Prokaryotic Cell Structure and Function

Prokaryotic Domains

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

• Bacteria: Ubiquitous microorganisms found in diverse environments, including soil, water, and as symbionts or pathogens in other organisms.

• Archaea: Microorganisms often found in extreme environments (e.g., high temperature, salinity, or acidity), but also present in moderate habitats.

Differences between Bacteria and Archaea:

• Cell wall composition: Bacteria have peptidoglycan; Archaea lack peptidoglycan and may have pseudopeptidoglycan or other polymers.

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

• Genetic machinery: Archaea have some genes and metabolic pathways more similar to eukaryotes than to bacteria.

Similarities:

• Both lack a membrane-bound nucleus.

• Both have circular DNA.

• Both reproduce asexually, typically by binary fission.

Structure of a Prokaryotic Cell

Prokaryotic cells are structurally simpler than eukaryotic cells, lacking membrane-bound organelles. Their structures can be divided into intracellular and extracellular components.

• Intracellular structures:

  • Nucleoid (region containing circular DNA)

  • Ribosomes (70S type)

  • Cytoplasm

• Extracellular structures:

  • Cell membrane (plasma membrane)

  • Cell wall

  • Capsule or slime layer (in some species)

  • Flagella (for motility, in some species)

  • Pili or fimbriae (for attachment, in some species)

Size of Prokaryotic Cells

The average diameter of a prokaryotic cell is approximately 0.5–5.0 μm. Prokaryotes are generally much smaller than eukaryotic cells, which allows for a high surface-area-to-volume ratio, facilitating efficient nutrient uptake and waste removal.

Gram-Positive vs. Gram-Negative Bacteria

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

• Gram-Positive Bacteria:

  • Thick peptidoglycan layer

  • Teichoic acids present

  • No outer membrane

  • Stain purple in Gram stain

• Gram-Negative Bacteria:

  • Thin peptidoglycan layer

  • Outer membrane present (contains lipopolysaccharide, LPS)

  • Periplasmic space between inner and outer membranes

  • Stain pink/red in Gram stain

Implications of Cell Envelope Differences:

• Gram-negative bacteria are generally more resistant to antibiotics due to the outer membrane barrier.

• Gram-positive bacteria are more susceptible to antibiotics targeting peptidoglycan synthesis (e.g., penicillins).

• Differences affect susceptibility to drying, staining, and immune system recognition.

Transport Mechanisms in Prokaryotes

Prokaryotic cells exchange substances with their environment using passive and active transport mechanisms.

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, facilitated diffusion, osmosis).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP or proton motive force).

Example: Uptake of glucose via a specific transporter protein.

Osmosis and Tonicity

Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to higher 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).

Motility and Adhesion Structures

Prokaryotes use specialized structures for movement and attachment.

• Flagella: Long, whip-like appendages used for motility. Rotation of the flagellum propels the cell.

• Pili (Fimbriae): Short, hair-like structures used for attachment to surfaces or other cells. Some pili (sex pili) are involved in conjugation.

Example: Treponema pallidum (a spirochete) uses periplasmic flagella for corkscrew motility, aiding in tissue penetration and survival.

Organization of Genetic Material

Prokaryotic DNA is located in the nucleoid region, not enclosed by a membrane. DNA replication is coordinated with cell division.

• Nucleoid: Irregularly-shaped region containing the circular chromosome.

• Plasmids: Small, circular DNA molecules that replicate independently of the chromosome.

• DNA replication begins at a single origin and proceeds bidirectionally.

Difference from Eukaryotes: Eukaryotes have a membrane-bound nucleus and multiple linear chromosomes.

Specialized Structures: Thylakoids, Storage Granules, Magnetosomes

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

• Storage Granules: Reserve deposits of nutrients (e.g., polyphosphate, glycogen, sulfur).

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

Bacterial Endospores

Endospores are highly resistant, dormant structures formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium) in response to harsh conditions.

• Resistant to heat, desiccation, chemicals, and radiation.

• Medically important endospore-formers include Bacillus anthracis (anthrax) and Clostridium botulinum (botulism).

• Endospores can survive in the environment for years and cause disease when conditions become favorable for germination.

Antibiotics Targeting Bacterial Structures

Many antibiotics exploit differences between prokaryotic and eukaryotic cells.

• Ribosome-targeting antibiotics: (e.g., tetracyclines, aminoglycosides) bind to the 70S ribosome, inhibiting protein synthesis in bacteria but not affecting the 80S ribosomes of eukaryotes.

• Peptidoglycan synthesis inhibitors: (e.g., penicillins, cephalosporins) block cell wall synthesis, which is unique to bacteria.

Why these antibiotics are selective: Eukaryotic cells lack peptidoglycan and have structurally different ribosomes, so these drugs are less toxic to human cells.

Table: Comparison of Gram-Positive and Gram-Negative Bacteria

Additional info: Some explanations and examples were expanded for clarity and completeness, including details on antibiotic mechanisms and the structure of specialized prokaryotic organelles.