In this video we're going to talk about redox
reactions and start looking into the specifics
of the light reactions of photosynthesis.
What we are going to see is that as a metabolic
pathway, both photosynthesis and cellular
respiration involve many redox reactions.
Chemical reactions which transfer electrons
from one substance to another are called oxidation
reduction reactions or redox reactions for
short.
During these chemical reactions, some atoms
give away electrons or lose electrons whereas
other atoms gain or receive electrons.
We call the loss of electrons during a redox
reaction oxidation, whereas the acceptance
of electrons during a redox reaction is called
reduction.
So oxidation is loss of electrons, reduction
is gain of electrons.
This is a bit counterintuitive because why
would gaining electrons be called reduction?
Well, it goes back to our understanding that
electrons have negative electrical charges.
So if a molecule is gaining electrons, its
overall electrical charge is being reduced
or made more negative.
Now, one way to possibly remember the names
of these two steps in a redox reaction is
through the acronym O.i.L R.i.G.
So, for O.i.L R.i.G, oxidation is loss of
electrons and reduction is gain of electrons.
So O.i.L, oxidation is loss, R.i.G, reduction
is gain, and remember that we're talking about
the loss or gain of electrons.
Hopefully this memory aid will help you remember
the difference between the oxidation and reduction
steps of a redox reaction.
So let's see how these redox reactions are
used during the light reactions of photosynthesis.
You may wonder, why is transferring electrons
important or significant for photosynthesis?
Well, we discussed that potential energy can
be stored in an object based on its position
or location, the same is true for electrons.
They can store more or less potential energy
based on their position.
If an electron is at a high-energy state,
we call that an excited state and that means
that it has more stored energy.
Now, the electron, it still has its negative
charge, it's still made out of the same materials,
it's just in a position in which it has higher
energy.
Electrons are less stable when they store
this extra energy, so they will try to release
that energy soon after they have absorbed
it.
Often this comes in the form of fluorescence,
which is a release of light and heat.
In photosynthesis, the pigment chlorophyll
absorbs energy from light.
That energy is transferred to the electrons,
taking it to that excited state.
Now, if nothing were to be done with that
electron, it would simply release that energy
and return back to its ground state.
But, what happens in photosynthesis is that
when that electron becomes energized, before
it has a chance to release that energy, it
is then taken away from the pigment.
Transferred through a series of proteins in
an electron transport chain.
This electron transfer is a series of redox
reactions.
Light is absorbed by the pigment chlorophyll.
The energy of that absorbed light is then
transferred to the electrons of that chlorophyll.
That high-energy electron is then taken away
from chlorophyll.
Now what we'll see is that there are actually
two photosystems that operate during the light
reactions.
One photosystem splits water into oxygen atoms,
hydrogen protons, and electrons and then it
steals those electrons from water.
We'll see why that is in just a moment.
The other photosystem receives electrons from
the first and uses them to reduce molecules
of NADP+, again, these are the empty shuttle
buses, into NADPH or a filled shuttle bus.
Light is used in both these photosystems to
energize the electrons.
So we have our two photosystems, the water-splitting
photosystem also known as Photosystem II and
the NADPH-producing photosystem or Photosystem
I.
Now, you may be wondering why Photosystem
II seems to be acting before Photosystem I,
and it is true, the activity of Photosystem
II does happen first.
Unfortunately, these photosystems were named
based on the order in which they were discovered
and so the NADPH-producing photosystem was
the first discovered.
It wasn't until later that we discovered the
water-splitting photosystem.
So I apologize on behalf of science for the
poor naming of these photosystems.
This is one of the reasons why I prefer the
more descriptive names of water-splitting
photosystem and NADPH-producing photosystem.
Now let's look at what's happening in each
of these photosystems.
So with the water-splitting photosystem, light
is absorbed, electrons are energized, those
electrons are then taken away from that chlorophyll,
passing down a series of electron transport
chains, all the while releasing energy.
