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 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. What happens in Acetyl CoA Entry is that the two carbon atoms of the Acetyl CoA molecules are going to enter the Krebs cycle and react with a molecule that's called oxaloacetate that must be present inside of the mitochondria. When the two carbon atoms of Acetyl CoA enter and react with oxaloacetate, it ends up producing citrate or citric acid. This is exactly 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 very important to note that the CoA portion of Acetyl CoA does not enter the Krebs Cycle. It's just the two carbon atoms that enter the Krebs Cycle, but not the CoA portion.
If we take a look at our image of the Krebs Cycle, over here on the left-hand side is what we're going to be focusing on first. We know that the previous stage, pyruvate oxidation, ends up producing two Acetyl CoA Molecules. But we're going to look at this one acetyl CoA molecule at a time. Here at the top, what we have is the acetyl CoA molecule, which has two carbon atoms and this CoA portion, and notice that the CoA portion does not enter the Krebs Cycle, the CoA portion is going to be recycled and will go back to be part of another pyruvate oxidation reaction. Instead, it's just these two carbons that are going to enter the Krebs cycle. These two carbons react with the four carbons of the oxaloacetate and it ends up creating this six carbon molecule which is, once again called citrate. This is again why the Krebs cycle is also commonly referred to as the citric acid cycle because citrate is the very first molecule that's produced in the cycle. That is it for the first phase of the Krebs Cycle here, phase A creates citrate.
Moving on to the second phase of the Krebs Cycle, we have phase B, which is going to be citrate oxidation. Recall oxidation is when it loses electrons, so citrate is going to lose electrons and become oxidized. As its name implies, citrate oxidation involves the rearrangement and the oxidation of citrate. Ultimately, citrate oxidation is going to end up producing a little bit of ATP, one ATP via substrate level phosphorylation, two NADH molecules, and two carbon dioxide molecules. When we take a look at phase B down below here in our image, notice once again that it's producing two NADH molecules, two carbon dioxide molecules, and it's also going to produce one ATP molecule via substrate level phosphorylation. After phase B, what we have is the final phase, phase C, which is Oxaloacetate regeneration. Recall Oxaloacetate was one of the starting molecules here that reacted to form Citrate. In order for the Krebs Cycle to be a cycle, it needs to start and end at the same place, and in order for it to start and end at the same place, there needs to be a part that's dedicated to regeneration. Phase C here is Oxaloacetate regeneration, which as its name implies is going to involve the regeneration of oxaloacetate by continuing this oxidation process. Ultimately, oxaloacetate regeneration produces one NADH and one FADH2 molecule. Let's take a look down below at our phase C over here, and notice that phase C is gonna produce one FADH2 and one NADH, and it ends up regenerating the starting molecule here, oxaloacetate.
This concludes our introduction to the phases of the Krebs cycle. It's important to note that it's actually going to require two rounds of the Krebs cycle to occur for every one glucose molecule that enters the cell, and that's because one glucose molecule that enters the cell is going to get split into two pyruvate, which gets converted into two acetyl CoA. So here in this image, we only looked at one acetyl CoA, but there has to be one round of the Krebs cycle per Acetyl CoA. So we have to remember that there are two acetyl CoA's that need to go through the Krebs cycle. This here is just showing for one revolution for one Acetyl CoA, but we need to remember that there's actually going to be two revolutions of the Krebs cycle. So when we consider two revolutions, all we need to do is take all of these products that we talked about here and double them. The total output is really something that you want to be able to focus on. When it comes to the total output, there's going to be a total of two FADH2s produced, a total of two ATPs that are produced, a total of six NADHs that are produced, and a total of four CO2s that are produced, and those CO2s are ultimately going to be exhaled.
At this point, what you can see is that we've got a little memory tool to help you guys remember the total output for the Krebs Cycle. When you think of the Krebs Cycle, you can think about a fan and you can think about a Krebs fan company. When you think about a fan company, what you'll notice is that the 'F' in fan is for the F in FADH2. The 'A' in fan is for the A in ATP, and the 'N' in fan is for the N in NADH. Then, of course, the company part, the 'C' in company, is for the C in CO2. If you can remember 'Krebs Fan Company', then you'll be able to remember the products that are being produced here. Ultimately, this is a number that associates with each of the products. The number is 2264. If you can remember 2264 and FANC, then you'll be able to remember the four products along with the numbers of each product. This is once again the total output. What you'll notice here is that we've color coordinated some of the things that you should really commit to memory here in a bluish color. You should know that oxaloacetate is the very first molecule that's going to react to produce citrate, and you should also know the total output numbers over here. Because if you know the total output numbers, then in order to get the numbers for one revolution, all you need to do is cut these numbers in half.
This here really concludes our introduction to the phases of the Krebs cycle. One thing that's important to note is that all of the FADH2s and NADHs are going to make their way to the final stage of aerobic cellular respiration, which is the electron transport chain or the ETC. This is the next phase that we're going to be talking about as we follow these electron carriers. The CO2 molecules, once again, they're being exhaled, so we're not going to follow those. This here concludes our introduction to the phases of the Krebs cycle and we'll 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.