Prokaryotic Cell Basics

Major Characteristics of Prokaryotic Cells

• Prokaryotic cells are unicellular organisms lacking a membrane-bound nucleus and organelles.

• Genetic material is located in a region called the nucleoid.

• Cell division occurs primarily by binary fission.

• Two main domains: Bacteria and Archaea.

Monomorphic vs. Pleomorphic Bacteria

• Monomorphic bacteria maintain a single, consistent shape.

• Pleomorphic bacteria can alter their shape or size in response to environmental conditions.

Common Bacterial Shapes and Arrangements

• Coccus (spherical), Bacillus (rod-shaped), Vibrio (comma-shaped), Spirillum (rigid spiral), Spirochete (flexible spiral).

• Arrangements include: single, diplo- (pairs), strepto- (chains), staphylo- (clusters).

Cell Size and Surface Area-to-Volume Ratio

• Small cell size and high surface area-to-volume ratio facilitate efficient nutrient uptake and waste removal.

• This allows rapid growth and adaptation to environmental changes.

Binary Fission

Steps of Binary Fission

• Replication of the circular DNA molecule.

• Cell elongation and separation of DNA copies.

• Formation of a septum (dividing wall).

• Division into two genetically identical daughter cells.

Example:

Escherichia coli divides every 20 minutes under optimal conditions by binary fission.

Plasma Membrane

Structure and Function

• Composed of a phospholipid bilayer with embedded proteins.

• Functions as a selective barrier, controlling entry and exit of substances.

• Site of metabolic processes such as respiration and photosynthesis in prokaryotes.

Selective Permeability

• Allows passage of small, nonpolar molecules (e.g., O2, CO2).

• Ions and polar molecules require transport proteins for movement across the membrane.

Membrane Proteins

• Transport of molecules, signal transduction, enzymatic activity, and structural support.

Membrane Fluidity

• Influenced by temperature and fatty acid composition (more unsaturated fatty acids increase fluidity).

Cell Walls

Function and Composition

• Provides structural support and protection against osmotic pressure.

• Peptidoglycan is the main component in bacterial cell walls.

• Archaea have cell walls but lack true peptidoglycan; may contain pseudopeptidoglycan or other polymers.

Peptidoglycan as an Antibacterial Target

• Many antibiotics (e.g., penicillins) inhibit peptidoglycan synthesis, weakening the cell wall and causing cell lysis.

Gram-Positive vs. Gram-Negative Bacteria

Comparison Table

Acid-Fast Bacteria

Key Features

• Contain mycolic acid in their cell walls, making them waxy and resistant to desiccation and chemicals.

• After acid-fast staining, cells appear red/pink.

• Grow slowly and are difficult to treat due to impermeable cell wall.

Example:

Mycobacterium tuberculosis is an acid-fast bacterium.

Mycoplasma and L-Forms

Cell Wall Deficiency

• Mycoplasma species lack a cell wall entirely, making them naturally resistant to antibiotics targeting cell wall synthesis.

• L-forms are bacteria that have lost their cell wall due to mutation or environmental conditions.

• Loss of cell wall increases susceptibility to osmotic lysis but confers resistance to certain antibiotics.

Transport Across the Membrane

Types of Transport

• Simple diffusion: Movement of molecules from high to low concentration without energy or proteins.

• Facilitated diffusion: Passive movement via transport proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active transport: Movement against a concentration gradient, requiring energy (often ATP).

Passive vs. Active Transport

• Passive transport: No energy required; moves substances down their concentration gradient.

• Active transport: Requires energy; moves substances against their concentration gradient.

Primary vs. Secondary Active Transport

• Primary: Direct use of ATP (e.g., ATP-binding cassette transporters).

• Secondary: Uses energy from an ion gradient (e.g., symport and antiport systems).

Symport vs. Antiport

• Symport: Two substances move in the same direction across the membrane.

• Antiport: Two substances move in opposite directions.

Phosphotransferase Systems

• Group translocation system that chemically modifies a substance as it is transported into the cell (e.g., phosphorylation of sugars).

Osmosis and Tonicity

Effects of Different Environments

• Isotonic: No net water movement; cell remains stable.

• Hypertonic: Water leaves the cell; may cause plasmolysis (cell membrane pulls away from cell wall).

• Hypotonic: Water enters the cell; may cause lysis if the cell wall is damaged or absent.

External Structures

Flagella

• Provide motility; rotate like propellers.

• Arrangements: Monotrichous (single), Lophotrichous (tuft at one end), Amphitrichous (one at each end), Peritrichous (all over surface).

• Spirochetes use periplasmic flagella (axial filaments) for corkscrew motion.

Fimbriae and Pili

• Fimbriae: Short, numerous; aid in attachment to surfaces.

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

Glycocalyx, Capsule, and Slime Layer

• Glycocalyx: General term for polysaccharide/protein coating outside the cell wall.

• Capsule: Organized, firmly attached glycocalyx; protects against phagocytosis.

• Slime layer: Loosely attached, unorganized glycocalyx; aids in adherence and biofilm formation.

Intracellular Structures

Nucleoid

• Region containing the single, circular DNA chromosome.

Ribosomes

• Sites of protein synthesis; prokaryotic ribosomes are 70S (composed of 50S and 30S subunits).

Cytoskeleton

• Maintains cell shape, aids in division, and organizes cellular components.

Inclusion Bodies

• Storage sites for nutrients, such as glycogen, polyphosphate, or sulfur granules.

Endospores

Formation and Function

• Endospores are dormant, highly resistant structures formed by certain bacteria for survival under harsh conditions (not for reproduction).

• Resistant to heat, desiccation, chemicals, and radiation due to tough outer layers and low water content.

• Sporulation: Process of endospore formation.

• Germination: Return of endospore to vegetative state when conditions improve.

• Clinically important genera: Bacillus, Clostridium, Clostridioides.

Summary Table: Key Prokaryotic Structures and Functions

Additional info:

• Be prepared to apply these concepts to clinical scenarios, such as antibiotic resistance, infection control, and bacterial identification.