Photosynthesis: Crash Course Biology #8

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

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