Hi. It's Mr. Andersen and welcome to my podcast on Gibbs Free Energy. Whenever
I get to Gibbs free energy in the year I can see my kids eyes gloss over and they're confused.
And even Willard Gibbs talks about this in this quote. He said the idea of entropy and
the second law of thermodynamics will seem far fetched and may repel beginners as obscure
and difficult for comprehension. So what does that mean? I'm going to try and tackle something
that's been a problem for years. I'm going to try to make Gibbs free energy more understandable.
So that you really know what it is and understand the power of it. Especially in biology. And
so I think the first place to start is the name itself. The idea of free kind of gets
in the way. And so what I'd like to go back to is old name, which it used to be called
available energy. And so let's thing of delta G as available energy. Or energy that we can
actually use to do work. The other thing that I think makes it hard is the equation itself
is confusing. We've got delta G. We've got enthalpy. We've got entropy. We've got words
that you're maybe not understanding. And so what I'm going to try to do is pare this down
to the easiest explanation that I can. And then we can get into more of the specifics
a little bit later. And so the best place to start when you're trying to understand
available energy or free energy is to look at spontaneous reactions. Spontaneous reactions
are reactions that once you give them a little bit of a push, they'll actually go on their
own. These reactions also will tend to release energy. And they give energy to their surroundings.
Quintessential example in life is cellular respiration. But let's not get there yet.
So the three reactions that I'm going to talk about here are, number one a ball rolling
down a slide. Number two, diffusion. And then number three, a cherry bomb that's exploding.
And so we're going to apply Gibbs free energy to each of these. So let's start with a ball
at the top of a slide. And the ball at the top of the slide is going to roll down to
the bottom. And so the first thing that I want to give you is what's called enthalpy
or total energy of a system. And so if we say this is a system. What happens to the
total energy of the system as the ball rolls down the slide? Well if you know anything
about potential energy, the amount of potential energy that we have up here is going to be
greater than the amount of potential energy that we have down here. And so the total energy
of a system, we'll call that H or enthalpy. The total energy of the system has gone from
a big H to a little H. In other words the total energy of the system has decreased when
the ball rolls down. Now if I had to push it up again, then we would add energy to the
system. But this is a spontaneous reaction. And the enthalpy or the total energy is decreasing.
Now in biology we don't move around because of potential energy or mechanical potential
energy. In biology our energy or our potential energy is actually in these bonds. In other
words there's a huge amount of potential energy in this bond between the carbon and the hydrogen.
And so this right here is glucose. And if we can release some of that energy, we can
do it to do work. And so again, what is H? H stands for the total energy of the system
and it looks like in a spontaneous reaction that's actually going to get smaller. Or decrease.
Okay. Next one. Let's talk about diffusion. So in diffusion imagine it right here that
we've got a bunch of molecules in this container and they're bouncing around. And I remove
this wall. So if I remove that wall, what's going to happen to the molecules? The molecules
are going to spread out to fill that area. We call that diffusion. Now entropy is, we
use the symbol S for that, entropy is a measure or the disorder of a system. Or sometimes
we call that the randomness of the system. And so let's compare this. Right here we've
got a bunch of molecules on this side. And then we've got a lot of space over here. So
what happens to the disorder of that system as I do diffusion? Well it's becoming more
disordered. In other words the entropy is increasing. What's an example of that? Let's
say I go into your room and your room's a mess. It looks like this. If I say clean up
your room, then you could go like that. And so what happens through diffusion or in the
spontaneous reaction, well let's say remove this wall. What's going to happen now? We're
going to get even more disorder. So we're going to even get a bigger S value. And so
in this spontaneous reaction it looks like the S value is increasing. Okay. Last one
is that cherry bomb. So let's say we have a cherry bomb. We put it on the desk. Does
it explode? No. And one of the reasons it doesn't is the temperature is really low.
And so let's say I add a bonfire to the situation. So I increase the temperature. Does that make
it a more spontaneous reaction or is the reaction more likely to happen or less? Well it's more
likely to happen if I increase the temperature. More likely to get an explosion. Okay. So
those three things, total energy or enthalpy, entropy or S, and temperature can effect spontaneous
reactions. And so now let's apply that to Gibbs free energy. And so before we actually
get to the equation, let's do a little algebra here. So let's say I wrote this equation.
X = Y - AB. Okay. So let's say we had this equation right here and I were to decrease
this value, the Y value. What would that do to the X value? It would decrease it, right?
Let's say I were to increase the A value and increase the B value, what would that do to
the X value? Well since we're subtracting right here that would decrease it as well
or make it go even farther down. And so let's go back and summarize those three spontaneous
reactions. In the first one the ball rolled down. So what happened to our H value? Well
our enthalpy of the system decreased. What should that do to our Gibbs free energy or
our available energy? It should decrease that value. What happened here when we increased
the entropy of the system or increased the delta S? Well if we increase the delta S that
should also decrease the Gibbs free energy. So now we have two things decreasing that.
