To understand how organic carbon saves energy, stores energy,
we have to know something about what's called the oxidation
and reduction of the carbon. There are some simple rules.
1. When you put together a molecule, that has an
oxygen in it, that oxygen is greedy for two electrons
in the chemical bond. It pulls them very tightly towards itself.
Those electrons have a negative charge and so,
this oxygen has whats called, an Oxidation state
of -2 equal, to the -2 charge of
those two electrons that it's pulling close to itself.
2. Hydrogen, in contrast, is very generous with
one electron, when it forms a chemical bond.
Hydrogen in say, water, where its bond is with
oxygen, will have a deficit of one electron.
And so that means that it has an
oxidation state, kind of like a charge, of plus one.
However, there's an exception to this.
If you have two identical atoms together in a bond, like O2, two oxygens
are the same as each other, in that case they have to share equally.
In that case, the oxidation
state of those oxygens is each equal to zero.
And, then the other thing you have to know is that, the sum of the oxidation states
of all of the atoms in a molecule has to equal the charge in the molecule.
Using these simple rules, we can look at the various forms of
carbon that we're talking about and understand
how its oxidation state changes, when the photosynthesis
reaction happens. This is how the energy is stored.
You can understand the energy
storage, by looking at this bookkeeping of electrons.
The most oxidized form of carbon is CO2.
Each oxygen here has minus two oxidation state,
because each oxygen is greedy for those two electrons and there are two of
them, so there's a total of minus four oxidation state, in those oxygens.
The total charge on CO2 is zero, so carbon has to fill in the gap.
And it has to have an oxidation state of plus four, in order to make this work.
Methane, CH4, is the most reduced of the carbon species. That's because
each hydrogen that it's bonded to is donating one electron, so that's a charge
on the hydrogen of plus one, and there are four hydrogens, so then the carbon has to
be minus four, in order to have the overall charge on the molecule be zero.
Going from CO2, the oxidation state of carbon is plus four.
Here it's minus four.
The word oxidation kind of makes
sense, just by looking at where the oxygens
are. Carbon CO2 is oxidized carbon, Oxidized iron
is rust which is an iron oxide.
That makes sense, but if
you didn't know about the electrons, you wouldn't
make any sense out of the word reduced.
The word reduced comes from the oxidation state on the carbon.
This number is reduced, relative to that one.
This is a reduced carbon atom.
CH2O is actually right in the middle of these two.
It's half way between the oxidized and the reduced ends of the spectrum.
Oxygen has a charge
of minus two, there's two positive charges on the hydrogens,
so, the oxidation state of this carbon here is zero.
Now the reason why we're doing all this is because, the oxidized form of
carbon is the lowest energy form of carbon, if you have oxygen around.
Carbon in CO2 is like a dead battery.
You couldn't get any energy out of it, because it's already dead.
The more reduced the carbon gets, the more energy it's storing.
This will be important for fossil fuels, which we'll learn
about next because some fossil fuels are more reduced, like natural gasses.
Methane is very reduced.
Oil is in between, and coal is about here.
There are different amounts of energy
per carbon, in the different types of fossil fuels,
because of the different oxidation states of the carbon.
You can also see why the different fossil fuels yield different
amounts of energy, just by looking at the reaction of combustion.
Coal is mostly just elemental carbon and it combines with O2 to make CO2.
The formation of the CO2 is what yields the energy.
Organic Carbon
is the same oxidation state as coal, but it has got hydrogens and oxygens
in it, the H2 and the O here balance each other In the
oxidation state because it's
pulling two electrons closer, and two electrons are donating to electron.
This part washes, when you burn that, you get CO2 plus the water
but there is also water here, so you are not
really getting energy from H2O on the two sides there.
This is like coal.
You just have this H2O kind of tagging along on both sides.
It doesn't really effect things. But methane now, CH4, when you combust
that, you need more oxygen and you make this water that didn't exist before.
And so the extra energy
you get from methane is basically the energy you get from making water.