The Transition Reaction converts the two molecules of the 3-carbon pyruvate from glycolysis into two molecules of the 2-carbon molecule acetyl Coenzyme A (acetyl-CoA).
Transition Reaction is also called Oxidative Decarboxylation. It takes place in the matrix of the mitochondria. It is the beginning of aerobic respiration, meaning this phase will continue to take place as long as there is a sufficient amount of oxygen available in the mitochondria. If there is not then the process of fermentation will begin.