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- [Voiceover] So what I wanna do in this video
is give ourselves an overview of cellular respiration.
It can be a pretty involved process, and even the way
I'm gonna do it, as messy as it looks,
is going to be cleaner than actually what goes
on inside of your cells, and other organs themselves,
because I'm going to show clearly from
going from glucose, and then see how we can
produce ATP through glycolysis, and the Krebs cycle,
and oxidative phosphorylation,
but in reality, all sorts of molecules can jump in
at different parts of the chain, and then jump out
at different parts of the chain, to go along other pathways.
But I'll show, kind of the traditional narrative.
So we're gonna start off, for this narrative,
we're gonna start off with glucose.
We have a six-carbon-chain right over here.
And we have the process of glycosis, which is occurring
in the cytosol, the cytosol of our cells.
So if this is a cell right over here,
you can imagine, well the glycolysis,
the glycolysis could be occurring right over there.
And that process of glycolysis is essentially splitting up
this six-carbon glucose molecule
into two three-carbon molecules,
and these three-carbon molecules,
we go into detail in another video, we call these pyruvate.
Pyruvate.
And in the process of doing so, and this is,
I guess you could say, the point of glycolysis,
we're able to, on a net basis, produce two ATP's.
We actually produce four, but we have to use two,
so on a net basis, we produce two ATP's.
I'm gonna keep a little table here, to keep track.
So we produce two ATP's, and we are also,
we're also, in the process of that,
we reduce two NAD molecules to NADH.
Remember, reduction is gaining of electrons.
And you see over here, this is positively charged,
this is neutrally charged, it essentially gains a hydride.
So this is reduction.
Reduction.
And if we go all the way through the pathway,
all the way to oxidative phosphorylation,
the electronic transport chain, these NADH's, the reduced
form of NAD, they can be, then, oxidized,
and in doing so, more energy is provided to produce
even more ATP's, but we'll get to that.
So you're also gonna get two NADH's.
Two NADH's get produced.
Now at that point, you could kind of think of it
as a little bit of a decision point.
If there's no oxygen around, or if
you're the type of organism that doesn't
want to continue, for some reason,
with cellular respiration, or doesn't know how,
this pyruvate can be used for fermentation.
We have videos on fermentation,
lactic acid fermentation, alcohol fermentation,
and fermentation is all about using the pyruvates
to oxidize your NADH back into NAD,
so it could be re-used again, for glycolysis.
So even though the NADH has energy
that could eventually be converted into ATP,
and even though pyruvates have energy
that could eventually be converted into ATP,
when you do fermentation, you kinda give up on that,
and you just view them as waste products,
and you use the pyruvate to convert the NADH back into NAD,
And then, glycolysis can occur again.
But let's assume we're not gonna go down
the fermentation pathway, and we're
gonna continue with traditional
aerobic cellular respiration, using oxygen.
Well, the next thing that's going to happen,
is that the carboxyl group, and
and everything I'm going to show now,
it's going to happen for each of these pyruvates.
So, you can imagine these things all happening twice.
So I'm gonna multiply a bunch of things, times two.
But what happens in the next step,
is this carboxyl group, this carboxyl group is stripped
off of the pyruvate, and it, essentially,
is going to be released as carbon dioxide.
So this is our carbon dioxide being released here,
and then the rest of our pyruvate, which is, essentially,
an acetyl group, that latches onto coenzyme A.
And you'll hear a lot about coenzyme A.
Sometimes I'll write just CoA, like this.
Sometimes I'll do CoA, and then
the sulfur, bonded to the hyrdrogen.
And the reason why they'll draw the sulfur part,
is because the sulfur is what bonds
with the acetyl group, right over here.
So, you have the carbon dioxide being released, and
then the acetyl group, bonding with that sulfur,
and by doing that, you form acetyl-CoA.
And acetyl-CoA, just so you know, you only see
three letters here, but this is
actually a fairly involved molecule.
This is actually a picture of acetyl-CoA,
I know it's really small, but hopefully
you'll appreciate that it's a more involved molecule.
That, the acetyl group that we're talking about
is just this part, right over here, and it's a coenzyme.
It's really acting to transfer that acetyl group,
and we'll see that in a second.
But it's also fun to look at these molecules,
because once again, we see these patterns
over and over again in biology or biochemistry.
Acetyl-CoA, you have an adenine right over here.
It's hard to see, but you have a ribose,
and you also have two phosphate groups.
So this end of the acetyl-CoA is essentially,
is essentially an ADP.
But it's used as a coenzyme.
Everything that I'm talking about,
this is all going to be facilitated
by enzymes, and the enzymes will have
cofactors, coenzymes, if we're talking about organic
cofactors, that are gonna help facilitate things along.
And as we see, the acetyl group joins on
to the coenzyme A, forming acetyl-CoA,
but that's just a temporary attachment.
The acetyl-CoA is, essentially, gonna transfer
the acetyl group over to, and now we're going to
enter into the citric acid cycle.
It's gonna transfer these two carbons
over to oxaloacetic acid, to form citric acid.
So it's gonna transfer these two carbons
to this one, two, three, four carbon molecule,
to form a one, two, three, four, five, six carbon molecule.
But before we go into the depths
of the citric acid cycle, I wanna make sure that
I don't lose track of my accounting,
because, even that step right over here,
where we decarboxylated the pyruvate,
we went from pyruvate to acetyl-CoA,
that also reduced some NAD to NADH.
Now, this is gonna happen once for each pyruvate,
but we're gonna-
all the accounting we're gonna
say, is for one glucose molecule.
So for one glucose molecule, it's
gonna happen for each of the pyruvates.
So this is going to be times-
