Bacterial and Archaeal Cell Structure

Prokaryote Controversy

The term "prokaryote" is controversial because bacteria and archaea are distinct but grouped together.
Originally defined by the lack of a membrane-bound nucleus, cytoskeleton, and internal membraneous structures.

Common shapes include rods (bacilli), cocci (spheres), comma-shaped (vibrio), spiral (spirillum & spirochetes), filamentous, and pleomorphic.
Frequently observed structures: cell wall, nucleoid, plasma membrane, cytoplasm, inclusions, ribosomes, fimbriae, capsule, and flagella.
Factors determining size & shape: evolution and surface area to volume ratio (S/V ratio).
S/V ratio affects nutrient uptake and diffusion efficiency, facilitating rapid growth.

Cell envelope: plasma membrane and surrounding layers.
Plasma membrane: semipermeable barrier; carries out respiration & photosynthesis; detects chemicals.
Cell wall: additional layer.
Bacterial membranes are bilayers of phospholipids with fatty acids connected to glycerol by ester linkage.

Microorganisms need nutrients for energy conservation and biosynthesis.
Macronutrients are needed in large quantities; micronutrients (trace elements) are needed in small amounts.
Growth factors (amino acids, purines & pyrimidines, vitamins) are obtained from the environment.
Passive diffusion: movement down the concentration gradient without energy input.
Facilitated diffusion: uses transport proteins (channels or carriers) without metabolic energy.
Group translocation: active transport modifying organic molecules during transport.
Secretion of siderophores binds ferric irons, enabling iron accumulation.

Most bacteria have a cell wall outside the plasma membrane for shape and osmotic protection.
Walls contain peptidoglycan.
Gram-positive: thick peptidoglycan layer and teichoic acids.
Gram-negative: thin peptidoglycan layer surrounded by a complex outer membrane containing lipopolysaccharides (LPSs).
Endotoxin: lipid A portion of LPS, which can lead to septic shock.
Gram stain mechanism depends on peptidoglycan thickness, preventing its loss during the ethanol wash.

Capsules, slime layers, and glycocalyces protect cells and aid in surface attachment.
S-layers are the outermost layer in some bacteria and are composed of proteins or glycoproteins.

A concentrated solution of biochemicals contains proteins similar to eukaryotic cytoskeletal proteins.
Some bacteria have internal membrane systems for photosynthesis and respiration.
Inclusions store organic or inorganic substances (e.g., PHB inclusions, polyphosphate granules, magnetosomes, gas vacuoles).
Microcompartments contain enzymes for reactions like CO2 fixation (carboxysomes).
Bacterial ribosomes are 70S in size, composed of proteins & rRNA molecules.
Nucleoid: contains genetic material (usually a double-stranded, covalently closed circular DNA molecule).
Plasmids: extrachromosomal DNA that can provide selective advantages; some are episomes that integrate into the chromosome.

Types: swimming, swarming, spirochete motility, twitching, and gliding.
Swimming: flagellar filament rotates like a propeller.
Swarming: group movement on moist surfaces, mediated by flagella.
Spirochete motility: flagella wound around the cell within the periplasmic space rotate.
Twitching: jerky movement via type IV pili.
Gliding: smooth movement.
Chemotaxis: response to gradients of attractants and repellents.

Endospores are dormant structures resistant to heat and desiccation, formed within mother cells during adverse conditions.
Germination is the breaking of endospore dormancy in response to signals.

Many archaea have been identified through 16S rRNA or partial genome sequencing.
Common shapes: rods, cocci, curved rods, branched, pleomorphic, and spirals (no spirochetes or mycelial forms).
Similar in size to bacteria, but extremely small or large archaea have also been identified.

Consist of a plasma membrane and cell wall (no peptidoglycan), often with an S-layer.
Membranes are composed of glycerol diether and diglycerol tetraether lipids.
Diether lipids form bilayers; tetraether lipids form monolayers.

Contains cytoskeletal proteins: FtsZ (tubulin homologue), MreB (actin homologue), and Crenactin (unique archaeal actin homologue).
Inclusions include gas vesicles.
Ribosomes are 70S in size and are more similar to eukaryotic ribosomes.
Genetic material is in a nucleoid (not enclosed by a membrane) and consists of a single, double-stranded, covalently closed, circular DNA molecule.
Some archaea are polyploid.

Have pili (similar to bacterial type IV pili).
Motile by archaella (flagella).
Engage in relocate-and-seek swimming behaviors, and archaella-related Type IV Pili may aid in positioning cells optimally in a temperature gradient.
Archaella: rigid helices that rotate; rotation direction determines forward or backward movement.
Some are phototactic.
Rotations are powered by ATP hydrolysis; archaeal taxis machinery is similar to bacterial.