Pyruvate and fatty acids enter the mitochondrion, are broken down to acetyl CoA, and are then metabolized by the citric acid cycle, which produces NADH (and FADH2, which is not shown). In the process of oxidative phosphorylation, high-energy electrons from NADH (and FADH2) are then passed to oxygen by means of the respiratory chain in the inner membrane, producing ATP by a chemiosmotic mechanism.
NADH generated by glycolysis in the cytosol also passes electrons to the respiratory chain (not shown). Since NADH cannot pass across the mitochondrial inner membrane, the electron transfer from cytosolic NADH must be accomplished indirectly by means of one of several "shuttle" systems that transport another reduced compound into the mitochondrion; after being oxidized, this compound is returned to the cytosol, where it is reduced by NADH again.