Lung mitochondrion. Transmission electron micrograph (TEM) of a section through lung tissue showing a mitochondrion. Mitochondria are a type of organelle found in the cytoplasm of eukaryotic cells. They oxidise sugars and fats to produce energy in a process called respiration. A mitochondrion has two membranes, a smooth outer membrane and a folded inner membrane (dark lines). The folds of the inner membrane are called cristae, and it is here that the chemical reactions to produce energy take place.