• Intermediate filaments

Functions

• Cell shape
• Cell movement
• Intracellular transport

Section XIII: Centrosome and Centrioles

Occurrence

• Present in animal cells
• Absent in higher plant cells

Functions

• Formation of spindle fibres
• Cell division

Section XIV: Cilia and Flagella

Structure

• 9+2 arrangement of microtubules

Functions

• Locomotion
• Movement of substances over cell surface

Section XV: Vacuoles

Characteristics

• Large central vacuole in plant cells
• Contains cell sap

Functions

• Storage
• Maintains turgidity
• Osmoregulation

Revision Notes: Exam-Focused Key Points

• Cell is the basic unit of life
• Prokaryotic cells lack membrane-bound organelles
• Fluid Mosaic Model explains membrane structure
• Mitochondria and plastids have their own DNA
• Ribosomes are non-membranous organelles
• Diagrams of cell organelles are high-weightage

Conclusion

The chapter Cell – The Unit of Life provides fundamental knowledge essential for understanding all biological processes. Clear understanding of cell theory, cell types, and organelle functions, supported by comparisons and diagrams, is crucial for success in CBSE Class 11 Biology examinations. Mastery of this chapter also forms the foundation for advanced topics in genetics, physiology, and biotechnology.

✅ 100% NCERT Based

✅ CBSE Board Exam Oriented

✅ Conceptual, Comparative & Diagram-Focused