Mitochondrion. Coloured high resolution scanning electron micrograph (SEM) of a single mitochondrion in the cytoplasm of an intestinal epithelial cell. The cylindrical mitochondrion (pink, centre) has two membranes: an outer surrounding membrane; an inner membrane which forms folds called cristae. It is on the cristae that chemical reactions occur. Mitochondria are sites of cell respiration: sugars and fats are oxidised to produce energy which is then stored. They are thus called “powerhouses” of the cell. Magnification: x120,000 at 10 centimetres tall.