Mitochondria are present in the living eukaryotic cells and absent in prokaryotic cells like bacteria and blue-green algae. Form, size and number- Mitochondria are either filamentous or granular, but can change their shape. The diameter varies from 0.5-2.0µ. The number varies from one-person cell to 3000, 000-person cell. Mitochondria are less in number in green plant cells.


The electron microscope reveals two parts of the mitochondria, an outer envelope and central cavity filled with matrix. The outer envelope is made up of two unit membranes. The space between two membranes is called perimitochondrial space. The outer membrane is smooth but the inner membrane has many infoldings which extend into matrix. These infoldings are called cristae. These cristae provide an increased surface area within the mitochondrion for enzyme activity.

The inner surface of the inner membrane is studded by small tennis racket- like particles with a head and stalk known as elementary particles or oxysomes or Electron transporting particles (ETP). They are placed in regular intervals.

Each particle consists of a head called F1 sub-unit and a base piece called F0 sub-unit. These particles project into the matrix. The inner membrane has all the enzymes required for electron transport. The F1-F0with a head and combination has special ATPase enzyme, which helps in oxidative phosphorylation.


Mitochondria are known as the powerhouse of the cell. This is because of the following facts:


(i) Formation and storage of energy-rich compound ATP as a result of oxidation. ATP is readily available source of energy for numerous cellular reactions.

(ii) Mitochondria are concerned with the oxidation and phosphorylation reactions of Krebs cycle during aerobic respiration.

(iii) Besides oxidation of carbohydrates, mitochondria are also involved in the oxidation of proteins and fats.


(iv) Mitochondria are absent in cells which do not respire aerobically.