Introduction to cellular respiration | Cellular respiration | Biology | Khan Academy

In my humble opinion, the
single most important 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 doing is, they're taking– and
I know this gets complicated here, but I think over the
course of the next few videos we'll get an intuition for it–
in these two parts of the reaction, glycolysis and the
Krebs cycle, we're constantly taking NAD– I'll write it as
NAD plus– and we're adding hydrogens to it to form NADH.

And this actually happens for
one molecule of glucose, this happens to 10 NADs. Or 10 NAD plusses
to become NADHs. And those are actually
what drive the electron transport chain. And I'll talk a lot more about
it and kind of how that happens and why is energy being
derived and how is this an oxidative reaction
and all of that. And what's getting oxidized
and what's being reduced. But I just wanted to
give due credit. These guys aren't just producing
two ATPs in each of these stages. They're also producing, actually
combined, 10 NADHs, which each produce three ATPs
in an ideal situation, the electron transport chain. And they're also doing it to
this other molecule, FAD, which is very similar. But they're producing FADH. Now I know all of this
is very complicated. I'll make videos on this
in the future. But the important thing to
remember is cellular respiration, all it is is taking
glucose and kind of repackaging the energy in
glucose, and repackaging it in the form of, your textbooks
will tell you, 38 ATPs. If you're doing an exam, that's
a good number to write.

It tends to, in reality
be a smaller number. It's also going to
produce heat. Actually most of it is
going to be heat. But 38 ATPs, and it does it
through three stages. The first stage is glycolysis,
where you're just literally splitting the glucose
into two. You're generating some ATPs. But the more important thing
is, you're generating some NADHs that are going to be used
later in the electron transport chain. Then those byproducts are split
even more in the Krebs cycle, directly producing
two ATPs. But that produces a
lot more NADHs. And all of those NADHs are used
in the electron transport chain to produce the
bulk of your energy currency, or your 34 ATPs..

You May Also Like