- [Instructor] I love that picture.
Alright, so, glucose metabolism,
AKA eating and breaking
all organic molecules down.
I hate snakes by the way,
but that's just so cool
as far as being able to eat
something larger than itself.
So in this lecture,
we're going to look at the other organelle
that is so important for most organisms.
And that is to mitochondria.
Now, one thing I want to
make abundantly clear,
'cause you people
usually think that plants
just have chloroplasts
and then all other
organisms have mitochondria.
Even plants have mitochondria.
If you think about it,
only the leaves of the plants
are actually undergoing
photosynthesis and making glucose,
but then the plant then
still has to turn around
and turn that glucose into ATP
when it's undergoing metabolism.
Now, the plant's metabolism is much slower
than yours and I's,
so therefore it doesn't
need to produce as much ATP
in the moment like you and I,
but they still have mitochondria.
This is something that
people always just overlook
and say, oh no, plants have chloroplast,
that's what makes their energy.
Nope.
They make the glucose.
And then in those same
cells can turn around
and break the glucose
down when they need it.
Remember the plant's not always
being exposed to sunlight,
so it only makes sugars
during that period of
time that the sun is up.
And then it stores
those sugars as starches
and as sucrose and things like that.
And then it'll break it down
as it grows and as it needs it.
Now you and I, on the other hand,
we cannot make the glucose.
We cannot undergo photosynthesis.
So the only way to be able to get energy
for our cells' needs is
to eat something else
that has that energy already made.
Now you can eat the plant
that is making the glucose,
you can eat an animal that ate the plant,
you can eat an animal
that ate the animal that ate the plant,
I'd rather do the latter,
that ultimately allows you
to get your fuel molecules.
Now, when we say glucose metabolism,
we don't just mean sugars.
We actually mean all metabolism,
but you'll see in today's lecture
that it starts with glucose.
And then as you go through
the process of metabolism,
all organic molecules are incorporated
in one spot or another.
In fact, let me jump ahead, and just...
Right here.
It starts with glucose,
but then the amino acids can come in
at different stages of the game
and fatty acids can come in at
different stages of the game.
So we're really looking
at all organic molecules.
Now we really don't use
nucleic acids as a fuel source
because their primary role
is for information storage,
but we can metabolize proteins,
carbohydrates and fats
all through this same pathway.
So that's why we call
it glucose metabolism,
is because it starts with glucose,
but then everything else can come in
at any point through it.
Now, not every living
organism needs mitochondria
to undergo glucose metabolism,
but it is inefficient if you
don't have mitochondria, okay.
So all living organisms, all cells,
have the ability to break
down sugars into ATP.
But the difference is in the
presence of mitochondria,
you can get somewhere upwards
of 36 to 40 ATP per glucose.
In the absence of mitochondria,
you get two, okay.
So big difference from
the same glucose molecule.
That's really the difference
between having mitochondria
and not having mitochondria, okay.
ATP, as we know, is that battery
that fuels protein synthesis
and muscle contractions
and all other metabolic processes.
If you remember from the cell video
where that vesicle's being
walking along the microtubials,
every step takes ATP.
So even in the simplest of cells,
every little process that's going on
requires at least some energy
in order for that to be able to occur.
Now, some cells are just
fine making two ATP.
You'll find that bacteria
and very simple cells,
they're fine doing this.
They don't have mitochondria.
So therefore they just break down glucose,
get two ATP out of it, dump
the rest of the energy outward,
and don't even bother using it.
You and I, on the other hand,
use so much ATP that without
mitochondria we can't function.
Our cells would not be able to produce
the amount of energy required
for our metabolic needs.
So there are three ways actually,
in which ATP can be made.
We have aerobic respiration,
anaerobic respiration, and fermentation.
And they go in that order
as far as efficiency.
So what is the most efficient
way of producing ATP?
It's not only with mitochondria,
but with oxygen as a substrate.
That's why we call it aerobic respiration.
So organisms that use
mitochondria and oxygen,
they produce the mass quantities of ATP.
