A mitochondrion (from Greek mitos thread + khondrion granule) is an organelle found in most eukaryotic cells, including those of plants, animals, fungi, and protists. A few cells, such as the trypanosome protozoan, have a single large mitochondrion, but usually a cell has hundreds or thousands of mitochondria. The exact number of mitochondria depends on the cell's level of metabolic activity: more activity means more mitochondria. Mitochondria can occupy up to 25% of the cell's cytosol.
Mitochondria are sometimes described as "cellular power plants", because their primary function is to convert organic materials into energy in the form of ATP via the process of oxidative phosphorylation.
The outer and inner membranes of the mitochondria are composed of phospholipid bilayers studded with proteins, much like a typical cell membrane. The two membranes, however, have very different properties. The outer mitochondrial membrane, which encloses the entire organelle, is composed of about 50% phospholipids by weight and contains a variety of enzymes involved in such diverse activities such as the oxidation of epinephrine (adrenaline), the degradation of tryptophan, and the elongation of fatty acids. The outer mitochondrial membrane contains numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm) that allow ions and small molecules to move in and out of the mitochondrion. Large molecules, however, cannot traverse the outer membrane without being actively transported.
The inner mitochondrial membrane, in contrast, contains more than 100 different polypeptides, and has a very high protein to phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). Additionally, the inner membrane is rich in an unusual phospholipid, cardiolipin, which is usually characteristic of bacterial plasma membranes. The inner membrane does not contain porins, however, and is highly impermeable; almost all ions and molecules require special membrane transporters to enter or exit the matrix. The inner membrane also has foldings called the cristae which allow the inner membrane to have a greater surface area for the electron transport chains.
In addition to various enzymes, the mitochondrial matrix also contains ribosomes and several molecules of DNA. Thus, mitochondria possess their own genetic material, and the machinery to manufacture their own RNAs and proteins. This nonchromosomal DNA encodes a small number of mitochondrial peptides (13 in humans) that are integrated into the inner mitochondrial membrane, along with polypeptides encoded by genes that reside in the host cell's nucleus.
Although the primary function of mitochondria is to convert organic materials into cellular energy in the form of ATP, mitochondria play an important role in many metabolic tasks, such as:
Apoptosis
Cellular proliferation
Regulation of the cellular redox state
Heat production (enabling the organism to stay warm)
Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. A mutation in the genes regulating any of these functions can result in a variety of mitochondrial diseases.
As stated above, the primary function of the mitochondria is the production of ATP. This is done by metabolizing the major products of glycolysis: pyruvate and NADH (glycolysis is performed outside the mitochondria, in the host cell's cytosol). This metabolism can be performed in two very different ways, depending on the type of cell and the presence or absence of oxygen.
As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it has been suggested that they were originally derived from endosymbiotic prokaryotes. This endosymbiotic theory is now generally accepted. Studies of mitochondrial DNA, which is circular and employs a variant genetic code, show their ancestor was a member of the Proteobacteria. In particular, the pre-mitochondrion was probably related to the rickettsias.
A few groups of unicellular eukaryotes lack mitochondria: the symbiotic microsporidians, metamonads, and entamoebids, and the free-living pelobionts. On rRNA trees these groups appeared as the most primitive eukaryotes, suggesting they appeared before the origin of mitochondrion, but this is now known to be an artifact of long branch attraction - they are apparently derived groups and retain genes or organelles derived from mitochondria. Thus it appears that there are no primitively amitochondriate eukaryotes, and so the origin of mitochondria may have played a critical part in the development of eukaryotic cells.