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 main differences among these three types are summarized below:
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.
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.
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:
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.
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.
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.
For information on biologists' ideas about how the various flagella may have evolved, see evolution of flagella.
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.
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