Hi, everybody! This is going to be about
light independent reactions; the
"synthesis" part of photosynthesis. Just to
make sure we're all on the same page,
we're going to look at this diagram one
more time. So, here we have a chloroplast
which has a space inside called the
thylakoid membrane and that's where the
light-dependent reactions are going to
occur. In the light-dependent
reactions, we add together photons of
light and water and then we create ATP
and NADPH and then oxygen as a
by-product. Then for the light
independent reactions, which occur out
in the stroma, we're going to add
together carbon dioxide, ATP, and NADPH
along with more water in order to get
PGAL, the precursor of glucose. So
this is our framework for drawing out
how the light independent reactions work.
We're going to include some aspects of
the light-dependent reactions so we can
see how they all work together. This
looks pretty complicated, but you have
this exact template in your dropbox
folder.
The first thing we're going to do is
we're going to label all the different
reagents that are necessary for
photosynthesis. Since every one of these
reagents are going to be added into the
light-dependent reactions, the first part
of photosynthesis, we're going to start
by labeling the box in the middle as the
light-dependent reactions. So, here are
three major reagents of photosynthesis;
water, carbon dioxide, and photons of
light, and those are all going to enter
in different ways. Photons of light will
be absorbed by the two photosystems;
photosystem 2 and photosystem 1, and then
will also be involved in the photolysis
of water in the light-dependent
reactions.
However, they don't directly play a role
in the light independent reactions. The
next reagent, water, will be crucial in
both the light-dependent and the light
independent reactions. It is pulled from the
soil and enters the plant tubes called
the xylem. Carbon dioxide is not involved
in the light-dependent reactions, but it
does become important in the light
independent reactions. It enters through
the openings called stomata. Each
molecule of carbon dioxide has only one
carbon atom. In order to create one molecule of PGAL and combine them to make
glucose, we need to use six carbon dioxides.
The light independent reactions happened
in the thylakoids, however the light
independent reactions are going to
happen in the stroma. Since the light
independent reactions are a cycle, it's
hard to know where to start but the
easiest place for us is probably going
to be to start up here at the top. This
process starts with molecule called RUBP.
Now, RUBP stands for Ribulose 1-5 biphosphate,
so we're just going to stick with calling it
RUBP. One molecule of
RUBP contains five carbons and it's
shaped like a Pentagon, and in
this process we're actually going to use
six molecules of RUBP for a total of
thirty carbons. Now, if you take the 5
carbon RUBP and add together the single
carbon from carbon dioxide, what you get
is a six-carbon intermediate molecule.
This long chain of carbon is highly
unstable, and so it doesn't stay together
for very long. Because these 6 carbon
intermediates are so unstable, they
almost immediately break apart. When the
intermediates break apart, they rearrange
themselves into shorter molecules which
are called PGA. The real name for PGA is
phosphoglyceric acid, so we're
going to call it PGA. Just to make sure
we're on the same page in terms of the
number of carbons, let's do some quick
math. During the intermediate step of the
calvin cycle, we had six six carbon
molecules for a total of thirty six
carbons. When they rearrange themselves
after breaking apart from their more
unstable form, they form twelve smaller
molecules each with three carbons. So, 6
times 6 equals 36, and 3 times 12 is
also equal to 36. In our next step of the
process, each molecule of PGA will
combine with inorganic phosphate,
hydrogen an electron and energy. So, now
we need to ask ourselves "where do we get
the inorganic phosphate, H+, the electron
and the energy?" and this is where the
product of the light-dependent reactions
are going to come into play. One molecule
of NADPH
will carry one hydrogen ion one and
one electron. This accounts for our first
two ingredients. Our final ingredient,
inorganic phosphate, will come from our
good friend ATP from the light-dependent
reactions.
Once the ATP and NADPH have dropped off
the important components to this
chemical reaction, they are in their
lower energy form. After ATP has lost its
phosphate molecule, it becomes ADP the
lower energy form and must return to the
light-dependent reactions to be
"recharged". Once NADPH has dropped off at
hydrogen and its electron, it becomes
something called NADP+ and must also
return to the light-dependent reactions
to be "recharged". In addition to providing
an inorganic phosphate molecule to
combine with the PGA, the hydrogen, and
the electron, the ATP molecule will also
provide a boost of energy for this
reaction to occur. When we have combined
all these ingredients together, we wind
up with a total of twelve PGAL. Each PGAL
has three carbons on it.
Of the 12 PGAL we've created, we're
going to borrow two of them to convert
into a glucose molecule. 1PGAL plus 1 PGAL
equals 1 glucose. However we now have
to ask ourselves "what do we do with the
10 PGAL that are then left over after we
steal two of them to create a glucose?".
The answer is that we re-invest the
remaining PGAL order to keep the cycle
turning. So here my 10 PGAL and I'm going to
have to rearrange them in order to
reinvest them in the cycle and somehow
transform them back into RUBP. In the
next step of the process,
10 gal re-arrange themselves in a
process that we will simply refer to as
"10 PGAL get crazy" and the reason that
we're keeping it simple because people
have spent decades studying how all these
crazy reactions happen. But for our
intents and purposes, they're simply
getting a little help from ATP and they're
rearranging themselves again. So, a little
through ATP and its consequential boost
of energy, the molecules will rearrange
themselves.
The 10 PGAL start off as 10 groups of
3 carbons each for a total of 30
carbon. After the "get crazy" step, we
still have 30 carbons, but they've
rearranged themselves to become 6
groups of 5 carbons each, also known
as 6 RUBP and the cycle can begin
again.
So, like this. This process also releases
water as a byproduct in the form of water
vapor. The water vapor will then diffuse
out through the stomata just like we
saw with the other gasses. The only thing we haven't addressed
is what happens to the oxygen
from the light-dependent reactions.
Since oxygen is a gas, it can also
diffuse out through the stomata.
So, there you have it! That is the Calvin
Cycle, also known as the light
independent reactions. I know it's a lot
so if you need to go back and watch it again, please
feel free to do so. Please make sure to
bring questions to class and I'll see
you all soon. Bye!
