Photosynthesis: Light Reactions 1

Details on the light-dependent reactions of photosynthesis

In the last video we learned
a little bit about

photosynthesis.

And we know in very general
terms, it's the process where

we start off with photons and
water and carbon dioxide, and

we use that energy in the
photons to fix the carbon.

And now, this idea of carbon
fixation is essentially taking

carbon in the gaseous form, in
this case carbon dioxide, and

fixing it into a solid
structure.

And that solid structure we fix
it into is a carbohydrate.

The first end-product of
photosynthesis was this

3-carbon chain, this
glyceraldehyde 3-phosphate.

But then you can use that to
build up glucose or any other

carbohydrate.

So, with that said, let's try to
dig a little bit deeper and

understand what's actually going
on in these stages of

photosynthesis.

Remember, we said there's
two stages.

The light-dependent reactions
and then you have the light

independent reactions.

I don't like using the word
dark reaction because it

actually occurs while
the sun is outside.

It's actually occurring
simultaneously

with the light reactions.

It just doesn't need the
photons from the sun.

But let's focus first on the
light-dependent reactions.

The part that actually uses
photons from the sun.

Or actually, I guess, even
photons from the heat lamp

that you might have in
your greenhouse.

And uses those photons in
conjunction with water to

produce ATP and reduce
NADP plus to NADPH.

Remember, reduction is gaining
electrons or hydrogen atoms.

And it's the same thing,
because when you gain a

hydrogen atom, including its
electron, since hydrogen is

not too electronegative, you
get to hog its electron.

So this is both gaining a
hydrogen and gaining electron.

But let's study it a
little bit more.

So before we dig a little
deeper, I think it's good to

know a little bit about the
anatomy of a plant.

So let me draw some
plant cells.

So plant cells actually have
cell walls, so I can draw them

a little bit rigid.

So let's say that these are
plant cells right here.

Each of these squares, each of
these quadrilaterals is a

plant cell.

And then in these plant cells
you have these organelles

called chloroplasts.

Remember organelles are
like organs of a cell.

They are subunits,

membrane-bound subunits of cells.

And of course, these cells have
nucleuses and DNA and all

of the other things you normally
associate with cells.

But I'm not going to
draw them here.

I'm just going to draw
the chloroplasts.

And your average plant cell--
and there are other types of

living organisms that perform
photosynthesis, but we'll

focus on plants.

Because that's what we tend
to associate it with.

Each plant cell will contain
10 to 50 chloroplasts.

I make them green on purpose
because the chloroplasts

contain chlorophyll.

Which to our eyes,
appear green.

But remember, they're green
because they reflect green

light and they absorb red
and blue and other

wavelengths of light.

That's why it looks green.

Because it's reflecting.

But it's absorbing all the
other wavelengths.

But anyway, we'll talk more
about that in detail.

But you'll have 10 to 50 of
these chloroplasts right here.

And then let's zoom in on one
chloroplast. So if we zoom in

on one chloroplast. So
let me be very clear.

This thing right here
is a plant cell.

That is a plant cell.

And then each of these green
things right here is an

organelle called the
chloroplast. And let's zoom in

on the chloroplast itself.

If we zoom in on one
chloroplast, it has a

membrane like that.

And then the fluid inside of the
chloroplast, inside of its

membrane, so this fluid
right here.

All of this fluid.

That's called the stroma.

The stroma of the chloroplast.
And then within the

chloroplast itself, you have
these little stacks of these

folded membranes, These
little folded stacks.

Let me see if I can
do justice here.

So maybe that's one, two,
doing these stacks.

Each of these membrane-bound--
you can almost view them as

pancakes-- let me draw
a couple more.

Maybe we have some over here,
just so you-- maybe you have

some over here, maybe
some over here.

So each of these flattish
looking pancakes right here,

these are called thylakoids.

So this right here
is a thylakoid.

That is a thylakoid.

The thylakoid has a membrane.

And this membrane is especially
important.

We're going to zoom in
on that in a second.

So it has a membrane, I'll color
that in a little bit.

