In this video, we're going to introduce the phases of the Krebs Cycle. The Krebs Cycle, which is the third stage of aerobic cellular respiration, consists of a series of multiple reactions. All of the reactions of the Krebs Cycle can actually be grouped into three phases that we have labeled down below as phase A, phase B, and phase C. In the first phase of the Krebs Cycle, phase A, we've titled it "Acetyl CoA Entry." In Acetyl CoA Entry, the two carbon atoms of the Acetyl CoA molecules enter the Krebs cycle and react with a molecule called Oxaloacetate that must be present inside the mitochondria. When the two carbon atoms of Acetyl CoA enter and react with oxaloacetate, it produces citrate or citric acid. This is why the Krebs cycle is commonly referred to as the citric acid cycle because citrate is the very first molecule that's produced. It's important to note that the CoA portion of Acetyl CoA does not enter the Krebs Cycle; it's just the two carbon atoms.

Looking at our image of the Krebs Cycle, we focus on the left-hand side. The previous stage, pyruvate oxidation, produces two Acetyl CoA molecules, but we're going to look at one Acetyl CoA molecule at a time. Here at the top, we have the Acetyl CoA molecule, which has two carbon atoms and this CoA portion. Notice that the CoA portion does not enter the Krebs cycle; it is recycled and goes back to be part of another pyruvate oxidation reaction. The two carbons enter the Krebs Cycle and react with a molecule already present called Oxaloacetate. These two carbons react with the four carbons of oxaloacetate and create a six-carbon molecule called citrate. That is it for the first phase of the Krebs Cycle, phase A creates Citrate.

Moving on to the second phase of the Krebs Cycle, we have phase B, which is Citrate Oxidation. Recall that oxidation involves losing electrons, so citrate loses electrons and becomes oxidized. Citrate Oxidation involves the rearrangement and oxidation of citrate and ultimately produces a little bit of ATP, one ATP via substrate-level phosphorylation, two NADH molecules, and two carbon dioxide molecules or CO₂ molecules. In phase B, notice that it produces two NADH molecules, two carbon dioxide molecules, and also one ATP molecule via substrate-level phosphorylation.

After phase B, we have the final phase, phase C, Oxaloacetate Regeneration. Recall Oxaloacetate was one of the starting molecules here that reacted to form citrate. For the Krebs Cycle to be a cycle, it needs to start and end at the same place, requiring a part dedicated to regeneration. Phase C, Oxaloacetate Regeneration, involves the regeneration of oxaloacetate by continuing the oxidation process. It produces one NADH, and one FADH₂ molecule.

It is important to note that it requires two rounds of the Krebs cycle for every one glucose molecule that enters the cell because one glucose molecule gets split into two pyruvate, which gets converted into two Acetyl CoA. So, there are two Acetyl CoA that need to go through the Krebs Cycle. When considering two revolutions, the total output is really something to focus on. The total output includes two FADH₂s, two ATPs, six NADHs, and four CO₂ molecules that are exhaled. A memory tool for the total output is "Krebs Fan Company," where "FAN" stands for FADH₂, ATP, and NADH, and "company" for CO₂. The number 2264 helps remember the products and their quantities: 2 FADH₂, 2 ATP, 6 NADH, 4 CO₂.

All the FADH₂s and NADHs make their way to the final stage of aerobic cellular respiration, which is the electron transport chain. The CO₂ molecules are exhaled. This concludes our introduction to the phases of the Krebs cycle. We will be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.