Krebs Cycle: Study with Video Lessons, Practice Problems & Examples

The Krebs Cycle, also known as the citric acid cycle or TCA cycle, is the third stage of aerobic cellular respiration. It begins with Acetyl CoA entering the cycle, combining with oxaloacetate to form citrate. The cycle consists of three phases: Acetyl CoA Entry, citrate oxidation, and oxaloacetate regeneration. Each complete cycle produces 2 CO₂, 1 ATP, 3 NADH, and 1 FADH₂. For every glucose molecule, two rounds of the cycle occur, yielding a total of 4 CO₂, 2 ATP, 6 NADH, and 2 FADH₂.

Krebs Cycle

In this video, we're going to begin our introduction to the Krebs Cycle. And so the Krebs Cycle is the third 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 FADH₂. 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.

Phases of The Krebs Cycle

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.

Krebs Cycle Example 1

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 CoA molecules. And so we have 2 acetyl CoA molecules, and each of those acetyl CoA 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.

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.

• A. Citrate.
• B. ATP.
• C. Acetyl-CoA.
• D. Oxaloacetate.

Taking one molecule of glucose through glycolysis, pyruvate oxidation, and the Krebs cycle generates:

a) 6 CO₂, 8 NADH, 2 FADH₂ and 4 ATP.

b) 6 CO₂, 8 NADH, 1 FADH₂ and 2 ATP. 

c)  6 CO₂, 10 NADH, 2 FADH₂ and 4 ATP.  

d) 6 CO₂, 10 NADH, 2 FADH₂ and 2 ATP.

• A. 6 CO₂, 8 NADH, 2 FADH₂ and 4 ATP.
• B. 6 CO₂, 8 NADH, 1 FADH₂ and 2 ATP.
• C. 6 CO₂, 10 NADH, 2 FADH₂ and 4 ATP.
• D. 6 CO₂, 10 NADH, 2 FADH₂ and 2 ATP.

Here’s what students ask on this topic:

What is the Krebs Cycle and why is it important in cellular respiration?

The Krebs Cycle, also known as the citric acid cycle or TCA cycle, is the third stage of aerobic cellular respiration. It plays a crucial role in energy production by oxidizing Acetyl CoA to produce ATP, NADH, and FADH₂. These molecules are essential for the electron transport chain, which generates the majority of ATP in cellular respiration. The cycle also produces CO₂ as a byproduct, which is exhaled. Understanding the Krebs Cycle is vital for grasping how cells convert nutrients into usable energy.

What are the main phases of the Krebs Cycle?

The Krebs Cycle consists of three main phases: Acetyl CoA Entry, Citrate Oxidation, and Oxaloacetate Regeneration. In the first phase, Acetyl CoA combines with oxaloacetate to form citrate. The second phase involves the oxidation of citrate, producing ATP, NADH, and CO₂. The final phase regenerates oxaloacetate, producing additional NADH and FADH₂. These phases ensure the continuous production of energy carriers and the regeneration of cycle intermediates.

What are the key outputs of the Krebs Cycle per glucose molecule?

For each glucose molecule, the Krebs Cycle produces 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂. These outputs are crucial for the electron transport chain, where NADH and FADH₂ are used to generate a significant amount of ATP. The CO₂ produced is expelled from the body through exhalation. These outputs highlight the Krebs Cycle's role in cellular energy metabolism.

Why is the Krebs Cycle also called the citric acid cycle?

The Krebs Cycle is also known as the citric acid cycle because the first molecule produced in the cycle is citrate, or citric acid. When Acetyl CoA enters the cycle, it combines with oxaloacetate to form citrate. This naming highlights the importance of citrate as a key intermediate in the cycle's series of reactions.

How does the Krebs Cycle contribute to the electron transport chain?

The Krebs Cycle contributes to the electron transport chain by producing NADH and FADH₂, which are electron carriers. These molecules transport high-energy electrons to the electron transport chain, where they are used to generate a large amount of ATP through oxidative phosphorylation. This connection underscores the Krebs Cycle's role in the overall process of cellular respiration and energy production.