Now, just below that,
the same mechanics apply
to some other organisms
that have mitochondria,
but they don't use oxygen.
They use something else, like nitrogen,
or they could even use carbon
dioxide during this process,
carbon dioxide, nitrogen
gas, and the like,
that's what we call anaerobic respiration.
So there is a difference
between what substrate they use
to break glucose down into ATP.
Oxygen is by far the most efficient way.
And that's what you and I do.
That's why when we breathe
in, we breathe in oxygen.
Now, other organisms,
like bacteria in the soil,
that do have the capability
of undergoing this
process with mitochondria,
can undergo a higher rate of metabolism,
produce ATP by using
nitrogen instead of oxygen.
And that's what we call
anaerobic respiration.
So when we go through this process,
we're not really going to distinguish
between aerobic and anaerobic,
because they are virtually the same.
The only difference is
what's used at the end.
Is it oxygen?
Is it nitrogen?
Is it carbon dioxide?
Is it something other than oxygen?
You know, that's really the
main difference between the two.
On the other hand,
fermentation is the
process that is coupled
when there is no mitochondria.
Or under extreme circumstances
where our body is not producing enough ATP
through aerobic respiration alone.
That's when our cells will
kick in with fermentation.
That will be the last
concept we talk about.
We won't get to that today.
So our cells do undergo fermentation,
but only under very stressful conditions.
You know, like when you're exercising.
I'm not kidding.
So as long as you have enough oxygen,
your body doesn't need
to undergo fermentation.
But when you start using more energy
than you could produce through
aerobic respiration alone,
that's when fermentation kicks in.
That's when you start
getting the muscle cramps
and things like that.
So we'll talk about lactic
acid buildup and other things.
Now, what is the process?
What's the overall process?
Well, guess what,
it is the exact opposite
of photosynthesis.
And I mean, exact.
We take in glucose, we take in oxygen.
Those now are the substrates.
Those are the reactants.
That's what we metabolize.
And the products are
carbon dioxide, water,
we make water during this process,
but instead of being luminescent beams,
and giving off light, as we
know how photosynthesis works,
we make ATP, which is the same concept.
It's energy that's being produced
through the breaking down
of these covalent bonds.
So they are literally
reverses of one another.
So you see right here, photosynthesis,
you have carbon dioxide,
water and light going in,
glucose and oxygen coming out.
Aerobic metabolism, glucose,
and oxygen going in,
carbon dioxide, water, and ATP coming out.
But one of the fascinating things,
and I will test you on this concept,
is when you look at the energy
that is contained within our cells,
where does it have its origins?
What's the origin of all
the energy in your body?
We are literally sun driven.
We just, not directly, but indirectly,
the energy that is in our
cells originated from our sun.
So we're not Superman in
the sense that we absorb it
and get super powers and whatnot,
but every ounce of energy
that is holding your cells together,
those covalent bonds
that hold your organic molecules together,
they have their origins in sunlight.
So we are really children of the stars,
our atoms and our energy.
Alright.
One of the things I
want to illustrate here,
is you're already familiar
with photosynthesis,
you had the light reactions
where water goes in,
water gets split and oxygen comes out,
you make ATP and ATPH,
Calvin cycle takes in carbon
dioxide, turns it into glucose.
Well, going from right to left,
this is the overview
of glucose metabolism.
Notice it's exact opposite.
Glucose gets broken down,
as it's being broken down
carbon dioxide is released.
That's why we have to
breathe out carbon dioxide,
is those are the byproducts of metabolism.
So as we breathe out carbon dioxide,
plants just take it back up.
And hence the circle of life.
As we undergo the last and final process,
this is where oxygen is required,
where we breathe in oxygen
and it gets turned back into water.
And then, so really we're
just recycling these atoms
and exchanging energy
between one and another.
All right, so this is
just a simple diagram
of the three methods of metabolism
that I just talked about.
Notice aerobic and anaerobic respiration
are exactly the same.
The only difference is
aerobic uses oxygen,
anaerobic uses something
other than oxygen,
but the mechanics are exactly the same.
So we call this cellular
respiration, okay.
