biochemical reaction, especially to us, is cellular

respiration.

And the reason why I feel so strongly about that is because

this is how we derive energy from what we

eat, or from our fuel.

Or if we want to be specific, from glucose.

At the end of the day, most of what we eat, or at least

carbohydrates, end up as glucose.

In future videos I'll talk about how we derive energy

from fats or proteins.

But cellular respiration, let's us go from glucose to

energy and some other byproducts.

And to be a little bit more specific about it, let me

write the chemical reaction right here.

So the chemical formula for glucose, you're going to have

six carbons, twelve hydrogens and six oxygens.

So that's your glucose right there.

So if you had one mole of glucose-- let me write that,

that's your glucose right there-- and then to that one

mole of glucose, if you had six moles of molecular oxygen

running around the cell, then-- and this is kind of a

gross simplification for cellular respiration.

I think you're going to appreciate over the course of

the next few videos, that one can get as involved into this

mechanism as possible.

But I think it's nice to get the big picture.

But if you give me some glucose, if you have one mole

of glucose and six moles of oxygen, through the process of

cellular respiration-- and so I'm just writing it as kind of

a big black box right now, let me pick a nice color.

So this is cellular respiration.

Which we'll see is quite involved.

But I guess anything can be, if you want to be particular

enough about it.

Through cellular respiration we're going to produce six

moles of carbon dioxide.

Six moles of water.

And-- this is the super-important part-- we're

going to produce energy.

We're going to produce energy.

And this is the energy that can be used to do useful work,

to heat our bodies, to provide electrical

impulses in our brains.

Whatever energy, especially a human body needs, but it's not

just humans, is provided by this

cellular respiration mechanism.

And when you say energy, you might say, hey Sal, on the

last video didn't you just-- well, if that was the last

video you watched, you probably saw that I said ATP

is the energy currency for biological systems. And so you

might say, hey, well it looks like glucose is the energy

currency for biological systems. And to some degree,

both answers would be correct.

But to just see how it fits together is that the process

of cellular respiration, it does produce energy directly.

But that energy is used to produce ATP.

So if I were to break down this energy portion of

cellular respiration right there, some of it

would just be heat.

You know, it just warms up the cell.

And then some of it is used-- and this is what the textbooks

will tell you.

The textbooks will say it produces 38 ATPs.

It can be more readily used by cells to contract muscles or

to generate nerve impulses or do whatever else-- grow, or

divide, or whatever else the cell might need.

So really, cellular respiration, to say it

produces energy, a little disingenuous.

It's really the process of taking glucose and producing

ATPs, with maybe heat as a byproduct.

But it's probably nice to have that heat around.

We need to be reasonably warm in order for our cells to

operate correctly.

So the whole point is really to go from glucose, from one

mole of glucose-- and the textbooks will

tell you-- to 38 ATPs.

And the reality is, this is in the ideal circumstances that

you'll produce 38 ATPs.

I was reading up a little bit before doing this video.

And the reality is, depending on the efficiency of the cell

in performing cellular respiration, it'll probably be

more on the order of 29 to 30 ATPs.

But there's a huge variation here and people are really

still studying this idea.

But this is all cellular respiration is.

In the next few videos we're going to break it down into

its kind of constituent parts.

And I'm going to introduce them to you right now, just so

you realize that these are parts of cellular respiration.

The first stage is called glycolysis.

Which literally means breaking up glucose.

And just so you know, this part, the glyco for glucose

and then lysis means to break up.

When you saw hydrolysis, it means using water to break up

a molecule.

Glycolysis means we're going to be breaking up glucose.

And in case you care about things like word origins,

glucose comes from, the gluc part of glucose comes from

Greek for sweet.

And glucose is indeed sweet.

And then all sugars, we put this ose ending.

So that just means sugar.

So you might think it's kind of a redundant statement to

say sweet sugar.

But there are some sugars that aren't sweet.

For example, lactose.

Milk, it might be a little bit, but when you actually

digest lactose then you can turn it into an actual sweet

sugar, but it doesn't taste sweet like glucose or fructose

or sucrose would taste.

But anyway, that's an aside.

But the first step of cellular respiration is glycolysis,

breaking up of glucose.

What it does is, it breaks up the glucose from a 6-carbon

molecule-- so it literally takes it from a 6-carbon

molecule-- let me draw it like this-- a 6-carbon molecule

that looks like this.

And it's actually a cycle.

