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Video transcript
In the last video we discovered what seems like a problem with the Calvin Cycle. That you have this big protein here, enzyme, that facilitates the Calvin Cycle. All of the molecules that are involved bond to this and then it twists and turns and it jams things together so that they react properly. And we know what this is, the RuBisCo. RuBisCo enzyme, or ribulose bisphosphate carboxylase oxygenase. And we know when the Calvin Cycle operates properly you'll have some carbon dioxide attached in one part of this enzyme. And then you'll have some RuBP, or maybe you could call it the proper word, ribulose-1 5-bisphosphate. And they're going to react. And then after they react, if everything with the Calvin Cycle is going properly, they're going to react and form-- they're going to be jammed together and then split into-- for one molecule of that and one molecule of that, you're going to have two molecules of 3-phosphoglycerate. In the last video I started with three of these and three of these so I ended up with six of these. But for every one of these you end up with two of these. This is the proper Calvin Cycle. Then these turn into your phosphoglyceraldehydes. These turn into two phosphoglyceraldehydes, or PGALs. And then for every six of these that are produced-- and maybe I should write a three here, a three here and then I'll have six of these-- and for every six of these that are produced, five go back into the cycle to produce. So five PGALs, or glyceraldehyde 3-phosphate go back into the cycle to produce the ribulose bisphosphate. And one of them is kind of our end product of photosynthesis that can be used to produce other carbohydrates. So, one PGAL. And the whole problem, we saw with the Calvin Cycle, is that RuBisCo does not only fix carbon dioxide. That instead of carbon dioxide we might have an oxygen molecule. We might have an oxygen molecule that jumps in here. And it can also attach to the RuBisCo enzyme. And in that situation, the oxygen and the ribulose bisphosphate react. So if we had three ribulose bisphosphates and three oxygens, instead of producing six phosphoglycerates we're only going to produce five phosphoglycerates and we're going to produce five phosphoglycolates. Which is that byproduct that gets processed later on. And then these five. You're going to have five here. You can't have one left over. And then you're not going to produce anything. In doing this whole cycle you have to use up a bunch of ATPs and NADHs. So this is a problem. If there's a lot of oxygen present, or even a little bit of oxygen present, it's going to make this a little bit less efficient. Because every now and then an oxygen's going to jump in where a carbon dioxide is needed to actually produce an actual carbohydrate in the end. So how do plants solve this problem? Well, one solution would be to operate the Calvin Cycle in an environment where there is very, very little oxygen. Or you can almost say no oxygen. And this is exactly what some plants do. You're like, wait, how do I do that? Do I have to go to a planet where there's no oxygen? No! What they do is-- and to understand this we'll have to understand a little bit of the actual make-up of the leaf of a plant. And that doesn't hurt because everything we've been doing now has been biochemistry. it's nice to see leaves. So if I draw-- let's say that that is a leaf. That is a leaf. I can make it look nice like a leaf. That's a nice looking leaf. On your leaf's surface, and actually on both sides of it, you have these little pores, these little holes on the leaf's surface. They are actually surrounded by these things called guard cells. But the important thing is that these little pores-- and they're actually much smaller than that; you'd have to get a microscope to actually see them-- they're called stomata. Or one individual of these pores or holes is called a stoma. And this is where the oxygen and the-- mainly the carbon dioxide-- but this is where the air enters the cell and this is actually where water vapor is also released from the cell. If we draw a cross section of a leaf, so let me do my best to draw a cross section like this. Let me draw it like that and maybe that's the bottom of the leaf. This would be my stoma. This is the actual opening. This is the actual opening and plants can actually open and close their stomata. The plural of stoma is stomata. They can open and close their stomata. But the important thing to realize is what's going on inside the cell. So most plants, you have this whole photosynthetic process, or photosynthesis process, occurring in these mesophyll cells, which are really just these middle layer cells. And I'll do a detailed video