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Lactic acid fermentation

Glycolysis breaks down glucose into two pyruvate molecules, producing ATP and reducing NAD to NADH. If oxygen is unavailable, lactic acid fermentation occurs, recycling pyruvate and NADH to produce more NAD+ for glycolysis. This process is common in organisms like lactobacillus, which is used in making yogurt, kimchi, and sauerkraut. Created by Sal Khan.

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Video transcript

- When we first learned about glycolysis, we saw that if you start with a molecule of glucose, and you carry forward with glycolysis, that glucose, which is a six carbon sugar, it's got oxygens and hydrogens as well, but it's a six carbon sugar, it gets split into two pyruvate molecules, and each pyruvate has three carbons. And in the process of doing so, we're able to produce a net of two ATPs. We use two ATPs in the investment phase, then we produce four ATPs in the payoff phase for a net of two ATPs. But that's not all that happens. You also have two NAD molecules, nicotinamide adenine dinucleotide, getting reduced to NADH. And why is it getting reduced? Well we see it's a positive, it's a positive molecule here, it becomes a neutral molecule, it gains electrons. So this over here becomes reduced. Now, the next question you might have is, "Well what happens next?" Well if you're me or you, you might continue on and if you have enough oxygen you'll continue on with cellular respiration. Things move on to the mitochondria because the pyruvates and the NADHs can be can also be used to produce more ATP. The pyruvate gets more broken down in the Krebs cycle, also known as the citric acid cycle. And also and that produces ATPs and NADHs and the NADHs can participate in the electron transport chain which eventually leads to even more ATPs being produced. But what if you're in a situation that maybe we don't have oxygen, or maybe you're just like, you're just the type of organism that doesn't like to use oxygen or doesn't know how to use oxygen, what happens next? Well what we're gonna talk about in this video is one potential pathway, and that's lactic acid fermentation, which is one of the two major forms of fermentation. Lactic acid fermentation. Fermen, lactic acid fermenation. And lactic acid fermentation isn't so much about producing more ATPs, it's more about recycling the pyruvate and the NADH. Even though the pyruvate and NADH can, it has free energy to give that could be converted to ATP, if we're gonna be doing lactic acid fermentation, we kind of give up on that, and then we actually use the pyruvate to oxidize the NADH to become NAD+ so that we have more NAD+ for glycolysis to occur again. So organisms that do fermentation, their main energy source is the glycolysis, and then the fermentation is all about recycling what it views as waste materials, pyruvate and NADH, so that you can have more NAD to have glycolysis occurring again. Now, you might say, "Oh is this some strange thing "that we don't encounter much in life?" Probably everyday or maybe at least every week, you probably consume some organisms that perform lactic acid fermentation. This right over here, this is a picture of yogurt. Yogurt is what we get when you have species of lactobacillus digesting the sugars in the milk and then they're performing glycolysis and then they perform lactic acid fermentation, converting the pyruvate into lactate. Or if you view the conjugate acid version of it, lactic acid, you could say pyruvic acid, lactic acid. Pyruvate is the conjugate base for pyruvic acid. Lactate is the conjugate base for lactic acid. But that's what's giving it its uniquely yogurt taste, it's this bacteria here, the lactobacillus, this is just one variation of it, and there's slightly different variations of lactobacillus that do each of these foods. But this is yogurt, this right over here, if you're into Korean food, this is kimchi, this uses a variation of lactobacillus to once again perform lactic acid fermentation on the sugars in the vegetables. This is sauerkraut, once again, a variation of lactobacillus, a species of lactobacillus performing lactic acid fermentation on the sugars in the cabbage. Sauerkraut literally means, "sour cabbage." That's what it is. So let's think a little bit more about what's going on. So as I mentioned, it's all about taking your pyruvate or pyruvic acid, the way I've drawn it right over here, this is pyruvic acid. Pyruvic, pyruvic acid, right over here. Because we have our hydrogen proton, if we lose our hydrogen proton, this is the same thing drawn again, but now we don't have the hydrogen proton here, this oxygen kept that electron, and all of the other hydrogens, all the hydrogens here, they are implicit. So the three hydrogens here, they're implicit on this carbon. I'm just drawing it with a different notation. And so this one, where we've lost the proton, we would call this pyruvate. Pyruvate. And what we have happening is that the pyruvic acid, or the pyruvate is used to oxidize the NADH, take away a hydride, take away an electron from, actually more than just electron but net, you have the NADH losing electrons. And so if it's losing