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Course: Biology library > Unit 13
Lesson 3: The Calvin cyclePhotosynthesis: Calvin cycle
The Calvin cycle, or the light-independent (dark) reactions of photosythesis. Created by Sal Khan.
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- What is PGAL? And why does it have carbon yet no C's in its name? Confusing..(6 votes)
- PGAL is 3-PhosphoGlycerALdehyde. Most molecules in the body contain carbon and there's no rule saying the names should contain a 'C'. The 'glycer'- part tells you it's related to glucose, which contains carbon.(40 votes)
- PGAL is the same as G3P right? But why the different name then?(15 votes)
- Phosphoglyceraldehyde is the informal name of the molecule. Glyceraldehyde 3 Phosphate is a little more descriptive but still not the real IUPAC name. Glyceraldehyde 3 Phosphate describes a glycerol molecule (3 carbon backbone) that had an aldehyde group (R-CH=O) and also has a phosphate group (PO3) on the third carbon.(18 votes)
- how do you know that the electrons in PGAL are in a higher energy state than the other two molecules?(9 votes)
- We know this because of experimentation, and the fact that you can calculate, with reasonable accuracy, the relative energy levels of any molecule if its structure and bonding patterns are known! Hope this helps!(5 votes)
- Is this the same thing as Gluconeogenesis? I heard that gluconeogenesis means 'to generate glucose from non-carbohydrate molecules'.(5 votes)
- "Gluconeogenesis is the process by which glucose is made, primarily in the liver, from non-carbohydrate sources. The body is able to make glucose from amino acids (protein), glycerol (the backbone of triglycerides, the primary fat storage molecule), and glucose metabolism intermediaries like lactate and pyruvate. " Generally speaking, our body produces glucose in the liver when we haven't had a sufficient amount of glucose intake so the body basically produces it for us, however it can only do it for so long.(8 votes)
- if you're only reacting the carbons from CO2 with the ribulose-1,5-biphosphate what happens to the O2?(8 votes)
- the oxygen is taken as a waste product(2 votes)
- RUBISCO= Ribolos BIS CARBOXYLASE/OXYGENASE
CARBOXYLASE/OXYGENASE Is this because it can reduce as well as oxidase(4 votes) - How much energy does one photon contain/pass to an electron?(3 votes)
- That depends on the frequency or wavelength of the photon.
E = hν = hc/λ
Where h is Planck's constant (6.626 x 10⁻³⁴ J·s), c is the speed of light in a vacuum, ν is the frequency and λ is the wavelength(3 votes)
- atwhere does the RuBp come from? is it already in the cell? if so, where is it produced? 4:45(3 votes)
- Ribulose biphosphate comes from ribulose phosphate. Ribulose phosphate is being synthesized in the chloroplasts.
RuBP is formed by taking a phosphate, coming from the splitting of ATP, and joining it with RuP, changing RuP (ribulose phosphate) into RuBP. RuBP is then able to join with CO2 and form an unstable 6C molecule which is the basis for the dark reaction.(1 vote)
- where does the RUBP come from?(2 votes)
- RUBP means Ribulose-1,5-bisphosphate.
for more information go to the following link
http://en.wikipedia.org/wiki/Ribulose-1,5-bisphosphate(3 votes)
- how long will they last without the light-dependent cycle?(3 votes)
- As long as there are sufficient reserves of ATP and NADPH.(1 vote)
Video transcript
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-bisphosphate. 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 bisphosphate. Or sometimes, ribulosee-1-- let
me write this-- that's the first carbon. 5-bisphosphate. We have two phosphates. So that's ribulose-1
5-bisphosphate. 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 bisphosphate; 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
bisphosphate. 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
bisphosphate, 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. And 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
bisphosphate 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 bisphosphate 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 bisphosphate 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 bisphosphate
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-bisphophate. This RuBisCo, this is short for
ribulose-1 5-bisphophate 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-bisphophate. 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.