This is going to be times two.
So we're gonna produce two,
two NADH's in this step, going
from pyruvate to acetyl-CoA.
Now, the bulk of, I guess you could say,
the catabolism, of the carbons, or the things that
are eventually going to produce our ATP's,
are going to happen in what we call
the citric acid, or the Krebs cycle.
It's called the citric acid cycle because, when we
transferred the acetyl group from the coenzyme A
to the oxaloacetic acid, we formed citric acid.
And citric acid, this is the thing
that you have in lemons, or orange juice.
It is this molecule right over here.
And the citric acid cycle, it's also called the Krebs cycle,
when you first learn it, seems very, very complex,
and some could argue that it is quite complex.
But I'm just gonna give you an overview of what's going on.
The citric acid, once again, six-carboned,
it keeps getting broken down, through multiple steps,
and I'm really not showing all of the detail here,
all the way back to oxaloacetic acid,
where, then, it can accept the two carbons again.
And just to be clear, once the two carbons
are released by the coenzyme A,
then that coenzyme A can be used
again, to decarboxylate some pyruvates.
So there's a bunch of cycles going on.
But the important take-away, is as we go through the citric
acid cycle, as we go from one intermediary to the next,
we keep reducing NAD to NADH,
in fact, we do this three times for each cycle
of the citric acid cycle, but remember,
we're gonna do this for each acetyl-CoA.
For each pyruvate.
So all of this stuff is going to happen twice.
So we're going to go through it twice
for each original glucose molecule.
So, here we have one, two, three NADH's being produced,
but since we're going to go through it twice,
and we're gonna be accounting for
the original glucose molecule, we could say
that we have six
six NADH's,
or you could say, six NAD's get reduced to NADH.
Now, you also, in the process,
as you're breaking down, going from
the six-carbon molecule to four-carbon molecule,
you're releasing carbon, as carbon dioxide,
and you also have, traditionally GDP being converted
into GTP, or sometimes ADP converted into ATP,
but functionally, it's equivalent to ATP, either way.
So, we could also say that we're gonna directly-
Remember, we're gonna do all of this stuff twice.
So, we could say that two,
I'll just say two ATP's, to make it simple.
We could say GTP, but I'll say two ATP's.
Because once again, this happens once in each cycle,
but we're gonna do two cycles, for each glucose.
And then, we have this other coenzyme right over here,
FAD, that gets reduced to FADH2,
but that stays covalently attached to the enzymes
that are facilitating it, so eventually,
that's being used to reduce .
coenzyme Q to QH2.
So I'm just gonna write the QH2 here, but
once again, you're gonna get two of these.
So two QH2's.
Now let's think about what the
net product, over here, is going to be.
And to think about it, we should just, we'll just-
I'll do a little bit of a shorthand.
We'll go into more detail in future videos.
These coenzymes, the NADH,
the QH2, these are going to be oxidized,
during oxidative phosphorylation,
and the electron transport chain,
to create a proton gradient across
the inner membrane of mitochondria.
We're gonna go into much more detail in the future,
but that proton gradient is going
to be used to produce more ATP.
And one way to think about it, is
each NADH is going to produce,
and I've seen accounts, it depends on the efficiency,
and where the NADH is actually going
to be produced, but it's going to produce
anywhere between two and three ATP's.
Each of the reduced coenzyme Q's,
so QH2,
that's going to each produce about one and a half ATP's.
And people are still getting a good handle
on exactly how this is happening.
It depends on the efficiency of the cell,
and what the cell is actually trying to do.
So, using these ranges, actually I'll say
one and a half to two ATP's.
And these are approximate numbers.
So let's think about what our total accounting is.
So if we just count up the ATP or the GTP's,
we're gonna get two there, two there.
So we're gonna have four direct,
or very close to direct, ATP's net, being created.
And then how many NADH's?
We have two, four, and then we add six.
We have ten NADH's.
Ten NADH's.
And then, we have two of the coenzyme Q's.
Two QH2's.
So that's gonna be four ATP's,
this is going to be between-
this is going to be between 20 and 30 NADH's.
Sorry, 20 and 30 ATP's.
20 to 30 ATP's.
And then, this is going to be three to four.
Three to four ATP's.
So if you add them all together,
if you add the low ends of the range, you get,
let's see, 20 plus three, plus four.
That's 27 ATP's.
27 ATP's.
And the high end of the range, let's see.
You have four plus 30 plus four.
You have 38.
38 ATP's.
And 38 ATP's is currently considered to become
the theoretical maximum, but when we actually
observe things in cells, it looks like
it comes right at around 29 to 30 ATP's.
And once again, it depends what the cell's
trying to do, the type of cells, and the type of efficiency.
But all of this is happening through cellular respiration.
And just to get a better sense
of where all of this is occurring.
Where all of this is occuring,
we said glycolosis is occurring in the cytosol.
The citric acid cycle, this is occurring in
the matrix of the mytochondria.
So this space right over here,
that is the citric acid cycle,
in that little magenta space that I've drawn.
So that's the matrix.
In the video on mitochondria, we go
into much more detail on that.
And then, the actual conversion of
the coenzymes, of the electron transport chain,
that's occurring across the membrane of the crista.
And the crista are these folds, these kind of,
inner membrane folds, of our mitochondria.
So, it's occurring across the membranes
of those, actually the plural is cristae.
Crista is the singular of the cristae.
And we'll go into more detail into that, in other videos.
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B1 中級 英

Overview of cellular respiration | Cellular respiration | Biology | Khan Academy

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Sophia Chung 2020 年 7 月 2 日 に公開

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