Citric Acid Cycle

Figure 43.

The ten NADH molecules made per glucose molecule during glycolysis, the transition phase, and the Krebs cycle carry enough energy to make 30 ATP molecules, while the 2 FADH2 molecules made in the process make an extra four ATP molecules. This is a theoretical yield of 34 molecules of ATP, except that there is leakage of electrons so the actual yield is less.

CITRIC ACID CYCLE

There is a transition phase that occurs before glycolysis enters the Krebs cycle or the citric acid cycle. Pyruvate gets transported into the mitochondria in eukaryotes, where it gets turned into acetyl-CoA by removing carbon dioxide from it and adding coenzyme A. It delivers the acetyl group to the Krebs cycle. This “transition phase” makes a molecule of NADH in the process.

The next part is the Krebs cycle or citric acid cycle. It takes place in the mitochondrial matrix and is a closed loop system. There are eight steps to the cycle, which makes 2 carbon dioxide molecules, one ATP molecule, NADH, and FADH2. Part of this is oxygen-requiring because NADH and NADH2 need to transfer their electrons to the next part of the system—which does require oxygen. It will not work if oxygen is not present.

There are two carbon atoms entering the citric acid cycle. Two CO2 molecules are released by the cycle but these are not the same carbon atoms coming from the acetyl group. It takes two turns of the cycle in order to process one glucose molecule. Each turn makes three NADH molecules, one ATP molecule, and one FADH2 molecule. Figure 44 shows the Krebs cycle:

Figure 44.

So far, not many high energy molecules are made. It takes oxidative phosphorylation in order to make most of the necessary ATP molecules. Oxidative phosphorylation can happen in prokaryotes and eukaryotes.