Pronunciation: fla-DZHE-lum

Etymology: Flagellum is the Latin word for whip.

Flagellum (n., pl. flagella) is a whip-like locomotory structure that is found in both prokaryotic and eukaryotic groups.

Eukaryotic Flagella: The eukaryotic flagellum, called undulapodium by some, is a complex organelle with a 9+2 structure of associated microtubules (made of actin and myosin) that allow for movement of the structure. The flagellum is anchored by a basal body, which usually has flagellar roots. The modes of insertion and orientation vary enormously in the eukaryotes.

Prokaryotic Flagella: Bacterial flagella are simpler with a single filament made mainly of flagellin. This flagellum is corkscrew-like and turns by a “motor” embedded in the membrane. The flagellum is directed by a basal collar. The prokaryotic flagellum does not ripple or bend like a eukaryotic flagellum. Prokaryotic flagella may be inserted in different locations: polar (at the ends of the cell) or peritrichous (around the sides of the cell).

The eukaryotic flagellum showing the star-like triplets of the basal body and the 9+2 structure of the whip-like extension.	The bacterial flagellum showing the “basal motor”, the collar that bends the flagellum in a particular direction and the terminal whip of flagellin.

By Jack R. Holt. Last revised: 01/05/2009