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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- At time3:00, how come when pyruvate loses a carboxyl group to form acetaldehyde , there is an extra hydrogen atom on the acetaldehyde? Where does this hydrogen come from?(19 votes)
- Good question. This hydrogen most likely comes from the water in the cytosol, where glycolysis/fermentation is performed. Remember that there are always small amounts hydrogen protons and hydronium ions (hydrogen donators) in an aqueous (water-based) solution. :)(24 votes)
- Why don't mammals have alcoholic fermentation?
Why lactate fermentation, instead?
What's wrong going the other way round?(10 votes)
- Willson, Alcoholic fermentation produces ethanol. Ethanol produces more adverse effects in smaller concentrations than lactic acid. Instead of ethanol, lactic acid is produced, which is far less harmful to muscles and is easily converted into sugar in the liver. Maybe it works for other organisms, but due to how our other systems function, it's more harmful than efficient, so lactic acid was the more evolutionarily favored option for vertebrates.
Hope this helped!(20 votes)
- Where does the pyruvate gain a hydride molecule in place of the carboxyl group to become acetaldehyde?(7 votes)
- Thiamine diphosphate (ThDP) is an important cofactor in the pyruvate, acetaldehyde reaction. ThDP binds the substrate during the first step of the reaction at C2 of the pyruvate. It is this ThDP that changes the environment of the active site which leads to the protonation or deprotonation of Glu473.
And that hydride is present in the cell so protonation can occur.
This paper can give you an overview:
There is also another pathway via Acetyl-Coa which includes oxidation-reduction of Ferredoxin.(5 votes)
- what is pyruvate molecule?(5 votes)
- Pyruvate is an intermediate in metabolic reactions which can be used to start the process of fermentation when oxygen is not available or Krebs Cycle when aerobic respiration is available.(6 votes)
- Which organisms produce lactic acid? Which organisms produce alcohol?(5 votes)
- Well, humans produce lactic acid. When you have strenuous exercise, the sort of "burning" sensation you feel is the lactic acid in your bloodstream.
On the other hand, yeast produces alcohol, which can be found in wine and beer(6 votes)
- At around3:53, when NADH is being oxidized, does it lose an electron to the acetaldehyde, and thus lose the H+ to it? If so, how come in the end product of ethanol, 2 extra hydrogens are added? Is this because both NADHs are being oxidized? I guess my question is, in a process that involves one molecule of glucose that undergoes glycolysis, two NADHs are produced. Are both of these used to make one molecule of ethanol?(4 votes)
- it loses a hydride(proton plus 2 electrons)
I'm not sure but it is also possible that the carbon or oxygen grabs a hydrogen. I'm thinking this is it because there are 2 pyruvates too(4 votes)
- What's the difference between Alcohol fermentation and lactic acid fermentation ?(4 votes)
- Lactic acid fermentation happens in our muscle cells when we are exercising feverishly, while alcoholic fermentation is used in yeast cells and is what leads to beer, bread, and wine.
Different cells (organisms) and different products in the end.(5 votes)
- Can fermentation in bread occur without adding succrose? Does flour contain any sugars? What Sugars?(5 votes)
- Making breads the natural way, i.e. culturing naturally occurring yeasts and organisms with a simply flour + water starter, by all means it is absolutely possible to make yeast breads without the addition of sugar. Even before yeast comes on the scene, there's usually some of one or more types of sugar in the flour. This is mostly glucose but perhaps also some sucrose - the sugar you get in a bag from the supermarket. hope this helps(2 votes)
- so if you were to put more sugar in a loaf or bread would that mean it would get more and more fluffy the more sugar you added as the yeast has more to feed on?(4 votes)
- Hypothetically yes, but again, it depends on the capacity of yeast to grow.
Yes, we pout minimum sugar, just to make sure yeast grows - we do not want to get sweet bread as a result.
However, growing capacity depends on many criteria such as carbon dioxide, oxygen, medium, metabolic compounds other yeast cells produce, etc.
Sometimes metabolic compounds from other cells may affect the growth of our cells of interest. They may compete with each other for space and nutrients.(4 votes)
- [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.