Organelle

In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name organelle comes from the idea that these structures are parts of cells, as organs are to the body, hence organelle, the suffix -elle being a diminutive. Organelles are either separately enclosed within their own lipid bilayers (also called membrane-bound organelles) or are spatially distinct functional units without a surrounding lipid bilayer (non-membrane bound organelles). Although most organelles are functional units within cells, some function units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.

Organelle
Details
Pronunciation	/ɔːrɡəˈnɛl/
Part of	Cell
Identifiers
Latin	Organella
MeSH	D015388
TH	H1.00.01.0.00009
FMA	63832
Anatomical terms of microanatomy [edit on Wikidata]

Organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. They include structures that make up the endomembrane system (such as the nuclear envelope, endoplasmic reticulum, and Golgi apparatus), and other structures such as mitochondria and plastids. While prokaryotes do not possess eukaryotic organelles, some do contain protein-shelled bacterial microcompartments, which are thought to act as primitive prokaryotic organelles;^[1] and there is also evidence of other membrane-bounded structures.^[2] Also, the prokaryotic flagellum which protrudes outside the cell, and its motor, as well as the largely extracellular pilus, are often spoken of as organelles.

History and terminology

Cell biology
Animal cell diagram
Components of a typical animal cell: Nucleolus Nucleus Ribosome (dots as part of 5) Vesicle Rough endoplasmic reticulum Golgi apparatus (or, Golgi body) Cytoskeleton Smooth endoplasmic reticulum Mitochondrion Vacuole Cytosol (fluid that contains organelles; with which, comprises cytoplasm) Lysosome Centrosome Cell membrane

In biology organs are defined as confined functional units within an organism.^[3] 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.

In the 1830s, Félix Dujardin refuted Ehrenberg theory which said that microorganisms have the same organs of multicellular animals, only minor.^[4]

Credited as the first^[5][6][7] to use a diminutive of organ (i.e., little organ) for cellular structures was German zoologist Karl August Möbius (1884), who used the term organula (plural of organulum, the diminutive of Latin organum).^[8] 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.^[8][9]

Types

While most cell biologists consider the term organelle to be synonymous with cell compartment, a space often bound by one or two lipid bilayers, some cell biologists choose to limit the term to include only those cell compartments that contain deoxyribonucleic acid (DNA), having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.^[10][11][12]

Under this definition, there would only be two broad classes of organelles (i.e. those that contain their own DNA, and have originated from endosymbiotic bacteria):

• mitochondria (in almost all eukaryotes)
• plastids^[13] (e.g. in plants, algae, and some protists).

Other organelles are also suggested to have endosymbiotic origins, but do not contain their own DNA (notably the flagellum – see evolution of flagella).

A second, less restrictive definition of organelles is that they are membrane-bound structures. However, even by using this definition, some parts of the cell that have been shown to be distinct functional units do not qualify as organelles. Therefore, the use of organelle to also refer to non-membrane bound structures such as ribosomes is common and accepted.^{[14][verification needed][15][16]} This has led many texts to delineate between membrane-bound and non-membrane bound organelles.^[17] The non-membrane bound organelles, also called large biomolecular complexes, are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. Many of these are referred to as "proteinaceous organelles" as their main structure is made of proteins. Such cell structures include:

• large RNA and protein complexes: ribosome, spliceosome, vault
• large protein complexes: proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, symmetric viral capsids, complex of GroEL and GroES; membrane protein complexes: porosome, photosystem I, ATP synthase
• large DNA and protein complexes: nucleosome
• centriole and microtubule-organizing center (MTOC)
• cytoskeleton
• flagellum
• nucleolus
• stress granule
• germ cell granule
• neuronal transport granule

The mechanisms by which such non-membrane bound organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.^[18]

Eukaryotic organelles

Eukaryotic cells are structurally complex, and by definition are organized, in part, by interior compartments that are themselves enclosed by lipid membranes that resemble the outermost cell membrane. The larger organelles, such as the 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 eukaryotic cells have each of the organelles listed below. Exceptional organisms have cells that do not include some organelles that might otherwise be considered universal to eukaryotes (such as mitochondria).^[19] 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.

