hi guys in this video we'll be looking at an overview of the light independent stage the fixation of carbon dioxide formation of organic molecules in the calvin cycle the regeneration of rubp and then we'll finish with a summary the light independent stage is the second stage of photosynthesis and occurs in a different region to the light dependence stage this one happens in the stroma of the chloroplasts so remember we've got the grana with the thylakoid's membranes which is where the first stage happened the light dependent stage and then in the stroma which is the fluid around the membranes is where the second stage happens the light independent stage in the light independent stage the products of the light dependence stage and carbon dioxide are both used to synthesize organic molecules so we've had the light dependent stage produce particular molecules in its reactions and these molecules are atp and reduced nadp these can then quickly pass into the stroma where the second stage happens the light independent stage and they combine with carbon dioxide that the plant has taken up from the atmosphere and eventually through a series of reactions they produce the final organic product which is very useful which is normally glucose the main metabolic pathway of the light independent stage has a name and it's called the calvin cycle so we'll go into more detail about these steps but this overall cycle is called the calvin cycle and this is in the light independent stage so it's a cycle of metabolic reactions although the calvin cycle itself does not require any sunlight energy it does depend on the products from the light dependent stage so sunlight is still important for photosynthesis so in the light dependence stage certain products were made atp and reduced nadp because of the presence of light and then these feed into the calvin cycle which is the light independent stage so even though the calvin cycle itself doesn't need light it relies on products which were made using light so if there is no sunlight then the products of the light dependence stage do not form unless the calvin cycle cannot take place so usually what would happen is the sunlight through the light dependence stage would form atp and reduced nadp and these would then feed into the calvin cycle but of course if we take away the sunlight the light dependent stage doesn't happen no atp is made no nadp is made and so the whole calvin cycle stops working the pumping of the protons into the thylakoid space in the light dependent stage increases the ph of the stroma to about eight so remember in the light independent stage there is a pumping of hydrogen ions into the thylakoid space the ph depends on the level of hydrogen if there's a high hydrogen concentration in a solution the ph is very low if there's a low concentration of hydrogen ions the ph goes higher so because they're being pumped out of the stroma the ph starts rising to about eight which is slightly alkaline a ph of 8 is actually the optimum ph for the enzymes involved in the calvin cycle so because of this proton pumping the enzymes had to evolve to optimize their conditions to be around ph 8 which is just above neutral and this is ideal for the reactions of the calvin cycle to happen so let's go through the steps of the calvin cycle one by one first of all we have the fixation of carbon dioxide carbon dioxide from the atmosphere diffuses into the leaf tissues via the stomata so remember fixation is when we take co2 and we add it in to make organic molecules the co2 comes from the environment or the atmosphere and it enters the leaf tissue by diffusion through stomata this carbon dioxide diffuses across the plant cell wall and the chloroplast envelope and then into the stroma so just imagine the barriers that the carbon dioxide has to go through it's in the leaf tissue so first of all it needs to get into the plant cells so it passes through the cell wall and then of course it needs to get to the stroma so it needs to pass through the envelope that's the outer and inner membranes of the chloroplast and therefore it will then be in the stroma which surrounds the thylakoid membranes the first step of the calvin cycle involves carbon dioxide combining with a five-carbon compound known as ribulose bisphosphate so here we have co2 that's just arrived into the chloroplast and it combines with a five carbon compound which is ribulose bisphosphate and you see this written as rubp you can see it's got one two three four five carbons the addition of co2 to ribose bisphosphate is catalyzed by the enzyme rubisco ribulose bisphosphate carboxylase oxygenase so this can be shortened to rubisco so here's the structure of the enzyme and you'll often see it written as this form so this reaction causes the ribulose bisphosphate to be carboxylated and this forms an unstable intermediate compound that has six carbons so remember the ribulose bisphosphate has five carbons and carbon dioxide as a molecule has one carbon so the addition of these two together is obviously going to make a compound with six carbons but because the structure of the molecule has changed it's quite unstable and it doesn't really last long so the sixth bond has made the molecule very energetic and it wants to change its form so what happens is the unstable six carbon compound immediately breaks down and it splits in half to form two molecules of glycerate three phosphate often written as gp so here's our unstable intermediate and it has one two three four five six carbons it's very unstable so it splits in half and therefore makes two compounds each with three carbons because six divided by two is three and each of these are called glycerate three phosphate or gp so we