In cell biology, a mitochondrion (plural mitochondria) (from Greek mitos thread + khondrion granule) is an organelle, variants of which are found in most eukaryotic cells. 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. Usually a cell has hundreds or thousands of mitochondria, which can occupy up to 25% of the cell's cytoplasm. Mitochondria usually have their own DNA, and, according to the generally accepted Endosymbiotic theory, they were originally derived from external organisms.

Mitochondrion structure

A mitochondrion contains outer and inner membranes 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, contains numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm) that is permeable to all molecules of 5000 daltons or less. Larger molecules can only tranverse the outer membrane by active transport. The outer mitochondrial membrane is composed of about 50% phospholipids by weight and contains a variety of enzymes involved in such diverse activities as the elongation of fatty acids, oxidation of epinephrine (adrenaline), and the degradation of tryptophan.

The inner membrane contains proteins with three types of functions:

- those that carry out the oxidation reactions of the respiratory chain
- ATP synthase, which makes ATP in the matrix
- specific transport proteins that regulate the passage of metabolites into and out of the matrix.

It 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. Unlike the outer membrane, the inner membrane does not contain porins, and is highly-impermeable; almost all ions and molecules require special membrane transporters to enter or exit the matrix.

The mitochondrial matrix

Mitochondria structure :
1) Inner membrane
2) Outer membrane
3) Crista
4) Matrix

The matrix is the space enclosed by the inner membrane. The matrix contains a highly concentrated mixture of hundreds of enzymes, in addition to the special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. Of the enzymes, the major functions include oxidation of pyruvate and fatty acids, and the citric acid cycle.

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.

The inner mitochondrial membrane is folded into numerous cristae (see diagram above), which expand the surface area of the inner mitochondrial membrane, enhancing its ability to generate ATP. In typical liver mitochondria, for example, the surface area, including cristae, is about five times that of the outer membrane. Mitochondria of cells which have greater demand for ATP, such as muscle cells, contain even more cristae than typical liver mitochondria.


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