in the future about the anatomy of a plant. But these are the mesophyll cells. This is where photosynthesis normally occurrs. And because they use carbon dioxide, or they need air-- actually I drew it wrong. Let me draw it a little bit better. There's actually space between them, so that air can get to them. There's mesophyll, I'm doing a very rough drawing. But in this situation air can enter through a stoma and then it can fill the space between these mesophyll cells and it can provide air to the mesophyll cells. And when I say air, that air is made up of carbon dioxide and oxygen and nitrogen and all the things that are in our air. And of course, it needs the carbon dioxide to actually perform the Calvin Cycle. Now, we just said that it's not just getting CO2. If it was just getting CO2 you wouldn't worry about photorespiration. It's also getting oxygen. It's also getting molecular oxygen. So what can the plant to do to prevent this? And not all plants-- you know, most plants just deal with photorespiration. It's just a little less efficient than the ideal. But some plants have-- I guess we could say-- evolved past the photorespiration problem. And these are called C-4 plants. Or they perform C-4 photosynthesis. And we'll understand, hopefully in a few minutes, why it's called C-4. Just as a reminder, when we go up to the mechanism up here. The classic Calvin Cycle, the first byproduct is this phosphoglycerate, this is a 3-carbon chain. So it's saying that the first time that you fix carbon dioxide, or actually the first time you fix carbon dioxide or oxygen, but let's say the first time you fix carbon dioxide you end up with a 3-carbon chain molecule. That's why this is called C-3 photosynthesis. So, that's a clue. On C-4 plants, the very first time that they fix carbon dioxide they must end up with a 4-carbon molecule. And what they do-- and this is the interesting thing-- is you have all your mesophyll cells that are out here. They're getting air. They're getting carbon dioxide. You know, they're getting carbon dioxide and they're getting oxygen and whatever else. So you have all your mesophyll cells that are getting air. But you also have cells that are deeper within, more embedded within the leaf that aren't being exposed directly to the air coming through the stomata. So you have these bundle-sheath cells. And these are actually the cells that surround the actual pipes in the plant that distribute the fluid up and down the plant. And we'll do a whole video on the anatomy of the plant. I really just want you to understand what's going on in C-4 photosynthesis. So you have these other cells that are more embedded. They don't have direct access to the air. So these are bundle-sheath cells. And what these plants do is, the carbon dioxide comes in and-- so in the standard Calvin Cycle, everything happens in the mesophyll cells and you have to deal with photorespiration. In your C-4 plant, or your plants performing C-4 photosynthesis, what happens is the carbon dioxide comes in-- so this is in the mesophyll cell. Let me be neat about it. So in our mesophyll cell, that's that right there, you have CO2 coming in and it reacts. Instead of reacting with RuBP or ribulose bisphosphate, it reacts with another very hairy sounding compound, we'll just call it PEP. But it's phosphoenolpyruvate So that's PEP. You just have to remember, it is a 3-carbon chain. Now let me write down the word, because sometimes you might want to know, what does PEP stand for? It's phosphoenolpyruvate or phosphoenolpyruvic acid Either way. 3-carbon molecule, it's got other stuff hanging off of this but we just have to remember the carbons. So when these two react what are you going to end up with? Well you could guess. You have one carbon, you have three carbons. You're going to end up with a 4-carbon molecule. And this reaction right here is facilitated, not by RuBisCo, ribulose bisphosphate carboxylase oxygenase. It's facilitated by a different enzyme. And this is the key. This is the key for C-4. This is a different enzyme. This is called PEP carboxylase. Let me write it down. PEP carboxylase. And that's a fitting name. Remember, RuBisCo or ribulose bisphosphate carboxylase, it reacted ribulose bisphosphate with carbon or oxygen. That's where that oxygenase comes from. But now we have something that reacts PEP, our phosphoenolpyruvate with carbon dioxide. So it's called PEP carboxylase. Actually this is carboxylase, not carboxylate. It's an enzyme. This is PEP carboxylase. And what's special about PEP carboxylase, and why it's useful in preventing photorespiration, is that it can only fix carbon. Not oxygen. So you can imagine, this is occurring in the mesophyll cell. You have oxygen and carbon dioxide running