flagellated

Flagellated is an adjective used in biology to describe cells or organisms that bear one or more flagella, slender whip-like appendages used for locomotion and sometimes for sensing the environment. Flagella are found across domains, including Bacteria, Archaea, and Eukaryota, and they enable movement through liquids by rotating or beating in characteristic patterns.

In bacteria, flagella are extracellular structures consisting of a basal body embedded in the cell envelope, a hook, and a hollow filament made mainly of flagellin subunits. Bacterial flagella rotate like propellers, powered by the proton motive force across the membrane or, in some species, a sodium gradient. This rotary motion propels the cell and can be used for chemotaxis, guiding movement toward nutrients or away from harmful substances. In Archaea, flagella (often called archaella) are structurally distinct from bacterial flagella and are typically powered by ATP hydrolysis rather than a proton motive force, fulfilling a similar locomotor role.

In most Eukaryota, flagella are larger, enclosed by the cell membrane, and share a conserved axoneme structure of nine outer microtubule doublets with a central pair (the 9+2 arrangement). They beat in whip-like waves, driven by dynein motors, enabling locomotion in many protists and propulsion in sperm cells. Some organisms have multiple flagella, while others rely on a single flagellum.

Flagellated locomotion supports dispersal, environmental exploration, and interactions with hosts and surroundings. The term flagellated thus indicates the presence of one or more flagella, rather than a specific evolutionary relationship, and is applied across diverse life forms.