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.