The inside of the thylakoid, so
the space, the fluid inside

of the thylakoid, right
there that area.

This light green color
right there.

That's called the thylakoid
space or the thylakoid lumen.

And just to get all of our
terminology out of the way, a

stack of several thylakoids
just like that,

that is called a grana.

That's a stack of thylakoids.

That is a grana.

And this is an organelle.

And evolutionary biologists,
they believe that organelles

were once independent
organisms that then,

essentially, teamed up with
other organisms and started

living inside of their cells.

So there's actually, they
have their own DNA.

So mitochondria is another
example of an organelle that

people believe that one time
mitochondria, or the ancestors

of mitochondria, were
independent organisms. That

then teamed up with other cells
and said, hey, if I

produce your energy maybe
you'll give me

some food or whatnot.

And so they started
evolving together.

And they turned into
one organism.

Which makes you wonder what we
might evolve-- well anyway,

that's a separate thing.

So there's actually ribosomes
out here.

That's good to think about.

Just realize that at one point
in the evolutionary past, this

organelle's ancestor
might have been

an independent organism.

But anyway, enough about
that speculation.

Let's zoom in again on one of
these thylakoid membranes.

So I'm going to zoom in.

Let me make a box.

Let me zoom in right there.

So that's going to be
my zoom-in box.

So let me make it really big.

Just like this.

So this is my zoom-in box.

So that little box is the same
thing as this whole box.

So we're zoomed in on the
thylakoid membrane.

So this is the thylakoid
membrane right there.

That's actually a phospho-bilipd
layer.

It has your hydrophilic,
hydrophobic tails.

I mean, I could draw it
like that if you like.

The important thing from the
photosynthesis point of view

is that it's this membrane.

And on the outside of the
membrane, right here on the

outside, you have the fluid
that fills up the entire

chloroplast. So here you
have the stroma.

And then this space right here,
this is the inside of

your thylakoid.

So this is the lumen.

So if I were to color it
pink, right there.

This is your lumen.

Your thylakoid space.

And in this membrane, and this
might look a little bit

familiar if you think about
mitochondria and the electron

transport chain.

What I'm going to describe in
this video actually is an

electron transport chain.

Many people might not consider
it the electron transport

chain, but it's the same idea.

Same general idea.

So on this membrane you have
these proteins and these

complexes of proteins
and molecules

that span this membrane.

So let me draw a
couple of them.

So maybe I'll call this
one, photosystem II.

And I'm calling it that because
that's what it is.

Photosystem II.

You have maybe another
complex.

And these are hugely
complicated.

I'll do a sneak peek of
what photosystem II

actually looks like.

This is actually what
photosystem II looks like.

So, as you can see, it
truly is a complex.

These cylindrical things,
these are proteins.

These green things are
chlorophyll molecules.

I mean, there's all sorts
of things going here.

And they're all jumbled
together.

I think a complex probably
is the best word.

It's a bunch of proteins, a
bunch of molecules just

jumbled together to perform a
very particular function.

We're going to describe
that in a few seconds.

So that's what photosystem
II looks like.

Then you also have
photosystem I.

And then you have other
molecules, other complexes.

You have the cytochrome B6F
complex and I'll draw this in

a different color right here.

I don't want to get too
much into the weeds.

Because the most important thing
is just to understand.

So you have other protein
complexes, protein molecular

complexes here that also
span the membrane.

But the general idea-- I'll tell
you the general idea and

then we'll go into the
specifics-- of what happens

during the light reaction, or
the light dependent reaction,

is you have some photons.

Photons from the sun.

They've traveled 93
million miles.

so you have some photons that
go here and they excite

electrons in a chlorophyll
molecule, in a

chlorophyll A molecule.

And actually in photosystem II--
well, I won't go into the

details just yet-- but they
excite a chlorophyll molecule

so those electrons enter into
a high energy state.

Maybe I shouldn't draw
it like that.

They enter into a high
energy state.

And then as they go from
molecule to molecule they keep

going down in energy state.

