Organelle

An organelle is a specialized subunit, within a biological 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 bounded organelles). Although most organelles are functional units within cells, some functional units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst (these could be referred to as membrane bound in the sense that they are attached to (or bound to) the membrane).

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.

## Contents

* 1 History and terminology
* 2 Types
* 3 Eukaryotic organelles
* 4 Prokaryotic organelles
* 5 See also
* 6 References
* 7 External links

## History and terminology

Cell biology

The animal cell

Components of a typical animal cell:

1. Nucleolus
2. Nucleus
3. Ribosome (little dots)
4. Vesicle
5. Rough endoplasmic reticulum
6. Golgi apparatus (or, Golgi body)
7. Cytoskeleton
8. Smooth endoplasmic reticulum
9. Mitochondrion
10. Vacuole
11. Cytosol (fluid that contains organelles, comprising the cytoplasm)
12. Lysosome
13. Centrosome
14. Cell membrane

In biology, organs are defined as confined functional units 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.

In the 1830s, Félix Dujardin refuted Ehrenberg's theory that microorganisms have the same organs as multicellular animals, only smaller.[3]

Credited as the first[4][5][6] 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).[7] 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.[7][8]

## Types

In the broadest definition, an organelle is any part of the cell that acts as a distinct functional unit.[9] This includes membrane-bound as well as non-membrane-bound, or membrane-less organelles.[10] In a more restrictive definition, only membrane-bound ones are included. In the most restrictive definition, only the endosymbiotic membrane-bound ones are included.[11]

The membrane-bound organelles include the endosymbiotic (mitochondria and plastids)[12] and components formed by the endomembrane system such as the lysosome. An endomembrane system and mitochondria are found in almost all eukaryotes. Plants, algae, and some protists additionally have chloroplasts. A very small minority of bacteria also have a sort-of endomembrane system.[13][14]

The membrane-less organelles (MLOs), also called biomolecular complexes, are large assemblies of macromolecules that carry out particular and specialized functions, but are membrane-less. Many of these are referred to as "proteinaceous organelles" as their main structure is made of proteins.[15] MLOs 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 bounded organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.[16]

## Eukaryotic organelles

File:Typical human cell.tifDiagram showing the organelles of a generalised animal cell

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 (such as mitochondria) that might otherwise be considered universal to eukaryotes.[17] The several plastids including chloroplasts are distributed among some but not all eukaryotes.

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. The cell membrane and cell wall are not organelles.

Major eukaryotic organelles  Organelle  | Main function  | Structure  | Organisms  | Notes

|
|
|
|

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 (which are bound to the ribosome membrane), 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)[18]
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 |
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".[19] | 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[20] | mRNP complexes  | most eukaryotes
TIGER domain | mRNA encoding proteins  | network of RNA-binding proteins  | most organisms
vault | unclear; possibly nuclear-cytoplasmic transport[21] | RNA-protein  | most eukaryotes (including all higher eukaryotes)
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.[22] Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane

Prokaryotes are not as structurally complex as eukaryotes, and were once thought to have 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.[23]

However, there is increasing evidence of compartmentalization in at least some prokaryotes.[2] Research has revealed that at least some bacteria 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 magnetotactic bacteria, reported in 2006.[24][25]

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).[26] Membrane-bounded anammoxosomes have been discovered in five Planctomycetota "anammox" genera, which perform anaerobic ammonium oxidation.[27] In the Planctomycetota species Gemmata obscuriglobus, a nucleus-like structure surrounded by lipid membranes has been reported.[26][28]

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]

Advances in synthetic biology have enabled the construction of artificial bacterial organelles that are more reminiscent to eukaryotic ones, including structures formed through liquid-liquid phase separation of "RNA organelle" reported in 2017.[29] These RNA systems termed TEARS is capable of regulating compartmentalize cellular processes, scaffolding and sequestering metabolic pathways.[30][31] These synthetic organelles can be repurposed as their eukaryotic counterparts, to isolate purify proteins within prokaryotes, enabling a technology termed PandaPure for chromatography-free purification.[32]

Prokaryotic non-membranous organelles and cell components  Organelle/macromolecule  | Main function  | Structure  | Organisms

|
|
|

nucleoid | DNA maintenance, transcription to RNA | DNA-protein | bacteria and archaea
ribosome (70S) | translation of RNA into proteins | RNA-protein | bacteria and archaea
plasmid | DNA exchange | circular DNA | some bacteria and archaea
carboxysome | carbon fixation | protein-shell bacterial microcompartment | some bacteria
flagellum | movement in external medium | protein filament | some 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 | some prokaryotes
Prokaryotic membranous organelles and cell components  Organelle/macromolecule  | Main function  | Structure  | Organisms

|
|
|

anammoxosome | anaerobic ammonium oxidation | ladderane lipid membrane | "Candidatus" bacteria within Planctomycetota
chlorosome | photosynthesis | phospholipid-bounded light-harvesting complex attached to cell membrane | green sulfur bacteria
magnetosome | magnetic orientation | inorganic crystal, lipid membrane | magnetotactic bacteria
thylakoid | photosynthesis | photosystem proteins and pigments in membranes | mostly cyanobacteria
pepin | containing DNA and ribosomes | membrane | "Ca. Thiomargarita magnifica"[14]

