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Photosynthesis Stages Overview · Acedra
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Photosynthesis Stages Overview
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0:00
in the last video we learned a little bit about photo photosynthesis 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 in the photons to fix the carbon and now uh 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
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three carbon chain this glycer aldhy 3 phosphate but then you can use that to build up glucose or any other carbohydrate so uh 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 it's actually occurring
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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 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 NAD d p+ 2 NAD D pH remember reduction is gaining electrons or hydrogen atoms and it's the same
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thing because when you grain a hydrogen atom including its electron since hydrogen is not two electr negative 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 a 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
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are plant cells right here each of these squares each of these quadrilaterals is a plant cell and then in the plant cells you have these organel called chloroplasts remember organel are like organs of a cell they're subunits membranebound SE subunits of cells and of course these cells have nucleuses and DNA and all the other things you normally associate with cells but I'm not going to draw them here I'm just going to draw the chloroplast and your average plant cell
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and there are other an 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 I make them on in green on purpose cuz the chloroplast contain chlorophyll which to our eyes appear green but remember remember they're green because they reflect green light and they absorb uh red and blue and other wavelengths of light that's
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why it looks green CU 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 so 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 organel called the chloroplasts chloroplast and let's zoom in on the chloroplast
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itself if we zoom in on one chloroplast it has a it has a membrane like that has a membrane that looks like that and then the fluid inside of the chloroplast inside of its membrane so this fluid right here 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
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these membrane bound you can almost view them as pancakes let me draw a couple of more maybe you have some over here here just so you maybe you have some over here maybe some over here so each of these flattish looking pancakes right here these are called thids so this right here is a thid that is a thid that's a thid the thid 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
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the thid so the space the fluid inside of the thilo covid right there that area this light green color right there that's called the thoy space or the thid Lumen Lumen and just to get all of our terminology out of the way a stack of several thids so this stack of several thids just like that that is called a Grana that's a stack of thids that is a that is a and this is an organel and you know evolutionary biologists they believe that organel were once independent organisms that
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then uh uh 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 organel that people beli that at one time mitochondria or the ancestors of mitochondria were 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 whatnot not and and so they started evolving together and they turned into
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one organism which makes you wonder what you know we might evolve well anyway that's a separate thing so there's actually ribosomes out here so you know that's good to think about just to realize that at one point in The evolutionary in evolutionary past uh this organism's or this organ's 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 thid membranes so I'm going to zoom in let me make a
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box let me well I I made that let me zoom in right there so that's going to be my zoom in box 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 thid membrane so this is the thid membrane right there and it's actually a phospho bilipid layer you know it's it has it has your hydrophilic hydrophobic Tails like that I mean I could draw it like that if you like but the important thing
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for photosynthesis point of 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 base right here is this is the inside of your thid so this is the Lumen so if I were to color it pink right there this is your Lumen your th oid space and in this membrane and this might look a little bit familiar when you if you think about mitochondria
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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 me I'll call this one photo system 2 and I'm calling it that because that's what it is photos
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system 2 you have maybe another complex and these are hugely complicated I'll do I'll do a sneak peek of what photos system 2 actually looks like this is actually what photos system 2 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 to you know I think a complex probably is the best word it's
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a bunch of proteins bunch of molecules uh uh uh just jumbled together to to perform a very particular function and we're going to describe that in a few seconds so that's what photos system 2 looks like then you also have photos system photos system one and then you have other molecules that other complexes you have the you know the cytochrome b6f complex and I you I'll draw that in this we do in a different color right here I don't want to get too much into the weeds because the most
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important thing is just to understand so you we have other uh protein complexes protein molecular complexes here that also span the membrane but the general idea I I'll tell you the general idea and 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 in a
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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 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 an 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
