Introduction to Prokaryotic and Eukaryotic Cells: Structure, Function, and Laboratory Techniques

Introduction to Prokaryotic and Eukaryotic Cells

Overview

This module covers the comparative biology of bacteria, archaea, and eukaryotes, focusing on cell structure, taxonomy, and laboratory techniques essential for microbiology. Understanding these foundational concepts is critical for exploring microbial diversity, physiology, and laboratory identification methods.

Comparing Bacteria, Archaea, and Eukaryotes

Taxonomy and Nomenclature

• Genus and Species: The scientific name of a microbe consists of two parts: the genus (capitalized) and the species (lowercase). Both should be italicized (e.g., Escherichia coli) or underlined if handwritten.

• Taxonomic Hierarchy: The classification system from broadest to most specific is: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.

• Strain: A strain is a genetic variant or subtype of a microbial species. Strains may differ in pathogenicity, metabolism, or other characteristics.

Major Differences Between Eukaryotic and Prokaryotic Organisms

• Prokaryotes: Include bacteria and archaea. They lack a membrane-bound nucleus and organelles.

• Eukaryotes: Include protists, fungi, plants, and animals. They possess a true nucleus and membrane-bound organelles.

• Cell Size: Prokaryotic cells are generally smaller (0.5–5 μm) than eukaryotic cells (10–100 μm).

• Cell Division: Prokaryotes divide by binary fission; eukaryotes divide by mitosis or meiosis.

Binary Fission

• Definition: A form of asexual reproduction in prokaryotes where a single cell divides into two identical daughter cells.

• Steps:

  1. DNA replication

  2. Chromosome segregation

  3. Cytokinesis (division of the cytoplasm)

Key Bacterial Structures and Their Functions

• Nucleoid: Region containing the bacterial chromosome (DNA); not membrane-bound.

• Flagella: Long, whip-like structures used for motility.

• Pili: Hair-like appendages involved in attachment and conjugation (transfer of DNA).

• Fimbriae: Short, bristle-like fibers for attachment to surfaces.

• Ribosome: Site of protein synthesis (70S in prokaryotes).

• Capsule: Gelatinous outer layer that protects against desiccation and phagocytosis.

Endosymbiotic Theory and Evolution of Eukaryotic Organelles

• Endosymbiotic Theory: Proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence:

  • Mitochondria and chloroplasts have their own circular DNA.

  • They possess 70S ribosomes, similar to bacteria.

  • They replicate independently by binary fission.

Key Eukaryotic Structures and Their Functions

• Nucleus: Contains the cell's genetic material; site of transcription.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Site of aerobic respiration and ATP production.

• Chloroplast: Site of photosynthesis in plants and algae.

• Lysosome: Contains digestive enzymes for breaking down macromolecules.

• Peroxisome: Breaks down fatty acids and detoxifies hydrogen peroxide.

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick (multiple layers)	Thin (single layer)
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains lipopolysaccharide)
Staining	Retains crystal violet (purple)	Counterstained with safranin (pink/red)

Osmosis and Bacterial Cells

• Osmosis: The movement of water across a semipermeable membrane from an area of lower solute concentration to higher solute concentration.

• Effects of Different Solutions:

  • Hypertonic: Water leaves the cell; cell shrinks (plasmolysis).

  • Hypotonic: Water enters the cell; cell may burst (lysis) if the wall is weak.

  • Isotonic: No net movement of water; cell remains stable.

Eukaryotic Plasma Membranes

• Structure: Composed of a phospholipid bilayer with embedded proteins, cholesterol (in animals), and carbohydrates.

• Differences Among Kingdoms:

  • Animal cells contain cholesterol; plant cells contain phytosterols.

  • Fungal membranes contain ergosterol.

Laboratory Techniques in Microbiology

Aseptic Technique

• Best Practices:

  • Flame sterilization of inoculating loops and needles before and after use.

  • Minimize exposure of sterile media and instruments to air.

  • Work near a flame or in a biosafety cabinet to reduce contamination.

  • Disinfect work surfaces before and after procedures.

• Inverting Plates: Plates are inverted during incubation to prevent condensation from dripping onto the agar surface, which could spread colonies and cause contamination.

Microbial Growth Media

• General Purpose Medium: Nutrient agar or nutrient broth supports the growth of a wide variety of bacteria.

• Common Formats of Media:

  1. Broth (liquid)

  2. Agar plates (solid)

  3. Agar slants or deeps (semi-solid)

Streak Plate Method for Isolation

• Purpose: To obtain isolated colonies from a mixed or pure culture.

• Procedure:

  1. Flame the loop and cool it.

  2. Obtain a small amount of culture and streak it across one quadrant of the agar plate.

  3. Flame the loop again, drag through the first quadrant into the second, and streak.

  4. Repeat for third and fourth quadrants, flaming the loop between each.

  5. Incubate the plate inverted.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Size	0.5–5 μm	10–100 μm
Cell Division	Binary fission	Mitosis/meiosis
Examples	Bacteria, Archaea	Fungi, Protists, Plants, Animals

Additional info:

• Understanding these foundational concepts is essential for interpreting microbial diversity, physiology, and laboratory identification methods.