A thread-like structure found on some bacteria and on or in many eucaryotic organisms. Flagella usually function in locomotion, bacterial flagella rotating and eucaryotic flagella undulating as they beat. In eucaryotic organisms there are two main kinds of flagellum: a smooth whip-like type, and a tinsel type with rows of long hairs along its length.
The flagellum of eukaryotes usually moves with an “S” motion, and is surrounded by cell membrane.Prokaryotes may have one or many flagella for locomotion, always outside the cell membrane and normally visible only with the aid of the electron microscope. In some bacterial species the flagella twine together helically outside the cell body to form a bundle, large enough to be visible in the light microscope.
A eukaryote cell usually only has about one or two flagella. Flagellates have one or more flagella, they move by whipping the flagella on the flagellate side to side. Cilia or flagella can also extend out from the stationary cells that are held in place as part of a layer of tissue in a multicellular organism. In Eukaryotic cells, flagella are active in movements involving feeding and sensation.
Eukaryotic flagella are not the same as flagella of bacteria. These proteins include dynein, a molecular motor which can cause flagella to bend and propel the cell relative to its environment, or propel water or mucus relative to the cell. 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.
Arthropod flagellum
In Chelicerata, the flagellum is a non-segmental pluri-articulated whip, present in the arachnid orders Schizomida, Thelyphonida and Palpigradi.
Bacterial flagellum
The filament is composed of the protein flagellin and is a hollow tube 20 nanometers thick. 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 components of the flagellum are capable of self-assembly in which the component proteins associate spontaneously without the aid of enzymes or other factors. It was thought that bacterial flagella may have evolved from such pores, though it is now known that these pores are derived from flagella.
Different species of bacteria have different numbers and arrangements of flagella. 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.(example:Escherichia coli)
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.
Archaeal flagellum
The archaeal flagellum is superficially similar to the bacterial (or eubacterial) flagellum; 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; archaeal flagella are almost certainly powered by ATP. While bacterial cells often have many flagellar filaments, each of which rotates independently, the archaeal flagellum is composed of a bundle of many filaments that rotate as a single assembly. Bacterial flagella grow by the addition of flagellin subunits at the tip; archaeal flagella grow by the addition of subunits to the base. Bacterial flagella are thicker than archaeal flagella, and the bacterial filament has a large enough hollow "tube" inside that the flagellin subunits can flow up the inside of the filament and get added at the tip; Many components of bacterial flagella share sequence similarity to components of the type III secretion systems, but the components of bacterial and archaeal flagella share no sequence similarity. Instead, some components of archaeal flagella share sequence and morphological similarity with components of type IV pili, which are assembled through the action of type II secretion systems (the nomenclature of pili and protein secretion systems is not consistent).
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. Therefore many assume erroneously that there is only one basic kind of prokaryotic flagellum, and that archaeal flagella are homologous to it (e.g., Cavalier-Smith (2002), who is aware of the differences in archaeal and bacterial flagellins, but retains the misconception that the basal bodies are homologous).
Eukaryotic flagellum
The eukaryotic flagellum is completely different from the prokaryote flagella in both structure and evolutionary origin. The only shared characteristics among bacterial, archaeal, and eukaryotic flagella is their superficial appearance;
A eukaryotic flagellum is a bundle of nine fused pairs of microtubule doublets surrounding two central single microtubules. The flagellum is encased within the cell's plasma membrane, so that the interior of the flagellum is accessible to the cell's cytoplasm.
Motile flagella serve for the propulsion of single cells (e.g.
Additionally, immotile flagella are vital organelles in sensation and signal transduction across a wide variety of cell types (e.g.
Intraflagellar transport (IFT), the process by which axonemal subunits, transmembrane receptors, and other proteins are moved up and down the length of the flagellum, is essential for proper functioning of the flagellum, in both motility and signal transduction.
For information on biologists' ideas about how the various flagella may have evolved, see evolution of flagella.
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