But as they go down in energy
state, you have hydrogen

atoms, or actually I should say
hydrogen protons without

the electrons.

So you have all of these
hydrogen protons.

Hydrogen protons get pumped
into the lumen.

They get pumped into the lumen
and so you might remember this

from the electron
transport chain.

In the electron transport chain,
as electrons went from

a high potential, a high energy
state, to a low energy

state, that energy
was used to pump

hydrogens through a membrane.

And in that case it was in the
mitochondria, here the

membrane is the thylakoid
membrane.

But either case, you're creating
this gradient where--

because of the energy from,
essentially the photons-- the

electrons enter a high energy
state, they keep going into a

lower energy state.

And then they actually go to
photosystem I and they get hit

by another photon.

Well, that's a simplification,
but that's how you

can think of it.

Enter another high energy
state, then they go to a

lower, lower and lower
energy state.

But the whole time, that energy
from the electrons

going from a high energy state
to a low energy state is used

to pump hydrogen protons
into the lumen.

So you have this huge

concentration of hydrogen protons.

And just like what we saw in the
electron transport chain,

that concentration is then-- of
hydrogen protons-- is then

used to drive ATP synthase.

So the exact same-- let me see
if I can draw that ATP

synthase here.

You might remember ATP
synthase looks

something like this.

Where literally, so here you
have a huge concentration of

hydrogen protons.

So they'll want to
go back into the

stroma from the lumen.

And they do.

And they go through
the ATP synthase.

Let me do it in a new color.

So these hydrogen protons are
going to make their way back.

Go back down the gradient.

And as they go down the
gradient, they literally--

it's like an engine.

And I go into detail on this
when I talk about respiration.

And that turns, literally
mechanically turns, this top

part-- the way I drew it--
of the ATP synthase.

And it puts ADP and phosphate
groups together.

It puts ADP plus phosphate
groups

together to produce ATP.

So that's the general,
very high overview.

And I'm going to go into more
detail in a second.

But this process that I just
described is called

photophosphorylation.

Let me do it in a nice color.

Why is it called that?

Well, because we're
using photons.

That's the photo part.

We're using light.

We're using photons to excite
electrons in chlorophyll.

As those electrons get passed
from one molecule, from one

electron acceptor to another,
they enter into lower and

lower energy states.

As they go into lower energy
states, that's used to drive,

literally, pumps that allow
hydrogen protons to go from

the stroma to the lumen.

Then the hydrogen protons
want to go back.

They want to-- I guess you could
call it-- chemiosmosis.

They want to go back into the
stroma and then that drives

ATP synthase.

Right here, this is
ATP synthase.

ATP synthase to essentially
jam together ADPs and

phosphate groups
to produce ATP.

Now, when I originally talked
about the light reactions and

dark reactions I said, well the
light reactions have two

byproducts.

It has ATP and it also has--
actually it has three.

It has ATP, and it
also has NADPH.

NADP is reduced.

It gains these electrons
and these hydrogens.

So where does that show up?

Well, if we're talking about
non-cyclic oxidative

photophosphorylation, or
non-cyclic light reactions,

the final electron acceptor.

So after that electron keeps
entering lower and lower

energy states, the
final electron

acceptor is NADP plus.

So once it accepts the electrons
and a hydrogen

proton with it, it
becomes NADPH.

Now, I also said that part of
this process, water-- and this

is actually a very interesting
thing-- water gets oxidized to

molecular oxygen.

So where does that happen?

So when I said, up here in
photosystem I, that we have a

chlorophyll molecule that has
an electron excited, and it

goes into a higher
energy state.

And then that electron
essentially gets passed from

one guy to the next, that begs
the question, what can we use

to replace that electron?

And it turns out that we use,
we literally use, the

electrons in water.

So over here you literally
have H2O.

And H2O donates the hydrogens
and the electrons with it.

So you can kind of imagine it
donates two hydrogen protons

and two electrons to replace the
electron that got excited

by the photons.

Because that electron got passed
all the way over to

photosystem I and eventually
ends up in NADPH.

So, you're literally stripping
electrons off of water.

