Photophosphorylation

Transcription

I want to review a little bit of what we did in the last video. And maybe draw a larger, more spread-out diagram. Because I think in the last video I started to cram things on the right-hand side here. And this is a very important concept, so I want to do it nice and spread-out in a way that we can breathe. And maybe in the process I can fill in some more blanks. So let's go back and draw the membrane of a thylakoid that's sitting inside of a chloroplast. I'm going to draw this same membrane here. So let me draw it nice and spread out. So let me draw a nice big membrane like that. That's the inside of the membrane. So you can imagine that this loops around and that would form the thylakoid. On this side of the membrane we have the lumen. And on the outside of the membrane we have the stroma, where all the fluid that fills up the choloroplast. So this is the stroma right there. And this is just a kind of standard membrane that we see in a lot of organelles. But this is actually a membrane within an organelle. And then maybe there will be a phospho-bilipid layer. And I just say that, or I'm pointing that out because I want to think a little bit about, in this video, how protons can actually get across this thing. How do we use the energy from these electrons going to lower energy states to actually pump protons across this membrane. So you know when you have these bilipid layers, your outside is hydrophilic. And of course, it's hydrophilic because it operates well in a polar environment. And then the insides are non-polar or they're hydrophobic and you have these tales. So I could draw the whole membrane like that, but I won't do that. It will take me forever. But let me draw some of the components that I did in the last video. So we have these complexes that span across this membrane. And the place we started off with was the photosystem II complex. And then later on we have the photosystem I complex. And let me draw the ATP synthase right here. So ATP synthase also spans across it. Then it has little motor part of it. And the hydrogens go through and it spins the motor and it crams the phosphate groups into the ADP to make ATP. I'll talk about that in a second. But the first thing I want to point out is, as I said in the last video, the first place where the electrons get excited is in the chlorophyll and photosystem II. And then it gets less and less and less excited, it gets headed off from one complex to another complex. And eventually ends up in photosystem I. It gets excited again. Then it gets handed off, handed off, the whole time that energy is being used to transfer hydrogen protons from the stroma into the lumen . But the first question that I would ask is, why is this called photosystem II, while this is called photosystem I when we're starting over here? And the reason is, this was discovered first. Even though in the light reaction it actually comes into use, or it comes into play, second. This was discovered first. That's why they call it photosystem I. But the reality is photosystem II is where everything starts from. Now in some textbooks you'll also see this written as P680. And you'll see photosystem I written as P700. And these numbers come from the wavelength of light that is best absorbed by the chlorophyll in these photosystems. So 680 corresponds to 680 nanometers. That's the wavelength of light that this absorbs the best. 700 corresponds to 700 nanometers. That's the wavelength of light it absorbs best. Now what I want to do here is draw a little diagram below here to kind of talk more about the electron energy states. I just kind of handwaved it a little bit in the last video. So I'm going to draw a little diagram here. And over here I'm going to write the different things that the electron can be a part of. So right now the electrons can be part of H2O. It could be part of chlorophyll A. It could be a part of-- I'll talk more about this in a little bit-- pheophytin And then you have all of the molecules or the complexes it can become a part of. I'll actually write them down here. So let me write. I don't want to take up too much space. Plastoquinone and then there's a cytochrome B6F complex. I'll just write B6F. Then you have plastocyanin . I'll just write as PC. You don't have to memorize these. You'll forget them in a week if you do. But unless you're studying photosynthesis, then it might make sense to memorize them. And this is in photosystem II. Then you have chlorophyll in photosystem I. And then you have some other, you know you have ferredoxin I'll just write FD for ferredoxin Some other molecules, you keep going and then you have your eventual electron acceptor NADP plus. Which, once it accepts the electron, becomes NADPH. Now, electrons are very-- so this is, if we go up that's a high energy state, down it's a low energy state. So electrons are already very comfortable. in water. And in chlorophyll A they're even more comfortable. At least this is how I view it. But left to its own devices, this electron will never leave chlorophyll A. But we know what happens. A photon comes in from 93 million miles away. You could imagine photons as little light packets or you could view it as a light wave. Either way. And it excites-- not necessarily directly the chlorophyll A. It might excite other antenna chlorophyll or other pigment molecules. And