Everything about Flagellum totally explained
A
flagellum (plural:
flagella) is a long, slender organelle that projects from the
cell body, and can be directly seen (or rendered visible after appropriate treatment) with the light or electron microscope. Its function is usually to propel a unicellular or small multicellular organism by beating with a
whip-like motion. In larger animals, flagella are often arranged en masse at the surface of a stationary cell—anchored within an organ—and serve to move
fluids along
mucous membranes, such as the lining of the
trachea.
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, for example, 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 don't 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's 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's now known that these pores are derived from flagella.
Different species of bacteria have different numbers and arrangements of flagella. Monotrichous bacteria have a single flagellum (for example,
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 (for example,
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's 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 (for example, 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 hasn't 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 isn't 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 (for example by Berg), archaeal flagella have only recently begun to get serious scientific attention. Therefore, many assume erroneously that there's only one basic kind of prokaryotic flagellum, and that archaeal flagella are homologous to it. For example, Cavalier-Smith (2002)
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 only two rotary structures found in nature (the other being ATP synthase,) and it's billions of years older than Behe's other two examples, which exist in many homologous forms, simplifying the explanation of their origin.
Potentially 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's 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.
Further Information
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