Prokaryotic and Eukaryotic Cells

Key Differences and Features

Prokaryotic and eukaryotic cells are the two fundamental cell types in biology, distinguished by their structural and functional characteristics.

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. Examples: Bacteria, Archaea.

• Eukaryotic Cells: Possess a true nucleus and various membrane-bound organelles. Examples: Fungi, Protozoa, Plants, Animals.

• Unique to Eukaryotes: Organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.

• Unique to Prokaryotes: Plasmids, nucleoid region, and often cell walls with peptidoglycan.

Example: Escherichia coli is a prokaryote; Saccharomyces cerevisiae (yeast) is a eukaryote.

The Microscope

Principles of Light Microscopy

Microscopy is essential for visualizing microorganisms. The compound light microscope uses visible light to magnify specimens.

• Path of Light: Light passes through the condenser, specimen, objective lens, and ocular lens to the observer's eye.

• Magnification: Total magnification = (Objective lens) × (Ocular lens).

• Resolution: The ability to distinguish two points as separate; higher resolution allows for clearer images.

Example: Observing bacterial cell shape and arrangement using oil immersion at 1000× magnification.

Gram Positive vs. Gram Negative Cell Wall Structure

Layers, Molecules, and Structural Differences

The Gram stain differentiates bacteria based on cell wall structure, which affects staining properties and antibiotic susceptibility.

• Peptidoglycan: A mesh-like polymer forming the bacterial cell wall.

• Gram Positive: Thick peptidoglycan layer, teichoic acids, no outer membrane.

• Gram Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), periplasmic space.

• Antibiotic Sensitivity: Gram-positive bacteria are generally more susceptible to antibiotics targeting peptidoglycan (e.g., penicillin).

• Clinical Relevance: Gram-negative bacteria are often more resistant due to the outer membrane barrier.

Example: Staphylococcus aureus (Gram-positive), Escherichia coli (Gram-negative).

Gram Stain Technique

Principles and Interpretation

The Gram stain is a differential staining technique that classifies bacteria based on cell wall properties.

1. Crystal violet (primary stain)

2. Iodine (mordant)

3. Alcohol/acetone (decolorizer)

4. Safranin (counterstain)

• Gram-positive: Retain crystal violet, appear purple.

• Gram-negative: Lose crystal violet, take up safranin, appear pink/red.

• Acid-fast bacteria: Have unique cell walls (e.g., Mycobacterium tuberculosis), resist Gram stain, require acid-fast staining.

Example: Gram stain distinguishes Streptococcus (Gram-positive) from Neisseria (Gram-negative).

Cellular Structures: Flagella, Pili, Capsules, and Endospores

Functions and Types

• Flagella: Motility structures; types include monotrichous, lophotrichous, amphitrichous, peritrichous, and atrichous (no flagella).

• Pili (Fimbriae): Hair-like appendages for attachment and conjugation (DNA transfer).

• Capsule: Polysaccharide layer outside the cell wall; protects against phagocytosis and desiccation, aids in immune evasion.

• Endospores: Highly resistant, dormant structures formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium); survive extreme conditions.

Example: Bacillus anthracis forms endospores; Klebsiella pneumoniae has a prominent capsule.

Metabolism

Catabolism vs. Anabolism and Energy Production

Metabolism encompasses all chemical reactions in a cell, divided into catabolism (breakdown) and anabolism (biosynthesis).

• Catabolism: Breaks down molecules, releases energy (e.g., glycolysis, Krebs cycle).

• Anabolism: Synthesizes complex molecules, requires energy (e.g., protein synthesis).

• ATP: Main energy currency; produced by substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.

• Redox Reactions: Involve electron transfer; oxidation (loss of electrons), reduction (gain of electrons).

Example: Glycolysis converts glucose to pyruvate, generating ATP and NADH.

Bioenergetics

Electron Transport and Energy Yield

Bioenergetics studies how cells obtain and use energy, focusing on electron transport chains and ATP synthesis.

• Final Electron Acceptor:

  • Aerobic respiration: Oxygen (O2)

  • Anaerobic respiration: Inorganic molecules (e.g., nitrate, sulfate)

  • Fermentation: Organic molecules (e.g., pyruvate)

• ATP Yield: Complete oxidation of one glucose molecule yields up to 38 ATP in prokaryotes.

• FADH2 and NADH: Electron carriers; NADH yields more ATP than FADH2 in the electron transport chain.

Equation:

Example: During aerobic respiration, oxygen is reduced to water at the end of the electron transport chain.

Enzymes and Inhibition

Mechanisms and Regulation

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Active Site: Region where substrate binds and reaction occurs.

• Competitive Inhibition: Inhibitor competes with substrate for active site.

• Allosteric Inhibition: Inhibitor binds elsewhere, changing enzyme shape and function.

• Environmental Factors: Temperature, pH, and inhibitors affect enzyme activity and bacterial growth.

Example: Sulfa drugs act as competitive inhibitors of folic acid synthesis in bacteria.

Definitions and Key Concepts

Important Terms in Microbiology

• Pathogenic Microbes: Microorganisms that cause disease.

• Spontaneous Generation: Disproven theory that life arises from non-living matter.

• Genetic Engineering: Manipulation of DNA for practical purposes.

• Bioremediation: Use of microbes to clean up environmental contaminants.

• Pilus: Structure involved in bacterial conjugation (DNA transfer).

• Archaea: Prokaryotes distinct from bacteria, often extremophiles.

Example: Thermophiles are archaea that thrive in high-temperature environments.

Taxonomy

Naming and Classification

Taxonomy is the science of classifying organisms, using a binomial system (genus and species).

• Genus: Capitalized, first part of the name (e.g., Escherichia).

• Species: Lowercase, second part (e.g., coli).

• Order: Genus comes first, then species.

Example: Staphylococcus aureus (genus: Staphylococcus, species: aureus).

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curricula.