Mitochondria | Definition, Structure, and its Function

Mitochondria | Definition, Structure, and its Function

These are cylindrical or sausage – shaped double membraned organelles distributed in the cytoplasm. They are not easily visible under the microscope. Thus, they are stained by a vital stain Janus green to make visible for observation. Mitochondria show a great degree of variability in their shape, size & number.

Size : A typical mitochondrion is a sausage shaped or cylindrical having a diameter in the range of 0.2 – 1.0  µm (average 0.5 µm) and length of 1.0 – 4.1 µm (show considerable degree of variability with shape and size).

Number : Their number also varies in different cell types depending upon the amount of work done by the cell and its energy requirement.


(i) Mitochondria are surrounded by two membranes designated as other & inner membrane. The outer membrane is smooth whereas the inner membrane forms a number of infoldings called the cristae (sing : crista). Due to the presence of two membranes, the organelle is partitioned into two distinct chambers filled with aqueous fluid.

(a) Outer compartment or intermembrane space : It lies between the two mitochondrial membranes. It is also called peri – mitochondrial space.

 (b) Inner compartment or matrix : It lies inside the inner membrane. The cristae are infoldings of inner membrane & are formed towards the matrix which increase the surface area for enzyme action.

(ii) The matrix contains single circular dsDNA molecule (with high G = C content), a few RNA molecules, 70S ribosome & the components required for the synthesis of proteins. The matrix also contains enzymes for TCA (Tricarboxylic acid) cycle.

(iii) The two membranes have their own specific enzymes associated with the mitochondrial function. The mitochondria divide by fission. The enzyme & electron carries for formation of ATP are present only in the inner membrane.

Mitochondria | Definition, Structure, and its Function

The cristae & the inner face of the inner membrane is studded with numerous spherical or knob like protuberance called elementry particles or Particals of Fernandez & Moran or F1 particles or oxysomes. Each oxysome is differentiated into base, stalk & head piece. The head piece contains enzyme ATP synthetase which brings about oxidative phosphorylation coupled with release of ATP.

Detailed structure of cristae and oxysomes


(i) Mitochondria are main sites of aerobic respiration. They are miniature biochemical factories where food stuffs or respiratory substrate are completely oxidised to carbon dioxide & water. The energy liberated in the process is stored in the form of ATP. These bring about the oxidation of carbohydrates, proteins & β-oxidation of fats.

(ii) The energy (ATP) produced in the mitochondria helps to perform various energy – requiring processes the cell like muscle muscle contraction, nerve impulse conduction etc. Because of the formation of ATP mitochondria are called power house of the cell. 

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