False-colour transmission electron micrograph of mammalian mitochondria (green) in cross-section. Mitochondria are the sites of cell respiration - the chemical process which uses molecular oxygen to oxidise sugars & fats to produce energy. The energy is stored as adenosine triphosphate (ATP) & is used by the cell to drive chemical reactions such as protein formation. Mitochondria are bound by a double membrane; the inner membrane is folded to produce ingrowths (red lines) called cristae, which are where the chemical reactions of respiration occur. Other cellular organelles are visible in the surrounding cytoplasm. Magnification: x18,200 at 6x4.5cm size.