Hi folks, we're going to finish up our discussion of photosynthesis by talking
about three different classes of plants; c3, c4 and cam plants. As we go through,
each of the three types of plants, we're going to take a look at the cells and
processes that are involved in each one.
A common term that you hear a lot when you're talking about photosynthesis is
this idea of carbon fixation. And, all that really means is that you take carbon
from the atmosphere and you transform it into part of an organic molecule. So, in
our case, we're transforming carbon dioxide into glucose and this is
basically what the process of photosynthesis does.
Here's our equation right down here. So, essentially we're taking the carbons
from carbon dioxide,
we're combining it with a couple of other things and we're generating sugar.
This is our friendly light independent reaction diagram that we've
been looking at for the last couple of days and so we're going to just quickly
look at carbon fixation in here. We're taking carbon dioxide out of the
atmosphere we're using that carbon to add to the carbons of RUBP and
then eventually we're making this 3 carbon product down here called PGA and
this is why we call these reactions the "c3 reactions". And then eventually we're
going to wind up making a molecule of glucose.
We've successfully taken carbon dioxide carbon out of the air and then made it
into glucose which has six carbons.
Now, this reaction works really, really well provided that you have a constant
supply of carbon dioxide that's coming out of the atmosphere right up here.
Let's imagine that this is the cross section of a leaf and right now we have
a greater concentration of carbon dioxide and oxygen on the outside of the
leaf than we do on the inside of the leaf.
Right now, we have a whole bunch of stomata that are open and so eventually
the carbon dioxide and the oxygen are going to diffuse from the outside of the leaf
to the inside of the leaf kind of like this.
Provided that the stomata stay open, eventually will reach a state of dynamic
equilibrium where we have an equal concentration on both sides of the leaf.
Now, unfortunately, one of the problems with having your stomata open is that
they can actually cause water loss via evaporation. For most c3 plants, they live
in areas where it's not that hot, so this isn't a huge issue most of the time. But,
if you get a really hot day, this could be a problem, or if you put a c3 plant in
an area where it's much hotter and drier than normal. So, to prevent water
loss, the plant might close the guard cells of its stomates.
Ok, so now all of my stomata are closed and so I'm not losing any more water via
evaporation.
However, now i have a limited supply of carbon dioxide available and so if I
continue to do photosynthesis, eventually I'm going to lose all of my carbon
dioxides. So this puts us in a real quandary here because the plant can't
open it stomata, so it's not going to be able to survive if it continues to
lose water via evaporation. But, it will also go into an extremely inefficient
process called photorespiration if it allows oxygen to bond with rubisco.
Remember that rubisco allows carbon dioxide to bond with RUBP. However are
RUBP is actually not particularly picky, and if there is oxygen around,
RUBP will bond with that instead of with carbon dioxide which can
actually bring the process to a halt.
Take a minute to look back through this diagram and see if you can figure out
what would happen to the calvin cycle if you suddenly didn't have any more carbon
dioxide. So, hopefully you've figured out by now that if you stop adding in carbon
dioxide you don't have enough carbons to run the rest of the calvin cycle.
This particular situation would be extremely difficult for c3 plants, but
there are actually a couple of classes of other plans that do really well in
this particular situation, so let's take a look at that. We're going to take a
look at this plant next the "C4 pathway". This picture by now should look
really familiar to you; this is a standard c3 plant, so it's sort of like what we
would see here in New England where it's pretty cold most of the time.
This is the key for all of these structures here starting with the waxy
cuticle and working all the way down to the center of the leaf cross-section all
the way down to the other waxy cuticle on the bottom. But now let's take a look
at what happens when you have a c4 plant and how that looks similar and different.
This is a c4 plant, so before I zoom outwards, take a look at this and see if
you can identify a few similarities and differences between this and the c3
plant. The two biggest differences that I want you to be familiar with are that
the c4 plants, these grasses that grow in really hot dry areas, tend to have a lot
more palisade mesophyll cells, and they're also much more tightly packed
together than in the c3 plants. And, they also have much bigger bundle sheath
cells which is going to be this purple cells that are surrounding the vascular
bundle right in the center here. Look at how much bigger they are in this c4
plants rather than in the c3 plant. That's going to be important. Notice also that
the bundle sheath cells and the palisade mesophyll cells are packed very closely
together. We've been very familiar with the palisade mesophyll and we've also
been looking at the chloroplasts in a lot of detail, but bundle sheath cells are going
to become very important in a c4 plant. In the c3 plants that were familiar with,
carbon dioxide combines with RUBP, then it forms a three-carbon
intermediate called PGA which is why this is called the c3 pathway.
Ultimately, it forms a six carbon molecule called glucose and then that
gets carried around the plant by the phloem. Notice that this all takes place
in the palisade mesophyll cells. Now let's go over to the C4 plant in
the middle here and notice there are two different kinds of cells we're working
with. We have our palisade mesophyll cells up here
but down here we have something called the bundle sheath cells and those are
going to be extremely important.
