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Current time:0:00Total duration:8:37

Video transcript

a couple of videos ago we saw that in just classic c3 photosynthesis and once again it's called c3 because the first time that carbon is fixed it's fixed into a three or the first time carbon dioxide is fixed it's fixed into a three carbon molecule but we saw the problem with c3 photosynthesis is that the enzyme that does the carbon fixation it can also react with oxygen and when oxygen essentially reacts with your rival OHS by phosphate instead of your carbon you get an unproductive reaction not only is an unproductive it'll actually suck up your ATP in your NADPH and you'll go nowhere so every now and then when an oxygen bonds here instead of a carbon dioxide you get you get nothing produce a net net everything becomes less efficient and so in in the the very in the I think it was exactly the last video we saw that some plants have evolu they've evolved a way to get around this and what they do is they fix their carbon on outsides on cells that are actually exposed to the air and then once they fix the carbon and they actually fix it into a four carbon molecule into oxaloacetate and then they put and that gets turned into malli then they pump the malate deeper within the leaf where you don't you aren't exposed to oxygen and then they take the carbon dioxide off the malate and this is where they actually perform the calvin cycle and even though you do have your abisco still there your abisco isn't going to have the photo respiration is not going to occur because it only has access to carbon dioxide it does not have access to this oxygen out here now and that's a very efficient way of producing sugars and that's why some of the plants that we associate with being very strong sugar or even you know ethanol producers all perform c4 photosynthesis a corn sugar I could write these down corn sugar cane sugar cane and and crabgrass and these are all very very efficient sugar producers because they don't have to worry too much about photorespiration now some plants have a slightly different problem they're not so worried about the efficiency of the process they're more worried about losing water and you can imagine what plants these are these are plants that are in the desert because these stomata these pores that are on the leaves they let in they let in air but they can also let out water right I mean if I'm in the rainforest I don't care about that but if I'm in the middle of the desert I don't want to let out water vapor through my stomata so the ideal situation is I would want my stomata closed during the daytime right this is what I want so I want if I'm in the desert let me make this clear if I'm in the desert I want stomata closed I want stomata closed during the day right for obvious reasons I don't want all my water to vaporize out of these holes in my leaf but at the same time but the problem is that photosynthesis can only occur during the daytime and that includes the dark reactions remember I've said multiple times the dark reactions are badly named they're more like the light independent reactions but they both occur simultaneously the light independent and light dependent and only during the daytime so photosynthesis photosynthesis synthesis only at day and if your stomata are is closed you need I mean you know to perform photosynthesis especially the Calvin cycle you need co2 so how can you get around this if I want to close my stomata during the day but I need co2 during the day how can I solve this problem and what what desert plants or many desert plants have evolved to do essentially does photosynthesis but instead of fixing instead of fixing the carbon in outer cells and then pushing it into inner cells and then performing the Calvin cycle they do it instead of outer and inner cells they do it at the night time and in the daytime so in camp plants and these are called camp plants because I could tell you what it stands for it stands for Kraft soul a.cian acid metabolism and that's because it was first observed and that in that species of plants the crassulacean plant but these are just called you know you could call it cam photosynthesis or cam plants they're essentially a subset of c4 plants but instead of performing c4 photosynthesis kind of an outside cells and inside cells they do it at the night time in the day so they're subset of c-4 plants or c4 photosynthetic plants and what they do is at night maybe at night they keep their stomata open they keep their stomata open and they perform they keep their stomata open and they're able to fix so and this everything occurs in the mesophyll cells and the cam cells and the cam plants so in the date at the night time when they're not afraid of losing water so let's say this is a mesophyll cell right here my stomata is open let's say that this is my stomata right there and so it lets in carbon dioxide I'm not worried about losing water vapor it's night time right now so carbon dioxide comes in here and then it fixes the carbon dioxide it fixes it the exact same way that the c4 plants do so you have your co2 come in CEO 2 you have your pep you have your pep it's all facilitated by pep carboxylase that's the enzyme that can only fix co2 that can only react with co2 not with oxygen and then that is used to produce and we saw it here in our cam 4 and our cam 4 diagram in the last video that is to use to produce malate this for carbon of four carbon molecule so that is used to produce malate and then the malate and then this is what's key the malate gets stored in other organelles in the cell and vacuoles which are you can kind of view them as large storage containers in the cell so I drew this as the whole cell I mean this is actually all occuring you know it in in your in your chloroplast but you could imagine your cell having a big storage set center where the malate gets stored at night and you can view malate is almost you know it's like a carbon dioxide store because later on we can access the malate and get the carbon dioxide and that's exactly what these cam plants are going to do so this is nighttime then the Sun comes up so now we're in the daytime this desert plant well maybe it's a cactus it doesn't want to lose its water vapor so it's closes its stomata so it's closed it's the stoma this particular Stormo now is closed it's now closed and you say oh boy how's it going to perform photosynthesis well it can perform photosynthesis in that very same cell in that very same cell because it stored up all of this malate at night and so now the malate can be pumped out of the vacuoles into the stroma of our chloroplast and then this that you can have the pyruvate break off but the more important thing is you have co2 break off so you have a ready supply of co2 and now we can from warm our standard calvin cycle in an environment only with co2 our stomata our stoma is closed so we're ready to go our co2 reacts with ribulose biphosphate and then catalyzed by Rubisco it's the whole calvin cycle and we produce our sugar so this is this is kind of a neat adaptation in these hi very efficient sugar producing plants that aren't that worried about water they do they perform carbon fixation on the on things that are exposed to the air and then they they pump kind of a stored version of the carbon deeper into the leaf to actually perform the calvin cycle so that you it's not la C so that photorespiration doesn't occur because it down here you have no oxygen the desert plants I mean they benefit from that property as well but their whole concern is I don't want to keep my store model open in the daytime so what I do is I fix my carbon at night but I use the exact same process but the process I use pep carboxylase and I store it I store my carbon dioxide at night and then the daytime in the daytime I can actually when you know my light reactions my light dependent reactions are occurring and they're producing my ATP in my NADH I can also perform my dark reactions and the daytime as I said the dark reactions always occur in the daytime or my light independent reactions because even though my stomata is closed I have a store of carbon dioxide in the form of malate
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