There is no oxygen present or no oxygen necessary for glycolysis.
If there is oxygen present, we can continue on down into
Krebs cycle and the oxidative piece of this puzzle.
But if there is no oxygen present,
we have a little bit of a problem.
You may be aware that when we first begin exercise,
we have not much oxygen in the tissues,
and so we will produce a little bit of ATP.
Glycolysis is fast, it's readily available to make some ATP.
But again, just a tiny bit of ATP.
In order to get more ATP in the absence of oxygen,
we produce lactic acid in order to recycle those NAD cars
so we can keep making ATP and glycolysis.
Not much, but at least we can keep it going.
But lactic acid causes a few problems for us. You may have felt
muscle soreness when you initially start to run up a stack of stairs.
So let's look at how this process works.
Again we have glucose, the process of glycolysis,
we make some pyruvate. That's excellent.
We've got pyruvate. But there's no oxygen to drive the flow into
Krebs cycle and oxidative phosphorylation and so we're at a standstill.
In order to recycle it, we need to make some lactate.
We can recycle the NAD so that they can fuel the glycolysis.
There are two lactates produced per each glucose.
This process is reversible such the little lactate can go back
and form pyruvates and they can go on down into
the Krebs cycle and oxidative phosphorylation.
This is a neat system that you'll cover
in much more detail again in the biochemistry course.
You may cover it more in physiology also.
You've probably felt this in your body
but the production of lactate has a number of different effects.
First of all, lactate is going to lower pH. And this is because
we have NADH is dropping off hydrogen ions in the solution
and also lactate can become lactic acid.
So we're seeing an accumulation of hydrogen ions
and as you'll recall from our discussion on pH,
the hydrogen ion increase is going to cause more acidity
and that is a lower level of pH.
This lower level of pH will affect enzyme activity
because all of our enzymes are targetted for
specific temperature as well as pH.
Once we decrease the pH, enzyme activity is slowed,
and that's where we feel the muscle fatigue.
Things are literally getting slower in there
and enzymes are not happy.
So yeast have a slightly different mechanism of
dealing with this problem.
Yeast you probably know are not oxidative as in
they cannot use oxygen to survive.
And so they have to recycle their NAD in a different fashion.
Somehow they've got to get the taxis, the limousines empty
so that they can continue glycolysis.
So the only reason that we have that lactic acid thing going on
is because we need to continue glycolysis
to have a little bit of energy being made
in the form of ATP from glycolysis.
Yeast only have glycolysis for making energy.
Yeast undergo glycolysis in just the same way we do.
They'll produce their pyruvates, two from each 6 carbon glucose.
And then in order to recycle the NADH taxis
to keep glycolysis going and making ATP,
they have to produce ethanol,
which is a pretty neat reaction.
But the whole point is to recycle NADs
so that they can continue to undergo glycolysis
and yield a tiny bit of ATP and go about their cellular processes.