Major eukaryotic organelles

Organelle	Main function	Structure	Organisms	Notes
cell membrane	separates the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment.	double-layered, fluid sheet of phospholipids	all eukaryotes
cell wall	The cell wall is a rigid structure composed of cellulose that provides shape to the cell, helps keep the organelles inside the cell, and does not let the cell burst from osmotic pressure.	various	plants, protists, rare kleptoplastic organisms
chloroplast (plastid)	photosynthesis, traps energy from sunlight	double-membrane compartment	plants, algae, rare kleptoplastic organisms	has own DNA; theorized to be engulfed by the ancestral archaeplastid 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
flagellum	locomotion, sensory	protein	some eukaryotes
Golgi apparatus	sorting, packaging, processing 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 from the oxidation of glucose substances and the release of adenosine triphosphate	double-membrane compartment	most eukaryotes	constituting element of the chondriome; has own DNA; theorized to have been engulfed by an ancestral eukaryotic cell (endosymbiosis)^[20]
nucleus	DNA maintenance, controls all activities of the cell, RNA transcription	double-membrane compartment	all eukaryotes	contains bulk of genome
vacuole	storage, transportation, helps maintain homeostasis	single-membrane compartment	all eukaryotes

Mitochondria and plastids, including chloroplasts, have double membranes and their own DNA. According to the endosymbiotic theory, they are believed to have originated from incompletely consumed or invading prokaryotic organisms.

Minor eukaryotic organelles and cell components

Organelle/Macromolecule	Main function	Structure	Organisms
acrosome	helps spermatozoa fuse with ovum	single-membrane compartment	most animals (including sponges)
autophagosome	vesicle that sequesters cytoplasmic material and organelles for degradation	double-membrane compartment	all eukaryotes
centriole	anchor for cytoskeleton, organizes cell division by forming spindle fibers	Microtubule protein	animals
cilium	movement in or of external medium; "critical developmental signaling pathway".^[21]	Microtubule protein	animals, protists, few plants
cnidocyst	stinging	coiled hollow tubule	cnidarians
eyespot apparatus	detects light, allowing phototaxis to take place		green algae and other unicellular photosynthetic organisms such as euglenids
glycosome	carries out glycolysis	single-membrane compartment	Some protozoa, such as Trypanosomes.
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	animals
melanosome	pigment storage	single-membrane compartment	animals
mitosome	probably plays a role in Iron–sulfur cluster (Fe–S) assembly	double-membrane compartment	a few unicellular eukaryotes that lack mitochondria
myofibril	myocyte contraction	bundled filaments	animals
nucleolus	pre-ribosome production	protein–DNA–RNA	most eukaryotes
ocelloid	detects light and possibly shapes, allowing phototaxis to take place	double-membrane compartment	members of the family Warnowiaceae
parenthesome	not characterized	not characterized	fungi
peroxisome	breakdown of metabolic hydrogen peroxide	single-membrane compartment	all eukaryotes
porosome	secretory portal	single-membrane compartment	all eukaryotes
proteasome	degradation of unneeded or damaged proteins by proteolysis	very large protein complex	all eukaryotes, all archaea, and some bacteria
ribosome (80S)	translation of RNA into proteins	RNA-protein	all eukaryotes
stress granule	mRNA storage^[22]	membraneless (mRNP complexes)	most eukaryotes
TIGER domain	mRNA encoding proteins	membraneless	most organisms
vesicle	material transport	single-membrane compartment	all eukaryotes

Other related structures:

• cytosol
• endomembrane system
• nucleosome
• microtubule

Prokaryotic organelles

 

(A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.^[23]

 

Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane

Prokaryotes are not as structurally complex as eukaryotes, and were once thought as having little internal organization, and lack cellular compartments and internal membranes; but slowly, details are emerging about prokaryotic internal structures that overturn these assumptions.^[2] An early false turn was the idea developed in the 1970s that bacteria might contain cell membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy.^[24]

However, there is increasing evidence of compartmentalization in at least some prokaryotes.^[2] Recent research has revealed that at least some prokaryotes have microcompartments, such as carboxysomes. These subcellular compartments are 100–200 nm in diameter and are enclosed by a shell of proteins.^[1] Even more striking is the description of membrane-bound magnetosomes in bacteria, reported in 2006.^[25][26]

The bacterial phylum Planctomycetota has revealed a number of compartmentalization features. The Planctomycetota cell plan includes intracytoplasmic membranes that separates the cytoplasm into paryphoplasm (an outer ribosome-free space) and pirellulosome (or riboplasm, an inner ribosome-containing space).^[27] Membrane-bound anammoxosomes have been discovered in five Planctomycetota "anammox" genera, which perform anaerobic ammonium oxidation.^[28] In the Planctomycetota species Gemmata obscuriglobus, a nucleus-like structure surrounded by lipid membranes has been reported.^[27][29]

Compartmentalization is a feature of prokaryotic photosynthetic structures.^[2] Purple bacteria have "chromatophores", which are reaction centers found in invaginations of the cell membrane.^[2] Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes.^[2] Cyanobacteria have internal thylakoid membranes for light-dependent photosynthesis; studies have revealed that the cell membrane and the thylakoid membranes are not continuous with each other.^[2]

