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, such as hot springs, salt lakes, and anaerobic conditions.

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)

Average Size of Prokaryotic Cells:

• Most prokaryotic cells range from 0.5 to 5 micrometers (μm) in diameter.

• They are small to maximize surface area-to-volume ratio, facilitating efficient nutrient uptake and waste removal.

Gram-Positive vs. Gram-Negative Bacteria

Gram staining differentiates bacteria based on cell wall structure, which affects their staining properties and susceptibility to antibiotics.

Feature	Gram-Positive	Gram-Negative
Cell Wall Thickness	Thick peptidoglycan layer	Thin peptidoglycan layer
Outer Membrane	Absent	Present (contains lipopolysaccharides)
Teichoic Acids	Present	Absent
Gram Stain Result	Purple (retains crystal violet)	Pink/red (loses crystal violet, retains safranin)
Antibiotic Susceptibility	More susceptible to penicillins	More resistant due to outer membrane

Applications:

• Gram-positive bacteria are often more sensitive to antibiotics targeting peptidoglycan synthesis.

• Gram-negative bacteria's outer membrane can act as a barrier to certain drugs and detergents.

Transport Mechanisms in Prokaryotic Cells

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 (usually 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).

Motility and Adhesion Structures

Prokaryotes possess specialized structures for movement and attachment.

• Flagella: Long, whip-like appendages used for motility. Powered by a rotary motor at the base, allowing bacteria to swim toward or away from stimuli (chemotaxis).

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

Example: Treponema pallidum (spirochete) uses periplasmic flagella for corkscrew motility, aiding in tissue penetration and survival.

Organization of Genetic Material

Prokaryotic DNA is organized in a single, circular chromosome located in the nucleoid region. Some bacteria also contain plasmids (small, circular DNA molecules).

• Nucleoid: Not membrane-bound; contains the main genetic material.

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

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

Specialized Structures: Thylakoids, Storage Granules, 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 certain bacteria (e.g., Bacillus, Clostridium) in response to harsh conditions.

• Function: Ensure survival 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 for germination.

Antibiotics Targeting Prokaryotes

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

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

• Peptidoglycan synthesis inhibitors: (e.g., penicillins, cephalosporins) block cell wall synthesis, which is absent in eukaryotic cells.

• Selective toxicity: These antibiotics are generally ineffective against eukaryotic cells due to structural differences in ribosomes and cell walls.

Example: Penicillin inhibits transpeptidase, an enzyme involved in cross-linking peptidoglycan, leading to bacterial cell lysis.

Additional info: Some antibiotics, such as chloramphenicol, can affect mitochondrial ribosomes in eukaryotic cells due to their prokaryotic origin, leading to potential side effects.