Cell Structure and Function in Microbiology

Introduction

This study guide covers the fundamental differences between prokaryotic and eukaryotic cells, focusing on their structures, functions, and the significance of these differences in microbiology. Key terms, examples, and applications are provided to support understanding and exam preparation.

Prokaryotic vs. Eukaryotic Cells

Definitions and Key Differences

• Prokaryotes: Organisms whose cells lack a true nucleus and membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: Organisms whose cells contain a true nucleus and membrane-bound organelles. Includes algae, protozoa, fungi, animals, and plants.

Prokaryotic Cell Structures

Capsule

• Composed of polysaccharide (most common) or protein (rarely).

• Firmly attached to the cell surface.

• Protects bacteria from recognition by host immune system.

• Associated with increased virulence.

Flagella

• Function: Motility (movement).

• Structure: Long, whip-like; rotate 360°.

• Rotation can be clockwise or counterclockwise (reversible).

• Not covered by cell wall or membrane in prokaryotes.

• Movement in response to stimuli is called taxis (e.g., chemotaxis, phototaxis).

Fimbriae and Pili

• Fimbriae: Short, bristle-like protein fibers; hundreds per cell; important in adhesion and biofilm formation.

• Pili: Longer, fewer per cell; involved in conjugation (transfer of DNA between cells).

Cell Wall

• Provides structure and protection from osmotic shock.

• Composed mainly of peptidoglycan (in bacteria).

• Peptidoglycan: Polymer of sugars (NAG and NAM) cross-linked by peptide bridges.

Types of Bacterial Cell Walls

• Lipopolysaccharide (LPS): Found in Gram-negative bacteria; can cause septic shock if released into the bloodstream.

Cytoplasmic Membrane

• Phospholipid bilayer with embedded proteins (fluid mosaic model).

• Functions: Selective permeability, energy harvesting, transport of molecules.

• Maintains concentration gradients and osmotic balance.

Ribosomes

• Sites of protein synthesis.

• Prokaryotic ribosomes: 70S (composed of 50S and 30S subunits).

Endospores

• Dormant, highly resistant structures formed by some bacteria (e.g., Bacillus anthracis).

• Allow survival in harsh conditions (heat, desiccation, chemicals).

• Significant in disease transmission and bioterrorism (e.g., anthrax spores in mail).

Eukaryotic Cell Structures

Glycocalyx

• Less organized than prokaryotic capsules.

• Helps cells adhere to each other and provides protection against dehydration.

Cell Wall and Cytoplasmic Membrane

• Present in fungi, algae, plants, and some protists.

• Composed of polysaccharides (cellulose in plants, chitin in fungi).

• Cytoplasmic membrane: Phospholipid bilayer with proteins and sterols; controls movement into and out of the cell.

Flagella and Cilia

• Flagella: Different structure from prokaryotic flagella; covered by cell membrane; move in a whip-like fashion.

• Cilia: Shorter and more numerous than flagella; coordinated movement propels cells or moves substances along surfaces.

Ribosomes

• Larger than prokaryotic ribosomes (80S, composed of 60S and 40S subunits).

• Sites of protein synthesis.

Membrane-Bound Organelles

• Nucleus: Contains most of the cell's DNA; surrounded by a nuclear envelope; contains nucleolus (site of rRNA synthesis).

• Endoplasmic Reticulum (ER): Network of membranes; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids.

• Golgi Body: Modifies, sorts, and packages proteins and lipids for transport.

• Mitochondria: Site of ATP production via cellular respiration.

• Chloroplasts: Site of photosynthesis in plants and algae.

Key Scientists

• Hans Christian Gram: Developed the Gram stain, a critical technique for classifying bacteria based on cell wall structure.

• Lynn Margulis: Proposed the endosymbiotic theory for the origin of eukaryotic organelles.

• Bruce Ivins: Associated with the investigation of the 2001 anthrax bioterrorism attacks.

Additional Information

• Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration.

• Critical Thinking Example: Bacteria do not grow in honey, pickles, or foods with high salt content due to osmotic effects.

• Clinical Relevance: Gram stain is essential for diagnosing bacterial infections and guiding antibiotic therapy.

Summary Table: Bacterial Cell Wall Types

Key Equations and Terms

• Osmosis:

• Peptidoglycan Structure: (chains cross-linked by peptide bridges)

Additional info: Some context and definitions have been expanded for clarity and completeness based on standard microbiology textbooks.