Mitochondrion. Coloured transmission electron micrograph (TEM) of mitochondria (blue/green) in heart muscle. Mitochondria are organelles found in the cytoplasm of eukaryotic cells. They oxidise sugar and fats to produce energy. Mitochondria have two membranes (blue), 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: 16,200x when printed 10 centimetres tall.