Organelle
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In [[cell biology]], an '''organelle''' (pronunciation: {{IPA|/ɔː(r)gəˡnɛl/}}) is a specialized subunit within a [[cell (biology)|cell]] that has a specific function, and is separately enclosed within its own [[lipid]] membrane.
[[Image:biological cell.svg|thumb|right|350px|A typical animal cell. Within the [[cytoplasm]], the major organelles and cellular structures include: (1) [[nucleolus]] (2) [[cell nucleus|nucleus]] (3) [[ribosome]] (4) [[vesicle (biology)|vesicle]] (5) rough [[endoplasmic reticulum]] (6) [[Golgi apparatus]] (7) [[cytoskeleton]] (8) smooth [[endoplasmic reticulum]] (9) [[mitochondrion|mitochondria]] (10) [[vacuole]] (11) [[cytosol]] (12) [[lysosome]] (13) [[centriole]].]]
The name ''organelle'' comes from the idea that these structures are to cells what an [[Organ (anatomy)|organ]] is to the [[body]] (hence the name ''organelle,'' the suffix ''-elle'' being a [[diminutive]]). Organelles are identified by [[microscopy]], and can also be purified by [[cell fractionation]].
There are many types of organelles, particularly in the [[eukaryote|eukaryotic]] cells of higher organisms. [[Prokaryote]]s were once thought not to have organelles, but some examples have now been identified.<ref name="pref">{{cite journal |author=Kerfeld CA, Sawaya MR, Tanaka S, ''et al'' |title=Protein structures forming the shell of primitive bacterial organelles |journal=Science |volume=309 |issue=5736 |pages=936–8 |year=2005 |pmid=16081736 |doi=10.1126/science.1113397}}</ref>
== History and Terminology ==
In biology, an ''[[Organ (anatomy)|organ]]'' is defined as a confined functional unit within an [[organism]]. The [[analogy]] of bodily organs to microscopic cellular substructures is obvious, as from even early works, authors of respective textbooks rarely elaborate on the distinction between the two.
Credited as the first<ref>{{cite book |last= Bütschli|first= O. |authorlink= Otto Bütschli|coauthors= |editor= |others= |title=Dr. H. G. Bronn's Klassen u. Ordnungen des Thier-Reichs wissenschaftlich dargestellt in Wort und Bild. Erster Band. Protozoa. Dritte Abtheilung: Infusoria und System der Radiolaria.|origdate= |origyear= |origmonth= |url= |format= |accessdate= |accessyear= |accessmonth= |edition= |date= |year= 1888 |month= |publisher= |location= |language= |id= |doi = |pages=1412 |chapter= |chapterurl= |quote = Die Vacuolen sind demnach in strengem Sinne keine beständigen Organe oder O r g a n u l a (wie Möbius die Organe der Einzelligen im Gegensatz zu denen der Vielzelligen zu nennen vorschlug).}}</ref><ref>Amer. Naturalist. 23, 1889, S. 183: ''„It may possibly be of advantage to use the word organula here instead of organ, following a suggestion by Möbius. Functionally-differentiated multicellular aggregates in multicellular forms or metazoa are in this sense organs, while, for functionally-differentiated portions of unicellular organisms or for such differentiated portions of the unicellular germ-elements of metazoa, the diminutive organula is appropriate.“'' Cited after : Oxford English Dictionary online, entry for „organelle“.</ref><ref>[http://books.google.com/books?id=yAQwAAAAIAAJ&q=Organulum+OR+Organula+OR+Organella+date:1800-1900&dq=Organulum+OR+Organula+OR+Organella+date:1800-1900&as_brr=0&pgis=1 'Journal de l'anatomie et de la physiologie normales et pathologiques de l'homme et des animaux' at Google Books]</ref> to use a [[diminutive]] of ''organ'' for respective cellular structures was German zoologist [[Karl August Möbius]] (1884), who used the term "organula" <ref>
{{cite journal | last = Möbius | first = K. | authorlink = Karl August Möbius | coauthors = | year = 1884 | month = September | title = Das Sterben der einzelligen und der vielzelligen Tiere. Vergleichend betrachtet | journal = Biologisches Centralblatt | volume = 4 | issue = 13,14 | pages = 389–392, 448 | quote = Während die Fortpflanzungszellen der vielzelligen Tiere unthätig fortleben bis sie sich loslösen, wandern und entwickeln, treten die einzelligen Tiere auch durch die an der Fortpflanzung beteiligten Leibesmasse in Verkehr mit der Außenwelt und viele bilden sich dafür auch besondere Organula." Footnote on p. 448: "Die Organe der Heteroplastiden bestehen aus vereinigten Zellen. Da die Organe der Monoplastiden nur verschieden ausgebildete Teile e i n e r Zelle sind schlage ich vor, sie „Organula“ zu nennen | url = http://www.dietzellab.de/goodies/history/}}
</ref> (plural form of ''organulum'', the diminutive of [[latin]] ''organum''). From the context, it is clear that he referred to reproduction related structures of [[protist]]s. In a footnote, which was published as a correction in the next issue of the journal, he justified his suggestion to call organs of unicellular organisms "organella" since they are only differently formed parts of one cell, in contrast to multicellular organs of multicellular organisms. Thus, the original definition was limited to structures of unicellular organisms.
It would take several years before ''organulum'', or the later term ''organelle'', became accepted and expanded in meaning to include subcellular structures in multicellular organisms. Books around 1900 from [[Valentin Häcker]],<ref>{{cite book |last=Häcker |first=Valentin |authorlink=Valentin Haecker |title=Zellen- und Befruchtungslehre |year=1899 |publisher=Verlag von Gustav Fisher |location=Jena }}</ref> [[Edmund Beecher Wilson|Edmund Wilson]]<ref>
{{cite book |last=Wilson |first=Edmund B. |authorlink=Edmund Beecher Wilson |title= The cell in Development and Inheritance|url= |edition=second edition |year=1900 |publisher=The Macmillan Company |location=New York |pages= |chapter= |chapterurl= |quote = }}</ref>
and [[Oscar Hertwig]]<ref> {{cite book |last=Hertwig |first=Oscar |authorlink=Oscar Hertwig |coauthors= |title=Allgemeine Biologie. Zweite Auflage des Lehrbuchs „Die Zelle und die Gewebe“ |year=1906 |publisher=Verlag von Gustav Fischer |location=Jena |id= }}</ref> still referred to cellular ''organs''.
Later, both terms came to be used side by side:
[[Bengt Lidforss]] wrote 1915 (in German) about "Organs or Organells".<ref>{{cite book |last=Lidforss |first=B. |authorlink= Bengt Lidforss |editor=Paul Hinneberg |title=Allgemeine Biologie |edition= |year=1915 |publisher=Verlag von B.G.Teubner |location=Leipzig, Berlin |pages=227 (218-264)|chapter=Protoplasma|quote = Eine Neubildung dieser Organe oder Organellen findet wenigstens bei höheren Pflanzen nicht statt}}</ref>
Around 1920, the term organelle was used to describe propulsion structures ("motor organelle complex", i.e., [[flagella]] and their anchoring)<ref>{{cite journal |author=[[Charles Atwood Kofoid|Kofoid CA]], Swezy O |title=Flagellate Affinities of Trichonympha |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=5 |issue=1 |pages=9–16 |year=1919 |pmid=16576345|doi= 10.1073/pnas.5.1.9}}</ref> and other protist structures, such as [[ciliates]].<ref>Cl. Hamburger, Handwörterbuch der Naturw. Bd. V, .S. 435. Infusorien. ''cited after'' {{cite journal | last = Petersen | first = Hans | authorlink = | coauthors = | year = 1919
| month = | title = Über den Begriff des Lebens und die Stufen der biologischen Begriffsbildung | journal = Archiv für Entwicklungsmechanik der Organismen (now: Development Genes and Evolution) | volume = 45 | issue = 3 | pages = 423–442 | doi =10.1007/BF02554406 }} </ref>
[[Alfred Kühn]] wrote about [[centriole]]s as division organelles, although he stated that, for [[Vahlkampfia]]s, the alternative 'organelle' or 'product of structural build-up' had not yet been decided, without explaining the difference between the alternatives.<ref>{{cite journal | last = Kühn | first = Alfred | authorlink = Alfred Kühn | coauthors = | year = 1920 | month = | title = Untersuchungen zur kausalen Analyse der Zellteilung. I. Teil: Zur Morphologie und Physiologie der Kernteilung von Vahlkampfia bistadialis | journal = Archiv für Entwicklungsmechanik der Organismen (now: Development Genes and Evolution) | volume = 46 | issue = | pages = 259–327 | doi =10.1007/BF02554424 |quote = die Alternative: Organell oder Produkt der Strukturbildung}}</ref>
In his 1953 textbook, [[Max Hartmann]] used the term for extracellular (pellicula, shells, cell walls) and intracellular skeletons of protists.<ref>{{cite book |last=Hartmann |first=Max |authorlink=Max Hartmann |title=Allgemeine Biologie |edition=4. Aufl. |year=1953 |publisher=Gustav Fisher Verlag |location=Stuttgart |id= }}</ref>
