Topic 9: Prokaryotes

Comprehensive vocabulary flashcards covering the structural characteristics, reproduction, horizontal gene transfer, metabolic modes, major phylogenetic groups, and ecological applications of prokaryotes.

Peptidoglycan

A structural polymer consisting of modified sugars cross-linked by short polypeptides; it is a unique component found only in bacterial cell walls.

Gram stain

A staining method used to differentiate bacteria based on cell wall composition, distinguishing Gram-positive cells with thick peptidoglycan from Gram-negative cells with thin peptidoglycan and an outer lipopolysaccharide membrane.

Gram-negative bacteria

Bacteria that possess a thin peptidoglycan layer enclosed by an outer lipopolysaccharide membrane, which acts as a protective barrier, can harbor toxins, enhances antibiotic resistance, and stains pink or red.

Capsule

A dense, sticky layer of polysaccharides or proteins surrounding the prokaryotic cell wall that mediates adherence to substrates, facilitates biofilm formation, prevents desiccation, and assists pathogens in evading host immune defenses.

Biofilm

A surface-coating colony of prokaryotes embedded in a sticky extracellular matrix, exemplified by dental plaque, allowing cells to adhere and cooperate.

Fimbriae

Short, hair-like cellular appendages on the surface of certain prokaryotes that function in anchoring cells to surfaces or to other individuals.

Pili (Sex pili)

Elongated, tube-like surface appendages that pull two prokaryotic cells together to facilitate the unidirectional exchange of DNA during conjugation.

Endospore

A dormant, metabolically inactive cell structure enclosed in a resilient multi-layered protective coat, capable of surviving extreme temperatures, desiccation, and chemical stress for up to centuries.

Taxis

An oriented movement toward or away from an environmental stimulus, such as phototaxis in response to light or chemotaxis in response to chemical gradients.

Thylakoid membranes (prokaryotic)

Specialized internal photosynthetic membranes formed from plasma membrane infoldings in cyanobacteria that perform light reactions.

Nucleoid

A non-membrane-enclosed, irregularly shaped central region in a prokaryotic cell where the single circular chromosome is organized and condensed.

Plasmid

A small, circular, autonomous DNA molecule separate from the main bacterial chromosome that carries non-essential accessory genes, such as antibiotic resistance determinants.

Binary fission

A mode of asexual reproduction in which a single prokaryotic cell replicates its circular genome simultaneously with cell elongation, synthesizes a dividing septum, and splits into two identical daughter cells.

Vertical evolution

The transmission of genetic information and newly arisen mutations directly from an ancestral parent cell to its descendant daughter cells through successive rounds of binary fission.

Horizontal gene transfer (HGT)

The transfer of genetic material between different cells through non-reproductive mechanisms such as transformation, transduction, or conjugation, rather than by direct descent.

Transformation

A horizontal gene transfer mechanism where a competent prokaryotic cell imports and integrates naked foreign DNA fragments or plasmids from the surrounding extracellular environment.

Transduction

The horizontal transfer of host bacterial genetic sequences from a donor cell to a recipient cell mediated by a bacteriophage error during viral packaging.

Conjugation

The direct, unidirectional horizontal transfer of genetic material (usually a plasmid) between a donor bacterium and a recipient bacterium through a mating bridge facilitated by a pilus.

Phototroph

An organism that harnesses light energy to drive ATP production and cellular work.

Chemotroph

An organism that derives its metabolic energy from the oxidation of chemical compounds rather than from sunlight.

Autotroph

An organism capable of synthesizing organic molecules from inorganic carbon precursors such as $CO_2$ or $HCO_3^-$.

Heterotroph

An organism that requires at least one organic nutrient, such as glucose, as its carbon source to build new organic matter.

Chemoautotroph

A nutritional mode unique to prokaryotes in which organisms oxidize inorganic chemicals (such as $H_2S$, $NH_3$, or $Fe^{2+}$) for energy while utilizing inorganic carbon sources like $CO_2$ or $HCO_3^-$.

Photoheterotroph

A nutritional mode unique to specific aquatic and extremophilic prokaryotes that uses light to produce energy but requires exogenous organic molecules as a source of carbon.

Obligate aerobe

An organism that strictly depends on molecular oxygen ($O_2$) as the terminal electron acceptor for cellular respiration and cannot survive in its absence.

Obligate anaerobe

An organism that is poisoned or inhibited by molecular oxygen ($O_2$) and relies exclusively on fermentation or anaerobic respiration.

Facultative anaerobe

An organism capable of aerobic cellular respiration in the presence of oxygen ($O_2$), but capable of switching to fermentation or anaerobic respiration when oxygen is absent.

Biological nitrogen fixation

The biochemical reduction of atmospheric dinitrogen gas ($N_2$) to ammonia ($NH_3$), carried out exclusively by specific bacteria and methanogenic archaea.

Proteobacteria

A large, diverse phylum of Gram-negative bacteria divided into five sub-lineages (alpha, beta, gamma, delta, and epsilon), encompassing photoautotrophs, chemoautotrophs, heterotrophs, and significant human and plant symbionts.

Alpha-proteobacteria

A sub-lineage of Proteobacteria that commonly forms close mutualistic partnerships with eukaryotic hosts (such as nitrogen-fixing Rhizobium in legume root nodules) and represents the ancestral lineage of eukaryotic mitochondria.

Chlamydias

A bacterial phylum of Gram-negative obligate intracellular parasites that lack peptidoglycan in their cell walls and live exclusively within animal host cells, such as Chlamydia trachomatis.

Spirochetes

A phylum of spiral-shaped, helical heterotrophic bacteria that move with a distinctive corkscrew motility using axial filaments, enabling them to burrow through viscous host fluids and tissues.

Cyanobacteria

Oxygenic photoautotrophic Gram-negative bacteria, also known as blue-green algae, that precipitated the Great Oxidation Event and gave rise to eukaryotic chloroplasts via endosymbiosis.

Actinomycetes

A group of Gram-positive, soil-dwelling filamentous decomposer bacteria (such as Streptomyces) renowned for producing medically important antibiotic compounds including streptomycin.

Methanogens

Obligate anaerobic archaea that inhabit anoxic settings (such as marshes, swamps, and herbivore digestive tracts) and release methane ($CH_4$) as a metabolic byproduct.

Extreme halophiles

Archaea specially adapted to inhabit and flourish in hypersaline environments containing extreme concentrations of dissolved salts.

Extreme thermophiles

Archaea with thermostable cellular adaptations allowing them to grow optimally in superheated thermal environments, such as hot springs and hydrothermal vents near $100\,^\circ\text{C}$.

Bioremediation

The biotechnological deployment of prokaryotic decomposers to detoxify, degrade, and remove environmental contaminants such as petroleum hydrocarbons from oil spills.

Polyhydroxyalkanoates (PHA)

Intracellular lipid-like carbon and energy storage polymers synthesized by certain bacteria, harvested industrially as building blocks for biodegradable plastics.

Taq DNA polymerase

A thermostable enzyme isolated from the thermophilic bacterium Thermus aquaticus, essential for synthesizing complementary DNA strands in the polymerase chain reaction (PCR).