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The mitochondrion (plural mitochondria) is a double membrane-bound organelle found in most eukaryotic cells. The word mitochondrion comes from the Greek , , i.e. "thread", and , , i.e. "granule"〔(【引用サイトリンク】url=http://www.etymonline.com/index.php?term=mitochondria&allowed_in_frame=0 )〕 or "grain-like". Mitochondria can range from 0.5 to 1.0 μm in diameter. A considerable variation can be seen in the structure and size of this organelle. Unless specifically stained, they are not visible. These structures are described as "the powerhouse of the cell" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth. Mitochondria have been implicated in several human diseases, including mitochondrial disorders, cardiac dysfunction, and heart failure. A recent University of California study including ten children diagnosed with severe autism suggests that autism may be correlated with mitochondrial defects as well.〔(Study Confirms Mitochondrial Deficits in Children with Autism ). biosciencetechnology.com. May 2014〕 Several characteristics make mitochondria unique. The number of mitochondria in a cell can vary widely by organism, tissue, and cell type. For instance, red blood cells have no mitochondria, whereas liver cells can have more than 2000. The organelle is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, and the cristae and matrix. Mitochondrial proteins vary depending on the tissue and the species. In humans, 615 distinct types of protein have been identified from cardiac mitochondria, whereas in rats, 940 proteins have been reported. The mitochondrial proteome is thought to be dynamically regulated. Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome. Further, its DNA shows substantial similarity to bacterial genomes. ==History== The first observations of intracellular structures that probably represent mitochondria were published in the 1840s. Richard Altmann, in 1894, established them as cell organelles and called them "bioblasts".〔 The term "mitochondria" itself was coined by Carl Benda in 1898.〔 Leonor Michaelis discovered that Janus green can be used as a supravital stain for mitochondria in 1900. Friedrich Meves, in 1904, made the first recorded observation of mitochondria in plants (''Nymphaea alba'')〔〔Ernster's citation is wrong, correct citation is , cited in Meves' 1908 paper and in , with confirmation of Nymphaea alba〕 and in 1908, along with Claudius Regaud, suggested that they contain proteins and lipids. Benjamin F. Kingsbury, in 1912, first related them with cell respiration, but almost exclusively based on morphological observations.〔 In 1913, particles from extracts of guinea-pig liver were linked to respiration by Otto Heinrich Warburg, which he called "grana". Warburg and Heinrich Otto Wieland, who had also postulated a similar particle mechanism, disagreed on the chemical nature of the respiration. It was not until 1925, when David Keilin discovered cytochromes, that the respiratory chain was described.〔 In 1939, experiments using minced muscle cells demonstrated that cellular respiration using one oxygen atom can form two adenosine triphosphate (ATP) molecules, and, in 1941, the concept of the phosphate bonds of ATP being a form of energy in cellular metabolism was developed by Fritz Albert Lipmann. In the following years, the mechanism behind cellular respiration was further elaborated, although its link to the mitochondria was not known.〔 The introduction of tissue fractionation by Albert Claude allowed mitochondria to be isolated from other cell fractions and biochemical analysis to be conducted on them alone. In 1946, he concluded that cytochrome oxidase and other enzymes responsible for the respiratory chain were isolated to the mitchondria. Over time, the fractionation method was further developed, improving the quality of the mitochondria isolated, and other elements of cell respiration were determined to occur in the mitochondria.〔 The first high-resolution micrographs appeared in 1952, replacing the Janus Green stains as the preferred way of visualising the mitochondria. This led to a more detailed analysis of the structure of the mitochondria, including confirmation that they were surrounded by a membrane. It also showed a second membrane inside the mitochondria that folded up in ridges dividing up the inner chamber and that the size and shape of the mitochondria varied from cell to cell. The popular term "powerhouse of the cell" was coined by Philip Siekevitz in 1957. In 1967, it was discovered that mitochondria contained ribosomes. In 1968, methods were developed for mapping the mitochondrial genes, with the genetic and physical map of yeast mitochondria being completed in 1976.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mitochondrion」の詳細全文を読む スポンサード リンク
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