Prokaryotic organelles and cell components

Organelle/macromolecule	Main function	Structure	Organisms
anammoxosome	anaerobic ammonium oxidation	ladderane lipid membrane	"Candidatus" bacteria within Planctomycetota
carboxysome	carbon fixation	protein-shell bacterial microcompartment	some bacteria
chlorosome	photosynthesis	light harvesting complex attached to cell membrane	green sulfur bacteria
flagellum	movement in external medium	protein filament	some prokaryotes
magnetosome	magnetic orientation	inorganic crystal, lipid membrane	magnetotactic bacteria
nucleoid	DNA maintenance, transcription to RNA	DNA-protein	prokaryotes
pilus	Adhesion to other cells for conjugation or to a solid substrate to create motile forces.	a hair-like appendage sticking out (though partially embedded into) the plasma membrane	prokaryotic cells
plasmid	DNA exchange	circular DNA	some bacteria
ribosome (70S)	translation of RNA into proteins	RNA-protein	bacteria and archaea
thylakoid membranes	photosynthesis	photosystem proteins and pigments	mostly cyanobacteria

See also

• CoRR hypothesis
• Ejectosome
• Endosymbiotic theory
• Organelle biogenesis
• Membrane vesicle trafficking
• Host-pathogen interface
• Vesiculo-vacuolar organelle

