糖分解 (英)

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糖分解 (英) : 糖分解概述

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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.