- In this video, I want to
talk about fermentation.
Now a lot of you may be familiar
with the term fermentation
as it applies to, like,
making sauerkraut or making
beer or wine.
But fermentation, really in
a much more general sense,
just simply means making
ATP without oxygen.
And there is a particular
type of fermentation
that does actually
happen in your body
under very specialized
conditions,
and that is lactic
acid fermentation.
Lactic acid fermentation
only happens
in cells of your
skeletal muscles
when they are operating
at such a strenuous level
that the blood supply to
those muscles can't keep up.
So remember, blood is
what brings your oxygen
into your muscle cells.
And if your muscle cells are
using that oxygen more quickly
than blood can bring
oxygen in, your muscles
start converting glucose
into lactic acid, or lactate.
It's not my intention
in this video
to go into all the deep,
biochemical details
of what happens during
lactic acid fermentation,
but I want to cover it at
about the level of thinking
about what goes into
lactic acid fermentation
and what comes back out.
OK.
I've written out a quick
summary of cellular respiration
over here on the left.
Remember, in
cellular respiration,
you have glucose being
converted to pyruvate,
and this is a step
known as glycolysis.
Once that glucose is
converted to pyruvate,
the pyruvate is further
converted to acetyl CoA,
acetyl CoA goes into
the Krebs cycle,
and you have high
energy electrons
from these steps going into
the electron transport chain.
And those high energy
electrons of course
are carried on NADH and FADH2,
getting the electron transport
chain started.
So that's what happens
in cellular respiration.
You've got glycolysis,
you have the Krebs cycle,
and you have the
electron transport chain.
But here's the kicker.
The conversion
of pyruvate to acetyl
CoA and the Krebs cycle and
the electron transport chain--
they all require
oxygen. All right?
Now remember lactic
acid fermentation
happens in the
absence of oxygen,
and in the absence of
oxygen, the only thing
that can really
happen is glucose
being converted to pyruvate,
that glycolysis step.
That does not require oxygen.
So if you have no oxygen
around, glucose gets
converted to pyruvate,
and that pyruvate just
accumulates and accumulates
in the cell because
there's no oxygen
to be used to convert
it to acetyl CoA
or to start up the Krebs cycle.
Now this is-- it's
an OK process.
It generates a
little bit of ATP.
You get 2 ADP being
converted to ATP
through that glycolysis
process, the same
as glycolysis in the presence
of oxygen makes 2 ATP.
But you're generating some
high energy electrons as well.
You're making 2 NADH through
that glycolysis process,
but there's nowhere for those
high energy electrons to go.
The electron transport
chain can not operate.
So you've got all this
pyruvate accumulating,
and you're depleting the NAD+
This is a coupled reaction--
glucose to pyruvate
conversion-- that makes ATP.
That's not going to happen
unless there's some NAD+ around
in order to accept
electrons from the glucose.
Remember, this is
a redox reaction
so these reactions
have to be coupled.
If you are depleting the NAD+,
you're going to have a problem.
So your cell has this dilemma.
It's accumulating pyruvate.
It's making small amounts
of ATP in the process,
but it's running out of NAD+.
What it needs to do is it
needs to recycle the NAD+.
Now NAD is getting reduced.
It's accepting electrons so it's
getting reduced in this step.
What we need is another step
that's going to allow the NADH
to donate some electrons and be
converted back to NAD+ so that
that NAD+ can go back and
be coupled to another round
of glycolysis.
And this is where the
fermentation step comes in.
This is the
NAD+ recycling step,
and that's where the pyruvate
is converted to lactate.
You make two pyruvate
molecules out of one glucose,
and you make two
lactate molecules.
So the complete process of
lactic acid fermentation
involves this
glycolysis step, which
is the same as the first
step in cellular respiration.
But it also has
this additional step
where these two
pyruvate molecules
are converted to lactate.
And this additional step allows
NADH to lose its electrons,
be converted back to NAD+
so that the NAD+ can be used
to drive more glycolysis.
And remember, this only happens
in the absence of oxygen.
So it only happens in
your skeletal muscles
when you are exercising
so hard that you can't get
enough oxygen to your muscles.
If there is oxygen
present, this pyruvate
is just going to be
converted to acetyl CoA,
and you'll get the Krebs
cycle, and you'll get
the electron transport chain.
And it will be much
more efficient.
You'll get a lot more ATP
in the presence of oxygen
than you do in the
absence of oxygen.
In the presence of oxygen, you
get about 36 ATP per glucose.
In the absence of
oxygen, when your muscles
are doing lactic
acid fermentation,
you only get 2 ATP per glucose.
That NADH recycling
stuff does not
generate any additional ATP.
All right. Right.
This is a quick,
broad strokes overview
of lactic acid fermentation
and how it compares
to cellular respiration.
