Popularly called ‘power house’ of cells, these granular or filamentous organelles are p resent in all eukaryotes. They are characterised by their special structure, and unique physiological function concerning intracellular energy transformation.
Historically-, were first observed by Altmann (1894) who called them bioblasts. The present name was coined by Benda (1897) who developed the crystal violet staining.
Bensely and Hoerr (1934) first isolated the liver cell mitochondria. The discovery that mitochondria are seats of cellular respiration was made by Hogeboom et al in 1948.
It is difficult to observe mitochondria in living cells due to their low refractive index, but they can be studied under phase contrast microscope after staining with a dilute solution of janus green. Mitochondria may be isolated from cells by centrifugatican between 20,000 g -40.000 g.
Generally mitochondria are granular or filamentous, even through the shape is variable. During certain conditions a long mitochondrion may swell at one end lo become club shaped or look like a tennis racket (De Robertis et al 1970). Mitochondria may also become vesicular.
Size also is variable. The width is about 0.5pm and the length may reach a maximum of 7 m.m. The size depends on the osmotic pressure.
In general, mitochondria are distributed evenly in the cytoplasm, but in some instances they may accumulate around the nucleus or at the periphery of the cytoplasm. During cell division, mitochondria accumulate near the spindles.
The distribution of mitochondria within the cell is in relation to the energy demand. For example in skeletal muscles they are grouped like ‘ rings or braces around the I-band of the myofibril.
Mitochondria have more or less a specific orientation. In cylindrical cells, their long axis is parallel to the long axis of the cell. In leucocytes, mitochondria are arranged radically in respect to the centrioles.
Mitochondria are absent in prokaryotic cells. The quantity of mitochondria pier cell varies, but in liver cells it is estimated that mitochondria contains 30-35 % of the total protein content of the cell. A normal liver cell contains about 1000-1500 mitochondria. Some oocytes contain a many as 300000 mitochondria.