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Current time:0:00Total duration:20:16

Photosynthesis: Overview of the light-dependent reactions

Video transcript

in the last video we learned a little bit about both photosynthesis it and we know in very general terms it's the process where we start off with photons and water and carbon dioxide and we use that energy and the photons to fix the carbon and now there's this idea of carbon fixation is essentially taking carbon in a gaseous form in this case carbon dioxide and fixing it into a solid structure and that solid structure we fix it into is a carbohydrate the first end product of photosynthesis was this three carbon chain this glyceraldehyde 3-phosphate but then you can use that to build up glucose or any other carbohydrate so with that said let's try to dig a little bit deeper and understand what's actually going on in these stages of photosynthesis remember we said there's two stages the light dependent reactions and then you have the light independent reactions I don't like using the word dark reaction because it actually occurs while the Sun is outside is actually occurring simultaneously with the light reactions it just doesn't need the photons from the Sun but let's focus first on the light dependent reactions the part that actually uses photons from the Sun or maybe I guess even photons from your from you know the the heat lamp that you might have in your greenhouse and uses those photons in conjunction with water to produce ATP and reduced reduce nad plus nadp+ to nadph remember reduction is gaining electrons or hydrogen atoms and it's the same thing because when you grain a hydrogen atom including its electron since hydrogen is not too electronegative you get to hog its electron so this is both gaining a hydrogen and gaining electron but let's study it a little bit more so before we dig little deep I think it's good to know a little bit about the anatomy of a plant so let me draw some some plant cells so plant cells actually have cell walls so I can draw them a little bit rigid so let's say that these are plant cells right here each of these squares these quadrilaterals is a plant cell and then in the plant cells you have these organelles called chloroplasts remember organelles are like organs of a cell they're subunits membrane-bound sets subunits of cells and of course these cells have nucleus is in DNA and all of the other things you normally associate with cells but I'm not going to draw them here I'm just going to draw the chloroplasts and your average plant cell and there are other and there are other types of living organisms that perform photosynthesis but we'll focus on plants because that's what we tend to associate with these plants each plant cell will contain 10 to 50 chloroplasts chloroplasts make them one in green on purpose because the chloroplasts contain chlorophyll which to our eyes appear green but remember they're green because they reflect green light and they absorb red and blue and other wavelengths of light that's why it looks green because it's reflecting but it's absorbing all the other wavelengths but anyway we'll talk more about that in detail but you'll have 10 to 50 of these chloroplasts right here and then let's zoom in on one chloroplast so if we zoom in on one chloroplast let me be very clear this thing right here is a plant cell that is a plant cell and then each of these green things right here is an organelle called the chloroplasts chloroplast and let's zoom in on the chloroplast itself if we zoom in on one chloroplast it has a it has a membrane like that as a membrane that looks like that and then the fluid inside of the chloroplast inside of its membrane so this fluid right here all of this fluid that's called the stroma the stroma of the chloroplast and then within the chloroplast itself you have these little stacks of these folded membranes these little folded stacks let me see if I can do justice here so maybe that's one two I'm doing these stacks each of these membrane bound you can almost view them as pancakes draw a couple more maybe you have some over here just so yeah maybe you have some over here maybe some over here so each of these flattish looking pancakes right here these are called thylakoids so this right here is a thylakoid that is a thylakoid that's a thylakoid the thylakoid has a membrane and this membrane is especially important we're going to zoom in on that in a second so it has a membrane I'll color that in a little bit the inside of the thylakoid so the space the fluid inside of the thylakoid right there that area that's light green color right there that's called the thylakoid space or the thylakoid lumen lumen and just to get all of our terminology out of the way a stack of several thylakoids so the stack of several thylakoids just like that that is called a grana that's a stack of thylakoids that is a that is a grand and this is an organelle and you know evolutionary biologists they believe that organelles were once independent organisms that then essentially teamed up with other organisms and started living inside of their cells so there's actually they have their own DNA so mitochondria is another example of an organelle that people believe that at one time mitochondria or the ancestors of mitochondria we're independent organisms and then teamed up with other cells and said hey if I produce your energy maybe you'll produce give me some food or or what not and and so they started evolving together they turn into one organism which makes you wonder what you know we might evolve well anyway that's the separate thing so there's actually ribosomes out here so you know that's good to think about just to realize it at one point in the evolutionary in evolutionary past this organisms or this organelles ancestor might have been an independent organism but anyway enough about that speculation let's zoom in again let's zoom in again on one of on one of these thylakoid membranes so I'm going to zoom in let me make a box let me well I made that let me zoom in right there so that's going to be my zoom in so let me make it really big just like this so this is my zoom in box so that little box is the same thing as this whole