- [Voiceover] We've seen in previous videos
that photosynthesis can be broken down
into the light-dependent reactions and the Calvin cycle.
And the light-dependent reactions is
where we take light as an input along with water,
and we'll see the water is actually a source of electrons,
and we can use that to store energy
in the form of ATP and NADPH,
and as a by-product we produce molecular oxygen,
which is very important for us to breathe.
And then that ATP and that NADPH
can be used in the Calvin cycle,
along with carbon dioxide, to actually synthesize sugar.
What we're gonna focus on in this video
are the light-dependent reactions.
How does this process right here work?
And to help us think about this,
we're going to zoom in onto a thylakoid membrane.
So this is a thylakoid right over here,
sitting inside of the chloroplast.
And if we zoom in on its membrane,
we see it's a phospholipid bilayer,
like many membranes that we see in biology.
And at first glance, this might seem
like a very complex diagram,
and that's because it is a complex diagram,
and you will often see things like this
in your biology textbooks.
They can be very intimidating,
these proteins and molecules and complexes
have very complicated-sounding names,
but the general idea of what's going on is,
you'll hopefully find, pretty straightforward.
You have the energy from light,
photons from light are going to,
either directly or indirectly, excite electrons.
Those excited electrons, they're in a high-energy state.
They're gonna be transferred from one molecule to another,
and they're going to go to lower energy states.
That's what allows those transfers
to be as spontaneous, for them to actually occur.
They're going from a high-energy state
to a lower-energy state.
The electrons are getting more
and more and more comfortable,
and some of that energy that's released
as the electron goes from a high-energy state
to a lower-energy state
is used to pump hydrogen ions across the membrane.
From the outside of the membrane, in the stroma,
to the inside of the membrane,
to within the thylakoid lumen.
So you are building a hydrogen ion concentration gradient.
Concentration gradient.
Where you have a higher concentration inside
than you have outside.
And this by itself,
this concentration gradient as we'll see,
can be used to fuel the production
of ATP by ATP synthase,
that those hydrogen ions want to get back out.
They wanna go down their concentration gradient,
and as they go back out through the ATP synthase
it essentially turns that motor
that can jam the phosphate group onto ADP to produce ATP.
So one way to think about it,
this is producing a hydrogen ion gradient.
So we could do it this way,
we could say H plus gradient,
which is then being used to produce the actual ATP.
Now the electrons going from a high-energy state
to a lower-energy state in this part
of the light-dependent reactions,
that by itself isn't the only thing that is contributing
to the hydrogen ion concentration gradient.
Once that electron gets donated,
you might say, well how does it get replaced?
Well the thing that's doing the donating,
the thing that eventually gets excited
and donates that electron,
it's a chlorophyll a variation called P680.
P680 is referring to the P stands for pigment,
680 stands for 680 nano-meters,
the wavelength of light that it absorbs best.
And so when it gets excited, it becomes
you'll see the notation off of P680*,
that's when it has an excited electron.
And then after it gives away its electron,
it becomes P680+
with a positive charge.
And this P680, we could call it P680 plus
right over here, maybe a P680 ion,
this is actually a very strong oxidizing agent.
One of the strongest, if not the strongest,
that we know in biological systems.
And so it really likes to grab electrons from other things.
And the thing that is around
that it can grab electrons from is actually water.
And so this is such a strong oxidizing agent
that it can essentially oxidize the oxygen in water,
and oxygen as itself.
I mean, oxidizing is named after oxygen
because oxygen is such a strong,
it's so electronegative, it's such a strong,
it's the thing that's normally doing the oxidizing.
So anyway, it grabs its electrons, once it gets this P680+,
grabs an electron from water,
and then the water essentially falls apart
so you're left just with the oxygen
and then the hydrogen ions.
And so those hydrogen ions also contribute
to the increased hydrogen ion concentration on the inside.
And this is where we get the oxygen by-product
right over here.
Here we have one half of an O2, so if you do this twice
you're going to have a molecular oxygen.
So, so far we've talked about how the oxygen gets produced,
we've talked about how the ATP gets produced.
What about the NADPH?
Well we've started our process in photosystem II.
You might say, why's it called photosystem II
if that's where we start?
Well it's actually that's because
that's the second photosystem to be discovered.
You might say, what is a photosystem?
Well these photosystems and complexes,
they're combinations of proteins and molecules,
and photosystem in particular has chlorophyll
and variations of chlorophyll and pigment molecules
that are responsive to light that are very easy,
that have electrons that can get excited by light,
and they can transfer that energy
back down to the P680 chlorophyll a pair,
which then can have its electron excited
and then it can give that to an acceptor molecule
and then it can go to lower, lower energy states
and pump those hydrogen ions out.
But that's not the entire light-dependent reactions.
That electron can eventually make its way
over to photosystem I, and why's it called photosystem I?
Well it's because the first one that was discovered.
In photosystem I, there's another
chlorophyll a pair called P700,
and that's because it optimally absorbs
light of a wavelength of 700 nano-meters.
And you have something similar that happens,
that light can either directly
or indirectly excite its electron.
And then that electron, as it goes to a lower-energy level,
it goes from one molecule to another,
it can be used to reduce NADP+ into NADPH.
And so that's where the NADPH comes from.
And then once again, once the P700 has given its electron,
it wants an electron,
and well it can get that from the electron
that's been going from lower to lower, lower energy states,
that's essentially been making its way from,
you can conceptualize it as the electron
that's been making its way from photosystem II.
And so that's why you'll often see these diagrams.
Lights come in, electron gets energized, it gets excited,
it goes to lower and lower energy states.
As it's doing that it's being transferred
from one molecule to another,
being facilitated by enzymes.
That energy, part of that energy
is being used to transfer hydrogen ions
into the thylakoid lumen, into the interior.
Then in photosystem I, you have another excitation event.
That thing that got excited can grab that electron
that went to lower, lower energy states,
and its excited electron can once again
be transferred from one molecule to another
in order to fuel or provide energy
for NADP+ being converted into NADPH.
And once again the whole idea
of the hydrogen ion concentration increasing here
can fuel ATP synthase, which allows us to jam a phosphate
onto ADP to produce ATP.
So that is where we actually get all of these things
and the by-product of course is our oxygen.
And if you wanted to see that same idea
but kind of just thinking from an energetic point of view
without all of the complexity
of seeing the physical components involved,
you see it right over here.
Where you have light energy comes, excites the electrons.
Once the P680 has given that electron away,
it wants an electron really badly.
It gets it from the water.
And then as that electron goes
to lower and lower and lower energy states,
it can eventually be grabbed by P700
that has given away its own electron.
And then that electron that was excited at P700
by, once again, more light energy,
that can be transferred from one molecule to another
to fuel the creation of NADPH.
And this part right over here, this phase right over here,
as the energy goes from a high-energy state to a-
as the electron goes from a high-energy state
to a lower energy state,
fuels the pumping of hydrogen protons
into the actual thylakoid.