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ATP: Adenosine triphosphate | Energy and enzymes | Biology | Khan Academy

Sal: ATP or adenosine triposphate is often referred to as

the currency of energy, or the energy store, adenosine,

the energy store in biological systems.

What I want to do in this video is get

a better appreciation of why that is.

Adenosine triposphate.

At first this seems like a fairly complicated term,

adenosine triphosphate, and even when we look at its

molecular structure it seems quite involved, but if we break

it down into its constituent parts it becomes a little bit

more understandable and we'll begin to appreciate why,

how it is a store of energy in biological systems.

The first part is to break down this molecule between

the part that is adenosine and the part that

is the triphosphates, or the three phosphoryl groups.

The adenosine is this part of the molecule,

let me do it in that same color.

This part right over here is adenosine,

and it's an adenine connected to a ribose

right over there, that's the adenosine part.

And then you have three phosphoryl groups,

and when they break off they can turn into a phosphate.

The triphosphate part you have, triphosphate,

you have one phosphoryl group, two phosphoryl groups,

two phosphoryl groups and three phosphoryl groups.

One way that you can conceptualize this molecule which will

make it a little bit easier to understand how it's a store

of energy in biological systems is to represent this whole

adenosine group, let's just represent that as an A.

Actually let's make that an Ad.

Then let's just show it bonded to

the three phosphoryl groups.

I'll make those with a P and a circle around it.

You can do it like that, or sometimes you'll see it

actually depicted, instead of just drawing these

straight horizontal lines you'll see it depicted

with essentially higher energy bonds.

You'll see something like that to show

that these bonds have a lot of energy.

But I'll just do it this way for the sake of this video.

These are high energy bonds.

What does that mean, what does that

mean that these are high energy bonds?

It means that the electrons in this bond are in a

high energy state, and if somehow this bond could be

broken these electrons are going to go into a more

comfortable state, into a lower energy state.

As they go from a higher energy state into a lower, more

comfortable energy state they are going to release energy.

One way to think about it is if I'm in a plane and

I'm about to jump out I'm at a high energy state,

I have a high potential energy.

I just have to do a little thing and I'm going

to fall through, I'm going to fall down,

and as I fall down I can release energy.

There will be friction with the air, or eventually

when I hit the ground that will release energy.

I can compress a spring or I can move a turbine,

or who knows what I can do.

But then when I'm sitting on my couch

I'm in a low energy, I'm comfortable.

It's not obvious how I could go to a lower energy state.

I guess I could fall asleep or something like that.

These metaphors break down at some point.

That's one way to think about what's going on here.

The electrons in this bond, if you can give them just

the right circumstances they can come out of that bond

and go into a lower energy state and release energy.

One way to think about it, you start

with ATP, adenosine triphosphate.

And one possibility, you put it in the presence of water and

then hydrolysis will take place, and what you're going to

end up with is one of these things are going to be essentially,

one of these phosphoryl groups are going to be

popped off and turn into a phosphate molecule.

You're going to have adenosine, since you don't

have three phosphoryl groups anymore, you're only

going to have two phosphoryl groups, you're going to

have adenosine diphosphate, often known as ADP.

Let me write this down.

This is ATP, this is ATP right over here.

And this right over here is ADP, di for two,

two phosphoryl groups, adenosine diphosphate.

Then this one got plucked off, this one gets plucked

off or it pops off and it's now bonded to the oxygen

and one of the hydrogens from the water molecule.

Then you can have another hydrogen proton.

The really important part of this I have not drawn yet,

the really important part of it,

as the electrons in this bond right over here go into

a lower energy state they are going to release energy.

So plus, plus energy.

Here, this side of the reaction,

energy released, energy released.

And this side of the interaction

you see energy, energy stored.

As you study biochemistry you will see time and time

again energy being used in order to go from ADP and

a phosphate to ATP, so that stores the energy.

You'll see that in things like photosynthesis

where you use light energy to essentially,

eventually get to a point where this P is put back on,

using energy putting this P back on to the ADP to get ATP.

Then you'll see when biological systems need to use energy

that they'll use the ATP and essentially hydrolysis

will take place and they'll release that energy.

Sometimes that energy could be used just to generate heat,

and sometimes it can be used to actually forward

some other reaction or change the confirmation of

a protein somehow, whatever might be the case.