WEBVTT

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

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dark reactions.

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

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photosynthesis, photons came
in and excited electrons in

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chlorophyll in the
light reactions.

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and as those photons went to
lower and lower energy

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

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

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going on in the thylakoid
membrane right over here.

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

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You can imagine it occurring
right here.

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As they went into lower and
lower energy states, two

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things happened.

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

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across this membrane.

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And then when you had a high
concentration of hydrogens

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here, those went back through
the ATP synthase and drove

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that motor to produce ATP.

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And then the final electron
acceptor, or hydrogen

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acceptor, depending on how
you want to view it.

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The whole hydrogen atom
was NAD plus.

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

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going to continue using in
photosynthesis from our light

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cycle, from our light
reactions I guess.

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

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here-- ATP and NADPH.

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And then the byproduct was that
we needed the electron to

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replace that first
excited electron.

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So we take it away from water.

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

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byproduct of this reaction.

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

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proceed into the
dark reactions.

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

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

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It actually happens at the same
time as light reactions.

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It occurs while the
sun is out.

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The reason why they call it
the dark reactions is that

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they're light independent.

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They don't require photons.

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They only require ATP, NADPH,
and carbon dioxide.

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So let's understand what's
going on here

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a little bit better.

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So let me go down to
where I have some

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clean space down here.

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So we had our light reactions.

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

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ATP and produced
some and NADPH.

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

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atmosphere.

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

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independent reactions.

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Because dark reactions
is misleading.

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So the light independent
reactions, the actual

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mechanism is called
the Calvin Cycle.

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And that's what this video
is really about.

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

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

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video-- or G3P.

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This is glyceraldehyde
3-phosphate.

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This is phosphoglyceraldehyde
They are the exact same

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molecule, just different
names.

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

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phosphate group.

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

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

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You might remember in the first
stage of glycolysis, or

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the first time we cut a glucose
molecule we ended up

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with two phosphoglyceraldehyde
molecules.

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Glucose has six carbons.

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This has three.

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Let's study the Calvin
Cycle in just a

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little bit more detail.

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

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well let's start off with
six carbon dioxides.

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So this is independent of
the light reactions.

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

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I don't have to use these
exact numbers.

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

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And I could write a CO2 because
we really care about

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what's happening
to the carbon.

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

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oxygens on it, which
I could draw.

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

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Because I want to really
show you what

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happens to the carbons.

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Maybe I should draw this
in this yellow.

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Just to show you only
the carbons.

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I'm not showing you the
oxygens on here.

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

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

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

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

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molecule, you could
call it RuBP.

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That's short for ribulose
biphosphate.

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Sometimes called ribulose-1
5-biphosphate.

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

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5-carbon molecule.

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So, three, four five.

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

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So it's ribulose biphosphate.

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

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first carbon.

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5-biphosphate.

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We have two phosphates.

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So that's ribulose-1
5-biphosphate.

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

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phosphates on it.

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These two react together.

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And this is a simplification.

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These two react together.

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

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the big picture.

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to form, 12 molecules of PGAL,
of phosphoglyceraldehyde or

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

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

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

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properly, let's think
about what happens.

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We have 12 of these guys.

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

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3-- we have 36 carbons.

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Now did we start with
36 carbons?

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Well we have 6 times 5 carbons.

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That's 30.

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Plus another 6 here.

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So, yes.

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We have 36 carbons.

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They react with each other
to form this PGAL.

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

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energy state than the electrons
in this molecule.

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So we have to add energy
in order for

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this reaction to happen.

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This won't happen
spontaneously.

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And the energy for this
reaction, if we use the

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

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

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every carbon and every ribulose

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biphosphate; and 12 NADPHs.

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

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

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what goes on in respiration.

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

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

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

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

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

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So as it goes to lower
energy state, it

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helps drive a reaction.

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

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

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

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energy into a reaction.

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So then we have these
12 PGALs.

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

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imagine-- we studied
the Kreb Cycle.

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Cycles start reusing things.

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

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reuse, actually, most
of these PGALs.

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

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let me actually do
it this way.

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So we're going to
have 10 PGALs.

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10 phosphoglyceraldehydes 10
PGALs we're going to use to

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recreate the ribulose
biphosphate.

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And the counting works.

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Because we have ten 3-carbon
molecules.

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That's 30 carbons.

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Then we have six 5-carbon
molecules.

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30 carbons.

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

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This is going to take the
energy from six ATPs.

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

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phosphate group.

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The electrons enter
lower energy

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states, drive reactions.

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

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that get released.

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And so you see it as a cycle.

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

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What do I get out of it?

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Well I only used 10
out of the 12.

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So I have 2 PGALs left.

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

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and 6 is so that
I get 12 here.

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And I get 2 here.

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

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used to make a glucose.

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Which is a 6-carbon molecule.

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

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

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be glucose.

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It can then go off and generate
longer chained

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carbohydrates and starches,
anything that

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has a carbon backbone.

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So this is it.

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This is the dark reaction.

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We were able to take the
byproducts of the light

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

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there-- and use it
to fix carbon.

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This is called carbon
fixation.

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

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into a solid structure, that
is called carbon fixation.

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

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the energy comes from these
molecules generated from the

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light reaction.

