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.
Krebs Cycle - Online Tutor, Practice Problems & Exam Prep
Krebs Cycle
Video transcript
Phases of The Krebs Cycle
Video transcript
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
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 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.
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.
Do you want more practice?
Your Microbiology tutor
- Use the following diagrams (a), (b), and (c) for question 1.<IMAGE>Name pathways diagrammed in parts (a)...
- Which of the following reactions produces the most molecules of ATP during aerobic metabolism?a. glucose → glu...
- Which of the following is the best definition of the Krebs cycle?a. the oxidation of pyruvic acidb. the way ce...
- _______ is a cyclic series of eight reactions involved in the catabolism of acetyl-CoA that yields eight molec...