That includes both aerobic, and anaerobic.
You'll see me mention that sometimes
as cellular respiration,
which is the same process,
no matter what the end substrate is
that is used in that process.
Now notice over here,
it's missing pretty much everything.
So this process of fermentation
does not occur in the mitochondria.
In fact, it either only occurs
when there's no mitochondria
or when the mitochondria
are at max capacity,
like when you're exercising, okay.
So fermentation is by far one
of the most wasteful processes
that only extracts a tiny
bit of energy out of glucose.
And that will be for the end
of the lecture next time.
All right, so let's look
at mitochondria's overall
structure and shape.
Unlike the chloroplasts,
notice this actually shows
plant cell, which is good,
because it shows here's the
chlorophyll or the chloroplast,
which undergoes photosynthesis,
and then right next to
it is a mitochondria,
as the plant will make the glucose
and then it needs to break
it down as it needs the ATP.
Well, it has a double membrane.
It has an outer membrane,
and then it has this huge inner membrane
that's folded back in on itself.
What do you think this
folding over and over
and over and over does for the organelle?
What does that increase?
Surface area.
And by increasing the surface area,
this tiny organelle can actually
produce a lot more energy
than if it just had another
big double membrane here
and a huge open space in the middle.
By having those folds,
because that's where the
ADP is really made mostly,
is in those inner folds,
which we call the cristae,
that's why that increased surface area
increases its ATP production.
Now, this is probably one of
the most valuable pictures
you have for this lecture.
It illustrates over half of
the concepts or questions
that I'm going to give you on your quiz.
So make note of that,
that this is one that you
absolutely should have
in front of you as you're doing the quiz,
because it answers so
many different questions.
Now this gives an overview as
far as what are the four steps
of glucose metabolism
that we're going to cover.
The first one is called glycolysis.
What you need to know
about glycolysis for now,
we're going to go into more
detail in a little bit,
is every cell can do this.
Every cell, whether
it's a bacteria, fungus,
plant, animal, doesn't matter.
All organisms can undergo glycolysis.
Notice it's in the cytoplasm,
it's not in the mitochondria.
So it doesn't matter whether
you have mitochondria or not.
All organisms can undergo glycolysis.
And that will definitely be
something I test you on, okay?
So doesn't matter what your species is,
glycolysis is universal.
Notice glycolysis makes
some ATP, two ATP, in fact.
And for some organisms, that's it,
that's all they do to make
energy, is undergo glycolysis,
and then they waste the rest of the energy
that's still there in the
products of glycolysis,
just go off by the wayside.
Well, in more advanced cells
where they have mitochondria,
we can still extract tons of energy
from the products of glycolysis.
And that's where we get
into the last three steps.
So these last three steps
require mitochondria to occur.
What are they?
The preparatory reactions,
the citric acid cycle, AKA Krebs cycle,
named after the scientist
who discovered it.
Okay, so the citric acid cycle
essentially tells you what the cycle does,
it makes citric acid and whatnot,
but we call it the Krebs
cycle after the scientist.
And then, the last but
definitely not least,
the electron transport chain.
This is where the bulk of ATP is produced.
We're going to show how that's done.
So those three reactions,
preparatory reaction, citric acid cycle,
and the electron transport chain,
happen in the mitochondria.
So you have to have mitochondria
for that to be able to occur.
That's why these belong
to aerobic and anaerobic respiration.
Whereas glycolysis is universal.
Glycolysis is in any cell,
any organism, any time.
Alright, so let's start with glycolysis.
Glyco having to do a glucose,
lysis means to break apart.
So all glycolysis is,
is the initial stages
of breaking glucose down
and extracting energy by
breaking its covalent bonds.
But I want to point out,
this is a lot of organic chemistry,
none of which you're
going to have to know,
but I want to point out something.
In the initial stages of glycolysis,
the cell actually has to
add or put energy into it
before it can get energy out.
Do you remember what we call this energy
that is needed to get the ball rolling?
The activation energy, good.
The energy of activation.
So that gets the ball rolling.
And then the cell can get
four ATP out of each glucose,
but since it invested
two, the net gain is two.
