In this video, we're going to begin our introduction to the Krebs Cycle. And so the Krebs Cycle is the 3rd stage of aerobic cellular respiration, and it's important to know that the Krebs Cycle is also commonly known as the citric acid cycle, and sometimes it's also referred to as the TCA cycle. And so it's important to know that the citric acid cycle and the TCA cycle are synonyms for the same exact cycle, the Krebs cycle. And that's because your professor or your textbooks might use a different term to refer to the same exact cycle. And in our course, we're mainly going to be referring to this as the Krebs cycle. Now, as we'll learn moving forward in our course, the Krebs Cycle is going to oxidize Acetyl CoA, producing energy in the form of a little bit of ATP, but a lot of NADH and some FADH 2. And so down below, notice that we're showing you the image for all four stages of aerobic cellular respiration. And already in our course, we've covered glycolysis, which occurs outside the mitochondria producing 2 pyruvates. And, we also covered pyruvate oxidation which converts those pyruvates into molecules of Acetyl CoA. And so this is where the Krebs cycle is going to pick up with, right where we left off with pyruvate oxidation. And so, we'll talk more and more about the Krebs cycle as we move forward in our course. So I'll see you all in our next video.
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concept
Phases of The Krebs Cycle
Video duration:
9m
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Video transcript
In this video, we're going to introduce the phases of the Krebs Cycle. And so the Krebs Cycle, which is the 3rd stage of aerobic cellular respiration, actually consists of a series of multiple reactions. And all of the reactions of the Krebs cycle can actually be grouped into 3 phases that we have labeled down below as phase a, phase b, and phase c. And so in the first phase of the Krebs Cycle, phase a, we've titled it Acetyl CoA Entry. And what happens in Acetyl CoA Entry is that the 2 carbon atoms of the Acetyl CoA molecules are going to enter the Krebs cycle and react with a fancy molecule that's called Oxaloacetate that must be present inside of the mitochondria. And so when the 2 carbon atoms of Acetyl CoA enter and react with oxaloacetate, 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 CoA portion of Acetyl CoA actually does not enter the Krebs Cycle. It's just the 2 carbon atoms that enter the Krebs Cycle, but not the CoA 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, pyruvate oxidation, ends up producing 2 Acetyl CoA Molecules, but we're gonna look at this 1 Acetyl CoA Molecule at a time. And so here at top what we have is the Acetyl CoA molecule, which has 2 carbon atoms and this CoA portion. And notice that the CoA portion actually 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. But the CoA does not enter the Krebs Cycle, instead it's just these 2 carbons that are gonna enter the Krebs Cycle. And these 2 carbons are going to react with a molecule that's already present called Oxaloacetate. So these 2 carbons react together with, the 4 carbons of the oxaloacetate and it ends up creating this 6 carbon molecule which is once again called citrate. 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 Citrate. So then moving on to the 2nd 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 ATP, one ATP via substrate level phosphorylation, 2 NADH molecules, and 2 carbon dioxide molecules or CO 2 molecules. And so when we take a look at phase b down below here in our image, notice once again that it's producing 2 NADH molecules, 2 carbon dioxide or c 02 molecules, and it's also gonna produce 1 ATP molecule via substrate level phosphorylation. And so after, phase b, what we have is the final phase, phase c, which is Oxaloacetate regeneration. And recall Oxaloacetate was the starting one of the starting molecules here that reacted to form citrate. And so 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. And so phase c here is Oxaloacetate regeneration, which as its name implies is going to involve the regeneration of oxaloacetate by continuing this oxidation process. And ultimately, oxaloacetate regeneration produces 1 NADH NADH and 1 FADH 2 molecule. So let's take a look down below at our phase c over here, and notice that phase c is gonna produce 1 FADH 2 and 1 NADH, and it ends up regenerating the starting molecule here And so really that is it for just one revolution of the acetyl CoA in the Krebs cycle. And so, what's important to note is that, it's actually going to require 2 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 2 pyruvate, which gets converted into 2 acetyl co a. So here in this image, we only looked at 1 acetyl co a, but, there has to be one round of the Krebs cycle per Acetyl Co a. So we have to remember that there are 2 acetyl co that need to go through the Krebs Cycle. And so this here is just showing for one revolution for one Acetyl CoA, but we need to remember that there's actually going to be 2 revolutions of the citric, or of the Krebs cycle. And so there's gonna be 1 revolution per Acetyl CoA. So when we consider 2 revolution per Acetyl CoA. So when we consider 2 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 2 FADH2s produced, a total of 2 ATPs that are produced, a total of 6 NADHs that are produced, and a total of 4 c o twos that are produced, and those c o twos are 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 a Krebs fan company. You can think about a fan company. And so when you think about a fan company, what you'll notice is that the f in fan is for the f in FADH 2. The a in fan is for the A in ATP, and the N in FAN is for the N in NADH, And then, of course, the company part, the c in company, is for the c in CO2. 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, this is a number that associates with, each of the products. And so the number is 2264. And so if you can remember 2264 and, FANCO or FANC, then you'll be able to remember the, 4 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 a bluish color. So 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 1 revolution, all you need to do is cut these numbers here in half. And so, this here really Krebs cycle. And, one thing that's important to note is that all of the f a d h twos 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 ETC. And so really, this is the next phase that we're gonna be talking about is, the, electron transport chain. So we'll be following these electron carriers. And the c o 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.
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example
Krebs Cycle Example 1
Video duration:
1m
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Video transcript
So here we have an example problem that's asking how many turns of the Krebs Cycle are needed to completely break down just one molecule of glucose. And we've got these 5 potential answer options down below. And so what we need to recall from our previous lesson videos is that for every one molecule of glucose it's going to undergo glycolysis. And so the one molecule of glucose will undergo glycolysis and be broken up into 2 molecules of pyruvate. And then those 2 molecules of pyruvate undergo pyruvate oxidation and get converted into, acetyl co a molecules. And so we have 2 acetyl co a molecules, and each of those acetyl co a molecules has to undergo the Krebs Cycle revolution. And so what that means is that from every one molecule of glucose, there's actually going to be 2 turns or revolutions of the krebs cycle. 1 for each of the acetyl CoA molecules that are produced, and so the correct answer here for this example problem is going to be option a, which says 2 turns of the Krebs cycle are needed to completely break down just 1 molecule of glucose. And so this here concludes this example problem and I'll see you all in our next video.
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Problem
Problem
Which product of the Krebs cycle is also used as a reactant in the Krebs cycle?
A
Citrate.
B
ATP.
C
Acetyl-CoA.
D
Oxaloacetate.
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Problem
Problem
Taking one molecule of glucose through glycolysis, pyruvate oxidation, and the Krebs cycle generates: