Krebs / citric acid cycle | Cellular respiration | Biology | Khan Academy

So we already know that if we start off with a glucose

molecule, which is a 6-carbon molecule, that this

essentially gets split in half by glycolysis and we end up 2

pyruvic acids or two pyruvate molecules.

So glycolysis literally splits this in half.

It lyses the glucose.

We end up with two pyruvates or pyruvic acids.

ruby And these are 3-carbon molecules.

There's obviously a lot of other stuff

going on in the carbons.

You saw it in the past. And you could look up their

chemical structures on the internet or on Wikipedia and

see them in detail.

But this is kind of the important thing.

Is that it was lysed, it was cut in half.

And this is what happened in glycolysis.

And this happened in the absence of oxygen.

Or not necessarily.

It can happen in the presence or in the absence of oxygen.

It doesn't need oxygen.

And we got a net payoff of two ATPs.

And I always say the net there, because remember, it

used two ATPs in that investment stage, and then it

generated four.

So on a net basis, it generated four, used two, it

gave us two ATPs.

And it also produced two NADHs.

That's what we got out of glycolysis.

And just so you can visualize this a little bit better, let

me draw a cell right here.

Maybe I'll draw it down here.

Let's say I have a cell.

That's its outer membrane.

Maybe its nucleus, we're dealing with

a eukaryotic cell.

That doesn't have to be the case.

It has its DNA and its chromatin form all spread

around like that.

And then you have mitochondria.

And there's a reason why people call it the power

centers of the cell.

We'll look at that in a second.

So there's a mitochondria.

It has an outer membrane and an inner

membrane just like that.

I'll do more detail on the structure of a mitochondria,

maybe later in this video or maybe I'll do a

whole video on them.

That's another mitochondria right there.

And then all of this fluid, this space out here that's

between the organelles-- and the organelles, you kind of

view them as parts of the cell that do specific things.

Kind of like organs do specific things

within our own bodies.

So this-- so between all of the organelles you have this

fluidic space.

This is just fluid of the cell.

And that's called the cytoplasm.

And that's where glycolysis occurs.

So glycolysis occurs in the cytoplasm.

Now we all know-- in the overview video-- we know what

the next step is.

The Krebs cycle, or the citric acid cycle.

And that actually takes place in the inner membrane, or I

should say the inner space of these mitochondria.

Let me draw it a little bit bigger.

Let me draw a mitochondria here.

So this is a mitochondria.

It has an outer membrane.

It has an inner membrane.

If I have just one inner membrane we call it a crista.

If we have many, we call them cristae.

This little convoluted inner membrane, let

me give it a label.

So they are cristae, plural.

And then it has two compartments.

Because it's divided by these two membranes.

This compartment right here is called the outer compartment.

This whole thing right there, that's the outer compartment.

And then this inner compartment in here, is called

the matrix.

Now you have these pyruvates, they're not quite just ready

for the Krebs cycle, but I guess-- well that's a good

intro into how do you make them ready

for the Krebs cycle?

They actually get oxidized.

And I'll just focus on one of these pyruvates.

We just have to remember that the pyruvate, that this

happens twice for every molecule of glucose.

So we have this kind of preparation step

for the Krebs Cycle.

We call that pyruvate oxidation.

And essentially what it does is it cleaves one of these

carbons off of the pyruvate.

And so you end up with a 2-carbon compound.

You don't have just two carbons, but its backbone of

carbons is just two carbons.

Called acetyl-CoA.

And if these names are confusing, because what is

acetyl coenzyme A?

These are very bizarre.

You could do a web search on them But I'm just going to use

the words right now, because it will keep things simple and

we'llget the big picture.

So it generates acetyl-CoA, which is

this 2-carbon compound.

And it also reduces some NAD plus to NADH.

And this process right here is often given credit-- or the

Krebs cycle or the citric acid cycle gets

credit for this step.

But it's really a preparation step for the Krebs cycle.

Now once you have this 2-carbon chain, acetyl-Co-A

right here.

you are ready to jump into the Krebs cycle.

This long talked-about Krebs cycle.

And you'll see in a second why it's called a cycle.

Acetyl-CoA, and all of this is catalyzed by enzymes.

And enzymes are just proteins that bring together the

constituent things that need to react in the right way so

that they do react.

So catalyzed by enzymes.

This acetyl-CoA merges with some oxaloacetic acid.

A very fancy word.

But this is a 4-carbon molecule.

These two guys are kind of reacted together, or merged

together, depending on how you want to view it.

I'll draw it like that.

It's all catalyzed by enzymes.

And this is important.

Some texts will say, is this an enzyme catalyzed reaction?


