Flagellum

The Flagellum is a propulsive structure used by many single-celled organisms to move through a liquid medium. There are two main varieties of flagellum; the bacterial flagellum (a helical filament that rotates like a screw) and the eukaryotic flagellum (a whip-like structure that lashes back and forth).

Bacterial flagellum

The filament is composed of the protein flagellin and is a hollow tube 20 nanometers thick. It is helical, and has a sharp bend just outside the outer membrane called the "hook" which allows the helix to point directly away from the cell. A shaft runs between the hook and the basal struture, passing through protein rings in the cell's membranes that act as bearings. Gram positive organisms have 2 rings, one in the cell wall and one in the cell membrane. Gram negative organisms have 4 rings, 2 in the cell wall and 2 in the cell membrane.

The bacterial flagellum is driven by a rotary engine composed of protein, located at the flagellum's anchor point on the inner cell membrane. The engine is powered by proton motive force, ie, by the flow of protons across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism (In Vibro species the motor is a sodium ion pump, rather than a proton pump). The rotor transports protons across the membrane, and is turned in the process. The rotor by itself can operate at 6,000 to 17,000 rpm, but with a filament attached usually only reaches 200 to 1000 rpm.

The components of the flagellum are capable of spontaneous assembly in bacterial membranes. Both the basal structure and the fillament have a hollow core, through which the component proteins of the flagellum are able to move into their respective positions. The basal structure has many traits in common with some types of secretory pore which have a hollow rod-like "plug" in their centers extending out through the cell wall, and it is thought that bacterial flagella may have evolved from such pores.

Different species of bacteria have different numbers and arrangements of flagella. Monotrichous bacteria have a single flagellum, lophotrichous bacteria have multiple flagella located at the same spot on the bacteria's surface which act in concert to drive the bacteria in a single direction, amphitrichous bacteria have a single flagellum on two opposite ends (only one end's flagellum operates at a time, allowing the bacteria to reverse course rapidly by switching which flagellum is active) and peritrichous bacteria have flagella projecting in all directions.

Some species of bacteria (those of Spirochete body form) have internal flagella that lie between their inner and outer membranes, the rotation of which cause the entire bacterium to corkscrew through its medium.

Anticlockwise rotation of monotrichious polar flagella thrusts the cell forward with the flagellum trailing behind. Periodically the direction of rotation is briefly reversed, causing what is known as a "tumble", and results in reorientation of the cell.