View source for Flagellum

{{short description|Cellular appendage functioning as locomotive or sensory organelle}}
{{About||the insect anatomical structure|Antenna (biology)|the flagella of male Solifugae|Solifugae|Eukaryotic only|Cilia}}
{{Use dmy dates|date=October 2020}}
{{Infobox anatomy
| Name        = Flagellum
| Latin       =
| Image       = Flagellum base diagram-en.svg
| Caption     = Structure of [[bacterial]] flagellum
| Width       =
| Image2      = Chlamydomonas (10000x).jpg
| Caption2    = [[Scanning electron microscope|SEM]] image of flagellated [[eukaryote]] ''[[Chlamydomonas]]'' sp. (10000×)
| Precursor   =
| System      =
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
}}

A '''flagellum''' ({{IPAc-en|f|l|ə|ˈ|dʒ|ɛ|l|əm}}; {{plural form}}: '''flagella''') ([[Latin]] for 'whip' or 'scourge') is a hair-like [[appendage]] that protrudes from certain [[plant]] and animal [[sperm cell]]s, from [[fungus|fungal]] [[spore]]s ([[zoospore]]s), and from a wide range of [[microorganism]]s to provide [[Motility#Cellular level|motility]].<ref name="pmid12624192">{{cite journal |last1=Bardy |first1=Sonia L. |last2=Ng |first2=Sandy Y. M. |last3=Jarrell |first3=Ken F. |title=Prokaryotic motility structures |journal=Microbiology |date=1 February 2003 |volume=149 |issue=2 |pages=295–304 |doi=10.1099/mic.0.25948-0 |pmid=12624192 |doi-access=free}}</ref><ref name="Lefebvre_2001">{{cite journal |last1=Silflow |first1=Carolyn D. |last2=Lefebvre |first2=Paul A. |title=Assembly and Motility of Eukaryotic Cilia and Flagella. Lessons from Chlamydomonas reinhardtii |journal=Plant Physiology |date=1 December 2001 |volume=127 |issue=4 |pages=1500–1507 |doi=10.1104/pp.010807 |pmid=11743094 |pmc=1540183}}</ref><ref name= JarrellK>{{cite book |editor1-last=Jarrell |editor1-first=Ken F. |title=Pili and flagella: current research and future trends |date=2009 |publisher=Caister academic press |location=Norfolk |isbn=978-1-904455-48-6}}</ref><ref name="pmid17148374">{{cite journal |last1=Malo |first1=Aurelio F |last2=Gomendio |first2=Montserrat |last3=Garde |first3=Julian |last4=Lang-Lenton |first4=Barbara |last5=Soler |first5=Ana J |last6=Roldan |first6=Eduardo R.S |title=Sperm design and sperm function |journal=Biology Letters |date=22 June 2006 |volume=2 |issue=2 |pages=246–249 |doi=10.1098/rsbl.2006.0449 |pmid=17148374 |pmc=1618917}}</ref> Many [[protist]]s with flagella are known as [[flagellate]]s.

A microorganism may have from one to many flagella. A [[gram-negative bacterium]] ''[[Helicobacter pylori]]'', for example, uses its flagella to propel itself through the stomach to reach the [[gastric mucosa|mucous lining]] where it may colonise the epithelium and potentially cause gastritis, and [[peptic ulcer|ulcers]] – a risk factor for [[stomach cancer]].<ref name="pmid11584108">{{cite journal |last1=Lacy |first1=BE |last2=Rosemore |first2=J | title = Helicobacter pylori: ulcers and more: the beginning of an era | journal = The Journal of Nutrition | volume = 131 | issue = 10 | pages = 2789S–2793S | date = October 2001 | pmid = 11584108 | doi = 10.1093/jn/131.10.2789S | url = http://jn.nutrition.org/cgi/content/abstract/131/10/2789S | format = abstract page | access-date = 2 June 2008 | archive-url = https://web.archive.org/web/20090207110701/http://jn.nutrition.org/cgi/content/abstract/131/10/2789S | archive-date = 7 February 2009 | url-status = dead | doi-access=free}}</ref> In some [[Swarming motility|swarming bacteria]], the flagellum can also function as a sensory [[organelle]], being sensitive to wetness outside the cell.<ref>{{cite journal |last1=Wang |first1=Qingfeng |last2=Suzuki |first2=Asaka |last3=Mariconda |first3=Susana |last4=Porwollik |first4=Steffen |last5=Harshey |first5=Rasika M |title=Sensing wetness: a new role for the bacterial flagellum |journal=The EMBO Journal |date=1 June 2005 |volume=24 |issue=11 |pages=2034–2042 |doi=10.1038/sj.emboj.7600668 |pmid=15889148 |pmc=1142604}}</ref>

Across the [[Three-domain system|three domains]] of [[Bacteria]], [[Archaea]], and [[Eukaryota]], the flagellum has a different structure, protein composition, and mechanism of propulsion but shares the same function of providing motility. The [[Latin]] word {{wikt-lang|la|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.<ref name=Albers>{{cite journal |last1=Albers |first1=Sonja-Verena |last2=Jarrell |first2=Ken F. |title=The archaellum: how archaea swim |journal=Frontiers in Microbiology |date=27 January 2015 |volume=6 |page=23 |doi=10.3389/fmicb.2015.00023 |pmid=25699024 |pmc=4307647 |doi-access=free}}</ref><ref name="Quax">{{cite journal |last1=Quax |first1=TEF |last2=Albers |first2=SV |last3=Pfeiffer |first3=F |title=Taxis in archaea |journal=Emerging Topics in Life Sciences |date=14 December 2018 |volume=2 |issue=4 |pages=535–546 |doi=10.1042/ETLS20180089 |pmid=33525831 |pmc=7289035}}</ref>

Eukaryotic flagella and [[cilia]] are identical in structure but have different lengths and functions.<ref name="pmid6459327">{{cite journal |last1=Haimo |first1=L T |last2=Rosenbaum |first2=J L |title=Cilia, flagella, and microtubules. |journal=The Journal of Cell Biology |date=1 December 1981 |volume=91 |issue=3 |pages=125s–130s |doi=10.1083/jcb.91.3.125s |pmid=6459327 |pmc=2112827}}</ref> [[Prokaryotic]] [[Fimbria (bacteriology)|fimbriae]] and [[pilus|pili]] are smaller, and thinner appendages, with different functions. Surface-attached cilia and flagella are used to swim or move fluid from one region to another.<ref>{{Cite journal |last=Wan |first=Kirsty Y. |date=2018-11-21 |title=Coordination of eukaryotic cilia and flagella |url=http://dx.doi.org/10.1042/ebc20180029 |journal=Essays in Biochemistry |volume=62 |issue=6 |pages=829–838 |doi=10.1042/ebc20180029 |pmid=30464007 |pmc=6281475 |issn=0071-1365}}</ref>

==Types==
[[File:Difference Between Prokaryote and Eukaryote Flagella.svg|thumb|upright=1.2|[[Prokaryotic]] (bacterial and archaeal) flagella run in a rotary movement, while eukaryotic flagella run in a bending movement. The prokaryotic flagellum uses a [[Rotating locomotion in living systems|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).<ref>{{cite journal |last1=Streif |first1=Stefan |last2=Staudinger |first2=Wilfried Franz |last3=Marwan |first3=Wolfgang |last4=Oesterhelt |first4=Dieter |title=Flagellar Rotation in the Archaeon Halobacterium salinarum Depends on ATP |journal=Journal of Molecular Biology |date=December 2008 |volume=384 |issue=1 |pages=1–8 |doi=10.1016/j.jmb.2008.08.057 |pmid=18786541}}</ref>]]
The three types of flagella are bacterial, archaeal, and eukaryotic.

