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Current time:0:00Total duration:11:05

Video transcript

we have other videos that go into some depth on the Calvin cycle and will refer to that in this video is 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 over here whose shorthand name is Rubisco so to get a 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 to head and we can start with this 5 carbon this 5 carbon molecule and we're visualizing just the carbons here for simplicity so each of these gray 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 and well not only the Calvin cycle but also this variation this this diversion that we're going to see that we're going to call photo respiration so right over here I've set it up so that I have 6 molecules of this we call this ribulose 1 5 Biss phosphate because but because it's a mouthful the shorthand notation is rubp sometimes people might say Ruby P or I guess you could even say rube P of some somehow but each of these six rube P or rubp s can then react with a carbon dioxide so for if I have six if I have six rubp s well they're going to 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 the this gaseous carbon dioxide and fixing it as part of an organic molecule now you might be tempted to say well is going to create six 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 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 want to account for all of our carbons we should have 36 carbons over here and we do we have 12 3 carbon molecules this three carbon molecules when we go into into some into some detail here in the video on the Calvin cycle it's called 3 phosphoglycerate but that's not what the focus is on this video the focus of this video is the enzyme that actually does the 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 it's name you could call it ribulose 1/5 this phosphate 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 could learn a lot about 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 1/5 this phosphate and it does indeed involve that and then you see oxygenase - carboxylase well the carboxylase is what tells us that it will it can deal with the carbon dioxide right over here that carbon dioxide can be one of the substrates in a reaction with the robbie buhl owes 1/5 this phosphate and so that's exactly what it's doing in this reaction in a normal calvin cycle it's acting as a carboxylate 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 NADPH --is we use up some ATP's and we go down we through this cycle eventually we create some g3ps which are also three carbon molecules g3p is short for glyceraldehyde 3-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 1/5 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 cycles purpose is is to be able to have a store of energy in the form of glucose now you might have already got gotten a little bit of the foreshadowing from Rubisco x' name well maybe it sometimes acts as an oxygen ace 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 photorespiration instead of fixing carbon it fixes oxygen along with the ribulose 1/5 this phosphate 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 a it's in some ways detrimental to the plant it's it could be a side effect a legacy feature or side effect from ancient 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 photorespiration is more likely to happen with typical plants often referred to as c3 plants and c3 it's 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 I'll write it in a hot color hot hot conditions that's where it typically happens with typical with typical plants and why hot conditions well in hot conditions first of all Rubisco is just more has more affinity Rubisco x' Rubisco x' affinity affinity 202 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 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 Oh - can't diffuse out can't diffuse diffuse out so your ratio of o2 - co2 increases so Oh - 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 is 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 is 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 this is inefficient well in this case it's fixing the it's 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 you can account Paul I encourage you to keep pausing the video and account for the carbons but here and you can't you can no longer produce you can no longer produce your 12 3 carbon molecules because you're not getting these 6 carbons over here so instead you can only produce six of those 3 carbon molecules and then another six of the two a two carbon molecule called phospho glycolate and once again I'm not showing all 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 car right over there well evolution has given us pathways to at least start to salvage some of it but it's a pretty 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 you want to attempt to produce some glucose and you want to 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 that phosphorus to carbon 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 mitochondria 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 four the way I have accounted of it three molecules of carbon dioxide and this is 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 this is the mechanism it kind of makes the calvin cycle it disrupts it or it 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 twenty percent of the protein mass in those plant leaves so it's a very common it's a very common a protein slash enzyme but it's got this quirk this quirk that takes the plant down the path of photorespiration