The oxidation of pyruvate occurs in the mitochondria's inner membrane. During cellular respiration, all of the CO2 that occurs is generated in the mitochondria. This aids in the citric acid cycle by providing molecules of acetyl-CoA.
During pyruvate oxidation a two-carbon acetyl group occurs after the three-carbon pyruvate undergoes conversion, resulting in the third carbon exiting as carbon monoxide. The stripping of two hydrogen atoms via NAD+ from pyruvate occurs, making this a very explicit type of oxidation reaction. After a reaction with four carbons, or oxaloacetate, the acetyl CoA group goes into the Kreb's cycle to make six carbons, or citric acid.