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Photosynthesis! It is not some kind of abstract
scientific thing. You would be dead without
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plants and their magical- nay, SCIENTIFIC
ability to convert sunlight, carbon dioxide
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and water into glucose and pure, delicious
oxygen.
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This happens exclusively through photosynthesis,
a process that was developed 450 million years
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ago and actually rather sucks.
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It's complicated, inefficient and confusing.
But you are committed to having a better,
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deeper understanding of our world! Or, more
probably, you'd like to do well on your
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test...so let's delve.
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There are two sorts of reactions in Photosynthesis...light
dependent reactions, and light independent
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reactions, and you've probably already figured
out the difference between those two, so that's
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nice. The light independent reactions are
called the "calvin cycle"
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no...no...no...no...YES! THAT Calvin Cycle.
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Photosynthesis is basically respiration in
reverse, and we've already covered respiration,
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so maybe you should just go watch that video
backwards. Or you can keep watching this one.
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Either way.
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I've already talked about what photosynthesis
needs in order to work: water, carbon dioxide
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and sunlight.
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So, how do they get those things?
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First, water. Let's assume that we're
talking about a vascular plant here, that's
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the kind of plant that has pipe-like tissues
that conduct water, minerals and other materials
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to different parts of the plant.
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These are like trees and grasses and flowering
plants.
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In this case the roots of the plants absorb
water
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and bring it to the leaves through tissues
called xylem.
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Carbon dioxide gets in and oxygen gets out
through tiny pores in the leaves called stomata.
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It's actually surprisingly important that
plants keep oxygen levels low inside of their
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leaves for reasons that we will get into later.
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And finally, individual photons from the Sun
are absorbed in the plant by a pigment called
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chlorophyll.
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Alright, you remember plant cells? If not,
you can go watch the video where we spend
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the whole time talking about plant cells.
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One thing that plant cells
have that animal cells don't... plastids.
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And what is the most important plastid?
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The chloroplast! Which is not, as it is sometimes
portrayed, just a big fat sac of chlorophyl.
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It's got complicated internal structure.
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Now, the chlorophyll is stashed in membranous
sacs called thylakoids. The thykaloids are
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stacked into grana. Inside of
the thykaloid is the lumen, and outside the
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thykaloid (but still inside the
chloroplast) is the stroma.
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The thylakoid membranes are phospholipid bilayers, which, if you remember
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means they're really good at maintaining
concentration gradients of ions,
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proteins and other things. This means keeping
the concentration higher on one side
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than the other of the membrane. You're going
to need to know all of these things, I'm sorry.
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Now that we've taken that little tour of
the Chloroplast, it's time to get down to
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the actual chemistry.
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First thing that happens: A photon created
by the fusion reactions of our sun is about
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to end its 93 million mile journey by slapping
into a molecule of cholorophyll.
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This kicks off stage one, the light-dependent reactions proving
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that, yes, nearly all life on our planet is
fusion-powered.
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When Chlorophyll gets hit by that photon,
an electron absorbs that energy and gets excited.
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This is the technical term for electrons gaining
energy and not having anywhere to put it and
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when it's done by a photon it's called
photoexcitation, but let's just imagine,
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for the moment anyway, that every photon is
whatever
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dreamy young man 12 year old girls are currently
obsessed with, and electrons are 12 year old girls.
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The trick now, and the entire trick
of photosynthesis, is to convert the energy
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of those 12 year old-
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I mean, electrons, into something that the
plant can use.
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We are literally going to be spending the
entire rest of the video talking about that.
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I hope that that's ok with you.
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This first Chlorophyll is not on its own here,
it's part of an insanely complicated complex
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of proteins, lipids, and other molecules called
Photosystem II that contains at least 99 different
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chemicals including over 30 individual chlorophyll
molecules.
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This is the first of four protein complexes
that plants need for the light dependent reactions.
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And if you think it's complicated that we
call the first complex photosystem II instead
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of Photosystem I, then you're welcome to
call it by its full name, plastoquinone oxidoreductase.
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Oh, no? You don't want to call it that?
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Right then, photosystem II, or, if you want
to be brief, PSII.
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PSII and indeed all of the protein complexes
in the light-dependent reactions, straddle
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the membrane of the thylakoids in the chloroplasts.
