Prokaryotes vs. Eukaryotes Overview

Prokaryotes vs. Eukaryotes

• Prokaryotes lack a nucleus and their DNA is not wrapped around histones.
• Prokaryotic cell walls can be peptidoglycan (bacteria) or made of other chemicals (archaea).
• Prokaryotes lack complex, membrane-bound organelles.

Bacterial Cell Structures

• Common to all bacterial cells: cell membrane, cytoplasm, ribosomes, one (or a few) chromosomes.
• Common to most bacterial cells: cell wall, glycocalyx (surface coating)
• Common to some bacterial cells: flagella, pili, fimbriae, capsules, slime layers, inclusions, actin cytoskeleton, endospores.

Bacterial Arrangements and Sizes

• Bacterial cells carry out all necessary life activities (reproduction, metabolism, nutrient processing)
• Bacteria can also act as a group, forming colonies and biofilms.
• Cocci (spherical) exhibit diverse arrangements (single, pairs, tetrads, clusters).
• Bacilli (rod-shaped) are less varied in arrangement (single, pairs, chains, palisades).
• Spirilla (spiral-shaped) can be arranged singly or in short chains

Bacterial Morphology (Shapes)

• Coccus (round)
• Bacillus (rod-shaped)
• Spirillum (spiral-shaped)
• Vibrio (curved rod)
• Coccobacillus (short rod)

External Structures (Appendages)

• Appendages are cell extensions found in some but not all species.
• They can provide motility (flagella, axial filaments) or attachment/mating (pili, fimbriae).
• Flagella: filament, hook, basal body; vary in arrangement (polar, lophotrichous, peritrichous, amphitrichous).

Flagellar Function

• Chemotaxis (positive/negative) and phototaxis
• Motility through runs and tumbles (counter-clockwise/clockwise rotation).
• Axial filaments (periplasmic flagella) impart twisting/flexing motion to spirochetes.

Capsules and Slime Layers

• Glycocalyx is a sugar and/or protein coating.
• Slime layer is loosely attached; protects against desiccation, helps in attachment
• Capsules are firmly attached and more resistant; prevents recognition and destruction by hosts.

Cell Wall

• Rigid structure outside the cell membrane.
• Provides structural support, maintains shape, and prevents bursting.
• Gram-positive have a thick peptidoglycan layer.
• Gram-negative have a thin peptidoglycan layer, an outer membrane.
• The cell wall is the target of many antibiotics (penicillin, cephalosporins, lysozyme).

Gram-Positive Cell Wall

• Thick peptidoglycan layer with acidic polysaccharides.
• Retains crystal violet dye (appears purple in Gram staining).
• Contains teichoic and lipoteichoic acids (negatively charged).

Gram-Negative Cell Wall

• Thin peptidoglycan layer with an outer membrane.
• Loses crystal violet dye easily (appears pink in Gram staining).
• Lipopolysaccharide (LPS) in the outer membrane (endotoxin).
• Periplasmic space between the outer membrane and the peptidoglycan layer.

Bacterial Internal Structures

• Cytoplasm: gelatinous, site for biochemical activities.
• Chromosome: single circular DNA strand; aggregated in the nucleoid.
• Plasmids: nonessential, double-stranded DNA circles; often confer drug resistance.
• Ribosomes: RNA and protein complexes; 70S prokaryotic.
• Inclusions: storage granules for nutrients and other substances (polyphosphate, sulfur, gas vesicles).
• Actin cytoskeleton: long protein polymers; arranged in helical ribbons; contributes to cell shape.

Endospores

• Dormant cellular structures formed by some bacteria (Bacillus, Clostridium).
• Extremely resistant to heat, drying, radiation, and chemicals.
• Dehydrated state; very low metabolic activity, survival strategy.
• Sporulation/ germination.
• Medical Significance: several bacterial pathogens are endospore-forming.

Mycoplasmas and other Cell Wall-Deficient Bacteria

• Lack cell walls; cell membranes stabilized by sterols or hopanoids.
• Resist lysis.
• Very small; range in shape.
• Can be grown on artificial media.