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
And when oxygen essentially reacts with ribulose
biphosphate instead of your carbon, you get an
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
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
They're more the light independent reactions.
But they both occur simultaneously-- the light
independent and light dependent-- and only during
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
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
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
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
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.