Let's talk about
one of the most important biological processes. Frankly, if this process didn't
occur, we probably wouldn't have life on Earth, and
I wouldn't be making this video for you, because there'd
be no place for me to actually get food. And the process is called
photosynthesis. And you're probably reasonably
familiar with the idea. The whole idea is plants, and
actually bacteria and algae and other things,
but we normally associate it with plants. Let me make it in very simple
terms. So we normally associate it with plants. And it's a process that plants
use, and we might have learned this when we were very young. It's the process that plants
use to take carbon dioxide plus some water plus some
sunlight and turn it into some sugars or some maybe
carbohydrates. Carbohydrates or sugars
plus oxygen. Obviously, this has two very
profound pieces to it for us as a living species. One, we need carbohydrates or
we need sugars in order to fuel our bodies. You saw that in the cellular
respiration videos. We generate all of our ATP
by performing cellular respiration on glucose, which is
essentially a byproduct, or a broken down carbohydrate. It's the simplest one for us
to process in cellular respiration. And the second hugely
important part is getting the oxygen. Once again, we need to breathe
oxygen in order for us to break down glucose, in order
to respire, in order to perform cellar respiration. So these two things are key for
life, especially for life that breathes oxygen. So this process, other than
the fact that it's interesting, that there are
organisms around us, mostly plants, that are able to harness
actual sunlight. You have these fusion reactions
in the sun 93 million miles away, and it's
releasing these photons, and some small subset of
those photons reach the surface of Earth. They make their way through
clouds and whatever else. And then these plants and
bacteria and algae are able to harness that somehow and turn
them into sugars that we can then eat or maybe the cow eats
them and we eat the cow if we're not vegetarians, and we
can then use that for energy. Not that the cow is all
carbohydrates, but this is essentially what is used as the
fuel or the energy for all of the other important compounds
that we eat. This is where we get
all of our fuel. So this is fuel for animals. Or you know, if you eat a
potato directly, you are directly getting your
carbohydrates. But anyway, this is a very
simple notion of photosynthesis, but it's
not incorrect. I mean, if you had to know one
thing about photosynthesis, this would be it. But let's delve a little bit
deeper and try to get into the guts of it and see if we can
understand a little bit better how this actually happens. I find it amazing that somehow
photons of sunlight are used to create these sugar
molecules or these carbohydrates. So let's delve a little
bit deeper. So we can write the general
equation for photosynthesis. Well, I've almost
written it here. But I'll write it a little bit
more scientifically specific. You start off with some
carbon dioxide. You add to that some water, and
you add to that-- instead of sunlight, I'm going to say
photons because these are what really do excite the electrons
in the chlorophyll that go down, and you'll see this
process probably in this video, and we'll go in more
detail in the next few videos. But that excited electron goes
to a high energy state, and as it goes to a lower energy state,
we're able to harness that energy to produce ATPs,
and you'll see NADPHs, and those are used to produce
carbohydrates. But we'll see that
in a little bit. But the overview of
photosynthesis, you start off with these constituents, And
then you end up with a carbohydrate. And a carbohydrate
could be glucose, doesn't have to be glucose. So the general way we can write
a carbohydrates is CH2O. And we'll put an n over here,
that we could have n multiples of these, and normally, n
will be at least three. In the case of glucose,
n is 6. You have 6 carbons, 12 hydrogens
and 6 oxygens. So this is a general term for
carbohydrates, but you could have many multiples of that. You could have these
long-chained carbohydrates, so you end up with a carbohydrate
and then you end up with some oxygen. So this right here isn't so
different than what I wrote up here in my first overview of
how we always imagined photosynthesis in our heads. In order to make this equation
balance-- let's see, I have n carbons so I need
n carbons there. Let's see, I have two
n hydrogens here. Two hydrogens and I have n
there, so I need two n hydrogens here. So I'll put an n out there. And lets see how many oxygens. I have two n oxygens,
plus another n, so I have three n oxygens. So let's see, I have one n,
and you put an n here, and then I have two n, and I think
this equation balances out. So this is a 30,000-foot view
of what's going on in photosynthesis. But when you dig a little
deeper, you'll see that this doesn't happen directly, that
this happens through a bunch of steps that eventually gets
us to the carbohydrate. So in general, we can break
down photosynthesis. I'll rewrite the word. We can break down
photosynthesis-- and we'll delve deeper into future videos,
but I want to get you the overview first--
into two stages. We can call one the
light reactions. Or sometimes they are called the
light-dependent reactions, and that actually would
probably be a better way to write it. Let me write it like that. Light dependent means that
they need light to occur. Light-dependent reactions. And then you have something
called the dark reactions, and that's actually a bad name,
because it also occurs in the light. Dark reactions, I wrote in
a slightly darker color. And the reason why I said it's a
bad name is because it still occurs in the light. But the reason why they probably
