Prokaryotic Domains and Cell Structure

Prokaryotic Domains: Bacteria and Archaea

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

• Differences:

  • Cell Wall Composition: Bacteria typically have peptidoglycan in their cell walls, while Archaea possess pseudopeptidoglycan or other polymers.

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

  • Genetic Machinery: Archaea share similarities with eukaryotes in their transcription and translation machinery, whereas bacteria are distinct.

• Similarities:

  • Both lack a membrane-bound nucleus.

  • Both possess circular DNA.

  • Both reproduce asexually, typically by binary fission.

Prokaryotic Cell Structure

Prokaryotic cells are characterized by their simplicity and lack of membrane-bound organelles. Their structures can be categorized as intracellular and extracellular.

• Intracellular Structures:

  • Nucleoid: Region containing the circular DNA.

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

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

• Extracellular Structures:

  • Cell Wall: Provides shape and protection.

  • Plasma Membrane: Controls entry and exit of substances.

  • Flagella: Used for motility.

  • Pili/Fimbriae: Involved in attachment and conjugation.

  • Capsule: Protective outer layer, often involved in pathogenicity.

Size and Adaptations of Prokaryotic Cells

Prokaryotic cells are generally small, with an average diameter of 0.5–5 μm. Their small size allows for efficient nutrient uptake and rapid growth.

• Why Prokaryotes Are Small: High surface-area-to-volume ratio facilitates diffusion and metabolic efficiency.

Gram-Positive vs. Gram-Negative Bacterial Cell Envelopes

Structural Comparison

The cell envelope of bacteria is crucial for protection, nutrient transport, and interaction with the environment. Gram-positive and Gram-negative bacteria differ significantly in envelope structure.

Applications: Gram staining is used to differentiate bacterial species in clinical diagnostics.

Additional info: Gram-negative bacteria are generally more resistant to environmental stress and antibiotics due to their outer membrane.

Transport Mechanisms in Prokaryotic Cells

Active and Passive Transport

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

• Passive Transport:

  • Diffusion: Movement of molecules from high to low concentration without energy input.

  • Osmosis: Diffusion of water across a semi-permeable membrane.

  • Facilitated Diffusion: Transport via membrane proteins, still passive.

• Active Transport:

  • Requires energy (usually ATP) to move substances against their concentration gradient.

  • Examples include uptake of ions and nutrients via specific transporters.

Osmosis and Solutions

Osmotic Effects on Prokaryotic Cells

Osmosis affects cell volume and integrity depending on the surrounding solution.

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

• Hypotonic Solution: Lower solute concentration outside; water enters the cell, possibly causing lysis.

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

Motility and Adhesion Structures

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 to surfaces and conjugation.

• 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 organized in the nucleoid, a region without a surrounding membrane.

• Nucleoid: Contains the single, circular chromosome.

• Difference from Eukaryotes: No nuclear envelope; DNA replication and cell division are closely linked.

• DNA Replication: Begins at a single origin and proceeds bidirectionally, coordinated with cell growth.

Specialized Structures: Thylakoids, Storage Granules, Magnetosomes

Functions and Importance

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

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

• Magnetosomes: Membrane-bound crystals of magnetite, allowing orientation in magnetic fields.

Bacterial Endospores

Structure, Function, and Medical Relevance

Bacterial endospores are highly resistant, dormant structures formed by certain bacteria under stress.

• Function: Survival in extreme conditions (heat, desiccation, chemicals).

• Medically Important Genera: Bacillus and Clostridium.

• Examples:

  • Bacillus anthracis (anthrax)

  • Clostridium botulinum (botulism)

• Pathogenicity: Endospores can survive harsh environments and cause disease when conditions become favorable.

Antibiotics Targeting Prokaryotic Structures

Mechanisms and Selectivity

Antibiotics exploit differences between prokaryotic and eukaryotic cells to selectively inhibit bacterial growth.

• Ribosome-Targeting Antibiotics: Bacterial ribosomes (70S) differ from eukaryotic ribosomes (80S), allowing selective inhibition (e.g., tetracyclines, aminoglycosides).

• Peptidoglycan Synthesis Inhibitors: Drugs like penicillins and cephalosporins block cell wall synthesis, which is absent in human cells.

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

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