<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> <P> A dominant role for the mitochondria is the production of ATP, as reflected by the large number of proteins in the inner membrane for this task . This is done by oxidizing the major products of glucose: pyruvate, and NADH, which are produced in the cytosol . This type of cellular respiration known as aerobic respiration, is dependent on the presence of oxygen . When oxygen is limited, the glycolytic products will be metabolized by anaerobic fermentation, a process that is independent of the mitochondria . The production of ATP from glucose has an approximately 13 - times higher yield during aerobic respiration compared to fermentation . Recently it has been shown that plant mitochondria can produce a limited amount of ATP without oxygen by using the alternate substrate nitrite . ATP crosses out through the inner membrane with the help of a specific protein, and across the outer membrane via porins . ADP returns via the same route . </P> <P> Pyruvate molecules produced by glycolysis are actively transported across the inner mitochondrial membrane, and into the matrix where they can either be oxidized and combined with coenzyme A to form CO, acetyl - CoA, and NADH, or they can be carboxylated (by pyruvate carboxylase) to form oxaloacetate . This latter reaction" fills up" the amount of oxaloacetate in the citric acid cycle, and is therefore an anaplerotic reaction, increasing the cycle's capacity to metabolize acetyl - CoA when the tissue's energy needs (e.g. in muscle) are suddenly increased by activity . </P>

Mitochondria produce atp using what kind of process