Flagellum

Source: Wikipedia, the free encyclopedia.
(Redirected from
Flagella
)
For the insect anatomical structure, see
Cilia
.

Flagellum
bacterial flagellum.
SEM image of flagellated eukaryote Chlamydomonas sp. (10000×)
Identifiers
MeSHD005407
THH1.00.01.1.01032
FMA67472
Anatomical terminology]

A flagellum (

sperm cells, from fungal spores (zoospores), and from a wide range of microorganisms to provide motility.[1][2][3][4] Many protists with flagella are known as flagellates
.

A microorganism may have from one to many flagella. A

ulcers – a risk factor for stomach cancer.[5] In some swarming bacteria the flagellum can also function as a sensory organelle, being sensitive to wetness outside the cell.[6]

Across the

Eukaryota the flagellum has a different structure, protein composition, and mechanism of propulsion but shares the same function of providing motility. The Latin word flagellum means "whip" to describe its lash-like swimming motion. The flagellum in archaea is called the archaellum to note its difference from the bacterial flagellum.[7][8]

Eukaryotic flagella and

fimbriae and pili
are smaller, and thinner appendages, with different functions.

Types

Prokaryotic (bacterial and archaeal) flagella run in a rotary movement, while eukaryotic flagella run in a bending movement. The prokaryotic flagellum uses a rotary motor, and the eukaryotic flagellum uses a complex sliding filament system. Eukaryotic flagella are ATP-driven, while prokaryotic flagella can be ATP-driven (Archaea) or proton-driven (Bacteria).[10]

The three types of flagella are bacterial, archaeal, and eukaryotic.

The flagella in eukaryotes have dynein and microtubules that move with a bending mechanism. Bacteria and archaea do not have dynein or microtubules in their flagella, and they move using a rotary mechanism.[11]

Other differences among these three types are:

Bacterial

Structure and composition

The bacterial flagellum is made up of

plasma membrane, and the S ring is directly attached to the cytoplasm. The filament ends with a capping protein.[20][21]

The flagellar filament is the long, helical screw that propels the bacterium when rotated by the motor, through the hook. In most bacteria that have been studied, including the gram-negative

Salmonella typhimurium, Caulobacter crescentus, and Vibrio alginolyticus, the filament is made up of 11 protofilaments approximately parallel to the filament axis. Each protofilament is a series of tandem protein chains. However, Campylobacter jejuni has seven protofilaments.[22]

The basal body has several traits in common with some types of

type-three secretion system
(TTSS).

The atomic structure of both bacterial flagella as well as the TTSS

injectisome have been elucidated in great detail, especially with the development of cryo-electron microscopy. The best understood parts are the parts between the inner and outer membrane, that is, the scaffolding rings of the inner membrane (IM), the scaffolding pairs of the outer membrane (OM), and the rod/needle (injectisome) or rod/hook (flagellum) sections.[23]

Part of a series on
Microbial and microbot movement
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Molecular motors
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Related

Motor

Further information: Rotating locomotion in living systems

The bacterial flagellum is driven by a rotary engine (

FliG, in the rotor.[25] The flagellum is highly energy efficient and uses very little energy.[26] [unreliable source?] The exact mechanism for torque generation is still poorly understood.[27] Because the flagellar motor has no on-off switch, the protein epsE is used as a mechanical clutch to disengage the motor from the rotor, thus stopping the flagellum and allowing the bacterium to remain in one place.[28]

The cylindrical shape of flagella is suited to locomotion of microscopic organisms; these organisms operate at a low Reynolds number, where the viscosity of the surrounding water is much more important than its mass or inertia.[29]

The rotational speed of flagella varies in response to the intensity of the proton-motive force, thereby permitting certain forms of speed control, and also permitting some types of bacteria to attain remarkable speeds in proportion to their size; some achieve roughly 60 cell lengths per second. At such a speed, a bacterium would take about 245 days to cover 1 km; although that may seem slow, the perspective changes when the concept of scale is introduced. In comparison to macroscopic life forms, it is very fast indeed when expressed in terms of number of body lengths per second. A cheetah, for example, only achieves about 25 body lengths per second.[30]

