Biology: C4 Plants and CAM Plants

Excellent and enthousiastic

11/07/2011

~ MrMark

Some small mistakes are made in the heat of the explenation, but they are easily overlooked. Pyruvate is (looking at his own schedule) not diffused back into the mesophyll cell because it costs ATP to do so. Therefore active transport instead of diffusion.
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mahalo

10/27/2011

~ Aliah

mahalo nui

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So rubisco has created this problem. And the problem is it will bond with oxygen as well as carbon dioxide. And this creates this dilemma called photorespiration, which is an unbelievable efficiency destroy of photosynthesis, and particularly the Calvin cycle. But evolution comes to the rescue. Indeed, natural selection has selected for a group of plants, actually two different groups of plants that I want to tell you about right now. And one of those groups is called the C4 plants.
Now why are they called C4 plants? Well I can't tell you that yet, but I have to tell you this. The other plants, all of the plants that run into photorespiration are called C3 plants. Why? Well think about it. Where does carbon dioxide end up as soon as it is fixed into the RuBP? Well the first product of C3 photosynthesis pathway is G3P. That is not going to be our first product here. In our product here, we're going to make a four-carbon substance. So it's going to be called C4 photosynthesis. There are about 19 families of plants, and several thousand species that do C4 photosynthesis. Some of the good examples of these are two examples of grasses that you know about, corn and sugar cane. Those are both in the grass family, and these both are C4 plants, which might give you a clue as to why they are such efficient photosynthesizers.
Well let's take a look at this. We've got to take a look at structure following function. Remember structure always follows function. Let's go through some structure again. This is a C3 plant leaf. Why is it a C3 plant leaf? It's a C3 plant leaf, and I can tell just by looking at it, because I want to look over here at the vein. You see the vein has a very simplified surrounding layer of cells. But on a C4 plant, we're going to find a completely different structural adaptation that nature has selected for. And they're called bundle sheath cells. Now let's trace what that word means. The veins of a plant are called the vascular bundle. So these are the cells that sheath the bundle. Thus they are called bundle sheath cells. Now you can see that the bundle sheath cells are very much interior in the leaf. So you would think that not a whole lot of light reactions are going to happen there. And guess what, you would be correct. A lot of light reactions are not going to happen there. So they do not have highly developed grana. But they are loaded with rubisco. But they're inside this plant. They're deep. Well what's going to happen here? And now we come to the magic of C4 photosynthesis.
Here's the thing. The thing is PEP. Remember these cells out here? These are the mesophyll cells, and that's where the chloroplasts are. And you'll remember that the mesophyll is where, first of all, the light reactions are going to happen, and indeed the light independent reactions happen. So the Calvin cycle took place in the same place that the other reactions, the light reactions, occurred in. Not so in C4 plants, in C4 plants, we have a completely different story. Let me tell you about it.
In C4 plants, the light hits. When the light hits, an interesting thing happens. And you make NADPH. And you make ATP, but that's not the interesting thing. What the interesting thing is, is this. When you take that CO[2] and you go to fix it in the mesophyll of a C4 plant, guess what. Rubisco is not what it's going to be fixed on. It's going to be fixed on a three-carbon substance called phosphophenyl pyruvate, or PEP. That's why this is going to be called the PEP cycle. And so PEP, with its three carbons, picks up CO[2] and forms a four-carbon substance. Thus it's going to be called C4 photosynthesis. Now this C4 substance is going to now be shuttled. So here's what's going to happen.
In the mesophyll, we have PEP. PEP carboxylase, you know what carboxylases do. PEP carboxylase is going to take PEP three carbon, take CO[2], and make a four-carbon substance, malate. But here's the key. Wait, I've got to tell you about something. Do you remember cell junctions? Do you remember that plant cell walls have a little channel in between them, where the cell membranes join to each other? Do you remember that they're called desmosomes? Now watch what a desmosome does.
So here's the thing. I'm going to draw two plant cells, and I'm going to draw a desmosome in between them. And I'm going to make this particular one a bundle sheath cell. And this one is going to be in the middle. It's going to be one of those mesodermal cells there. So this particular mesophyll cell is going to make a C4 substance. PEP carboxylase is so fussy it will never bond with oxygen. So it's guaranteed to bond to that CO[2]. And now guess what. That C4 substance can come into the bundle sheath cell. And in coming into the bundle sheath cell through the desmosome, what are we going to do? We now have a four-carbon substance in here. And what do you think is going to happen here? That four-carbon substance is going to split. It's going to split into CO[2] and a three-carbon substance that's going to go back into the mesophyll. Check it out. You used a ferryboat. You used a four-carbon ferryboat to ferry carbon dioxide over here. And guess what enzyme is in here, rubisco. But guess what isn't in there, oxygen. Why? It's deep within the plant cell. So it's deep within the plant leaf. So since it's deep within the plant leaf, it's not going to be readily bonding to oxygen. You have literally brought CO[2] from the outside through PEP carboxylase, brought it to the rubisco. Rubisco can now fix it, and there will be less competition for oxygen. Let's take a look at that in a diagram form. How cool is this?
We have our mesophyll cell. We have carbon dioxide coming in. We have PEP carboxylase making a four-carbon substance, oxaloacetate. Oxaloacetate eventually is converted into malate. Malate is the stuff that diffuses, via the desmosome, into the bundle sheath cell, where the bundle sheath cell will now take the CO[2] released from that malate and use it in the Calvin cycle to generate sugar, which of course goes into the vein of the plant. Meanwhile, the three-carbon substance, pyruvate, diffuses back into the mesophyll cell and becomes PEP so that the whole thing can continue to cycle.
How efficient is this? Real efficient. Do you realize that about one percent of the solar energy that hits a C3, approximately in many estimates one percent of the solar energy is converted into chemical bond energy in C3. In C4, estimates are eight percent. Eight times more efficient, would you like to have an efficient money making scheme where you made eight times more money than you did anyway? Pretty good stuff, now I have one more thing I'm going to tell you about, CAM plants, another way to get around this.
C4 plants, I want you to understand that this was a location situation. In other words, the way C4 plants defeated photorespiration is they changed the location of the Calvin cycle. CAM plants changed the time of the Calvin cycle. Here's what CAM plants do. CAM plants close their stomates at a certain time of day. When do you think they're going to close their stomates? Well normally stomates are closed at night, right? You know that. But CAM plants close their stomates in the day. Stomates closed during the daytime, which is a real drag, because they can't get any carbon dioxide. But let's watch what happens. Stomates closed daytime, that being said, we're going to close those stomates during the day, but here's the problem. If you're closing them during the day, you can't get any water moving through. So you can't do photosynthesis. But here's what they do at night. At night they open the stomates. And in opening the stomates, they allow in CO[2]. And they make a series of organic acids, one of which is called crasulacean acid, crasulacean acid metabolism, CAM. So with CO[2 ]in, they make acids. Guess what. What do you think they're going to do the next day?
Well then the next day, when the light is coming in during the daytime, they do the light reactions, and they release CO[2] from the acids. So their CO[2] isn't the daytime atmosphere at all. Their CO[2] is the nighttime atmosphere. And therefore the nighttime atmosphere, it will be a much more efficient way of fixing carbon dioxide. You know evolution is a story in progress. Come back in a million years and I'll tell you about the next adaptation.
Photosynthesis
Photorespiration
C4 Plants and CAM Plants Page [1 of 2]