Later, the now-widely-used<ref>Nultsch, Allgemeine Botanik, 11. Aufl. 2001, Thieme Verlag</ref><ref>Wehner/Gehring, Zoologie, 23. Aufl. 1995, Thieme Verlag</ref><ref>
Alberts et al., Molecular Biology of the Cell, 4. ed. 2002, online via [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Books&itool=toolbar "NCBI-Bookshelf"]</ref><ref>
Brock, Mikrobiologie, 2. korrigierter Nachdruck (2003), der 1. Aufl. von 2001</ref>
definition of organelle emerged, after which only cellular structures with surrounding [[Biological membrane|membrane]] had been considered organelles.
However, the more original definition of subcellular functional unit in general still coexists.<ref>[[Eduard Adolf Strasburger|Strasburger]]s Lehrbuch der Botanik für Hochschulen, 35. Aufl. (2002), S. 42</ref><ref name=clam> {{cite journal | last = Alliegro | first = Mark C. | authorlink = | coauthors = Mary Anne Alliegro and Robert E. Palazzo | year = 2006 | month = June 13, | title = Centrosome-associated RNA in surf clam oocytes | journal = Proc. Nat. Acad. Sci. USA | volume =103 | issue = 24 | pages = 9034–9038 | doi =10.1073/pnas.0602859103 | pmid = 16754862 }}</ref>
In 1978, [[Albert Frey-Wyssling]] suggested that the term organelle should refer only to structures that convert energy, such as centrosomes, ribosomes, and nucleoli.<ref name="afw">{{cite journal | author = Albert Frey-Wyssling | year = 1978 | month = | title = Zur Definition des Organell-Begriffes | journal = Gegenbaurs morphologisches Jahrbuch, Leipzig | volume = 124 | issue = 3 | pages = 455–57 | pmid=689352 | language=German}}</ref><ref>Albert Frey-Wyssling: Concerning the concept "Organelle". Experientia 34, 547 (1978). {{DOI|10.1007/BF01935984}} The journal's new name is ''Cellular and Molecular Life Sciences''. </ref> This new definition, however, did not win wide recognition.
==Examples==
Whereas most cell biologists consider the term '''organelle''' to be synonymous with "[[cell compartment]]," other cell biologists choose to limit the term organelle to include only those that are DNA-containing, having originated from formerly-autonomous microscopic organisms acquired via [[endosymbiosis]].
The most notable of these organelles having originated from [[endosymbiont]] [[bacterium|bacteria]] are:
* [[mitochondria]] (in almost all eukaryotes)
* [[chloroplast]]s (in plants, algae and protists).
Other organelles are also suggested to have endosymbiotic origins, (notably the flagellum - see [[evolution of flagella]]).
Not all parts of the cell qualify as organelles, and the use of the term to refer to some structures is disputed. These structures are large assemblies of [[macromolecule]]s that carry out particular and specialized functions, but they lack membrane boundaries. Such cell structures, which are not formally organelles, include:
*[[ribosome]]
*[[cytoskeleton]]
*[[flagellum]].
===Eukaryotic organelles===
[[Eukaryotes]] are the most structurally complex cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost [[cell membrane]]. The larger organelles, such as the [[cell nucleus|nucleus]] and [[vacuoles]], are easily visible with the [[light microscope]]. They were among the first biological discoveries made after the invention of the [[microscope]].