have two times this molecule this whole process is referred to as the fixation of carbon dioxide so remember fixation means taking it from the atmosphere and fixing it into an organic compound so co2 plus a five carbon compound which is ribulose bisphosphate this is catalyzed by the rubisco enzyme making our unstable six carbon intermediate which then splits into two molecules of gp so this whole process is the fixation of co2 and that's the first part of the calvin cycle so now we'll move on to the next stage of the calvin cycle which is the formation of organic molecules after the carbon dioxide has been fixed the gp is reduced to form the organic molecule known as trios phosphate or tp so here's our gp that we made before and remember there are actually two of them and it gets reduced to form a new molecule having had a hydrogen atom added to it and this new molecule is tp trios phosphate we still have three carbons at this point the gp is reduced to tp by borrowing and using the hydrogen atoms from the reduced nadp that was formed in the light dependent reaction so remember one of the products was nadp which was reduced and by donating its hydrogen it goes back to being nadp which is not reduced but the hydrogen joins the gp and in this process we form tp the atp that was made in the light dependent reaction is also used at this stage to make two molecules of adp so whilst gp is being converted to the reduced trials phosphate molecule with its added hydrogen the atp that we made in the light dependent stage is being converted to two molecules of adp and remember because this is happening twice i we made two gp molecules for each carbon dioxide for each molecule of carbon dioxide we make two molecules of adp being produced so we have one carbon dioxide which eventually will make two molecules of gp and because we have two gp turning to tp we have two triphosphate molecules and in the process of doing this atp becomes adp but obviously this happens twice so we form two molecules of adp from the calvin cycle two molecules of triple phosphate are needed to synthesize the hexose sugar glucose so remember the overall purpose here is to form glucose which is the organic molecule because glucose has six carbons and triphosphate only has three we need two of them which then come together to form a six carbon glucose and then the plant can do with the glucose what it wants the glucose can be converted to sucrose which is a disaccharide starch or cellulose for use in the plant cell so the glucose can be made into the disaccharide known as sucrose which can travel up and down the plant as a sugar form it can also make starch which is a good storage of energy for the plant and it can also be used to make cellulose which is incorporated into the cell wall of the plant cells there are other uses as well the tp can also be used to synthesize amino acids fatty acids and glycerol so the triphosphate doesn't always need to go and make glucose if it's not needed it can go and make glycerol or fatty acids both of which can contribute to triglycerides or it can be used to make amino acids and therefore from that proteins we also need to talk about how ribulose birth phosphate is regenerated the chloroplasts themselves only contain low levels of ribulose bisphosphate the reason is because it is continuously being converted to gp so in the calvin cycle the ribulose bisphosphate which is a five carbon sugar is immediately going with carbon dioxide to form the three carbon gp molecule so there's never really a high level of ribulose bisphosphate at all inside the stroma but of course if the calvin cycle's going to keep turning and continue then the ribulose bisphosphate has to be regenerated so that it can keep going around so we have to keep building ribulose bisphosphate so because of this 10 tp molecules that get made out of every 12 tp molecules are used to regenerate six molecules of ribulose bisphosphate so essentially what's happening here is the calvin cycle is going round and round but if it kept going the ribose bisphosphate level would go down and the calvin cycle would have to stop so what we've worked out is that if you have six carbon dioxides you can form 12 gp molecules if you form 12 gp molecules you're going to form 12 tp molecules in this process you'll use 12 atp to make 12 adp and 12 reduced nadp to make 12 nadp so we've got 12 trios phosphates out of every 12 that get used 10 of these will go to form six rubylosbis phosphates because if you imagine the trios phosphate has three carbons and if we have 10 of them that's 30 carbons so from this we can make six rubios bisphosphates because each ribulose bisphosphate is five carbons and so if there's 30 carbons altogether we can have six lots of these and in this process we make 5 adp from 580p so by using this the cycle to turn lots and lots of times we can use more molecules to continue the cycle to go round but also allow rubidos bisphosphate to be regenerated the remaining two molecules out of every 12 molecules of triphosphate are the product and these ones are the ones used to make organic compounds for example glucose so although it sounds like a lot of tries phosphates are going to make the ribulosa bisphosphate again two of them out of those original 12 will go to make the glucose and the useful products of photosynthesis 10 of them will go to make ribose bisphosphate but it has to keep turning so this can happen and continue to make glucose hey guys i hope you enjoyed the video if you are looking for an amazing a level biology resource join me today in my series of engaging bite size video tutorials just click the snap revise smiley face and together let's make a level biology a walk in the park