The flagella in eukaryotes have [[dynein]] and [[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.<ref name="Alberts1">{{cite book |last1=Alberts |first1=Bruce |title=Molecular biology of the cell |date=2015 |location=New York, NY |isbn=9780815344643 |page=942 |edition=Sixth}}</ref>

Other differences among these three types are:

*Bacterial flagella are helical filaments, each with a [[rotating locomotion in living systems|rotary motor]] at its base which can turn clockwise or counterclockwise.<ref name=Silverman1>{{cite journal |last1=Silverman |first1=Michael |last2=Simon |first2=Melvin |title=Flagellar rotation and the mechanism of bacterial motility |journal=Nature |date=May 1974 |volume=249 |issue=5452 |pages=73–74 |doi=10.1038/249073a0 |pmid=4598030 | bibcode=1974Natur.249...73S |s2cid=10370084}}</ref><ref name=Meister1>{{cite journal |last1=Lowe |first1=Graeme |last2=Meister |first2=Markus |last3=Berg |first3=Howard C. |title=Rapid rotation of flagellar bundles in swimming bacteria |journal=Nature |date=February 1987 |volume=325 |issue=6105 |pages=637–640 |doi=10.1038/325637a0 |bibcode=1987Natur.325..637L |s2cid=4242129 }}</ref><ref name=Berg1>{{cite journal |last1=Berg |first1=Howard C. |last2=Anderson |first2=Robert A. |title=Bacteria Swim by Rotating their Flagellar Filaments |journal=Nature |date=October 1973 |volume=245 |issue=5425 |pages=380–382 |doi=10.1038/245380a0 |pmid=4593496 | bibcode=1973Natur.245..380B |s2cid=4173914 }}</ref> They provide two of several kinds of [[bacterial motility]].<ref name=Jahn1>{{cite journal |last1=Jahn |first1=T L |last2=Bovee |first2=E C |title=Movement and Locomotion of Microorganisms |journal=Annual Review of Microbiology |date=October 1965 |volume=19 |issue=1 |pages=21–58 |doi=10.1146/annurev.mi.19.100165.000321 |pmid=5318439}}</ref><ref name=Harshey1>{{cite journal |last1=Harshey |first1=RM | title = Bacterial motility on a surface: many ways to a common goal | journal = Annual Review of Microbiology | volume = 57 | pages = 249–73 | year = 2003 | pmid = 14527279 | doi = 10.1146/annurev.micro.57.030502.091014 }}</ref>
*Archaeal flagella ([[archaellum|archaella]]) are superficially similar to bacterial flagella in that it also has a rotary motor, but are different in many details and considered non-[[homology (biology)|homologous]].<ref name=Ng1>{{cite journal |last1=Ng |first1=Sandy Y.M. |last2=Chaban |first2=Bonnie |last3=Jarrell |first3=Ken F. |title=Archaeal Flagella, Bacterial Flagella and Type IV Pili: A Comparison of Genes and Posttranslational Modifications |journal=Microbial Physiology |date=2006 |volume=11 |issue=3–5 |pages=167–191 |doi=10.1159/000094053 |pmid=16983194 |s2cid=30386932}}</ref><ref name=Metlina1>{{cite journal |last1=Metlina |first1=A. L. |title=Bacterial and archaeal flagella as prokaryotic motility organelles |journal=Biochemistry (Moscow) |date=November 2004 |volume=69 |issue=11 |pages=1203–1212 |doi=10.1007/s10541-005-0065-8 |pmid=15627373 |s2cid=632440 }}</ref><ref name= Jarelletal>{{cite book |last1=Jarrell |first1=K |year=2009|chapter=Archaeal Flagella and Pili|title=Pili and Flagella: Current Research and Future Trends|publisher=Caister Academic Press|isbn= 978-1-904455-48-6}}</ref>
*Eukaryotic flagella—those of animal, plant, and protist cells—are complex cellular projections that lash back and forth. Eukaryotic flagella and [[motile cilia]] are identical in structure, but have different lengths, waveforms, and functions. [[Cilium#Non-motile cilia|Primary cilia]] are immotile, and have a [[Cilium#Structure|structurally different]] [[Axoneme#Structure|9+0 axoneme]] rather than the [[Axoneme#Structure|9+2 axoneme]] found in both flagella and motile cilia.

===Bacterial flagella===

====Structure and composition====
The bacterial flagellum is made up of [[protein]] subunits of [[flagellin]].<ref name="Alberts1"/> Its shape is a 20-[[nanometer]]-thick hollow tube. It is [[helix|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]], passing through protein rings in the cell's membrane that act as bearings. [[Gram-positive]] organisms have two of these basal body rings, one in the [[peptidoglycan]] layer and one in the [[plasma membrane]]. [[Gram-negative]] organisms have four 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 [[cytoplasm]]. The filament ends with a capping protein.<ref name=MacNab1>{{cite journal |last1=Macnab |first1=Robert M. |title=How Bacteria Assemble Flagella |journal=Annual Review of Microbiology |date=October 2003 |volume=57 |issue=1 |pages=77–100 |doi=10.1146/annurev.micro.57.030502.090832 |pmid=12730325}}</ref><ref name=dioszeghy1>{{cite journal |last1=Diószeghy |first1=Zoltán |last2=Závodszky |first2=Péter |last3=Namba |first3=Keiichi |last4=Vonderviszt |first4=Ferenc |title=Stabilization of flagellar filaments by HAP2 capping |journal=FEBS Letters |date=18 June 2004 |volume=568 |issue=1–3 |pages=105–109 |doi=10.1016/j.febslet.2004.05.029 |pmid=15196929 |s2cid=33886010 |bibcode=2004FEBSL.568..105D}}</ref>

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]], [[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.<ref name="SevenProtofilamentsGalkin">{{cite journal |last1=Galkin |first1=Vitold E. |last2=Yu |first2=Xiong |last3=Bielnicki |first3=Jakub |last4=Heuser |first4=John |last5=Ewing |first5=Cheryl P. |last6=Guerry |first6=Patricia |last7=Egelman |first7=Edward H. |title=Divergence of Quaternary Structures Among Bacterial Flagellar Filaments |journal=Science |date=18 April 2008 |volume=320 |issue=5874 |pages=382–385 |doi=10.1126/science.1155307 |pmid=18420936 |bibcode=2008Sci...320..382G |s2cid=7702002}}</ref>

The basal body has several traits in common with some types of [[secretion#In gram-negative bacteria|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|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 [[Cryogenic electron microscopy|cryo-electron microscopy]]. The best understood parts are the parts between the inner and outer [[Cell membrane|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.<ref>{{Cite journal |last1=Worrall |first1=Liam J. |last2=Majewski |first2=Dorothy D. |last3=Strynadka |first3=Natalie C.J. |date=2023-09-15 |title=Structural Insights into Type III Secretion Systems of the Bacterial Flagellum and Injectisome |journal=Annual Review of Microbiology |language=en |volume=77 |issue=1 |pages=669–698 |doi=10.1146/annurev-micro-032521-025503 |pmid=37713458 |s2cid=261963968 |issn=0066-4227|doi-access=free }}</ref>

