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, such as hot springs and salt lakes, but also present in more common 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.

• 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

The average diameter of a prokaryotic cell is approximately 0.5–5.0 μm. Prokaryotes are small to maximize 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.

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Transport Mechanisms in Prokaryotes

Prokaryotic cells use various transport mechanisms to exchange substances with their environment.

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

  • Simple diffusion

  • Facilitated diffusion (via transport proteins)

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

  • Primary active transport (e.g., ATP-binding cassette transporters)

  • Secondary active transport (e.g., symporters, antiporters)

Osmosis and Solutions

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).

Prokaryotic Motility and Adhesion

Prokaryotes use specialized structures for movement and attachment.

• Flagella: Long, whip-like appendages that rotate to propel the cell. Beneficial for seeking nutrients and escaping harmful environments.

• 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 survival and infection.

Organization of DNA in Prokaryotes

Prokaryotic DNA is located in the nucleoid, a region within the cytoplasm. Unlike eukaryotes, prokaryotes lack a membrane-bound nucleus.

• DNA is typically a single, circular chromosome.

• May contain plasmids (small, circular DNA molecules).

• DNA replication is coordinated with cell division (binary fission).

Comparison: Eukaryotic cells have linear chromosomes within a nuclear envelope; prokaryotes do not.

Specialized Structures: Thylakoids, Storage Granules, Magnetosomes

• 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 magnetic minerals, allowing orientation to Earth's magnetic field.

Bacterial Endospores

Endospores are highly resistant, dormant structures formed by some bacteria to survive extreme conditions.

• Formed by genera such as Bacillus and Clostridium.

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

• Medically important examples: Bacillus anthracis (anthrax), Clostridium botulinum (botulism).

• Endospores can cause disease when they germinate in a suitable host.

Antibiotics Targeting Bacteria

Some antibiotics specifically target bacterial structures or processes not found in eukaryotic cells.

• Ribosome-targeting antibiotics: e.g., tetracyclines, macrolides, aminoglycosides. These bind to bacterial ribosomes (70S), inhibiting protein synthesis.

• Peptidoglycan synthesis inhibitors: e.g., penicillins, cephalosporins. These block cell wall synthesis, leading to cell lysis.

• Eukaryotic cells have 80S ribosomes and lack peptidoglycan, so these antibiotics are selectively toxic to bacteria.

Example: Penicillin inhibits transpeptidase, an enzyme involved in cross-linking peptidoglycan chains. *Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curricula.*