Hi. It's Mr. Andersen. And in this podcast I'm going to talk about coupled reactions.
Coupled reactions are really important in biology. But it's hard to find a good definition
for what a coupled reaction is. And so whenever I'm failed for a definition I go to an analogy.
And so let's talk about this which is a water wheel or a water mill. Basically what happens
is as the water flows down it's going to power this giant wheel. And inside here that wheel
is going to be attached to a stone. And it's going to be used to grind grain into something
like flour. A windmill works the same way. And so basically what we're doing is we're
coupling the power of that flowing river with the power of this grinding of the mill. And
so in biology we don't use water. We use chemicals. And so the big chemical reaction that you
should be familiar with is the breakdown of ATP. So this is adenosine triphosphate. It's
triphosphate because you have three phosphate groups here on the end. And so basically it
has a delta G of negative 7.3 kilocals per mole. That means that it releases energy when
this third phosphate is broken off. And so if we look at what that looks like. Basically
this phosphate comes off. And we can use that to power other reactions. What happens next?
Well we have to reattach that phosphate. And so it's kind of like a rechargeable battery.
In fact this is the rechargeable battery inside life. And so we constantly are going from
adenosine triphosphate or ATP. We're releasing that phosphate to make ADP. And then we're
storing it again. Over and over and over again. And we do this constantly all day. And you
go through about a body's worth of weight in ATP everyday, which is a huge amount. And
so you're probably familiar with coupled reactions. But maybe not the term. And so what I'm going
to do is I'm going to from the sun to the thumb. In other words I'm going to show you
how energy is transferred from the sun to your thumb or the raising of your thumb. And
all of the coupled reactions that we see within that. And so we have to start of course with
plants. When we're talking about photosynthesis. The first part of photosynthesis is the light
reaction. Right here. We're inside the thylakoid membrane. And so as this electron moves through
here we're using the energy of that flowing electron to pump protons to the inside. So
we're coupling the reaction or that movement of the electron with the movement of the protons.
So that's a coupled reaction. Let me find another one. Right here we're moving this
electron here and we're eventually making NADPH. So that's a coupled reaction. Or right
here we're pumping those protons back through that thylakoid membrane and we're making ATP
out of it. And so that's a coupled reaction. We'd have another coupled reaction here. So
we have a number of coupled reactions. And we're still just in the light reaction. If
we look in the Calvin cycle you can see right here, basically we're breaking ATP down into
ADP. And we're using that to couple this reaction. Which is eventually going to make G3P. After
we have coupled another reaction. Breaking down NADPH into NADP+. And even if you look
back here as we make RUBP we're doing another coupled reaction. In this case we're taking
the energy that we'd stored in the light reaction. And we're actually releasing that to make
RUBP. So a bunch of coupled reactions. But we're not to the thumb yet. Because in the
thumb what we've got to do is we've got to take sugar and break it down. That's what
life does. And so basically we're talking now about cellular respiration. So where is
the coupled reaction here? As we break glucose down into pyruvate through the process of
glycolysis we're coupling that reaction with this other endergonic reaction where we're
storing that energy in NADPH. Or NADH, excuse me. Same thing here. As we breakdown and make
acetyl CoA we make more NADH. And we're doing the same thing in the Kreb cycle. Now if we
look at the oxidative phosphorylation, we're coupling the flow of these electrons with
the pumping of the protons. And we're coupling that reaction as the protons flow back in
with ATP synthase. And so what are you doing in a coupled reaction? You're releasing energy
in one reaction or one process. And then storing it in another. But let's keep going. We're
not to the thumb yet. In order to get your thumb to rise you have to actually use a nerve.
And to do that we're using another coupled reaction. This is the sodium potassium pump
in your nerves. And we're using that to establish a gradient. Or even if we get to the muscle
itself. As we breakdown that ATP into ADP, it's moving that myosin along the actin. And
that's actually what makes your muscle move and makes your thumb move up. And so what
are coupled reactions? Coupling the exergonic with the endergonic. The energy releasing
with the energy consuming reactions. But sometimes that occurs within an equation itself. And
so now we get to something that always confuses students. And these are called the redox reactions.
And so redox reactions are basically, in this case we're looking at electrons and where
the electrons flow. And so this right here is sugar with oxygen makes carbon dioxide
and water. And then it's going to make ATP. And so this is cellular respiration. And so
we call this a redox reaction because the electrons and the energy of those electrons
is being transferred from the sugar to the oxygen to make carbon dioxide. And the oxygen
is gaining those electrons. And as it's gaining those electrons it's eventually becoming water.
So we lose the electrons here. We gain the electrons here. So you can see it's like a
coupled reaction within a reaction. So we're losing electrons of the food. And we're gaining
the electrons of the oxygen. And when I say losing and gaining what I means is we are
physically transferring those electrons from one chemical to another. And they're high
energy electrons in our food. And then as they fall down to low energy bonded with oxygen
we release a lot of that energy. In this case in the form of ATP. But this doesn't have
to be sugar up here. This could be for example methane gas. What you burn in a bunsen burner.
And so as we lose those electrons in there we get energy in the form of heat. Or this
could be propane or gasoline or anything like that is a redox reaction. Losing electrons
and gaining electrons. Now that's sometimes confusing because we've got oxidation, reduction.
And so here's a quick way to remember that. The way I remember it is Oil Rig. And this
is what an oil rig looks like. And if you think about it the burning of oil is a redox
reaction. But why do I even talk about Oil Rig? Well if you remember O I L R I G. That
will remind you that oxidation is the the losing of electrons. In cellular respiration
that would be from our sugar. And reduction is gaining. In this case would be the gaining
of electrons by oxygen. And so that's just a coupling within a reaction. And so hopefully
that makes sense. What coupled reactions are. You just have to have a little bit of understanding
of thermodynamics. Exergonic and endergonic reactions. And I hope that's helpful.