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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 and the electron transport chain as electrons went from a high uh potential a high energy state to a low energy state that energy was used to pump hydrogens through a membrane and that's and in that case it was in the mitochondria here the membrane is the thid membrane but either case you're creating you're creating this gradient
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where uh because of the energy from from essentially the photons you enter a high the electrons enter a high energy State they keep going into a lower energy State and then they actually go to photos system one and then they get hit by another Photon or that's a simplification that's what you can think of it enter another high energy State then they go to a lower lower lower energy State the whole time that energy from the electrons going from a high energy state to a low energy state is
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used to pump hydrogen protons 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 of hydrogen protons is then used to drive ATP synthes so the exact same so let me see if I can draw that ATP synthes here so you might remember ATP synthes looks something like this where literally so here you have a huge concentration of hydrogen protons so they'll want to go back into the
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stroma from the Lumen and they do and they go through the ATP synthes let me do 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 when when I talk about respiration and that turns literally mechanically turns the this top part the way I drew it of the ATP synthes and it puts ADP and phosphate groups together it puts ADP plus phosphate groups together to
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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 phosphorilation let me do it in a nice color photo photoos foration 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
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into lower and lower energy states as they go into lower energy states at that's used to drive literally pumps that allow hydrogen protons to go from the stroma to the Lumen then the hydrogen protons want to go back they want to uh I guess you could call it chemiosmosis they want to go back into the stroma and then that drives ATP synthes right here this is ATP synthes ATP synthes to essentially Jam together adps and phosphate groups to perform to produce ATP
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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 hydrogen so where does that show up well if we're talking about noncyclic oxidated for phosphorilation or or nony light reactions the final electron acceptor so after that electron
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keeps entering lower and lower energy states the final electron acceptor is n dp+ so once it accepts the electrons and a hydrogen proton with it it becomes n a d pH now you I 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 up here in photosystem one that we have a chlorophyll atom that or sorry a chlorophyll molecule that has an electron excited and it goes into a
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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 hydrogens 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
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because that electron got passed all the way over to photos system one and eventually ends up in nadph So you you're literally stripping electrons off of water and when you strip off the electrons and the hydrogens you're just left with 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
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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 you means you're really taking electrons away from oxygen so you're oxidizing oxygen the only place that we know that an oxidation agent is strong enough to do this is in photosystem 2 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
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oxygen is a very electronegative uh 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 photos system one by another set of photons and then enter lower and lower and lower
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energy states and then finally end up at nadph and the whole time it entered lower and lower energy states that energy was being used to pump hydrogen across this membrane from the stroma the Lumen and then that that gradient is used to actually produce ATP so in the next video I'm going to 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
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electrons get excited electrons get excited those electrons eventually end up at nadph and uh as the electron gets excited and goes into lower and lower energy states it pumps hydrogen across a gradient produce and then that gradient is used to drive ATP synthes to generate ATP and then that original electron that got excited it had to be replaced and that's that that replaced electron is actually stripped off of H2O so the the hydrogen protons and the electrons of
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H2O is stripped away and you're just left with molecular oxygen and just to get a nice appreciation of the complexity of all of this I I showed you this earlier in the video but this is literally a I mean this isn't a picture of photos system 2 you actually don't have cylinders like this but these cylinders represent proteins uh these uh right here these green these green kind of uh scaffold like uh molecules that's chlorophyll a and what literally happens is you have
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photons you have photons Hing hitting actually it doesn't actually always have to hit chlorophyll a it 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 chlorop a it could be in some of these other types of chlorophyll or in some of these other uh uh I guess you call them
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pigment molecules that will absorb these photons and then there 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
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resonance Reen 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 fopen some people call it Foo 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 uh essentially uh
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excite electrons and then use those electrons to uh well to start the process of pumping hydrogens 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 2 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
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there in the next video I'll talk a little bit more about uh these energy states and and I'll talk I'll fill in a little bit of the gaps about what some of these other molecules that uh that act as as hydrogen acceptors or you can almost do as electron acceptors along the process along the way
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