Everything in the Krebs cycle is an

enzyme catalyzed reaction.

And they form citrate, or citric acid.

Which is the same stuff in your lemonade

or your orange juice.

And this is a 6-carbon molecule.

Which makes sense.

You have a 2-carbon and a 4-carbon.

You get a 6-carbon molecule.

And then the citric acid is then oxidized

over a bunch of steps.

And this is a huge simplification here.

But it's just oxidized over a bunch of steps.

Again, the carbons are cleaved off.

Both 2-carbons are cleaved off of it to get back to

oxaloacetic acid.

And you might be saying, when these carbons are cleaved off,

like when this carbon is cleaved off,

what happens to it?

It becomes CO2.

It gets put onto some oxygen and leaves the system.

So this is where the oxygen or the carbons, or the carbon

dioxide actually gets formed.

And similarly, when these carbons get cleaved

off, it forms CO2.

And actually, for every molecule of glucose you have

six carbons.

When you do this whole process once, you are generating three

molecules of carbon dioxide.

But you're going to do it twice.

You're going to have six carbon dioxides produced.

Which accounts for all of the carbons.

You get rid of three carbons for every turn of this.

Well, two for every turn.

But really, for the steps after glycolysis you get rid

of three carbons.

But you're going to do it for each of the pyruvates.

You're going to get rid of all six carbons, which will have

to exhale eventually.

But this cycle, it doesn't just generate carbons.

The whole idea is to generate NADHs and FADH2s and ATPs.

So we'll write that here.

And this is a huge simplification.

I'll show you the detailed picture in a second.

We'll reduce some NAD plus into NADH.

We'll do it again.

And of course, these are in separate steps.

There's intermediate compounds.

I'll show you those in a second.

Another NAD plus molecule will be reduced to NADH.

It will produce some ATP.

Some ADP will turn into ATP.

Maybe we have some-- and not maybe, this is what happens--

some FAD gets-- let me write it this way-- some FAD gets

oxidized into FADH2.

And the whole reason why we even pay attention to these,

you might think, hey cellular respiration is all about ATP.

Why do we even pay attention to these NADHs and these

FADH2s that get produced as part of the process?

The reason why we care is that these are the inputs into the

electron transport chain.

These get oxidized, or they lose their hydrogens in the

electron transport chain, and that's where the bulk of the

ATP is actually produced.

And then maybe we'll have another NAD get reduced, or

gain in hydrogen.

Reduction is gaining an electron.

Or gaining a hydrogen whose electron you can hog.


And then we end up back at oxaloacetic acid.

And we can perform the whole citric acid cycle over again.

So now that we've written it all out, let's account for

what we have. So depending on-- let me draw some dividing

lines so we know what's what.

So this right here, everything to the left of that line right

there is glycolysis.

We learned that already.

And then most-- especially introductory-- textbooks will

give the Krebs cycle credit for this pyruvate oxidation,

but that's really a preparatory stage.

The Krebs cycle is really formally this part where you

start with acetyl-CoA, you merge it

with oxaloacetic acid.

And then you go and you form citric acid, which essentially

gets oxidized and produces all of these things that will need

to either directly produce ATP or will do it indirectly in

the electron transport chain.

But let's account for everything that we have. Let's

account for everything that we have so far.

We already accounted for the glycolysis right there.

Two net ATPs, two NADHs.

Now, in the citric acid cycle, or in the Krebs cycle, well

first we have our pyruvate oxidation.

That produced one NADH.

But remember, if we want to say, what are we producing for

every glucose?

This is what we produced for each of the pyruvates.

This NADH was from just this pyruvate.

But glycolysis produced two pyruvates.

So everything after this, we're going to multiply by two

for every molecule of glucose.

So I'll say, for the pyruvate oxidation times two means that

we got two NADHs.

And then when we look at this side, the formal Krebs cycle,

what do we get?

We have, how many NADHs?

One, two, three NADHs.

So three NADHs times two, because we're going to perform

this cycle for each of the pyruvates produced from


So that gives us six NADHs.

We have one ATP per turn of the cycle.

That's going to happen twice.

Once for each pyruvic acid.

So we get two ATPs.

And then we have one FADH2.

But it's good, we're going to do this cycle twice.

This is per cycle.

So times two.

We have two FADHs.

Now, sometimes in a lot of books these two NADHs, or per

turn of the Krebs cycle, or per pyruvate this one NADH,

they'll give credit to the Krebs cycle for that.

So sometimes instead of having this intermediate step,

they'll just write four NADHs right here.

And you'll do it twice.

Once for each puruvate.

So they'll say eight NADHs get produced from the Krebs cycle.

