- [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.