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That excited electron is now going to go on
a journey designed to extract all of its new
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energy and convert that energy into useful
stuff. This is called the electron transport
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chain, in which energized electrons lose their
energy in a series of reactions that capture
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the energy necessary to keep life living.
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PSII's Chlorophyll now has this electron
that is so excited that, when a special protein
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designed specifically for stealing electrons
shows up,
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the electron actually leaps off of the chlorophyll
molecule onto the protein, which we call a
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mobile electron carrier because it's...
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...a mobile electron carrier.
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The Chlorophyll then freaks out like a mother
who has just had her 12 year old daughter
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abducted by a teen idol and is like "WHAT
DO I DO TO FIX THIS PROBLEM!"
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and then it, in cooperation with the rest
of PSII does something so amazing and important
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that I can barely believe that it keeps happening
every day.
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It splits that ultra-stable molecule, H2O,
stealing one of its electrons, to replenish
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the one it lost.
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The byproducts of this water splitting?
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Hydrogen ions, which are just single protons,
and oxygen. Sweet, sweet oxygen.
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This reaction, my friends, is the reason that
we can breathe.
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Brief interjection: Next time someone says
that they don't like it when there are chemicals
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in their food, please remind them that all
life is
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made of chemicals and would they please stop
pretending that the word chemical is somehow
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a synonym for carcinogen!
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Because, I mean, think about how chlorophyll
feels when you say that! It spends all of
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it's time and energy creating the air we
breathe and then we're like
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"EW! CHEMICALS ARE SO GROSS!"
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Now, remember, all energized electrons from
PSII have been picked up by electron carriers
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and are now being transported onto our second
protein complex
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the Cytochrome Complex!
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This little guy does two things...one, it
serves as an intermediary between
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PSII and PS I and, two, uses a bit of the
energy from the electron to
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pump another proton into the thylakoid.
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So the thylakoid's starting to fill up with
protons. We've created some by splitting water,
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and we moved one in using the Cytochrome complex.
But why are we doing this?
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Well...basically, what we're doing, is charging
the Thylakoid like a battery.
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By pumping the thylakoid full of protons,
we're creating a concentration gradient.
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The protons then naturally want to get the
heck away from each other, and so they push
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their way through an enzyme straddling the
thylakoid membrane called ATP Synthase, and
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that enzyme uses that energy to pack an inorganic
phosphate onto ADP, making ATP: the big daddy
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of cellular energy.
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All this moving along the electron transport
chain requires energy, and as you might expect
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electrons are entering lower and lower energy
states as we move along. This makes sense
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when you think about it. It's been a long
while since
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those photons zapped us, and we've been
pumping hydrogen ions to create ATP and splitting
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water and jumping onto different molecules
and I'm tired just talking about it.
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Luckily, as 450 million years of evolution
would have it, our electron is now about to
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be re-energized upon delivery to Photosystem I!
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So, PS I is a similar mix of proteins and
chlorophyll molecules that we saw in PSII,
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but with some different products.
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After a couple of photons re-excite a couple
of electrons, the electrons pop off, and hitch
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a ride onto another electron carrier.
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This time, all of that energy will be used
to help make NADPH, which, like ATP, exists
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solely to carry energy around.
Here, yet another enzyme
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helps combine two electrons and one hydrogen
ion with a little something called NADP+.
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As you may recall from our recent talk about
respiration, there are these sort of
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distant cousins of B vitamins that are crucial
to energy conversion. And in photosynthesis,
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it's NADP+, and when it
takes on those 2 electrons
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and one hydrogen ion, it becomes NADPH.
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So, what we're left with now, after the
light dependent reactions is chemical energy
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in the form of ATPs and NADPHs. And also of
course, we should not forget the most useful
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useless byproduct in the history of
useless byproducts...oxygen.
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If anyone needs a potty break, now would be
a good time...or if you want to go re-watch
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that rather long and complicated
bit about light
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dependent reactions, go ahead and do that...it's
not simple, and it's not going to get any
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simpler from here.
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Because now we're moving along
to the Calvin Cycle!
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The Calvin Cycle is sometimes called the dark
reactions, which is kind of a misnomer, because
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they generally don't occur in the dark. They
occur in the day along with the rest of the
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reactions, but they don't require energy
from photons. So it's more proper to say
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light-independent. Or, if you're feeling
non-descriptive...just say Stage 2.