And what happened here? Well if we increase the temperature that made it more spontaneous.
So if we increase that, we also decrease the Gibbs free energy. So what's the moral of
the story? Moral of the story is that if the delta G ever decreases or if it's ever less
than zero, that's a spontaneous reaction. Likewise, if the delta G is greater than zero,
let's turn to the next slide, that's going to not be a non-spontaneous reaction. Okay.
So in summary. If the delta G is less than zero that's a spontaneous reaction or we call
that an exergonic reaction or an energy releasing reaction. If it's greater than zero that's
an endergonic reaction. And then finally if nothing happens to the available free energy
then it's just at equilibrium. The quintessential example of a spontaneous reaction in life
is going to be cellular respiration. Photosynthesis is an example of an endergonic reaction. And
so let's talk about each of those. So let's say we're doing cellular respiration. So what
I said earlier is that glucose has a certain amount of energy within the bonds. And so
let's do cellular respiration. So I take in sugar, my body is going to combine that with
oxygen. We're going to convert that to carbon dioxide and water. So let's animate and see
what happens. Wow. So a lot of stuff happens. So let's go back. So first of all let's look
at the entropy of the system. What happened? Did it become more or less ordered? It seems
like it became more random for sure. What happened to the enthalpy of the system? Well
we're releasing energy. So we have energy out here. So we have released energy. That
means that the total energy that was contained within those bonds has actually decreased.
And so you can look at the value right down here. The delta G of cellular respiration
is -686 kcal/mol. That means if you had a bout a third of a pound of glucose or sugar,
you were to break that down in your body, you would release about 686 calories, big
C, of energy. So we call that an exergonic reaction. Or it's releasing energy. Now we
can actually plot that on a energy diagram. So glucose itself had a certain amount of
free energy to start with. But in this cellular respiration, we end up with less energy. So
this is a spontaneous or a exergonic reaction. Now it doesn't just explode into flames, the
sugar that's sitting on your counter or the sugar that's inside your body. And so in order
to get that to work, you have to put a certain amount of energy into the system. And that
amount of energy we call activation energy. And so if you look back to the first three
examples I gave you, the ball just didn't roll down the slide on its own. You had to
push it. And the cherry bomb just didn't explode on its own. You had to add a little bit of
energy to that. We call that activation energy. But there's a net loss in energy. And so we
call that an exergonic reaction. Let's think about photosynthesis. What do you need for
photosynthesis? Really only three things. Carbon dioxide, water and sunlight. Oh, four
things. Because you also need plants. And so in photosynthesis what happens, well we
store that energy in glucose. And so if we look at our delta G value it's a positive
or we call that a endergonic reaction. If we were to actually draw the energy diagram
of that, we'd have a lower amount of energy to start with. A greater amount of energy
at the end. And so we're storing energy. And then it has more energy at the end in the
form of that glucose. Now where do we get the activation energy? In photosynthesis we
get that from the sun. And so those two reactions, releasing energy and storing energy allow
life to exist. But our day to day life doesn't use glucose. Doesn't use, we do use glucose,
but our second to second kind of a functioning doesn't use glucose. We actually use something
called ATP. And so ATP or adenosine triphosphate is the energy coinage of our cells. It's what
you're using right now. We can store it as energy and then we can kind of cash it in.
So let me show you what happens. ATP is broken down into ADP and it releases a certain amount
of energy. If we look at that the breakdown of ATP to ADP or adenosine diphosphate, it
only has two phosphates right here, plus a phosphate group, is an exergonic or the delta
G value is going to be negative. If we go back and convert that ADP back into ATP, now
you can see right here our delta G value is actually positive. Or if we break it, we release
energy and then we can store that again when we make ATP again. And so right now when you
move your finger like this, what's actually powering that muscle is ATP releasing energy.
Allows that muscle to contract and then we can do that over and over and over again.
And so you use respiration to get energy. But we actually store that in ATP over time.
So what is the secret of life? And how does all of this work? Well energy comes from the
sun. So we have a certain amount of energy up here. And we store that energy through
photosynthesis in sugars. And so the release of energy by the sun, the delta G is going
to be less than zero. It's an exergonic or an energy releasing reaction. But photosynthesis,
photosynthesis is going to have a delta G value that is greater than zero. In other
words it requires energy and it pairs that to the energy coming from the sun. Next we
make bread out of that. And then through cellular respiration, respiration, we are going to
release energy. So the delta G value now is less than zero. We use that to store ATP.
Now that ATP is used to breakdown into ADP. And so that's going to be a delta G value
of less than zero and that's going to release energy. But to convert that ADP back into
ATP is going to be a delta G value greater than zero. And so available energy and free
energy is super important. It's converting energy in sun eventually into energy that
we can use. And eventually that energy ends as heat. So the energy is converted that whole
way through but it just becomes more and more disordered as we get to the end. And so that's
free energy. And I hope that's helpful.