Mitochondrion. Colored Transmission Electron Micrograph (TEM) of a single mitochondrion in a human pancreas cell. The cylindrical mitochondrion (blue) 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 oxidized to produce energy which is then stored. They are thus called "powerhouses" of the cell. In the cytoplasm can be seen rough endoplasmic reticulum (ER) at lower left. The elongated membranes of ER store and transport cell products. Ribosomes (dots) on the rough ER are sites of protein synthesis.