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Current time:0:00Total duration:16:58
At multiple times during the video, Sal says ribulose biphosphoglycerate but meant ribulose biSphosphoglycerate.

Video transcript

in the last video we discovered what seems like a problem with the Calvin cycle that you have this big protein here or enzyme that facilitates the calvin cycle all of the molecules that are involved bond to this and then it you know 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 biphosphate carboxylase oxygenase and we know when the Calvin cycle operates properly you'll have some carbon dioxide you'll have some carbon dioxide attached on one part of this enzyme and then you'll have some rube P or maybe you could call it or the proper word is ribulose one 5bi phosphate and they're going to react and then after they react if the 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 in two for one molecule of that in one molecule of that you're going to have two molecules of 3-phosphoglycerate phospho blister rate now 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 phospho glyceraldehyde z' these turn into two phosphoglyceraldehyde zorp ii gals pea gals and then for every six that are of these that are producing maybe I should write a three here a three here and then I'll have six of these 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 P gals or glyceraldehyde-3-phosphate go back into the cycle to per to produce the ribulose biphosphate and one of them is kind of our end product of photosynthesis that can be used to produce other carbohydrates so one P gal and the whole problem we saw with the calvin cycle is that Rubisco it does not only fix carbon dioxide that instead of carbon dioxide we can might have an oxygen molecule we might have an oxygen molecule that jumps in here and it can also - the Rubisco enzyme and in that situation the oxygen and the ribulose biphosphate react so if we said we had three ribulose biphosphate sand three oxygens instead of producing six phosphoglycerates we're only going to produce five five phosphoglycerates and we're going to produce five phospho glycolate sand you know which is that phospho gly collate which is that byproduct it gets processed later on so it's this huge and then these five you can have five here you can't have one left over and then you're not going to produce anything and doing this whole cycle you have to use up a bunch of ATP's and nadh s so this is a problem if there's a lot of oxygen present or even if there's just even a little bit of oxygen present it's going to make this a little bit less a little bit less efficient because every now and then an oxygen is going to jump in when you when 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 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 doing you're like wait how do I do that or do they have to go to a planet where you know there's no oxygen no what they do is and - to understand this we'll have to understand a little bit of of the actual makeup 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 you know let's say that that is a leaf that is a leaf right there can make it look nice like a leaf that's a nice looking leaf on your leaf surface and actually on both sides of it you have these little pores these little holes these little holes on the leaf surface they're 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 would have to get a microscope to actually see them they're called stomata or one individual of these pores or holes are called is called a stoma and this is where the oxygen and the carbon or 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 cells draw cross section of a leaf so if we draw a cross section 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 stoma and we'll talk a little or their stomata I shouldn't say stoma is 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 occurs mezzo fill and because they use carbon dioxide or they need air there's actually actually at Druid one let me draw a little bit better there's actually space between them so that air can get to them so there's mezzo fill in as well phil i'm doing a very rough drawing but in this situation air can enter through a stoma and then it can fill these the 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 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 you know 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 do to prevent this and not all plant 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 photo respiration problem and these are called c4 plants or another or C or they perform c4 photosynthesis photosynthesis and we'll understand hopefully in a few minutes why it's called c4 and just as a reminder when we go up to the mechanism up here classic Calvin cycle the FIR is to buy product is this phosphoglycerate this is a three carbon chain so it's saying that the 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 three carbon chain molecule that's why this is called c3 photosynthesis so that's a clue on c4 plants the very first time that they fix carbon dioxide they must end up with a four carbon molecule and what they do and this is the interesting thing is you have all your mesophyll 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 they're 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 sheet cells and these are actually the cells that surround you know the actual the actual pipes in the plant that 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 c4 photosynthesis so you have these other cells that are more embedded there