Definition of Flagellum
Flagellum - A long slender structure that protrudes from the cell surface of a bacterium enabling motility.
A Flagellum rotates from its base and propels bacterium along. They can be up to several micrometers in length.
A flagellum is constructed of numerous subunits of the protein flagellin forming a filament, while at its base a system of rings anchors the flagellum in the bacterial cell wall and plasma membrane. Surrounding the rings are paired motor proteins which impart a rotary motion to the filament, these can switch which can reverse the direction of rotation.
Flagella may be attached singularly or in groups and can be positioned at the poles of the bacterium or scattered over the whole cell surface.