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Photosynthesis: Overview of the light-dependent reactions

Overview of the light-dependent reactions, including the structure of the chloroplast, the photosystems, and how ATP is synthesized. Created by Sal Khan.

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Video transcript

In the last video we learned a little bit about 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, this idea of carbon fixation is essentially taking carbon in the 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 3-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. It's 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 actually, I guess, even photons from the heat lamp that you might have in your greenhouse. And uses those photons in conjunction with water to produce ATP and reduce NADP plus to NADPH. Remember, reduction is gaining electrons or hydrogen atoms. And it's the same thing, because when you gain 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 a little deeper, I think it's good to know a little bit about the anatomy of a plant. So let me draw 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, each of these quadrilaterals is a plant cell. And then in these plant cells you have these organelles called chloroplasts. Remember organelles are like organs of a cell. They are subunits, membrane-bound subunits of cells. And of course, these cells have nucleuses and 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 types of living organisms that perform photosynthesis, but we'll focus on plants. Because that's what we tend to associate it with. Each plant cell will contain 10 to 50 chloroplasts. I make them 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. 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 organelle called the chloroplast. And let's zoom in on the chloroplast itself. If we zoom in on one chloroplast, it has a membrane 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, doing these stacks. Each of these membrane-bound-- you can almost view them as pancakes-- let me draw a couple more. Maybe we have some over 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 thylakoids. So this right here is a thylakoid. That is 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. This light green color right there. That's called the thylakoid space or the thylakoid lumen. And just to get all of our terminology out of the way, a stack of several thylakoids just like that, that is called a grana. That's a stack of thylakoids. That is a grana. And this is an organelle. And 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 one time mitochondria, or the ancestors of mitochondria, were independent organisms. That then teamed up with other cells and said, hey, if I produce your energy maybe you'll give me some food or whatnot. And so they started evolving together. And they turned into one organism. Which makes you wonder what we might evolve-- well anyway, that's a separate thing. So there's actually ribosomes out here. That's good to think about. Just realize that at one point in the evolutionary past, this organelle's ancestor might have been an independent organism. But anyway, enough about that speculation. Let's zoom in again on one of these thylakoid membranes. So I'm going to zoom in. Let me make a box. 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 thylakoid membrane. So this is the thylakoid membrane right there. That's actually a phospho-bilipd layer. It has your hydrophilic, hydrophobic tails. I mean, I could draw it like that if you like. The important thing from the 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 space right here, this is the inside of your thylakoid. 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 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. 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 maybe I'll call this one, photosystem II. And I'm calling it that because that's what it is. Photosystem II. You have maybe another complex. And these are hugely complicated. I'll do a sneak peek of what photosystem II actually looks like. This is actually what photosystem II 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. I think a complex probably is the best word. It's a bunch of proteins, a bunch of molecules just jumbled together to perform a very particular function. We're going to describe that in a few seconds. So that's what photosystem II looks like. Then you also have photosystem I. And then you have other molecules, other complexes. You have the cytochrome B6F complex and I'll draw this 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 and then we'll go into the specifics-- of what happens during the light reaction, or the light 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 chlorophyll A molecule. And actually in photosystem II-- 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. Maybe 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. In the electron transport chain, as electrons went from a high potential, a high energy state, to a low energy state, that energy was used to pump hydrogens through a membrane. And in that case it was in the mitochondria, here the membrane is the thylakoid membrane. But either case, you're creating this gradient where-- because of the energy from, essentially the photons-- the electrons enter a high energy state, they keep going into a lower energy state. And then they actually go to photosystem I and they get hit by another photon. Well, that's a simplification, but that's how you can think of it. Enter another high energy state, then they go to a lower, lower and 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 protons into the lumen. So you have this huge concentration of hydrogen protons. And just like what we saw in the electron transport chain, that concentration is then-- of hydrogen protons-- is then used to drive ATP synthase. So the exact same-- let me see if I can draw that ATP synthase here. You might remember ATP synthase 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 stroma from the lumen. And they do. And they go through the ATP synthase. Let me do it in 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 I talk about respiration. And that turns, literally mechanically turns, 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. So that's the general, very high overview. And I'm going to go into more detail in a second. But this process that I just described is called photophosphorylation. Let me do it in a nice color. 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 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 ADPs and phosphate groups to produce ATP. Now, when I originally talked about the light reactions and dark reactions I said, well the light reactions have two byproducts. It has ATP and it also has-- actually it has three. It has ATP, 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 photophosphorylation, 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 NADP plus. So once it accepts the electrons and a hydrogen proton with it, it becomes NADPH. Now, I also said that part of this process, water-- and 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 I, that we have 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 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. Because that electron got passed all the way over to photosystem I and eventually ends up in NADPH. So, 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. Or at least on a chemistry level, something profound is happening. You're 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 an oxidation agent is strong enough to do this is in photosystem II. So it's a very profound idea, that normally electrons are very happy in water. They're very happy circulating around oxygens. 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've found something that can oxidize oxygen, that can strip electrons off of oxygen and then give those electrons to the chlorophyll. The electron gets excited by photons. Then those photons enter lower and lower and lower energy states. Get excited again in photosystem I by another set of photons and then enter lower and lower and lower 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 to lumen. And then that gradient is used to actually produce ATP. So in the next video I'm going to give a little bit more context about what this means in terms of energy states of electrons and what's at a higher or lower energy state. But this is essentially all that's happening. 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. And then that gradient is used to drive ATP synthase, to generate ATP. And then that original electron that got excited, it had to be replaced. And that replaced electron is actually stripped off of H2O. So the hydrogen protons and the electrons of H2O are 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 showed you this earlier in the video-- but this is literally a-- I mean this isn't a picture of photosystem II. You actually don't have cylinders like this. But these cylinders represent proteins. Right here, these green kind of scaffold-like molecules, that's chlorophyll A. And what literally happens, is you have photons hitting-- actually it doesn't 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, 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 could 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 resonance energy. They vibrate their way, eventually, to chlorophyll A. And then in chlorophyll A, you have the electron get excited. The primary electron acceptor is actually this molecule right here. Pheophytin. Some people call it pheo. And then from there, 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. Look how complicated this is. In order to essentially excite electrons and then use those electrons 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 II complex. And 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. And in the next video I'll talk a little bit more about these energy states. And I'll fill in a little bit of the gaps about what some of these other molecules that act as hydrogen acceptors. Or you can also view them as electron acceptors along the way.