Mitochondrial Matrix - Citric Acid Cycle

Citric Acid Cycle

The citric acid cycle (or Krebs cycle or TCA cycle) takes place within the mitochondrial matrix. Prior to the citric acid cycle, pyruvic acid generated from glycolysis is converted into acetyl coenzyme A (acetyl CoA) by losing a carbon dioxide molecule. It then combines with oxaloacetic acid to form citric acid (a six-carbon molecule) to begin the citric acid cycle. In total, it loses 2 CO₂ molecules and 8 electrons, of which 6 are accepted by three NAD+ molecules which get reduced to NADH, and the last two electrons are accepted by FAD to reduce to FADH₂ in redox reactions. In the end, oxaloacetate is regenerated to continue the citric acid cycle. In addition, a single GTP molecule is created from the combination of GDP and a phosphate group via substrate-level phosphorylation.

Since 2 pyruvic acid molecules are formed by glycolysis, each time a cell undergoes glycolysis two turns of the citric acid cycle will occur. That means that the citric acid cycle produces a total of 6 NADH, 2 FADH₂, and 2 GTP molecules.