References

1. Kerfeld CA, Sawaya MR, Tanaka S, Nguyen CV, Phillips M, Beeby M, Yeates TO (August 2005). "Protein structures forming the shell of primitive organelles". Science. 309 (5736): 936–8. Bibcode:2005Sci...309..936K. CiteSeerX 10.1.1.1026.896. doi:10.1126/science.1113397. PMID 16081736. S2CID 24561197.
2. Murat, Dorothee; Byrne, Meghan; Komeili, Arash (2010-10-01). "Cell Biology of Prokaryotic Organelles". Cold Spring Harbor Perspectives in Biology. 2 (10): a000422. doi:10.1101/cshperspect.a000422. PMC 2944366. PMID 20739411.
3. Peterson L (April 17, 2010). "Mastering the Parts of a Cell". Lesson Planet. Retrieved 2010-04-19.
4. Di Gregorio MA (2005). From Here to Eternity: Ernst Haeckel and Scientific Faith. Gottingen: Vandenhoeck & Ruprecht. p. 218.
5. Bütschli O (1888). 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. p. 1412. 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).
6. Ryder JA, ed. (February 1889). "Embryology: The Structure of the Human Spermatozoon". American Naturalist. 23: 184. 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.
7. Robin C, Pouchet G, Duval MM, Retterrer E, Tourneux F (1891). Journal de l'anatomie et de la physiologie normales et pathologiques de l'homme et des animaux. F. Alcan.
8. Möbius K (September 1884). "Das Sterben der einzelligen und der vielzelligen Tiere. Vergleichend betrachtet". Biologisches Centralblatt. 4 (13, 14): 389–392, 448. 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
9. Walker, Patrick (2009). Nuclear import of histone fold motif containing heterodimers by importin 13. Niedersächsische Staats-und Universitätsbibliothek Göttingen.
10. Keeling PJ, Archibald JM (April 2008). "Organelle evolution: what's in a name?". Current Biology. 18 (8): R345-7. doi:10.1016/j.cub.2008.02.065. PMID 18430636. S2CID 11520942.
11. Imanian B, Carpenter KJ, Keeling PJ (March–April 2007). "Mitochondrial genome of a tertiary endosymbiont retains genes for electron transport proteins". The Journal of Eukaryotic Microbiology. 54 (2): 146–53. doi:10.1111/j.1550-7408.2007.00245.x. PMID 17403155. S2CID 20393495.
12. Mullins C (2004). "Theory of Organelle Biogenesis: A Historical Perspective". The Biogenesis of Cellular Organelles. Springer Science+Business Media, National Institutes of Health. ISBN 978-0-306-47990-8.
13. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). "The Genetic Systems of Mitochondria and Plastids". Molecular Biology of the Cell (4th ed.). ISBN 978-0-8153-3218-3.
14. Campbell NA, Reece JB, Mitchell LG (2002). Biology (6th ed.). Benjamin Cummings. ISBN 978-0-8053-6624-2.
15. Nott TJ, Petsalaki E, Farber P, Jervis D, Fussner E, Plochowietz A, Craggs TD, Bazett-Jones DP, Pawson T, Forman-Kay JD, Baldwin AJ (March 2015). "Phase transition of a disordered nuage protein generates environmentally responsive membraneless organelles". Molecular Cell. 57 (5): 936–947. doi:10.1016/j.molcel.2015.01.013. PMC 4352761. PMID 25747659.
16. Banani SF, Lee HO, Hyman AA, Rosen MK (May 2017). "Biomolecular condensates: organizers of cellular biochemistry". Nature Reviews Molecular Cell Biology. 18 (5): 285–298. doi:10.1038/nrm.2017.7. PMC 7434221. PMID 28225081.
17. Cormack DH (1984). Introduction to Histology. Lippincott. ISBN 978-0-397-52114-2.
18. Brangwynne CP, Eckmann CR, Courson DS, Rybarska A, Hoege C, Gharakhani J, Jülicher F, Hyman AA (June 2009). "Germline P granules are liquid droplets that localize by controlled dissolution/condensation". Science. 324 (5935): 1729–32. Bibcode:2009Sci...324.1729B. doi:10.1126/science.1172046. PMID 19460965. S2CID 42229928.
19. Fahey RC, Newton GL, Arrick B, Overdank-Bogart T, Aley SB (April 1984). "Entamoeba histolytica: a eukaryote without glutathione metabolism". Science. 224 (4644): 70–2. Bibcode:1984Sci...224...70F. doi:10.1126/science.6322306. PMID 6322306.
20. Alberts B, Johnson A, Lewis J, Morgan D, Raff MC, Roberts K, Walter P, Wilson JH, Hunt T (2014-11-18). Molecular biology of the cell (Sixth ed.). Garland Science. p. 679. ISBN 978-0815345244.
21. Badano JL, Mitsuma N, Beales PL, Katsanis N (September 2006). "The ciliopathies: an emerging class of human genetic disorders". Annual Review of Genomics and Human Genetics. 7: 125–48. doi:10.1146/annurev.genom.7.080505.115610. PMID 16722803.
22. Anderson P, Kedersha N (March 2008). "Stress granules: the Tao of RNA triage". Trends in Biochemical Sciences. 33 (3): 141–50. doi:10.1016/j.tibs.2007.12.003. PMID 18291657.
23. Tsai Y, Sawaya MR, Cannon GC, Cai F, Williams EB, Heinhorst S, Kerfeld CA, Yeates TO (June 2007). "Structural analysis of CsoS1A and the protein shell of the Halothiobacillus neapolitanus carboxysome". PLOS Biology. 5 (6): e144. doi:10.1371/journal.pbio.0050144. PMC 1872035. PMID 17518518.
24. Ryter A (January–February 1988). "Contribution of new cryomethods to a better knowledge of bacterial anatomy". Annales de l'Institut Pasteur. Microbiology. 139 (1): 33–44. doi:10.1016/0769-2609(88)90095-6. PMID 3289587.
25. Komeili A, Li Z, Newman DK, Jensen GJ (January 2006). "Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK" (PDF). Science. 311 (5758): 242–5. Bibcode:2006Sci...311..242K. doi:10.1126/science.1123231. PMID 16373532. S2CID 36909813.
26. Scheffel A, Gruska M, Faivre D, Linaroudis A, Plitzko JM, Schüler D (March 2006). "An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria". Nature. 440 (7080): 110–4. Bibcode:2006Natur.440..110S. doi:10.1038/nature04382. PMID 16299495. S2CID 4372846.
27. Lindsay, M. R.; Webb, R. I.; Strous, M; Jetten, M. S.; Butler, M. K.; Forde, R. J.; Fuerst, J. A. (2001). "Cell compartmentalisation in planctomycetes: Novel types of structural organisation for the bacterial cell". Archives of Microbiology. 175 (6): 413–29. doi:10.1007/s002030100280. PMID 11491082. S2CID 21970703.
28. Jetten, Mike S. M.; Niftrik, Laura van; Strous, Marc; Kartal, Boran; Keltjens, Jan T.; Op den Camp, Huub J. M. (2009-06-01). "Biochemistry and molecular biology of anammox bacteria". Critical Reviews in Biochemistry and Molecular Biology. 44 (2–3): 65–84. doi:10.1080/10409230902722783. PMID 19247843. S2CID 205694872. Retrieved 2020-08-03.
29. Fuerst JA (October 13, 2005). "Intracellular compartmentation in planctomycetes". Annual Review of Microbiology. 59: 299–328. doi:10.1146/annurev.micro.59.030804.121258. PMID 15910279.

External links

•   Media related to Organelles at Wikimedia Commons
• Tree of Life project: Eukaryotes
• Organelle Databases