Photosynthesis: Calvin Cycle

The Calvin Cycle or the light-independent (dark) reactions of photosythesis

I think we're now ready to learn
a little bit about the

dark reactions.

But just to remember where we
are in this whole scheme of

photosynthesis, photons came
in and excited electrons in

chlorophyll in the
light reactions.

and as those photons went to
lower and lower energy

states-- we saw it over here
in the last video-- as they

went to lower and lower energy
states, and all of this was

going on in the thylakoid
membrane right over here.

You can imagine-- Let me do
it in a different color.

You can imagine it occurring
right here.

As they went into lower and
lower energy states, two

things happened.

One, the release of energy was
able to pump the hydrogens

across this membrane.

And then when you had a high
concentration of hydrogens

here, those went back through
the ATP synthase and drove

that motor to produce ATP.

And then the final electron
acceptor, or hydrogen

acceptor, depending on how
you want to view it.

The whole hydrogen atom
was NAD plus.

So the two byproducts, or the
two byproducts that we're

going to continue using in
photosynthesis from our light

cycle, from our light
reactions I guess.

I shouldn't call it the light
cycle-- were-- I wrote it up

here-- ATP and NADPH.

And then the byproduct was that
we needed the electron to

replace that first
excited electron.

So we take it away from water.

And so we also produce oxygen,
which is a very valuable

byproduct of this reaction.

But now that we have this ATP
and this NADPH, we're ready to

proceed into the
dark reactions.

And I want to highlight again,
even though it's called the

dark reactions it doesn't mean
that it happens at night.

It actually happens at the same
time as light reactions.

It occurs while the
sun is out.

The reason why they call it
the dark reactions is that

they're light independent.

They don't require photons.

They only require ATP, NADPH,
and carbon dioxide.

So let's understand what's
going on here

a little bit better.

So let me go down to
where I have some

clean space down here.

So we had our light reactions.

And they produced-- I just
reviewed this-- produced some

ATP and produced
some and NADPH.

And now we're going to take some
carbon dioxide from the

atmosphere.

And all of this will go into
the-- I'll call it the light

independent reactions.

Because dark reactions
is misleading.

So the light independent
reactions, the actual

mechanism is called
the Calvin Cycle.

And that's what this video
is really about.

It goes into the Calvin Cycle
and out pops-- whether you

want to call it PGAL-- we talked
about it in the first

video-- or G3P.

This is glyceraldehyde
3-phosphate.

This is phosphoglyceraldehyde
They are the exact same

molecule, just different
names.

And you can imagine it as
a 3-carbon chain with a

phosphate group.

And then this can then be used
to build other carbohydrates.

You put two of these together
you can get a glucose.

You might remember in the first
stage of glycolysis, or

the first time we cut a glucose
molecule we ended up

with two phosphoglyceraldehyde
molecules.

Glucose has six carbons.

This has three.

Let's study the Calvin
Cycle in just a

little bit more detail.

So let's say exiting the light
reactions, let's say we have--

well let's start off with
six carbon dioxides.

So this is independent of
the light reactions.

And I'll show you why I'm
using these numbers.

I don't have to use these
exact numbers.

So let's say I start
off with six CO2s.

And I could write a CO2 because
we really care about

what's happening
to the carbon.

We can just write it as a single
carbon that has two

oxygens on it, which
I could draw.

But I'm not going to draw
them right now.

Because I want to really
show you what

happens to the carbons.

Maybe I should draw this
in this yellow.

Just to show you only
the carbons.

I'm not showing you the
oxygens on here.

And what happens is the CO2,
the six CO2s, essentially

react with-- and I'll talk a
little bit about this reaction

in a second-- they react with
six molecules-- and this is

going to look a little bit
strange to you-- of this

molecule, you could
call it RuBP.

That's short for ribulose
biphosphate.

Sometimes called ribulose-1
5-biphosphate.

And the reason why it's called
that is because it's a

5-carbon molecule.

So, three, four five.

And it has a phosphate on
the 1 and 5 carbon.

So it's ribulose biphosphate.

Or sometimes, ribulosee-1-- let
me write this-- that's the

first carbon.

5-biphosphate.

We have two phosphates.

So that's ribulose-1
5-biphosphate.

Fancy name, but it's just
a 5-carbon chain with 2

phosphates on it.

These two react together.

And this is a simplification.

These two react together.

There's a lot more going on
here, but I want you to get

the big picture.

to form, 12 molecules of PGAL,
of phosphoglyceraldehyde or

glyceraldihyde 3-phosphate of
PGAL, which you can view as

a-- it has three carbons and
then it has a phosphate group.

And just to make sure we're
accounting for our carbons

properly, let's think
about what happens.

We have 12 of these guys.

You can think of it that
we have-- 12 times

3-- we have 36 carbons.

Now did we start with
36 carbons?

Well we have 6 times 5 carbons.

That's 30.

Plus another 6 here.

So, yes.

We have 36 carbons.

They react with each other
to form this PGAL.

The bonds or the electrons in
this molecule are in a higher

energy state than the electrons
in this molecule.

So we have to add energy
in order for

this reaction to happen.

This won't happen
spontaneously.

And the energy for this
reaction, if we use the

numbers 6 and 6 here, the energy
from this reaction is

going to come from 12 ATPs-- you
could imagine 2 ATPs for

every carbon and every ribulose

biphosphate; and 12 NADPHs.