The energy that's released by those electrons
ultimately is used to make ATP, and we'll
talk a bit more about that in just a moment,
but we've left Photosystem II with a problem.
It had electrons, those electrons were energized,
and now those electrons were taken away.
It needs to replace those electrons, and the
easiest way for it to do that is to split
water molecules into hydrogen protons, oxygen
atoms, and electrons.
It then takes those electrons away from the
water.
The hydrogen protons dissociate into the solution
and it's the oxygen atoms that end up combining
together to form oxygen gas.
So what happened to the electrons that were
taken away from Photosystem II?
Well, they travel down an electron transport
chain, releasing their energy that will ultimately
be used to make ATP.
When those electrons arrive at the NADPH-producing
photosystem, they're back to being low energy
electrons.
So more light is absorbed at Photosystem I.
Those electrons become energized and again
those energized electrons are taken away from
Photosystem I.
In this case, those high-energy electrons
are now loaded on to the shuttle bus molecules,
reducing NADP+ into NADPH.
And so actually, molecules of ATP and molecules
of NADPH, these are the two forms of chemical
energy made during the light reactions.
So there are two types of chemical energy
produced during the light reactions.
These are energized molecules of ATP and NADPH.
These chemical energy products will be used
during the Calvin cycle to form sugar.
So let's look at the activity of these two
photosystems, the water-splitting photosystem
or photosystem II, it splits water molecules
into electrons, oxygen gas, and hydrogen protons.
It then energizes the electrons, which end
up entering the electron transport chain.
Energy from electron transport is used to
pump Hydrogen protons across that thylakoid
membrane, so those protons are being pumped
from the stroma into the thylakoid membrane.
This generates a proton gradient.
So electrons are energized by absorbing light,
are sent down the electron transport chain
through a series of redox reactions, and more
electrons have to replace the ones that are
lost by the splitting of water molecules.
Once those electrons arrive at Photosystem
I, those electrons are energized again.
Once these electrons are energized a second
time, they are then loaded onto the empty
shuttle buses.
NADPH is produced from NADP+ in the stroma,
electrons from Photosystem I and hydrogen
ions also from the stroma.
This is how we get our energized molecules
of NADPH.
Through the actions of these two photosystems,
there are now a bunch of hydrogen protons
in the thylakoid space and very few hydrogen
protons in the stroma.
This is a concentration gradient and that
proton gradient represents stored energy.
Diffusion is going to want to equalize that
proton concentration on both sides of the
membrane, but ions cannot diffuse through
a membrane on their own.
They need to have a channel protein and in
this case, there's a very specific channel
protein known as ATP synthase.
ATP synthase is a channel for protons or hydrogen
ions to pass from one side of the membrane
to the other, but as those protons move through
ATP synthase, it is able to use the energy
of movement to charge molecules of ATP.
So proton movement through ATP synthase drives
phosphorylation or the addition of a phosphate,
in essence, recharging those molecules of
ADP into ATP, and so this is how we get our
second type of chemical energy, ATP.
So both NADPH and ATP, these are two forms
of chemical energy made during the light reactions.
To summarize the two steps of these photosystems,
Photosystem II, or the water-splitting photosystem
energizes electrons by absorbing light.
Those electrons are taken away, passed to
Photosystem I, and in doing so, that energy
ultimately results in the production of ATP.
In photosystem I, another photon of light
is absorbed, the electrons are energized,
and those electrons are then loaded on to
the electron shuttle buses or NADPH.
These two forms of chemical energy will then
be used during the Calvin cycle.
This concludes our discussion of the light
reaction.
I know this was a lot of information and so
there will be an overview video of the light
reactions next.
After that, we'll discuss the Calvin cycle
and we can see how the Calvin cycle is going
to use the chemical energy made by the light
reactions and convert it into glucose.
Thanks for your attention and I'll see you
in the next video.