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Stage 2 is all about using the energy from
those ATPs and NADPHs that we created in
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Stage 1 to produce something
actually useful for the plant.
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The Calvin Cycle begins in the stroma, the
empty space in the chloroplast, if you remember
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correctly. And this phase is called carbon
fixation because...yeah, we're about to
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fix a CO2 molecule onto our starting point,
Ribulose Bisphosphate or RuBP, which is always
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around in the chloroplast because, not only
is it the starting point of the Calvin Cycle,
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it's also the end-point...
which is why it's a cycle.
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CO2 is fixed to RuBP with the help of an enzyme
called ribulose 1,5 bisphosphate carboxylase
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oxidase, which we generally
shorten to RuBisCo.
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I'm in the chair again! Excellent!
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This time for a Biolo-graphy of RuBisCo.
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Once upon a time, a one-celled organism was
like "Man, I need more carbon so I can make
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more little me's so I can take over the
whole world."
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Luckily for that little organism, there was
a lot of CO2 in the atmosphere, and so it
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evolved an enzyme that could suck up that CO2 and convert inorganic carbon into organic carbon.
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This enzyme was called RuBisCo, and it wasn't
particularly good at its job, but it was a
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heck of a lot better than just hoping to run
into some chemically formed organic carbon,
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so the organism just made a ton of it to make
up for how bad it was.
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Not only did the little plant stick with it,
it took over the entire planet, rapidly becoming
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the dominant form of life.
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Slowly, through other reactions, known as
the light dependent reactions, plants increased
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the amount of oxygen in the atmosphere. RuBisCo, having been designed in a world with
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tiny amounts of oxygen in the
atmosphere, started getting confused.
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As often as half the time RuBisCo started
slicing Ribulose Bisphosphate with Oxygen
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instead of CO2, creating a toxic byproduct
that plants then had to deal with in creative
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and specialized ways.
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This byproduct, called phosphogycolate, is
believed to tinker with some enzyme functions,
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including some involved in the Calvin cycle,
so plants have to make other enzymes that
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break it down into an amino acid (glycine),
and some compounds that are actually useful
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to the Calvin cycle.
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But plants had already sort of gone all-in
on the RuBisCo strategy and, to this day,
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they have to produce huge amounts of it (scientists estimate that at any given time there
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are about 40 billion tons of RuBisCo on the planet) and plants just deal with that toxic byproduct.
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Another example, my friends, of unintelligent
design.
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Back to the cycle!
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So Ribulose Bisphosphate gets a CO2 slammed
onto it and then immediately the whole thing
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gets crazy unstable. The only way to regain
stability is for this new six-carbon chain
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to break apart creating two molecules of
3-Phosphoglycerate, and these are
the first stable products of the calvin cycle.
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For reasons that will become clear in a moment,
we're actually going to do this to three
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molecules of RuBP.
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Now we enter the second phase,
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Reduction.
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Here, we need some energy. So some ATP slams
a phosphate group onto the 3-Phosphoglycerate,
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and then NADPH pops some electrons on and,
voila, we have two molecules of Glyceraldehyde
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3-Phosphate, or G3P, this is a high-energy,
3-carbon compound that plants can convert
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into pretty much any carbohydrate. Like glucose
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for short term energy storage, cellulose for
structure, starch for long-term storage.
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And because of this, G3P is considered the
ultimate product of photosynthesis.
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However, unfortunately, this is not the end.
We need 5 G3Ps to regenerate the 3 RuBPs that
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we started with. We also need 9 molecules
of ATP and 6 molecules of NADPH, so with all
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of these chemical reactions, all of this chemical
energy,
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we can convert 3 RuBPs into 6 G3Ps but only
one of those G3Ps gets to leave the cycle,
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the other G3Ps, of course, being needed to
regenerate the original 3 Ribulose Bisphosphates.
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That regeneration is the last phase of the
Calvin Cycle.
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And that is how plants turn sunlight, water,
and carbon dioxide into every living thing
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you've ever talked to, played with, climbed
on, loved, hated, or eaten. Not bad, plants.
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I hope you understand. If you don't, not only
do we have some selected references below
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that you can check out, but of course, you
can go re-watch anything that you didn't get
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and hopefully, upon review, it will make a
little bit more sense.
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Thank you for watching. If you have questions,
please leave them down in the comments below.
