Chapter 5: Cell – The Unit of Life

Introduction

The cell is the fundamental structural and functional unit of all living organisms. The concept of cells was first proposed by Robert Hooke in 1665 when he observed cork cells under a microscope. The cell theory, formulated by Schleiden and Schwann and later modified by Rudolf Virchow, states that all living organisms are composed of cells, cells are the basic units of structure and function, and all cells arise from pre-existing cells.

Cell Theory

• All living organisms are composed of one or more cells
• Cell is the basic structural and functional unit of life
• All cells arise from pre-existing cells through cell division
• The function of an organism is the sum of the activities and interactions of its cells

Types of Organisms Based on Cellular Organization

• Unicellular organisms: Consist of a single cell (e.g., bacteria, protozoa)
• Multicellular organisms: Composed of many cells with division of labor (e.g., plants, animals)

Cell Size and Shape

• Most cells are microscopic in size (1-100 micrometers)
• Cell size is limited by surface area to volume ratio
• Cell shapes vary widely depending on function:
  • Spherical (red blood cells)
  • Spindle-shaped (muscle cells)
  • Branched (neurons)
  • Columnar (epithelial cells)
  • Discoidal (some algae)
  • Irregular (amoeba)

Types of Cells

Based on cellular organization, cells are classified into two major types:

Prokaryotic Cells

• Found in bacteria and archaea
• Lack a true nucleus and membrane-bound organelles
• Simpler internal organization
• Smaller in size (typically 0.5-5 μm)
• Examples: E. coli, cyanobacteria

Eukaryotic Cells

• Found in protists, fungi, plants, and animals
• Possess a true nucleus with nuclear envelope
• Contain membrane-bound organelles
• Larger in size (typically 10-100 μm)
• Examples: Plant cells, animal cells

Structure of Prokaryotic Cell

Cell Envelope

• Cell Wall: Provides shape and protection
  • Primarily composed of peptidoglycan
  • Gram-positive: Thick peptidoglycan layer
  • Gram-negative: Thin peptidoglycan layer with outer membrane
• Cell Membrane (Plasma Membrane): Controls movement of substances
  • Phospholipid bilayer with proteins
  • Selective permeability

Cytoplasm

• Semi-fluid matrix containing enzymes, ribosomes, and inclusions
• No membrane-bound organelles
• Contains 70S ribosomes for protein synthesis

Genetic Material

• Nucleoid: Region containing circular DNA
  • Not separated from cytoplasm by membrane
  • Single, circular, double-stranded DNA molecule
• Plasmids: Small, circular, extra-chromosomal DNA
  • Confer special properties (e.g., antibiotic resistance)

Appendages

• Flagella: Locomotory organs
  • Composed of flagellin protein
  • Movement controlled by basal body
• Pili: Hair-like structures
  • Shorter than flagella
  • Function in attachment and bacterial conjugation
• Fimbriae: Similar to pili but shorter and more numerous
  • Aid in attachment to surfaces

Special Structures

• Capsule: Gelatinous layer outside cell wall
  • Protection against phagocytosis
  • Helps in moisture retention
• Ribosomes: Sites of protein synthesis
  • 70S type (30S + 50S subunits)
• Inclusion Bodies: Storage granules
  • Store nutrients or metabolic byproducts
  • Examples: Glycogen granules,