## See also

* CoRR hypothesis
* Ejectosome
* Endosymbiotic theory
* Organelle biogenesis
* Membrane vesicle trafficking
* Host–pathogen interaction
* Vesiculo-vacuolar organelle

## References

1. ↑ 1.0 1.1 "Protein structures forming the shell of primitive organelles". Science 309 (5736): 936–8. August 2005. doi:10.1126/science.1113397. PMID 16081736. Bibcode: 2005Sci...309..936K.
2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Murat, Dorothee; Byrne, Meghan; Komeili, Arash (2010-10-01). "Cell Biology of Prokaryotic Organelles". Cold Spring Harbor Perspectives in Biology 2 (10). doi:10.1101/cshperspect.a000422. PMID 20739411.
3. ↑ From Here to Eternity: Ernst Haeckel and Scientific Faith.. Gottingen: Vandenhoeck & Ruprecht. 2005. p. 218.
4. ↑ 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. 1888. 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)."
5. ↑ "Embryology: The Structure of the Human Spermatozoon". American Naturalist 23: 184. February 1889. https://archive.org/stream/jstor-2451084/2451084_djvu.txt. "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.".
6. ↑ Journal de l'anatomie et de la physiologie normales et pathologiques de l'homme et des animaux. F. Alcan. 1891. https://archive.org/details/journaldelanato40robigoog.
7. ↑ 7.0 7.1 "Das Sterben der einzelligen und der vielzelligen Tiere. Vergleichend betrachtet". Biologisches Centralblatt 4 (13, 14): 389–392, 448. September 1884. http://www.dietzellab.de/goodies/history/. "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".
8. ↑ Walker, Patrick (2009). Nuclear import of histone fold motif containing heterodimers by importin 13. Niedersächsische Staats-und Universitätsbibliothek Göttingen.
9. ↑ "Phase transition of a disordered nuage protein generates environmentally responsive membraneless organelles". Molecular Cell 57 (5): 936–947. March 2015. doi:10.1016/j.molcel.2015.01.013. PMID 25747659.
10. ↑ Introduction to Histology. Lippincott. 1984. ISBN 978-0-397-52114-2. https://archive.org/details/unset0000unse_d5o8.
11. ↑ "Organelle evolution: what's in a name?". Current Biology 18 (8): R345-7. April 2008. doi:10.1016/j.cub.2008.02.065. PMID 18430636. Bibcode: 2008CBio...18.R345K.
12. ↑ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). "The Genetic Systems of Mitochondria and Plastids". Molecular Biology of the Cell (4th ed.). Garland Science. ISBN 978-0-8153-3218-3. https://www.ncbi.nlm.nih.gov/books/NBK26924/.
13. ↑ "Lamellar organization of pigments in chlorosomes, the light harvesting complexes of green photosynthetic bacteria". Biophysical Journal 87 (2): 1165–72. August 2004. doi:10.1529/biophysj.104.040956. PMID 15298919. Bibcode: 2004BpJ....87.1165P.
14. ↑ 14.0 14.1 Volland, Jean-Marie; Gonzalez-Rizzo, Silvina; Gros, Olivier; Tyml, Tomáš; Ivanova, Natalia; Schulz, Frederik; Goudeau, Danielle; Elisabeth, Nathalie H. et al. (24 June 2022). "A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles". Science 376 (6600): 1453–1458. doi:10.1126/science.abb3634. ISSN 0036-8075. PMID 35737788. Bibcode: 2022Sci...376.1453V.
15. ↑ "Biomolecular condensates: organizers of cellular biochemistry". Nature Reviews Molecular Cell Biology 18 (5): 285–298. May 2017. doi:10.1038/nrm.2017.7. PMID 28225081.
16. ↑ "Germline P granules are liquid droplets that localize by controlled dissolution/condensation". Science 324 (5935): 1729–32. June 2009. doi:10.1126/science.1172046. PMID 19460965. Bibcode: 2009Sci...324.1729B.
17. ↑ "Entamoeba histolytica: a eukaryote without glutathione metabolism". Science 224 (4644): 70–2. April 1984. doi:10.1126/science.6322306. PMID 6322306. Bibcode: 1984Sci...224...70F.
18. ↑ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Morgan, David; Raff, Martin C; Roberts, Keith; Walter, Peter; Wilson, John H et al. (2014-11-18). Molecular biology of the cell (Sixth ed.). Garland Science. p. 679. ISBN 978-0-8153-4524-4.
19. ↑ "The ciliopathies: an emerging class of human genetic disorders". Annual Review of Genomics and Human Genetics 7: 125–48. September 2006. doi:10.1146/annurev.genom.7.080505.115610. PMID 16722803.
20. ↑ "Stress granules: the Tao of RNA triage". Trends in Biochemical Sciences 33 (3): 141–50. March 2008. doi:10.1016/j.tibs.2007.12.003. PMID 18291657.
21. ↑ "Latest Advances in the Development of Eukaryotic Vaults as Targeted Drug Delivery Systems". Pharmaceutics 11 (7): 300. June 2019. doi:10.3390/pharmaceutics11070300. PMID 31261673.
22. ↑ "Structural analysis of CsoS1A and the protein shell of the Halothiobacillus neapolitanus carboxysome". PLOS Biology 5 (6). June 2007. doi:10.1371/journal.pbio.0050144. PMID 17518518.
23. ↑ "Contribution of new cryomethods to a better knowledge of bacterial anatomy". Annales de l'Institut Pasteur. Microbiology 139 (1): 33–44. January–February 1988. doi:10.1016/0769-2609(88)90095-6. PMID 3289587.
24. ↑ "Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK". Science 311 (5758): 242–5. January 2006. doi:10.1126/science.1123231. PMID 16373532. Bibcode: 2006Sci...311..242K. https://authors.library.caltech.edu/24658/2/KOMs06supp.pdf.
25. ↑ "An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria". Nature 440 (7080): 110–4. March 2006. doi:10.1038/nature04382. PMID 16299495. Bibcode: 2006Natur.440..110S.
26. ↑ 26.0 26.1 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. Bibcode: 2001ArMic.175..413L.
27. ↑ 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.
28. ↑ "Intracellular compartmentation in planctomycetes". Annual Review of Microbiology 59: 299–328. October 13, 2005. doi:10.1146/annurev.micro.59.030804.121258. PMID 15910279.
29. ↑ "Team:Paris Bettencourt/RNA - 2017.igem.org". https://2017.igem.org/Team:Paris_Bettencourt/RNA.
30. ↑ Barnes, Natalie G. (2022-11-08). "Organelle-like scaffolds in bacteria for synthetic biology" (in en). Nature Biotechnology 40 (11): 1574. doi:10.1038/s41587-022-01569-8. ISSN 1546-1696. PMID 36347988. https://www.nature.com/articles/s41587-022-01569-8.
31. ↑ Guo, Haotian; Ryan, Joseph C.; Song, Xiaohu; Mallet, Adeline; Zhang, Mengmeng; Pabst, Victor; Decrulle, Antoine L.; Ejsmont, Paulina et al. (2022-09-29). "Spatial engineering of E.coli with addressable phase-separated RNAs". Cell 185 (20): 3823–3837.e23. doi:10.1016/j.cell.2022.09.016. ISSN 0092-8674. PMID 36179672. https://www.sciencedirect.com/science/article/pii/S0092867422011746.
32. ↑ Guo, Haotian; Chen, Jionglin (2024-05-21). "Synthetic organelles enable protein purification in a single operation". bioRxiv 10.1101/2024.05.17.594729.

