Organelle

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In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. 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 functional units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.

OrganelleDetailsPronunciation/ɔːrɡəˈnɛl/Part ofCellIdentifiersLatinorganellaMeSH D015388 TH H1.00.01.0.00009 FMA 63832Anatomical terms of microanatomy
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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 identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. While prokaryotes do not possess intracellular organelles per se, some do contain protein-based bacterial microcompartments, which are thought to act as primitive prokaryotic organelles.^[1] 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.

Components of a typical animal cell:

Nucleolus
Nucleus
Ribosome (little dots)
Vesicle
Rough endoplasmic reticulum
Golgi apparatus (or, Golgi body)
Cytoskeleton
Smooth endoplasmic reticulum
Mitochondrion
Vacuole
Cytosol (fluid that contains organelles, comprising the cytoplasm)
Lysosome
Centrosome
Cell membrane

In biology organs are defined as confined functional units within an organism.^[2] 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.^[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]

TypesEdit

While most cell biologists consider the term organelleto 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.^[9]^[10]^[11]

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^[12] (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.^[13]^[14]^[15] This has led many texts to delineate between membrane-bound and non-membrane bound organelles.^[16] The non-membrane bound organelles, also called large biomolecular complexes, are large assemblies of macromoleculesthat carry out particular and specialized functions, but they lack membrane boundaries. Many of these are referred to as “proteinaceous organelles” as there many 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: 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.^[17]

Eukaryotic organellesEdit

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).^[18] 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 organellesOrganelleMain functionStructureOrganismsNotescell membraneseparates the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment.two-dimensional liquidall eukaryotescell wallThe cell wall is composed of peptidoglycan and is rigid, provides shape to the cell, helps to keeps the organelles inside the cell, and does not let the cell burst due to changes in osmotic pressure.celluloseplants, protists, rare kleptoplastic organismschloroplast(plastid)photosynthesis, traps energy from sunlightdouble-membrane compartmentplants, protists, rare kleptoplastic organismshas own DNA; theorized to be engulfed by the ancestral eukaryotic cell (endosymbiosis)endoplasmic reticulumtranslation and folding of new proteins (rough endoplasmic reticulum), expression of lipids (smooth endoplasmic reticulum)single-membrane compartmentall eukaryotesrough endoplasmic reticulum is covered with ribosomes, has folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubularflagellumlocomotion, sensoryproteinsome eukaryotesGolgi apparatussorting, packaging, processing and modification of proteinssingle-membrane compartmentall eukaryotescis-face (convex) nearest to rough endoplasmic reticulum; trans-face (concave) farthest from rough endoplasmic reticulummitochondrionenergy production from the oxidation of glucose substances and the release of adenosine triphosphatedouble-membrane compartmentmost eukaryotesconstituting element of the chondriome; has own DNA; theorized to have been engulfed by an ancestral eukaryotic cell (endosymbiosis)^[19]nucleusDNA maintenance, controls all activities of the cell, RNA transcriptiondouble-membrane compartmentall eukaryotescontains bulk of genome vacuolestorage, transportation, helps maintain homeostasissingle-membrane compartmenteukaryotes

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.

ReferencesEdit

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^ 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 of unicellular organismsor for such differentiated portions of the unicellular germ-elements of metazoa, the diminutive organula is appropriate.
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