- [Narrator] I want you to be
silent for like three seconds.
Now, chances are in those three seconds,
your heart beat somewhere
between three and five times.
Now, your heart's a pretty amazing muscle.
I mean, it beats all day,
every day for your entire life.
And in order to do so,
it requires a ton of energy.
But where does all this energy come from?
So I drew a picture of a sun, here,
to describe that all of the energy
that our body uses for work
is originally derived from the sun.
But humans can't just
stand outside in the sun
and get energy that way.
Energy has to be transformed
into a usable form of energy for our body
that will allow our muscles
and things like our heart to beat.
Now, where does this energy come from?
I'll just erase some of my work here.
Well, our story starts with plants.
Now, plants take the
energy from the sunlight
and they convert inorganic
compounds into glucose,
and the energy from the sunlight
is added to this living system
in the form of the
chemical bonds of glucose.
Animals and humans then eat
the glucose from these plants,
and that glucose is then converted
into a usable form of energy,
which is known as ATP.
And this process of adding
energy to the system
and creating glucose is
known as photosynthesis.
And this process of breaking down glucose
into a usable form of energy
is known as cellular respiration.
So let's find out how this all happens.
This process starts in the
chloroplast of the plant cells.
And then this step, known
as the light reaction,
the energy from light
disrupts the H20, or water,
causing it to kick off a hydrogen ion,
and this hydrogen ion is then
bound to NADP to form NADPH.
And in the process of
all this, ETP is formed.
So you can see that we
already have an energy form
from the light, but this is
a relatively smaller amount
of energy, and it's this
NADPH that's created
that is a high-energy electron carrier
that can be used to
produce a lot more energy.
And the next step in this process
is known as the Calvin Cycle.
And in this part of photosynthesis,
we start with carbon dioxide.
And NADPH and ATP are added,
and there's a series of
reactions that occur.
And the specifics of these reactions
are less important than the
outcome, which is glucose.
So in these two steps,
in the light reaction
and the Calvin Cycle,
we take the energy that's in sunlight,
we store it into the
bonds of NADPH and ATP
and we use those to run the Calvin Cycle
to store all of our energy in
the chemical bonds of glucose.
Now, what happens when
glucose is eaten by an animal
and that animal then
wants to use the energy?
I mentioned earlier that this process
is celled cellular respiration,
and cellular respiration is
very similar to photosynthesis,
just in a backwards direction.
Now the first step, we
have to break glucose down
into a molecule known as pyruvate.
And this step gives off ATP.
And ATP is the usable form of energy,
but we're not giving off a whole lot yet.
We still have a lot of energy forms
contained in the bonds of pyruvate.
So pyruvate is then broken
down into acetyl-CoA.
And then acetyl-CoA then
enters the TCA cycle.
And TCA stands for tricarboxylic acid.
And this cycle is also known
by a couple other names
like the Kreb cycle and
the citric acid cycle.
Once again, there's a series
of reactions that occur
in this cycle, but the
specifics of these reactions
are less important than the outcome,
which is production of C02
as well as NADH and FADH2.
Now these molecules are similar
to the NADPH in photosynthesis,
in that they're high-energy
election carriers.
Now they enter a series of reactions
known as the electron transport chain.
And in this reaction, the hydrogen
from these high-energy election
carriers is bumped off.
And we have oxygen over here,
which is combined with the
hydrogen to form water, or H20.
And these hydrogens here
drive an enzymatic pump
that produces ATP.
And you can see here that photosynthesis
and cellular respiration
are very similar reactions,
just in the opposite direction.
And although they may have
different intermediates,
actually the products of one
are the reactants of the other,
and vice versa, and so
let me demonstrate that.
In photosynthesis, our
reactants are H20 and C02,
and our products are oxygen and glucose,
whereas in cellular respiration
we have glucose as a
reactant, as well as oxygen.
We're now producing C02 and water,
and you can actually see this
if I write out the equations
for photosynthesis and
for cellular respiration.
In photosynthesis, the
equation is 6CO2 + 6H2O,
which produces glucose or C6 H12 06 + 602.
And cellular respiration is
really just the opposite of that
where we take glucose, which
is C6 H12 06 + 6 oxygen,
which will end up producing 6
carbon dioxide and 6 waters.
Now there's an important
reactant in product
that isn't added in
these chemical equations,
and that is energy.
And in photosynthesis,
the energy is a reactant,
putting this energy into the
chemical bonds of glucose.
Whereas in cellular respiration,
the energy is a product.
And we're taking that energy
from the chemical bonds of glucose.
So let me just show you one more way
to demonstrate how energy
changes in these two reactions.
Now, to do this I'm gonna
draw a reaction diagram.
And on the x-axis, here, we
have the reaction progress,
and on the y-axis we have free energy.
Which is also known as G.
I'm gonna just dim down
the reaction a little bit
so that we can work over the top of it.
So if you look at the free energy level
for where the reactants
of photosynthesis start,
to where they end with glucose,
you're adding energy to this.
So you have a low free energy level
and you're going to a
higher free energy level.
So energy is added, and
that's from the sunlight,
whereas in cellular respiration,
you go from the reactants of glucose
with lots of free energy,
to low free energy with ATP,
you are releasing energy.
And it's this release in free energy
that allows our body to do
work, like pump the heart.
