In an earlier article, I wrote about various obstacles to the evolutionary origins of the glycolytic pathway. As noted previously, the end result of glycolysis is pyruvate. Pyruvate is then transported into the mitochondria where it is converted into acetyl-CoA by the enzyme pyruvate dehydrogenase. This process also produces NADH and releases one molecule of carbon dioxide (CO₂). The acetyl-CoA then feeds into the Krebs cycle (also known as the citric acid or tricarboxylic acid cycle), where it is further oxidized, generating more NADH, FADH₂, and ATP (or GTP). Pyruvate is also converted to oxaloacetate in a reaction catalyzed by the enzyme pyruvate carboxylase. Acetyl-CoA then combines with oxaloacetate in the Krebs cycle to form citrate. The various reactions of the Krebs cycle are represented by the figure above.

The final yield, per turn of the Krebs cycle is 3 NADH, 1 FADH₂, 1 GTP (or ATP), and 2 CO₂. Since each glucose molecule produces two molecules of acetyl CoA (from two pyruvate molecules), the results of the Krebs cycle per molecule of glucose are 6 NADH, 2 FADH₂, 2 GTP (or ATP), and 4 CO₂. The NADH and FADH₂ are important because they carry high-energy electrons to the electron transport chain, where they help produce more ATP. Carbon dioxide (CO₂) is released as a waste product. The Krebs cycle requires the presence of at least nine enzymes, as well as three cofactors.

 The entire cycle is represented by the following animation, created by Australian animator Drew Berry: