- [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.