know that oxygen is essential for a flame. And so let's start by looking at a candle.
Why does a candle burn? Well what's really burning is paraffin wax which is a hydrocarbon.
But it's not the solid paraffin wax that makes up the candle. And it's not the liquid paraffin
wax that forms a puddle underneath the wick. It's a gas. In other words this is going to
be converted from a liquid to a gas. And that is what is igniting. It's not the wick. But
it's the gas around the outside of the wick that's actually on fire. Now you know this.
If I were to take a beaker and put it over the top of a candle, it will go out. But why
does it go out? Well you might say because there's no oxygen anymore. And you're right.
But what is it about the oxygen that's required to support that flame. Let's talk a little
bit about oxygen. Oxygen is the eighth atom on the periodic table. That means it has eight
protons and eight electrons. But what's interesting about it is that in nature it will form a
diatomic molecule. In other words it's going to have two oxygen molecules chemically bonded
together. But it still is highly electronegative. And what that means is that oxygen would love
to pull electrons towards it and form something like water. It's so highly electronegative
that it's almost the most highly electronegative atom on the periodic table. Only fluorine
beats out oxygen. And so when you're thinking about oxygen, thimk about an atom that we
would love to pull electrons towards it. And as it does that, it can release a certain
amount of energy. So think of it like gravity. Oxygen is like gravity. And it's pulling electrons
towards it. Just like an object is going to fall to earth when it's acted on by gravity.
And as it does that, the reactants in that chemical reaction, so the paraffin wax and
the oxygen, when we have that chemical reaction the reactants are going to have more potential
energy, energy of those chemical bonds, than the products are going to. And so we're going
to release that energy, like in a flame, as heat and light. Now you can see in living
systems that that would be a little bit problematic if we were to just burst into flames. It would
be hard for us to harness some of that energy. And so let's go back to the candle again.
Where is the energy? We said the energy is going to be found in the chemical bonds. In
other words the paraffin wax and the oxygen are going to have more potential energy then
when we break those bonds and reform bonds in the products that is water and carbon dioxide.
We're going to release energy. And so if we take the beaker and put it over the top of
the candle, it's eventually going to go out. Why is that? Well there's going to be an increase
in the amount of carbon dioxide inside the beaker. And there's going to be a decrease
in the amount of oxygen. And those two things together are going to decrease that potential
pull of the oxygen pulling on those electrons. And eventually the candle goes out. Now, what
does this have to do with life? Well life works the same way that a candle does. In
other words we have a certain amount of potential energy inside us. And we have a certain amount
of potential energy inside all the cells in our body. Where is that potential energy?
It's in our food. And when we combine that with oxygen, we're going to create products,
carbon dioxide and water, that are going to have less potential energy than was found
in the food. And so we're releasing energy. And in us we're going to use that energy to
form a molecule called ATP that has a certain amount of potential energy as well. Now if
you take that mouse and sadly cover it with a beaker, it's going to go out just like the
candle. Why is that? It's because we're removing that oxygen. We're increasing the amount of
carbon dioxide inside there, decreasing the amount of oxygen. And eventually the mouse
is going to die. And so to review how this works inside us and how it's a little bit
different than a candle, the energy remember is going to be found inside our food. And
a great example of that is glucose, which is going to be a monosaccharide. And we're
going to take that glucose. We're going to combine it with oxygen. And these high energy
electrons that are found in the glucose are going to be transferred to the oxygen and
we're going to release a certain amount of potential energy. In other words these reactants
are going to have more potential energy than the products, water and carbon dioxide. What
are we using that energy to do? We're not going to release it in light and in heat.
We're going to use it to make something called ATP. Where does this occur inside us? It's
going to occur in all of our cells in the mitochondria. And if I were to point to where,
it's going to be on this inner membrane on the inside of the mitochondria. And so let's
go back to talking about gravity again. So this electron has a certain amount of potential
energy. And it's still going to fall to earth and it's still going to be pulled by oxygen.
But instead of releasing all of that energy in one step, we're going to gradually step
those electrons down something called an electron transport chain. And instead of releasing
all the energy at once, we're going to release that energy in small little bits. And we're
going to use that to form a molecule that has ATP. ATP is going to store some of that
potential energy. In other words we're going to take some of the energy in those electrons
and use it to make a molecule called ATP, which has a certain amount of potential energy
which we can then release. Where does this occur? Again, on the inner membrane on the
inside of the mitochondria. And let's go take a look at that. So here we are on the inside
of the mitochondria. And it's folded over and over and over again to increase the surface
area. But here's those electrons again. How did they get there? They're going to be transported
by a carrier molecule called NADH. What happens to those electrons? Well let's watch. So the
electrons are going to be transferred to a protein. Now this protein is going to receive
those electrons. And it's going to use the energy of those electrons to pump a proton
to the outside of that membrane. And so again, this is essentially going to be a hydrogen
ion. It's a hydrogen atom that's lost one of its electrons. And let's keep watching
those electrons. As those electrons are transferred now to another protein, we're going to use
the energy of that to pump a proton to the outside of that membrane. Again as we watch
those electrons, every time they go through a protein, we're going to pump a proton to
the outside of that membrane. And so just like going down a staircase, we're releasing
the energy that was found in those electrons, bit after bit after bit. And we're using that
remember to pump protons to the outside. Now let's watch where those electrons go. Eventually
those electrons are going to be received by oxygen. And so oxygen is the final electron
acceptor. So it's still receiving those electrons at the end, and it's using that. So the electron,
the oxygen and then those protons that come through are going to form something called
water. This is the same thing that happens in a candle. However we've released a little
bit of energy at each step along the way. And so as those electrons work their way down
the electron transport chain, they eventually end up at oxygen. But where is the energy
now? The energy is going to be found in these protons out here, outside that inner membrane.
Where do they go? They're going to move through ATP synthase. So they're going to move through
this protein right here. It's ind of like a motor. And essentially as they flow through,
we're going to use the energy of that flowing molecule, we call that chemiosmosis, to form
ATP. So we're phosphorylating ADP. So we're taking that phosphate, attaching it to ADP
to make ATP. This has more potential energy than ADP. And that's going to be the energy
source that we use inside our body. Now some students think that ATP is energy. That it
is energy. It's not energy. It's going to contain a certain amount of potential energy.
In other words when we break ATP down into ADP and a phosphate, like when we're using
our muscles, we're releasing some of that potential energy that was found in the ATP.
Nice thing about cellular respiration is that ADP and phosphate can come back to the mitochondria
and we can make ATP again. And so what happens to the mouse when we cover it with a beaker?
Or what happens to our cells when they don't have enough oxygen? Well let's start over
again. If there's no oxygen here in the electron transport chain, let's watch what happens
to the electrons. Starts the same exact way. Those electrons are going to be transferred
to a protein. And as they move through that protein they're going to use the energy of
the flowing electrons to pump a proton to the outside of that membrane. But eventually
it stops. Eventually there's no place for these electrons to go. And the whole chain
will come to a halt because there's no oxygen. There's no final oxygen. There's no final
electron acceptor. There's no place for those electrons to go. And so the whole system is
going to shut down. Once it shuts down, we can't move these protons through ATP synthase.
We can't make ATP. And so the cell is going to die. Just like a candle going out. And
so what's the role of oxygen? Again, it's to receive those electrons. We eventually
make water and carbon dioxide, which have lower potential energy than our food, but
we can use that energy to make something called ATP. And we can use it to make cells live.
And I hope that was helpful.