====Motor====
{{Further |Rotating locomotion in living systems}}
[[File:Flagellar Motor Assembly.jpg|alt=Bacterial flagellar motor assembly|thumb|440x440px|Bacterial flagellar motor assembly: Shown here is the C-ring at the base with FliG in red, FliM in yellow, and FliN in shades of purple; the MS-ring in blue; the MotAB in brown; the LP-ring in pink; and the rod in gray.<ref name="Singh2024">{{Cite journal |last1=Singh |first1=Prashant K. |last2=Sharma |first2=Pankaj |last3=Afanzar |first3=Oshri |last4=Goldfarb |first4=Margo H. |last5=Maklashina |first5=Elena |last6=Eisenbach |first6=Michael |last7=Cecchini |first7=Gary |last8=Iverson |first8=T. M. |date=2024-04-17 |title=CryoEM structures reveal how the bacterial flagellum rotates and switches direction |journal=Nature Microbiology |volume=9 |issue=5 |language=en |pages=1271–1281 |doi=10.1038/s41564-024-01674-1 |issn=2058-5276|doi-access=free |pmid=38632342 |pmc=11087270 }}</ref>]]
The bacterial flagellum is driven by a rotary engine ([[MotA|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 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 (''[[Vibrio]]'' species have two kinds of flagella, lateral and polar, and some are driven by a sodium [[ion transporter|ion pump]] rather than a [[proton pump]]<ref name="Atsumi2">{{cite journal |last1=Atsumi |first1=Tatsuo |last2=McCartert |first2=Linda |last3=Imae |first3=Yasuo |title=Polar and lateral flagellar motors of marine Vibrio are driven by different ion-motive forces |journal=Nature |date=January 1992 |volume=355 |issue=6356 |pages=182–184 |doi=10.1038/355182a0 |pmid=1309599 |bibcode=1992Natur.355..182A |s2cid=4315167 }}</ref>). The rotor transports protons across the membrane, and is turned in the process. The rotor alone can operate at 6,000 to 100,000 [[revolutions per minute|rpm]],<ref>{{Citation |last1=Kojima |first1=Seiji |title=The Bacterial Flagellar Motor: Structure and Function of a Complex Molecular Machine |date=2004 |series=International Review of Cytology |volume=233 |pages=93–134 |url=https://linkinghub.elsevier.com/retrieve/pii/S0074769604330032 |access-date=2024-04-23 |publisher=Elsevier |language=en |doi=10.1016/s0074-7696(04)33003-2 |isbn=978-0-12-364637-8 |last2=Blair |first2=David F|pmid=15037363 |url-access=subscription }}</ref> 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 the rotor.<ref>{{cite journal |last1=Dean |first1=Tim |url=http://lifescientist.com.au/content/molecular-biology/news/inside-nature-s-most-efficient-motor-the-flagellar-1216235209 |title=Inside nature's most efficient motor: the flagellar |journal=Australian Life Scientist |date=2 August 2010 }}</ref> The torque is transferred from the MotAB to the torque helix on FliG's D5 domain and with the increase in the requirement of the torque or speed more MotAB are employed.<ref name="Singh2024"/> 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.<ref>{{cite journal|last1=Whitfield|first1=John |title=Bacterial engines have their own clutch|url=http://www.nature.com/news/2008/080619/full/news.2008.903.html|journal=Nature News|access-date=17 May 2017|language=en|doi=10.1038/news.2008.903|date=19 June 2008|pages=news.2008.903 |url-access=subscription}}</ref>{{microbial and microbot movement|biological}}

The production and rotation of a flagellum can take up to 10% of an  ''Escherichia coli'' cell's energy budget and has been described as an "energy-guzzling machine".<ref name="t665">{{cite journal |last1=Bhattacharyya |first1=Souvik |last2=Lopez |first2=Shelby |last3=Singh |first3=Abhyudai |last4=Harshey |first4=Rasika M. |year=2024 |title=Flagellar motility is mutagenic |journal=Proceedings of the National Academy of Sciences |volume=121 |issue=41 |pages=e2412541121 |doi=10.1073/pnas.2412541121 |issn=0027-8424 |doi-access=free|pmid=39352926 |pmc=11474059 |bibcode=2024PNAS..12112541B }}</ref> Its operation generates [[reactive oxygen species]] that elevate mutation rates.<ref name="t665" /> 

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.<ref name="isbn0-674-03116-4">{{cite book | last1 = Dusenbery |first1=DB |title= Living at Micro Scale: The Unexpected Physics of Being Small |publisher= Harvard University Press |location= Cambridge |year= 2009 |isbn= 978-0-674-03116-6 |chapter= Chapter 13}}</ref>

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&nbsp;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.<ref name="cbn">{{cite journal |title=Motions of the running Cheetah and Horse |first=Milton |last=Hildebrand |date=November 1959 |journal=[[Journal of Mammalogy]] |volume=44 |issue=4 |pages=481–495 |doi=10.2307/1376265 |jstor=1376265}} Although according to {{cite book |first1=Luke |last1=Hunter |first2=Dave |last2=Hamman |title=Cheetah |publisher=Struik Publishers |year=2003 |pages=37–38 |quote=the cheetah's fastest recorded speed was {{convert|110|km/h|abbr=on}}}}</ref>

Through use of their flagella, bacteria are able to move rapidly towards attractants and away from repellents, by means of a [[biased random walk (biochemistry)|biased random walk]], with [[Run-and-tumble motion|runs and tumbles]] brought about by rotating its flagellum [[counterclockwise]] and [[clockwise]], respectively. The two directions of rotation are not identical (with respect to flagellum movement) and are selected by a molecular switch.<ref>{{cite journal |last1=Meadows |first1=Robin |title=How Bacteria Shift Gears |journal=PLOS Biology |date=10 May 2011 |volume=9 |issue=5 |pages=e1001061 |doi=10.1371/journal.pbio.1001061 | pmid = 21572986 | pmc = 3091840 | doi-access = free }}</ref> Clockwise rotation is called the ''traction mode'' with the body following the flagella. Counterclockwise rotation is called the ''thruster mode'' with the flagella lagging behind the body.<ref name="Sun">{{cite journal |last1=Sun |first1=Qifang |last2=Yuan |first2=Chengzhi |last3=Zhou |first3=Sainan |last4=Lu |first4=Jing |last5=Zeng |first5=Meiyan |last6=Cai |first6=Xiong |last7=Song |first7=Houpan |title=Helicobacter pylori infection: a dynamic process from diagnosis to treatment |journal=Frontiers in Cellular and Infection Microbiology |date=19 October 2023 |volume=13 |doi=10.3389/fcimb.2023.1257817 |pmid=37928189 |pmc=10621068 |doi-access=free}}</ref>

====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.<ref name="MinaminoReview1">{{cite journal |last1=Minamino |first1=Tohru |last2=Imada |first2=Katsumi |last3=Namba |first3=Keiichi |title=Mechanisms of type III protein export for bacterial flagellar assembly |journal=Molecular BioSystems |date=2008 |volume=4 |issue=11 |pages=1105–1115 |doi=10.1039/b808065h |pmid = 18931786}}</ref> ''In vitro'', flagellar filaments assemble spontaneously in a solution containing purified flagellin as the sole protein.<ref name="Asakura1">{{cite journal |last1=Asakura |first1=Sho |last2=Eguchi |first2=Goro |last3=Iino |first3=Tetsuo |title=Reconstitution of bacterial flagella in vitro |journal=Journal of Molecular Biology |date=October 1964 |volume=10 |issue=1 |pages=42–IN9 |doi=10.1016/S0022-2836(64)80026-7 | pmid = 14222895}}</ref>

