Krebs Cycle - Online Tutor, Practice Problems & Exam Prep

The Krebs Cycle, also known as the citric acid cycle or TCA cycle, is the third stage of aerobic cellular respiration. It oxidizes Acetyl CoA, producing energy in the form of ATP, NADH, and FADH₂, while releasing CO₂. The cycle consists of three phases: Acetyl CoA Entry, Citrate Oxidation, and Oxaloacetate Regeneration. Each glucose molecule results in two rounds of the cycle, yielding a total of 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂. Understanding these outputs is crucial for grasping cellular respiration's energy production.

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 also 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, and that's because your professor or your textbooks might use a different term to refer to the same exact cycle. 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. Already in our course, we've covered glycolysis, which occurs outside of the mitochondria, producing 2 pyruvates. And, 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, titled 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 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 important to note that the CoA portion of Acetyl CoA does not enter the Krebs Cycle; it's just the two carbon atoms. If we look at our image below of the Krebs Cycle, on the left-hand side, we focus on one Acetyl CoA molecule at a time. Notice that the CoA portion does not enter the Krebs Cycle, but it gets recycled and goes back to be part of another pyruvate oxidation reaction; only the two carbons enter the Krebs Cycle and react with a molecule called oxaloacetate.

These two carbons react with the four carbons of oxaloacetate, creating a six-carbon molecule once again called citrate. This is why the Krebs cycle is also commonly referred to as the citric acid cycle because citrate is the very first molecule produced in the cycle. Moving on to the second phase of the Krebs cycle, we have phase b, which is citrate oxidation. Citrate loses electrons and becomes oxidized. Citrate oxidation involves the rearrangement and oxidation of citrate, ultimately producing a bit of ATP, one ATP via substrate level phosphorylation, two NADH molecules, and two carbon dioxide molecules or \( \text{CO}_2 \) molecules.

In phase c, Oxaloacetate regeneration, the cycle needs to start and end at the same place. Oxaloacetate regeneration involves continuing the oxidation process, ultimately producing one NADH and one FADH₂ molecule, and regenerating the starting molecule, oxaloacetate. Each round of the Krebs cycle is necessary for Acetyl CoA. Two rounds of the Krebs cycle occur for every one glucose molecule that enters, split into two pyruvate, which gets converted into two acetyl CoA. Each revolution of the Krebs cycle yields certain products which are then doubled reflecting the two rounds needed per glucose molecule.

The total output from the Krebs cycle for one glucose molecule involves two FADH₂s, two ATPs, six NADHs, and four \( \text{CO}_2 \) molecules, all of which except for the \( \text{CO}_2 \) will proceed to the next stage of aerobic cellular respiration, the Electron Transport Chain. The \( \text{CO}_2 \) molecules are exhaled. A mnemonic to remember the outputs is "Krebs Fan Company," where each letter signifies the compounds and their relationships. This memory tool and the color-coordinated aspects are crucial for understanding and remembering the intricate Krebs cycle.

This concludes our introduction to the phases of the Krebs cycle, and we'll continue to 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 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. One for each of the acetyl co a 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 one 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.

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.

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 oxidizing Acetyl CoA to produce energy. This cycle generates ATP, NADH, and FADH₂, which are essential for the electron transport chain, the final stage of cellular respiration. Additionally, it releases CO₂ as a byproduct. The energy carriers NADH and FADH₂ produced in the Krebs Cycle are vital for generating a significant amount of ATP in the electron transport chain, making this cycle essential for energy production in cells.

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 Acetyl CoA Entry phase, Acetyl CoA combines with oxaloacetate to form citrate. During Citrate Oxidation, citrate undergoes a series of reactions that produce ATP, NADH, and CO₂. Finally, in the Oxaloacetate Regeneration phase, the cycle regenerates oxaloacetate, producing NADH and FADH₂. These phases ensure the continuous production of energy carriers and the regeneration of oxaloacetate for the cycle to repeat.

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

For each glucose molecule, the Krebs Cycle completes two rounds, as one glucose molecule produces two Acetyl CoA molecules. In total, the Krebs Cycle produces 2 ATP, 6 NADH, and 2 FADH₂ molecules per glucose molecule. Additionally, it releases 4 CO₂ molecules as byproducts. These energy carriers (NADH and FADH₂) are crucial for the electron transport chain, where they contribute to the production of a significant amount of ATP.

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

The Krebs Cycle is also called 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 the initial product and emphasizes the cycle's role in cellular respiration. Understanding this synonym is crucial, as different textbooks and professors may use either term interchangeably.

What is the role of oxaloacetate in the Krebs Cycle?

Oxaloacetate plays a critical role in the Krebs Cycle by reacting with Acetyl CoA to form citrate, the first molecule in 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, as it allows the cycle to start and end at the same point, maintaining a steady supply of oxaloacetate for ongoing reactions.