Mitochondrion is a rod-like cytoplasmic organelle which is the main site of cellular respiration. They are sources of energy and are often called as the power house of the cell. They have average length of 3-4 µ and diameter of 0.5-1 µ. Under light microscope, they appear as rod shaped, filamentous or granular structures in majority of the cells.
Their average number varies from 200- 800 per cell. In some protozoa, the number has been recorded up to 500,000 per cell. Mitochondria, contain about 65-70 per cent proteins, 25-30 per cent lipids, 1 per cent RNA and less than 1 per cent DNA.
Mitochondria consists of three main parts, viz:
(2) Christae, and
Each mitochondrion is enclosed by two concentric unit membranes, the outer membrane and the inner membrane. Each of the two unit membranes is 60 Å thick. The space between these two membranes is 40-47 Å. The outer membrane contains about 50 per cent proteins and 50 per cent lipids, whereas inner membrane has 75 per cent proteins and 25 per cent lipids.
The inner membrane has a series of inside folds known as cristae. Cristae project into the inner chamber. The space between two membranes is known as outer chamber.
The sace between cristae into the inner chamber is called matrix. Each mitochondrion has several copies of ring or circular DNA molecule. The DNA is either present in the matrix or is attached to the inner membrane.
The enzymes associated with Kreb’s cycle are also present in the matrix. Since mitochondria have their own DNA, tRNA, RNA polymerase, ribosomes, amino acids, activating enzymes, etc. they are considered as semi-autonomous.
There are two main views about the origin of mitochondria. According to one view, mitochondria have self-replication capacity, and the new mitochondria are formed from the fission of pre-existing mitochondria. According to second view, they originate from nuclear envelope.
Mitochondria have two important functions. First, they are the sites of cell respiration. The oxidation of carbohydrates, lipids and proteins occurs in the mitochondria. They supply energy to various processes of cell in the form of ATP. Second, mitochondria contain some amount of DNA and thus are associated with cytoplasmic inheritance.