flagela

Flagela, often referred to as flagella (singular flagellum), are whip-like appendages that enable motility in many prokaryotic and eukaryotic cells. They vary in structure, energy source, and movement mechanism, but share the common role of helping cells move through liquids or across surfaces.

Bacterial flagella consist of a basal body embedded in the cell envelope, a hook, and a long helical filament made of the protein flagellin. The filament rotates like a propeller, driven by a motor in the basal body that uses a proton motive force or, in some species, a sodium gradient. This rotation can propel the cell at speeds of several body lengths per second. Bacterial flagella are arranged in patterns such as monotrichous (single flagellum), peritrichous (distributed over the surface), lophotrichous (tufts at one end), or amphitrichous (flagella at both ends). Movement is modulated by chemotaxis, allowing bacteria to move toward nutrients or away from harmful substances.

Eukaryotic flagella are structurally distinct from bacterial ones. They contain an axoneme with a characteristic 9+2 arrangement of microtubules, powered by dynein motors that generate bending waves. This whip-like motion, driven by ATP, enables propulsion and can differ in beat pattern from bacterial rotation. Eukaryotic flagella are typically longer and often singular or paired, and they share ancestry with cilia, though they may perform similar or divergent functions.

Flagella are central to microbial ecology, host interactions, and biofilm formation. They are also recognized by host immune systems, with flagellin proteins acting as important microbial-associated molecular patterns.