box so we're zoomed in on the thylakoid membrane so this is the thylakoid membrane right there and it's actually a phospholipid layer you know it has it has your hydrophilic hydrophobic at tails like that I mean I could draw it like that if you like but the important thing from photosystem supportive view is that it's this membrane and on the outside of the membrane right here on the outside you have the fluid that fills up the entire chloroplast so here you have the stroma and then this space right here is this is you inside of your thigh lockwood so this is the lumen so if I were to color it pink right there this is your lumen your thylakoid space and in this membrane and this might look a little bit familiar when you if you think about mitochondria and the electron transport chain what I'm going to describe in this video actually is an electron transport chain it might many people might not consider it the electron transport chain but it's the same idea same general idea so on this membrane you have these proteins and these complexes of proteins and molecules that span this membrane so let me draw a couple of them so let me I'll call this one photosystem two and I'm calling it that because that's what it is for the system - you have them maybe another complex and these are hugely complicated I'll do a sneak peek of what's know photosystem two actually looks like this is actually what photosystem two looks like so as you can see it truly is a complex these cylindrical things these are proteins these green things are chlorophyll molecules I mean there's all sorts of things going here and they're all jumbled together you know I think a complex probably is the best word it's a bunch of proteins bunch of molecules just jumbled together to perform a very particular function and we're going to describe that in a few seconds so that's what photosystem two looks like then you also have photosystem photosystem one and then you have other molecules that other complexes you have you know the cytochrome b6f complex and I'll draw that this column you'll do it in a different color right here I don't want to get too much into the weeds because the most important thing is just to understand so you have other protein complexes protein molecular complexes here that also span the membrane but the general idea I'll tell you the general idea then we'll go into the specifics of what happens during the light reaction or the light dependent reaction dependent reaction is you have some photons photons from the Sun they've traveled 93 million miles so you have some photons that go here and they excite electrons in a chlorophyll molecule and a chlorophyll a molecule and actually in photosystem 2 well I won't go into the details just yet but they excite a chlorophyll molecule so those electrons enter into a high energy state I mean I shouldn't draw it like that they enter into a high energy state and then as they go from molecule to molecule they keep going down in energy state but as they go down in energy state you have hydrogen atoms or actually I should say hydrogen protons without the electrons so you have all of these hydrogen protons hydrogen protons get pumped into the lumen they get pumped into the lumen and so you might remember this from the electron transport chain the electron transport chain has electrons went from a high potential a high energy state to a low energy state that energy was used to pump hydrogen's through a membrane and that's examined Rhea here the membrane is the thylakoid membrane but either case you're creating you're creating this gradient where because of the energy from from essentially the photons you enter a high at the electrons enter a high energy state to keep going into a lower energy State and then they actually go to photosystem one and then they get hit by another photon or that's a simplification that's what you can think of it and for another high energy state then they go to a lower lower lower energy state but the whole time that energy from the electrons going from a high energy state to a low energy state is used to pump hydrogen Pro turns into the lumen so you have this huge concentration of hydrogen protons and just like just like what we saw in in the electron transport chain that concentration is then of hydrogen protons is then used to drive ATP synthase so the exact same so let me see if I can draw that ATP synthase here so you might remember ATP synthase looks something like this where it literally so here you have a huge concentration of hydrogen protons so they'll want to go back into the stroma from the lumen and they do and they go through the ATP synthase let me do it a new color so these hydrogen protons are going to make their way back go back down the gradient and as they go down the gradient they literally it's like an engine and I go into detail on this one when I talk about respiration and that turns literally mechanically turns the this top part the way I drew it of the ATP synthase and it puts adp and phosphate groups together it puts ADP plus phosphate groups together to produce ATP ATP so that's the general very high overview and I'm going to go into more detail in a second but this process this process that we I just described is called photo phosphorylation let me do it in a nice color photo photo foss for relation why is it called that well because we're using photons that's the photo part we're using light we're using photons to excite electrons in chlorophyll as those electrons get passed from one molecule from one electron acceptor to another they enter into lower and lower energy states as they go into lower energy states that's used to drive literally pumps that let allow hydrogen protons to go from the stroma to the lumen then the hydrogen protons want to go back they want to I guess you could call it chemiosmosis they want to go back into the stroma and then that drives ATP synthase right here this is ATP synthase ATP synthase to essentially jam together a DPS and phosphate groups to perform to produce ATP now when I originally talked about when I originally talked about the light reactions the dark reactions I said well the light reactions has two byproducts it has ATP and it also has nad