A flagellum (plural, flagella) is a whip-like organelle that many unicellular organisms, and some multicellular ones, use to move about. They may also be involved in other processes. The name actually covers three different structures, found in each of the three domains. Bacterial flagella are helical filaments that rotate like screws. Archaeal flagella are superficially similar, but are different in many details and considered non-homologous. Eukaryotic flagella - those of animal, plant, and protist cells - are complex cellular projections that lash back and forth. Sometimes the last are called cilia or undulipodia to emphasize their distinctiveness.
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 body, passing through protein rings in the cell's membranes that act as bearings. Gram-positive organisms have 2 basal body rings, one in the peptidoglycan layer and one in the plasma membrane. Gram-negative organisms have 4 rings: L ring associates with the lipopolysaccharides, P ring associates with peptidoglycan layer, M ring is imbedded in the plasma membrane, and the S ring is directly attached to the plasma membrane. The filament ends with a capping protein.
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 , i.e., by the flow of protons (i.e., hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism (in Vibrio 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 self-assembly in which the component proteins associate spontaneously without the aid of enzymes or other factors. Both the basal body and the filament have a hollow core, through which the component proteins of the flagellum are able to move into their respective positions. The filament grows at its tip rather than at the base. The basal body 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 plasma membrane, 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 each 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). Peritrichous bacteria have flagella projecting in all directions.
Some species of bacteria (those of Spirochete body form) have a specialized type of flagellum called axial filament that is located in the periplasmic space, the rotation of which causes the entire bacterium to corkscrew through its usually viscous medium.
Anticlockwise rotation of monotrichous 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. The direction at the end of the tumble state is random. The length of the run state is extended when the bacteria moves through a favorable gradient.
The archaeal flagellum is another prokaryote flagellum that is found exclusively in the archaea (also known as archaeabacteria, depending on whether or not one believes that these prokaryotes constitute a fundamental domain of life (e.g., Woese), or a just a highly-derived bacterium with heavy adaptation to extremophily, particularly thermophily (e.g., Cavalier-Smith)).
The archaeal flagellum is superficially similar to the bacterial (or eubacterial) flagellum; in the 1980s they were thought to be homologous on the basis of gross morphology and behavior (Cavalier-Smith, 1987). Both flagella consist of filaments extending outside of the cell, and rotate to propel the cell.
However, discoveries in the 1990s have revealed numerous detailed differences between the archaeal and bacterial flagella; these include:
These differences mean that the bacterial and archaeal flagella are a classic case of biological analogy, or convergent evolution, rather than homology. However, in comparison to the decades of well-publicized study of bacterial flagella (e.g. by Berg), archaeal flagella have only recently begun to get serious scientific attention. Therefore many assume erroneously that there is only one basic kind of prokaryotic flagellum, and that archaeal flagella are homologous to it (an example is Cavalier-Smith (2002), who is aware of the differences in archaeal and bacterial flagellins, but retains the misconception that the basal bodies are homologous).
The eukaryotic flagellum, also called a cilium or undulipodium, is completely different from the prokaryote flagella in structure and in evolutionary origin. The only thing that the bacterial, archaeal, and eukaryotic flagella have in common is that they stick outside of the cell and wiggle to produce propulsion.
A eukaryotic flagellum is a bundle of nine fused pairs of microtubules doublets surrounding two central single microtubules. The so-called "9+2"" structure is the characteritics of the core of the eukaryotic flugellum called an axoneme. At the base of a eukaryotic flagellum is a basal body or kinetosome, which is the microtubule organizing center for flagellar microtubules and is about 500 nanometers long. Basal bodies are structually identical to and functionally interchaglable with centrioles in the animal cells. The flagellum is encased within the cell's plasma membrane, so that the interior of the flagellum is accessible to the cell's cytoplasm. This is necessary because the flagellum's flexing is driven by the protein dynein bridging the microtubules all along its length and forcing them to slide relative to each other, and ATP must be transported to them for them to function.
For information on biologists' ideas about how the various flagella may have evolved, see evolution of flagella.