Chapter 3: Cell Structure and Function

Introduction

This chapter explores the fundamental differences and similarities between prokaryotic and eukaryotic cells, focusing on their structures, functions, and the unique features that distinguish bacteria, archaea, and eukaryotes. Understanding cell structure is essential for studying microbial physiology, classification, and the mechanisms of microbial pathogenicity.

Processes of Life

• Growth: Increase in size.

• Reproduction: Increase in number, producing progeny.

• Responsiveness: Ability to respond to environmental stimuli.

• Metabolism: Controlled chemical reactions of organisms.

• Cellular Structure: All living things are made of cells.

Additional info: These processes are not exclusive to eukaryotes; bacteria and archaea also exhibit all these characteristics.

Characteristics of Life and Their Distribution in Microbes

Types of Cells: Prokaryotes vs. Eukaryotes

Prokaryotic Cells

• Lack a nucleus

• Genetic material is not membrane-bound

• Include Bacteria and Archaea

• Typically smaller and simpler than eukaryotic cells

Eukaryotic Cells

• Have a nucleus

• Genetic material is enclosed within a nuclear envelope

• Include Algae, Protozoa, Fungi, Animals, and Plants

• Contain membrane-bound organelles

Size Comparison

• Prokaryotic cells: typically 0.5–5 μm

• Eukaryotic cells: typically 10–100 μm

External Structures of Bacterial Cells

Glycocalyces

• Gelatinous, sticky substance surrounding the outside of the cell

• Composed of polysaccharides, polypeptides, or both

• Types: Capsule (organized, firmly attached) and Slime Layer (loose, water-soluble)

• Functions: Protection from desiccation, adherence to surfaces, evasion of host immune system

Flagella

• Responsible for motility

• Composed of filament, hook, and basal body

• Arrangements: Monotrichous, Lophotrichous, Amphitrichous, Peritrichous

• Movement: Rotation propels bacterium through environment; can be clockwise (tumble) or counterclockwise (run)

Fimbriae and Pili

• Fimbriae: Short, bristle-like structures for attachment

• Pili: Longer than fimbriae, usually one or two per cell; involved in DNA transfer (conjugation)

Bacterial Cell Walls

• Provide structure, shape, and protection

• Composed mainly of peptidoglycan

• Two main types: Gram-positive (thick peptidoglycan, teichoic acids) and Gram-negative (thin peptidoglycan, outer membrane with lipopolysaccharide)

Comparison of Gram-Positive and Gram-Negative Cell Walls

Bacterial Cytoplasmic Membranes

• Phospholipid bilayer with embedded proteins

• Functions: Selective permeability, energy generation, transport of substances

Transport Processes Across Membranes

• Passive Transport: Diffusion, facilitated diffusion, osmosis (no energy required)

• Active Transport: Requires energy (ATP); includes group translocation in bacteria

Cytoplasm of Bacteria

• Contains cytosol (liquid portion), inclusions (storage granules), ribosomes (protein synthesis), and sometimes endospores (dormant, resistant structures)

Endospores

• Produced by some Gram-positive bacteria (e.g., Bacillus, Clostridium)

• Highly resistant to heat, desiccation, chemicals, and radiation

Archaeal Cell Structure

• Similar to bacteria but with unique features

• Cell walls lack peptidoglycan; may contain pseudopeptidoglycan

• Distinctive membrane lipids (ether-linked)

• Some have unique external structures (e.g., hami)

Eukaryotic Cell Structure

• External structures: Glycocalyces (in some cells), cell walls (plants, fungi, algae)

• Plasma membrane: Contains sterols for stability

• Membrane-bound organelles: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, chloroplasts (in plants and algae)

• Cytoskeleton: Microtubules, microfilaments, intermediate filaments

• Motility: Flagella and cilia (structurally distinct from prokaryotic flagella)

Comparison Table: Archaea, Bacteria, and Eukaryotes

Nonmembranous and Membranous Organelles

• Nonmembranous: Ribosomes, cytoskeleton, centrioles (in animals)

• Membranous: Nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria, chloroplasts

Summary

• All cells share basic characteristics but differ in complexity and structure.

• Prokaryotes (bacteria and archaea) lack a nucleus and most organelles, while eukaryotes possess both.

• Cell walls, membranes, and external structures are key to microbial survival, pathogenicity, and classification.