<P> Recent advances in the identification of the tethers between the mitochondrial and ER membranes suggest that the scaffolding function of the molecular elements involved is secondary to other, non-structural functions . In yeast, ERMES, a multiprotein complex of interacting ER - and mitochondrial - resident membrane proteins, is required for lipid transfer at the MAM and exemplifies this principle . One of its components, for example, is also a constituent of the protein complex required for insertion of transmembrane beta - barrel proteins into the lipid bilayer . However, a homologue of the ERMES complex has not yet been identified in mammalian cells . Other proteins implicated in scaffolding likewise have functions independent of structural tethering at the MAM; for example, ER - resident and mitochondrial - resident mitofusins form heterocomplexes that regulate the number of inter-organelle contact sites, although mitofusins were first identified for their role in fission and fusion events between individual mitochondria . Glucose - related protein 75 (grp75) is another dual - function protein . In addition to the matrix pool of grp75, a portion serves as a chaperone that physically links the mitochondrial and ER Ca channels VDAC and IP3R for efficient Ca transmission at the MAM . Another potential tether is Sigma - 1R, a non-opioid receptor whose stabilization of ER - resident IP3R may preserve communication at the MAM during the metabolic stress response . </P> <P> The MAM is a critical signaling, metabolic, and trafficking hub in the cell that allows for the integration of ER and mitochondrial physiology . Coupling between these organelles is not simply structural but functional as well and critical for overall cellular physiology and homeostasis . The MAM thus offers a perspective on mitochondria that diverges from the traditional view of this organelle as a static, isolated unit appropriated for its metabolic capacity by the cell . Instead, this mitochondrial - ER interface emphasizes the integration of the mitochondria, the product of an endosymbiotic event, into diverse cellular processes . </P> <P> Mitochondria (and related structures) are found in all eukaryotes (except one--the Oxymonad Monocercomonoides sp .). Although commonly depicted as bean - like structures they form a highly dynamic network in the majority of cells where they constantly undergo fission and fusion . Mitochondria vary in number and location according to cell type . A single mitochondrion is often found in unicellular organisms . Conversely, numerous mitochondria are found in human liver cells, with about 1000--2000 mitochondria per cell, making up 1 / 5 of the cell volume . The population of all the mitochondria of a given cell constitutes the chondriome . The mitochondrial content of otherwise similar cells can vary substantially in size and membrane potential, with differences arising from sources including uneven partitioning at cell divisions, leading to extrinsic differences in ATP levels and downstream cellular processes . The mitochondria can be found nestled between myofibrils of muscle or wrapped around the sperm flagellum . Often, they form a complex 3D branching network inside the cell with the cytoskeleton . The association with the cytoskeleton determines mitochondrial shape, which can affect the function as well: different structures of the mitochondrial network may afford the population a variety of physical, chemical, and signalling advantages or disadvantages . Mitochondria in cells are always distributed along microtubules and the distribution of these organelles is also correlated with the endoplasmic reticulum . Recent evidence suggests that vimentin, one of the components of the cytoskeleton, is also critical to the association with the cytoskeleton . </P> <P> The most prominent roles of mitochondria are to produce the energy currency of the cell, ATP (i.e., phosphorylation of ADP), through respiration, and to regulate cellular metabolism . The central set of reactions involved in ATP production are collectively known as the citric acid cycle, or the Krebs cycle . However, the mitochondrion has many other functions in addition to the production of ATP . </P>

Where is the mitochondria located in an animal cell
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