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A flagellum (; ) is a hairlike appendage that protrudes from a wide range of
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
s termed as
flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like Appendage, appendages called flagellum, flagella. The word ''flagellate'' also describes a particular const ...
s. A flagellate can have one or several flagella. Certain cells such as the mammalian
sperm cell Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, "female" reproductive cell and a smaller, "male" one). Animals produce motile sperm with a tail known as a flagellu ...
is also flagellated, in order to propel itself through the female reproductive tract. The primary function of a flagellum is that of
motility Motility is the ability of an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (bio ...

motility
. In some bacteria the flagellum can also function as a sensory
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
, being sensitive to wetness, chemicals, and temperatures outside the cell. Flagella are organelles defined by function rather than structure. Flagella vary greatly among the three domains of
Bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

Bacteria
,
Archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

Archaea
, and
Eukaryota Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
. In all three kinds the flagella can be used for swimming but they differ greatly in protein composition, structure, and mechanism of propulsion. The
Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an appa ...

Latin
word means "
whip File:Cat o' nine.JPG, upright=1.35, A leather cat o' nine tails pictured with a U.S dollar bill for size comparison. '' reins, featuring a quirt at the end of the ''romal'' A whip is a tool designed to strike humans or other animals to exert cont ...
". The flagellum of archaea has a special name,
archaellum An archaellum (plural: ''archaella'', formerly archaeal flagellum) is a unique whip-like structure on the cell surface of many archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack ...
, to emphasize its difference from the bacterial flagellum. An example of a flagellated
bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sma ...

bacterium
is the ulcer-causing ''
Helicobacter pylori ''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, microaerophile, microaerophilic, spiral bacteria, spiral (helical) bacterium usually found in the stomach. Its helical shape (from which the genus name, ...

Helicobacter pylori
'', which uses multiple flagella to propel itself through the mucus lining to reach the stomach
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
. Eukaryotic flagella are structurally identical to eukaryotic
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

cilia
, although distinctions are sometimes made according to function or length. Prokaryotic fimbriae and pili are also thin appendages, but have different functions and are usually smaller.


Types

Three types of flagella have so far been distinguished: bacterial, archaeal, and eukaryotic. The flagella in eukaryotes have
dynein Dynein is a family of cytoskeletal 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking protein filaments pre ...

dynein
, and
microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, rep ...

microtubule
s, 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. Other differences among these three types are: *Bacterial flagella are helical filaments, each with a rotary motor at its base which can turn clockwise or counterclockwise. They provide two of several kinds of bacterial motility. *Archaeal flagella ( archaella) are superficially similar to bacterial flagella (in that it also has a rotary motor), 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. Eukaryotic flagella are classed along with eukaryotic motile cilia as undulipodiaA Dictionary of Biology
2004, accessed 2011-01-01.
to emphasize the role their distinctive, wavy appendage plays in cellular function or
motility Motility is the ability of an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (bio ...

motility
. Primary cilia are immotile, and are not undulipodia; they have a structurally different ''9+0 axoneme'' rather than the ''9+2 axoneme'' found in both flagella and motile cilia.


Bacterial


Structure and composition

The bacterial flagellum is made up of
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
subunits of
flagellin '' Helicobacter pylori'' electron micrograph, showing multiple flagella on the cell surface Flagellin is a globular protein that arranges itself in a hollow cylinder to form the filament in a bacterial flagellum. It has a mass of about 30,000 to ...
. Its shape is a 20-
nanometer file:EM Spectrum Properties edit.svg, 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its derived scales. The nanometre is often used to express dimensions on an atomic scale and mostly in the Mo ...
-thick hollow tube. It is
helical Helical may refer to: *Helix, the mathematical concept for the shape * Helical spring, a coilspring *Helical plc, a British property company, once a maker of steel bar stock * Helicoil, a mechanical thread repairing insert * H-el-ical//, stage name ...

helical
and has a sharp bend just outside the outer membrane; this "hook" allows the axis of the helix to point directly away from the cell. A shaft runs between the hook and the
basal body ), 4-Basal body, 5-Cross section of flagellum, 6-Triplets of microtubules of basal body. Image:Chlamydomonas TEM 09.jpg, Longitudinal section through the flagella area in ''Chlamydomonas reinhardtii''. In the cell apex is the basal body that is th ...
, passing through protein rings in the cell's membrane that act as bearings.
Gram-positive In bacteriology Bacteriology is the branch and specialty of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, m ...
organisms have two of these basal body rings, one in the
peptidoglycan Peptidoglycan or murein is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repea ...

