Mitochondria cross-section, illustration. Mitochondria are organelles found in the cytoplasm of eukaryotic cells. They oxidise sugars and fats to produce energy in a process called cellular respiration. A mitochondrion has two membranes, a smooth outer membrane and a folded inner membrane (both orange). The folds of the inner membrane are called cristae, and it is here that the chemical reactions to produce energy in the form of adenosine triphosphate (ATP) take place. Enclosed by the inner mitochondrial membrane is the mitochondrial matrix.