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Arguably, one of the most important molecules in all of
biology is ATP.
ATP, which stands for adenosine triphosphate.
Which sounds very fancy.
But all you need to remember, or any time you see ATP
hanging around in some type of biochemical reaction,
something in your brain should say, hey, we're dealing with
Or another way to think of ATP is the currency-- I'll put
that in quotes-- of biological energy.
So how is it a currency of energy?
Well ATP stores energy in its bonds.
And I'll explain what that means in a second.
And before we learn what an adenosine group or a
3-phosphate group looks like, you can just take a bit of a
leap of faith, that you could imagine ATP as being made up
of something called-- let me do it in a nice color-- an
adenosine group right there.
And then attached to it you'll have three phosphates.
Not might, you will.
You'll have three phosphates attached to it just like that.
And this is ATP.
Tri- meaning three phosphate groups.
Now if you take adenosine triphosphate and you hydrolyze
this bond, which means if you take this in
the presence of water.
So let me just throw some water in here.
Let's say I have H2O.
Then one of these phosphate groups will break off.
Essentially part of this water joins to this phosphate group,
and then part of it joins to this
phosphate group right there.
And I'll show you that in a little bit more detail.
But I want to give you the big picture first.
What you're left with is an adenosine group that now has
two phosphates on it.
And this is called adenosine diphosphate or ADP.
Before we had triphosphate, which means three phosphates.
Now we have diphosphate, adenosine triphosphate, so
instead of a tri here we just write a di.
Which means you have two phosphate groups.
And so the ATP has been hydrolyzed, or you have broken
off one of these phosphate groups.
And so now you're left with ADP and then an extra
phosphate group right here.
And-- and this is the whole key to everything that we talk
about when we're dealing with ATP-- and
you have some energy.
And so when I talk about ATP being the currency of
biological energy, this is why.
Is that if you have ATP, and if you were to-- through some
chemical reaction-- you pop off this phosphate right here.
It's going to generate energy.
That energy can be used for just general heat.
Or you could couple this reaction with other reactions
that require energy.
And then those reactions will be able to move forward.
So, I draw these circles.
Adenosine and phosphates.
And really, this is all you need to know.
Already, what I've shown you right here is really all you
need to know to operationally think of how ATP operates in
most biological systems. And if you want to
go the other way.
If you have energy and you want to generate ATP, the
reaction will just go this way.
Energy plus a phosphate group plus some ADP, you
can go back to ATP.
And so this is stored energy.
So this side of the equation is stored energy.
And this side of the equation is used energy.
And this is really all you--well this is 95% of what you
need to know to really understand the function of ATP
in biological systems. It's just a store of energy when
you-- ATP has energy.
When you break a phosphate off, it generates energy.
And then if you want to go from ADP and a phosphate back
to ATP, you have to use energy up again.
So if you have ATP, that's a source of energy.
If you have ADP and you want ATP, you need to use energy.
And so far I've just drawn a circle with an A around it and
said that's an adenosine.
But sometimes I think it's satisfying to see what the
molecule actually looks like.
So I cut and pasted this from Wikipedia.
And the reason why I didn't show this to you initially is
because this looks very complicated.
While the conceptual reason why ATP is the currency of
energy, I think is fairly straightforward.
When it has three phosphates, one phosphate can break off.
And then that'll result with some energy being
put into the system.
Or if you want to attach that phosphate you
have to use up energy.
That's just the basic principle of ATP.
But this is its actual structure.
But even here we can break it down and see that it's really
not too bad.
We said adenosine.
Let me draw the adenosine group.
We have adenosine.
This right here is adenosine.
This part of the molecule right there.
That is adenosine.
And for those of you that have really paid attention to some
of the other videos, you might recognize that this part of
adenosine-- so this is called adenosine, but this part right
here-- is adenine.
Which is the same adenine that makes up the nucleotides that
are the backbone of DNA.
So some of these molecules in biological systems have more
than one use.
This is the same adenine where we talk
about adenine and guanine.
This is a purine.
And there's also the pyrimidines, but I won't go
into that much.
But that's the same molecule.
So that's just an interesting thing.
The same thing that makes up DNA is also part of what makes
up these energy currency molecules.
So the adenine makes part of the adenosine part of ATP.
And then the other part right here is ribose.
Which you might also recognize from RNA, ribonucleic acid.
That's because you have ribose dealing
in the whole situation.
But I won't go into that much.
But ribose is just a 5-carbon sugar.
When they don't draw the molecule, it's implied that
it's a carbon.
So this is one carbon right there, two carbons, three
carbons, four carbons, five carbons.
And that's just nice to know.
It's nice to know that they share parts of their
molecules with DNA.
And these are familiar building blocks that we see
over and over again.
But I want to emphasize that knowing this, or memorizing
this, in no way will help you understand the simpler
understanding of ATP just being what
drives biological reactions.
And then here I drew 3-phosphate groups, and this
is their actual molecular structure.
Their Lewis structures right here.
That's one phosphate group.
This is the second phosphate group.
And this is a third phosphate group.
Just like that.
When I first learned this, my first question was, OK I can
take this as a leap of faith that if you take one of these
phosphate groups off or if this bond is hydrolyzed, that
somehow that releases energy.