called it the dark reaction is that you don't need
light, or that part of photosynthesis isn't dependent
on photons to occur. So a better term for
it would have been light-independent reaction. So just to be clear,
the light reactions actually need sunlight. They actually need photons
for them to proceed. The dark reactions do not need
photons for them to happen, although they do occur
when the sun is out. They don't need those photons,
but they need the byproducts from the light reaction to
occur, so that's why it's called the light-independent
reaction. They occur while the sun
is out, but they don't need the sun. This needs the sun, so let
me make it very clear. So this requires sunlight. This requires photons. And let me just make a very
brief overview of this. This'll maybe let us start
building a scaffold from which we can dig deeper. So the light reactions
need photons, and then it needs water. So water goes into the light
reactions and out of the other side of the light reactions. We end up with some
molecular oxygen. So that's what happens in the
light reactions, and I'm going to go much deeper into
what actually occurs. And what the light the actions
produce is ATP, which we know is the cellular or the
biological currency of energy. It produces ATP and
it produces NADPH. Now, when we studied cellular
respiration, we saw the molecule NADH. NADPH is very similar. You just have this P there. You just have this phosphate
group there, but they really perform similar mechanisms. That
this agent right here, this molecule right here, is
able to give away-- now let's think about what this means--
it's able to give away this hydrogen and the electron
associated with this hydrogen. So if you give away an electron
to someone else or someone else gains an electron,
that something else is being reduced. Let me write that down. This is a good reminder. OIL RIG. Oxidation is losing
an electron. Reduction is gaining
an electron. Your charge is reduced when
you gain an electron. It has a negative charge. So this is a reducing agent. It gets oxidized by losing
the hydrogen and the electron with it. I have a whole discussion
on the biological versus chemistry view of oxidation,
but it's the same idea. When I lose a hydrogen, I also
lose the ability to hog that hydrogen's electron. So this right here, when it
reacts with other things, it's a reducing agent. It gives away this hydrogen and
the electron associated with it, and so the other
thing gets reduced. So this thing is a
reducing agent. And what's useful about it is
when this hydrogen, and especially the electron
associated with that hydrogen, goes from the NADPH to, say,
another molecule and goes to a lower energy state, that energy
can also be used in the dark reactions. And we saw in cellular
respiration the very similar molecule, NADH, that through
the Kreb Cycle, or actually more importantly, that through
the electron transport chain, was able to help produce ATP as
it gave away its electrons and they went to lower
energy states. But I don't want to confuse
you too much. So the light reactions, you take
in photons, you take in water, it spits out oxygen, and
it spits out ATP and NADPH that can then be used in
the dark reactions. And the dark reactions, for
most plants we talk about, it's called the Calvin Cycle. And I'll go into a lot more
detail of what actually occurs in the Calvin Cycle, but it
takes in the ATP, the NADPH, and it produces-- it doesn't
directly produce glucose. It produces-- oh, you
probably saw this. You could call it PGAL. You could call it G3P. These all stand for-- let me
write these down-- this is phosphoglyceraldehyde. My handwriting broke down. Or you could call it
glyceraldehyde 3-phosphate. Same exact molecule. You can almost imagine it as--
this is a very gross oversimplification-- as three
carbons with a phosphate group attached to it. But this can then be used to
produce other carbohydrates, including glucose. If you have two of these,
you can use those two to produce glucose. So let's just take a quick
overview again because this is super important. I'm going to make videos on the
light reactions and the dark reactions. Those will be the next
two videos I make. So photosynthesis, you
start with photons. All of these occur when the sun
is out, but only the light reactions actually
need the photons. The light reactions take
photons-- we're going to go into more detail about what
actually occurs-- and it takes in water. Oxygen gets spit out. ATP and NADPH get spit out,
which are then used by the dark reaction, or the
Calvin Cycle, or the light-independent reaction,
because these still occur in the light. They just don't need photons. So they're the light-independent
reaction. And it uses that in
conjunction-- and we'll talk about other molecules that
are used in conjunction. Oh, and I forgot a very
important constituent of the dark reaction. It needs carbon dioxide. That's where you get your
carbons to keep producing these phosphoglyceraldehydes, or
glyceraldehyde 3-phosphate. So that's super important. It takes in the carbon dioxide,
the products from the light reactions, and then uses
that in the Calvin Cycle to produce this very simple
building block of other carbohydrates. And if you remember from
glycolysis, you might remember that this PGAL molecule, or this
G3P-- same thing-- this was actually the first product
when we split glucose in two when we performed
the glycolysis. So now we're going
the other way. We're building glucose
so that we can split it later for energy. So this is an overview of
photosynthesis, and in the next couple of videos, I'm
actually going to delve a little bit deeper and tell you
about the light reactions and the dark reactions and how
they actually occur.