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 (text with tooltip) A 9(2)+2 microtubular array is the standard form of the eukaryotic flagellum. In cross section, 9 microtubular doublets circle a free pair of microtubules. of associated microtubules (made of actin and myosin) that allow for movement of the structure. The flagellum is anchored by a basal body (text with tooltip) Basal bodies (also called kinetosomes) are organelles that are not membrane-bound. All eukaryotic flagella (also called undulipodia) are underlain or attached to the cell by the basal body. The basal body is a microtubular structure with the general formula 9(3)+0. They are indistinguishable from centrioles. , which usually has flagellar roots (text with tooltip) Flagellar roots are microtubular structures that arise from the basal bodies and elaborate to the inside of the cell. They may connect to the nucleus (as part of a karyomastigont) or course their way into the cell such that they form other structures like axostyles. . 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 (text with tooltip) Polar flagella are those that emerge from the poles of the cell (rods or spirals). (at the ends of the cell) or peritrichous (text with tooltip) Peritrichous flagella are inserted on the sides but not on the end of a rod or spiral cell. (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