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Subject - cell biology

The mitochondrion is the eukaryotic organelle in which all respiratory functions of the cell, including the citric acid cycle, the electron-transport chain, and oxidative phosphorylation, occur. Because the majority of the biochemical reactions involved in energy conversion are localized in the mitochondria, they are also referred to as the powerhouses of the eukaryotic cell.

The number of mitochondria in a given cell depends strongly on the type of cell in question: whereas liver cells contain about 1,000 mitochondria, large oocytes (egg cells) from some animals may contain more than 100,000. In contrast, many protozoa feature only a single very large mitochondrion. The average dimensions of a mitochondrion correspond roughly to those of typical bacterial cells. According to the endosymbiotic theory, mitochondria developed from ancestors of modern α-proteobacteria.

Like plastids, mitochondria have a double membrane. The inner membrane contains the respiratory chain enzymes, ATP synthase complexes, and numerous transport systems for low-molecular metabolites. The surface area of the inner membrane is vastly increased by folds, known as cristae. The volume contained within the inner membrane is called the mitochondrial matrix. This is where the enzymes involved in the citric acid cycle and the β-oxidation of fatty acids are located, among others.

Mitochondria have their own DNA (known as mtDNA), which is present as a ring-shaped molecule in the matrix, and thus their own genome (chondrome). The size of mitochondrial DNA varies considerably depending on the organism: for example, mtDNA from yeast contains about 78,000 base pairs (bp), while human mtDNA contains only about 16,500 bp. However, the number of encoded genes is generally the same and amounts to about 5 % of the proteins located in the mitochondrion. All other proteins are encoded by the cellular genome.

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Animated section of a mitochondrion

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Cytology is introduced by means of illustrations, animations, and a comprehensive collection of references and selected links.