In cell biology, an organelle (pron.:/ ɔrɡəˈnɛl/) is a specialized subunit within a cell that has a specific function, and is usually individually enclosed within its own lipid bilayer. The name organelle originates from the idea that these structures are to cells what an organ is to the body (thus the name organelle, the suffix -ellebeing a small). Organelles are identified by microscopy, and can likewise be purified by cell fractionation. There are lots of types of organelles, especially in eukaryotic cells.
While Prokaryotes do not have organelles per se, some do include protein-based microcompartments, which are believed to function as primitive organelles.
History and terminology
In biology organs are specified as restricted practical units within an organism. [2] The example of physical organs to microscopic cellular bases is apparent, as from even early works, authors of respective books hardly ever elaborate on the distinction between the 2. Credited as the very first [3] [4] [5] to utilize a diminutive of organ (i.e., little organ) for cellular structures was German zoologist Karl August Möbius (1884 ), who utilized the term organula (plural of organulum, the small of Latin organum). [6] From the context, it is clear that he referred to recreation associated structures of protists. In a footnote, which was published as a correction in the next problem of the journal, he justified his tip 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. Therefore, the original definition was limited to structures of unicellular organisms.
It would take several years before organulum, or the later term organelle, ended up being accepted and expanded in suggesting to include subcellular structures in multicellular organisms.
Books around 1900 from Valentin Häcker,[7] Edmund Wilson[8] and Oscar Hertwig[9] still referred to cellular organs. Later, both terms came to be used side by side: Bengt Lidforss wrote 1915 (in German) about “Organs or Organells”.[10] Around 1920, the term organelle was used to describe propulsion structures (“motor organelle complex”, i.e., flagella and their anchoring)[11] and other protist structures, such as ciliates.[12]Alfred Kühn wrote about centrioles as division organelles, although he stated that, for Vahlkampfias, the alternative ‘organelle’ or ‘product of structural build-up’ had not yet been decided, without explaining the difference between the alternatives.[13]
In his 1953 textbook, Max Hartmann used the term for extracellular (pellicula, shells, cell walls) and intracellular skeletons of protists.[14] Later, the now widely used[15][16][17][18] definition of organelle emerged, after which only cellular structures with surrounding membrane had been considered organelles. However, the more original definition of subcellular functional unit in general still coexists.[19][20] In 1978, Albert Frey-Wyssling suggested that the term organelle should refer only to structures that convert energy, such as centrosomes, ribosomes, and nucleoli.[21][22] This new definition, however, did not win wide recognition.
While most cell biologists consider the term organelle to be synonymous with “cell compartment”, other cell biologists choose to limit the term organelle to include only those that are DNA-containing, having originated from formerly autonomous microscopic organisms acquired via .[23][24][25] 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[26] (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). Under the more restricted definition of membrane-bound structures, some parts of the cell do not qualify as organelles. Nevertheless, the use of organelle to refer to non-membrane bound structures such as ribosomes is common.[27] This has led some texts to delineate between membrane-bound and non-membrane bound organelles.[28] These structures are large assemblies ofmacromolecules that carry out particular and specialized functions, but they lack membrane boundaries. Such cell structures include:
* ribosome
* cytoskeleton
* flagellum
* centriole and microtubule-organizing center (MTOC)
* proteasome
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 themicroscope. 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).[29] 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.
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