====Evolution====
{{Main|Evolution of flagella}}

At least 10 protein components of the bacterial flagellum share homologous proteins with the [[type three secretion system]] (T3SS) found in many gram-negative bacteria,<ref name="ReferenceA">{{cite journal |last1=Pallen |first1=Mark J. |last2=Matzke |first2=Nicholas J. |title=From The Origin of Species to the origin of bacterial flagella |journal=Nature Reviews Microbiology |date=October 2006 |volume=4 |issue=10 |pages=784–790 |doi=10.1038/nrmicro1493 | pmid = 16953248 | s2cid = 24057949 }}</ref> 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<ref name="Saier">{{cite journal |last1=Saier |first1=M |title=Evolution of bacterial type III protein secretion systems |journal=Trends in Microbiology |date=March 2004 |volume=12 |issue=3 |pages=113–115 |doi=10.1016/j.tim.2004.01.003 |pmid=15001186}}</ref> that the flagellum may have evolved first or the two structures evolved in parallel. Early single-cell organisms' need for [[motility]] (mobility) support that the more mobile flagella would be selected by evolution first,<ref name="Saier"/> but the T3SS evolving from the flagellum can be seen as 'reductive evolution', and receives no topological support from the [[phylogenetic]] trees.<ref name="Gophna2003">{{cite journal |last1=Gophna |first1=Uri |last2=Ron |first2=Eliora Z. |last3=Graur |first3=Dan |title=Bacterial type III secretion systems are ancient and evolved by multiple horizontal-transfer events |journal=Gene |date=July 2003 |volume=312 |pages=151–163 |doi=10.1016/S0378-1119(03)00612-7 | pmid = 12909351}}</ref> 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.<ref>{{cite journal |last1=McCann |first1=Honour C. |last2=Guttman |first2=David S. |title=Evolution of the type III secretion system and its effectors in plant–microbe interactions |journal=New Phytologist |date=January 2008 |volume=177 |issue=1 |pages=33–47 |doi=10.1111/J.1469-8137.2007.02293.X | pmid = 18078471 |doi-access=free|bibcode=2008NewPh.177...33M }}</ref>

====Flagella and the intelligent design debate====
{{Main|Intelligent design|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 "[[irreducible complexity|irreducibly complex]]".<ref>{{cite book |last1=Behe |first1=Michael J. |title=The edge of evolution: the search for the limits of Darwinism |date=2007 |publisher=Free Press |location=New York, NY |isbn=978-0-7432-9620-5}}</ref> However, many proteins can be deleted or mutated and the flagellum still works, though sometimes at reduced efficiency.<ref>{{cite journal |last1=Rajagopala |first1=Seesandra V |last2=Titz |first2=Björn |last3=Goll |first3=Johannes |last4=Parrish |first4=Jodi R |last5=Wohlbold |first5=Katrin |last6=McKevitt |first6=Matthew T |last7=Palzkill |first7=Timothy |last8=Mori |first8=Hirotada |last9=Finley |first9=Russell L |last10=Uetz |first10=Peter |title=The protein network of bacterial motility |journal=Molecular Systems Biology |date=January 2007 |volume=3 |issue=1 |page=128 |doi=10.1038/msb4100166 | pmid = 17667950 | pmc = 1943423}}</ref> Moreover, with many proteins unique to some number across species, diversity of bacterial flagella composition was higher than expected.<ref>{{cite journal |last1=Titz |first1=Björn |last2=Rajagopala |first2=Seesandra V. |last3=Ester |first3=Claudia |last4=Häuser |first4=Roman |last5=Uetz |first5=Peter |title=Novel Conserved Assembly Factor of the Bacterial Flagellum |journal=Journal of Bacteriology |date=November 2006 |volume=188 |issue=21 |pages=7700–7706 |doi=10.1128/JB.00820-06 | pmid = 16936039 | pmc = 1636259}}</ref> 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''.<ref>{{cite journal |last1=Kakkanat |first1=Asha |last2=Phan |first2=Minh-Duy |last3=Lo |first3=Alvin W. |last4=Beatson |first4=Scott A. |last5=Schembri |first5=Mark A. |title=Novel genes associated with enhanced motility of Escherichia coli ST131 |journal=PLOS ONE |date=10 May 2017 |volume=12 |issue=5 |pages=e0176290 |doi=10.1371/journal.pone.0176290 | pmid = 28489862 | bibcode = 2017PLoSO..1276290K | pmc=5425062| doi-access = free }}</ref> 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.<ref name="ReferenceA"/> 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.<ref>{{cite journal |last1=Pallen |first1=M.J. |last2=Gophna |first2=U. |title=Bacterial Flagella and Type III Secretion: Case Studies in the Evolution of Complexity |journal=Genome Dynamics |date=2007 |volume=3 |pages=30–47 |doi=10.1159/000107602 | pmid = 18753783 | isbn = 978-3-8055-8340-4 }}</ref>

====Flagellar arrangements====
Different species of bacteria have different numbers and arrangements of flagella,<ref name="ndvsu">{{cite web |title=Bacterial flagella |url=http://www.ndvsu.org/images/StudyMaterials/Micro/Bacterial-Flagella.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.ndvsu.org/images/StudyMaterials/Micro/Bacterial-Flagella.pdf |archive-date=2022-10-09 |url-status=live |access-date=29 December 2021}}</ref><ref name="Ruan">{{cite journal |last1=Ruan |first1=Juanfang |last2=Kato |first2=Takayuki |last3=Santini |first3=Claire-Lise |last4=Miyata |first4=Tomoko |last5=Kawamoto |first5=Akihiro |last6=Zhang |first6=Wei-Jia |last7=Bernadac |first7=Alain |last8=Wu |first8=Long-Fei |last9=Namba |first9=Keiichi |title=Architecture of a flagellar apparatus in the fast-swimming magnetotactic bacterium MO-1 |journal=Proceedings of the National Academy of Sciences |date=11 December 2012 |volume=109 |issue=50 |pages=20643–20648 |doi=10.1073/pnas.1215274109 |pmid=23184985 |pmc=3528567 |bibcode=2012PNAS..10920643R |doi-access=free }}</ref> named using the term ''tricho'', from the Greek ''trichos'' meaning ''hair''.<ref name="rxlist">{{cite web |title=tricho- prefix |url=https://www.rxlist.com/tricho-_prefix/definition.htm |access-date=26 March 2022}}</ref>
*'''Monotrichous''' bacteria such as ''[[Vibrio cholerae]]'' have a single '''polar flagellum'''.<ref name="Echazarreta">{{cite journal |last1=Echazarreta |first1=MA |last2=Klose |first2=KE |title=''Vibrio'' Flagellar Synthesis. |journal=Frontiers in Cellular and Infection Microbiology |date=2019 |volume=9 |pages=131 |doi=10.3389/fcimb.2019.00131 |pmid=31119103|pmc=6504787 |doi-access=free }}</ref>
*'''Amphitrichous''' bacteria have a single flagellum on each of two opposite ends (e.g., ''[[Campylobacter jejuni]]'' or ''[[Alcaligenes faecalis]]'')&mdash;both flagella rotate but coordinate to produce coherent thrust.
*'''Lophotrichous''' bacteria (''lopho'' Greek combining term meaning ''crest'' or ''tuft'')<ref name="Collins">{{cite web |title=Lopho |url=https://www.collinsdictionary.com/dictionary/english/lopho |access-date=26 March 2022}}</ref> 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|date=January 2009}}
*'''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]].