So it puts two in, gets four out,
so the net gain is two ATP.
So that's why glycolysis
only makes two ATP.
Now there's another
molecule that's made here
that will be important
for later on, called NADH.
It seems very familiar,
it's just missing the P.
It's not NADPH, but guess what?
It's pretty much exactly
the same as NADPH.
What is NADPH again, that we
just learned in photosynthesis?
What is it?
What do we call it?
It's going to be important
for this lecture.
Called an electron what?
Electron carrier, okay?
So make sure you remember that.
In photosynthesis the electron
carriers that they use
are called NADPH.
In glucose metabolism, the
electronic carriers that we use,
there's actually two,
but one of the first ones is called NADH.
So make sure you know that terminology.
It will show up on a lot of
these quiz questions here.
So NADH, so let's go back to here.
That's really the sum of the whole,
when glycolysis occurs,
glucose is broken down into pyruvate.
Now, what is pyruvate?
It's essentially glucose split in half.
There are these three carbon molecules
that you break a bond of glucose
and you get pyruvate, is what we call it.
They're just these
three carbon structures.
In the breaking of that
covalent bond, ATP is made.
As well as, remember when
you break a covalent bond,
you release electrons.
That's what these are.
They're electron carriers.
They literally grab the
electrons that used to be there
when the atoms were sharing them,
and pulls them away from
it, and stores them.
That's why they're
called electron carriers.
I also want to point out,
just for the sake of it,
we got here,
same stuff that was put
together to make glucose.
So when glucose is broken down,
you can see that it's
just the reverse process
as it goes through there.
And you're not gonna be tested on that,
but, just to illustrate,
this is just a reverse
of what the Calvin cycle essentially does.
All right.
Now, enzymes are studied
throughout this whole process
and you're not going to have
to know a single one of them,
the names of them or
anything like that, okay.
I'll mention one of them,
but it's not critical for
any of your questions, okay.
But every step of the
way, there's enzymes.
There's an enzyme that's
breaking this bond,
making ATP, doing this and that.
So we ignore most of them
for the sake of simplicity,
but just be aware that
every metabolic reaction,
there's an enzyme involved
in that process, okay.
Alright, now, if mitochondria
is present in the cell
and if the cell has the capacity
to bring those substrates in,
which is pyruvate,
then it will go through
the last three steps.
But as I mentioned,
sometimes the mitochondria
working at max capacity,
like in our cells, producing ATP,
and if that's the case,
there's no other way to make ATP
besides these three mechanisms,
except for more glycolysis.
So we'll explain why fermentation
kicks in in our cells
when we're pushing them
beyond their limits
of ATP production, so to speak.
All right, so let's assume
that everything's working fine
and we're not using tons of ATP.
We're just chilling on
our couch or whatnot.
We're able to take pyruvate
into the mitochondria
via what we call the preparatory reaction.
So that's the second stage of this.
Now this is the only step
that does not produce
any ATP directly, okay.
Notice there is no ATP
production here, okay.
Even though a covalent bond is broken,
there is no ATP production.
So what happens in the
preparatory reactions
is actually pretty simple.
The three carbon pyruvates are
brought into the mitochondria
and an enzyme breaks one
of the covalent bonds
and makes what we call these
electron carriers and ADH.
So remember, every time you
break the covalent bond,
it extracts those electrons
that were holding the atoms together,
and that make these electron carriers.
Well, this is the first stage too,
in which carbon dioxide is produced.
So as one of the carbons of
the pyruvates being broken off,
carbon dioxide starts building up.
Well, as carbon dioxide builds up,
it diffuses through simple diffusion
out of the mitochondria,
out of our cells, into our bloodstream,
to our lungs and into the air.
So that's really, again, same
process, simple diffusion.
As carbon dioxide builds up,
it gets picked up
as it goes from high to low concentrations
and diffuses out through all
the membranes of the cells
and into our blood.
Alright, now what's left over
after the pyruvate is cut,
you get this two carbon molecule,
which we call an acetol group, okay.
