Mitochondrion. Illustration of the internal structure of a mitochondrion, an organelle in eukaryotic cells that is the site of energy production. There are two membranes: an outer membrane (red) enclosing an inner membrane (orange) that forms folds (cristae). Cristae are the sites of aerobic cell respiration, where sugars and fats are oxidised to release energy. Enzyme complexes (green) are shown in the mitochondrial matrix. Between the two membranes is the intermembrane space. For this artwork with labels, see image C023/8747.