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

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

00:10:18.160 --> 00:10:21.580
glucose or other carbohydrates
while most of them continue on

00:10:21.580 --> 00:10:26.250
to be recycled into ribulose
biphosphate, which once again

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reacts with carbon dioxide.

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

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

00:10:33.060 --> 00:10:35.040
Actually if you want to know the
actual location where this

00:10:35.040 --> 00:10:39.510
is occurring, this is all
occurring in the stroma.

00:10:39.510 --> 00:10:42.770
In the fluid, inside the
chloroplast but outside of

00:10:42.770 --> 00:10:43.950
your thylakoid.

00:10:43.950 --> 00:10:47.130
So in your stroma, this is where
your light independent

00:10:47.130 --> 00:10:49.850
reactions are actually
occurring.

00:10:49.850 --> 00:10:54.640
And it's not just happening with
the ADP and the NADPH.

00:10:54.640 --> 00:10:59.600
There's actually a fairly decent
sized enzyme or protein

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that's facilitating it.

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That's allowing the carbon
dioxide to bond at certain

00:11:02.790 --> 00:11:05.900
points and the ribulose
biphosphate and the ATP to

00:11:05.900 --> 00:11:08.440
react at certain points, to
essentially drive these two

00:11:08.440 --> 00:11:10.380
guys to react together.

00:11:10.380 --> 00:11:15.560
And that enzyme, sometimes it's
called RuBisCo, I'll tell

00:11:15.560 --> 00:11:16.870
you why it's called RuBisCo.

00:11:16.870 --> 00:11:19.000
So this is RuBisCo.

00:11:19.000 --> 00:11:23.690
So rub-- let me get the
capitalization right--

00:11:23.690 --> 00:11:29.950
ribulose biphosphate rub--
bis-- co-- carboxylase.

00:11:29.950 --> 00:11:31.040
And this is what
it looks like.

00:11:31.040 --> 00:11:34.490
So it's a pretty big protein
enzyme molecule.

00:11:34.490 --> 00:11:37.710
You can imagine that you have
your ribulose biphosphate

00:11:37.710 --> 00:11:39.070
bonding at one point.

00:11:39.070 --> 00:11:41.690
You have your carbon dioxide
bonding at another point.

00:11:41.690 --> 00:11:43.120
I don't know what
points they are.

00:11:43.120 --> 00:11:45.550
ATP bonds at another point.

00:11:45.550 --> 00:11:46.640
It reacts.

00:11:46.640 --> 00:11:50.400
That makes this thing twist and
turn in certain ways to

00:11:50.400 --> 00:11:55.590
make the ribulose biphosphate
react with the carbon dioxide.

00:11:55.590 --> 00:11:57.460
NADPH might be reacting
at other parts.

00:11:57.460 --> 00:12:01.040
And that's what facilitates
this entire Calvin Cycle.

00:12:01.040 --> 00:12:06.600
And you might-- I told you over
here-- that this R U B P,

00:12:06.600 --> 00:12:11.410
this is ribulose-1
5-biphosphate.

00:12:11.410 --> 00:12:16.800
This RuBisCo, this is short for
ribulose-1 5-biphosphate

00:12:16.800 --> 00:12:18.290
carboxylase.

00:12:18.290 --> 00:12:19.880
I won't write it all out;
you could look it up.

00:12:19.880 --> 00:12:23.260
But it's just telling you, it's
an enzyme that's used to

00:12:23.260 --> 00:12:28.040
react carbon and ribulose-1
5-biphophate.

00:12:28.040 --> 00:12:28.900
But now we're done.

00:12:28.900 --> 00:12:30.750
We're done with photosynthesis.

00:12:30.750 --> 00:12:36.070
We were able to start off with
photons and water to produce

00:12:36.070 --> 00:12:40.100
ATP and NADPH because we had
those excited electrons, we

00:12:40.100 --> 00:12:45.610
had the whole chemiosmosis to
drive the-- that allowed the

00:12:45.610 --> 00:12:47.840
ATP synthase to produce ATP.

00:12:47.840 --> 00:12:50.660
NADPH was the final
electron acceptor.

00:12:50.660 --> 00:12:54.020
These are then used as the fuel
in the Calvin Cycle, in

00:12:54.020 --> 00:12:54.990
the dark reaction.

00:12:54.990 --> 00:12:56.820
Which is badly named, it should
be called the light

00:12:56.820 --> 00:12:57.650
independent reaction.

00:12:57.650 --> 00:12:59.270
Because it actually does
happen in the light.

00:12:59.270 --> 00:13:02.070
You take your fuel from the
light reactions with some

00:13:02.070 --> 00:13:05.840
carbon dioxide and you can fix
it using your-- I like to call

00:13:05.840 --> 00:13:08.090
it-- the RuBisCo enzyme
in the Calvin Cycle.

00:13:08.090 --> 00:13:11.210
And you end up with your
phosphoglyceraldehyde which

00:13:11.210 --> 00:13:14.000
could also be called your
glyceraldehyde 3-phosphate,

00:13:14.000 --> 00:13:17.860
which can then be used to
generate glucose, which we all

00:13:17.860 --> 00:13:21.400
use to eat and fuel
our bodies.

00:13:21.400 --> 00:13:23.780
Or we learn in cellular
respiration, that can then be

00:13:23.780 --> 00:13:27.550
converted into ATP
when we need it.