Mitochondria were first seen in 1880 by Kolliker but the present name, i.e., mitochondria was given by Benda (1897-98) to them.
These are granular or filamentous organelles found in the cytoplasm of almost all kinds of cells of plant and animal.
They are usually roughly ovoid or spherical, but they assume a filamentous shape or a vesicular form with a clear central zone depending upon the physiological conditions of the cells.
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Mitochondria vary in size. They are usually 0.5 to 1.0 µm in diameter and in length upto a maximum of 7 µm.
The number of them in a cell depends on its type and stage of development. In a liver cell as many as 2500 mitochondria may be present. They are numerous in brown fat cells while in plant cells their number is very small.
Mitochondria contain a limiting double unit membrane of lipoprotein. The outer membrane of the mitochondrial limiting membrane forms a bag-like structure around the inner unit membrane.
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The inner, unit membrane has a complex structure consisting numerous parallel, flattened, interdigitating invaginations, the mitochondrial cristae (composed of membrane pairs). These cristae project from both sides into the interior as sheets or tubules.
The unit membrane of the mitochondria appears to be similar to the cell membrane or plasma membrane.
It is also made up of three layers, a middle lighter layer less opaque to electrons-sand witched between two electron-opaque (darker) layers.
According to Robertson (1959) the lighter middle-inner layer is made up of two rows of lipid molecules with-their non-polar groups in the centre, the two denser outer layers are composed of protein molecules.
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Cristae are most common in the mitochondria of cells with a high metabolic activity. A finely granular, homogeneous matrix fills the lumen of the mitochondria.
It also contains some very dense granules which are formed from the precipitation of insoluble salts of calcium and other ions accumulated by the mitochondria. Adhering to their surface are a very large number of different kinds of enzymes.
Mitochondria are frequently located to intracellular lipid droplets. Sometimes the lipid droplets appear to be fused to the mitochondria, particularly after a short of starvation or in rapidly metabolizing tissue such as cardiac muscles.
They are also found located at the surface of cells particularly in those which are actively engaged in absorption or excretion, for example the intestinal epithelium or renal tubular cells.