Mitochondria were first described by Altmann, but named by Benda in 1898. They are small, Rod-shaped or spherical organelles found in large numbers. Their sizes usually between 0.2 to 2 * 3 to 5 (1 mm-1000). Each mitochondrion is bounded by two membrances-outer and inner. Outer members is smooth and the inner members is pushed inwards at intervals as crests called cristae. The cristae lie in a ground substance called matrix. The number of mitochondria varies from cell to cell.

Those cells which are most active in terms of movements or synthesis of complex substances contain the largest number of mitochondria. An average cell (e.g., liver cell) contains about 10000 mitochondria. Plant cell have fewer mitochondria than animal cells. Mitochondria is mainly composed of protein and phospholiped (a phosphorous-containing fatty substance). Mitochondria process enzymes necessary for the oxidation of carbohydrate and thus release energy in the form of ATP (adenosine triphosphate). This is the reason why they are known as power houses of the cell. The site for ATP formation is F1 particle.