The transition reaction, which occurs in the matrix of the mitochondrion, is the first step in the process of aerobic cellular respiration.

If little or no oxygen is available, pyruvate in the cytosol can be oxidized through one of two fermentation processes.

Transition reactionPyruvate crosses the mitochondrion’s outer membrane and then enters the matrix.

Once in the matrix, the pyruvate dehydrogenase complex aids the process of oxidative decarboxylation.

NAD+ removes two electrons, oxidizing pyruvate.

Carbon dioxide is removed, leaving a two-carbon acetyl group that combines with coenzyme A to form acetyl-CoA.

Coenzyme A is a compound that contains a sulfur-based functional group. This electronegative sulfur-based functional group binds to a carbon in the acetyl group to make the reactive acetyl-CoA.

Lipids and proteins can be broken down to acetyl-CoA in the cell, and produce ATP in the mitochondrion.

If ATP levels are high, acetyl-CoA can be directed into other metabolic pathways, such as the production of fatty acids that are required to produce lipids.