So there's still tons of energy,
even in those last two atoms
sharing that covalent bond,
there's still tons that
you can extract from that.
So that's the preparatory reactions.
Pyruvate is cut,
turns it into an inositol,
into carbon dioxide,
and the electrons are picked up,
and you make some electron carriers.
That's it, that's the
preparatory reactions.
So when you come back to
this picture right here,
that's why this is what
you're seeing right here.
It doesn't label it,
unfortunately in this picture,
but that's what you're seeing.
You're making electron carriers,
you're making carbon dioxide
and the pyruvate's being prepared
to go into the Krebs cycle,
AKA citric acid cycle.
Alright.
Now, why do we call this
the citric acid cycle?
Well, why do we call the
Calvin cycle the C3 cycle?
Because that first step of the cycle
really is what gives it its name,
the Calvin cycle, the C3 cycle,
because it makes those PGA molecules
that are a three carbon molecule.
Well guess what?
The first step of the citric acid cycle
is to make citric acid.
So here's, again, don't worry about
any of the organic
chemistry going on here,
but here's how it works.
An enzyme called coenzyme A,
binds this four carb
into the acetyl group,
making citric acid.
Then it goes through this complex process
of chopping these carbon atoms off
and extracting the electron energy
and making a little ATP
on the side as well.
So here's where the
remainder of the carbons
that made up the glucose
that you took into your cells
are being released.
By the end of the citric acid cycle,
the atoms that made up the glucose
have all been turned
back into carbon dioxide
and your body is getting rid of them
because of that buildup.
Now the citric acid cycle
not only makes these
electronic carriers NADH,
but another one that you'll
need to know called FADH2.
It does the same thing.
This is an electronic carrier,
FADH2, NADH, these are
all electronic carriers.
And then it makes a little ATP too,
well, one per acetol, but
for every glucose it's two.
So let's go back and sum it up.
All right.
Now notice if you have a
six carbon glucose come in,
the preparatory reaction's
released as a couple.
And if by the time the
Krebs cycle is over,
all of the remaining carbons
that made up that glucose,
they're now carbon dioxide.
Here, the Krebs cycle.
This is an important concept
that I'll test you on.
This is where the bulk
of what we call electron
carriers are made,
NADH and FADH2.
In fact, this is the only
spot where FADH2 is made.
You notice that glycolysis,
the preparatory reactions
and the Krebs cycle,
they all make electron carriers,
but this is where the bulk
of the electron carriers are made.
Now, the big question becomes,
glucose has gone, all
of the covalent bonds
that held the atoms together,
they're all broken, okay.
But you've only made
four ATP at this point.
What the hell?
Where is all the energy?
Well, that brings us to
the last and final step.
The electron transport chain,
the energy is stored in
these electron carriers.
So get ready to have your mind
blown by what happens next.
This is where it culminates.
Now this is the reason
why the mitochondria
have a double membrane,
the outer membrane and the inner membrane
form his little pocket
that surrounds the cristae
which is where the electron
transport chain is at.
Well guess what?
All the electron carriers
that are made in glycolysis,
the preparatory reactions
and the citric acid cycle,
they all dump their electrons
into the electron transport chain.
Now guess what happens?
As these electrons travel down,
these protein complexes,
the energy that's in
these electrons gets used
to pump hydrogen ions into
this inner membrane space.
And as the hydrogen ions build
up in their concentration,
they're allowed out of this membrane
back into the matrix of the mitochondria,
down their concentration
gradient facilitated diffusion
through an enzyme ATP
synthase, thus making ATP.
What do we call this again?
Stupid word, right?
Chemiosmotic phosphorylation.
The same process that
plants use in photosystem II
to make ATP.
The only difference is we're
getting our energized electrons
from the sugars and the fats
and the proteins that we eat,
so that those electrons were
energized back in the day
when the plant absorbed that
energy during photosynthesis
and made those electron carriers,
and that got transferred to the glucose,
and now they're being used
right here to make ATP.
By the way, again, this is the
enzyme that cyanide blocks.
This is the enzyme that
stops the hydrogen ions
from flowing out.
