Phases of The Krebs Cycle

by Jason Amores Sumpter
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and this video, we're going to introduce the phases of the Krebs cycle. And so the Krebs Cycle, which is the third stage of Arabic cellular respiration, actually consists of a Siris of multiple reactions. And all of the reactions of the Krebs cycle can actually be grouped into three phases that we have labeled down below us. Phase A Phase B and phase C. And so in the first phase of the Krebs cycle, Phase A. We've titled it A Seal Co. A entry, and what happens in a c. Delco a entry is that the two carbon atoms of the acetyl coa A molecules are going to enter the Krebs cycle and react with the fancy molecule that's called Uccello acetate that must be present inside of the mitochondria. And so when the two carbon atoms of acetyl coa a enter and react with ox alot acetate, it ends up producing citrate or citric acid. And 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 now, it's very important to note that the co a portion of acetyl coa A actually does not enter the Krebs cycle. It's just the two carbon atoms that enter the Krebs cycle, but not the CO a portion. And so, if we take a look at our image down below of the Krebs cycle, over here on the left hand side is what we're gonna be focusing on first. And so we know that the previous stage Piru oxidation ends up producing two acetyl coa a molecules. But we're gonna look at this one acetyl coa a molecule at a time. And so here at the top, what we have is the acetyl coa, a molecule which has two carbon atoms and this co a portion and notice that the CO a portion actually does not enter the Krebs cycle. The CO a portion is going to be recycled, and we'll go back to be part of another pair of oxidation reaction. But the CO A does not enter the Krebs cycle. Instead, it's just these two carbons that are gonna enter the Krebs cycle. And these two carbons are going to react with a molecule that's already present called Uccello acetate. So these two carbons react together with the four carbons of the ox alot acetate, and it ends up creating this six carbon molecule, which is once again called citric. And 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. And so really, that is it for the first phase of the the Krebs cycle here, phase a creates citric. So then moving on to the second phase of the Krebs cycle, we have Phase B, which is going to be citrate, oxidation and basically recall. Oxidation is when it loses electrons, so citrate is going to lose electrons and become oxidized. And so, as its name implies, citrate oxidation involves the rearrangement. And once again, the oxidation of citrate and ultimately citrate oxidation is gonna end up producing a little bit of a T p one a tpv, a substrate level foster relation two and A. D h molecules and to carbon dioxide molecules or CO two molecules. And so when we take a look at fades, be down below. Here in our image, notice once again that it's producing two and a D H molecules to carbon dioxide or co two molecules and it's also going to produce 1 80 p molecule via substrate level phosphor relation. And so after phase be what we have is the final phase Phase C, which is ox alot, acetate, regeneration and recall. Uccello acetate was the starting one of the starting molecules here that reacted to form citrate. And so, in order for the crab 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. And so face see, here's oxtail acetate regeneration, which, as its name implies, is going to involve the regeneration of ox alot acetate. By continuing this oxidation process and ultimately, oxtail acetate regeneration produces one and a D h and one f a d h two molecule. So let's take a look down below at our face. See over here and notice that face C is gonna produce one f a. D. H two and one and a d h. And it ends up regenerating the starting molecule here Uccello acetate And so really that is it for just one revolution of the acetyl coa A in the Krebs cycle. And so what's important to note is that, uh, 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 Piru bait, which gets converted into two acetyl coa A. So here in this image we only looked at one a c Delco way. But there has to be one round of the Krebs cycle per acetyl coa A. So we have to remember that there are two acetyl COA ways that need to go through the Krebs cycle. And so this year is just showing for one revolution for one acetyl COA way. But we need to remember that there's actually going to be two revolutions off the citric or of the Krebs cycle, and so there's gonna be one revolution per acetyl coa it. 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, and so the total output is really something that you really want to be able to focus on. And so when it comes to the total output, there's going to be a total of two. F A. D H two is produced a total of 2. 80 p s. That air produced a total of six and a. D. H is that are produced and a total of four co two's that are produced and those co two is air ultimately going to be exhaled. And so, at this point, right here, 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. And so when you think of the Krebs cycle, you can think of this circular rotation here, which is really the circular rotation of a fan. And so when you think about the Krebs cycle, you can think about a fan and you can think about ah Krebs fan company. You can think about a fan company. And so when you think about a fan company, which you'll notice is that the F and fan As for the F and F A. D. H two, the A N fan is for the A ATP and the N N fan is for the n n n a d h. And then, of course, the company part the sea and company is for the C and C 02 And so, if you can remember Krebs fan company, then you'll be able to remember the products that are being produced here. And ultimately, uh, this is a number that associates with each of the products, and so the number is 2 to 64 And so, if you can remember to 264 and fan coat or F A N C, then you'll be able to remember the four products along with the numbers of each product. And this is once again the total output. And so what you'll notice here is that we've color coordinated some of the things that you should really commit to memory here in, ah, bluish color. So you should know that Uccello Acetate 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 here in half. And so this year really concludes our introduction to the phases of the Krebs cycle. And one thing that's important to note is that all of the F A. D h two and N a. D H s are going to make their way to the final stage of aerobic cellular respiration, which is the electron transport chain or the E T. C. And so, really, this is the next phase that we're gonna be talking about is, uh, the electron transport chain. So we'll be following these electron carriers and the CO two molecules once again, they're being exhaled. And so we're not going to follow those. And so 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. So I'll see you all in our next video