Let me show you what glucose actually looks like.

This is glucose right here.

And notice you have one, two, three,

four, five, six carbons.

I got this off of Wikipedia.

Just look up glucose and you can see this diagram if you

want to kind of see the details.

You can see you have six carbons, six oxygens.

That's one, two, three, four, five, six.

And then all these little small blue

things are my hydrogens.

So that's what glucose actually looks like.

But the process of glycolysis, you're essentially just

taking-- I'm writing it out as a string, but you could

imagine it as a chain-- and it has oxygens and hydrogens

added to each of these carbons.

But it has a carbon backbone.

And it breaks that carbon backbone in two.

That's what glycolysis does, right there.

So you've kind of lysed the glucose and

each of these things.

And I haven't drawn all the other stuff

that's added on to that.

You know, these things are all bonded to other things, with

oxygens and hydrogens and whatever.

But each of these 3-carbon backbone

molecules are called pyruvate.

We'll go into a lot more detail on that.

But glycolysis, it by itself generates-- well,

it needs two ATPs.

And it generates four ATPs.

So on a net basis, it generates two-- let me write

this in a different color-- it generates two net ATPs.

So that's the first stage.

And this can occur completely in the absence of oxygen.

I'll do a whole video on glycolysis in the future.

Then these byproducts, they get

re-engineered a little bit.

And then they enter into what's called the Krebs cycle.

Which generates another two ATPs.

And then, and this is kind of the interesting point, there's

another process that you can say happens

after the Krebs cycle.

But we're in a cell and everything's bumping into

everything all of the time.

But it's normally viewed to be after glycolysis

and the Krebs cycle.

And this requires oxygen.

So let me be clear, glycolysis, this first step,

no oxygen required.

Doesn't need oxygen.

It can occur with oxygen or without it.

Oxygen not needed.

Or you could say this is called an anaerobic process.

This is the anaerobic part of the respiration.

Let me write that down too.

Anaerobic.

Maybe I'll write that down here.

Glycolysis, since it doesn't need oxygen, we

can say it's anaerobic.

You might be familiar with the idea of aerobic exercise.

The whole idea of aerobic exercise is to make you

breathe hard because you need a lot of oxygen

to do aerobic exercise.

So anaerobic means you don't need oxygen.

Aerobic means it needs oxygen.

Anaerobic means the opposite.

You don't need oxygen.

So, glycolysis anaerobic.

And it produces two ATPs net.

And then you go to the Krebs cycle, there's a little bit of

setup involved here.

And we'll do the detail of that in the future.

But then you move over to the Krebs cycle, which is aerobic.

It is aerobic.

It requires oxygen to be around.

And then this produces two ATPs.

And then this is the part that, frankly, when I first

learned it, confused me a lot.

But I'll just write it in order the way it's

traditionally written.

Then you have something called-- we're using the same

colors too much-- you have something called the electron

transport chain.

And this part gets credit for producing

the bulk of the ATPs.

34 ATPs.

And this is also aerobic.

It requires oxygen.

So you can see, if you had no oxygen, if the cells weren't

getting enough oxygen, you can produce a

little bit of energy.

But it's nowhere near as much as you can produce once you

have the oxygen.

And actually when you start running out of oxygen, this

can't proceed forward, so what happens is some of these

byproducts of glycolysis, instead of going into the

Krebs cycle and the electron transport chain, where they

need oxygen, instead they go through a side process called

fermentation.

For some organisms, this process of fermentation takes

your byproducts of glycolysis and

literally produces alcohol.

That's where alcohol comes from.

That's called alcohol fermentation.

And we, as human beings, I guess fortunately or

unfortunately, our muscles do not directly produce alcohol.

They produce lactic acid.

So we do lactic acid fermentation.

Let me write that down.

Lactic acid.

That's humans and probably other mammals.

But other things like yeast will do alcohol fermentation.

So this is when you don't have oxygen.

It's actually this lactic acid that if I were to sprint

really hard and not be able to get enough oxygen, that my

muscles start to ache because this lactic acid

starts to build up.

But that's just a side thing.

If we have oxygen we can move to the Krebs cycle, get our

two ATPs, and then go on to the electron transport chain

and produce 34 ATPs, which is really the bulk of what

happens in respiration.

Now I said this as an aside, that to some

degree this isn't fair.

Because while these guys are operating they're also

producing these other molecules.

They're not producing them entirely, but what they're