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Alcohol or ethanol fermentation

Glycolysis breaks down glucose into two pyruvate molecules, producing ATP and reducing NAD+ to NADH. When oxygen isn't available, organisms can use lactic acid fermentation or alcohol fermentation to recycle NADH back to NAD+. In alcohol fermentation, pyruvate loses a carboxyl group, becoming acetaldehyde, which is then reduced to ethanol. This process, performed by yeast, is used in bread and alcohol production. Created by Sal Khan.

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Video transcript

- [Voiceover] We've already seen multiple times that glycolysis is the process where we start with a glucose molecule which has six carbons and we're able to break it down into two pyruvate molecules which each have three carbons. And in the process of doing so, we produce a net of two ATPs, we reduce NAD+ to NADH, we're adding a hydride anion, so this is reduction, this is reduction going on right over there and then we say, "Well, what happens?" and, of course, we have the water, some water produced and some hydrogen ions. But then the next question is, well what happens next? And we could talk about cellular respiration and that's what will happen in many organisms including... It's happening in my body right now in order for me to generate as much ATP as possible. That's why I inhale oxygen. Because that oxygen is needed for cellular respiration. But sometimes there isn't oxygen around or I'm the type of organism that just doesn't use oxygen or doesn't conduct cellular respiration. Now we've already seen one example of that and that was lactic acid fermentation. It's a way of recycling these NADH's, oxidizing them back. Oxidizing them back to NAD+. So this oxidation, this oxidation from NADH to NAD+, this is what we do in fermentation. Actually, both variations of fermentation that we're going to talk about. And lactic acid fermentation, we talked about taking the pyruvates to oxidize the NADH and in doing so the pyruvate turns into lactic acid and that's used in yogurt production, in sauerkraut, it's also used in your muscles, when your muscles run out of oxygen. If you're kind of sprinting or your lifting some really really really heavy weights, you probably have some lactic acid fermentation going on. But now we're gonna talk about another type of fermentation and that is alcohol fermentation. Alcohol. Alcohol or sometimes called ethanol fermentation. Ethanol fermentation. Because when we talk about drinking alcohol, we're talking about ethanol. Ethanol fermentation. And as you might guess, this is the process by which alcohol is produced and things like bread. It's used in baking a lot. But let's first think about the reaction and then we'll talk about maybe where you might have encountered it last. So we saw that pyruvate... When we finish glycolysis, we have, for each molecule of glucose, we have two molecules of pyruvate and so each of these molecules of pyruvate... The first step is they lose their carboxyl group right over here and that's facilitated by pyruvate decarboxylase. This name is all about getting rid of a pyruvate's carboxyl group. And you can see, when you remove a carboxyl group, it's a carbon and two oxygens, that is going to be, and I'm not showing you the detailed mechanism here, but that's going to be a carbon dioxide that is released and then what's leftover, what's leftover, and once again I'm not showing you the detailed mechanism, but you can account at least for the various atoms, what's leftover is acetaldehyde. Acetaldehyde, I have trouble saying that. And then the next step, the acetaldehyde can be reduced. One way you can think about it, you can say it's reduced by the NADH to turn into ethanol. It's a similar mechanism to what we saw in lactic acid fermentation where we went from pyruvate to lactic acid, but now it's acetaldehyde going to ethanol. So this right over here, this is, this is reduced. Reduced. It gains a hydride. One way to think about it is it's gaining a hydride anion and in the process that this is being reduced, the NADH is being oxidized. So this, this is being oxidized. And the whole point of this, the whole reason why it occurs is so that you can oxidize the NADH to have more fresh NAD+ for glycolysis to occur again. And as I mentioned in the video on lactic acid fermentation, it's a little bit of a shame, because the pyruvate still has energy into it. If you had oxygen around, you could have cellular respiration, you could go into the Krebs cycle, the citric acid cycle, and derive more energy from it. The NADH also, the process of oxidizing it, in theory, you can use it to generate more energy. But we're not doing that when we do either type of fermentation whether ethanol fermentation or we're talking about lactic acid fermentation. But this is a process that has proven very useful for human civilization. This right over here, the most famous actor when we're talking about ethanol fermentation or alcohol fermentation is this character right over here. This is a yeast cell. Yeasts are categorized as fungi. That is a yeast cell. It's a handful of micometers in diameter, although they can vary. And variations of yeast are used in things like bread making and wine making or alcohol production. Beer, whatever you wanna talk about. Because what the yeast does, it uses, it digests the sugar, it performs glycolysis and then it performs alcohol fermentation. What makes bread so fluffy is this step right over here. When the carboxyl group gets stripped off of the pyruvate facilitated by the pyruvate decarboxylase, and the carbon dioxide gets released, it makes the bread fluffy. It bubbles through the dough and gives it its nice spongy flavor. But you also have ethanol being produced. So it's actually an interesting fact that a lot of bread... You know, I've been reading up on it, a lot of the ethanol might get baked off, but bread will usually have trace amounts, trace amounts of alcohol in it. So bread will have trace amounts, trace amounts of ethanol in it because the whole process, you're using yeast to leaven the bread. To give it this fluffiness, that yeast is performing glycolysis and alcohol fermentation is producing ethanol and in the process the carbon dioxide that gives the fluffiness but there's still going to be some trace amounts of ethanol. Now, of course, if you're talking about wine production, your goal is to get to the actual ethanol which we view as traditional drinking alcohol. So yeast plays a fairly important role in our society.