[[Spirochete]]s, in contrast, have flagella called [[endoflagella]] arising from opposite poles of the cell, and are located within the [[periplasmic space]] as shown by breaking the outer-membrane and also by [[electron cryotomography]] microscopy.<ref>{{cite journal |last1=Kudryashev |first1=Mikhail |last2=Cyrklaff |first2=Marek |last3=Baumeister |first3=Wolfgang |last4=Simon |first4=Markus M. |last5=Wallich |first5=Reinhard |last6=Frischknecht |first6=Friedrich |title=Comparative cryo-electron tomography of pathogenic Lyme disease spirochetes |journal=Molecular Microbiology |date=March 2009 |volume=71 |issue=6 |pages=1415–1434 |doi=10.1111/j.1365-2958.2009.06613.x | pmid = 19210619 | s2cid = 19650892 | doi-access = free }}</ref> 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 ''[[Selenomonad|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 "[[wikt:fascicle|fascicle]]".

In some ''Vibrio'' spp. (particularly ''[[Vibrio parahaemolyticus]]''<ref name="Kim1">{{cite journal |last1=Kim |first1=Yun-Kyeong |last2=McCarter |first2=Linda L. |title=Analysis of the Polar Flagellar Gene System of Vibrio parahaemolyticus |journal=Journal of Bacteriology |date=July 2000 |volume=182 |issue=13 |pages=3693–3704 |doi=10.1128/JB.182.13.3693-3704.2000 | pmid = 10850984 | pmc = 94540}}</ref>) 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.<ref name="Atsumi1">{{cite journal |last1=Atsumi |first1=T |last2=Maekawa |first2=Y |last3=Yamada |first3=T |last4=Kawagishi |first4=I |last5=Imae |first5=Y |last6=Homma |first6=M |title=Effect of viscosity on swimming by the lateral and polar flagella of Vibrio alginolyticus |journal=Journal of Bacteriology |date=August 1996 |volume=178 |issue=16 |pages=5024–5026 |doi=10.1128/jb.178.16.5024-5026.1996 |pmid=8759871 |pmc=178290}}</ref><ref name="McCarter2">{{cite journal |last1=McCarter |first1=Linda L. |title=Dual Flagellar Systems Enable Motility under Different Circumstances |journal=Microbial Physiology |date=2004 |volume=7 |issue=1–2 |pages=18–29 |doi=10.1159/000077866 |pmid = 15170400 | s2cid = 21963003 }}</ref><ref name="Merino1">{{cite journal |last1=Merino |first1=Susana |last2=Shaw |first2=Jonathan G. |last3=TomÃ¡s |first3=Juan M. |title=Bacterial lateral flagella: an inducible flagella system |journal=FEMS Microbiology Letters |date=October 2006 |volume=263 |issue=2 |pages=127–135 |doi=10.1111/j.1574-6968.2006.00403.x |pmid=16978346 |doi-access=free }}</ref><ref name="Belas1">{{cite journal |last1=Belas |first1=R |last2=Simon |first2=M |last3=Silverman |first3=M |title=Regulation of lateral flagella gene transcription in Vibrio parahaemolyticus |journal=Journal of Bacteriology |date=July 1986 |volume=167 |issue=1 |pages=210–218 |doi=10.1128/jb.167.1.210-218.1986 |pmid = 3013835 | pmc=212863}}</ref><ref name="Canals1">{{cite journal |last1=Canals |first1=Rocío |last2=Altarriba |first2=Maria |last3=Vilches |first3=Silvia |last4=Horsburgh |first4=Gavin |last5=Shaw |first5=Jonathan G. |last6=Tomás |first6=Juan M. |last7=Merino |first7=Susana |title=Analysis of the Lateral Flagellar Gene System of Aeromonas hydrophila AH-3 |journal=Journal of Bacteriology |date=February 2006 |volume=188 |issue=3 |pages=852–862 |doi=10.1128/JB.188.3.852-862.2006 |pmid = 16428388 | pmc = 1347325}}</ref><ref name="Canals2">{{cite journal |last1=Canals |first1=Rocío |last2=Ramirez |first2=Silvia |last3=Vilches |first3=Silvia |last4=Horsburgh |first4=Gavin |last5=Shaw |first5=Jonathan G. |last6=Tomás |first6=Juan M. |last7=Merino |first7=Susana |title=Polar Flagellum Biogenesis in Aeromonas hydrophila |journal=Journal of Bacteriology |date=15 January 2006 |volume=188 |issue=2 |pages=542–555 |doi=10.1128/JB.188.2.542-555.2006 |pmid = 16385045 | pmc = 1347287}}</ref> 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 [[Run-and-tumble motion|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.<ref name="pmid14671319">{{cite journal |last1=Kim |first1=MunJu |last2=Bird |first2=James C. |last3=Van Parys |first3=Annemarie J. |last4=Breuer |first4=Kenneth S. |last5=Powers |first5=Thomas R. |title=A macroscopic scale model of bacterial flagellar bundling |journal=Proceedings of the National Academy of Sciences |date=23 December 2003 |volume=100 |issue=26 |pages=15481–15485 |doi=10.1073/pnas.2633596100 |arxiv=cond-mat/0312562 |bibcode=2003PNAS..10015481K |pmc=307593 |pmid=14671319 |doi-access=free}}</ref><ref name="pmid264676">{{cite journal |last1=Macnab |first1=RM |date=January 1977 |title=Bacterial flagella rotating in bundles: a study in helical geometry |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=74 |issue=1 |pages=221–5 |bibcode=1977PNAS...74..221M |doi=10.1073/pnas.74.1.221 |pmc=393230 |pmid=264676 |doi-access=free}}</ref>

===Eukaryotic flagella===
[[File:Eukarya Flagella.svg|thumb|left|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]]
[[File:Eukaryotic flagellum.svg|thumb|200px|Cross section of an [[axoneme]]]]
[[File:Chlamydomonas TEM 09.jpg|thumb|left|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).]]

