Prokaryotic Domains and Cell Structure

Prokaryotic Domains: Bacteria and Archaea

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

• Differences:

  • Cell Wall Composition: Bacterial cell walls contain peptidoglycan, while archaeal cell walls do not; Archaea may have pseudopeptidoglycan or other polymers.

  • Membrane Lipids: Bacteria have ester-linked lipids; Archaea have ether-linked lipids, often with branched isoprenoid chains.

  • Genetic Machinery: Archaeal transcription and translation machinery are more similar to eukaryotes than to bacteria.

• Similarities:

  • Both lack a membrane-bound nucleus.

  • Both lack membrane-bound organelles.

  • Both reproduce asexually, typically by binary fission.

• Example: Escherichia coli (Bacteria), Halobacterium salinarum (Archaea)

Structure of a Prokaryotic Cell

Prokaryotic cells are structurally simpler than eukaryotic cells, lacking a nucleus and most organelles. They possess unique features that enable survival in diverse environments.

• Intracellular Structures:

  1. Nucleoid (region containing circular DNA)

  2. Ribosomes (70S type)

  3. Cytoplasm

• Extracellular Structures:

  1. Cell wall

  2. Plasma membrane

  3. Capsule or slime layer (in some species)

  4. Flagella (for motility)

  5. Pili or fimbriae (for attachment)

• Average Size: Prokaryotic cells typically range from 0.5 to 5 μm in diameter, much smaller than most eukaryotic cells.

• Why Prokaryotes Are Small: Their small size allows for a high surface area-to-volume ratio, facilitating efficient nutrient uptake and waste removal.

Cell Envelope and Gram Staining

Gram-Positive vs. Gram-Negative Bacteria

The cell envelope structure is a key distinguishing feature between Gram-positive and Gram-negative bacteria, affecting staining, antibiotic susceptibility, and environmental resistance.

Example: Staphylococcus aureus (Gram-positive), Escherichia coli (Gram-negative)

Transport Mechanisms in Prokaryotes

Active and Passive Transport

Prokaryotic cells exchange substances with their environment using various transport mechanisms across the plasma membrane.

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

  • Simple Diffusion: Small, nonpolar molecules (e.g., O2, CO2) pass directly through the membrane.

  • Facilitated Diffusion: Transport proteins assist movement of larger or polar molecules (e.g., glucose).

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

  • Example: Sodium-potassium pump, ABC transporters.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. The effect of osmosis on prokaryotic cells depends on the tonicity of the surrounding solution:

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

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

• Hypertonic: Higher solute concentration outside; water leaves the cell, causing shrinkage (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. Important for chemotaxis (movement toward or away from 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).

• Benefit: Flagella enable bacteria to move toward nutrients or away from harmful substances, enhancing survival.

Genetic Organization in Prokaryotes

Nucleoid and DNA Replication

Prokaryotic DNA is organized in a region called the nucleoid, which is not membrane-bound. DNA replication and cell division are tightly coordinated.

• Nucleoid: Contains a single, circular chromosome; lacks a nuclear envelope.

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

• DNA Replication: Begins at a single origin of replication and proceeds bidirectionally. Replication is coordinated with cell growth and division (binary fission).

Specialized Structures and Survival Mechanisms

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 (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 bacteria to survive extreme conditions.

• Function: Protect genetic material during harsh conditions (heat, desiccation, chemicals).

• Medically Important Genera: Bacillus (e.g., B. anthracis), Clostridium (e.g., C. botulinum).

• Human Disease: Endospores can survive in the environment and cause diseases such as anthrax, botulism, and tetanus when conditions become favorable.

Antibiotics and Cell Structure

Antibiotic Targets: Ribosomes and Peptidoglycan Synthesis

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

• Ribosome-Targeting Antibiotics: Prokaryotic ribosomes (70S) differ from eukaryotic ribosomes (80S). Antibiotics like tetracycline and streptomycin bind to bacterial ribosomes, inhibiting protein synthesis.

• Peptidoglycan Synthesis Inhibitors: Antibiotics such as penicillin inhibit enzymes involved in peptidoglycan synthesis, weakening the bacterial cell wall and causing lysis.

• Selective Toxicity: These antibiotics are generally ineffective against eukaryotic cells due to structural differences in ribosomes and absence of peptidoglycan.

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

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology knowledge.