Mitochondria. Coloured transmission electron micrograph (TEM) of mitochondria (blue) in an adipocyte (fat cell). 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. The folds of the inner membrane are called cristae, and it is here that the chemical reactions to produce energy take place. Magnification: x20,000 when printed at 10 centimetres wide.