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Mitochondria are organelles of eukaryote cells.
That is, they are parts of a cell wrapped in their own membrane. They serve two functions in a cell. They take in the waste products of fermentation and gently burn them to make simpler chemicals like carbon dioxide and water. During this burning some of the chemical energy of these waste products is converted into ATP , the molecule used by all living things to store and transport energy. So mitochondria are also the power stations of the cell.