So this is where the oxygen or the carbons, or the carbon dioxide actually gets formed. And similarly, when these carbons get cleaved off, it forms CO2. And actually, for every molecule of glucose you have six carbons. When you do this whole process once, you are generating three molecules of carbon dioxide. But you're going to do it twice. You're going to have six carbon dioxides produced.
Which accounts for all of the carbons. You get rid of three carbons for every turn of this. Well, two for every turn. But really, for the steps after glycolysis you get rid of three carbons. But you're going to do it for each of the pyruvates. You're going to get rid of all six carbons, which will have to exhale eventually. But this cycle, it doesn't just generate carbons. So we'll write that here.
And this is a huge simplification. I'll show you the detailed picture in a second. We'll do it again. And of course, these are in separate steps. There's intermediate compounds. I'll show you those in a second. It will produce some ATP. And the whole reason why we even pay attention to these, you might think, hey cellular respiration is all about ATP. The reason why we care is that these are the inputs into the electron transport chain.
These get oxidized, or they lose their hydrogens in the electron transport chain, and that's where the bulk of the ATP is actually produced. And then maybe we'll have another NAD get reduced, or gain in hydrogen. Reduction is gaining an electron. Or gaining a hydrogen whose electron you can hog. And then we end up back at oxaloacetic acid. And we can perform the whole citric acid cycle over again.
So now that we've written it all out, let's account for what we have. So depending on-- let me draw some dividing lines so we know what's what. So this right here, everything to the left of that line right there is glycolysis.
We learned that already. And then most-- especially introductory-- textbooks will give the Krebs cycle credit for this pyruvate oxidation, but that's really a preparatory stage.
The Krebs cycle is really formally this part where you start with acetyl-CoA, you merge it with oxaloacetic acid. And then you go and you form citric acid, which essentially gets oxidized and produces all of these things that will need to either directly produce ATP or will do it indirectly in the electron transport chain.
But let's account for everything that we have. Let's account for everything that we have so far. We already accounted for the glycolysis right there. Now, in the citric acid cycle, or in the Krebs cycle, well first we have our pyruvate oxidation. That produced one NADH. But remember, if we want to say, what are we producing for every glucose? This is what we produced for each of the pyruvates.
This NADH was from just this pyruvate. But glycolysis produced two pyruvates. So everything after this, we're going to multiply by two for every molecule of glucose. And then when we look at this side, the formal Krebs cycle, what do we get?
We have, how many NADHs? One, two, three NADHs. So three NADHs times two, because we're going to perform this cycle for each of the pyruvates produced from glycolysis.
So that gives us six NADHs. We have one ATP per turn of the cycle. That's going to happen twice. Once for each pyruvic acid. So we get two ATPs. And then we have one FADH2.
But it's good, we're going to do this cycle twice. This is per cycle. So times two. We have two FADHs. So sometimes instead of having this intermediate step, they'll just write four NADHs right here. And you'll do it twice. Once for each puruvate. But the reality is, six from the Krebs cycle two from the preparatory stage. Now the interesting thing is we can account whether we get to the 38 ATPs promised by cellular respiration.
So we have four ATPs. Four ATPs. How many NADHs do we have? We have 10 NADHs. And then we have 2 FADH2s. It should be FADH2, just to be particular about things. And these, so you might say, hey, where are our 38 ATPs? We only have four ATPs right now. But these are actually the inputs in the electron transport chain. These molecules right here get oxidized in the electron transport chain. So two of them are going to produce four ATPs in the electron transport chain. So we now see, we get four from just what we've done so far.
Glycolysis, the preparatory stage and the Krebs or citric acid cycle. And then eventually, these outputs from glycolysis and the citric acid cycle, when they get into the electron transport chain, are going to produce another So 34 plus 4, it does get us to the promised 38 ATP that you would expect in a super-efficient cell.
This is kind of your theoretical maximum. In most cells they really don't get quite there. But this is a good number to know if you're going to take the AP bio test or in most introductory biology courses.
There's one other point I want to make here. Everything we've talked about so far, this is carbohydrate metabolism. Or sugar catabolism, we could call it. We're breaking down sugars to produce ATP. Glucose was our starting point. But animals, including us, we can catabolize other things. We can catabolize proteins. We can catabolize fats. If you have any fat on your body, you have energy. In theory, your body should be able to take that fat and you should be able to do things with that.
You should be able to generate ATP. And the interesting thing, the reason why I bring it up here, is obviously glycolysis is of no use to these things. Although fats can be turned into glucose in the liver. But the interesting thing is that the Krebs cycle is the entry point for these other catabolic mechanisms. Proteins can be broken down into amino acids, which can be broken down into acetyl-CoA. Fats can be turned into glucose, which actually could then go the whole cellular respiration.
But the big picture here is acetyl-CoA is the general catabolic intermediary that can then enter the Krebs cycle and generate ATP regardless of whether our fuel is carbohydrates, sugars, proteins or fats.
Now, we have a good sense of how everything works out right now, I think. Now I'm going to show you a diagram that you might see in your biology textbook. Or I'll actually show you the actual diagram from Wikipedia. I just want to show you, this looks very daunting and very confusing. And I think that's why many of us have trouble with cellular respiration initially.
Because there's just so much information. It's hard to process what's important. But I want to just highlight the important steps here.
Just so you see it's the same thing that we talked about. From glycolysis you produce two pyruvates. That's the pyruvate right there. They actually show its molecular structure. This is the pyruvate oxidation step that I talked about. The preparatory step. And you see we produce a carbon dioxide. Then we're ready to enter the Krebs cycle. The acetyl-CoA and the oxaloacetate or oxaloacetic acid, they are reacted together to create citric acid.
They've actually drawn the molecule there. And then the citric acid is oxidized through the Krebs cycle right there. All of these steps, each of these steps are facilitated by enzymes. And it gets oxidized. But I want to highlight the interesting parts. So, so far, if you include the preparatory step, we've had four NADHs formed, three directly from the Krebs cycle.
That's just what I told you. Now we have, in this diagram they say GDP. Also during the Krebs cycle, the two carbon atoms of acetyl-CoA are released, and each forms a carbon dioxide molecule. Thus, for each acetyl-CoA entering the cycle, two carbon dioxide molecules are formed.
Two acetyl-CoA molecules enter the cycle, and each has two carbon atoms, so four carbon dioxide molecules will form. Add these four molecules to the two carbon dioxide molecules formed in the conversion of pyruvic acid to acetyl-CoA, and it adds up to six carbon dioxide molecules. These six CO 2 molecules are given off as waste gas in the Krebs cycle. They represent the six carbons of glucose that originally entered the process of glycolysis.
At the end of the Krebs cycle, the final product is oxaloacetic acid. This is identical to the oxaloacetic acid that begins the cycle.
Now the molecule is ready to accept another acetyl-CoA molecule to begin another turn of the cycle. Previous Quiz Glycolysis. Next Quiz Krebs Cycle. Removing book from your Reading List will also remove any bookmarked pages associated with this title. Are you sure you want to remove bookConfirmation and any corresponding bookmarks? My Preferences My Reading List. Krebs Cycle.
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