## External links

Library resources about
Organelle

* * *

* Resources in your library

* Tree of Life project: Eukaryotes
* Organelle Databases

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* t
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Structures of the cell / organelles

Endomembrane system|

* Cell membrane
* Nucleus
* Endoplasmic reticulum
* Golgi apparatus
* Parenthesome
* Autophagosome
* Vesicle
* Exosome
* Lysosome
* Endosome
* Phagosome
* Vacuole
* Acrosome
* Cytoplasmic granule
* Melanosome
* Microbody
* Glyoxysome
* Peroxisome
* Weibel–Palade body

Cytoskeleton|

* Microfilament
* Intermediate filament
* Microtubule
* Prokaryotic cytoskeleton
* Microtubule organizing center
* Centrosome
* Centriole
* Basal body
* Spindle pole body
* Myofibril

Endosymbionts|

* Mitochondrion
* Plastid
* Chloroplast
* Chromoplast
* Gerontoplast
* Leucoplast
* Amyloplast
* Elaioplast
* Proteinoplast
* Tannosome

Other internal|

* Nucleolus
* RNA
* Ribosome
* Spliceosome
* Vault
* Cytoplasm
* Cytosol
* Inclusions
* Proteasome

External|

* Undulipodium
* Cilium
* Flagellum
* Axoneme
* Radial spoke
* Archaellum
* Cell wall
* Extracellular matrix

* v
* t
* e

Hierarchy of life

* Biosphere > Biome > Ecosystem > Biocenosis > Population > Organism > Organ system > Organ > Tissue > Cell &#62; Organelle &#62; Biomolecular complex &#62; [[Physics:MacroMacromolecule > [[Physics:Biomolecule|Biomolecule]]

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