[Music] hi everybody this is andy from med school eu and in today's video we're gonna do part two of photosynthesis and more specifically we're gonna talk about the photosynthetic electron transport as well as the calvin cycle and we're going to see that through the calvin cycle we're finally going to produce our organic compound so in the last video we left off talking about photosystems and how light is going to be absorbed a photon of light is going to be absorbed on this structure on the thylakoid membrane called photosystem so here we're having a view of photosystem 2 more specifically and right now we're going to talk about the electron transport because there are several photosystems that are located in the thylakoid membrane and they're going to absorb photons of light and using this energy from the photon they're going to be able to pass on electrons down the thylakoid membrane and eventually the whole point of the electron transport the whole reason why we have it is to generate n a d p h molecule as well as the atp molecule because these two are the substrates needed for the calvin cycle to produce our organic compound so let's see how we make these two structures so what we've got is our photo system we already know the antenna complex and the reaction mechanism so what we have is the photosystem 2 is going to absorb light of 680 nanometers wavelength so at it specifically 680 nanometers the photon of light will be able to be absorbed at the antenna complex and passed down to the pigment molecule now keep in mind that the water here must be oxidized into our two protons and oxygen and because of this oxidation the electrons will be passed down to our special pigment molecule we call the chlorophyll however this could be any other pigment molecule that would accept the electron now what happens is it accepts the electron and when the photon of light hits the pigment molecule at 680 nanometers it's going to excite the electron and pass it down to the final electron acceptor and this is what we talked about in detail in the previous video on photo synthesis part 1. so what happens next well it's important to keep in mind is that we are building a proton motive force on the thylakoid lumen side of the membrane so there's going to be an increasingly positive charge here because we are adding these protons from hydrolysis of oxygen and there's going to be other mechanisms where the protons are being added to the lumen now looking at the photosystem the electron is going to be passed down to a molecule called plastiquinone so we're going to label this as plastiquinone plasti quinone and plastic there's multiple plasticquinones we call them plastiquinone pool like this plastic one on pool because there's multiple plastiquinone uh proteins they're going to be going around in between photosystem 2 and the cytochrome complex and what they do is they go through oxidation and reduction so for example right here right by the photosystem 2 the plastic one is going to be reduced by receiving the electron from the photosystem ii is going to be reduced so if we are receiving an electron we must also receive a positive charge a proton so the proton will be taken up by the plastic conor because of the reduction and plastiquinone is going to travel over to the cytochrome complex now in the cytochrome complex the plastiquinone is going to be oxidized releasing the proton and releasing the electron to the cytochrome complex now let's see what happens what we have is in the stroma the positive charge is going to continue to decrease because the proton is being removed by the plastiquinone and over here the the force is going to be continuously increasing because we're adding a proton from the plastic on each time it's being oxidized so that's the second mechanism how we're building our proton motive force now looking at the next step the electron is being passed down to cytochrome complex in the cytochrome complex the the electron will be passed down to a copper containing molecule called plastocyanin and this is a peripheral membrane protein because it exists on the lumen side of the thylakoid membrane and the electron will then be passed down to our photosystem one so don't get confused by the names photosystem one exists simply because as photosystem one simply because it was discovered first so this one was discovered first this one was discovered second therefore they labeled them first and second even though photosystem two would come earlier in our photosynthetic electron transport diagram so what we've got is now the electrons the electron is being received at 700 nanometer pigment molecule and keep in mind that water is not being hydrolyzed for this photosystem it's only being hydrolyzed for photosystem 2 but not for photosystem 1 because the electrons being passed down to photosystem 1 that was originally taken off the water now what happens is the photon of light of 700 nanometers is going to be absorbed by the pigment molecule and the electron will then be passed down to the primary electron acceptor now what happens next is that the electron will be passed over to a sulfur iron containing compound called ferredoxin and ferrodoxin receives the electron and is going to pass down electrons to nadp plus reductase the nadp plus reductase is an enzyme that catalyzes the reaction of a proton plus nadp plus and is going to equal nadph is going to produce our nadph molecule they'll be passed down to the calvin cycle so it's going to be important to recognize here as well that's the third mechanism of decreasing the positive charge and increasing the positive charge here as the proton is being taken up it's being taken away from the stroma therefore further decreasing that positive energy and over here we're going to continue to build up our protons from the hydrolysis of water and the placequinone pumping up our protons over to the other side now from the lumen so we got to see how our atp is being produced well because we built this proton motor force the protons are are wanna naturally by diffusion go over to the other side of the membrane because there is a lower concentration of