A couple of videos ago, we
saw that in classic C-3 photosynthesis– and once again
it’s called C-3 because the first time that carbon
dioxide is fixed, it’s fixed into a 3-carbon molecule. But we saw the problem with C-3
photosynthesis is that the enzyme that does the carbon
fixation, it can also react with oxygen. And when oxygen essentially
reacts with ribulose biphosphate instead of your
carbon, you get an unproductive reaction. Not only is it unproductive,
it’ll actually suck up your ATP and your NADPH and
you’ll go nowhere. So every now and then, when
oxygen bonds here instead of a carbon dioxide, you get
nothing produced, net. Everything becomes
less efficient. And so in the last video, we
saw that some plants have evolved a way to get
around this. And what they do is, they fix
their carbon on the outside, on cells that are actually
exposed to the air. And then once they fix the
carbon they actually fix it into a 4-carbon molecule, into
oxaloacetate And then that gets turned into malate Then
they pump the malate deeper within the leaf, where you
aren’t exposed to oxygen. And then they take the carbon
dioxide off the malate, and this is where they actually
perform the Calvin cycle. And even though you do have
your rubiscos still there, your rubisco isn’t going to
have– the photorespiration is not going to occur. Because it only has access
to carbon dioxide. It does not have access to
this oxygen out here. Now that’s a very efficient
way of producing sugars. And that’s why some of the
plants that we associate with being very strong sugar, or even
ethanol producers, all perform C-4 photosynthesis. Corn, sugarcane,
and crab grass. And these are all very, very
efficient sugar producers. Because they don’t have to
worry too much about photorespiration. Now some plants have a slightly
different problem. They’re not so worried about the
efficiency of the process. They’re more worried
about losing water. And you can imagine what
plants these are. These are plants that
are in the desert. Because these stomata, these
pores that are on the leaves, they let in air, but they
can also let out water. I mean, if I’m in the
rainforest, I don’t care about that. But if I’m in the middle of the
desert, I don’t want to let out water vapor through
my stomata. So the ideal situation is, I
would want my stomata closed during the daytime. This is what I want. So I want– if I’m in
the desert, let me make this clear. If I’m in the desert I want
stomata closed during the day. For obvious reasons. I don’t want all my water
to vaporize out of these holes in my leaves. But at the same time, the
problem is that photosynthesis can only occur during
the daytime. And that includes the
dark reactions. Remember, I’ve said multiple
times, the dark reactions are badly named. They’re more the light
independent reactions. But they both occur
simultaneously– the light independent and light
dependent– and only during the daytime. And if your stomata is closed,
you need to perform photosynthesis, especially the
Calvin cycle, you need CO2. So how can you get
around this? If I want to close my stomata
during the day, but I need CO2 during the day, how can
I solve this problem? And what desert plants, or
many desert plants, have evolved to do, essentially
does photosynthesis, but instead of fixing the carbon
in outer cells and then pushing it in to inner cells and
then performing the Calvin cycle, instead of outer and
inner cells, they do it at the nighttime and in the daytime. So in CAM plants– and these are
called CAM plants because, I could tell you what
it stands for. It stands for crassulacean
acid metabolism. And that’s because it was first
observed in that species of plants, the crassulacean
plant. But these are just called,
you could call it CAM photosynthesis or CAM plants. They’re essentially a subset
of C-4 plants. But instead of performing C-4
photosynthesis, kind of an outside cells and inside cells,
they do it at the nighttime and the day. And what they do is, at night
they keep their stomata open. And they perform, and they’re
able to fix– and everything occurs in the mesophyll
cells and the CAM cells, in the CAM plants. So at nighttime, when they’re
not afraid of losing water– let’s say this is a mesophyll
cell right here– my stomata is open. Let’s say that this is my
stomata right there. And so it lets in
carbon dioxide. I’m not worried about
losing water vapor. It’s night time right now. So carbon dioxide
comes in here. And then it fixes the
carbon dioxide. It fixes it the exact same way
that the C-4 plants do. So you have your CO2 come in. You have your PEP . It’s all facilitated by PEP
carboxylase That’s the enzyme. That can only fix CO2, that can
only react with CO2, not with oxygen. And then that is used to
produce– and we saw it here in our CAM-4 diagram in the last
video, that is to used to produce malate. A 4-carbon molecule. And then the malate– and then
this is what’s key– the malate get stored in other
organelles in the cell. In the vacuoles , which are, you
can kind of view them as large storage containers
in the cell. So I drew this as
the whole cell. I mean, this is actually all
occurring in your chloroplast. But you can imagine your cell
having a big storage center where the malate gets
stored at night. And you can view malate as
almost a carbon dioxide store. Because later on we can
access the malate and get the carbon dioxide. And that’s exactly what these
CAM plants are going to do. So this is nighttime. Then the sun comes up. So now we’re in the daytime. This desert plant, well maybe
it’s a cactus, it doesn’t want to lose its water vapor. So it closes its stomata. This particular stoma
now is closed. It’s now closed. And you say, oh boy, how
is it going to perform photosynthesis? Well, it can perform
photosynthesis in that very same cell. Because it stored up all of
this malate at night. And so now the malate can be
pumped out of the vacuoles into the stroma of our
chloroplast. And then you can have pyruvate break off. But the more important thing
is you have CO2 break off. So you have a ready
supply of CO2. And now we can perform our
standard Calvin cycle. And in an environment only
with CO2, our stomata is closed, so we’re ready to go. Our CO2 reacts with ribulose
bisphosphate It is catalyzed by rubisco It’s the whole
Calvin cycle and we produced our sugar. So this is kind of a
neat adaptation. In these high, very efficient
sugar-producing plants that aren’t worried about water, they
perform carbon fixation on things that are exposed to
the air and then they pump kind of a stored version of
the carbon deeper into the leaf to actually perform the
Calvin cycle so that it’s not lossy, so that photorespiration doesn’t occur. Because down here you
have no oxygen. The desert plants benefit from
that property as well, but their whole concern is, I don’t
want to keep my stomata open in the daytime. So what I do is, I fix
my carbon at night. But I use the exact
same process. I use PEP carboxylase
And I store my carbon dioxide at night. And in the daytime, I can
actually– when my light-dependant reactions are
occurring, they’re producing my ATP and my NADH I can also
perform my dark reactions in the daytime. As I said, the dark reactions
always occur in the daytime. Or my light-independent
reactions. Because even though my stomata
is closed, I have a store of carbon dioxide in the
form of malate.