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Current time:0:00Total duration:13:28

Photosynthesis: Calvin cycle

Video transcript

I think we're now ready to learn a little bit about the dark reactions but just to remember where we are in this whole scheme of photosynthesis photons came in and excited electrons and chlorophyll in the light reactions and as those photons went to lower and lower energy states we saw it over here in the last video as they went to lower and lower energy states and all of this was going on in the thylakoid membrane right over here you could imagine let me do it in a different color you can imagine it occurring right here is they went into lower and lower energy states two things happen one the release of energy was able to pump the hydrogen's across this membrane and then when you had a high concentration of hydrogen's here those went back through the ATP synthase and drove that motor to produce ATP and then the final electron acceptor or a hydrogen acceptor depending on how you want to view it the whole hydrogen atom was nad plus so the two byproducts or the two byproducts that we're going to continue using in photosynthesis from our light cycle from our light reactions I guess I shouldn't call it the light cycle were I wrote it up here ATP and NADPH and then the byproduct was is that we needed an electron to replace that first excited electron so we take it away from water and so we also produce oxygen which is a very valuable byproduct of this reaction but now that we have this ATP and this NADPH we're ready to proceed into the dark reactions and I want to highlight again even though it's called the dark reactions it doesn't mean that it happens at night it actually happens at the same time as the light reactions it occurs while the Sun is out the reason why they call it the dark reactions is that they're light independent they don't require photons they only require ATP NADPH and carbon dioxide so let's understand what's going on here a little bit better so let me down to where I have some clean space down here so we had our light reactions light reactions and they produced I just reviewed this produced some ATP and produce some na dhih pH and now we're going to take some carbon dioxide from the atmosphere carbon dioxide from the atmosphere and all of this will go into the alcohol the light independent reactions because dark reactions is misleading so the light independent independent reactions the actual the actual mechanism is called the Calvin cycle and then we're going to that's what this video is really about goes into the Calvin cycle and out pops and out will pop whether you want to call it P gal we talked about in the first video or g3p this is glyceraldehyde 3-phosphate this is phospho glyceraldehyde they are the exact same molecule just different names and you can imagine it as a three carbon chain with a phosphate group with a phosphate group and then this can then be used to build other carbohydrates you put two of these together you can get a glucose you might remember in the first stage of glycolysis or the first time that we cut a glucose molecule we ended up with two phosphoglyceraldehyde molecules glucose has six carbons this has three let's study the calvin cycle in a little bit in just a little bit more detail so let's say exiting the light reactions let me write this well I'll keep that there so exiting the light reactions let's say we have well let's start off with six carbon dioxide so this is independent of the car with the light reactions and I'll show you why I'm using these numbers I don't have to use these exact numbers so let's say I start off with six co2 s and I could write a co2 because we really care about what's happening with the carbon we can just write it as a you know a single carbon has two oxygens on it which I could drop but I'm not going to draw them right now because I want to really show you what happens to the carbons maybe I should draw this in this yellow just to show only the carbons I'm not showing you the oxygens on here and happens is the co2 the co2 the six co2 s essentially react with and I'll talk a little bit about this reaction in a second they react with six molecules six molecules and this is going to look a little bit strange to you of this molecule you could call it our you be P that's short for ribulose biphosphate sometimes called ribulose one five by phosphate and the reason why it's called that is because it's a five carbon it's a five carbon molecule so three four five and it has a phosphate on the one and five carbons so it's ribulose by phosphate or sometimes rob you Lowe's one let me write this one that's the first carbon 5 by phosphate we have two phosphates so that's ribulose one five by phosphate fancy name but it's just a five carbon chain with two phosphates on it these two react together and this is a this is a simplification these two react together there's a lot more going on here but I want you to get the big picture to form to form 12 molecules 12 molecules of P gal of phosphoglyceraldehyde or glyceraldehyde-3-phosphate of P of P gal which you can view as a it has three carbons and then it has a phosphate group it has a phosphate group and just to make sure we're counting for our carbons properly let's think about what happens we have 12 of these guys so we can think of it that we have 12 times 3 we have 36 carbons now did we start with 36 carbons well we have 6 times 5 carbons that's 30 plus another 6 here so yes we have 36 carbons they react with each other to form this P gal now this the this the bonds or the electrons in this molecule are in a higher energy state than the bot then the electrons in this molecule so we have to add energy in order for this reaction to happen this won't happen spontaneously and the energy for this reaction the enter if we use the numbers 6 & 6 here the