around here. But only carbon dioxide can react with the PEP via the PEP carboxylase. So then they react, they actually produce oxaloacetic acid or oxaloacetate And you might remember this from the Kreb Cycle. This was the thing that was the first reactive species in our Kreb Cycle. So all of these molecules, they keep showing up in our chemical pathways. And that's interesting if you find that type of thing interesting. But the important thing is, they form oxaloacetate and then oxaloacetate gets converted. Let me make it, I made this not as neatly as I would like to. But then that gets converted to either malate or aspartate but these are all 4-carbon molecules. They're a little different; they're going to have different oxygens and hydrogens hanging off of them. But this is either malate or aspartate Most books will just say, oh it will eventually just form into malate only. And then this, this malate, will then essentially react to produce carbon dioxide. And you're like, wait. That doesn't make sense. I have carbon dioxide, it gets fixed on oxaloacetic acid and then it gets turned into malate or aspartate And then later I'm going to turn into carbon dioxide again. What's the whole point? And this is the key. This is the whole crux of the issue. So now this malate is going to be converted back into PEP and carbon dioxide. You're like, what was the whole point of this whole reaction? I just ended up with carbon dioxide and PEP again. I'm just going in circles. But the neat thing about this, and the reason why this prevents photorespiration is that this part, this part of the reaction right here-- maybe I should do it like this-- this part of the reaction occurs in the mesophyll cell. It occurs up here. It occurs over here in the mesophyll cell. So you have this malate. And then the malate actually gets transferred into these bundle-sheath cells. So the malate gets transferred into the bundle-sheath cells via little tubes that connect the cells called plasmodesmata Sounds like the name of a horror movie. So let me draw this a little bit neater. So over here you have, with exposure to the air, you have your mesophyll cell. Air is coming in, O2, CO2, everything is coming in. But only CO2 can be fixed with PEP. So you have the PEP here, the phosphoenolpyruvate So you have your Pep here. Only CO2 can be reacted with PEP because of the PEP carboxylase. This is the enzyme that's operating. So this is much more specific than the ribulose bisphosphate carboxylase, or the RuBisCo. So the oxygen just gets ignored. Even though it's hanging around these mesophyll cells. And then these, this gets converted. Oxaloacetic acid. And then to malate. But once it gets converted to malate, the malate gets transferred deeper into the actual cell via these plasmodesmata So this might be butting right up against bundle-sheath cell, that's deeper in the cell. This bundle-sheath cell has no access to oxygen. So the malate comes in. You have these little pipes that connect the cell. Maybe I'll just draw one pipe. So let's say there's one pipe. So the malate can come here. And then, within this deeper cell, within this bundle-sheath cell, it can then react to form carbon dioxide and pyruvate. And then the pyruvate-- so let's just say this is the pyruvate. That pyruvate can then later go back to actually form the PEP again. So this can go back through your plasmodesmata and form the PEP. So the whole value here is, now in the bundle-sheath cell, I have an environment that only has carbon dioxide. I have no oxygen here. I was able to essentially select for the carbon dioxide outside. Or closer to the air in the mesophyll cell. And now that I'm deeper within the plant, I'm in an environment that only has CO2, because I've already selected for it. And now I can perform the Calvin Cycle. So now, inside this deeper cell, inside this bundle-sheath cell, I can fix the carbon dioxide with the ribulose bisphosphate using RuBisCo, just like we learned in the original Calvin Cycle. And we have the whole cycle and we produce our sugars. We produce our phosphoglyceraldehydees or our PGALs, whatever you want to call them. And the whole value this is, that we were able to avoid the photorespiration problem. Because now the Calvin Cycle is occurring in an environment that only has carbon dioxide. And I think I already mentioned it, but this is called C-4 photosynthesis. And it's an adaption to make sure that you don't waste cycles of your Calvin Cycle through photorespiration. And of course it was called C-4, because the first time that carbon gets fixed it doesn't happen in the Calvin Cycle. It happens up here, with PEP carboxylase. And it gets fixed with PEP into a 4-carbon chain. And that's why it's called C-4 photosynthesis.