Through use of their flagella, bacteria are able to move rapidly towards attractants and away from repellents, by means of a

counterclockwise and clockwise, respectively. The two directions of rotation are not identical (with respect to flagellum movement) and are selected by a molecular switch.[31]

Assembly

During flagellar assembly, components of the flagellum pass through the hollow cores of the basal body and the nascent filament. During assembly, protein components are added at the flagellar tip rather than at the base.[32] In vitro, flagellar filaments assemble spontaneously in a solution containing purified flagellin as the sole protein.[33]

Evolution

Main article: Evolution of flagella

At least 10 protein components of the bacterial flagellum share homologous proteins with the

phylogenetic trees.[36] The hypothesis that the two structures evolved separately from a common ancestor accounts for the protein similarities between the two structures, as well as their functional diversity.[37]

Flagella and the intelligent design debate

Main articles: Intelligent design and Irreducible complexity

Some authors have argued that flagella cannot have evolved, assuming that they can only function properly when all proteins are in place. In other words, the flagellar apparatus is "irreducibly complex".[38] However, many proteins can be deleted or mutated and the flagellum still works, though sometimes at reduced efficiency.[39] Moreover, with many proteins unique to some number across species, diversity of bacterial flagella composition was higher than expected.[40] Hence, the flagellar apparatus is clearly very flexible in evolutionary terms and perfectly able to lose or gain protein components. For instance, a number of mutations have been found that increase the motility of E. coli.[41] Additional evidence for the evolution of bacterial flagella includes the existence of vestigial flagella, intermediate forms of flagella and patterns of similarities among flagellar protein sequences, including the observation that almost all of the core flagellar proteins have known homologies with non-flagellar proteins.[34] Furthermore, several processes have been identified as playing important roles in flagellar evolution, including self-assembly of simple repeating subunits, gene duplication with subsequent divergence, recruitment of elements from other systems ('molecular bricolage') and recombination.[42]

Flagellar arrangements

Different species of bacteria have different numbers and arrangements of flagella,[43][44] named using the term tricho, from the Greek trichos meaning hair.[45]

  • Monotrichous bacteria such as Vibrio cholerae have a single polar flagellum.[46]
  • Amphitrichous bacteria have a single flagellum on each of two opposite ends (e.g., Alcaligenes faecalis)—only one flagellum operates at a time, allowing the bacterium to reverse course rapidly by switching which flagellum is active.
  • Lophotrichous bacteria (lopho Greek combining term meaning crest or tuft)[47] have multiple flagella located at the same spot on the bacterial surface such as Helicobacter pylori, which act in concert to drive the bacteria in a single direction. In many cases, the bases of multiple flagella are surrounded by a specialized region of the cell membrane, called the polar organelle.[citation needed]
  • Peritrichous bacteria have flagella projecting in all directions (e.g., E. coli).

Counterclockwise rotation of a monotrichous polar flagellum pushes the cell forward with the flagellum trailing behind, much like a corkscrew moving inside cork. Water on the microscopic scale is highly

viscous, unlike usual water
.

electron cryotomography microscopy.[48][49][50]
The rotation of the filaments relative to the cell body causes the entire bacterium to move forward in a corkscrew-like motion, even through material viscous enough to prevent the passage of normally flagellated bacteria.

In certain large forms of Selenomonas, more than 30 individual flagella are organized outside the cell body, helically twining about each other to form a thick structure (easily visible with the light microscope) called a "fascicle".

In some Vibrio spp. (particularly Vibrio parahaemolyticus[51]) and related bacteria 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 meet too much resistance to turn.[52][53][54][55][56][57] These provide swarming motility on surfaces or in viscous fluids.

Bundling

Bundling is an event that can happen in multi-flagellated cells, bundling the flagella together and causing them to rotate in a coordinated manner.

Flagella are left-handed helices, and when rotated counter-clockwise by their rotors, they can bundle and rotate together. When the rotors reverse direction, thus rotating clockwise, the flagellum unwinds from the bundle. This may cause the cell to stop its forward motion and instead start twitching in place, referred to as tumbling. Tumbling results in a stochastic reorientation of the cell, causing it to change the direction of its forward swimming.

