Editing Flagellum (section)

===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)