Prokaryotic Cell Basics

Overview of Prokaryotic Domains

• Prokaryotic Domains: The two main prokaryotic domains are Bacteria and Archaea.

• Key Difference: Bacteria have peptidoglycan in their cell walls, while Archaea do not.

• Key Similarity: Both lack a membrane-bound nucleus and membrane-bound organelles.

Characteristics of Prokaryotic Cells

• Basic Description: Prokaryotic cells are unicellular organisms without a true nucleus. Their genetic material is located in a region called the nucleoid.

• Cell Size: Prokaryotes are generally small (0.2–2.0 μm in diameter) due to the absence of complex internal structures, which allows for efficient nutrient uptake and waste removal.

Shapes and Arrangements

• Common Shapes: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral), Vibrios (comma-shaped), Spirochetes (flexible spirals).

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-), and single cells.

Pleomorphism

• Definition: Pleomorphic organisms can alter their shape or size in response to environmental conditions.

• Clinical Impact: Pleomorphism can enhance a pathogen’s ability to evade the immune system and adapt to different host environments.

Binary Fission

• Definition: Binary fission is the primary method of reproduction in prokaryotes, involving the division of one cell into two genetically identical daughter cells.

• Summary of Process: DNA replication → Chromosome segregation → Cytokinesis (cell splits).

Extracellular Structures

Prokaryotic Plasma Membranes

• Structure: Composed of a phospholipid bilayer with embedded proteins.

• Function: Regulates transport of substances, site of metabolic processes.

Bacteria vs. Archaea: Plasma Membranes and Cell Walls

• Bacteria: Plasma membrane contains ester-linked phospholipids; cell wall contains peptidoglycan.

• Archaea: Plasma membrane contains ether-linked phospholipids; cell wall lacks peptidoglycan (may contain pseudopeptidoglycan).

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-Positive: Thick peptidoglycan layer, teichoic acids, no outer membrane.

• Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides (LPS).

Acid-Fast Genera

• Examples: Mycobacterium and Nocardia.

• Acid-Fast Stain: Detects mycolic acid in cell walls; useful for diagnosing tuberculosis and leprosy.

L-forms vs. Mycoplasma

• L-forms: Bacteria that have lost their cell wall but can revert to walled forms.

• Mycoplasma: Naturally lack a cell wall; resistant to antibiotics targeting cell wall synthesis.

Transport Mechanisms

• Passive Transport: Diffusion, facilitated diffusion, osmosis (no energy required).

• Active Transport: Requires energy (ATP); includes primary and secondary active transport.

Extracellular Structures (continued)

Osmosis and Tonicity

• Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration.

• Effects on Bacteria:

  • Hypertonic Solution: Cell shrinks (plasmolysis).

  • Hypotonic Solution: Cell swells (may burst if wall is weak).

  • Isotonic Solution: No net water movement.

Surface Structures

• Flagella: Motility structures; arrangements include monotrichous, lophotrichous, amphitrichous, and peritrichous.

• Fimbriae: Short, hair-like; attachment to surfaces.

• Pili: Longer; involved in conjugation (DNA transfer).

• Glycocalyx: Capsule or slime layer; protection and adherence.

Flagella Arrangements

• Regular Flagella: Extend from cell surface.

• Periplasmic Flagella: Located in the periplasmic space; characteristic of spirochetes, enabling corkscrew motion.

Examples of Medically Important Gram-Negative Bacteria

Examples of Medically Important Gram-Positive Bacteria

Intracellular Structures

Nucleoid

• Definition: The nucleoid is the region in a prokaryotic cell where the circular DNA chromosome is located, not surrounded by a membrane.

Prokaryotic Cytoskeleton

• Structure: Composed of protein filaments (e.g., FtsZ, MreB, CreS).

• Function: Maintains cell shape, assists in cell division, and organizes cellular components.

Prokaryotic Ribosomes

• Function: Sites of protein synthesis; 70S ribosomes (50S + 30S subunits).

• Endosymbiotic Theory: Structural similarity to eukaryotic organelle ribosomes supports the theory that mitochondria and chloroplasts evolved from prokaryotes.

Inclusion Bodies

• Role: Storage of nutrients and building blocks (e.g., glycogen, polyphosphate, sulfur granules).

• Examples: Gas vesicles, magnetosomes.

Endospores

• Structure: Highly resistant, dormant structures formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium).

• Function: Enable survival in harsh conditions (heat, desiccation, chemicals).

• Healthcare Challenge: Endospores are difficult to eradicate and can survive standard disinfection procedures.