peptidoglycan
layer and one in the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
.
Gram-negative Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few microm ...
organisms have four such rings: the
L ring The L-ring of the bacterium, bacterial flagellum is the ring in the lipid outer cell membrane through which the Axis of rotation, axial filament (rod, hook, and flagellum) passes. References

{{bacteria-stub Bacteria ...
associates with the
lipopolysaccharides Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide composed of O-antigen, outer core and inner core joined by a covalent bond; they are found in the Bacterial outer membrane, outer membrane of Gram-negative ...
, the
P ring The P ring forms part of the basal body of the bacterial appendage known as the flagellum A flagellum (; ) is a hairlike appendage that protrudes from a wide range of microorganism A microorganism, or microbe,, ''mikros'', "small") and '' ...
associates with
peptidoglycan Peptidoglycan or murein is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repea ...

peptidoglycan
layer, the M ring is embedded in the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
, and the S ring is directly attached to the plasma membrane. The filament ends with a capping protein. 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 ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaer ...

Escherichia coli
,
Salmonella typhimurium ''Salmonella enterica'' subsp. ''enterica'' is a subspecies In Taxonomy (biology), biological classification, the term subspecies refers to one of two or more populations of a species living in different subdivisions of the species' range an ...

Salmonella typhimurium
,
Caulobacter crescentus ''Caulobacter crescentus'' is a Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacterial differentiation. They are characterized by their cell envelo ...

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 ''Campylobacter jejuni'' () is one of the most common causes of food poisoning in Europe and in the United States. The vast majority of cases occur as isolated events, not as part of recognized outbreaks. Active surveillance through the Foodborne ...

Campylobacter jejuni
'' has seven protofilaments. 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. The similarities between bacterial flagella and bacterial secretory system structures and proteins provide scientific evidence supporting the theory that bacterial flagella evolved from the type-three secretion system.


Motor

The bacterial flagellum is driven by a rotary engine ( Mot complex) made up of protein, located at the flagellum's anchor point on the inner cell membrane. The engine is powered by
proton motive forceChemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient An electrochemical gradient is a gradient In vector calculus, the gradient of a scalar-valued function, scalar-valued dif ...
, i.e., by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a
concentration gradient Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) of ...
set up by the cell's metabolism (''
Vibrio ''Vibrio'' is a genus of Gram-negative bacteria, possessing a curved-rod (comma) shape, several species of which can cause foodborne illness, foodborne infection, usually associated with eating undercooked seafood. Typically found in Seawater, s ...
'' species have two kinds of flagella, lateral and polar, and some are driven by a sodium
ion pump An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump which operates by sputtering In physics, sputtering is a phenomenon in which microscopic particle In the Outline of physical science, physical sciences, a particle ( ...
rather than a
proton pump A proton pump is an integral membrane protein An integral membrane protein (IMP) is a type of membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several b ...

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 Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is the number of turns in one minute The minute is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the ...
, but with the flagellar filament attached usually only reaches 200 to 1000 rpm. The direction of rotation can be changed by the flagellar motor switch almost instantaneously, caused by a slight change in the position of a protein,
FliG In molecular biology, the flagellar motor switch is a protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', ...
, in the rotor. The flagellum is highly energy efficient and uses very little energy. The exact mechanism for torque generation is still poorly understood. 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. The cylindrical shape of flagella is suited to locomotion of microscopic organisms; these organisms operate at a low
Reynolds number The Reynolds number () helps predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers flows tend to be turbulent In fluid dynam ...
, where the viscosity of the surrounding water is much more important than its mass or inertia. 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. Although according to Through use of their flagella, '''' is able to move rapidly towards attractants and away from repellents, by means of a biased random walk, with 'runs' and 'tumbles' brought about by rotating its flagellum
counterclockwise Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite sen ...
and
clockwise Two-dimensional rotation A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary line ...

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


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. ''In vitro'', flagellar filaments assemble spontaneously in a solution containing purified flagellin as the sole protein.


