A flagellum (plural: flagella) is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and it functions in locomotion. An example of a eukaryotic flagellated cell is the sperm cell, which uses its flagellum to propel itself toward and through the female reproductive tract. An example of a flagellated bacterium is the ulcer-causing Helicobacter pylori, which uses its multiple flagella to propel itself through the mucus lining to reach the stomach epithelium. Prokaryotic and eukaryotic flagella have some notable differences, such as protein composition, structure, and mechanism of propulsion. Flagella are structurally identical to eukaryotic cilia, although distinctions are sometimes made according to function and/or length. Flagella are cellular structures, not organelles.

The word flagellum comes from the Latin for whip.

Types

Three quite distinct types of flagella have so far been distinguished; bacterial, archaeal and eukaryotic.

The main differences among these three types are summarized below:

• Bacterial flagella are helical filaments that rotate like screws. They provide two of several kinds of bacterial motility.
• Archaeal flagella are superficially similar to bacterial flagella, 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 eukaryotic flagella are called cilia or undulipodia to emphasize their distinctiveness.

Bacterial

The bacterial flagellum is made up of the protein flagellin. Its shape is a 20 nanometer-thick hollow tube. It is helical and has a sharp bend just outside the outer membrane; this "hook" 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 membrane that act as bearings. Gram-positive organisms have 2 of these basal body rings, one in the peptidoglycan layer and one in the plasma membrane. Gram-negative organisms have 4 such rings: the L ring associates with the lipopolysaccharides, the P ring associates with peptidoglycan layer, the M ring is embedded 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 made up of protein (Mot complex), 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 (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism (in Vibrio species there are two kinds of flagella, lateral and polar, and some are driven by a sodium ion pump rather than a proton pump). The rotor transports protons across the membrane, and is turned in the process. The rotor alone can operate at 6,000 to 17,000 rpm, but with the flagellar filament attached usually only reaches 200 to 1000 rpm.

Flagella do not rotate at a constant speed but instead can increase or decrease their rotational speed in relation to the strength of the proton motive force. Flagellar rotation can move bacteria through liquid media at speed of up to 60 cell lengths/second (sec). Although this is only about 0.00017 km/h, when comparing this speed with that of higher organisms in terms of number of lengths moved per second, it is extremely fast. The fastest land animal, the cheetah, moves at a maximum rate of about 110 km/h, but this represents only about 25 body lengths/sec. Thus, when size is accounted for, prokaryotic cells swimming at 50-60 lengths/sec are actually much faster than larger organisms.

The components of the bacterial flagellum are capable of self-assembly 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. During assembly, protein components are added at the flagellar tip rather than at the base.

The basal body has several traits in common with some types of secretory pores, such as the hollow rod-like "plug" in their centers extending out through the plasma membrane. Given the structural similarities, it was thought that bacterial flagella may have evolved from such pores; however, it is now known that these pores are derived from flagella.

Flagella arrangement schemes

Different species of bacteria have different numbers and arrangements of flagella. Monotrichous bacteria have a single flagellum (e.g., Vibrio cholerae). Lophotrichous bacteria have multiple flagella located at the same spot on the bacteria's surfaces which act in concert to drive the bacteria in a single direction. In many cases, the bases of multiple flagella are surrounded a specialized region of the cell membrane; the so-called polar membrane. Amphitrichous bacteria have a single flagellum on each of two opposite ends (only one 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 (e.g., Escherichia coli).

In some bacteria, such as the larger forms of Selenomonas, the individual flagella are organized outside the cell body, helically twining about each other to form a thick structure called a "fascicle". Other bacteria, such as Spirochetes, have a specialized type of flagellum called an "axial filament" that is located in the periplasmic space, the rotation of which causes the entire bacterium to move forward in a corkscrew-like motion.

Counterclockwise rotation of monotrichous polar flagella thrust the cell forward with the flagella trailing behind. Periodically, the direction of rotation is briefly reversed, causing what is known as a "tumble" in which the cell seems to thrash about in place. This results in the reorientation of the cell. When moving in a favorable direction, "tumbles" are unlikely; however, when the cell's direction of motion is unfavorable (e.g., away from a chemical attractant), a tumble may occur, with the chance that the cell will be thus reoriented in the correct direction.

In some Vibrio (particularly Vibrio parahemolyticus) and related proteobacteria such as Aeromonas, two flagellar systems co-exist, using different sets of genes and different ion gradients for energy. The polar flagella are constitutively expressed and provide motility in bulk fluid, while the lateral flagella are expressed when the polar flagella meets too much resistance to turn. These provide swarming motility on surfaces or in viscous fluids.

