Krebs Cycle - Video Tutorials & Practice Problems

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. The citric acid cycle and the TCA cycle are synonyms for the same exact cycle, the Krebs cycle. That's 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 be referring to this as the Krebs cycle.

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. Already in our course, we've 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 the reactions of the Krebs cycle can be grouped into three phases, labeled as phase a, phase b, and phase c. In the first phase, phase a, we've titled it Acetyl CoA Entry. Here, the two carbon atoms of the Acetyl CoA molecules enter the Krebs cycle and react with a molecule called Oxaloacetate, which must be present inside the mitochondria. When the two carbons 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 first molecule produced. It is crucial to note that the CoA portion of Acetyl CoA does not enter the Krebs Cycle; only the two carbon atoms do.

Looking at our image below of the Krebs Cycle, we focus first on the left-hand side. Pyruvate oxidation produces two Acetyl CoA molecules, but we examine one Acetyl CoA molecule at a time. At the top, we have the Acetyl CoA molecule, consisting of two carbon atoms and the CoA part. The CoA portion does not enter the Krebs Cycle; rather, it is recycled and returns to be part of another pyruvate oxidation reaction. The two carbons from the Acetyl CoA react with Oxaloacetate (which has four carbons), producing a six-carbon molecule once again called citrate. Thus, phase a creates citrate.

Moving to the second phase, phase b, entitled Citrate Oxidation, involves the rearrangement and oxidation of citrate, resulting in the production of one ATP via substrate-level phosphorylation, two NADH molecules, and two carbon dioxide molecules. During Citrate oxidation in phase b, it produces one ATP molecule, two NADH molecules, and two carbon dioxide molecules.

The final phase, phase c, is Oxaloacetate Regeneration. Since Oxaloacetate is one of the starting molecules that reacted to form citrate, for the Krebs Cycle to complete its cycle, it must start and end at the same place; hence, phase c is dedicated to regeneration. Phase c involves continuing the oxidation process and generates one NADH and one FADH₂ molecule, regenerating the starting molecule, Oxaloacetate.

Importantly, it requires two rounds of the Krebs cycle for each glucose molecule entering the cell, as one glucose molecule splits into two pyruvate, which are converted into two Acetyl CoA. Thus, there are two revolutions of the Krebs Cycle per glucose molecule. The total output for both revolutions includes 2 FADH₂s, 2 ATPs, 6 NADHs, and 4 CO₂s, which are ultimately exhaled. A mnemonic to remember this is "Krebs Fan Company," where 'FAN' stands for FADH₂, ATP, NADH, and 'company' for CO₂. The numbers 2264 correspond to the quantities of each product. Knowing the total output numbers, one revolution’s products can be calculated by halving these values.

This concludes our introduction to the phases of the Krebs cycle. As we progress, we will apply these concepts more practically. I'll see you all in our next video where we’ll discuss the electron transport chain, following these electron carriers. The CO₂ molecules, being exhaled, will not be tracked further.

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 five 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.

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.