It is not known which stimuli drive the switch between bundling and tumbling, but the motor is highly adaptive to different signals. In the model describing chemotaxis ("movement on purpose") the clockwise rotation of a flagellum is suppressed by chemical compounds favorable to the cell (e.g. food). When moving in a favorable direction, the concentration of such chemical attractants increases and therefore tumbles are continually suppressed, allowing forward motion; likewise, when the cell's direction of motion is unfavorable (e.g., away from a chemical attractant), tumbles are no longer suppressed and occur much more often, with the chance that the cell will be thus reoriented in the correct direction.

Even if all flagella would rotate clockwise, however, they often cannot form a bundle due to geometrical and hydrodynamic reasons.[58][59]

Eukaryotic

Eukaryotic flagella. 1–axoneme, 2–cell membrane, 3–IFT (IntraFlagellar Transport), 4–Basal body, 5–Cross section of flagella, 6–Triplets of microtubules of basal body
Cross section of an axoneme
Longitudinal section through the flagella area in Chlamydomonas reinhardtii. In the cell apex is the basal body that is the anchoring site for a flagellum. Basal bodies originate from and have a substructure similar to that of centrioles, with nine peripheral microtubule triplets (see structure at bottom center of image).
The "9+2" structure is visible in this cross-section micrograph of an axoneme.

Terminology

Aiming to emphasize the distinction between the bacterial flagella and the eukaryotic cilia and flagella, some authors attempted to replace the name of these two eukaryotic structures with "

Cavalier-Smith), preserving "flagella" for the bacterial structure. However, the discriminative usage of the terms "cilia" and "flagella" for eukaryotes adopted in this article (see § Flagella versus cilia below) is still common (e.g., Andersen et al., 1991;[63] Leadbeater et al., 2000).[64]

Internal structure

The core of a eukaryotic flagellum, known as the

plasma membrane, so that the interior of the flagellum is accessible to the cell's cytoplasm
.

Besides the axoneme and basal body, relatively constant in morphology, other internal structures of the flagellar apparatus are the transition zone (where the axoneme and basal body meet) and the root system (microtubular or fibrilar structures that extend from the basal bodies into the cytoplasm), more variable and useful as indicators of phylogenetic relationships of eukaryotes. Other structures, more uncommon, are the paraflagellar (or paraxial, paraxonemal) rod, the R fiber, and the S fiber.[65]: 63–84  For surface structures, see below.

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 produce force through ATP hydrolysis. The flagellar axoneme also contains radial spokes, polypeptide complexes extending from each of the outer nine 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.[66]

Flagella versus cilia

Beating pattern of eukaryotic "flagellum" and "cillum", a traditional distinction before the structures of the two are known.

The regular beat patterns of eukaryotic

spermatozoa to the transport of fluid along a stationary layer of cells such as in the respiratory tract.[67]

Although eukaryotic

cilia and flagella are ultimately the same, they are sometimes classed by their pattern of movement, a tradition from before their structures have been known. In the case of flagella, the motion is often planar and wave-like, whereas the motile cilia often perform a more complicated three-dimensional motion with a power and recovery stroke.[67] Yet another traditional form of distinction is by the number of 9+2 organelles on the cell.[66]

Intraflagellar transport

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.[68]

Evolution and occurrence

Further information: Evolution of flagella

Eukaryotic flagella or cilia, probably an ancestral characteristic,[69] are widespread in almost all groups of eukaryotes, as a relatively perennial condition, or as a flagellated life cycle stage (e.g., zoids, gametes, zoospores, which may be produced continually or not).[70][71][62]

The first situation is found either in specialized cells of multicellular organisms (e.g., the

metazoans), as in ciliates and many eukaryotes with a "flagellate condition" (or "monadoid level of organization", see Flagellata
, an artificial group).

Flagellated lifecycle stages are found in many groups, e.g., many

chytrid
fungi (zoospores and gametes).

Flagella or cilia are completely absent in some groups, probably due to a loss rather than being a primitive condition. The loss of cilia occurred in

chytrids
).

Typology

A number of terms related to flagella or cilia are used to characterize eukaryotes.[71][74][65]: 60–63 [75][76] According to surface structures present, flagella may be:

  • whiplash flagella (= smooth, acronematic flagella): without hairs, e.g., in
    Opisthokonta
  • hairy flagella (= tinsel, flimmer, pleuronematic flagella): with hairs (= mastigonemes sensu lato), divided in:
    • with fine hairs (= non-tubular, or simple hairs): occurs in
      Pavlovales
      )
    • with stiff hairs (= tubular hairs, retronemes, mastigonemes sensu stricto), divided in:
      • bipartite hairs: with two regions. Occurs in
        Heterokonta
      • tripartite (= straminipilous) hairs: with three regions (a base, a tubular shaft, and one or more terminal hairs). Occurs in most
        Heterokonta
  • stichonematic flagella: with a single row of hairs
  • pantonematic flagella: with two rows of hairs
  • acronematic: flagella with a single, terminal mastigoneme or flagellar hair (e.g., bodonids);[77] some authors use the term as synonym of whiplash
  • with scales: e.g.,
    Prasinophyceae
  • with spines: e.g., some brown algae
  • with undulating membrane: e.g., some
    kinetoplastids, some parabasalids
  • with proboscis (trunk-like protrusion of the cell): e.g.,
    apusomonads, some bodonids[78]

According to the number of flagella, cells may be: (remembering that some authors use "ciliated" instead of "flagellated")[62][79]

According to the place of insertion of the flagella:[80]

According to the beating pattern:

  • gliding: a flagellum that trails on the substrate[78]
  • heterodynamic: flagella with different beating patterns (usually with one flagellum functioning in food capture and the other functioning in gliding, anchorage, propulsion or "steering")[82]
  • isodynamic: flagella beating with the same patterns

Other terms related to the flagellar type:

  • isokont: cells with flagella of equal length. It was also formerly used to refer to the Chlorophyta
  • anisokont: cells with flagella of unequal length, e.g., some
    Prasinophyceae
  • heterokont: term introduced by Luther (1899) to refer to the
    Heterokonta
  • stephanokont: cells with a crown of flagella near its anterior end, e.g., the gametes and spores of
    Oedogoniales
  • akont: cells without flagella. It was also used to refer to taxonomic groups, as Aconta or Akonta: the
    Rhodophyceae
    (Christensen, 1962)

Archaeal

The archaellum possessed by some species of Archaea is superficially similar to the bacterial flagellum; in the 1980s, they were thought to be homologous on the basis of gross morphology and behavior.[83] Both flagella and archaella consist of filaments extending outside the cell, and rotate to propel the cell. Archaeal flagella have a unique structure which lacks a central channel. Similar to bacterial type IV pilins, the archaeal proteins (archaellins) are made with class 3 signal peptides and they are processed by a type IV prepilin peptidase-like enzyme. The archaellins are typically modified by the addition of N-linked glycans which are necessary for proper assembly or function.[3]

Discoveries in the 1990s revealed numerous detailed differences between the archaeal and bacterial flagella. These include:

These differences support the theory that the bacterial flagella and archaella are a classic case of biological analogy, or convergent evolution, rather than homology.[86][87][88] Research into the structure of archaella made significant progress beginning in the early 2010s, with the first atomic resolution structure of an archaella protein, the discovery of additional functions of archaella, and the first reports of archaella in Nanoarchaeota and Thaumarchaeota.[89][90]

Fungal

The only

plasmalemma, and a terminal plate is present in the transitional zone. An inner ring-like structure attached to the tubules of the flagellar doublets within the transitional zone has been observed in transverse section.[91]

Additional images

  • Multiple flagella in lophotrichous arrangement on surface of Helicobacter pylori
    Multiple flagella in lophotrichous arrangement on surface of Helicobacter pylori
  • Physical model of a bacterial flagellum
    Physical model of a bacterial flagellum

See also

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Further reading

External links

Wikimedia Commons has media related to Flagella.

Public Domain This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al. {{cite encyclopedia}}: Missing or empty |title= (help)

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