Prokaryotic Cell Structure and Function: Study Notes for Microbiology

Prokaryotic Domains and Cell Structure

Prokaryotic Domains

Prokaryotes are classified into two major domains: Bacteria and Archaea. These domains represent distinct evolutionary lineages with unique characteristics.

• Differences between Bacteria and Archaea:

  • Cell Wall Composition: Bacteria typically have peptidoglycan in their cell walls, while Archaea do not.

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

  • Genetic Machinery: Archaea have some genetic processes (e.g., transcription, translation) more similar to eukaryotes than bacteria.

• Similarities between Bacteria and Archaea:

  • Both lack a membrane-bound nucleus.

  • Both are generally unicellular organisms.

  • Both reproduce asexually, typically by binary fission.

Prokaryotic Cell Structure

A typical prokaryotic cell consists of several intracellular and extracellular structures that contribute to its function and survival.

• Intracellular Structures:

  • Nucleoid: Region containing the circular DNA molecule.

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

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

• Extracellular Structures:

  • Cell Wall: Provides shape and protection; composition varies between Gram-positive and Gram-negative bacteria.

  • Plasma Membrane: Controls movement of substances in and out of the cell.

  • Capsule or Slime Layer: Offers protection and aids in adherence to surfaces.

  • Flagella: Used for motility.

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

Size of Prokaryotic Cells

Prokaryotic cells are generally small, with an average diameter of 0.5–5.0 μm. Their small size allows for efficient nutrient uptake and waste removal.

• Why are prokaryotes so small? High surface-area-to-volume ratio facilitates rapid exchange of materials with the environment.

Gram-Positive vs. Gram-Negative Bacteria

Structural Components

Gram-positive and Gram-negative bacteria differ in their cell envelope structure, which affects their staining properties and susceptibility to antibiotics.

Feature	Gram-Positive	Gram-Negative
Cell Wall	Thick peptidoglycan layer	Thin peptidoglycan layer
Outer Membrane	Absent	Present (contains lipopolysaccharide)
Teichoic Acids	Present	Absent
Gram Stain Result	Purple	Pink/Red
Antibiotic Penetration	Generally more susceptible	Outer membrane restricts penetration

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Transport Mechanisms in Prokaryotes

Active and Passive Transport

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

• Passive Transport:

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

  • Facilitated Diffusion: Transport via membrane proteins without energy input.

• Active Transport:

  • Primary Active Transport: Uses ATP to move substances against their concentration gradient.

  • Secondary Active Transport: Uses energy from ion gradients.

Osmosis and Solutions

Osmotic Environments

Osmosis is the movement of water across a semipermeable membrane. Prokaryotic cells respond differently to various osmotic environments:

• Isotonic Solution: No net movement of water; cell volume 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 Structures and Adhesion

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.

Benefit: Flagella enable bacteria to move toward nutrients (chemotaxis) and away from harmful substances, enhancing survival.

Genetic Organization in Prokaryotes

Nucleoid and DNA Replication

Prokaryotic DNA is located in the nucleoid region, not enclosed by a membrane. 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 in Prokaryotes

Thylakoids, Storage Granules, Magnetosomes

• 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

Endospore Formation and Medical Importance

Endospores are highly resistant, dormant structures formed by some bacteria under adverse conditions.

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

• Medically Important Genera:

  • Bacillus (e.g., Bacillus anthracis)

  • Clostridium (e.g., Clostridium botulinum, Clostridium difficile)

• Diseases: Anthrax, botulism, tetanus, and pseudomembranous colitis.

Antibiotics Targeting Prokaryotes

Mechanisms of Action

Antibiotics can target unique features of prokaryotic cells, such as ribosomes and cell wall synthesis.

• Ribosome-Targeting Antibiotics: Bind to 70S ribosomes, inhibiting protein synthesis (e.g., tetracyclines, aminoglycosides).

• Peptidoglycan Synthesis Inhibitors: Block cell wall formation (e.g., penicillins, cephalosporins).

• Selective Toxicity: Eukaryotic cells have 80S ribosomes and lack peptidoglycan, so these antibiotics are less toxic to human cells.

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

Equation:

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

Additional info: Antibiotic resistance can arise through mutation or acquisition of resistance genes, emphasizing the importance of prudent antibiotic use.