Prokaryotic Cell Structure and Function: Study Guide

Prokaryotic Domains

Classification and Comparison

Prokaryotes are classified into two major domains: Bacteria and Archaea. These domains share fundamental cellular features but also exhibit important differences.

• Differences:

  1. Cell Wall Composition: Bacteria typically have peptidoglycan in their cell walls, while Archaea possess pseudopeptidoglycan or other polymers.

  2. Membrane Lipids: Bacterial membranes contain ester-linked fatty acids; archaeal membranes have ether-linked isoprenoids.

  3. Genetic Machinery: Archaea's transcription and translation machinery resemble those of eukaryotes more than bacteria.

• Similarities:

  1. Both lack a membrane-bound nucleus.

  2. Both possess circular DNA.

  3. Both reproduce asexually, typically by binary fission.

Prokaryotic Cell Structure

Intracellular and Extracellular Components

A typical prokaryotic cell contains several key structures that are essential for its function and survival.

• Intracellular Structures:

  1. Nucleoid: Region containing the circular DNA.

  2. Ribosomes: Sites of protein synthesis (70S type).

  3. Cytoplasm: Gel-like matrix where metabolic reactions occur.

• Extracellular Structures:

  1. Cell Wall: Provides shape and protection.

  2. Plasma Membrane: Controls entry and exit of substances.

  3. Capsule (sometimes present): Offers additional protection and aids in adherence.

  4. Flagella: Used for motility.

  5. Pili/Fimbriae: Involved in attachment and genetic exchange.

Size and Adaptations of Prokaryotic Cells

Cell Size and Evolutionary Success

Prokaryotic cells are generally small, with diameters ranging from 0.5 to 5 micrometers. Their small size allows for rapid nutrient uptake and efficient metabolic processes, contributing to their evolutionary success.

• Why Prokaryotes are so Small:

  • High surface area-to-volume ratio facilitates efficient exchange of materials.

  • Rapid growth and division rates.

  • Adaptation to diverse environments.

Gram-Positive vs. Gram-Negative Bacteria

Structural Comparison and Implications

Bacteria are classified as Gram-positive or Gram-negative based on their cell envelope structure and response to Gram staining.

Feature	Gram-Positive	Gram-Negative
Cell Wall	Thick peptidoglycan layer	Thin peptidoglycan layer
Outer Membrane	Absent	Present (contains lipopolysaccharides)
Teichoic Acids	Present	Absent
Gram Stain Result	Purple	Pink/Red
Antibiotic Penetration	Generally more susceptible	Outer membrane restricts penetration
Pathogenicity	Varies; some produce exotoxins	Often produce endotoxins

Applications: Gram staining is used in clinical microbiology to guide antibiotic therapy and identify pathogens.

Transport Mechanisms in Prokaryotes

Active and Passive Transport

Prokaryotic cells exchange substances with their environment through various transport mechanisms.

• Passive Transport:

  • Diffusion: Movement of molecules from high to low concentration.

  • Osmosis: Diffusion of water across a semipermeable membrane.

  • Facilitated Diffusion: Transport via membrane proteins.

• Active Transport:

  • Requires energy (ATP) to move substances against their concentration gradient.

  • Examples: Sodium-potassium pump, transport of sugars and amino acids.

Osmosis and Solutions

Effects on Prokaryotic Cells

Osmosis affects prokaryotic cells differently depending on the surrounding solution.

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

• Hypotonic Solution: Water enters the cell; risk of lysis if cell wall is compromised.

• Hypertonic Solution: Water leaves the cell; cell may undergo plasmolysis.

Motility and Adhesion Structures

Flagella and Pili

Prokaryotes use specialized structures for movement and attachment.

• Flagella: Long, whip-like appendages that rotate to propel the cell.

• Pili (Fimbriae): Short, hair-like structures for attachment and genetic exchange.

• Benefits: Flagella enable movement toward nutrients (chemotaxis) and away from harmful substances; pili facilitate colonization and horizontal gene transfer.

Genetic Organization in Prokaryotes

Nucleoid and DNA Replication

Prokaryotic DNA is organized in a region called the nucleoid, which is not membrane-bound. DNA replication is coordinated with cell division.

• Nucleoid: Contains a single, circular chromosome.

• Difference from Eukaryotes: No nuclear envelope; replication and transcription occur in the same compartment.

• DNA Replication: Begins at a single origin and proceeds bidirectionally.

Specialized Structures: Thylakoids, Storage Granules, Magnetosomes

Functions and Examples

• Thylakoids: Membranous structures in photosynthetic bacteria for light-dependent reactions.

• Storage Granules: Reserve materials such as glycogen, polyphosphate, or sulfur.

• Magnetosomes: Membrane-bound crystals of magnetite; allow orientation along magnetic fields.

Bacterial Endospores

Formation, Function, and Medical Importance

Bacterial endospores are highly resistant, dormant structures formed by certain bacteria under adverse conditions.

• Function: Survival during extreme heat, desiccation, chemicals, and radiation.

• Medically Important Genera: Bacillus and Clostridium.

• Examples:

  • Bacillus anthracis (anthrax)

  • Clostridium botulinum (botulism)

• Pathogenicity: Endospores can survive in harsh environments and cause disease when conditions become favorable.

Antibiotics Targeting Prokaryotes

Mechanisms and Selectivity

Antibiotics can selectively target prokaryotic cells by interfering with unique cellular structures or processes.

• Ribosome-Targeting Antibiotics: Prokaryotic ribosomes (70S) differ from eukaryotic ribosomes (80S), allowing selective inhibition (e.g., tetracyclines, aminoglycosides).

• Peptidoglycan Synthesis Inhibitors: Drugs like penicillins and cephalosporins block cell wall synthesis, which is absent in animal cells.

• Effectiveness: These antibiotics are generally ineffective against eukaryotic cells due to structural differences.

Example: Penicillin inhibits transpeptidase, preventing cross-linking of peptidoglycan.

Equation:

$\text{Peptidoglycan synthesis:} \, NAG + NAM \xrightarrow{transpeptidase} \text{Peptidoglycan polymer}$

Additional info: Antibiotic selectivity is a cornerstone of antimicrobial therapy, minimizing harm to host cells while targeting pathogens.