Follow US:

Practice English Speaking&Listening with: CAM Plants

Normal
(0)
Difficulty: 0

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.

The Description of CAM Plants