Not all [[Eukaryotes|eukaryotic cells]] have every one of the organelles listed below. Exceptional organisms have cells which do not include some organelles that might otherwise be considered universal to eukaryotes
(such as mitochondria).<ref>{{cite journal |author=Fahey RC, Newton GL, Arrack B, Overdank-Bogart T, Baley S |title=Entamoeba histolytica: a eukaryote without glutathione metabolism |journal=Science |volume=224 |issue=4644 |pages=70–72 |year=1984 |pmid=6322306 |doi=10.1126/science.6322306}}</ref> There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below (e.g., some that are listed as double-membrane are sometimes found with single or triple membranes). In addition, the number of individual organelles of each type found in a given cell varies depending upon the function of that cell.
{| class="wikitable" align="center"
|+'''''Major eukaryotic organelles'''''
!Organelle
!Main function
!Structure
!Organisms
!Notes
|-
|[[chloroplast]] ([[plastid]])||[[photosynthesis]]||double-membrane compartment||plants, protists||has some genes; theorized to be engulfed by the ancestral eukaryotic cell (endosymbiosis)
|-
|[[endoplasmic reticulum]]||translation and folding of new proteins (rough endoplasmic reticulum), expression of lipids (smooth endoplasmic reticulum)||single-membrane compartment||all eukaryotes||rough endoplasmic reticulum is covered with ribosomes, has folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubular
|-
|[[Golgi apparatus]]||sorting and modification of proteins||single-membrane compartment||all eukaryotes||cis-face (convex) nearest to rough endoplasmic reticulum; trans-face (concave) farthest from rough endoplasmic reticulum
|-
|[[mitochondrion]]||energy production||double-membrane compartment||most eukaryotes||has some DNA; theorized to be engulfed by the ancestral eukaryotic cell (endosymbiosis)
|-
|[[vacuole]]||storage, [[homeostasis]]||single-membrane compartment||eukaryotes||
|-
|[[cell nucleus|nucleus]]||DNA maintenance, RNA [[Transcription (genetics)|transcription]]||double-membrane compartment||all eukaryotes ||has bulk of [[genome]]
|-
|}
Mitochondria and chloroplasts, which have double-membranes and their own [[DNA]], are believed to have originated from incompletely consumed or invading [[prokaryotes|prokaryotic organisms]], which were adopted as a part of the invaded cell. This idea is supported in the [[Endosymbiotic theory]].
{| class="wikitable" align="center"
|+'''''Minor eukaryotic organelles and cell components'''''
!Organelle/Macromolecule
!Main function
!Structure
!Organisms
|-
|[[acrosome]]||helps spermatoza fuse with ovum||single-membrane compartment||many animals
|-
|[[autophagy|autophagosome]]||vesicle which sequesters cytoplasmic material and organelles for degradation||double-membrane compartment||all eukaryotic cells
|-
|[[centriole]]||anchor for [[cytoskeleton]]||[[Microtubule]] protein||animals
|-
|[[cilium]]||movement in or of external medium||[[Microtubule]] protein||animals, protists, few plants
|-
|[[glycosome]]||carries out [[glycolysis]]||single-membrane compartment||Some [[protozoa]], such as ''[[Trypanosome]]s''.
