Prokaryotic Cell Structure and Function: Study Notes for Microbiology

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., hot springs, salt lakes), but also present in moderate habitats.

Differences between Bacteria and Archaea:

• Cell wall composition: Bacteria have peptidoglycan in their cell walls; Archaea do not.

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

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

Similarities:

• Both lack a membrane-bound nucleus.

• Both are generally unicellular and microscopic.

• Both reproduce asexually, typically by binary fission.

Structure of a Prokaryotic Cell

Prokaryotic cells have a simple structure compared to eukaryotic cells. They lack membrane-bound organelles and a true nucleus.

• Intracellular structures:

  • Nucleoid (region containing DNA)

  • Ribosomes (site of protein synthesis)

  • Cytoplasm (gel-like matrix)

• Extracellular structures:

  • Cell wall (provides shape and protection)

  • Plasma membrane (controls entry and exit of substances)

  • Capsule or slime layer (protection, adhesion)

  • Flagella (motility)

  • Pili or fimbriae (attachment, conjugation)

Size of Prokaryotic Cells

Prokaryotic cells are typically much smaller than eukaryotic cells, usually ranging from 0.5 to 5 micrometers in diameter.

• Why are prokaryotes so small? Their small size 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 wall 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	Retain crystal violet (purple)	Lose crystal violet, take up safranin (pink/red)
Antibiotic Penetration	Generally more susceptible	Outer membrane can block antibiotics
Pathogenicity	Varies	Often more resistant to environmental stress

Applications: Gram staining is a key diagnostic tool in microbiology for identifying bacterial species and guiding antibiotic therapy.

Transport Mechanisms in Prokaryotic Cells

Prokaryotic cells use both passive and active transport mechanisms to exchange substances with their environment.

• 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 (e.g., ATP-driven pumps, group translocation).

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 that rotate to propel the cell. Important for chemotaxis (movement toward or away from chemical stimuli).

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

Example: Periplasmic flagella in spirochetes allow movement through viscous environments, aiding in survival and infection.

Organization of Genetic Material

Prokaryotic DNA is located in the nucleoid, a region within the cytoplasm that lacks a surrounding membrane.

• Prokaryotic nucleoid: Single, circular DNA molecule; may also contain plasmids (small, circular DNA).

• Eukaryotic nucleus: DNA enclosed within a nuclear membrane; multiple linear chromosomes.

• DNA replication: In prokaryotes, replication is coordinated with cell division and occurs rapidly due to the simple structure.

Specialized Structures: Thylakoids, Storage Granules, Magnetosomes

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

• Storage granules: Reserve deposits of nutrients (e.g., glycogen, polyphosphate, 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 bacteria (e.g., Bacillus, Clostridium) to survive harsh conditions.

• Function: Protect genetic material during extreme heat, desiccation, chemicals, and radiation.

• Medically important examples:

  • Bacillus anthracis (anthrax)

  • Clostridium botulinum (botulism)

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

Antibiotics Targeting Bacterial Structures

Some antibiotics specifically target structures unique to bacteria, making them effective against prokaryotes but not eukaryotic cells.

• Ribosome-targeting antibiotics: Many antibiotics (e.g., tetracyclines, aminoglycosides) bind to the 70S ribosome of bacteria, inhibiting protein synthesis. Eukaryotic ribosomes are 80S, so these drugs are selective.

• Peptidoglycan synthesis inhibitors: Beta-lactam antibiotics (e.g., penicillins) inhibit enzymes involved in peptidoglycan synthesis, weakening the bacterial cell wall. Eukaryotic cells lack peptidoglycan.

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

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