[[File:Chlamydomonas TEM 17.jpg|thumb|200px|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 "[[undulipodium|undulipodia]]" (e.g., all papers by [[Lynn Margulis|Margulis]] since the 1970s)<ref name="pmid14657097">{{cite journal |last1=Taylor |first1=F J R Max |title=The collapse of the two-kingdom system, the rise of protistology and the founding of the International Society for Evolutionary Protistology (ISEP) |journal=International Journal of Systematic and Evolutionary Microbiology |date=1 November 2003 |volume=53 |issue=6 |pages=1707–1714 |doi=10.1099/ijs.0.02587-0 | pmid = 14657097 |doi-access=free}}</ref> or "cilia" for both (e.g., Hülsmann, 1992;<ref>{{cite journal |last1=Hülsmann |first1=Norbert |title=Undulipodium: End of a useless discussion |journal=European Journal of Protistology |date=August 1992 |volume=28 |issue=3 |pages=253–257 |doi=10.1016/s0932-4739(11)80231-2 | pmid = 23195228}}</ref> Adl et al., 2012;<ref name = "Adl_2012">{{cite journal |last1=Adl |first1=Sina M. |last2=Simpson |first2=Alastair G. B. |last3=Lane |first3=Christopher E. |last4=Lukeš |first4=Julius |last5=Bass |first5=David |last6=Bowser |first6=Samuel S. |last7=Brown |first7=Matthew W. |last8=Burki |first8=Fabien |last9=Dunthorn |first9=Micah |last10=Hampl |first10=Vladimir |last11=Heiss |first11=Aaron |last12=Hoppenrath |first12=Mona |last13=Lara |first13=Enrique |last14=le Gall |first14=Line |last15=Lynn |first15=Denis H. |last16=McManus |first16=Hilary |last17=Mitchell |first17=Edward A. D. |last18=Mozley-Stanridge |first18=Sharon E. |last19=Parfrey |first19=Laura W. |last20=Pawlowski |first20=Jan |last21=Rueckert |first21=Sonja |last22=Shadwick |first22=Laura |last23=Schoch |first23=Conrad L. |last24=Smirnov |first24=Alexey |last25=Spiegel |first25=Frederick W. |title=The Revised Classification of Eukaryotes |journal=Journal of Eukaryotic Microbiology |date=September 2012 |volume=59 |issue=5 |pages=429–514 |doi=10.1111/j.1550-7408.2012.00644.x | pmid = 23020233 | pmc = 3483872}}</ref> most papers of [[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 {{section link||Flagella versus cilia}} below) is still common (e.g., Andersen et al., 1991;<ref>{{cite journal |last1=Andersen |first1=R. A. |last2=Barr |first2=D. J. S. |last3=Lynn |first3=D. H. |last4=Melkonian |first4=M. |last5=Moestrup |first5=Ø. |last6=Sleigh |first6=M. A. |title=Terminology and nomenclature of the cytoskeletal elements associated with the flagellar/ciliary apparatus in protists |journal=Protoplasma |date=February 1991 |volume=164 |issue=1–3 |pages=1–8 |doi=10.1007/bf01320809 |bibcode=1991Prpls.164....1A |s2cid=40755371}}</ref> Leadbeater et al., 2000).<ref>{{cite book |editor1-first=Barry S. C. |editor1-last=Leadbeater |editor2-first=John C. |editor2-last=Green |title=Flagellates: Unity, Diversity and Evolution |url=https://books.google.com/books?id=GURZDwAAQBAJ&pg=PP1 |date=2000 |publisher=Taylor and Francis |isbn=978-1-4822-6822-5 |series=The Systematics Association Special Volume |volume=59}}</ref>

====Internal structure====
The core of a eukaryotic flagellum, known as the [[axoneme]] is a bundle of nine fused pairs of [[microtubule]]s known as ''doublets'' surrounding two central single microtubules (''singlets''). This [[9+2 axoneme]] is characteristic of the eukaryotic flagellum. At the base of a eukaryotic flagellum is a [[basal body]], "blepharoplast" or kinetosome, which is the [[microtubule organizing center]] for flagellar microtubules and is about 500 nanometers long. Basal bodies are structurally identical to [[centriole]]s. The flagellum is encased within the cell's [[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.<ref name = "Barsanti_2006" />{{rp|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 spoke]]s, 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.<ref name=pmid20145000/>

====Flagella versus cilia====
[[File:Flagellum-beating.svg|thumb|350px|Beating pattern of eukaryotic "flagellum" and "cillum", a traditional distinction before the structures of the two are known.]]

The regular beat patterns of eukaryotic [[cilia]] and flagella generate motion on a cellular level. Examples range from the propulsion of single cells such as the swimming of [[spermatozoa]] to the transport of fluid along a stationary layer of cells such as in the [[respiratory epithelium#Mucociliary Escalator|respiratory tract]].<ref name=Lodish00/>

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.<ref name=Lodish00>{{cite book |last1=Lodish |first1=Harvey |last2=Berk |first2=Arnold |last3=Zipursky |first3=S. Lawrence |last4=Matsudaira |first4=Paul |last5=Baltimore |first5=David |last6=Darnell |first6=James |title=Cilia and Flagella: Structure and Movement |date=2000 |isbn=0-7167-3136-3 |url=https://www.ncbi.nlm.nih.gov/books/NBK21698/ |chapter=Section 19.4Cilia and Flagella: Structure and Movement}}</ref> Yet another traditional form of distinction is by the number of 9+2 organelles on the cell.<ref name=pmid20145000>{{cite journal |last1=Lindemann |first1=CB |last2=Lesich |first2=KA |title=Flagellar and ciliary beating: the proven and the possible. |journal=Journal of Cell Science |date=15 February 2010 |volume=123 |issue=Pt 4 |pages=519–28 |doi=10.1242/jcs.051326 |pmid=20145000 |s2cid=18673550 |doi-access=}}</ref>

====Intraflagellar transport====
[[Intraflagellar transport]], 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.<ref name="pmid15466257">{{cite journal |last1=Pazour |first1=Gregory J. |title=Intraflagellar Transport and Cilia-Dependent Renal Disease: The Ciliary Hypothesis of Polycystic Kidney Disease |journal=Journal of the American Society of Nephrology |date=October 2004 |volume=15 |issue=10 |pages=2528–2536 |doi=10.1097/01.ASN.0000141055.57643.E0 | pmid = 15466257 | doi-access = free }}</ref>

====Evolution and occurrence====
{{further|Evolution of flagella}}

Eukaryotic flagella or cilia, probably an ancestral characteristic,<ref>{{cite journal |last1=Yubuki |first1=Naoji |last2=Leander |first2=Brian S. |title=Evolution of microtubule organizing centers across the tree of eukaryotes |journal=The Plant Journal |date=July 2013 |volume=75 |issue=2 |pages=230–244 |doi=10.1111/tpj.12145 | pmid = 23398214 |doi-access=free}}</ref> are widespread in almost all groups of eukaryotes, as a relatively perennial condition, or as a flagellated life cycle stage (e.g., [[zoid]]s, [[gamete]]s, [[zoospore]]s, which may be produced continually or not).<ref>{{cite book |first=J.A. |last=Raven |chapter=The flagellate condition |chapter-url=https://books.google.com/books?id=GURZDwAAQBAJ&pg=PA27 |title={{harvnb|Leadbeater|Green|2000|pp=27–48}} |year=2000 |publisher=CRC Press |isbn=9781482268225 }}<!-- Pages from table of contents --></ref><ref name = "Webster_Weber_2007">{{cite book |last1=Webster |first1=John |last2=Weber |first2=Roland | title = 2007 | chapter = Spores of Fungi | date = 25 January 2007 | edition = 3rd | location = Cambridge | publisher = Cambridge University Press | pages = 23–24 | isbn = 9781139461504 | chapter-url = https://books.google.com/books?id=SApIn7IEnucC&pg=PT64 }}</ref><ref name = "Adl_2012" />