protons on this side compared to the lumen side and because the protons want to be pumped over to the stroma they're going to be using a protein called atp synthase and atp synthase is specifically designed to catalyze the reaction of adp and phosphate and produce our atp molecule and the atp will be moved over to the kelvin cycle so this entire process is going to be called the linear linear electron transport because the electron is being transported from photosystem 2 down all the way down to nadp plus reductase and we're able to produce our nadph however what often happens is that our nadp plus is not available it's not there therefore atp plus reductase cannot use the electron that was originally given by the ferridoxin so the ferridoxin keeps the electron in that case and it migrates over to plastiquinone and it donates the electron over to plastiquinone to keep the cycle going and to at the minimum keep producing our atp even though nadph is not going to be produced we will keep producing our atp so that the cell could undergo its functions so now that we have produced our nadph from the electron transport and we also produced our atp we're able to go through the calvin cycle that occurs in the stroma of the chloroplast so what we've got is we have three phases the first phase is called the fixation phase and what this really means is that we're going to be fixating the carbon from carbon dioxide onto an organic molecule something that is uh organic 3-phosphoglycerate for example so what we've got is we have three carbon dioxides coming in from the atmosphere that were let in by the stomata and we have three molecules of five carbons called rubp and they're going to go through a carboxylation reaction in order to produce six molecules of three phosphoglycerate now this whole thing occurs through the action of an enzyme called rubisco and rubisco is actually a really fascinating enzyme because it is considered the most important enzyme in the biosphere because catalyzing co2 fixation in all autoautotrophs it provides the source of organic carbon for most of the world's organisms so that's why it's considered the most important enzyme in our biosphere as it's able to fixate the carbons from carbon dioxide onto an organic molecule and then all of these organic molecules will be able to use be used by animals and other consumers that will take the products of it and be able to hydrolyze it to create a lot of energy so the significance of rubisco is uh very famous however one important thing to know is that rubisco is not a very good catalytically only because it is not specific the binding site for carbon dioxide is not specific for the molecule of carbon dioxide because oxygen can also bind to it and undergo oxygenation reaction with rubp now this only occurs a quarter of the time and of course our carbon dioxide carboxylation reaction occurs three quarters of the time so it is more specific for the carbon dioxide however the oxygen could also bind to rubisco to undergo our this process the oxygenation and the oxygenation of of the rubp through rubisco does not create our preferable 3-phosphoglycerate it creates a molecule called glycolate which will then be just simply converted to carbon dioxide and released into the atmosphere so it is not catalytically the oxygen will not be useful if it binds to our rubisco so we can then consider that oxygen is a competitive inhibitor of rubisco because it's going to compete for the binding site with carbon dioxide and this is why plant cells have their waxy cuticle to prevent any water loss and to prevent gas exchange and this is why they only have the stomata that will promote the carbon dioxide going in and the oxygen going out to prevent our oxygenation reaction with rubisco so let's assume everything is perfect and we've got our three carbon dioxides being fixated onto organic molecules and another thing to consider is that in this diagram i am showing three carbon dioxides going in at the same time and combining with three rubp molecules however in real the car the calvin cycle occurs with one carbon dioxide at a time i just have to show you with three carbon dioxides because this is after only three cycles were able to produce one unit of our organic compound that will be used to produce glucose or fructose or any other organic compounds that the plant uses so again what we have is our 3-phosphoglycerate it is going to be phosphorylated by atp so atp will be hydrolyzed into adp and the phosphate will be taken up by the organic molecule and we have now created our 1 3 bisphosphoglycerate now looking at the next step of the reaction is that our 1 3 bisphosphoglycerate is going to undergo phase two which is reduction stage and reduction involves the uptake of electrons so we're going to gain electrons with this molecule and it's going to do that by gaining the electrons of nadph so we have six molecules of one three bisphosphoglycerate they're going to be reduced by six molecules of nadph and the phosphate will be released and what we get is the production of six g3p molecules and because it's six g3p molecules we're producing one g3p molecule in excess right here so we're producing one extra and this one extra will then go on to produce our organic compounds c6h12o6 so here we have glucose for example so our organic compound three carbons will be this would be the substrate used to produce all kinds of organic compounds so that's the reduction phase and now what we get is from the six g3ps one will be eliminated because it's in excess and we only need five g3ps to keep the kelvin cycle going and what we get is the last stage is the five g3ps undergo regeneration so we're going to use three atps and that's going to go undergo another phosphorylation reaction where phosphate will be added on to the carbon molecules and we produce three molecules of ru bp and this can undergo the cycle once more by fixating more carbon dioxide so this concludes our two-part lecture on photosynthesis and in the next video we're going to begin looking at cellular respiration and the first step of cellular respiration will be glycolysis [Music] you