The second stage of aerobic respiration is the Krebs cycle, which occurs within the matrix of the mitochondria

• The Krebs cycle is also commonly referred to as the citric acid cycle or the tricarboxylic acid (TCA) cycle
  

In the Krebs cycle, acetyl CoA transfers its acetyl group to a 4C compound (oxaloacetate) to make a 6C compound (citrate)

• Coenzyme A is released and can return to the link reaction to become loaded with another acetyl group
  

Over a series of reactions, the 6C compound is broken down to reform the original 4C compound (hence, a cycle)

• Two carbon atoms are released via decarboxylation to form two molecules of carbon dioxide (CO2)
  

• Four oxidation reactions result in the reduction of hydrogen carriers (3 × NADH ; 1 × FADH2)
  

• One molecule of ATP is produced directly via substrate level phosphorylation
  

As the link reaction produces two molecules of acetyl CoA (one per each pyruvate), the Krebs cycle occurs twice

• Per glucose molecule, the Krebs cycle produces ATP (×2), CO2 (×4) and a large yield of hydrogen carriers (×8)