Biology:Organelle

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Short description: Specialized subunit within a cell
Organelle
Details
Pronunciation/ɔːrɡəˈnɛl/
Part ofCell
Identifiers
Latinorganella
Anatomical terms of microanatomy

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-bounded 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 function 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.

History and terminology

Cell biology
The animal cell
Animal Cell.svg

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 bounded 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]

The first, broader conception of organelles is that they are membrane-bounded 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 bounded structures such as ribosomes is common and accepted.[11][verification needed][12][13] This has led many texts to delineate between membrane-bounded and non-membrane bounded organelles.[14] The non-membrane bounded 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:

The mechanisms by which such non-membrane bounded organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.[15]

The second, more restrictive definition of organelle includes only those cell compartments that contain deoxyribonucleic acid (DNA), having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.[10][16][17]

Using 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):

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

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 (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)[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
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:

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 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.[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-bounded 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-bounded 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

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): a000422. doi:10.1101/cshperspect.a000422. PMID 20739411. 
  3. ↑ Peterson, Lynsey (April 17, 2010). "Mastering the Parts of a Cell". Lesson Planet. http://www.lessonplanet.com/directory_articles/biology_lesson_plans/19_April_2010/363/mastering_the_parts_of_a_cell. 
  4. ↑ From Here to Eternity: Ernst Haeckel and Scientific Faith.. Gottingen: Vandenhoeck & Ruprecht. 2005. p. 218. 
  5. ↑ 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. pp. 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. ↑ "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.". 
  7. ↑ 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. 
  8. ↑ 8.0 8.1 "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". 
  9. ↑ Walker, Patrick (2009). Nuclear import of histone fold motif containing heterodimers by importin 13. Niedersächsische Staats-und Universitätsbibliothek Göttingen. 
  10. ↑ 10.0 10.1 "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. 
  11. ↑ Campbell, Neil A; Reece, Jane B; Mitchell, Lawrence G (2002). Biology (6th ed.). Benjamin Cummings. ISBN 978-0-8053-6624-2. https://archive.org/details/biologyc00camp. 
  12. ↑ "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. 
  13. ↑ "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. 
  14. ↑ Introduction to Histology. Lippincott. 1984. ISBN 978-0-397-52114-2. https://archive.org/details/unset0000unse_d5o8. 
  15. ↑ "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. 
  16. ↑ "Mitochondrial genome of a tertiary endosymbiont retains genes for electron transport proteins". The Journal of Eukaryotic Microbiology 54 (2): 146–53. March–April 2007. doi:10.1111/j.1550-7408.2007.00245.x. PMID 17403155. 
  17. ↑ Mullins, Christopher (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. 
  18. ↑ 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.). ISBN 978-0-8153-3218-3. https://www.ncbi.nlm.nih.gov/books/NBK26924/. 
  19. ↑ "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. 
  20. ↑ 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 978-0815345244. 
  21. ↑ "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. 
  22. ↑ "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. 
  23. ↑ "Structural analysis of CsoS1A and the protein shell of the Halothiobacillus neapolitanus carboxysome". PLOS Biology 5 (6): e144. June 2007. doi:10.1371/journal.pbio.0050144. PMID 17518518. 
  24. ↑ "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. 
  25. ↑ "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. 
  26. ↑ "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. 
  27. ↑ 27.0 27.1 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. 
  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. https://pubmed.ncbi.nlm.nih.gov/19247843/. Retrieved 2020-08-03. 
  29. ↑ "Intracellular compartmentation in planctomycetes". Annual Review of Microbiology 59: 299–328. October 13, 2005. doi:10.1146/annurev.micro.59.030804.121258. PMID 15910279. 

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