Anorganelle is a specialized subunit, within a biological cell, that has a specific function. The nameorganelle comes from the idea that these structures are parts of cells, asorgans are to thebody, henceorganelle, the suffix-elle being adiminutive. Organelles are either separately enclosed within their ownlipid 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 ascilia, theflagellum andarchaellum, and thetrichocyst (these could be referred to as membrane bound in the sense that they are attached to (or bound to) the membrane).
In biology,organs are defined as confined functional units within anorganism. Theanalogy 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 refutedEhrenberg's theory that microorganisms have the same organs as multicellular animals, only smaller.[3]
Credited as the first[4][5][6] to use adiminutive oforgan (i.e., little organ) for cellular structures was German zoologistKarl August Möbius (1884), who used the termorganula (plural oforganulum, the diminutive ofLatinorganum).[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]
In the broadest definition, an organelle is any part of the cell that acts as a distinct functional unit.[9] This includesmembrane-bound as well asnon-membrane-bound 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 andplastids)[12] and components formed by theendomembrane system such as thelysosome. An endomembrane system and mitochondria are found in almost alleukaryotes. Plants, algae, and some protists additionally have chloroplasts. A very small minority of bacteria also have a sort-of endomembrane system.[13][14]
The non-membrane bound organelles, also calledbiomolecular complexes, are large assemblies ofmacromolecules 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] Such cell structures include:
The mechanisms by which such non-membrane bounded organelles form and retain their spatial integrity have been likened to liquid-liquidphase separation.[16]
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 outermostcell membrane. The larger organelles, such as thenucleus andvacuoles, are easily visible with thelight microscope. They were among the first biological discoveries made after the invention of themicroscope.
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 severalplastids includingchloroplasts 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.
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
Prokaryotes are not as structurally complex as eukaryotes, and were once thought to have little internal organization, and lackcellular compartments and internalmembranes; 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 containcell membrane folds termedmesosomes, but these were later shown to beartifacts produced by the chemicals used to prepare the cells forelectron microscopy.[23]
However, there is increasing evidence of compartmentalization in at least some prokaryotes.[2] Research has revealed that at least some bacteria havemicrocompartments, such ascarboxysomes. 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-boundmagnetosomes inmagnetotactic bacteria, reported in 2006.[24][25]
The bacterial phylumPlanctomycetota 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-boundedanammoxosomes have been discovered in five Planctomycetota "anammox" genera, which performanaerobic ammonium oxidation.[27] In the Planctomycetota speciesGemmata obscuriglobus, a nucleus-like structure surrounded by lipid membranes has been reported.[26][28]
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
^Di Gregorio MA (2005).From Here to Eternity: Ernst Haeckel and Scientific Faith. Gottingen: Vandenhoeck & Ruprecht. p. 218.
^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).
^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 ofunicellular organisms or for such differentiated portions of the unicellular germ-elements of metazoa, the diminutive organula is appropriate.
^abMöbius 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
^Walker, Patrick (2009).Nuclear import of histone fold motif containing heterodimers by importin 13. Niedersächsische Staats-und Universitätsbibliothek Göttingen.
^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.ISBN978-0-8153-4524-4.
^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.PMID16722803.
^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.PMID18291657.
^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.PMID3289587.
^abLindsay, 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.Bibcode:2001ArMic.175..413L.doi:10.1007/s002030100280.PMID11491082.S2CID21970703.
^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.hdl:2066/75127.PMID19247843.S2CID205694872.