And thus ATP is not produced,
which is why you die, because
you don't make any ATP
when cyanide blocks those enzymes.
All right.
Here we run into the opposite problem
that photosynthesis had.
Photosynthesis had the issue
of having a source of electrons.
We have the opposite problem.
We need to get rid of the
electrons, enter oxygen.
This is why oxygen is
the most efficient way
of producing ATP,
because it has the highest
affinity for electrons.
It loves electrons more
than any other molecule,
which means that it can
remove the electrons
faster than any other molecule.
Hence the difference
between aerobic and anaerobic respiration.
So when oxygen comes into
the electron transport chain,
it picks up the electrons,
combines with some hydrogen ions,
and lo and behold turns back into water.
Now, as water builds up,
it just leaks out through osmosis
and the oxygen leaks into the mitochondria
through simple diffusion.
So that's just how we could exchange.
That's why when we breathe,
we pick up more oxygen,
it goes to our cells, it
gets picked up by the cells,
goes to the mitochondria,
gets turned into water
and so on and so forth.
And as I mentioned, this
one too, right here,
you're not going to have to
memorize what goes in where,
but it does illustrate
that all organic molecules
can be used in the glucose
metabolism process.
You've got fats,
which when they're broken off,
turn into acetyl groups,
that enters right into the Krebs cycle,
you've got glycerol,
which can enter into the glycolysis cycle,
you've got amino acids
that can enter in multiple
stages of glucose metabolism,
all in the end to produce ATP.
So whether you're breaking down
carbohydrates, fats or
proteins, in the end,
they all turn into the same
energy molecule, doesn't matter.
Now, fermentation is probably
the most difficult part
of this lecture as far
as the questions go,
and people understanding
what's going on here.
So just pay close attention.
Some organisms have no mitochondria.
So the only way in which
they could produce energy
is through glycolysis,
because they don't have
the preparatory reactions
and the citric acid cycle and
the electron transport chain.
Without that organelle, you
don't do those processes.
You don't have the enzymes for it.
So the only way in which
they can produce ATP
is through glycolysis, okay.
So there's something about glycolysis
that you have to understand,
is ATP is only produced
after the NADH, is produced,
or the electron carriers are produced.
Let me go back to that
overall dynamic here,
here's glycolysis.
And like I said,
you're not going to have to
know the steps of glycolysis,
but I want to point something out.
The first stage of what we call
energy extraction of glycolysis,
after you put some energy in,
is first, you have to get some electrons,
you remove those electrons,
and then the glycolysis process,
the organic chemistry, can produce ATP.
So, this is what we would
call the rate limiting step,
meaning only as fast as you
make these electron carriers,
can you make ATP.
Well, normally these electron carriers
dumped their electrons where?
If you have mitochondria,
where do they normally dump them?
Where do they get rid
of them in this step?
Good, the electron transport chain.
So normally these go to the
electron transport chain,
dump their electrons,
they come back and they pick up some more.
So as long as there's a constant recycling
of these electron carriers,
glycolysis can just keep going
over and over and over and over again.
However, if there's no
electron transport chain,
what's the cell to do with
these electron carriers?
That's where fermentation comes into play.
Fermentation is a electron
carrier recycling process,
that essentially recycles
these electron carriers
so that they can go back
and pick up more electrons.
So, whereas the electron transport chain
is normally the most efficient place to go
for these electron carers
to dump their electrons,
if there are no electron transport chains,
'cause there's no mitochondria,
well, what are they to do?
They essentially, instead
of turning pyruvate
into carbon dioxide through
the preparatory reactions
and through the Krebs cycle,
and then dumping the electrons
in the electron transport chain,
the electrons use that energy
to split pyruvate into alcohol.
Ethanol and carbon dioxide.
And this is where
we usually think of the
fermentation process,
where bread is made, where
alcohol is made and the like.
This isn't the only
fermentation process though,
for example, yogurt is
made through fermentation.
That's a completely different
fermentation process,
but it's still fermentation.
So there's two main types of fermentation.
