Mitochondrion (Greek: mitos - "thread", chondrios - "grain") is the main spot of aerobic respiration activity of the cell. The number of mitochondria in a cell is not constant. It depends on the species of an organism and the nature of a cell. A cell with strong need in energy (for example, a liver cell) can contain up to a thousand of mitochondria. Mitochondria can be spiral, round, rod-shaped, cup-shaped, even branched. The length of mitochondria may vary from 1.5 to 10.0 microns, diameter from 0.25 to 1.00 micron.

Mitochondrion has a double membrane dividing it into two compartments. The area limited by the inner membrane thrown into folds (the folds are called cristae) is known as mitochondrial matrix. It contains ribosomes and mitochondrial DNA, circular double-stranded molecule encoding some of mitochondrial proteins. Cristae in the inner membrane significantly increase its surface, thus, providing the site for placing multi-enzymatic systems and facilitating the access to the enzymes in the mitochondrial matrix.

The compartment between the outer and inner membranes, which is called intermembrane space, contains substrates, enzymes and some metabolites.

Mitochondrion is the main site of ATP synthesis in animals. The most part of oxidizing cell processes is accomplished in mitochondria. Mitochondria resemble bacteria in shape and size. They contain DNA and replicate. It is thought that eukaryotic cells are descendants of primitive anaerobic organisms who survived in the world rich in oxygen having devoured aerobic bacteria. Instead of digesting the bacteria, these organisms fed them and kept in symbiosis for the sake of their ability to use atmospheric oxygen and produce energy.

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