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