Editing Flagellum (section)

====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 }}</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 }}</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>