The structures and pattern of movement of prokaryotic (prokaryote) and eukaryotic flagella are different. Eukaryotes (eukaryote) have one to many flagella, which move in a characteristic whiplike manner. The flagella closely resemble the cilium (q.v.) in structure. The core is a bundle of nine pairs of microtubules surrounding two central pairs of microtubules (the so-called nine-plus-two arrangement); each microtubule is composed of the protein tubulin. The coordinated sliding of these microtubules confers movement. The base of the flagellum is anchored to the cell by a basal body.

Bacterial flagella are helically shaped structures containing the protein flagellin. The base of the flagellum (the hook) near the cell surface is attached to the basal body enclosed in the cell envelope. The flagellum rotates in a clockwise or counterclockwise direction, in a motion similar to that of a propeller.

The movement of eukaryotic flagella depends on adenosine triphosphate (ATP) for energy, while that of the prokaryotes derives its energy from the proton-motive force, or ion gradient, across the cell membrane.