1 (in prokaryotes) A long slender structure that protrudes from the cell surface of a bacterium. It rotates from its base and propels the bacterium along. Up to several micrometres in length, a flagellum is constructed of numerous subunits of the protein flagellin, while at the base a system of rings anchors the flagellum in the cell wall and plasma membrane. Surrounding these rings are paired motor proteins, which impart a rotary motion to the filament, and switch proteins, which can reverse the direction of rotation. Flagella may be attached singly or in groups, for example at the poles of the bacterial cell, or scattered over the cell surface.
2 (in eukaryotes) See undulipodium; tinsel flagellum.