Prokaryotic Cell Structure and Function: Study Notes

Prokaryotic Domains and Cell Structure

Prokaryotic Domains: Bacteria and Archaea

Prokaryotes are divided into two major domains: Bacteria and Archaea. These domains represent fundamental divisions in the tree of life, each with unique characteristics.

• Differences:

  • Cell Wall Composition: Bacteria typically have peptidoglycan in their cell walls, while Archaea do not; Archaea may have pseudopeptidoglycan or other polymers.

  • Membrane Lipids: Bacterial membranes contain ester-linked lipids; archaeal membranes have ether-linked lipids, often forming monolayers.

  • Genetic Machinery: Archaea have some genes and metabolic pathways more similar to eukaryotes than to bacteria, such as those involved in transcription and translation.

• Similarities:

  • Both lack a membrane-bound nucleus.

  • Both lack membrane-bound organelles.

  • 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 (3):

  • Nucleoid: Region containing the circular DNA chromosome.

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

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

• Extracellular Structures (5):

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

  • Plasma Membrane: Selectively permeable barrier controlling entry and exit of substances.

  • Capsule (Glycocalyx): Protective outer layer that aids in adherence and evasion of host defenses.

  • Flagella: Structures for motility.

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

Average Size and Simplicity of Prokaryotic Cells

Prokaryotic cells are generally small, typically ranging from 0.5 to 5 micrometers in diameter. Their small size allows for a high surface-area-to-volume ratio, facilitating efficient nutrient uptake and waste removal. Prokaryotes are considered simple because they lack membrane-bound organelles and a true nucleus.

Gram-Positive vs. Gram-Negative Bacteria

Structural Components and Gram Staining

Gram-positive and Gram-negative bacteria differ in their cell envelope structure, which affects their staining properties, antibiotic susceptibility, and environmental resilience.

Feature	Gram-Positive	Gram-Negative
Cell Wall	Thick peptidoglycan layer	Thin peptidoglycan layer
Outer Membrane	Absent	Present (contains lipopolysaccharide, LPS)
Teichoic Acids	Present	Absent
Gram Stain	Retains crystal violet (purple)	Counterstained with safranin (pink/red)
Antibiotic Penetration	Generally more susceptible to antibiotics targeting peptidoglycan	Outer membrane can block some antibiotics
Desiccation Resistance	More resistant due to thick wall	Less resistant
Detergent Susceptibility	Less susceptible	More susceptible due to outer membrane disruption

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

Transport Mechanisms in Prokaryotic Cells

Active and Passive Transport

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

• Passive Transport: Movement of molecules down their concentration gradient without energy input.

  • Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2).

  • Facilitated Diffusion: Movement via specific membrane proteins (channels or carriers).

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

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

  • Primary Active Transport: Direct use of ATP (e.g., ABC transporters).

  • Secondary Active Transport: Uses energy from an ion gradient (e.g., symporters, antiporters).

Osmosis and Solutions

Isotonic, Hypotonic, and Hypertonic Solutions

Osmosis is the movement of water across a semipermeable membrane from an area of low solute concentration to high solute concentration.

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell, which may cause swelling or lysis.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, causing shrinkage (plasmolysis).

Prokaryotic cells often possess rigid cell walls to prevent lysis in hypotonic environments.

Prokaryotic Motility and Adhesion

Structures for Motility and Adhesion

• Flagella: Long, whip-like appendages that rotate to propel the cell. Flagellar motility enables bacteria to move toward nutrients (chemotaxis) or away from harmful substances.

• Pili (Fimbriae): Short, hair-like structures that facilitate attachment to surfaces and other cells. Specialized pili (sex pili) are involved in conjugation (DNA transfer).

Benefit of Flagella: Enhance survival by enabling movement toward favorable environments and away from threats.

Organization of Genetic Material in Prokaryotes

Nucleoid and DNA Replication

Prokaryotic DNA is located in the nucleoid, a region within the cytoplasm that lacks a surrounding membrane. The DNA is typically a single, circular chromosome.

• Difference from Eukaryotes: Eukaryotic DNA is enclosed within a nuclear membrane; prokaryotic DNA is not.

• Coordination of DNA Replication and Cell Division: DNA replication begins at a single origin and proceeds bidirectionally. Replication is tightly coordinated with cell division (binary fission) to ensure each daughter cell receives a complete genome.

Specialized Structures in Prokaryotes

Thylakoids, Storage Granules, and Magnetosomes

• Thylakoids: Membranous structures in photosynthetic bacteria (e.g., cyanobacteria) where light-dependent reactions occur.

• Storage Granules: Reserve deposits of nutrients such as glycogen, polyphosphate, or sulfur.

• Magnetosomes: Membrane-bound iron-containing structures that allow bacteria to orient along magnetic fields.

Bacterial Endospores

Function and Medical Importance

Endospores are highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus and Clostridium species) in response to adverse conditions.

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

• Medically Important Examples:

  • Bacillus anthracis (causes anthrax)

  • Clostridium botulinum (causes botulism)

• Pathogenicity: Endospores can persist in the environment and cause disease when conditions become favorable for germination.

Antibiotics Targeting Bacterial Structures

Mechanisms and Selectivity

Many antibiotics exploit differences between prokaryotic and eukaryotic cells to selectively target bacteria.

• Ribosome-Targeting Antibiotics: Bacterial ribosomes are 70S (composed of 50S and 30S subunits), while eukaryotic ribosomes are 80S. Antibiotics such as tetracyclines and aminoglycosides bind to bacterial ribosomes, inhibiting protein synthesis.

• Peptidoglycan Synthesis Inhibitors: Antibiotics like penicillins and cephalosporins inhibit enzymes involved in peptidoglycan synthesis, weakening the bacterial cell wall and causing lysis. Eukaryotic cells lack peptidoglycan, so they are unaffected.

Example: Penicillin is effective against Gram-positive bacteria due to their thick peptidoglycan layer.

Additional info: Some antibiotics may have side effects on eukaryotic mitochondria, which have ribosomes similar to those of bacteria.