energy for this reaction is going to come from 12 ATP's 12 ATP so you could imagine two ATP's for every carbon in every ribulose biphosphate and 12 nadh --is + 12 na dphh I should say I don't wanna get you confused with it's very similar to NADH but I want to get you confused with what goes on in respiration and then these leave as 12a DPS a DPS plus 12 phosphate groups and then you're going to have plus 12 na dhih nadp+ --is and the reason why this is a source of energy is because the electrons in NADPH or you can say the hydrogen with the electron in NADPH is at a higher energy state so as it goes to a lower energy state it helps drive a reaction and of course ATP's when they lose their phosphate groups those electrons in a very high energy state they enter a lower energy State help drive a reaction help put energy into reaction so then we have these 12 P gals now the reason why it's called a Calvin cycle as you can imagine we studied the krebs cycle cycles we start reusing things the reason why it's called the calvin cycle is because we do reuse actually most of these pee gals so of the 12 P gals we're going to use ten of them we're going to use ten of them to let me actually let me do it this way so we're going to have 10 P gals 10 phosphoglyceraldehyde x' 10 P gals we're going to use to recreate the ribulose biphosphate and the counting works because we have 10 3 carbon molecules that's 30 carbons then we have 6 5 carbon molecules 30 carbons but this once again is going to take energy this is going to take the energy from 6 ATP's so you're going to have 6 atps essentially losing their phosphate group the electrons enter lower energy states drive reaction you're going to have six a DPS plus six phosphate groups that get released and so you see it is a cycle but no the question is well you know Angie I used all of these what do I get out of it well I only use 10 out of the 12 so I have to pee gals left to pee gals left and these can then be used and the reason why I used six and six is so that I get twelve here and I get two here and the reason why I have two years because to pee gals can be used to make a glucose to make a glucose which is a six six carbon molecule it's small its formula we've seen it for c6h12o6 but it's important to memorize to remember that doesn't have to just be glucose it could then go off and generate longer change carbohydrates and and and start to anything that has a carbon backbone so this is it this is the dark reaction we were able to take the byproducts the byproduct of the light reactions with the ATP and the NADH is there's some more ATP there and use it to fix carbon this is called carbon fixation when you take carbon in a gaseous form and you put it into a solid structure that is called carbon fixation so through this Calvin cycle we were able to fix carbon and the energy comes from these molecules generated from the light reaction and of course it's called a cycle because we generate these pee gals some of them can be used to actually produce glucose or other carbohydrates while most of them continue on to be recycled into ribulose biphosphate which once again reacts with carbon dioxide and then you get the cycle happening over and over again now we said it doesn't happen in a in a vacuum actually if you want to know the actual location where this is occurring this is all occurring in the stroma in the fluid inside the chloroplast but outside of your thylakoid so in your stroma this is where your dot light independent reactions are actually occurring and it's not just happening it's not just happening with the ATP and NADPH there's actually a fairly decent-sized enzyme or protein that's facilitating it that's allowing the carbon dioxide to bond it's ER points and the ribulose biphosphate and then the ATP to react at certain points to essentially drive these two guys to react together and that enzyme that enzyme sometimes it's called Rubisco I'll tell you why it's called Rubisco so this is Rubisco so rube I mean I get the capitalisation right ribulose by phosphate ooh this co carboxylase and this is what it looks like so it's a pretty big protein enzyme molecule but you can imagine that you have your ribulose biphosphate bonding at one point you have your carbon dioxide bonding at another point I don't know what points they are ATP a bonds at another point it reacts that that makes this thing twist and turn in certain ways to make the ribulose biphosphate react with the carbon dioxide NADPH might be reacting at other parts and that's what facilitates this entire calvin cycle and you might you know we we know we I told you over here that this rubp this is ribulose 1/5 by phosphate this Rubisco this is short for ribulose 1/5 by phosphate carboxylase i won't write it all out you could look it up but it's just telling you look i'm i'm it's it's an enzyme that's used to react carbon and ribulose 1/5 by phosphate but now we're done we're done with photosynthesis we were able to start off with photons and water to produce ATP and nadph because we had that those excited electrons we had the whole chemiosmosis to drive the that allowed the ATP synthase to produce ATP NADPH was the final electron acceptor these are then used as the fuel in the calvin cycle in the dark reaction which is badly named it should be called the light independent reaction because it actually does happen in the light you take your fuel from the light reactions with some carbon dioxide and you can fix it using your I like to call it the Rubisco enzyme in the calvin cycle and you end up with your phospho glyceraldehyde which could also be called your glyceraldehyde 3-phosphate which can then be used to generate glucose which we all use to eat and and fuel our bodies or any we learn in cellular respiration that can then be converted into ATP when we need it
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