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!