But the reality is, six from the Krebs cycle two from the

preparatory stage.

Now the interesting thing is we can account whether we get

to the 38 ATPs promised by cellular respiration.

We've directly already produced, for every molecule

of glucose, two ATPs and then two more ATPs.

So we have four ATPs.

Four ATPs.

How many NADHs do we have?

2, 4, and then 4 plus 6 10.

We have 10 NADHs.

And then we have 2 FADH2s.

I think in the first video on cellular

respiration I said FADH.

It should be FADH2, just to be particular about things.

And these, so you might say, hey, where are our 38 ATPs?

We only have four ATPs right now.

But these are actually the inputs in the electron

transport chain.

These molecules right here get oxidized in the electron

transport chain.

Every NADH in the electron transport chain

produces three ATPs.

So these 10 NADHs are going to produce 30 ATPs in the

electron transport chain.

And each FADH2, when it gets oxidized and gets turned back

into FAD in the electron transport chain,

will produce two ATPs.

So two of them are going to produce four ATPs in the

electron transport chain.

So we now see, we get four from just what

we've done so far.

Glycolysis, the preparatory stage and the Krebs or citric

acid cycle.

And then eventually, these outputs from glycolysis and

the citric acid cycle, when they get into the electron

transport chain, are going to produce another 34.

So 34 plus 4, it does get us to the promised 38 ATP that

you would expect in a super-efficient cell.

This is kind of your theoretical maximum.

In most cells they really don't get quite there.

But this is a good number to know if you're going to take

the AP bio test or in most introductory biology courses.

There's one other point I want to make here.

Everything we've talked about so far, this is carbohydrate


Or sugar catabolism, we could call it.

We're breaking down sugars to produce ATP.

Glucose was our starting point.

But animals, including us, we can catabolize other things.

We can catabolize proteins.

We can catabolize fats.

If you have any fat on your body, you have energy.

In theory, your body should be able to take that fat and you

should be able to do things with that.

You should be able to generate ATP.

And the interesting thing, the reason why I bring it up here,

is obviously glycolysis is of no use to these things.

Although fats can be turned into glucose in the liver.

But the interesting thing is that the Krebs cycle is the

entry point for these other catabolic mechanisms. Proteins

can be broken down into amino acids, which can be broken

down into acetyl-CoA.

Fats can be turned into glucose, which actually could

then go the whole cellular respiration.

But the big picture here is acetyl-CoA is the general

catabolic intermediary that can then enter the Krebs cycle

and generate ATP regardless of whether our fuel is

carbohydrates, sugars, proteins or fats.

Now, we have a good sense of how everything works out right

now, I think.

Now I'm going to show you a diagram that you might see in

your biology textbook.

Or I'll actually show you the actual diagram from Wikipedia.

I just want to show you, this looks very

daunting and very confusing.

And I think that's why many of us have trouble with cellular

respiration initially.

Because there's just so much information.

It's hard to process what's important.

But I want to just highlight the important steps here.

Just so you see it's the same thing that we talked about.

From glycolysis you produce two pyruvates.

That's the pyruvate right there.

They actually show its molecular structure.

This is the pyruvate oxidation step that I talked about.

The preparatory step.

And you see we produce a carbon dioxide.

And we reduce NAD plus into NADH.

Then we're ready to enter the Krebs cycle.

The acetyl-CoA and the oxaloacetate or oxaloacetic

acid, they are reacted together to

create citric acid.

They've actually drawn the molecule there.

And then the citric acid is oxidized through the Krebs

cycle right there.

All of these steps, each of these steps are

facilitated by enzymes.

And it gets oxidized.

But I want to highlight the interesting parts.

Here we have an NAD get reduced to NADH.

We have another NAD get reduced to NADH.

And then over here, another NAD gets reduced to NADH.

So, so far, if you include the preparatory step, we've had

four NADHs formed, three directly from the Krebs cycle.

That's just what I told you.

Now we have, in this diagram they say GDP.

GTP gets formed from GDP.

The GTP is just guanosine triphosphate.

It's another purine that can be a source of energy.

But then that later can be used to form an ATP.

So this is just the way they happen to draw it.

But this is the actual ATP that I drew in the

diagram on the top.

And then they have this Q group.

And I won't go into it.

And then it gets reduced over here.

It gets those two hydrogens.

But that essentially ends up reducing the FADH2s.

So this is where the FADH2 gets produced.

So as promised, we produced, for each pyruvate that

inputted-- remember, so we're going to do it twice-- for

each pyruvate we produced one, two, three, four NADHs.

We produced one ATP and one FADH2.

That's exactly what we saw up here.

I'll see you in the next video.