The reactions which help in converting pyruvic acid to carbon dioxide and water in mitochondria are called the Krebs cycle. It is also known as the citric acid or tricarboxylic acid cycle (TCA cycle).

In the first reaction of the citric acid cycle, acetyl CoA combines with the oxaloacetic acid to form citric acid. This reaction is catalyzed by citric acid synthetase.

Citric acid contains three carboxylic acid groups. Citric acid is dehydrated to form cis-aconitic acid in the presence of aconitase. The same enzyme aconitase catalyzes the formation of isocitric acid from cis-aconitic acid by the addition of the molecule of water.

Citric acid, cis-aconitic acid, and isocitric acid contain three carboxylic acid groups. The isocitric acid is oxidatively decarboxylated to α-ketoglutaric acid. This reaction is catalyzed by isocitric dehydrogenase.

During this reaction, one NADH₂ is formed. The α-ketoglutaric acid is oxidatively decarboxylated to form succinyl co-a. This reaction is catalyzed by α-Ketoglutaric dehydrogenase. The energy released during this reaction is conserved in NADH₂.

The succinyl-CoA is hydrolyzed to succinic acid in the presence of succinyl co-a synthetase. In this reaction, ADP is phosphorylated to ATP. This is called substrate-level phosphorylation. The succinic acid is oxidized to form fumaric acid by succinic dehydrogenase. FAD⁺ is reduced to

FADH₂. The fumaric acid is converted to malic acid by the addition of a molecule of water. This reaction is catalyzed by fumarase. The malic acid is oxidized to oxaloacetic acid by the enzyme malic dehydrogenase.