Evolution

At least 10 protein components of the bacterial flagellum share homologous proteins with the
type three secretion system Type three secretion system (often written Type III secretion system and abbreviated TTSS or T3SS, also called Injectisome) is a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino a ...
(T3SS) found in many gram-negative bacteria, hence one likely evolved from the other. Because the T3SS has a similar number of components as a flagellar apparatus (about 25 proteins), which one evolved first is difficult to determine. However, the flagellar system appears to involve more proteins overall, including various regulators and chaperones, hence it has been argued that flagella evolved from a T3SS. However, it has also been suggested that the flagellum may have evolved first or the two structures evolved in parallel. Early single-cell organisms' need for
motility Motility is the ability of an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (bio ...

motility
(mobility) support that the more mobile flagella would be selected by evolution first, but the T3SS evolving from the flagellum can be seen as 'reductive evolution', and receives no topological support from the
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

phylogenetic
trees. 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.


Flagella and the intelligent design debate

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". However, many proteins can be deleted or mutated and the flagellum still works, though sometimes at reduced efficiency. In addition, the composition of flagella is surprisingly diverse across bacteria, with many proteins only found in some species, but not others. 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''. 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. 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.


Flagellar arrangement schemes

Different species of bacteria have different numbers and arrangements of flagella. *Monotrichous bacteria have a single flagellum (e.g., ''
Vibrio cholerae ''Vibrio cholerae'' is a species of Gram-negative Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain o ...

Vibrio cholerae
''). *Lophotrichous bacteria have multiple flagella located at the same spot on the bacterial surfaces (e.g., ''
Helicobacter pylori ''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, microaerophile, microaerophilic, spiral bacteria, spiral (helical) bacterium usually found in the stomach. Its helical shape (from which the genus name, ...

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. *Amphitrichous bacteria have a single flagellum on each of two opposite ends (e.g., ''
Alcaligenes faecalis ''Alcaligenes faecalis'' is a species of Gram-negative, rod-shaped bacteria commonly found in the environment. It was originally named for its first discovery in feces, but was later found to be common in soil, water, and environments in associat ...
'')—only one flagellum operates at a time, allowing the bacterium to reverse course rapidly by switching which flagellum is active. *Peritrichous bacteria have flagella projecting in all directions (e.g., ''E. coli''). 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".
Spirochete A spirochaete () or spirochete is a member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...
s, in contrast, have flagella called endoflagella arising from opposite poles of the cell, and are located within the
periplasmic space 400px, cell_wall.html"_;"title="Gram-negative_cell_wall">Gram-negative_cell_wall_ The_periplasm_is_a_concentrated_gel-like_matrix_(biology).html" ;"title="cell_wall_.html" ;"title="cell_wall.html" ;"title="Gram-negative cell wall">Gram-negative ce ...
as shown by breaking the outer-membrane and also by
electron cryotomography Electron cryotomography (CryoET) is an imaging technique used to produce high-resolution (~1–4 nm) three-dimensional views of samples, typically biological macromolecules and Cell (biology), cells. CryoET is a specialized application of trans ...
microscopy. 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. Counterclockwise rotation of a monotrichous polar flagellum pushes the cell forward with the flagellum trailing behind, much like a corkscrew moving inside cork. Indeed, water on the microscopic scale is highly
viscous The viscosity of a fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...
, very different from our daily experience of water. Flagella are left-handed helices, and bundle and rotate together only when rotating counterclockwise. When some of the rotors reverse direction, the flagella unwind and the cell starts "tumbling". Even if all flagella would rotate clockwise, they likely will not form a bundle, due to geometrical, as well as hydrodynamic reasons. Such "tumbling" may happen occasionally, leading to the cell seemingly thrashing about in place, resulting in the reorientation of the cell. The clockwise rotation of a flagellum is suppressed by chemical compounds favorable to the cell (e.g. food), but the motor is highly adaptive to this. Therefore, when moving in a favorable direction, the concentration of the chemical attractant increases and "tumbles" are continually suppressed; however, 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. In some ''Vibrio'' spp. (particularly '' Vibrio parahaemolyticus'') and related
proteobacteria Proteobacteria is a major phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plu ...
such as ''
Aeromonas ''Aeromonas'' is a genus of gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, ...
'', 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. These provide swarming motility on surfaces or in viscous fluids.


Archaeal

The
archaellum An archaellum (plural: ''archaella'', formerly archaeal flagellum) is a unique whip-like structure on the cell surface of many archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack ...
possessed by some
archea Archaea ( ; singular archaeon ) constitute a domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacteria, recei ...
e is superficially similar to the bacterial flagellum; in the 1980s, they were thought to be homologous on the basis of gross morphology and behavior. 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
pilin Pilin refers to a class of fibrous proteins that are found in pilus structures in bacteria. These structures can be used for the exchange of genetic material, or as a cell adhesion mechanism. Although not all bacteria have pili or fimbriae, bacter ...

pilin
s, the archaeal flagellins (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
glycan The terms glycan and polysaccharide , a beta-glucan is an example of a (1→4)-β-D-glucan composed of glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharid ...

glycan
s which are necessary for proper assembly or function. 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 ions); archaeal flagella are almost certainly powered by
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
. The
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment, moment of force, rotational force or turning effect, depending on the field of study. The concept originated with the studies ...

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 rotates 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 archaella, 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 archaellum is too thin (12-15 nm) to allow this. *Many components of bacterial flagella share sequence similarity to components of the type III secretion systems, but the components of bacterial flagella and archaella share no sequence similarity. Instead, some components of archaella share sequence and morphological similarity with components of
type IV pili A pilus (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it bec ...
, which are assembled through the action of (the nomenclature of pili and protein secretion systems is not consistent). These differences could mean that the bacterial flagella and archaella could be a classic case of biological
analogy Analogy (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximate ...

analogy
, or
convergent evolution Convergent evolution is the independent evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; eit ...
, rather than homology. However, in comparison to the decades of well-publicized study of bacterial flagella (e.g. by Howard Berg), archaella have only recently begun to garner scientific attention.


Eukaryotic


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 " undulipodia" (e.g., all papers by since the 1970s) or "cilia" for both (e.g., Hülsmann, 1992; Adl et al., 2012; most papers of
Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes ...
), preserving "flagella" for the bacterial structure. However, the discriminative usage of the terms "cilia" and "flagella" for eukaryotes adopted in this article is still common (e.g., Andersen et al., 1991; Leadbeater et al., 2000).


Internal structure

A eukaryotic flagellum is a bundle of nine fused pairs of
microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, rep ...

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 An axoneme is the microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecule ...
. At the base of a eukaryotic flagellum is a
basal body ), 4-Basal body, 5-Cross section of flagellum, 6-Triplets of microtubules of basal body. Image:Chlamydomonas TEM 09.jpg, Longitudinal section through the flagella area in ''Chlamydomonas reinhardtii''. In the cell apex is the basal body that is th ...
, "blepharoplast" or kinetosome, which is the
microtubule organizing centerThe microtubule-organizing center (MTOC) is a structure found in eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that e ...
for flagellar microtubules and is about 500 nanometers long. Basal bodies are structurally identical to
centriole In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...

centriole
s. The flagellum is encased within the cell's
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
, so that the interior of the flagellum is accessible to the cell's
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
. 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 which extends 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. For surface structures, see below.


Mechanism

Each of the outer 9 doublet microtubules extends a pair of
dynein Dynein is a family of cytoskeletal 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking protein filaments pre ...

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.


Flagella versus cilia

The regular beat patterns of eukaryotic
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

cilia
and flagella generate motion on a cellular level. Examples range from the propulsion of single cells such as the swimming of
spermatozoa A spermatozoon (pronounced , alternate spelling spermatozoön; plural spermatozoa; from grc, σπέρμα ("seed") and grc, ζῷον ("living being")) is a motile Motility is the ability of an organism In biology, an organism (from An ...

spermatozoa
to the transport of fluid along a stationary layer of cells such as in the
respiratory tract The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network which connects several biologically re ...
. Although eukaryotic
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

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. Yet another traditional form of distinction is by the number of 9+2 organelles on the cell.


Intraflagellar transport

Intraflagellar transport Intraflagellar transport or IFT is a bidirectional motility along axonemal microtubules that is essential for the formation (ciliogenesis Ciliogenesis is defined as the building of the cell's antenna (primary cilium, primary cilia) or extracellul ...
, the process by which axonemal subunits,
transmembrane receptors Cell surface receptors (membrane receptors, transmembrane receptors) are receptor (biochemistry), receptors that are embedded in the cell membrane, plasma membrane of cell (biology), cells. They act in cell signaling by receiving (binding to) ext ...
, 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.


Evolution and occurrence

Eukaryotic flagella or cilia, probably an ancestral characteristic, are widespread in almost all groups of eukaryotes, as a relatively perennial condition, or as a flagellated life cycle stage (e.g.,
zoid In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the An ...
s,
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
s,
zoospore A zoospore is a motile Motility is the ability of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of li ...
s, which may be produced continually or not). The first situation is found either in specialized cells of multicellular organisms (e.g., the
choanocyte Choanocytes (also known as "collar cells") are cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious r ...

choanocyte
s of
sponges Sponges, the members of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. T ...

sponges
, or the ciliated
epithelia Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
of
metazoan Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
s), as in
ciliate The ciliates are a group of protozoan Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a c ...

ciliate
s and many eukaryotes with a "flagellate condition" (or "monadoid level of organization", see flagellate#Flagellates as organisms: the Flagellata, Flagellata, an artificial group). Flagellated lifecycle stages are found in many groups, e.g., many green algae (zoospores and male gametes), bryophytes (male gametes), pteridophytes (male gametes), some gymnosperms (cycads and ''Ginkgo'', as male gametes), centric diatoms (male gametes), brown algae (zoospores and gametes), oomycetes (assexual zoospores and gametes), hyphochytrids (zoospores), labyrinthulomycetes (zoospores), some apicomplexans (gametes), some radiolarians (probably gametes), foraminiferans (gametes), Phytomyxea, plasmodiophoromycetes (zoospores and gametes), myxogastrids (zoospores),
metazoan Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
s (male gametes), and 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 red algae, some green algae (Zygnematophyceae), the gymnosperms except cycads and ''Ginkgo'', angiosperms, pennate diatoms, some apicomplexans, some amoebozoans, in the sperm of some
metazoan Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
s, and in fungi (except chytrids).


Typology

A number of terms related to flagella or cilia are used to characterize eukaryotes. 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 Euglenophyceae, Dinoflagellata, some Haptophyceae (Pavlovales) **with stiff hairs (= tubular hairs, retronemes, mastigonemes ''sensu stricto''), divided in: ***bipartite hairs: with two regions. Occurs in Cryptophyceae, Prasinophyceae, and some 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., Bodonida, bodonids); 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 Bodonida, bodonids According to the number of flagella, cells may be (remembering that some authors use "ciliated" instead of "flagellated": *uniflagellated: e.g., most Opisthokonta *biflagellated: e.g., all Dinoflagellata, the gametes of Charophyceae, of most bryophytes and of some
metazoan Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
s *triflagellated: e.g., the gametes of some Foraminifera *quadriflagellated: e.g., some Prasinophyceae, Collodictyonidae *octoflagellated: e.g., some Diplomonads, Diplomonada, some Prasinophyceae *multiflagellated: e.g., Opalinata, Ciliophora, ''Stephanopogon'', Parabasalida, Hemimastigophora, Caryoblastea, ''Multicilia'', the gametes (or
zoid In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the An ...
s) of Oedogoniales (Chlorophyta), some pteridophytes and some gymnosperms According to the place of insertion of the flagella: *opisthokont: cells with flagella inserted posteriorly, e.g., in Opisthokonta (Vischer, 1945). In Haptophyceae, flagella are laterally to terminally inserted, but are directed posteriorly during rapid swimming. *akrokont: cells with flagella inserted apically *subakrokont: cells with flagella inserted subapically *pleurokont: cells with flagella inserted laterally According to the beating pattern: *gliding: a flagellum that trails on the substrate *heterodynamic: flagella with different beating patterns (usually with one flagellum functioning in food capture and the other functioning in gliding, anchorage, propulsion or "steering") *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 Euglenophyceae and Prasinophyceae *heterokont: term introduced by Luther (1899) to refer to the Xanthophyceae, due to the pair of flagella of unequal length. It has taken on a specific meaning in referring to cells with an anterior straminipilous flagellum (with tripartite mastigonemes, in one or two rows) and a posterior usually smooth flagellum. It is also used to refer to the taxon Heterokonta *stephanokont: cells with a crown of flagella near its anterior end, e.g., the gametes and spores of Oedogoniales, the spores of some Bryopsidales. Term introduced by Blackman & Tansley (1902) to refer to the Oedogoniales *akont: cells without flagella. It was also used to refer to taxonomic groups, as Aconta or Akonta: the Zygnematophyceae and Bacillariophyceae (Oltmanns, 1904), or the Rhodophyceae (Christensen, 1962)


See also

*Ciliopathy *RpoF


References


Further reading

* * * *


External links


Cell Image Library - Flagella
{{Authority control Cell movement Organelles Protein complexes Bacteria