And when you strip off the
electrons and the hydrogens,

you're just left with
molecular oxygen.

Now, the reason why I want to
really focus on this is that

there's something profound
happening here.

Or at least on a chemistry
level,

something profound is happening.

You're oxidizing water.

And in the entire biological
kingdom, the only place where

we know something that is strong
enough of an oxidizing

agent to oxidize water, to
literally take away electrons

from water.

Which means you're
really taking

electrons away from oxygen.

So you're oxidizing oxygen.

The only place that we know
that an oxidation agent is

strong enough to do this
is in photosystem II.

So it's a very profound idea,
that normally electrons are

very happy in water.

They're very happy circulating
around oxygens.

Oxygen is a very electronegative
atom.

That's why we even call it
oxidizing, because oxygen is

very good at oxidizing things.

But all of a sudden we've
found something that can

oxidize oxygen, that can strip
electrons off of oxygen and

then give those electrons
to the chlorophyll.

The electron gets excited
by photons.

Then those photons enter
lower and lower and

lower energy states.

Get excited again in photosystem
I by another set

of photons and then enter lower
and lower and lower

energy states.

And then finally,
end up at NADPH.

And the whole time it entered
lower and lower energy states,

that energy was being used to
pump hydrogen across this

membrane from the
stroma to lumen.

And then that gradient is used
to actually produce ATP.

So in the next video I'm going
to give a little bit more

context about what this means
in terms of energy states of

electrons and what's at a higher
or lower energy state.

But this is essentially
all that's happening.

Electrons get excited.

Those electrons eventually
end up at NADPH.

And as the electron gets excited
and goes into lower

and lower energy states,
it pumps

hydrogen across the gradient.

And then that gradient is used
to drive ATP synthase, to

generate ATP.

And then that original electron
that got excited, it

had to be replaced.

And that replaced electron is
actually stripped off of H2O.

So the hydrogen protons and
the electrons of H2O are

stripped away and you're just
left with molecular oxygen.

And just to get a nice
appreciation of the complexity

of all of this-- I showed you
this earlier in the video--

but this is literally a-- I mean
this isn't a picture of

photosystem II.

You actually don't have
cylinders like this.

But these cylinders represent
proteins.

Right here, these green kind
of scaffold-like molecules,

that's chlorophyll A.

And what literally happens, is
you have photons hitting--

actually it doesn't always have
to hit chlorophyll A.

It can also hit what's called
antenna molecules.

So antenna molecules are other
types of chlorophyll, and

actually other types
of molecules.

And so a photon, or a set of
photons, comes here and maybe

it excites some electrons,
it doesn't have to be in

chlorophyll A.

It could be in some of these
other types of chlorophyll.

Or in some of these other I
guess you could call them,

pigment molecules that will
absorb these photons.

And then their electrons
get excited.

And you can almost imagine
it as a vibration.

But when you're talking about
things on the quantum level,

vibrations really don't
make sense.

But it's a good analogy.

They kind of vibrate their
way to chlorophyll A.

And this is called
resonance energy.

They vibrate their way,
eventually, to chlorophyll A.

And then in chlorophyll A, you
have the electron get excited.

The primary electron acceptor
is actually this molecule

right here.

Pheophytin.

Some people call it pheo.

And then from there, it keeps
getting passed on from one

molecule to another.

I'll talk a little bit more
about that in the next video.

But this is fascinating.

Look how complicated this is.

In order to essentially excite
electrons and then use those

electrons to start the
process of pumping

hydrogens across a membrane.

And this is an interesting
place right here.

This is the water
oxidation site.

So I got very excited about the
idea of oxidizing water.

And so this is actually where
it occurs in the photosystem

II complex.

And you actually have this very
complicated mechanism.

Because it's no joke to actually
strip away electrons

and hydrogens from an actual
water molecule.

I'll leave you there.

And in the next video I'll talk
a little bit more about

these energy states.

And I'll fill in a little bit of
the gaps about what some of

these other molecules that act
as hydrogen acceptors.

Or you can also view
them as electron

acceptors along the way.