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 glucose molecule results in two revolutions of the cycle, producing 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂. These products are crucial for the subsequent electron transport chain, where oxidative phosphorylation occurs.

Krebs Cycle

In this video, we're going to begin our introduction to the Krebs Cycle. 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 referred to as the TCA cycle. It's important to know that the citric acid cycle and the TCA cycle are synonyms for the same exact cycle, the Krebs cycle. This is because your professor or your textbooks might use different terms to refer to the same exact cycle. In our course, we're mainly going to refer 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. Notice that we're showing you the image for all four stages of aerobic cellular respiration below. In our course, we've already covered glycolysis, which occurs outside the mitochondria, producing 2 pyruvates. We also covered pyruvate oxidation, which converts those pyruvates into molecules of Acetyl CoA. This is where the Krebs cycle is going to pick up, right where we left off with pyruvate oxidation. 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 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.

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 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 are actually going to be 2 turns or revolutions of the Krebs cycle. One 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 products are essential for the electron transport chain, where most of the cell's ATP is generated. The cycle also produces CO₂ as a byproduct, which is exhaled. Understanding the Krebs Cycle is vital for comprehending 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 NADH and FADH₂. These phases ensure the continuous operation of the cycle, allowing for efficient energy production.

How many ATP, NADH, and FADH₂ molecules are produced per glucose molecule in the Krebs Cycle?

For each glucose molecule, the Krebs Cycle produces 2 ATP, 6 NADH, and 2 FADH₂. This is because one glucose molecule generates two Acetyl CoA molecules, each undergoing one revolution of the cycle. These products are crucial for the electron transport chain, where they contribute to the generation of a significant amount of ATP through oxidative phosphorylation.

Why is the Krebs Cycle also known as 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.

What is the role of oxaloacetate in the Krebs Cycle?

Oxaloacetate plays a crucial role in the Krebs Cycle by combining with Acetyl CoA to form citrate, initiating the cycle. It is also regenerated in the final phase of the cycle, ensuring that the cycle can continue. This regeneration is essential for the cycle's continuity and for maintaining the balance of intermediates required for efficient energy production.