Photorespiration: C3, C4, and CAM plants
Current time:0:00Total duration:11:05
- [Voiceover] We have other videos that go into some depth on the Calvin cycle, and we'll refer to that in this video as the normal Calvin cycle, and the focus of this video is really a quirk that diverts us from the normal Calvin cycle, and it's a quirk due to this enzyme right here whose shorthand name is rubisco. So to get an appreciation for that quirk, let's first do a very quick overview of a normal Calvin cycle. So we can start at any point, but I'll start at the point that is typically started at, and we can start with this five carbon molecule. And we're visualizing just the carbons here for simplicity. So each of these grey circles represent a carbon. There's other atoms a part of this molecule, but we're not drawing them, and that's because the carbon accounting is what is interesting in-- Well, not only the Calvin cycle, but also this variation, this diversion that we're going to see, that we're gonna call photo-respiration. So, right over here, I've set it up so that I have six molecules of this. We call this ribulose one, five, bisphosphate, but because it's a mouthful, the shorthand notation is R-U-B-P. Sometimes people might say Roo-B-P, or I guess you could even say Rube-P somehow, but each of these six Rube-P, or RuBPs, can then react with a carbon dioxide. So if I have six RuBPs, well, they're gonna react with six carbon dioxides, and so one way to think about it is, it's fixing the carbon in that carbon dioxide. It's taking this carbon that's part of this gaseous carbon dioxide, and fixing it as part of an organic molecule. Now, you might be tempted to say, well, it's gonna create six carbon molecules, but then those will immediately become 12 three carbon molecules. And notice, and it's important to keep doing this. Pause the video if you need to. You can make sure that the carbons are all accounted for. Right over here, how many carbons do we have? Well, we have six times five, so that's 30 carbons right over here, and here we have six times one carbon, so that's six carbons right over here. So if we wanna account for all of our carbons, we should have 36 carbons right over here, and we do. We have 12 three carbon molecules. This three carbon molecules, when we go into some detail here in the video on the Calvin cycle, it's called three phosphoglycerate, but that's not what the focus is on this video. The focus of this video is the enzyme that actually does the fixing of the carbon along with the RuBP. And that enzyme, that character, the character with the quirks that we're going to talk about, the shorthand, its name, you could call it ribulose one, five, bisphosphate oxygenase-carboxylase, but that's even more of a mouthful than RuBP, so people call it the nice friendly name rubisco, rubisco for short. But you can learn a lot about what rubisco does from its name right over here, and you can even learn a little bit about its quirk that we're about to talk about. So it obviously involves ribulose one, five bisphosphate, and it does indeed involve that, and then you see oxygenase, dash, carboxylase. Well, the carboxylase is what tells us that it can deal with the carbon dioxide right over here. The carbon dioxide can be one of the substrates in a reaction with the ribulose one, five, bisphosphate. And so that's exactly what it's doing in this reaction. In a normal Calvin cycle, it's acting as a carboxylase. It is fixing that carbon. It's making it part of, if you view, you know, if you view that carbon-- Actually, I won't do it that way because here we have 12 as many. But it's taking these carbon molecules, and it's fixing them into organic molecules, some of which can eventually be used to create glucose. And that's what happens in a typical Calvin cycle. We use up some NadPhs. We use up some ATPs, and we go down. Through this cycle, eventually, we create some G3Ps, which are also three carbon molecules. G3P is short for glyceraldehyde three phosphate, for those of you who are interested, and then, if we use this accounting of those 12, 10 go back through the Calvin cycle to regenerate our ribulose one, five, bisphosphate, and two of them exit the Calvin cycle, and then can be used to produce one six carbon glucose. And so that's what happens when everything is fine and dandy. That's what the Calvin cycle's purpose is, is to be able to have a store of energy in the form of a glucose. Now, you might have already gotten a little bit of the foreshadowing from rubisco's name. Well, maybe it sometimes acts as an oxygenase. So instead of fixing carbon, maybe sometimes it fixes oxygen, and that is indeed the quirk that I'm talking about of rubisco. So in photo-respiration, instead of fixing carbon, it fixes oxygen along with the ribulose one, five, bisphosphate. And you might say, "Why does it do that?" And the answer is, well, that's a really good question. Some folks think, well, that's just one of these inefficiencies of a biological process. It really shouldn't do it. It's in some ways detrimental to the plant. It could be a side effect, a legacy feature, or side effect from ancient evolution when there was very little oxygen in the atmosphere, and so this didn't seem like that bad of an inefficiency. But it does happen, and in particular, the times where photo-respiration is more likely to happen with typical plants, often referred to as C three plants, and C three is referred to because the first product when you fix the carbon is a three carbon molecule. But this typically happens, or this happens with typical plants in hotter than normal weather. So let me write this down, and I'll write it in a hot color. Hot, hot conditions. That's where it typically happens with typical plants, and why hot conditions? Well, in hot conditions, first of all, rubiso has more affinity. Rubisco's affinity, affinity to O2 increases. So under normal conditions, it tends to have more affinity for carbon dioxide, but under hot conditions, these proteins are-- No protein is perfect. It can morph a little bit, so it has more affinity to molecular oxygen, and also, under hot conditions, plants are worried about conserving water, and so they will close their stomata, stomata, stomata closed, to preserve water, but when the stomata are closed, you have CO2 can't diffuse in, can't diffuse in, and O2 can't diffuse out, can't diffuse out. So your ratio of O2 to CO2 increases. So O2 to CO2 ratio, ratio increases. So under hot weather, the rubisco just wants to work with the oxygen more. It typically wants to work with the carbon dioxide, and also, because the stomata's closed, and you don't have as easy diffusion, well, this ratio is going to increase. And so, things are just more likely to react with the oxygen, especially the rubisco's more likely to bump into it in the right way than it is with the carbon dioxide. But let's think a little bit about why this is inefficient. Well, in this case, it's fixing the oxygen, and so it's not gaining those carbons like we just saw in the typical or the normal Calvin cycle, and so here, and you can account-- I encourage you to keep pausing the video and account for the carbons, but here, you can no longer produce your 12 three carbon molecules, 'cause you're not getting these six carbons over here, so instead, you can only produce six of those three carbon molecules, and then another six of a two carbon molecule called phosphoglycolate, and once again, I'm not showing all of the oxygens and I'm not showing all the phosphates. I'm just accounting for the carbons. And so, this seems like a pretty bad loss. You're not able to use those carbons right over there. Well, evolution has given us pathways to at least start to salvage some of it. But it's a pretty intense pathway to get back some of those carbons, and the reason why it wants to get back some of those carbons is because, remember, at the end of the day, you want to attempt to produce some glucose, and you wanna have the typical or the normal Calvin cycle continue to happen, but just so you get a sense of what the salvage pathway looks like, this two carbon molecule right over here, it has to be converted to glycolate, then that has to go to the peroxisomes where it becomes glycine to the mitochondrion, and you see this whole process here just to be able to salvage it into a few more of the three carbon molecules, and that's essentially what happens right over here. We get three more of the three carbon molecules, but we do lose, for the way I've accounted of it, three molecules of carbon dioxide, and this one way that why it's called, or one reasoning for why it's called photo-respiration. Respiration, we use oxygen, and we produce carbon dioxide, and that's exactly what's happening. We're using oxygen, and we're producing carbon dioxide. And so, as you see, the mechanism, it kind of makes the Calvin cycle-- It disrupts it, or makes it less efficient. And so, once again, why does this happen? Well, it could just be a biological quirk that has not been selected strongly enough against, or some people believe it actually has some not so well understood mechanism, and it somehow helps the plant in some way, but it's a really interesting thing that goes on, and, you know, rubisco is not just some very fringe molecule. As you see, it's central to the Calvin cycle, and if you look at plant matter, in particular plant leaves, it'll represent roughly 20% of the protein mass in those plant leaves. So it's a very common protein slash enzyme, but it's got this quirk, this quirk that takes the plant down the path of photo-respiration.
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