they don't have direct access to the air so these are bundle sheath cells bundle sheath and what these plants do is the carbon dioxide comes in - so in the standard calvin cycle everything happens in the mesophyll cells and you have to deal with photorespiration in your c4 plants or your plants performing c4 photosynthesis what happens is is the carbon dioxide comes in so this is in the mesophyll cell let me be neat about it so in our mesophyll and our mesophyll cell mesophyll that's that right there you have co2 coming in and it reacts instead of reacting with Rupiah ribulose biphosphate it reacts with another very hairy sounding compound we'll just call it pep but it's phosphine all pyruvate so--that's pep and just you just have to remember it is a three carbon it is a three carbon chain now let me write down the word just because sometimes you might want to know what does what does pep stand for its Foss phenol phenol pyruvate or phosphine all pyruvic acid either way pyruvate three 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're going to end up you have one carbon you have three carbons you're going to end up with a four carbon molecule you're going to end up with a four carbon molecule and this this reaction right here is facilitated not by Rubisco ribulose biphosphate oxygen carboxylase oxygenase all it's facilitated by a different enzyme and this is the key this is the key for C this so this is a different enzyme this is called pep carboxylase let me write it down pep-pep carboxylase and that's a fitting name remember right Rubisco or ribulose biphosphate carboxylase it reacted ribulose biphosphate with carbon now or oxygen that's where that oxygen is comes from but now we have something that reacts tap our phosphine all pyruvate with carbon dioxide so it's called pep carboxylase actually this is carboxylase not carboxylate carboxylase 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 it can only fix carbon not not oxygen so you can imagine this is occurring in the mesophyll cell you have oxygen and carbon dioxide running around here but only only carbon dioxide can react with the pep via the pep carboxylase so then you they react they perfect they actually produce a solo acetic acid or excel acid eight Solow let me write that oxaloacetate and you might remember this from the krebs cycle this was what this was the thing that was the first reactive species in our Krebs cycle so all of these molecules you know they're there are they keep showing up in our in our chemical pathways and that's interesting if you find that type of thing interesting but the important thing is they felt form oxaloacetate then oxaloacetate gets converted 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 four carbon molecules they're a little different they're going to have different oxygens and hydrogen's hanging off of them but this is either malate or or aspartate did I just say carboxylase no there's going to form either malate or aspartate most books will just say Oh a little it'll eventually just form into malate only and then this this malate will then essentially react to produce carbon dioxide you're like wait with that that that doesn't make sense I have carbon dioxide it gets fixed in 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 this is 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 bundle sheath cells via little tubes that connect the cells called plasmodesmata sounds like the name of our horror movie so let me draw this let me draw this a little bit neater so over here you have with exposure to the air with exposure there you have your mesophyll cell right air is coming in O to co2 everything is coming in but only co2 can be fixed with pep so you have the pep here the phosphine old pyruvate so you have your pep here pep only co2 can be reacted with pep because of the pep carboxylase the pep carboxylase this is the enzyme that's operating so this is much more specific than the ribulose biphosphate carboxylase or the Rubisco so pep carboxylase so the oxygen just gets ignored even though it's hanging around these metal fuel 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 deeper into the actual cell via these plasmodesmata so this might be budding right up against right up against a bundle sheet cell that's deeper in the cell this bundle sheet cell has no access to oxygen right so the malate comes in you have these little pipes that connect the cell maybe I'll just draw one pipe actually 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 bundle sheath it can then react to form carbon dioxide and pyruvate and then the pyruvate so let's say then this is the pyruvate and with 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 sheet 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 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 sheet cell I can fix the carbon dioxide with the ribulose biphosphate 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 phosphoglyceraldehyde x' or you know pee gals whatever you want to call them and the whole value of this is that we were able to avoid the photorespiration problem because now the Calvin cycle is occurring an environment that only has carbon dioxide and I think I already mentioned it but this is called c4 photosynthesis c4 photosynthesis and it's an adaption to essentially make sure that you don't waste waste cycles of your calvin cycle by photo through photorespiration of course it was called c4 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 c4 photosynthesis
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