actually it has three it has ATP it and it also has NADPH NADP is reduced it gains these electrons and these hydrogens so where does that show up well if we're talking about non cyclic oxidative phosphorylation or non cyclic light reactions the final electron acceptor so after that electron keeps entering lower and lower energy states the final electron acceptor is na D P plus so once it accepts the electrons and a hydrogen proton with it it becomes n a D pH now you are I also said that part of this process water and this is this is actually a very interesting thing water gets oxidized to molecular oxygen so where does that happen so when I said appear in photosystem one that we have a chlorophyll atom that work sorry a chlorophyll molecule that has an electron excited and it goes into a higher energy state and then that electron essentially gets passed from one guy to the next that begs the question what can we use to replace that electron and it turns out that we use we literally use the electrons in water so over here you literally have you literally have h2o and h2o donates the hydrogen's and the electrons with it so you can kind of imagine it donates two hydrogen protons and two electrons to replace the electron that got excited by the photons because that electron got passed all the way over to photosystem one and eventually ends up in NADPH so you're literally stripping electrons off of water and when you strip off the electrons and the hydrogen's you're just left with the molecular oxygen now the reason why I want to really focus on this is that there's something profound happening here at least on a chemistry level something profound is happening here you're oxidizing oxidizing water and in the entire biological Kingdom the only place where we know something that is strong enough of an oxidizing agent to oxidize water to literally take away electrons from water which means you're really taking electrons away from oxygen so you're oxidizing oxygen the only place that we know that it's an oxidation agent is strong enough to do this is in photosystem two so it's a very profound idea that you know normally electrons are very happy in water they're very happy circulating around oxygens right oxygen is a very electronegative atom that's why we even call it oxidizing because oxygen is very good at oxidizing things but all of a sudden we found something that can oxidize oxygen that can strip electrons off of oxygen and then use and then give those electrons to the chlorophyll the electron gets excited by photons then those photons enter lower and lower lower energy states get excited again in photosystem one by another set of photons and then enter lower and lower and lower energy states and then finally end up at NADPH and then the whole time it entered lower and lower energy states that energy was being used to pump hydrogen across this membrane from the stroma of the lumen and then that that gradient is used to actually produce ATP so the next thing I'm going to give a little bit more context about what this means in terms of energy states of electrons and and what's at a higher or lower energy state but this is essentially all that's happening electrons get excited electrons get excited those electrons eventually end up at NADPH and as the electron gets excited and goes into lower and lower energy states it pumps hydrogen across the gradient produce and then that gradient is used to drive ATP synthase to generate ATP and then that original electron they get excited it had to be replaced and that's act that replaced electron is actually stripped off of h2o so the the hydrogen protons and the electrons of h2o is stripped away and you're just left with molecular our oxygen and just to get a nice appreciation of the complexity of all of this I showed you this earlier in the video but this is literally a I mean this isn't a picture of photosystem two you actually don't have cylinders like this but these cylinders represent proteins these right here these green these green kind of scaffold like molecules that's chlorophyll a and what literally happens is you have photons you have photons hitting hitting actually it doesn't actually always have to hit chlorophyll a can also hit what's called antenna molecules so antenna molecules are other types of chlorophyll and actually other types of molecules and so a photon or a set of photons comes here and maybe it excites some electrons that are in it doesn't have to be in chlorophyll a it could be in some of these other types of chlorophyll or in some of these other I guess you call them pigment molecules that will absorb these photons and then their electrons get excited and you can almost imagine it as a vibration but when you're talking about things on the quantum level vibrations really don't make sense but it's a good analogy they kind of vibrate their way to chlorophyll a and this is called this is called resonance energy resonance Ranas resonance energy they vibrate their way eventually to chlorophyll a and then in chlorophyll a you have the electron get excited the original the primary electron acceptor is actually this molecule right here pheophytin some people call it co and then from there it gets it keeps getting passed on from one molecule to another I'll talk a little bit more about that in the next video but this is fascinating I mean look how complicated this is in order to essentially excite electrons and then use those electrons to well to start the process of pumping hydrogen's across a membrane and this is an interesting place right here this is the water oxidation site so I got very excited about the idea of oxidizing water and so this is actually where it occurs in the photosystem ii complex that you actually have this very complicated mechanism because it's no joke to actually strip away electrons and hydrogens from an actual water molecule i'll leave you there in the next video I'll talk a little bit more about these energy states and I'll talk I'll fill in a little bit of the gaps about what some of these other molecules that act as as hydrogen acceptors or you can almost sumit electron acceptors along the process along the way
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