Flagellum

A long, whip-like cell appendage that produces movement or fluid flow. Bacterial and archaeal flagella differ in structure and motion from the eukaryotic flagellum.

Bacterial flagellum: A rotary appendage made of a filament, hook, and membrane-embedded motor in many bacteria. Its rotating filament propels bacteria through liquid.
Flagellin: The principal protein subunit that assembles into many bacterial flagellar filaments. Repeated flagellin subunits form the long external filament.
Cilium: A short, hair-like cell projection that can move fluid or sense signals. Eukaryotic cilia share an axonemal structure with flagella but are typically shorter and more numerous.
Bacterial motility: The capacity of bacteria to move, often through liquid or across surfaces. Flagella provide one of the main mechanisms bacteria use to move.
Endosymbiotic theory: The theory that mitochondria and plastids originated from bacteria incorporated into ancestral eukaryotic cells. It explains organelle origins, but not the origin of eukaryotic flagella.
Eukaryotic flagellum: A cell appendage whose microtubule-based core bends through dynein-driven motion. Its bending strokes move cells or surrounding fluid.
Hook-basal body: The bacterial flagellum’s curved connector and envelope-spanning structure linking filament to motor. It couples motor rotation to the filament while anchoring the apparatus.
Archaeal flagellum: A rotating surface filament in archaea, built from archaellins and powered by ATP. Despite its name and rotary motion, it is structurally distinct from the bacterial flagellum.
Sperm motility: The movement of sperm cells, commonly driven by beating eukaryotic flagella. Flagellar beating propels many sperm cells through fluid.
Evolution of eukaryotic flagella: The evolutionary history of the microtubule-based motility structures found in eukaryotes. Their shared axonemal architecture raises questions about how the system evolved.

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