then through resonance energy, you can imagine them vibrating, and it eventually will excite the photophorylation A directly. Or excite the electrons in chlorophyll A directly. And this dude right here gets excited. Let me do that in a brighter color. So it goes to a high energy state. So the electron here is in a high energy state. Ignore that lumen right there. It has nothing to do with this electron. And then it goes-- and actually when it goes to the high energy state, maybe I should draw it like this, it actually shows up in pheophytin. That is the primary electron acceptor. And it's actually a chlorophyll A molecule. Actually, let me show you what a chlorophyll A molecule looks like. This is what a chlorophyll A molecule looks like. In general, it has a hydrocarbon tail. You see that right here. And it has a porphyrin ring. Or porphyrin head, I guess you could call it. This little group right here is called a porphyrin. And right in the center of it, you have a magnesium. That green right there, that's a magnesium ion. And when the photon comes in or when the resonance energy comes in from some of the antenna molecules, electrons in the double bond sitting here in the porphyrin head get excited. Those are the electrons that we're talking about. And they get excited. And the first electron acceptor is this pheophytin that I just talked about. Pheophytin. It actually looks just like a chlorophyll, but it has no magnesium ion in the middle. And maybe I'm getting a little bit into the weeds a little bit too much. But the pheophytin, you actually see in this diagram right here. It's part of this photosystem complex. So the electron, you can imagine, jumping from the chlorophyll to the pheophytin that does not have that magnesium in the center. And when it sits in the pheophytin it's at a very, very, very, very, high energy state. And then it keeps being transferred from the pheophytin. It goes to the plastoquinone So maybe we go to a slightly lower energy state here. We keep using the electron in green. Then it keeps going to a slightly lower energy state in the cytochrome B6F complex. And then you have the plastocyanin complex, lower energy state. And then eventually it goes into photosystem I at an even lower energy state. Maybe slightly higher than the energy state that it was originally in the chlorophyll A molecule in photosystem II. Another photon or another set of photons comes and hits photosystem I. Maybe its antenna molecules, through resonance energy, that excites the electron. It might directly hit the chlorophyll in photosystem in its reaction center. And then this gets excited again. And so once again we have an electron with a high potential that can keep going to, from one molecule to another as it gets more and more comfortable. And this releases energy that can drive the proton pump. And it eventually ends up in the NADPH. At a fairly high level of energy still. This electron can still be transferred to other things and release energy. And we'll see that when we talk about the light independent reactions. Now the whole point of me showing you this is, I wanted to kind of depict graphically that the electron is starting off at a pretty low energy state. And the only way this happens is by energy from light. This would not happen on its own. Going from a low energy state to a higher energy state. And I touched on in the last video, you have this electron going here and it gets transferred from one molecule to another. Gets excited again, then keeps going all the way, eventually being accepted by the NAD plus to become NADPH. And you're like, where did that H come from? You could say, well that H is a proton. It gets that electron and then they merge together and you have NADPH. But either way. But the question is, what replaces this electron? And that's where that amazing thing that I talked about in the last video happens. Water gets oxidized. Oxidizing is losing electrons. OIL RIG. So water gets oxidized by the water oxidation on photosystem II. And that electron ends up and replaces the electron in the chlorophyll. So once again, that's an amazing idea, that you're oxidizing oxygen. So the net effect of what happens is, is you're using energy, using this photon energy right here, to essentially strip electrons off of water. And as you know, when it's on water it's spending most of its time on the oxygen. So it essentially strips electrons off of oxygen and put them in a higher energy state and have them end up on NADPH. And in the process, it had gone to an even higher energy state. And then as it goes down to NADPH, you are pumping protons across the membrane. We learned in the last video, through chemiosmosis, eventually goes through the ATP synthase channel, turns around this part of this protein complex or enzyme complex and actually generates ATP. ATP from ADP and phosphate groups. And in the electron transport chain video, when I talk about cellular respiration, I give a visual concept of how this actually might happen. How you could, as these go through, you actually can jam together the ATP and the ADP. So that's another question in my head is, we talk about these electrons going from one molecule to another. But how does that actually pump hydrogen through? And I'm just going to do a very gross oversimplification. I'm sure it's much more complicated in actual plant cells. But you