Now the last thing I wanna consider
here is the fate of pyruvate.
Pyruvate is the end product of this pathway but we
remember that pathways don't occur an isolation.
Pyruvate goes on to something else. And so
pyruvate can go on to a variety of things
and the things that it goes into depends upon
the circumstances in the cell in which its found.
Okay. So let's look a little bit closer
to see what happens with this pathway?
As I say it's not an end point and it's important for
a variety of things. It's important to make alanine.
It's important for energy
and it's an important source
of keeping glycolysis going as we
are going to see in just a second.
Okay? Now let's consider the three different things
that pyruvate can go to that are shown on the screen.
Pyruvate in bacterial
cells and yeast cells
can be converted into a
molecule called acetaldehyde.
An acetaldehyde can be
converted into ethanol.
Beer drinkers of course like that reactions because
that reaction is known as fermentation and
its the way we brew. We make beer. We
make wine so forth with this reaction.
And we make this reaction by closing
off the vessel in which the
reaction is occurring and the loss of oxygen in
that vessel favors the production of ethanol.
Now you probably didn't think about
this that your cells that is human,
also ferment. Now fermentation
in us is different than it is
in yeast and bacteria otherwise we wouldn't
have to go out and get beer. Right?
To convert pyruvate into lactate, there is
a reason we are doing this as we will see.
But our cells when they run out
of oxygen, they make lactate.
Also known as lactic acid and that becomes important
for the production of NAD as we shall see.
You notice that in the acetaldehyde ethanol
reaction that NAD was also produced.
Now what about aerobic conditions?
Under aerobic conditions
acetyl-CoA is sent to the citric acid cycle
for oxidation. Oxidation requires
oxygen and so this make sense.
No oxygen we have alternative pathways.
We have oxygen we uses it best as we can.
Now the difference between oxidation
using the citric acid cycle
and oxidation using the fermentation
pathways is enormous.
If we start with glucose and we
go through a fermentation pathway
the net products of that reaction are 2 ATPs.
If we start with glucose and we go through
the citric cycle and oxidize it completely
we get 38 ATPs.
That's 19 times more energy under
oxygen conditions than under oxygen absence.
You can see why cells really like to have oxygen
and you can see why our body are setup
to deliver oxygen as much as it can.
It doesn't always succeed in that.
But it does as much as it can.
Okay, well why is that
fermentation is even necessary?
Well you probably didn't think about it as
I was going though describing the lecture
but I noted that glyceraldehyde-3-phosphate is
converted into 1,3-bisphosphoglycerate
in the only oxidation
reaction of glycolysis.
That reaction is important because any
oxidation requires an electron carrier
and the electron carrier used
here is NAD+ which becomes NADH.
The cell doesn't have
an infinite amount of NAD.
If it converts its NAD into NADH it has
to be able to convert NADH back to NAD
Fine and dandy. Okay? How does a cell
do that? Usually the cell does that
in a process that requires oxygen.
It's called the electron transport system. And, what
does the electron transport system require? Oxygen.
So if there is no oxygen, the electron
transport system can't function.
And what will happen to the concentration of NADH?
It will go high and the
concentration of NAD will go low.
Glycolysis can't keep running under those conditions.
But imagine you are out jogging and your blood supply
is not delivering oxygen as rapidly as you need it,
then what's going to happen in
that case? When that happens
then glycolysis is gonna stop and glycolysis is the only
thing that's providing those muscles cells with energy.
You got a problem. Well fortunately muscle cells
and yeast cells, and all these things have
ways of regenerating that NAD
and that's fermentation.
We can see here that the end
product of glycolysis is pyruvate.
So pyruvate in a bacterial system is
converted into acetaldehyde as I noted
and then acetaldehyde is converted into
ethanol. That's why the ester is making ethanol.
In the ethanol reaction, NAD+ is
produced and guess where it's used.
Back in the glycolysis reaction. So by cycling
back and forth between those two reactions,
NAD is generated, glycolysis is kept
going and the cell stays alive.
Now animal cells don't convert
pyruvate into acetaldehyde
instead they convert pyruvate into lactate or
lactic acid. You can see this reaction here
catalyzed by the lactate dehydrogenase. And the same thing
is happening here that was happening in the yeast.
It takes electrons from NADH
and transfers them onto pyruvate to make lactate.
That regenerates NAD and that NAD that's been made
goes right back to the glycolysis and keeps it going.
So we ferment for the same reasons that yeast and
bacteria do but we don't have the same end product.
In this set of talks, well I hope I have given you
is a good overview of the pathway of glycolysis
the energy needs of the cell and
how energy factors into the overall
ways in which the reaction
occurs and the overall
controls that enable the cells to
efficiently make this process happen.