The cytoplasm of nearly all eukaryotic cells contain mitochondria, although there is at least one exception, the protist Chaos (Pelomyxa) carolinensis. They are especially abundant in cells and parts of cells that are associated with active processes. For example, in flagellated protozoa or in mammalian sperm, mitochondria are concentrated around the base of the flagellum or flagella. In cardiac muscle, mitochondria surround the contractile elements.