There's the fermentation
that produces alcohol,
which occurs in yeast and
some bacteria and the like.
And then there's the fermentation
that doesn't produce alcohol,
called lactic acid fermentation.
That's the one that you and I
do as well as other microbes,
for example, in cheese production,
and yogurt production
and things like that.
This process right here is irreversible.
Why?
Because when the pyruvate
is split into ethanol and carbon dioxide,
carbon dioxide just starts diffusing out.
In fact, that's where the foam comes from.
That's where the bubbles
and the fermentation process come from.
That's where the bread rises
during the yeast fermentation process,
is the buildup of carbon
dioxide in the dough.
That's why you have to let it sit there
and wait for a little
bit as the bread rises,
because the carbon dioxide
that's being produced
during the fermentation process
is puffing up the dough.
But the whole reason for
fermentation is not for this.
This is the only way that it can recycle
these electron carriers.
The whole reason
is so that it can keep on
making ATP through glycolysis.
So let me be clear on this,
fermentation doesn't produce any ATP.
It merely recycles the electron carriers.
It's glycolysis that makes the ATP.
So if an organism doesn't
have any mitochondria,
the only way they can make
ATP is through glycolysis.
Well, the only way that
they can do glycolysis
is to recycle these electron carriers.
So that's one way that
fermentation occurs.
The other way is called
lactic acid fermentation.
Now this process is reversible.
You and I undergo this
process when we exercise
and when we stress ourselves out.
So why, why would we ever do this
if we have mitochondria?
Because you're like, huh,
why don't the electron
carriers get recycled
at the electron transport
chain where they should?
Well, let me show you why.
So, when we have mitochondria,
this is the most efficient
way of producing ATP,
where you've got lots of ATP
coming out of mitochondria
through this process,
and the electron carriers
essentially dump their electrons,
and then they go back and
they pick up some more
and so on and so forth.
Well, let's say that you're
stressing your muscles out
by using lots and lots of ATP.
There's a limit to how fast this can go.
So let's just say for the sake of it,
that you're producing 40 ATP a second,
it's more than that,
but let's just say for the
sake of it, 40 ATP per second,
but let's say you're
using 50 ATP per second.
Well, this can't go any faster.
So how else are you going to make energy?
Well, remember glycolysis is
the process that pumps out ATP,
in fact, two ATP for
every glucose molecule.
Well, if this is completely saturated,
you've got plenty of oxygen,
you're making lots and lots of ATP,
but you're pushing yourself faster
than aerobic respiration
alone can provide,
then you have to do some
glycolysis and fermentation
on the side, okay.
So what ends up happening
is you're recycling
these electron carriers through
lactic acid fermentation,
and for every round of
glycolysis you make two ATP.
So in fact, to make up that 10 ATP,
how many rounds of
glycolysis and fermentation
do you need to do extra on top of this?
Five, okay.
To make that final ATP.
So the long and the short of it is,
the more you push yourself,
the more you'll have to undergo
glycolysis and fermentation
in addition to aerobic respiration.
Well, what's the
byproduct of fermentation?
Lactic acid.
Guess what?
This prevents your
muscles from contracting.
It builds up in your muscles.
You get cramps and your
liver will recycle this
and turn it back into pyruvate.
So it does leak into your blood.
Your blood takes it to your liver,
your liver recycles it back into pyruvate,
goes back into your cells,
and can be turned into ATP,
but that requires that
you stop pushing yourself.
So anyway, that's why
we undergo fermentation.
And that's why lactic
acid builds up, okay,
is when we are exercising
more and using more energy
than aerobic respiration
alone can provide.
That's when fermentation kicks in.
Now, there obviously is conditioning.
You can condition your muscles
so that they use less energy
for the force of contraction.
That's where conditioning comes into play,
where you can push yourself
faster and farther.
There's a lot of physiological
things that can change
as you exercise,
so that you undergo less
and less fermentation,
but that's kind of the
fundamentals behind it,
is that if you're not
producing as much ATP
as you're using in your cells,
then the only other way to make ATP,
is by doing glycolysis and fermentation
in addition to aerobic respiration.
