Glycolysis

Overview of glycolysis

We've already learned that
cellular respiration can be

broken down into roughly
three phases.

The first is glycolysis, which
literally means the breaking

down of glucose.

And then this can occur with
or without oxygen.

If we don't have oxygen, then
we go over to fermentation.

We'll talk about that
in the future.

Go over to fermentation
and in humans it

produces lactic acid.

In other types of organisms
it might

produce alcohol or ethanol.

But if we have oxygen-- and for
the most part we're going

to assume that we can proceed
forward with oxygen-- if there

is oxygen, then we could
proceed forward

to the Krebs cycle.

Sometimes called the citric acid
cycle because it deals

with citric acid.

The same thing that's in
orange juice or lemons.

And then from there
we proceed to the

electron transport chain.

And we learned in the first
overview video of cellular

respiration that this is where
the bulk of the ATP is

actually produced.

Although it uses raw materials
that came out of

these phases up here.

Now what I want to do in this
video is just focus on

glycolysis.

And this is kind of-- it's
sometimes a challenging task

because you can really get
stuck in the weeds.

And I'll show you the weeds
in a little bit,

and the actual mechanism.

And it can be very daunting.

But what I want to do is
simplify it for you so you can

have the big take-aways.

And then we can appreciate, and
then maybe when we look at

the weeds of glycolysis we
can make a little bit

more sense of it.

So glycolysis, or really
cellular respiration, it

starts off with glucose.

And glucose, we know
its formula.

It's C6H12O6.

And I could draw its whole
structure; it would take a

little time.

But I'm just going to focus
on the carbon backbone.

So it is a ring, or
can be a ring.

But I'm just going to draw it
as six carbons in a row.

Now there's two kind of
important phases of glycolysis

that are good to know.

One, I call the investment
phase.

And the investment phase
actually uses two ATPs.

So you know, the whole purpose
of cellular respiration is to

generate ATPs, but right from
the get-go I actually have to

use two ATPs.

But I use two ATPs and then I'm
essentially going to break

up the glucose into two 3-carbon
compounds right here

that actually also have a
phosphate group on them.

The phosphate groups are
coming from those ATPs.

They also have a phosphate
group on them and this is

often called-- well, there's
a lot of names for it.

Sometimes it's called PGAL.

You really don't have
to know this.

Or phosphoglyceraldehyde, really
challenging my spelling

skills right here.

That's not that important
to know.

All you have to know
is in this first

phase you use two ATPs.

That's why I call it the
investment phase.

If we use a business analogy,
investment phase.

And then each of these two PGAL
molecules can then go

into the payoff phase.

So in the payoff phase,
each of these

PGALs turn into pyruvate.

Which is another 3-carbon,
but it's reconfigured.

But the process of it going to
pyruvate-- and let me write

pyruvate in blue, because this
is something that, at least

it's good to know the word.

And I'll show you the structure
in a second.

Pyruvate.

Sometimes it's called
pyruvic acid.

Same thing.

And that's essentially the end
product of glycolysis.

So you start off with glucose
in the investment phase.

You end up in this
phosphoglyceraldehyde, which

essentially you broke up your
glucose and you put a

phosphate on either end of it.

And then those each
independently go through the

payoff phase.

So you end up with two molecules
of pyruvate for

every molecule of glucose
you started off with.

Now you're saying, hey, Sal,
there was a payoff phase, what

was our payoff?

Well our payoff, we got, for
each-- let me write this down

as a payoff phase.

This is our payoff phase.

And I apologize for the
white background.

I did it because, the mechanism
I'm showing you, I

copy-and-pasted it from
Wikipedia, and they had a

white background so I just ran
with the white background for

this video.

But I, personally at least, like
the black background a

lot better.

But this is the payoff
phase right here.

And so when we go from the
phosphoglyceraldehyde to the

pyruvate or the pyruvic acid,
we produce two things.

Or I guess we could say we
produce three things.

We produce, each of
these PGALs to

pyruvates produce two ATPs.

So I'm going to produce two
ATPs there, I'm going to

produce two ATPs there.

And then they each
produce an NADH.

And I'll do it in
a darker color.

NADH.

And of course they're not
producing the whole molecule

in a vacuum.

Essentially what they're doing
is they're starting with the

raw material of an NAD plus-- so
they start off with an NAD

plus-- and they essentially
reduce

it by adding a hydrogen.

Remember, we learned a couple
of videos ago that you could

view reduction as a
gain in hydrogen.

So the NAD gets reduced
to NADH.

And then later on, these NADHs
are used in electron transport

chain to actually
produce ATPs.

So the big take-away here, if
I were to write the reaction

that we get for glycolysis,
is that you

start off with a glucose.

And you need some NAD plus.

And actually, for every mole
of glucose, you're going to

need two NAD plusses.

You're going to need two ATPs.

So I'm just writing all the
ingredients that we need to

start off with.

And then you're going to need--
well, let me say, these

guys are going to be ADPs before
we turn them to ATPs.

So I'll write plus four ADPs.

And then, after performing
glycolysis-- and

let me write it here.

Let me write also-- sorry
that was ADPs.

Let me just rewrite that
part right there.

Four ADPs.