C4 plants start out being pretty different from c3 reactions because they
actually use a different kind of compound to start with.
They use this thing called PEP instead of are you BP and PEP stands for phosphoenal pyruvate
but you can just call it PEP. PEP is much more selective than RUBP
and will only combine with whatever carbon dioxide is left in the palisade
mesophyll cells, so it will completely ignore oxygen. Unlike RUBP, PEP has three
carbons. Then it combines with a molecule of carbon dioxide which has one carbon.
The two of them together rearrange to form a molecule called oxaloacetate
which has a total of four carbons.
This is why this pathway is called a c4 pathway.
Its first compound here has four carbons. So we have now "fixed" carbon dioxide once;
we've pulled it out of the atmosphere, combined it with PEP, and made this
oxaloacetate molecule. Oxaloacetate is then converted into a new 4-carbon
molecule called malate, and malate is actually going to exit this palisade
mesophyll cell and diffuse into the bundle sheath cells.
Once it's inside the bundle sheath cells, malate will split into two new molecules;
pyruvate which has three carbons, and carbon dioxide which has one carbon.
Waiting right there to pick up that carbon dioxide molecule is our old
friend RUBP, which has five carbons. The calvin cycle can then proceed
normally and produce glucose.
So, before we move on, we should think about what happens to this molecule of
pyruvate up here.
Look back at the diagram and see if you can figure out what other molecule
also has three carbons might enable us to RECYCLE this pyruvate. If you guessed
that we might convert pyruvate back into PEP, you would be completely right.
This process of reconverting pyruvate back into PEP does require a little bit
of ATP, though. The pyruvate diffuses back into the palisade mesophyll cell and
then becomes PEP again.
Overall, it's important for us to remember that we wind up "fixing" carbon
or converting it two different times.
First, right up here, we took the co2 out of the atmosphere added that the PEP, and
then a second time down here.
We took this carbon dioxide molecule from the malate and added it to RUBP
right here.
Now we have to ask ourselves "why on earth would be bother going through all
of these steps to create carbon dioxide down here if we already have a little
bit of carbon dioxide up here?"
That's actually a really valid question, but the important thing to remember is
that this palisade mesophyll cell that's up top right here is actually
full of two different kinds of gases. So you have the co2, which you really want
but you also have the oxygen, which you really don't want.
And the good news is that with this pathway you're keeping this are RUBP
molecule, which will pick up either oxygen or carbon dioxide,
you're keeping it away from all of the oxygen which is up here in this cell
right here.
So even if you have only a tiny bit of carbon dioxide,
you can use this c4 pathway,
and even though this pathway exists in C4 plants, they can also do the c3
normal photosynthesis process that we've been talking about. The way that I like
to think about the c-4 pathway is a selective carbon pump; it can engage
this pathway
if conditions require it. It's not going to do this all the time;
it's actually a little bit more efficient to do this pathway right here
the c3 pathway, but C4 plants are pretty flexible. They can do C3 if
conditions are great and then they can go in to C4 mode of things are not
so great. Now let's talk about CAM plants. "CAM" stands for crassulacean acid
metabolism but we're just going to call them "CAM" plants. Cam plants tend to live
in areas where it's very hot and dry and so they can to keep their stomata
closed during the daytime and then only open them up at night.
This is great because it helps to prevent water loss, because at night it's a lot
cooler but it also really limits when they can do photosynthesis because, of course,
at nighttime when they're bringing in carbon dioxide through the open stomata
you also don't have any sunlight out and so you can't do regular photosynthesis.
They solve this problem by doing photosynthesis in two different stages;
so i'm going to divide this palisade mesophyll up into two stages right
here. The top one is going to be what happens at night, and then the bottom
part is going to be what happens during the daytime.
CAM plants are going to behave a lot like c4 plants at night; they're going to
bring in carbon through their open stomata in the form of carbon dioxide
and they're going to "fix it" into this creature right here called malate that
we saw in our last bit.
Malate is then stored until the light comes back again during the day.
From there on out, these plants behave a lot like c4 plants. The malate splits into
pyruvate and carbon dioxide, and we can use the carbon dioxide to run the calvin
cycle now that the light is back again.
We produce glucose just like we did earlier, and we send the pyruvate back to
be converted into PEP. Notice that in CAM plants, everything is taking place within
the mesophyll cells; we're not using any bundle sheath cells. CAM plants don't
have too much oxygen, so we don't need to worry about keeping the oxygen away from
the RUBP.
All we have to worry about is preventing water loss; this pathway allows the CAM
plant to
take advantage of having the stomata open at night time to bring in carbon
dioxide and then having them closed during the day to prevent water loss
while still being able to do the photosynthesis process and make glucose.
So, there you have it; that's c3 c4 and CAM plants all on one screen. If
you're confused about any of these processes make sure you go back and
re-watch or bring your questions to class. Thanks for watching and don't forget to subscribe!