Functional Anatomy of Prokaryotic and Eukaryotic Cells

Introduction

This chapter explores the structural and functional differences between prokaryotic and eukaryotic cells, with a focus on their relevance in microbiology. Understanding these differences is essential for identifying microorganisms, their physiology, and their roles in health and disease.

Prokaryotic vs. Eukaryotic Cells

Cells are the basic structural, functional, and biological units of all living organisms. They can be classified into two major types: prokaryotic and eukaryotic cells.

• Prokaryotic cells (Bacteria & Archaea): Lack a true nucleus; genetic material is in a nucleoid region. Typically have a single, circular chromosome. Cell division occurs by binary fission. Organelles are generally absent.

• Eukaryotic cells (Animals, Plants, Fungi, Protists): Possess a true nucleus surrounded by a nuclear membrane. Chromosomes are paired and linear. Cell division occurs by mitosis. Many functional organelles are present (e.g., mitochondria, endoplasmic reticulum).

Main distinguishing feature: Presence or absence of a membrane-bound nucleus.

Basic Shapes of Bacteria

Bacteria exhibit three primary shapes:

• Coccus (plural: cocci): Spherical shape.

• Bacillus (plural: bacilli): Rod-shaped.

• Spirillum (plural: spirilla): Spiral or helical shape.

These shapes are important for bacterial identification and classification.

Glycocalyx: Structure and Function

The glycocalyx is a gelatinous, sticky substance that surrounds the outside of some bacterial cells.

• Capsule: Organized and firmly attached to the cell wall; protects against phagocytosis and desiccation.

• Slime layer: Loosely attached; aids in adherence to surfaces and biofilm formation.

Function: Protection, adherence, and evasion of host immune responses.

Cell Appendages: Flagella, Axial Filaments, Fimbriae, and Pili

• Flagella: Long, whip-like structures for motility; rotate to propel the cell. Movement can be toward or away from stimuli (chemotaxis).

• Axial filaments: Found in spirochetes; wrap around the cell, enabling corkscrew movement.

• Fimbriae: Short, hair-like structures; allow attachment to surfaces and other cells, important in biofilm formation.

• Pili: Longer than fimbriae; involved in motility and transfer of genetic material (conjugation).

Cell Walls: Gram-Positive, Gram-Negative, Acid-Fast, Archaea, and Mycoplasmas

Bacterial cell walls provide shape, protection, and are key in classification.

• Gram-positive: Thick peptidoglycan layer, contains teichoic acids. More permeable to antibiotics.

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides (LPS), no teichoic acids. Less permeable to antibiotics.

• Acid-fast: Contains mycolic acid (waxy); resists staining and desiccation (e.g., Mycobacterium).

• Archaea: Cell walls lack peptidoglycan; may contain pseudopeptidoglycan or proteins.

• Mycoplasmas: Lack cell walls; have sterols in plasma membrane for stability.

Comparison Table: Gram-Positive vs. Gram-Negative Cell Walls

Gram Stain Mechanism

The Gram stain is a differential staining technique used to classify bacteria:

• Gram-positive: Retain crystal violet stain (purple).

• Gram-negative: Do not retain crystal violet; counterstained with safranin (red/pink).

Steps: Application of crystal violet, iodine (mordant), alcohol (decolorizer), and safranin (counterstain).

Plasma Membrane: Structure and Function

The plasma membrane is a phospholipid bilayer with embedded proteins, acting as a selective barrier for transport of materials.

• Selective permeability: Controls entry and exit of substances.

• Transport mechanisms: Simple diffusion, facilitated diffusion, osmosis, active transport.

Transport Mechanisms

• Simple diffusion: Movement from high to low concentration; no energy required.

• Facilitated diffusion: Movement via transport proteins; no energy required.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active transport: Movement against concentration gradient; requires energy (ATP).

Osmosis equation:

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Osmotic Solutions and Effects on Cells

Organelles and Their Functions (Eukaryotic Cells)

• Nucleus: Contains DNA; controls cellular activities.

• Mitochondria: Site of ATP (energy) production.

• Endoplasmic reticulum: Protein and lipid synthesis.

• Ribosomes: Protein synthesis; found in both prokaryotes and eukaryotes.

• Golgi apparatus: Modification and transport of proteins.

Ribosomes: Structure and Function

Ribosomes are the sites of protein synthesis. They consist of two subunits (large and small) and are found in both prokaryotic and eukaryotic cells.

• Prokaryotic ribosomes: 70S (smaller)

• Eukaryotic ribosomes: 80S (larger)

Endospores: Formation and Function

Endospores are dormant, tough, non-reproductive structures formed by some bacteria (e.g., Bacillus, Clostridium) under harsh conditions.

• Resistant to drying, heat, starvation, and chemicals.

• Allow bacteria to survive until conditions improve.

Comparison: Archaea and Mycoplasmas

• Archaea: Cell walls lack peptidoglycan; unique membrane lipids; often extremophiles.

• Mycoplasmas: No cell wall; smallest free-living organisms; contain sterols in membrane.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Example: Escherichia coli is a prokaryotic bacterium with a single circular chromosome, a cell wall containing peptidoglycan, and divides by binary fission.

Additional info: Academic context and table entries have been expanded for clarity and completeness.