And then you maybe need
two phosphate groups.

Because we're going to need
four phosphate groups.

Plus four-- I'll just call
them, sometimes they're

written like that.

But maybe I'll write
it like this.

Four phosphate groups.

And then once you perform
glycolysis, you have two

pyruvates, you have two NADHs.

The NAD has been reduced.

It gained a hydrogen.

RIG.

OIL RIG.

Reduction is gain an electron.

But in the biological
sense, we think of

it gaining the hydrogen.

Because hydrogen is very
non-electronegative, so you're

hogging its electrons.

You've gained its electrons.

So two NADHs and then plus these
two ATPs get used in the

investment phase.

That's why I kind of wrote
them a little separately.

So these two get used.

So then you're left
with two ADPs.

And then these guys,
essentially,

get turned into ATPs.

So plus four ATPs.

I guess we didn't need four.

We only needed a net of
two phosphate groups.

Because two jump off of here.

And then we need a total
of two more to get

four jumping on there.

But the big picture is, you
start with a glucose, you end

up with two pyruvates.

You use up two ATPs.

You get four ATPs.

So you have a net of
two ATPs formed.

Let me write that very big.

Net, what you get out of
glycolysis, is two ATPs.

You get two NADHs that can
each later be used in the

electron transport chain
to produce three ATPs.

You get two NADHs and you get
two pyruvates, which are going

to be re-engineered into
acetyl-CoAs that are going to

be the raw materials for
the Krebs cycle.

But these are the outputs
of glycolysis.

So now that we have that big
picture, let's actually look

at the mechanism.

Because this is a little
bit more daunting

when you see it here.

But we'll see the same themes
that I just talked about.

We're starting with a
glucose right there.

It is a six chain.

It's in a circle, in a ring.

One, two, three, four,
five, six carbons.

I could write it like that,
just to make a huge

oversimplification.

It goes through a few steps.

I use an ATP here.

So let me do that in a color.

Let me do it in orange whenever
I use an ATP.

I use one ATP there.

I use one ATP there.

And just like I told you,
they have a slightly

different name for it.

But this is the

phosphoglyceraldehyde right here.

They call it glyceraldehyde
3-phosphate.

It's the exact same molecule.

But as you can see, just when I
drew it very roughly before,

you've got one, two three
carbons there.

And it also has a phosphate
group on it.

The phosphate group's actually
attached to the oxygen.

But for just for simplification
I draw the

phosphate group just
like that.

And I showed that right here.

This was the

phosphoglyceraldehyde right here.

This is the actual structure
up here.

But I think sometimes when you
look at the structure it's

easy to miss the big picture.

And there are two of these.

They kind of say that you can
go back and forth with this,

with this other kind
of isomer of this.

But the important thing is that
you have two of these

compounds that are now
3-carbon compounds.

Glucose has been split.

And now we're ready to enter
the payoff phase.

Remember you have two of these
compounds right here.

That's why, when they drew this
mechanism, they wrote

times two right there.

Because the glucose has
been split into

two of these molecules.

So each of the molecules
are now going to

do this right here.

And for each of the
glyceraldehyde 3-phosphates,

or PGALs, or
phosphoglyceraldehyde, we can

look at the mechanism and say,
OK look here, there's going to

be an ADP turning into
an ATP there.

So this is plus one ATP.

And then we see it again
happening here

on our way to pyruvate.

On our way to pyruvate right,
there then we have another

plus one ATP.

So for each of the PGALs, or
the phosphoglyceraldehydes

that were produced, we're
producing two ATPs in the

payoff phase.

Now there were two of these.

So total for one glucose, we're
going to produce four

ATPs in the payoff phase.

So in the payoff phase,
four ATPs.

In the investment phase
we used one, two ATPs.

So total net ATPs directly
generated from

glycolysis is two ATPs.

Four, gross produced.

But we had to invest two in
the investment phase.

And then the NADs and the NADHs,
we see right here.

For each phosphoglyceraldehyde,
or

glyceraldehyde 3-phosphates or
PGALs or whatever you want to

call them, at this stage right
here you see that we are

reducing NAD plus to NADH.

So this happens once for each
of these compounds.

And obviously there
are two of these.

Glucose got split into
two of these guys.

So two NADHs are going
to be produced.

And later these are going to
be used in the electron

transport chain to actually
each produce three ATPs.

And then finally, when
everything is said and done,

we're left with the pyruvates.

And it's nice, at least that
they made it nice and big.

We can take a look at what
a pyruvate looks like.

And just as promised, we can
look at all the oxygen bonds

and all that.

But it's a 3-carbon structure.

It has a 3-carbon backbone.

So the end result is that the
carbon, that the glucose got

split in half.

It got oxidized.

Some of the hydrogens got
stripped off of it.

As you can see there's only
three hydrogens here.

We started off with 12
hydrogens in glucose.

And now it has its carbons
bonding more

strongly with oxygen.

So it's essentially having its
electrons stolen by the

oxygens, or hogged
by the oxygens.

So carbon has gotten oxidized
in this process.

There's going to be more
oxidation left to do.

And in the process we were able
to generate two net ATPs

and two NADHs that can later
be used to produce ATPs.

Anyway, hopefully you
found that helpful.