Normally when we talk about the
production of energy in a cell,
glucose and ATP are the main
characters of the story.
But in this video,
we're going to talk
about a behind-the-scene
player called
electron-carrier
molecules that really
do play a vital role in this
energy-production process
as well.
But in order to talk about
electron-carrier molecules,
we need to first review the
main character of our story.
So let's start off with glucose.
Glucose, which has the chemical
formula of 6 carbons, 12
hydrogens, and 6
oxygens, we know
is broken down in our
body to produce energy.
And specifically,
this breakdown process
is an oxidation process-- a
process of losing electrons.
And glucose is oxidized
to carbon dioxide.
And if the flow of electrons
isn't very clear here to you,
definitely review one of the
previous videos on oxidation
reduction from a
biological viewpoint.
But the key idea is to notice
that this carbon is far more
oxidized, or electron
poor, when it is attached
to highly electronegative
oxygen molecule,
than when it's
attached to hydrogen.
So that's the key idea here.
Now in our bodies,
this reaction does not
take place in a single step.
Instead, it takes
place in many steps.
And so I'm going to
actually literally draw
a staircase with many
steps to indicate this.
And I just want to note
that this collective process
of breaking down
glucose in several steps
is called cellular respiration,
and that there are videos just
devoted to the details
of this process.
Now we're ready to talk about
our electron-carrier molecules.
And we can think about our
electron-carrier molecules
a bit like a molecular shuttle.
I'm drawing a picture
of an actual shuttle
here just to remind
us of this analogy.
At each step of glucose's
breakdown or oxidation,
a new breakdown
product of glucose
is formed that is given a
fancy name of a metabolite.
And just for short, I'm going
to abbreviate the rest of these
as "M."
Now the key point
to realize here
is that as glucose
is broken down,
the metabolites are
more and more oxidized.
That is to say, they have
less electron density.
And this is where our
electron carriers come in.
So recall that energy is
extracted from the oxidation
process by harnessing
the flow of electrons.
And it is the job of
the electron carriers
to harness the
electrons that are
lost at each step of
the breakdown process.
And so I'm going to go ahead
and draw a metaphorical shuttle
for our electron-carrier
molecule
at each stage of
glucose's breakdown.
And these
electron-carrier molecules
are of course carrying
the electrons that
are lost from the
oxidation process.
Ultimately, all of these
electron-carrier molecules
are going to shuttle
the electrons
that are harnessed from
the breakdown of glucose
to something called
the electron transport
chain, which is in
the mitochondria.
And at this point,
there are enzymes
that can facilitate the
transfer of these electrons
to the final electron acceptor
in our body, which is oxygen.
And remember, it is this flow
of electrons in the electron
transport chain that allows
the production of energy
to produce ATP-- the
cell's currency of energy.
So another question
that you might have is,
why does the body such
a complex mechanism
that involves many steps and
all of these electron-carrier
molecules to fuel the
production of ATP?
Well, let's reconsider
the chemical equation that
describes the overall
breakdown of glucose.
So glucose combines with oxygen
to produce carbon dioxide
and water.
And just so that we
conserve our mass,
let's put in our stoichiometry.
This describes the overall
breakdown of glucose.
And we know that this
reaction releases energy
to fuel the production of ATP.
Now another way to
view this process
is as a simple
combustion process.
And combustion is exactly
what it sounds like.
It's what you think of when
you think of something burning.
And a combustion process
just requires a fuel.
And in this case, our
carbon rich or electron
rich molecule of glucose
can be considered a fuel.
And it also requires
molecular oxygen,
which we also have here.
So to reword the
question that I just
posed, why doesn't
glucose, the fuel,
spontaneously
combust in one step,
instead of being broken
down into multiple steps
inside our body?
Let's first think
about the consequences
of combustion in our body
if glucose spontaneously
combusted in our body to
produce a large burst of energy,
likely in the form of heat.
All of that would
be unusable energy.
It would be unusable because
remember, the only energy
that our body really is able
to use is in the form of ATP.
And luckily for us, this
combustion processes
does not take place in one
step inside of our body.
Because even though it is
energetically favorable--
that is to say, it
releases energy--
it is actually
kinetically unfavorable.
That is to say, it has a
very high activation energy.
And you know this
actually intuitively,
because the combustion or
burning of sugar in food, only
happens when you
overcook something
at a high temperature.
And a high temperature is able
to overcome the activation
energy to allow this
combustion process to occur.
Now our bodies are, of
course, at a temperature
much lower than that needed
for a combustion process, which
is thankful.
Because otherwise,
we would be producing
all of this unusable energy.
Instead, our body overcomes
the high activation energy
of this reaction
by using enzymes
at each step of the reaction.
So I'm going to abbreviate
this here as "E."
And there are two benefits of
using a multitude of enzymes
to break down glucose.
The first benefit-- I'm going
to write benefits of enzymes.
The first benefit is that
we're able to produce
a large number of metabolites.
And this is important,
because it allows our body
to essentially reshuffle all
of the metabolic products
into many different pathways,
so that we can reuse and recycle
these products as
much as possible.
And the second benefit of using
many enzymes to break down
glucose is that we have a
slow and controlled oxidation
of glucose, which allows us
to harness all that energy,
which is in the form
of the electrons that
are being oxidized in a
very controlled manner.
As opposed to the
big burst of heat
that we would get in a
single-step combustion process.
And electron-carrier
molecules play a big role
in this controlled
oxidation process
that is facilitated by enzymes,
as you can see in our diagram
above, because they
are essentially
serving as a temporary storage
for all of the electrons that
are being lost by glucose.
And in fact, because of
they're close association
with the enzymes that facilitate
the breakdown at each step,
electron carriers are
also called coenzymes.
And coenzyme is exactly
what it sounds like.
It's a molecule or it's a
chemical functional group
that helps enzymes
perform their function.
Now the enzymes involved in
the breakdown of glucose,
for the most part,
are in the class
of enzymes that have a special
name called dehydrogenases.
And these dehydrogenase
enzymes do exactly what
their name implies-- they
dehydrogenate the glucose.
That is to say that
they take away hydrogen.
And when they're
taking away hydrogens,
they also take away electrons.
And most often they
do this by taking away
two electron along
with a proton, which
is what chemists call a hydride.
And this hydride has
a negative charge,
so it's often referred
to the hydride ion.
And just another
way to think of this
is as one proton
plus two electrons.
Now the two
electron-carrier molecules
are coenzymes that are
most commonly discussed
in the breakdown of
glucose, are two molecules
that go by the name
of NAD and FAD.
And I want to write
down the reaction that
occurs between these
electron-carrier molecules
and the electrons that they're
accepting from the glucose
molecule through this
dehydrogenase enzyme.
So first, it's notable that
NAD has a positive charge
in its most oxidized
form, while FAD does not.
And each co-enzyme reacts as two
protons and two electrons each.
In other words,
reacting with a hydride
ion plus an extra proton.
And once the
electron-carrier molecules
accept the electrons in this
way, they become reduced.
And the reduction products are
slightly different for each.
In the case of NAD,
it's reduced into NAD H.
So notice that we only
have one hydrogen.
And the reason for that is
NAD can only except to one
hydride ion.
On the other hand, FAD
can accept both hydrogens,
so it's reduced to FAD H2.
The extra proton is just donated
to solution in the case of NAD.
And it is these molecules
here-- these reduced form
of our electron-carrier
molecules--
that shuttle the electrons to
the electron transport chain
to allow for the
production of ATP.