electrons, it's getting oxidized. So it's oxidized. So that the NAD+ can be reused in glycolysis. And when pyruvic acid does this to the NADH, it gets reduced, it gets reduced, it gets reduced, it gains electrons and, if we're thinking about the acid forms, it would turn into this right over here is lactic acid. Lactic, lactic, lactic acid. And that's why we call it lactic acid fermentation, 'cause you're taking that pyruvate, if you had oxygen around, or if you knew how to do it, use the oxygen, you might continue on with cellular respiration and use that for energy. But lactic acid fermentation, we use it to oxidize the NADH so we get more NAD+. And let's just now get a better appreciation for all of this happens. So the first thing that I want to show you because a lot of times in biology classes, you just learn NAD, NADH, and it just seems like this somewhat abstract molecule. But this is a picture of it. This is nicotinamide adenine dinucleotide. And it's kind of a mouthful, but when you break it down, you see these patterns that you see repeatedly in biology. This is, this right over here is what gives us the nicotinamide. This right over here is our good friend adenine. We see that in ATP, we see that also as one of the nitrogenous bases in DNA and RNA. You have ribose right over here, this is derived from ribose. You have a phosphate group, you have a phosphate group. So, nicotinamide, adenine, you have a nucleotide right over there, you have another nucleotide right over there, so it's nicotine adenine dinucleotide. So the name makes a lot of sense. But I wanted to show this to you to get an appreciation that it's a fairly involved molecule over here. You know sometimes when you just see the letters, NAD, you don't get a full appreciation for it. And it's a coenzyme, and we learned about coenzymes in other videos. Where the enzyme that catalyzes this is lactic acid dehydrogenase. And remember enzymes are for the most part just these big protein structures, so all folded up in all different ways. And then you have the NAD, the NAD, or in the case of lactic acid fermentation right over here, you have the NADH, so this is the NADH right over here, and I'm just kind of drawing what it could look like. This isn't actually what it looks like. It's going to react with the pyruvate. And let me do that in a... It's going to react with the pyruvate and by doing so, even though the pyruvate you might formally consider to be the substrate of the enzyme, the whole purpose is to get your NADH to be oxidized, to lose a hydrogen and an extra electron. So a hydrogen proton, a hydrogen electron, and another electron. So really lose a hydride. So how does that happen? Well it happens because you have this nitrogen here, it has an extra lone pair of electrons, so it can form, that lone pair can form a double bond right over there. Well if that carbon, if this now has a double bond, this carbon has to let go of this double bond, so that goes over there. That goes over there. And then if there's now a double bond over here, is which we see in the end product, this carbon's gonna have to let go of a bond. And it lets go of the entire covalent bond with this hydrogen, so both of the electrons. So it's gonna let go of both of these electrons, right over here. And then both of those electrons can attach to this carbon right over there. Now that carbon forms a new covalent bond, it has to let go of one of its covalent bonds and so it could let of one of these double bonds with this oxygen. And those could either go back to that oxygen, or more likely, they can be used to grab a hydrogen proton maybe from a passing water molecule or hydronium molecule. And so, I can draw it like this. Could grab a hydrogen, a hydrogen proton. And so what do we end up with? Well, that lone pair of electrons is now there, we now have a double bond right over there, now we've lost one of these hydrogens, I obviously haven't drawn all of the hydrogens in these molecules. So now we have, now we are back to NAD. And since this nitrogen has essentially, it was neutral before, but now it is instead of keeping these two electrons, it's sharing these two electrons, so now it's going to have a positive charge. So this is why we call it NAD+. It lost a hydrogen and an electron. A hydrogen, the hydrogen's electron, and another electron so now it has a positive charge. And the pyruvate is now the conjugate, with the way I've drawn it here, since we've, I show it deprotonated, I would say that this is now lactate. This is now lactate. If we had our proton over here, we would call it lactic acid. So anyway, hopefully you get a kick out of this, I know I do, it's kind of interesting that all of this can happen. Lactobacillus isn't the only organism that does this, but this is a fairly useful organism for all sorts of delicious food that we have. I just get an appreciation, my mind's always blown that these fairly complex processes are constantly occurring in nature all around us, sometimes even in our own bodies. And these organisms that we would consider quite small, for example, this lactobacillus right over here, this is on the order of five to 10 micrometers. Five micrometers, so five millionths of a meter.