Archaeal

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. Both flagella consist of filaments extending outside of the cell, and rotate to propel the cell.

However, discoveries in the 1990s revealed numerous detailed differences between the archaeal and bacterial flagella; these include:

• Bacterial flagella are motorized by a flow of H⁺ ions (or occasionally Na⁺ ions); archaeal flagella are almost certainly powered by ATP. The torque-generating motor that powers rotation of the archaeal flagellum has not been identified.
• 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; the archaeal flagellum is too thin to allow this.
• 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. 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. For example, Cavalier-Smith (2002) is aware of the differences between archaeal and bacterial flagellins, but retains the misconception that the basal bodies are homologous.

Eukaryotic

The eukaryotic flagellum is completely different from the prokaryote flagellum in both structure and evolutionary origin. The only shared characteristics among bacterial, archaeal, and eukaryotic flagella are their superficial appearance; they are intracellular extensions used in creating movement. Along with cilia, flagella make up a group of organelles known as undulipodia.

Structure

A eukaryotic flagellum is a bundle of nine fused pairs of microtubule doublets surrounding two central single microtubules. The so-called "9+2" structure is characteristic of the core of the eukaryotic flagellum called an axoneme. At the base of a eukaryotic flagellum is a basal body, "blepharoplast" or kinetosome, which is the microtubule organizing center (MTOC) for flagellar microtubules and is about 500 nanometers long. Basal bodies are structurally identical to centrioles. The flagellum is encased within the cell's plasma membrane, so that the interior of the flagellum is accessible to the cell's cytoplasm.

Mechanism

Each of the outer 9 doublet microtubules extends a pair of dynein arms (an "inner" and an "outer" arm) to the adjacent microtubule; these dynein arms are responsible for flagellar beating, as the force produced by the arms causes the microtubule doublets to slide against each other and the flagellum as a whole to bend. These dynein arms produce force through ATP hydrolysis. The flagellar axoneme also contains radial spokes, polypeptide complexes extending from each of the outer 9 microtubule doublets towards the central pair, with the "head" of the spoke facing inwards. The radial spoke is thought to be involved in the regulation of flagellar motion, although its exact function and method of action are not yet understood.

Flagella vs Cilia

Though eukaryotic flagella and cilia are ultrastructurally identical, the beating pattern of the two organelles can be different. In the case of flagella (e.g. the tail of a sperm) the motion is propeller-like. In contrast, beating of cilia consists of coordinated back-and-forth cycling of many cilia on the cell surface. Thus, motile flagella serve for the propulsion of single cells (e.g. swimming of protozoa and spermatozoa), and cilia for the transport of fluids (e.g. transport of mucus by stationary ciliated cells in the trachea). However, cilia are also used for locomotion (through liquids) in organisms such as Paramecium.

Intraflagellar Transport

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.

Irreducible complexity

In his 1996 book Darwin's Black Box, intelligent design proponent Michael Behe cited the bacterial flagellum as an example of an irreducibly complex structure that could not have evolved through naturalistic means. Behe argued that the flagellum becomes useless if any one of its constituent parts is removed, and thus could not have arisen through numerous, successive, slight modifications; therefore, it is hopelessly improbable that the proteins making up the flagellar motor could have come together all at once, by chance.

While Behe discussed the immune system and the blood clotting cascade in greater detail, the bacterial flagellum has become a "poster child" for intelligent design proponents and other creationists. It is one of two identified rotary structures found in nature (the other being ATP synthase) and it is billions of years older than Behe's other two examples, which exist in many homologous forms, simplifying the explanation of their origin.

Viable evolutionary pathways have since been proposed for the bacterial flagellum.

In addition, the Type III secretory system, a molecular syringe which bacteria use to inject toxins into other cells, appears to be a simplified sub-set of the bacterial flagellum's components, meaning that it is much less likely to be irreducibly complex.

Behe's arguments have been examined and rejected by the scientific community at large. Exaptation explains how systems with multiple parts can evolve through natural means.

See also

• Evolution of flagella

References

External links

• Molecular Machines Museum Index. Access Research Network. Retrieved on 2008-05-18..
• Howard Berg Motile Behavior of Bacteria. Physics Today on the Web. American Institute of Physics. Retrieved on 2008-05-18..
• Charles Lindemann Mechansims of sperm motility. Oakland University. Retrieved on 2008-05-18..