Surrounded by two membranes and that is the site of cellular respiration, which produces ATP

Mitochondrion. A mitochondrion (mitochondria, plural), is a membrane-enclosed organelle that is found in most eukaryotic cells. Mitochondria are called the "power plants" of the cell because they are the sites of cellular respiration, where they use energy from organic compounds to make ATP (adenosine triphosphate)