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Mitochondria are bacteria-sized membrane-bound organelles found within eukaryotic cells. In fact, according to what is called the endosymbiotic theory, they are thought to have arisen from early bacteria that became engulfed within another cell and established a symbiotic relationship. The gross structural organization of a mitochondrion is shown as a schematic drawing in the figure and can be described as follows:

Schematic drawing of a mitochondrion   The outer membrane surrounds a highly invaginated inner membrane. Between the two membranes lies the intermembrane space. Due to the presence of the integral membrane protein porin, the outer membrane is permeable to ions and small to medium sized molecules (up to as much as 10 kD).
Therefore, the intermembrane space is close to equilibrium with the cytosol with respect to these cellular constituents. The invaginations of the inner membrane, called cristae, create an extensive surface area. In addition, the inner membrane is quite protein-rich, especially compared with the smooth-contoured outer membrane. Both features reflect the function of the inner membrane in respiration. Protein complexes of the inner mitochondrial membrane carry out the process of oxidative phosphorylation, which couples the exergonic electron transfer reactions of the electron transport chain to generation of a transmembrane electrochemical gradient and the the synthesis of ATP by ATP synthase. In addition, the inner membrane is generally impervious to ions and charged or polar metabolites. The cross-membrane transfer of such species must be carried out by other integral membrane proteins. The compartment on the inside of the inner mitochondrial membrane is the matrix.

The matrix is a viscous soup of soluble enzymes and metabolites, including pyruvate dehydrogenase complex, and all but one of the enzymes of the citric acid cycle. Mitochondria also have their own genome and protein-synthesizing machinery (i.e., ribosomes). The mitochondrial DNA, RNA, and ribosomes are all also in the matrix.

Related entry: chloroplast .

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© 2012 Jeff D Cronk, Gonzaga University