Brent Cornell

    
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 form another molecule of acetyl CoA

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)
• Multiple oxidation reactions result in the reduction of hydrogen carriers (3 × NADH + H⁺ ; 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:  4 × CO₂  ;  2 × ATP  ;  6 × NADH + H⁺  ;  2 × FADH₂

The Krebs Cycle