The first situation is found either in specialized cells of multicellular organisms (e.g., the [[choanocyte]]s of [[sponges]], or the ciliated [[epithelia]] of [[metazoan]]s), as in [[ciliate]]s and many eukaryotes with a "flagellate condition" (or "monadoid [[:de:Organisationsstufe|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), [[bryophyte]]s (male gametes), [[pteridophyte]]s (male gametes), some [[gymnosperm]]s ([[cycad]]s and ''[[Ginkgo]]'', as male gametes), centric [[diatom]]s (male gametes), [[brown algae]] (zoospores and gametes), [[oomycete]]s (assexual zoospores and gametes), [[hyphochytrid]]s (zoospores), [[labyrinthulomycetes]] (zoospores), some [[apicomplexan]]s (gametes), some [[radiolarian]]s (probably gametes),<ref>{{cite journal |last1=Lahr |first1=Daniel J. G. |last2=Parfrey |first2=Laura Wegener |last3=Mitchell |first3=Edward A. D. |last4=Katz |first4=Laura A. |last5=Lara |first5=Enrique |title=The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms |journal=Proceedings of the Royal Society B: Biological Sciences |date=22 July 2011 |volume=278 |issue=1715 |pages=2081–2090 |doi=10.1098/rspb.2011.0289 |pmid = 21429931 | pmc = 3107637}}</ref> [[foraminiferan]]s (gametes), [[Phytomyxea|plasmodiophoromycete]]s (zoospores and gametes), [[myxogastrid]]s (zoospores), [[metazoan]]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 [[gymnosperm]]s except cycads and ''Ginkgo'', [[angiosperm]]s, pennate [[diatom]]s, some [[apicomplexan]]s, some [[amoebozoan]]s, in the sperm of some [[metazoan]]s,<ref name = "Austin_1995">{{cite book |last1=Austin |first1=CR |editor1-last=Grudzinskas |editor1-first=Jurgis Gediminas |editor2-last=Yovich |editor2-first=J L |title=Gametes - the spermatozoon |date=1995 |publisher=Cambridge University Press |location=Cambridge |isbn=9780521479967 |url = https://books.google.com/books?id=UEvbQcZ7e1oC}}</ref> and in [[fungi]] (except [[chytrid]]s).

====Typology====
A number of terms related to flagella or cilia are used to characterize eukaryotes.<ref name = "Webster_Weber_2007" /><ref>{{cite book |last1= South |first1=GR |last2=Whittick |first2=A | year = 1987 | title = Introduction to Phycology | publisher = Blackwell Scientific Publications | location = Oxford | page = 65 | url = https://books.google.com/books?id=dOaODP4Oo5kC&pg=PA65 | isbn = 9781444314205 }}</ref><ref name = "Barsanti_2006">{{cite book | last1 = Barsanti | first1 = Laura | last2 =Gualtieri |first2=Paolo |year = 2006 | title = Algae: Anatomy, Biochemistry, and Biotechnology | location = Florida, USA | publisher = CRC Press |url=https://books.google.com/books?id=t4ZQRWvr510C&pg=PA60 |isbn=9780203492598}}</ref>{{rp|60–63}}<ref>{{cite book |last1 = Dodge |first1=JD | year = 1973 | title = The Fine Structure of Algal Cells | publisher = Academic Press | location = London | pages = 57–79 | url = https://books.google.com/books?id=5e6FqXpRlv8C&pg=PA57 | isbn = 9780323158237 }}</ref><ref>{{cite book |last1= Lee |first1=RE | year = 2008 | title = Phycology | edition = 4th | publisher = Cambridge University Press | page = [https://archive.org/details/phycology00leer_0/page/7 7] | url = https://archive.org/details/phycology00leer_0 | url-access = registration | quote = lee tubular hairs. | isbn = 9781139469876 }}</ref> 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 (= [[mastigoneme]]s ''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|bodonid]]s);<ref>{{cite book |last1=Corliss |first1=J.O. |last2=Lom |first2=J |chapter=An annotated glossary of protozoological terms |editor-last=Lee |editor-first=J.J. |editor2-last=Leedale |editor2-first=G.F. |editor3-last=Bradbury |editor3-first=P. |title=An illustrated guide to the protozoa |edition=2nd |volume=2 |publisher=Society of Protozoologists |year=2000 |isbn=1891276239 |pages=1346–85 }}</ref> 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 [[kinetoplastid]]s, some [[parabasalid]]s
*with proboscis (trunk-like protrusion of the cell): e.g., [[apusomonad]]s, some [[Bodonida|bodonid]]s<ref name="Jeuck"/>

According to the number of flagella, cells may be: (remembering that some authors use "ciliated" instead of "flagellated")<ref name="Adl_2012"/><ref>{{cite book |last1= Sleigh |first1=M | year = 1989 | title = Protozoa and other Protists | publisher = Edward Arnold | location = London | pages = 98–99 | url = https://books.google.com/books?id=K2Y4AAAAIAAJ&pg=PA98 | isbn = 9780521428057 }}</ref>
*uniflagellated: e.g., most [[Opisthokonta]]
*biflagellated: e.g., all [[Dinoflagellata]], the gametes of [[Charophyceae]], of most [[bryophyte]]s and of some [[metazoan]]s<ref name = "Austin_1995" />
*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]]'', [[Parabasalid]]a, [[Hemimastigophora]], [[Caryoblastea]], ''[[Multicilia]]'', the gametes (or [[zoid]]s) of [[Oedogoniales]] ([[Chlorophyta]]), some [[pteridophyte]]s and some [[gymnosperm]]s

According to the place of insertion of the flagella:<ref>{{cite book | last1=Sparrow |first1=FK | year = 1960 | title = Aquatic phycomycetes | edition = 2nd | location = Ann Arbor | publisher = Michigan: University of Michigan Press | page = [https://archive.org/details/aquaticphycomyce00spar/page/15 15] | url = https://archive.org/details/aquaticphycomyce00spar }}</ref>
*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.<ref>{{cite journal |last1= Hibberd |first1=DJ | year = 1976 | title = The ultrastructure and taxonomy of the Chrysophyceae and Prymnesiophyceae (Haptophyceae): a survey with some new observations on the ultrastructure of the Chrysophyceae | journal = Journal of the Linnean Society of London, Botany | volume = 72 | issue = 2| pages = 55–80 | doi=10.1111/j.1095-8339.1976.tb01352.x }}</ref>
*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<ref name="Jeuck">{{cite journal |last1=Jeuck |first1=Alexandra |last2=Arndt |first2=Hartmut |title=A Short Guide to Common Heterotrophic Flagellates of Freshwater Habitats Based on the Morphology of Living Organisms |journal=Protist |date=November 2013 |volume=164 |issue=6 |pages=842–860 |doi=10.1016/j.protis.2013.08.003 | pmid = 24239731 | doi-access = free }}</ref>
*heterodynamic: flagella with different beating patterns (usually with one flagellum functioning in food capture and the other functioning in gliding, anchorage, propulsion or "steering")<ref>{{cite journal | last1 = Sleigh | first1 = MA | year = 1985 | title = Origin and evolution of flagellar movement | url = http://opensample.info/fundamental-problems-of-movement-of-cilia-eukaryotic-flagella-and-related-systems-a-seminar-held-under-the-u-s-japan-cooperative-science-program | journal = Cell Motil | volume = 5 | pages = 137–138 | access-date = 21 February 2016 | archive-date = 3 March 2016 | archive-url = https://web.archive.org/web/20160303230957/http://opensample.info/fundamental-problems-of-movement-of-cilia-eukaryotic-flagella-and-related-systems-a-seminar-held-under-the-u-s-japan-cooperative-science-program | url-status = dead }}</ref>
*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)

===Archaeal flagella===
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.<ref name="Cavalier-Smith ">{{cite journal | last1 = Cavalier-Smith |first1=T | title = The origin of eukaryotic and archaebacterial cells | journal = Annals of the New York Academy of Sciences | volume = 503 | issue = 1 | pages = 17–54 | year = 1987 | pmid = 3113314 | doi = 10.1111/j.1749-6632.1987.tb40596.x | url = http://www.annalsnyas.org/cgi/content/citation/503/1/17 | bibcode = 1987NYASA.503...17C | s2cid = 38405158 | url-access = subscription }}{{Dead link|date=December 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> 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 [[Pilus#Type IV pili|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 [[glycan]]s which are necessary for proper assembly or function.<ref name="JarrellK"/>