|-
|[[glyoxysome]]||conversion of fat into sugars||single-membrane compartment||plants
|-
|[[hydrogenosome]]||energy & hydrogen production||double-membrane compartment||a few unicellular eukaryotes
|-
|[[lysosome]]||breakdown of large molecules (e.g., proteins + polysaccharides)||single-membrane compartment||most eukaryotes
|-
|[[melanosome]]||pigment storage||single-membrane compartment||animals
|-
|[[mitosome]]||not characterized||double-membrane compartment||a few unicellular eukaryotes
|-
|[[myofibril]]||muscular contraction||bundled filaments||animals
|-
|[[nucleolus]]||ribosome production||protein-DNA-RNA||most eukaryotes
|-
|[[parenthesome]]||not characterized||not characterized||fungi
|-
|[[peroxisome]]||breakdown of metabolic hydrogen peroxide||single-membrane compartment||all eukaryotes
|-
|[[ribosome]]||[[Translation (genetics)|translation]] of RNA into proteins||RNA-protein|| eukaryotes, prokaryotes
|-
|[[Vesicle (biology)|vesicle]]||material transport||single-membrane compartment||all eukaryotes
|}
Other related structures:
* [[cytosol]]
* [[endomembrane system]]
* [[nucleosome]]
* [[microtubule]]
* [[cell membrane]]
===Prokaryotic organelles===
[[Prokaryote]]s are not as structurally or metabolically complex as eukaryotes, and were once thought not to have any internal structures enclosed by [[Lipid bilayer|lipid membranes]]. In the past, they were often viewed as having little internal organization; but, slowly, details are emerging about prokaryotic internal structures. An early false turn was the idea developed in the 1970's that bacteria might contain membrane folds termed [[mesosome]]s, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for [[electron microscopy]].<ref>{{cite journal |author=Ryter A |title=Contribution of new cryomethods to a better knowledge of bacterial anatomy |journal=Ann. Inst. Pasteur Microbiol. |volume=139 |issue=1 |pages=33–44 |year=1988 |pmid=3289587 |doi=10.1016/0769-2609(88)90095-6}}</ref> However, more recent research has revealed that at least some prokaryotes have ''[[microcompartment]]s'', which are compartments enclosed by proteins.<ref name="pref">{{cite journal |author=Kerfeld CA, Sawaya MR, Tanaka S, ''et al'' |title=Protein structures forming the shell of primitive bacterial organelles |journal=Science |volume=309 |issue=5736 |pages=936–8 |year=2005 |pmid=16081736 | doi = 10.1126/science.1113397 <!--Retrieved from CrossRef by DOI bot-->}}</ref> Even more striking is the description of magnetosomes,<ref>{{cite journal |author=Komeili A, Li Z, Newman DK, Jensen GJ |title=Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK |journal=Science |volume=311 |issue=5758 |pages=242–5 |year=2006 |pmid=16373532 |doi=10.1126/science.1123231}}</ref><ref>{{cite journal |author=Scheffel A, Gruska M, Faivre D, Linaroudis A, Plitzko JM, Schüler D |title=An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria |journal=Nature |volume=440 |issue=7080 |pages=110–4 |year=2006 |pmid=16299495 |doi=10.1038/nature04382}}</ref> as well as the nucleus-like structures of the ''Planctomycetes'' that are surrounded by [[Lipid bilayer|lipid membranes]].<ref>{{cite journal |author=Fuerst JA |title=Intracellular compartmentation in planctomycetes |journal=Annu. Rev. Microbiol. |volume=59 |issue= |pages=299–328 |year=2005 |pmid=15910279 |doi=10.1146/annurev.micro.59.030804.121258}}</ref>
{| class="wikitable" align="center"
|+'''''Prokaryotic organelles and cell components'''''
!Organelle/Macromolecule
!Main function
!Structure
!Organisms
|-
|[[carboxysome]]||[[carbon fixation]]||protein-shell compartment||some bacteria
|-
|[[chlorosome]]||[[photosynthesis]]||light harvesting complex||[[green sulfur bacteria]]
|-
|[[flagellum]]||movement in external medium||protein filament||some prokaryotes and eukaryotes
|-
|[[magnetosome]]||magnetic orientation||inorganic crystal, lipid membrane||[[magnetotactic bacteria]]
|-
|[[nucleoid]]||DNA maintenance, [[Transcription (genetics)|transcription]] to RNA||DNA-protein||prokaryotes
|-
|[[plasmid]]||DNA exchange||circular DNA||some bacteria
|-
|[[ribosome]]||[[Translation (genetics)|translation]] of RNA into proteins||RNA-protein|| eukaryotes, prokaryotes
|-
|[[thylakoid]]||[[photosynthesis]]||photosystem proteins and pigments||mostly cyanobacteria
|-
|}
==See also==
*[[Cell (biology)|Cell]]
*[[Endosymbiotic theory]]
==References==
<div style="-moz-column-count:2">
{{reflist}}
</div>
* Alberts, Bruce et al. (2003). ''Essential Cell Biology,'' 2nd ed., Garland Science, 2003, ISBN 081533480X.
* Alberts, Bruce et al. (2002). ''The Molecular Biology of the Cell,'' 4th ed., Garland Science, 2002, ISBN 0-8153-3218-1.
{{organelles}}
{{Composition}}
[[Category:Organelles|*]]
[[Category:Cell anatomy]]
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