could imagine that we have our pheophytin right here that has that electron on it right there. Maybe it has its electron right there. This is a gross oversimplification. And then you have your plastoquinone right here. That's the next acceptor. Now maybe on this protein complex right there, the point that wants to accept the electron is right there. And let's say that there's another point on it that can accept a hydrogen. Maybe it accepts a hydrogen proton there. So you can imagine when it's on this side of the membrane, a hydrogen can become attached right there. And this guy will want to be attracted to that. So he'll rotate around. So you can imagine this-- if this is kind of a wheel-- this attraction. Because the electron wants to go into a lower energy state right here. It'll rotate around. That'll allow, essentially, this hydrogen as it rotates. As this molecule, as this protein rotates around this hydrogen will be able to cross the barrier. And then once this guy and that guy meet, then the hydrogen will be on the other side. And so it can freely go away again. So that's, at least in my head, how I imagine the electrons going from a high energy state to a lower energy state, how that can actually drive a reaction. Remember, the electrons want to do this. So they'll attract the different parts of the molecules together. And as those molecules turn and rotate and move, that can help facilitate hydrogen going from the stroma, the outside of the thylakoid, to the inside of the thylakoid. That will drive the chemiosmosis later on. Now there's one other point I want to touch on here. Everything I've described so far, we started with an electron in water. And obviously when water loses its hydrogens, it loses both the hydrogen protons and electrons associated with it. You end up with just water. So you start off with hydrogens and then you end up with just O2. And then the hydrogen protons-- the electron got taken up by the chlorophyll. When you start off with that, we've seen already that you end up with the electron sitting in NADPH. The electron sitting out here in NADPH. At some point you have NAD as the final acceptor. Let me do it in the right color. You have NAD plus as the final acceptor. And it becomes NADPH. You can imagine it accepts maybe a hydrogen proton from out here. It accepts the electron from this electron transport chain in photosynthesis. And it becomes NADPH and that travels in the stroma, which is where the dark reactions occur that actually produce the carbohydrate. But this idea of an electron going from water to NADPH, this is called non-cyclic photophosphorylation And it's called non-cyclic because you're not reusing the same electrons over and over again. The electrons start off, and depending how you view it, in the chlorophyll or the water. And they end up in the NADPH. Now there's another type of photophosphorylation and you might guess what it's called. It's called cyclic. Cyclic photophosphorylation We'll see when we study the dark cycles or the Calvin cycle or the dark reactions or the light independent reactions, that it uses a lot of ATP. Actually ATP in disproportion to the amount of NADPH it uses. It uses both, but it uses a ton of ATP. So cyclic phosphorylation only produces ATP and actually does not oxidize water. So what happens in that situation is this electron, after it gets activated or after it gets excited in photosystem I, it's the electron, it eventually ends up-- instead of at NADPH, it ends up at photosystem II. So instead of this guy having to be replaced by electrons from water, this guy, in cyclic photophosphorylation ends up-- well, maybe I should do it from here-- ends up getting replaced by the original electrons. It gets excited here. It goes from molecule to molecule, lower energy states, hydrogen gets pumped into the lumen. Gets excited again in photosystem I and it enters lower and lower energy states. But then ends up again in photosystem II. That is cyclic photophosphorylation. So you can imagine in this situation, since the electron never ends up at NAD plus, you don't end up producing NADPH. And since you're replacing this electron from the photosynthesis or from the electron transport chain directly, you don't have to strip the electrons off of the water. So you're not going to produce your oxygen. So, in this situation-- so this non-cyclic phosphorylation, which is kind of what most photophosphorylation, is what most people associate with photophosphorylation, this produces O2 and NADPH. And of course it produces ATP. While cyclic photophosphorylation, because it doesn't have to strip electrons off of water and the electrons don't end up at NADPH, only produces ATP. So I think we now have a very good understanding, hopefully, of the light reactions in photosynthesis. We're now ready to take the products of this. Now let's remember what the products were. Well, the oxygen just gets eliminated. We don't need the oxygen anymore. But that goes into the atmosphere and you and I can breathe that and we can use that for cellular respiration. But in the photosynthesis context, we've now generated a bunch of ATP. And now we have a bunch of NADPH. And we can use that in conjunction with carbon dioxide to produce actual carbohydrates in the stroma. Outside the thylakoids, but we're still inside of the chloroplast. And I'll cover that in the next video.