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

*Bacterial flagella rotation is powered by the [[proton motive force]] – a flow of [[hydron (chemistry)|H<sup>+</sup> ion]]s or occasionally by the [[sodium-motive force]] – a flow of [[sodium|Na<sup>+</sup>]] ions; archaeal flagella rotation is powered by [[adenosine triphosphate|ATP]].<ref name="Brock1">{{cite book |last1=Madigan |first1=Michael T. |title=Brock biology of microorganisms |date=2019 |location=NY, NY |isbn=9781292235103 |pages=70–71 |edition=Fifteenth, Global}}</ref> 
*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&nbsp;nm) to allow this.<ref name="Archaeal flagella">{{cite journal |last1=Ghosh |first1=Abhrajyoti |last2=Albers |first2=Sonja-Verena |title=Assembly and function of the archaeal flagellum |journal=Biochemical Society Transactions |date=1 February 2011 |volume=39 |issue=1 |pages=64–69 |doi=10.1042/BST0390064 |pmid = 21265748}}</ref>
*Many components of bacterial flagella share sequence similarity to components of the [[type three secretion system|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]], which are assembled through the action of [[secretion#Type II secretion system (T2SS)|type II secretion systems]] (the nomenclature of pili and protein secretion systems is not consistent).<ref name="Archaeal flagella"/>

These differences support the theory that the bacterial flagella and archaella are a classic case of biological [[analogy]], or [[convergent evolution]], rather than [[homology (biology)|homology]].<ref>{{cite journal |last1=Thomas |first1=Nikhil A. |last2=Bardy |first2=Sonia L. |last3=Jarrell |first3=Ken F. |title=The archaeal flagellum: a different kind of prokaryotic motility structure |journal=FEMS Microbiology Reviews |date=April 2001 |volume=25 |issue=2 |pages=147–174 |doi=10.1111/j.1574-6976.2001.tb00575.x |pmid = 11250034 |s2cid=34411164}}</ref><ref>{{Cite journal |last1=Chimileski |first1=Scott |last2=Papke |first2=R. Thane |date=2015 |title=Getting a hold on archaeal type IV pili: an expanding repertoire of cellular appendages implicates complex regulation and diverse functions |journal=Frontiers in Microbiology |volume=6 |page=362 |doi=10.3389/fmicb.2015.00362 |issn=1664-302X |pmc=4419858 |pmid=25999922 |doi-access=free}}</ref><ref>{{Cite journal |last1=de Sousa Machado |first1=J. Nuno |last2=Vollmar |first2=Leonie |last3=Schimpf |first3=Julia |last4=Chaudhury |first4=Paushali |last5=Kumariya |first5=Rashmi |last6=van der Does |first6=Chris |last7=Hugel |first7=Thorsten |last8=Albers |first8=Sonja-Verena |date=2021 |title=Autophosphorylation of the KaiC-like protein ArlH inhibits oligomerization and interaction with ArlI, the motor ATPase of the archaellum |journal=Molecular Microbiology |language=en |volume=116 |issue=3 |pages=943–956 |doi=10.1111/mmi.14781 |issn=0950-382X|doi-access=free |pmid=34219289 }}</ref> 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.<ref>{{Cite journal |last1=Nuno de Sousa Machado |first1=João |last2=Albers |first2=Sonja-Verena |last3=Daum |first3=Bertram |date=2022 |title=Towards Elucidating the Rotary Mechanism of the Archaellum Machinery |journal=Frontiers in Microbiology |volume=13 |doi=10.3389/fmicb.2022.848597 |issn=1664-302X |doi-access=free|pmid=35387068 |pmc=8978795 }}</ref><ref>{{Cite journal |last1=Jarrell |first1=Ken F |last2=Albers |first2=Sonja-Verena |last3=Machado |first3=J Nuno de Sousa |date=2021 |title=A comprehensive history of motility and Archaellation in Archaea |url=https://doi.org/10.1093/femsmc/xtab002 |journal=FEMS Microbes |volume=2 |pages=xtab002 |doi=10.1093/femsmc/xtab002 |issn=2633-6685 |pmc=10117864 |pmid=37334237}}</ref>

===Fungal===
The only [[fungi]] to have a single flagellum on their [[Zoospore|spores]] are the [[chytrid]]s. In ''[[Batrachochytrium dendrobatidis]]'' the flagellum is 19–20&nbsp;μm long.<ref name=Longcore1999>{{cite journal |last1=Longcore |first1=Joyce E. |last2=Pessier |first2=Allan P. |last3=Nichols |first3=Donald K. |title=''Batrachochytrium Dendrobatidis'' gen. et ''sp. nov.'', a Chytrid Pathogenic to Amphibians |journal=[[Mycologia]] |date=March 1999 |volume=91 |issue=2 |pages=219–227 |doi=10.2307/3761366 |jstor=3761366}}</ref> A nonfunctioning [[centriole]] lies adjacent to the [[kinetosome]]. Nine interconnected props attach the kinetosome to the [[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.<ref name=Longcore1999 />

==Additional images==
<gallery>

File:EMpylori.jpg|Multiple flagella in lophotrichous arrangement on surface of ''Helicobacter pylori''
File:Physical model of a bacterial flagellum.jpg|Physical model of a bacterial flagellum
</gallery>

== See also ==
*[[Ciliopathy]]
*[[RpoF]]

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* {{cite journal |first=Howard C. |last=Berg |title=Motile Behavior of Bacteria |journal=Physics Today |volume=53 |issue=1 |pages=24–29 |date=January 2000 |doi=10.1063/1.882934 |bibcode=2000PhT....53a..24B |s2cid=178516210 |doi-access=free }}
* {{cite web |url= http://www2.oakland.edu/biology/lindemann/ |title= Mechanisms of sperm motility |first= Charles |last= Lindemann |date= 2008-04-04 |publisher= Oakland University |access-date= 2008-05-18 |archive-url= https://web.archive.org/web/20080516113827/http://www2.oakland.edu/biology/lindemann/ |archive-date= 16 May 2008 |url-status= dead }}
* {{cite journal |last1=Purcell |first1=EM |title=Life at Low Reynolds Number |journal=American Journal of Physics |volume=45 |issue=1 |pages=3–11 |year=1977 |doi=10.1119/1.10903 |url=http://www.nada.kth.se/~annak/Low_RE.pdf |bibcode=1977AmJPh..45....3P |hdl=2433/226838 |access-date=19 October 2009 |archive-url=https://web.archive.org/web/20110605051839/http://www.nada.kth.se/~annak/Low_RE.pdf |archive-date=5 June 2011 |url-status=dead |hdl-access=free }}
* {{cite web |url= http://www.talkdesign.org/faqs/flagellum.html |title= Evolution in (Brownian) space: a model for the origin of the bacterial flagellum |last1= Matzke |first1=NJ |date= 2003-11-10 |publisher= talkdesign.org }}
{{refend}}

== External links ==
{{Commons category|Flagella}}

*[https://web.archive.org/web/20180215083922/http://cellimagelibrary.org/images?k=flagella&simple_search=Search Cell Image Library - Flagella]

{{Cellular structures}}
{{Plankton}}
{{Protist structures}}
{{Cyclopaedia 1728}}

{{Authority control}}

[[Category:Cell movement]]
[[Category:Organelles]]
[[Category:Protein complexes]]
[[Category:Bacteria]]