I don't want to get you confused
with-- it's very

similar to NADH, but I don't
want to get you confused with

what goes on in respiration.

And then these leave as 12 ADPs
plus 12 phosphate groups.

And then you're going to have
plus 12 NADP pluses.

And the reason why this is a
source of energy is because

the electrons in NADPH, or you
could say the hydrogen with

the electron in NADPH, is at
a higher energy state.

So as it goes to lower
energy state, it

helps drive a reaction.

And of course ATPs, when they
lose their phosphate groups,

those electrons are in a very
high energy state, they enter

a lower energy state, help drive
a reaction, help put

energy into a reaction.

So then we have these
12 PGALs.

Now the reason why it's called
a Calvin Cycle-- as you can

imagine-- we studied
the Kreb Cycle.

Cycles start reusing things.

The reason why it's called the
Calvin Cycle is because we do

reuse, actually, most
of these PGALs.

So of the 12 PGALs, we're going
to use 10 of them to--

let me actually do
it this way.

So we're going to
have 10 PGALs.

10 phosphoglyceraldehydes 10
PGALs we're going to use to

recreate the ribulose
biphosphate.

And the counting works.

Because we have ten 3-carbon
molecules.

That's 30 carbons.

Then we have six 5-carbon
molecules.

30 carbons.

But this, once again, is
going to take energy.

This is going to take the
energy from six ATPs.

So you're going to have six ATPs
essentially losing their

phosphate group.

The electrons enter
lower energy

states, drive reactions.

And you're going to have six
ADPs plus six phosphate groups

that get released.

And so you see it as a cycle.

But the question is, well
gee I used all of these.

What do I get out of it?

Well I only used 10
out of the 12.

So I have 2 PGALs left.

And these can then be used--
and the reason why I used 6

and 6 is so that
I get 12 here.

And I get 2 here.

And the reason why I have 2 here
is because 2 PGALs can be

used to make a glucose.

Which is a 6-carbon molecule.

It's formula, we've seen
it before, is C6H12O6.

But it's important to remember
that it doesn't have to just

be glucose.

It can then go off and generate
longer chained

carbohydrates and starches,
anything that

has a carbon backbone.

So this is it.

This is the dark reaction.

We were able to take the
byproducts of the light

reactions, the ATP and the
NADHs-- there's some more ATP

there-- and use it
to fix carbon.

This is called carbon
fixation.

When you take carbon in a
gaseous form and you put it

into a solid structure, that
is called carbon fixation.

So through this Calvin Cycle we
were able to fix carbon and

the energy comes from these
molecules generated from the

light reaction.

And of course, it's called a
cycle because we generate

these PGALs, some of them can
be used to actually produce

glucose or other carbohydrates
while most of them continue on

to be recycled into ribulose
biphosphate, which once again

reacts with carbon dioxide.

And then you get this cycle
happening over and over again.

Now we said it doesn't
happen in a vacuum.

Actually if you want to know the
actual location where this

is occurring, this is all
occurring in the stroma.

In the fluid, inside the
chloroplast but outside of

your thylakoid.

So in your stroma, this is where
your light independent

reactions are actually
occurring.

And it's not just happening with
the ADP and the NADPH.

There's actually a fairly decent
sized enzyme or protein

that's facilitating it.

That's allowing the carbon
dioxide to bond at certain

points and the ribulose
biphosphate and the ATP to

react at certain points, to
essentially drive these two

guys to react together.

And that enzyme, sometimes it's
called RuBisCo, I'll tell

you why it's called RuBisCo.

So this is RuBisCo.

So rub-- let me get the
capitalization right--

ribulose biphosphate rub--
bis-- co-- carboxylase.

And this is what
it looks like.

So it's a pretty big protein
enzyme molecule.

You can imagine that you have
your ribulose biphosphate

bonding at one point.

You have your carbon dioxide
bonding at another point.

I don't know what
points they are.

ATP bonds at another point.

It reacts.

That makes this thing twist and
turn in certain ways to

make the ribulose biphosphate
react with the carbon dioxide.

NADPH might be reacting
at other parts.

And that's what facilitates
this entire Calvin Cycle.

And you might-- I told you over
here-- that this R U B P,

this is ribulose-1
5-biphosphate.

This RuBisCo, this is short for
ribulose-1 5-biphosphate

carboxylase.

I won't write it all out;
you could look it up.

But it's just telling you, it's
an enzyme that's used to

react carbon and ribulose-1
5-biphophate.

But now we're done.

We're done with photosynthesis.

We were able to start off with
photons and water to produce

ATP and NADPH because we had
those excited electrons, we

had the whole chemiosmosis to
drive the-- that allowed the

ATP synthase to produce ATP.

NADPH was the final
electron acceptor.

These are then used as the fuel
in the Calvin Cycle, in

the dark reaction.

Which is badly named, it should
be called the light

independent reaction.

Because it actually does
happen in the light.

You take your fuel from the
light reactions with some

carbon dioxide and you can fix
it using your-- I like to call

it-- the RuBisCo enzyme
in the Calvin Cycle.

And you end up with your
phosphoglyceraldehyde which

could also be called your
glyceraldehyde 3-phosphate,

which can then be used to
generate glucose, which we all

use to eat and fuel
our bodies.

Or we learn in cellular
respiration, that can then be

converted into ATP
when we need it.