And then I kind of went on and answered all the questions
that I had to answer.
But why does it release energy?
What is it about this bond that releases energy?
Remember all bonds are are electrons being shared with
So the best way you could think about it is right here.
These electrons that are being shared right across this bond,
or this electron that's being shared right across this bond,
and it's coming from the phosphate.
I won't draw the Periodic Table right now.
But you know the phosphate has five electrons to share.
It's less electronegative than oxygen, so oxygen will kind of
hog the electron.
But this electron is very uncomfortable.
There's a couple of reasons why it is uncomfortable.
It's in a high energy state.
One reason why is, you have all these
negative oxygens here.
So they kind of want to push away from each other.
So these electrons in this bond really can't kind of get
close to the nucleus.
They'll go into kind of a low energy state.
All of this is more of an analogy than the reality.
We all know that electrons can get quite complex.
And there's a whole quantum mechanical world.
But that's a good way to think of it.
That these molecules want to be away from each other.
But you have these bonds, so this electron, it's kind of in
a high energy state.
It's further from the nucleuses of these two atoms
than it might want to be.
And when you pop this phosphate group off, all of a
sudden these electrons can enter into a
lower energy state.
And that generates energy.
So this energy right here is always-- in fact in any
chemical reaction where they say energy is generated, it's
always from electrons going to a lower energy state.
That's what it's all about.
And later in future videos when we do cellular
respiration and glycolysis and all that, whenever we show
energy, it's really from electrons going from
uncomfortable states to more comfortable states.
And in the process they generate energy.
If I'm in a plane or I'm jumping out of a plane, I have
a lot of potential energy right when I
jump out of the plane.
And you can view that as an uncomfortable state.
And then when I'm sitting on my couch watching football, I
have a lot less potential energy, so that's a very
And I could have generated a lot of energy
falling to my couch.
But I don't knows.
My analogies always break down at some point.
Now, the last thing I want to go over for you is exactly how
this reaction happens.
So far you could turn off this video and you could already
deal with ATP as it is used in 95% of biology,
especially AP Bio.
But I want you to understand how this
reaction actually happens.
So to do that, what I'm going to do is copy and
paste parts of these.
So I already told you that this guy right here is going
to break off of the ATP.
So that's the phosphate group that breaks off.
And then you have the rest of it.
You have the ADP that's left over.
So this is the ADP.
I don't even have to copy and paste all of this stuff.
You can just accept that that's the adenosine group.
Just like that.
So we've already said that this thing gets hydrolyzed
off, or gets cut off and that generates energy.
But what I want to do is actually
show you the mechanism.
A little bit of hand-wavy mechanism of how
this actually happens.
So I said this reaction occurs in the presence of water.
So let me draw some water here.
So I have an oxygen and a hydrogen.
And then I have another hydrogen.
That's water right there.
So hydrolysis is just a reaction where you say, hey,
this guy here, he wants to bond with something or he
wants to share someone else's electrons.
So maybe this hydrogen right here goes down here and shares
its electron with this oxygen right here.
And then this phosphorus, it has an extra electron that it
needs to share.
Remember it has five valence electrons; it wants to share
them with oxygen.
It has one, two, three, four being shared right now.
Well, if this hydrogen goes to this guy, then you're left
with this blue OH right here.
And this guy can share one of the
phosphorus' extra electrons.
So you get the OH just like that.
So that's the actual process that happens.
And it could go the other way as well.
I could've cleaved it here.
I could have cleaved the whole thing here.
And so this guy would have kept the oxygen and the
hydrogen would have gone to him.
And then this guy would have taken the OH.
It could happen in either order.
And so either order would be fine.
And there's one other point I want to make.
And this is a little bit more complex.
And I was even wondering whether I wanted to make it.
My whole reason why you're kind of in a lower energy
state is, once you break apart--actually let me go
down here-- is because I said, hey, this electron is happier
when it's-- so let's say this electron that was part of this
phosphorus is happier now.
It's in a lower energy state because
it's not being stretched.
It's not having to spend time between that guy and that guy
because this molecule and this molecule want to spread apart
because they have negative charges.
That's part of the reason.
The other reason why, and we'll talk about this in a lot
more detail when we learn more about organic chemistry, is
that this has more resonance.
More resonance structures or resonance configurations.
And all that means is that these electrons, these extra
electrons here, they can kind of move about between the
different atoms. And that makes it even more stable.
So if you imagine that this oxygen right here has an extra
electron with it.
So that extra electron right there, it could come down here
and then form a double bond with the phosphorus.
And then this electron right here can then jump back up to
And then that could happen on this side and on that side.
And I won't go into the details, but that's another
reason why it makes it more stable.
If you've already taken organic chemistry, you can
kind of appreciate that more.
But I don't want to get all into the weeds.
The most important thing to remember about ATP is that
when you cleave off a phosphate group it generates
energy that can drive all sorts of biological functions,
like growth and movement, muscle movement, muscle
contraction, electrical impulses in
nerves and the brain.
So this is the main battery or currency of energy in
biological systems. That's the main thing that you really
just need to remember about ATP.